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The real-time monitoring of the pH values of alcoholic beverages was performed with a compact wireless device based on a colorimetric detection method with the Hue (H) as the analytical signal working in a pH range of 2.50–6.50. This device represents the first colorimetric pH meter reported in the literature monitoring in real-time the pH value of colored solutions. This pH meter consists of I) a nitrocellulose membrane impregnated with a pH-sensitive gel; II) a CCD camera for color acquisition; III) an electronic board with the calibration profiles of H vs. pH, and IV) a display to read the measured pH. It was applied to the pH determination of a white wine, a prosecco white wine, and a double malt beer leading to the values of pH wine = 3.30, pH prosecco = 3.33, pH beer = 4.29. The analytical performance is comparable to the glass electrode with an accuracy error ≤ 0.05 pH units. [Display omitted] • First colorimetric pH meter with real-time monitoring of wine and beer pH. • The nitrocellulose support enables the in-line detection of pH. • Sensor reusable, robust, and stable over one month. • pH errors comparable to the ones of the glass electrode. [ABSTRACT FROM AUTHOR]

• Optical fiber sensor for pH measurement as low-cost solution. • pH sensitive material as polyaniline using synthetic oxidative polymerization on optical fiber surface. • Transition between protonated and deprotonated polyaniline increasing RI of the layer as the pH increases. • FBGs inscribed by UV laser in tapering fiber and D-shaped fiber as optical solution. • Sensors with good reproducibility and repeatability with excelent sensitivities for tapers and D-shape fibers. Optical fiber sensors show potential for monitoring multiple parameters essential to the growth and health of fish, particularly the pH value of the aquaculture medium. Two types of optical fiber sensors, using tapered silica single mode fibers with a Bragg grating inscribed and D-shaped plastic optical fibers (home-made and reused) were produced. A pH-sensitive layer made of polyaniline (PANI) was deposited on the optical fibers' surface. The sensors demonstrated reproducibility and repeatability with a sensitivity of −0.22 ± 0.02 dB/pH, −472 ± 9 Counts/pH, and −(26 ± 2) × 102 Counts/pH for tapers, D-shape home-made, and reused, respectively, but only for increasing pH. Stabilization time tests indicated that a minimum of 60 s is required for a stable response, with no significant cross-sensitivities regarding temperature and ionic strength. [ABSTRACT FROM AUTHOR]

The measurement of arterial oxygen saturations, end-tidal carbon dioxide and pH are all key to modern anaesthetic practice. They can all be measured in a variety of ways, but the most common are discussed in this article. The understanding of the underlying physical principles and how the anaesthetist monitors function to measure these variables is discussed in this article, including limitations and inaccuracies of each technique. [ABSTRACT FROM AUTHOR]

[Display omitted] • The stability domain of Be(OH) 2 was detremined by thermodynamic calculations and pH measurements. • The degree of passivation of Be(OH) 2 was evaluated by open circuit potential measurements and electrochemical impedance spectroscopy as a function of the pH solution. • Be is protected against the corrosion from neutral to basic pH. The corrosion is the lowest around pH 12. Beryllium wastes produced by nuclear industry could be managed by encapsulation in cements. The main risk is the aqueous corrosion, which leads to the hydrogen production and cracks causing a loss of radioactivity confinement. However, the corrosion can be limited by the formation of the hydroxide solid phase Be(OH) 2(s). A study on the influence of the pH on the reactivity of beryllium was carried out in aqueous solution. The solubility diagram was drawn and showed that the hydroxide solid phase is stable until very high basic pH of 14. The pH variation of solution at different initial pH was measured after immersion of metallic beryllium. The results are in good agreement with the calculated solubility diagram. The open circuit potential (OPC) and the electrochemical impedance spectra (EIS) were recorded at a beryllium electrode from pH 2 to 15. The simulation of the EIS by using an electric equivalent circuit allowed to evaluate the Be reactivity by measuring the charge transfer resistance as a function of the pH. The experimental results confirm that the corrosion of beryllium is limited at neutral to basic pH and this low reactivity was attributed to the formation of the hydroxide solid phase Be(OH) 2. The OCP and EIS measurements put also in evidence that the corrosion of beryllium is the lowest in solution having pH around 12. In this pH range, the Be metal would be protected against the corrosion both by the formation of the hydroxide solid Be(OH) 2 and by the adsorption of hydroxide ions (OH−) at the metal surface. [ABSTRACT FROM AUTHOR]

A hydrogel-coated optical fiber surface plasmon resonance (SPR) sensor based on MMF-SMF-MMF structure has been fabricated for pH measurements. When the solutions with difference pH values contacting the sensing probe, the amount of the dissociated carboxylic ions in the hydrogel will change according to the pH value of the solution. This process will modify the volume and refractive index of the hydrogel, which results in a shift of the SPR wavelength. The experimental sensor is able to indicate pH variations in a wide pH range from 1 to 12, and the maximum pH sensitivity of the proposed sensor can reach 13 nm/pH at higher pH region (8–10 pH). The influence of temperature (in the range of 20 °C–40 °C) on SPR wavelength is found to be weak. Further, the sensor possesses remarkable repeatability and excellent stability. Due to the wide measuring range and simple fabrication process, the sensor is promising to be used for the pH measurements. [ABSTRACT FROM AUTHOR]

The measurement of arterial oxygen saturations, end-tidal carbon dioxide and pH are all key to modern anaesthetic practice. They can all be measured in a variety of ways but with the most common being discussed in the article. The understanding of the underlying physical principles and how the anaesthetist monitors function to measure these variables is discussed in this article, including limitations and inaccuracies of each technique. [ABSTRACT FROM AUTHOR]

Electrolyte-insulator-semiconductor (EIS) devices were fabricated using titanium nitride (TiN), gold (Au) and platinum (Pt) as integrated reference electrodes. Here, Au and Pt were deposited by RF sputtering and TiN was deposited by DC sputtering. The devices were developed with different geometries and using silicon (Si) n and p type wafers. A titanium oxide (TiO 2) film was used as a pH sensitive membrane in all EIS devices. Capacitance versus voltage (C-V) measurements were used to determine the sensitivity of the devices from their interaction with solutions of pH: 1, 3, 4, 7, 9, 10 and 12. From the analysis of the C-V curves for the EIS capacitors with integrated Pt and Au reference electrodes, two well-defined sensitivity regions are observed: one for acidic and neutral pH solutions, and one for basic pH solutions. This type of behavior is due to the difference between the values of the work functions of Au and Pt, in relation to that of Si. This difference between the work functions causes an increase in the curvature of the energy bands, as the pH value of the solution to be analyzed increases. As a result, leakage currents are generated by Schottky emission and Fowler-Nordheim (F-N) tunneling, causing incorrect displacements in C-V responses when interacting with different pH solutions, and with this, the formation of two regions of sensitivity. For EIS capacitors with TiN as an integrated reference electrode, this effect is not observed because the value of its work function is remarkably close to that of Si. • Ease of packaging. • Integrated reference electrode. • Developed in the same silicon wafer. • Device with quick and easy handling. [ABSTRACT FROM AUTHOR]

