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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]

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 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]