A novel micro-structured bubble column reactor (MSBC) is studied by a combined experimental and simulation approach, for the chemisorption of CO 2 into NaOH. To understand the reactor behavior different reactor configurations are tested for chemisorption by varying the internals (single wire mesh, staged wire meshes, Sulzer packing). Bubble size distribution, pH and holdup data are obtained from chemisorption experiments using an advanced digital image analysis technique. The effect of superficial gas velocities ranging from 5 to 25 mm/s and varying number of wire mesh stages (1, 6 and 10) is deduced from the experiments. The experimental results are used for the validation of the detailed VoF-DBM simulations. [ABSTRACT FROM AUTHOR]

CO 2 sequestration is among the most anticipated methods to mitigate the already detrimental concentrations of CO 2 in the atmosphere. Among sequestration methods, CO 2 injection into oceans is of great significance due to the oceans' large sequestration capacity. However, there are concerns about the changes in water pH as CO 2 is injected into oceans. Previous studies in conditions representative of CCS in the ocean are scarce. In the current study, we experimentally measure the pH and solubility at pressures up to 400 atm, temperatures between 283 and 298 K, and different aqueous solutions in a high-pressure autoclave reactor. The results indicated that increasing pressure increases the solubility of CO 2 in aqueous solutions, resulting in lower pH values. In contrast, increasing salinity and temperature lowers the solubility and, as a result, increases the system's pH. Among all the tested aqueous solutions, the synthetic seawater mimicked that of a potential injection point in the South China sea, exhibiting the highest salting-out effect and, therefore, the lowest solubility (i.e., the highest pH). The experimental dataset of this study was fed to a machine learning algorithm, Group modeling data handling(GMDH), to develop an explainable, white-box solubility model. The model could predict the pH as a function of solubility, temperature, pressure, and salinity with an accuracy of 0.87. The pH values from the model were compared to those from previous studies, and a good agreement among the values was found. Lastly, a parameter importance analysis was conducted to shed further light on the model's performance. Pressure and temperature were found to be the most and the least influential factors, respectively. As the implantation of the technology is currently being considered in China, the current study can pave the way to better understand the interactions and mechanisms involved in conditions representative of ocean sequestration before large-scale operations. • Solubility and pH of CO2 in aqueous solutions was experimentally investigated. • Both parameters are essential for different CCS operations specifically CCS in oceans. • An accurate, data-driven pH equation was developed based on the experimental data. [ABSTRACT FROM AUTHOR]

Tungsten/tungsten oxide electrodes were prepared by four kinds of methods and their pH response behaviors in lithium borate buffer solutions at 25 °C have been investigated by electrochemical measurements. It was found that tungsten/tungsten oxide electrode made by immersion in HNO 3 solution was unsuited to the testing environment because it tended to exfoliate or dissolve during tests. All the other tungsten/tungsten oxide electrodes showed linear response to pH values and good stability during the long-term potential monitoring. The lower dissolution rate of tungsten/tungsten oxide electrode made by heat-treatment made it the best candidate for the long-term monitoring due to its compact oxide film formed on the surface. X-ray photoelectron spectroscopy and thermodynamic analysis have been done to evaluate pH response mechanisms of the corresponding electrodes. [ABSTRACT FROM AUTHOR]

[Display omitted] • The impact of pH error on p K a values was analyzed. • The electrophoretic and thermophoretic methods were considered. • Resulting uncertainty of p K a can be more significant than assumed. • A way for estimating p K a error has been proposed. • Thermodynamic analysis is less susceptible to errors. The accuracy of the acid dissociation constant (p K a) determined by experimental techniques depends on the potential errors in the pH measurement. This dependence is obvious, but, according to common practice, this effect is either ignored or treated in a greatly simplified manner when discussing the credibility of obtained p K a values. This paper investigates the influence of incorrect pH measurement on the p K a values ​​obtained by capillary electrophoresis (CE) and microscale thermophoresis (MST). A simple self-control procedure has been proposed to control and reliably predict the corresponding total uncertainty of the acidity constant. The significance of the pH measurement error was also investigated in relation to the thermodynamic analysis, which requires determining the thermal dependency of p K a values. The obtained results clearly indicate that the investigated effect should not be ignored, and the actual accuracy of methods using electrophoretic separation may be worse than commonly assumed. It also points at the need to develop alternative methods that do not require measuring pH, such as a known internal standard-based approach. Besides pH-related effects, other sources of inaccuracy of p K a constant should also be considered. [ABSTRACT FROM AUTHOR]

A new pH sensing poly(vinyl chloride) (PVC) membrane electrode was developed by using recently synthesized 5,17-bis(4-benzylpiperidine-1-yl)methyl-25,26,27,28-tetrahydroxy calix[4]arene as an ionophore. The effects of membrane composition, inner filling solution and conditioning solution on the potential response of the proposed pH sensing membrane electrode were investigated. An optimum membrane composition of 3% ionophore, 67% o -nitrophenyl octyl ether ( o -NPOE) as plasticizer, 30% PVC was found. The electrode exhibited a near-Nernstian slope of 58.7±1.1 mV pH −1 in the pH range 1.9–12.7 at 20±1 °C. It showed good selectivity for H + ions in the presence of some cations and anions and a longer lifetime of at least 12 months when compared with the other PVC membrane pH electrodes reported in the literature. Having a wide working pH range, it was not only applied as a potentiometric indicator electrode in various acid–base titrations, but also successfully employed in different real samples. It has good reproducibility and repeatability with a response time of 6–7 s. Compared to traditional glass pH electrode, it exhibited excellent potentiometric response after being used in fluoride-containing media. [ABSTRACT FROM AUTHOR]

(W/WO 3 –Ag/AgCl) and (Pt/IrO 2 –Ag/AgCl) potentiometric microdevices were developed for pH measurement in liquid phase using a (Pt–Pt–Ag/AgCl) electrochemical microcells (ElecCell) silicon-based technological platform. A special emphasis was placed on the mass fabrication of the W/WO 3 microelectrode using sputtering deposition and oxygen plasma processes. Compared to the Pt/IrO 2 -based one, the W/WO 3 -based microelectrodes showed lower performances regarding the pH measurement. Nevertheless, since W/WO 3 microelectrodes yielded quasi-Nernstian sensitivity (around 55 mV/pH) in the [2–12] pH range, tungsten oxide WO 3 can be considered as a good candidate for the mass fabrication of pH microsensors using silicon technologies, even if important temporal drift (at least 6 mV/h) and high hysteresis (around 50 mV) were also evidenced. [ABSTRACT FROM AUTHOR]

An autonomous electrochemical biosensor with three electrodes integrated on the same silicon chip dedicated to the detection of herbicides in water was fabricated by means of silicon-based microfabrication technology. Platinum (Pt), platinum black (Pt Bl), tungsten/tungsten oxide (W/WO3) and iridium oxide (Pt/IrO2) working ultramicroelectrodes were developed. Ag/AgCl and Pt electrodes were used as reference and counter-integrated electrodes respectively. Physical vapor deposition (PVD) and electrodeposition were used for thin film deposition. The ultramicroelectrodes were employed for the detection of O2, H2O2 and pH related ions H3O+/OH-, species taking part in photosynthetic and metabolic activities of algae. By measuring the variations in consumption-production rates of these electroactive species by algae, the quantity of herbicides present at trace level in the solution can be estimated. Fabricated ultramicroelectrodes were electrochemically characterized and calibrated. Pt Black ultramicroelectrodes exhibited the greatest sensitivity regarding O2 and H2O2 detection while Pt/IrO2 ultramicroelectrodes were more sensitive for pH measurement compared to W/WO3 ultramicroelectrodes for pH measurement. Bioassays were then conducted to detect traces of Diuron herbicide in water samples by evaluating disturbances in photosynthetic and metabolic activities of algae caused by this herbicide. [ABSTRACT FROM AUTHOR]

Background: Blind placement of a nasogastric feeding tube is a common nursing procedure. Confirmation of the correct position in the stomach is warranted to avoid serious complications such as misplacement in the lung. Testing pH of aspirate from a tube is one of the techniques to confirm the tip position. The purpose of this study was to evaluate the auscultatory method and pH measurement with a pH cut-off point of 5.5 after tube insertion and to compare this with the 'gold standard': an abdominal X-ray. Also the feasibility of the pH method was evaluated. Materials and methods: Large prospective observational study in a general hospital. In adult hospitalised patients, the positioning of 331 feeding tubes was tested using two different methods to predict tube position in the stomach. Results: In 98.9% (n = 178) of aspirate samples with a pH ≤ 5.5, the tube was located in the stomach. If an aspirate could be obtained, the results of pH measurements showed a sensitivity of 78.4% and a specificity of 85.7%. Obtaining aspirate initially after placement was possible in approximately half of cases but after taking additional measures (including administration of air into the tube, side-positioning of the patient and re-aspiration after one hour) this increased to 81.6%. The sensitivity of the auscultatory method was 79% while the specificity was 61%. Conclusions: A pH of ≤ 5.5 from tube aspirate is adequate to check the position of the tube in the stomach. Additional measures improve the success to obtain an aspirate from the tube. The auscultatory method is unreliable. [ABSTRACT FROM AUTHOR]

Objectives This study was conducted to investigate the following: (1) the effects of chewing honey on plaque formation in orthodontic patients, (2) the effect of chewing honey on dental plaque bacterial counts, (3) determine if honey possesses antibacterial effects on bacteria recovered from plaques. Methods Female orthodontic patients (n=20, 12-18years of age) participated in this randomized controlled study. The effects of honey were compared to treatment with either 10% sucrose or 10% sorbitol that served as positive and negative controls, respectively. The pH of plaque was measured using a digital pH meter prior to baseline and at 2, 5, 10, 20, and 30min after chewing honey or rinsing with control solutions and the numbers of Streptococcus mutans, Lactobacilli, and Prophymonas gingivalis in respective plaques were determined. The antibacterial activity of honey was tested against commonly used antibiotics using the disk diffusion method. Results Significant differences in pH were observed in the honey and sucrose groups compared to the pH observed in the sorbitol group (p⩽0.001). The maximum pH drop occurred at 5min in both the honey and sucrose groups; however the pH in the honey group rapidly recovered 10-20min after exposure and did not drop below the critical decalcification pH of 5.5. On the other hand, the pH following sucrose exposure fell <5.5 and was associated with a 30min recovery time. The pH observed for the sorbitol group did not change over time. Bacterial counts were significantly reduced in the honey group compared to the other treatment groups (p⩽0.001) and honey significantly inhibited the growth of all studied strains compared to inhibition observed with antibiotics (p⩽0.001). Conclusions Honey can be used as an alternative to traditional remedies for the prevention of dental caries and gingivitis following orthodontic treatment. [ABSTRACT FROM AUTHOR]

Abstract: Carbon dots with unique characters of chemical inertness, low cytotoxicity and good biocompatibility, demonstrate important applications in biology and optoelectronics. In this paper we report the optical properties of pH-sensitive carbon dots with different surface modifications. The as-prepared carbon dots can be well dispersed in water by modifying with acid, alkali or metal ions though they tend to form a suspension when being directly dispersed in water. We find that the carbon dots dispersed in water show a new emission and absorption character which is tunable due to the pH-sensitive nature. It is firstly proved that the addition of bivalent copper ions offers a high color contrast for visual colorimetric assays for pH measurement. The effect of surface defects with different modification on the performances of the carbon dots is also explored with a core–shell model. The hydro-dispersed carbon dots can be potentially utilized for cellular imaging or metal ion probes in biochemistry. [Copyright &y& Elsevier]

Abstract: The coating of substrate materials with different geometry (gauze, plate, foam) and composition (Pt and FeCrAlloy) with Layered Double Hydroxides (Ni/Al, Mg/Al) was performed using the base electrogeneration method. The pH established by the cathodic reduction of nitrates in the proximity of a Pt gauze was measured with a glass electrode. In order to study the morphology, composition, and structure of the electrodeposited solids, at first Pt plates were used as substrates; the characterization was performed by SEM/EDS and XRD. Lastly, the study was focused on the FeCrAlloy foam more complex substrate. The more negative the applied potential was, the higher the pH values measured were; however, very negative potentials (−1.1 to −1.3V vs SCE) promoted the formation of H2 bubbles. Optimum potential for precipitating Layered Double Hydroxides depended on their chemical composition and even if the potential applied was adequate for the formation of pure phases, SEM/EDS analyses revealed that a sequential precipitation occurred at long synthesis times. [Copyright &y& Elsevier]

Abstract: The Internet of Things (IoT) envisions a future based on billions of smart connected devices. Devices that sense their surroundings and which when combined with cloud computing services will provide information on scales ranging from the microscopic to global. We demonstrate here for the first time the development and characterisation of an ultra low-power radio-frequency identification (RFID) wireless sensor tag with potentiometric input for use with pH and ion-selective electrodes. Tags such as this will be critical to the practical realisation of the Internet of Things. The tag is able to autonomously measure and store electrode potential in its internal memory, and then transfer logged data wirelessly by RFID to a nearby reader or by near field communication (NFC) to a smart phone. The unique features of the sensor tag include its ultra low-power consumption, potentiometric sensing ability, autonomous data logging, and RFID/NFC compatible air interface. These features make the tag suitable for use where pH or ion-selective electrodes are deployed as stand-alone chemical sensors or as part of larger chemical sensor networks that will underpin the IoT. pH determination in solution is used as an experimental example for illustrating measurement capability of the tag, and its analytical performance is compared to a standard laboratory pH meter. [Copyright &y& Elsevier]

Abstract: Quantification of a pH change that was observed over the course of the transesterification reaction that converts vegetable oil to biodiesel may provide a simple method to monitor the reaction. Transesterification of canola oil at 6:1 methanol to oil ratio with 0.5wt.% KOH as catalyst was studied at 25, 35, and 45°C. Reaction conversion was correlated to pH measurements and the results were shown to be in agreement with an independent measure of conversion using an enzymatic assay for glycerol. Rate constants obtained from these measurements are consistent with those in the literature. The measured pH change appears to be related to dilution of OH− ions as the oil is converted to products rather than to depletion of OH− due to reaction. [Copyright &y& Elsevier]

Abstract: We use fluorescence spectroscopy to measure the pH sensed by proteins which are immobilized in mesoporous silica particles by covalently attaching the pH-probe SNARF1 to the proteins. In contrast to previous attempts where pH-probes were attached to the pore surface, the present approach allows the pH to be measured without altering the properties of the silica material. When the particles are suspended in aqueous solutions of various pH the emission spectra of the labeled proteins indicate an environment inside the pores which is closer to neutral compared to the bulk solution. In a 1-butanol/water mixture (92.5/7.5%) the emission spectra of the immobilized proteins report a pH-behavior typical of the aqueous suspensions, in contrast to the blue-shifted emission spectrum of the protein in the outside organic solvent. This observation shows that the immobilized proteins experience an aqueous pore environment even though the surrounding solution is poor in water. The spectra of SNARF1-labeled bovine serum albumin and feruloyl esterase generated similar results suggesting that the method can be applied to different types of proteins. Taken together the results show that spectroscopic probes carried by immobilized macromolecules can be used to characterize the environment inside mesoporous particles without perturbing the properties of the material. [Copyright &y& Elsevier]

Abstract: The reaction between dilute fluorine gas and cresolsulfonphthalein in acetic acid was investigated. The mono-, di-, and trifluorinated cresolsulfonphthalein derivatives were isolated and characterized. These compounds possessed the properties of pH indicators with biologically relevant pK a values (6.4–7.5) and the absorption maxima of the basic forms at 582–592nm. This method can be used for synthesis of positron-emitting 18F-labeled pH indicators with potential application for non-invasive in vivo pH measurement in biological objects. [Copyright &y& Elsevier]

Abstract: The increased interest in architecting nanocomposites and in clarifying their impact on health and environment necessitates a standardized method of surface characterization, because the behaviour of nanoparticles is governed largely by their surface free energy and interfacial chemistry. Potentiometric acid–base titration has been used for decades to determine surface charge density of oxides, tremendously useful in understanding and modelling oxide/electrolyte interfaces. The method has solid theoretical and experimental foundation. Yet, published values of the point of zero charge (PZC) of chemically identical oxides scatter widely: the deviation of seven units of pH is the rule, rather than an exception. The PZC must reflect the inherent charging behaviour, linked exclusively to the protonation, and determined by the chemistry of the oxides. Assignment of PZC (just like assignment of equilibrium constant of any chemical reaction) requires strict conditions and consistent protocol. We present here such a protocol, and some examples of application. The protocol leads through the basic steps: pH buffer calibration, proton concentration calibration in the background electrolytes, and titrations of oxides. Built-in tests help to eliminate artificial side effects simultaneously from the measurement of pH and from the oxide/electrolyte interfacial equilibria. Although these may seem self-evident, literature survey proves the need to go back to basics in order to obtain precise, reliable and comparable pH-dependent charging of oxides. It is most general that the oxides contain some unrecognized surface impurities and have a strongly pH-dependent dissolution. These distort the results of titrations; their effect increases with decreasing particle size and can become dominant for nanoparticles. The protocol presented here contains the necessary feedback loops to easily recognize and exclude these side effects. [Copyright &y& Elsevier]

Abstract: This study demonstrates that tungsten rotating disk electrode can be used for pH or H+/OH− concentration measurements in practically entire pH range, using the combination of voltammetric and potentiometric techniques. In strong acidic (c(H+) > 10−3 moldm−3), and alkaline (c(OH−)>10−3 moldm−3) solutions, rotating tungsten disk electrode can be used as a maintenance-free electrode for voltammetric determination of H+ and OH− ion concentration. After an appropriate electrochemical oxidation, the same electrode, in a stationary regime, may be used as a potentiometric sensor in the range 3

Abstract: The performance of a flow-through electrochemical cell with an yttria-stabilized zirconia (YSZ) sensor for pH measurement in high-temperature nickel laterite pressure acid leach (PAL) process solutions was evaluated. Very good agreement was observed between the measured pH values and those theoretically predicted using the OLI Systems software package calibrated independently based on solubility measurements. PAL process solutions were simplified to ternary MgSO4–Al2(SO4)3–H2SO4 solutions by combining all the divalent metal sulphates into one with the properties of MgSO4. The experimental results support the postulation that the high-temperature behaviour of nickel, cobalt and manganese sulphates can be satisfactorily approximated with that of MgSO4. The experimental findings also support the postulation that acid should be added to a PAL process so that the solution pH is around 1 at the leach temperature, regardless of the feed composition. [Copyright &y& Elsevier]

Abstract: The residual liquid junction potential (RLJP) needs to be accounted for in pH uncertainty estimation. Attempts to do this and the currently available methods for evaluating the RLJP are critically discussed and their weak sides are pointed out. In this work an empirical approach to the problem is proposed. It is based on the use of the RLJP bias estimated on a variety of measurement conditions for a specific class of analytical objects essentially differing in ionic strength from the pH calibration buffers. The data from five independent studies, including interlaboratory comparisons, on pH measurement in low ionic strength waters were used to find the overall bias observed in the 10−4 moldm−3 strong acid solution. The procedure includes quantifying the uncertainty of bias values from separate studies by combination of the relevant uncertainty components and testing the consistency of the data. The weighted mean bias in pH was found to be 0.043±0.007 (k =2). With this estimate, the pH measurement uncertainties calculated according to the previously suggested procedure (I. Leito, L. Strauss, E. Koort, V. Pihl, Accredit. Qual. Assur. 7 (2002) 242–249.) can be enlarged to take the uncorrected bias into account. The resulting uncertainties on the level of 0.10–0.14 (k =2) are obtained in this way for pH measurement in water and poorly buffered aqueous solutions in the range of pH 7.5–3.5. [Copyright &y& Elsevier]

Abstract: Objectives: Several in vitro laboratory tests to assess the quality control of platelet concentrates (PC) are available. Some of them have a good correlation with the platelet recovery index. To assess the quality control of standard PC prepared in our blood bank, we measured the blood gas and the degree of platelet activation. Materials and methods: SPC were prepared by the PRP method. Fifty-five SPC (45 SPC at day one of storage and 20 SPC at day five of storage) were analysed. Blood gas (pH, PO2, PCO2 and bicarbonate concentration) in the SPC were measured by blood gas automate. Platelet activation profile were determined by measuring the percentage of platelet expressing the CD62p (% CD62) and the percentage of platelet-leukocyte aggregate (% PLA). Results: The pH values of all studied SPC were comprised between 7.0 and 7.6. SPC at day 1 of storage have a significantly higher pH than those at day 5 of storage (7.5±0.05 versus 7.3±0.14; p <0.001). The % CD62p were higher in SPC at day five compared to the SCP at day one without reaching a statistical significance (28.4±15% versus 24.3±9.7%, p =0.052). The percentage of PLA were higher in SPC at day one compared to SCP at day five although this difference is not statistically significant (22.2±7.5% versus 17.9±8.0%; p =0.23). Conclusion: Preparation and storage procedure adopted in our centre did not significantly affect the quality SPC. Our study is the first to assess the PLA in PC. Studies assessing the PLA are warranted to appreciate the clinical impact of this parameter. [Copyright &y& Elsevier]

Abstract: The electrochemical potentials of tungsten nanowire samples, covered with their own oxide, were measured in dependence of the pH value. The samples were prepared by selective etching of a directionally solidified eutectic NiAl–W alloy. Directional solidification in a Bridgman-type crystal growth furnace yields nanostructured two-phase materials. Electrochemical processing allows selective etching of the phases exposing the nanoscale structures. In this work, pointed samples with a single wire 200nm in diameter protruding from the tip were produced. Subsequently the tungsten oxide layer on these single nanowires was electrochemically modified to optimize their pH sensing capabilities. The method has a potential for further downsizing since the wire diameter and exposed length can be controlled by the process parameters during solidification and during electrochemical processing. The advantages of these nanowire pH probes along with possible applications such as the pH measurement in ultra small cavities and other small systems of interest such as corrosion pits and biological cells are discussed. [Copyright &y& Elsevier]

Abstract: We describe an aggregation-based growth mechanism for formation of silver nanowires at room temperature. It is found that the pH of solution and the concentration of l-cysteine capping molecules have an important effect on the formation and growth of nanowires. Characterization by atomic force microscopy (AFM) and UV–vis spectroscopy recorded as time clearly shows that the silver nanowires are grown at the expense of nanoparticles. [Copyright &y& Elsevier]

Abstract: The calix[4]-aza-crown derivatives have been used in polymeric ion selective electrodes (ISEs) for pH sensing. The membranes containing ionophores and cation exchangers showed near Nernstian response to hydrogen ions and wide dynamic range. New carrier based electrodes exhibited high selectivity over alkali metal ions. The binding properties of ionophores with hydrogen ions (acidity constants pK a) in solvent polymeric membranes were characterized in situ by the so-called sandwich membrane method. The influence of membrane solvents (plasticizers) and the types of the ion exchangers were studied. [Copyright &y& Elsevier]

Abstract: In this work, metal oxide microelectrodes were developed to monitor pH change in agar media during plant tissue culture. An antimony wire was produced by a new approach “capillary melt method”. The surface of the obtained antimony wire was oxidized in a potassium nitrate melt to fabricate an antimony oxide film for pH sensing. Characterization results show that the oxide layer grown on the wire surface consists of Sb2O3 crystal phase. The sensing response, open-circuit potential, of the electrode has a good linear relationship (R 2 =1.00) with pH value of the test solution. Adding organic compounds into the test media would not affect the linear relationship, although the slope of the lines varied with different ingredients added. The antimony oxide electrodes were employed to continuously monitor pH change of agar culture media during a 2-week plant tissue culture of Dendrobium candidum. The antimony oxide electrode fabricated this way has the advantages of low cost, easy fabrication, fast response, and almost no contamination introduced into the system. It would be suitable for in situ and continuous pH measurement in many bio applications. [Copyright &y& Elsevier]

While there is a considerable interest in the food industry in determining various analytes using ion-selective electrodes (ISEs), only few reports describe their use for direct measurements in food. In this study, the suitability of glass electrodes and ionophore-based solvent polymeric ISEs for the determination of pH in Process cheese, Cheddar cheese and milk was investigated. The liquid junction potential between a 3 M KCl bridge electrolyte and diluted as well as undiluted Process cheese was found to be negligible. Reference electrodes with ceramic plug and sleeve-type junctions performed well, although precautions needed to be taken to prevent plugging at the junctions. While the protein rennet casein posed no problems in pH measurements, the extraction of neutral lipophilic compounds or hydrophobic peptides into solvent polymeric membranes was evident, resulting in some loss of selectivity for monovalent cations upon exposure to cheese. However, it was found that ISEs based on tridodecylamine (R3N) as ionophore and o-nitrophenyl octyl ether (oNPOE) as plasticizer can be used to accurately measure the pH of milk and, after desensitization of the electrodes in a cheese emulsion, of diluted Process cheese. Since pH measurements with a glass electrode showed that emulsions of cheese moderately diluted to a cheese content of 70% have the same pH as undiluted cheeses, it is possible to determine the pH in cheese with ionophore-based ISEs. R3N membranes also performed well in undiluted milk. [Copyright &y& Elsevier]

A simple pH probe is developed based upon the covalent chemical derivatisation of carbon particles with anthraquinone. The amperometric response of electrodes constructed from this material is examined and shown to produce a Nernstian linear response to pH from 1 to 9, over a range of temperatures from 20 to 70 °C, consistent with a two-electron, two-proton electrochemical process. [Copyright &y& Elsevier]

A flow-through yttria-stabilized zirconia (FTYSZ) electrode has been employed for pH measurements at 250 °C in concentrated electrolyte systems relevant to hydrometallurgical processing of nickeliferous laterites. Experiments were conducted using a custom-made flow-through titanium electrochemical cell specially designed for high-temperature potentiometric pH measurements. In the present work, we assess the accuracy and validity of pH measurements with theory at 250 °C on concentrated sulfate process solutions. First, binary sulfuric acid–water solutions were measured at concentrations ranging from 0.05 to 0.5 mol kg−1 and showed 0.3 pH unit increase on the average between 25 and 250 °C. The measurements were then extended to ternary and quaternary systems containing 0.0076–0.038m Al2(SO4)3 and 0.0276–0.276m MgSO4, respectively. Autoclave passivation against corrosion was successfully achieved using sodium chromate or hydrogen peroxide. In this technique, the potentials are measured against a Ag/AgCl flow-through external reference electrode (FTEFE). A reference solution of 0.1 mol kg−1 NaCl(aq) flows permanently through the electrode at a constant flow rate, so that the thermal liquid junction and streaming potentials are kept constant. The experimental results were compared with theory using available thermodynamic data for estimating the quality of pH measurements. An average difference of 0.15 pH unit was observed between measured and theoretical values at temperature. [Copyright &y& Elsevier]

The pH of aqueous phenol solutions at subcritical and supercritical conditions was measured for the first time at temperatures from 296.4 to 673.4 K and pressures from 29.8 to 34.9 MPa by using a flow-through electrochemical cell. From the results, we determined the equilibrium constant for the dissociation reaction of phenol (KPhOH). The dissociation constants of phenol varied from 1.0×10−12 mol/kg at 673.4 K and 30.2 MPa to 7.9×10−10 mol/kg at 473.1 K and 29.9 MPa. The semi-empirical model for correlation of the equilibrium constant in subcritical and supercritical water was applied to correlate the data and to discuss the temperature, pressure and density effects on the phenol dissociation constant. [ABSTRACT FROM AUTHOR]

The acid–base constants of several bases (aniline, pyridine, ammonia, ethanolamine, and triethylamine) in acetonitrile–water mixtures up to 60% of acetonitrile by volume have been potentiometrically determined. Two different pH scales have been used: the pH scale relative to each acetonitrile–water mixture as standard state solvent (sspH) and the absolute pH scale in reference to water as standard state solvent (wspH). The two sets of pK values are in a reasonable good agreement. In contrast with the variation of pKa values of neutral and anionic acids, the pKa values of the bases decrease with the increase of the acetonitrile contents in the solvent mixture. In the studied range of solvent composition, linear relationships have been obtained between the wspKa values of the bases and the volume fraction of acetonitrile in the mixture. [Copyright &y& Elsevier]

[Display omitted] • A SERS active sensor to measure microdroplets pH was designed. • The effects of acid species and ionic strength were investigated. • The pH detection of size-resolved micron-scale single droplet was realized. • Our method's applicability to ambient samples was showcased. Microdroplets present unique physiochemical properties, which can influence chemical reactions and atmospheric aerosol processes. The microdroplets' pH is one of the most important factors that dictate chemical processes, such as heterogeneous and condensed reactions. However, currently, there is no well-established method for directly measuring the pH in microdroplets. Herein, we develop a straightforward method, for the first time, to directly measure the pH of size-resolved, micron-sized single droplets with Surface Enhanced Raman Scattering (SERS)-activated pH-indicator paper; realized by embedded Au nanoparticles (20, 40 and 60 nm). We establish the key parameters for optimizing the method, and demonstrate its accuracy by benchmarking various acids and solutions with different ionic strength. With the established method, we report the pH dependence on the size of microdroplets. Microdroplets of sizes down to 230 nm can be analyzed by our method. We also showcase our method's applicability to ambient samples, establishing its promising potential beyond the laboratory environment. [ABSTRACT FROM AUTHOR]

In this study, a novel solid-state pH sensor based on an untreated platinum sheet was investigated. Chronopotentiometry using positive and negative currents was applied to effect the interconversion of Pt and PtO. To eliminate the influence of concentration polarization, a galvanostatic double-pulse method was applied for pH measurement. The consecutively applied currents were +100, +3, −100, and −3 μA for excitation times of 0.5, 1, 0.5, and 1 s, respectively. Stable potentials under the small currents were obtained which could be used to determine pH. The platinum sheet electrode exhibited near-Nernstian behavior in the pH range 2–12, with fitted lines having slopes of −44.2 and −51.1 mV/pH unit under the positive and negative currents, respectively. Overall, excellent linearity and reproducibility could be achieved. Moreover, the temperature responses for the method in pH 4, 6.86, and 9.18 buffer solutions obeyed the Nernst equation, and further supported the proposed mechanism for pH determination. In conclusion, this new detection method provides a solid-state, long-lived, and adaptable solution for pH measurement. [ABSTRACT FROM AUTHOR]

• Thermal anneal leads to atomic rearrangement/crystallization into alumina structure. • PEALD Al 2 O 3 films presents various characteristics according to anneal temperature. • Dielectric gate properties are optimized for annealed polycrystalline Al 2 O 3 films. • pH-detection properties are optimized for as-deposited amorphous Al 2 O 3 films. The dielectric properties of aluminium oxide (Al 2 O 3) thin films obtained by plasma-enhanced atomic layer deposition (PEALD) from Al(CH 3) 3 /O 2 precursors were investigated while focusing on the influences of thermal annealing under a dioxygen (O 2) ambient. PEALD-Al 2 O 3 -based metal-insulator-silicon structures/capacitors and pH-sensitive chemical field effect transistors were fabricated to deal respectively with microelectronic applications and measurement in liquid phase. Antagonist results were thus evidenced. On the one hand, dealing with high-k gate materials, optimized dielectric properties, i.e. low fixed charge density (4 × 1012 cm−2), high dielectric constant (ε r = 10.2), Fowler-Nordheim conduction and high breakdown electric fields (E bd = 8.75 MV/cm) were obtained for polycrystalline PEALD Al 2 O 3 films annealed at high temperature (T > 750 °C). On the other hand, pH detection properties, i.e. quasi-Nernstian sensitivity (59 mV/pH), low drift (< 1 mV/day) and long lifetime (> 180 days), were optimized for unannealed amorphous PEALD Al 2 O 3 films. These phenomena were associated germination/crystallization phenomena in the deposited amorphous alumina structure as well as to related charge trapping and leakage currents in water-based solutions related to the final Al 2 O 3 polycrystalline structure. [ABSTRACT FROM AUTHOR]

Optical, and especially fiber optic techniques for chemical sensing have become very attractive in recent decades for a wide variety of biomedical and industrial processes and considerable progress in research in this field has been seen, evidenced by the significant number of papers published over several decades, commercial products developed and marketed and which continuing to be produced. This work extends the body of knowledge in the field and focuses on two industrially-important 'chemical' measurands: the determination of pH level and oxygen concentration (O 2) - both are critically important for a broad range of applications globally, in fields as diverse at the life sciences, environmental monitoring, biomedical research and thus widely across industry. The many different optical platform designs and fabrication methods that have been developed are considered, including those for commercial applications, recognizing the wide range of industrial and scientific uses, and their performance compared. Further, the effect of specific fiber structures on sensor performance, e.g. on sensitivity, response time and long-term stability, and possible applications also has been analyzed. Applications are seen in difficult and 'niche' measurement environments to which conventional sensors are often not well suited, taking advantage of their lightweight nature, ease of miniaturization, potential to be multiplexed and low cost. Through a discussion of representative techniques that have reached commercial development, a comprehensive and state-of-the-art view of this exciting and important field is possible. [ABSTRACT FROM AUTHOR]

• 3D printed sensor brought the possibility of measuring ionic strength in solutions. • Detection was based on colorimetric methods and usage of common materials. • Accuracy, fast-responsiveness, user-friendliness makes it ideal to be used in situ. • Can detect ionic strength variation in solutions with pH difference of 0.25 unit. • Can detect the buffer capacity of 0.1 M. A miniaturized 3D-printed device has been designed, manufactured and validated to perform as a low-cost sensor for compositional analysis of buffered and non-buffered solutions in industrial or remote areas. The proposed sensor takes advantage of the transport phenomenon and colorimetric measurements. The novel design can simultaneously detect the ionic strength of the solution by measuring the diffusion width of the ions and the pH by image analysis of the pH indicator color change. The results showed that it can detect pH variations of 0.25 and ionic measure difference of 0.1 M in non-buffer solutions. In addition, the design showed its adaptability to be used as a self-referencing sensor. The 3D-printed sensor presented here is not only successful in the evaluation of some important chemical characteristics but also brings flexibility, cost-effectiveness, swiftness and user-friendliness. [ABSTRACT FROM AUTHOR]

• Synergy precursors-surfactants tested for pH colorimetric sensors arrays (CSAs). • Study of two families of pH indicators to improve CSA performance. • Management of the preparation parameters to obtain "on-off" sensors. The behavior of pH indicators belonging to two different families, Sulfonephthalein and Azo in the Organically modified silicates (OrMoSil) polymer matrix, was investigated. The fraction and the dimension of the OrMoSil organic moiety were co-modulated with the cationic surfactant Hexadecyltrimethylammonium p-toluenesulfonate concentration. Two matrices were tested: the former based on Tetraethyl orthosilicate/ Trimethoxyphenylsilane (TEOS/FTMS), the latter, more lipophilic, based on TEOS/Dodecyl-TEOS. The working interval of Bromothymol Blue (BB) and Nitrazine Yellow (NY), with the co-variation of cationic surfactant amount, C , was wider for the matrix based on TEOS/Dodecyl-TEOS. The overall working interval of BB was between pH 5.80 and 12.50 and between 6.00 and 11.30 for NY. Among the pH indicators characterized by transitions between non-complementary colors, the most important results could be achieved with Methyl Red (MR). With the addition of Tetrabromophenol Blue (TBB), the sensitivity gradually rises with a linear hue profile between pH = 4.20 and pH = 11.90 with a small amount of dye for only one spot. In correspondence of the molar ratio n TBB /n MR = 0.233, there is the inversion point characterizing a spot working as an "on-off" sensor centered at pH = 4.15 suitable for pH measurement of acidic soils. [ABSTRACT FROM AUTHOR]

Abstract: An NMR spectroscopic technique has been developed to give rapid, accurate pH measurements on tenth-milliliter samples of concentrated acidic aqueous solutions buffered by fluoride ion in the pH 1.5–4.5 range. The fluoride 19F chemical shift has been calibrated as a function of pH at 0.1 and 1.0M concentration by reference to an internal 3-fluoropyridine standard. Subsequent measurements of fluoride buffer pH required no additives and only two NMR spectra in the presence of an external reference standard. [ABSTRACT FROM AUTHOR]

Abstract: pH is commonly measured using a pH meter containing a glass electrode and a reference electrode. These electrodes measure the electrochemical potential of two different liquids: one reference liquid with a known hydrogen ion concentration and other measuring liquid of unknown hydrogen ion concentration. The electrochemical potential is proportional to the hydrogen ion concentration of the measuring liquid. The pulse oximeter relies on absorption spectrophotometric analysis of red and infrared lights. These lights are absorbed differently by pulsatile and non-pulsatile components of the tissue. An absorption ratio of these lights is calculated and is used to calculate the oxygen saturation of the arterial blood. Capnography is also based on infrared absorption spectrophotometric analysis of light passing through a measuring compartment. The light is monochromatic and it has a carbon dioxide-specific wavelength. The amount of light absorbed is proportional to the carbon dioxide concentration of the measuring sample. This article briefly discusses the advantages, disadvantages and limitations of each of these techniques. [Copyright &y& Elsevier]

A pH colorimetric sensor array (CSA) with fast response time (<1 min) using only one acid-base indicator, Bromothymol Blue (BB), was prepared and characterized by modulating the amount, C , of the surfactant Hexadecyltrimethylammonium p-toluenesulfonate between 0 and 0.3725 g CTApTs /g precursor with a constant amount of the OrMoSil precursors. The effect of the C increase is a continuous acidic shift of the calibration position, i.e. a huge variation of the p K a value of BB in the pH range 5.80-13.50. The precision error decreased with increasing C from 0.096 pH units (lower C values) to 0.023 pH units (larger C values). This result led to the development of a model to determine the number of spots with suitable C values required for having a similar value of precision in the entire working interval of the CSA. By selecting only 4 spots the precision error is < 0.100 pH units in the pH range 5.80–13.50. With 256 spots (diameter of each spot ≈ 3 mm), the model predicted an error almost constant (≈0.010) in the entire pH range. Image 1 • Reversible pH CSA working within 7 pH units by using only one pH indicator. • Use of a suitable surfactant to vary the pK a of the indicator over 7 pH units. • Error minimization (0.01 pH units) with a model computing the optimal spots number. • A pH CSA with analytical performance comparable to the glass electrode. [ABSTRACT FROM AUTHOR]

Absorption spectra of pH indicators in aqueous solutions were reproduced from RGB values of microscopic images utilizing principal component analysis (PCA) and linear algebraic treatments. The reproduction of absorption spectra comprises the following three steps: (1) determining the loading spectra by PCA, (2) determining the conversion matrix from the RGB values to the score vectors, and (3) reproducing the absorption spectra by linear combination of the loading spectra and the score vectors. The reproducibility of the absorption spectra was demonstrated by employing bromothymol blue and methyl red solutions as pH indicators. The reproduced spectra of both indicators were in good agreement with the spectra measured with a conventional spectrophotometer. The p K a values of both indicators calculated from the reproduced spectra are in good agreement with those obtained from the spectrophotometric spectra and the literature values, confirming validity of the reproduction. This approach was applied to measure pH of freeze concentrated solutions in micro drains formed in ice. A change in pH was successfully observed on freezing and was compared with that reported in previous literature. Since this method does not necessitate the use of grating systems, spectral changes can be traced in milliseconds; this elucidates the phenomena occurring in fluctuating fields. Image 1 • A method to reproduce absorption spectra from RGB values of microscopic images is proposed. • The absorption spectra are reproduced by the linear combination of score values determined from the RGB values and loading spectra determined by principal component analysis. • A change in pH of freeze concentrated solutions on freezing was confirmed by reproducing absorption spectra of the indicator therein. • Non-grating microspectroscopy can be achieved with the present method. [ABSTRACT FROM AUTHOR]

pH colorimetric sensors with prediction error comparable to the potentiometric measurements in the working range of the indicator employed, were prepared. Bromothymol Blue (BB), Tetrabromophenol Blue (TBB), Cresol Red (CR) and Brilliant Yellow (BY) in PVDF (Polyvinylidene Fluoride)-supported OrMoSil (organically modified silicate) matrix, were used. Sensor preparation was reproducible. Signal was based on a suitably modified Hue coordinate from HSV (Hue, Saturation, Value) color space, H. Although H profile was not influenced by the indicator concentrations, the prediction error depended on the product of saturation and luminance, Δ. By using H , the response of every spot was less affected by shape and optical inhomogeneity. The H calibration held for all the spots of the same lot and remained identical in time giving an advantage with respect to a glass electrode requiring calibration before each set of measurements. The prediction errors both in solution and in the OrMoSil matrix were similar: for BB, s p H i = 0.03 in both environments; for TBB, s p H i = 0.02 in the sensing spot and 0.05 in solution; for CR, s p H i = 0.11 in the sensing spot and 0.04 in solution; for BY, s p H i = 0.17 in the sensing spot and 0.07 in solution. Indeed, H values of TBB and BB increased their variances near the inflection point where Δ has the minimum value (heteroscedastic behavior). Image 1 • Highly accurate pH colorimetric sensors. • Hue coordinate as a robust signal for pH colorimetric response. • Prediction error comparable to that of potentiometric data. • Sensor calibration made once and constant in time. • New theoretical model for Heteroscedastic behavior of some pH sensors. [ABSTRACT FROM AUTHOR]

An in-depth study was conducted to determine the influence of some important parameters to produce a pH colorimetric sensor characterized by long-term stability, lack of leaching, high reversibility, repeatability and extended working range. Bromothymol Blue (BB) was used as pH sensitive molecule. Its performance was monitored while tailoring the concentration of some added cationic surfactants, together with the organic fraction of the sol-gel composition to avoid leaching phenomena, low homogeneity and to increase reversibility. Five cationic surfactants with different linear alkyl chain length (from C 8 to C 18) were added to a sol-gel containing BB. The C 16 was chosen as optimal. The effect of the surfactant counterion was also investigated. The C 16 surfactant had a pH working interval of 3 pH unit by using chloride, 1.5 with bromide and 1 pH unit with p -toluenesulfonate as counterions, respectively. Chloride was characterized by a larger inhomogeneity while p -toluenesulfonate gave homogeneity and high repeatability. The hexadecyltrimethylammonium p -toluenesulfonate was therefore chosen as the best surfactant. Leaching in alkaline medium was avoided by using the dodecyl moiety among three organic groups linked to the OrMoSil matrix (dodecyl, phenyl and methyl). Colorimetric sensor arrays made with these specifications remained stable and reversible for months. Image 1 • Reversible pH colorimetric sensor. • Surfactants and their counterions make the difference for the pH response. • OrMoSil composition avoids leaching. • Sensor calibration made once and constant in time. • Portability and Low cost. [ABSTRACT FROM AUTHOR]

• Accuracy enhancement of a two-indicators based pH colorimetric sensors. • Hue coordinate as robust signal for pH colorimetric responses. • Prediction error comparable to that of potentiometric data. • The CIE-xy plane as a tool to choose the modifier dye. • Significant margin of maneuver for sensor optimization. A pH colorimetric sensor was prepared and characterized by combining two acid-base indicators, Tetrabromophenol blue (TBB) and Phenol Red (PR) with molar ratios ranging from 0 to 0.47. Indicators were embedded in Organically Modified Silicate (OrMoSil) spots supported in Polyvinylidene fluoride (PVDF) together with a suitable cationic surfactant. The deposition procedure was very repeatable allowing a single calibration for all membranes built in the same preparation lot. The signal was based on the Hue profile (H) vs. pH. The CIE-xy color space contains an achromatic point that is the key point of the colorimetric sensor behavior as it causes the inversion of the slope of the H calibration curve at a specific inversion point. For this reason, the identification of the effective conditions of the improvement of the PR analytical performances are not trivial, 0.186 < n TBB / n PR < 0.324 with prediction error in the range 0.01–0.16 pH unit. The proposed adjustment allowed the construction of pH colorimetric sensors with prediction error comparable to that of potentiometric measurements. The obtained results indicated the route to widen the availability even to those acid-base indicators lacking of suitable color transitions but having suitable p K a values. [ABSTRACT FROM AUTHOR]