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In this work we propose a thread-based microfluidic device (μTAD) for the preconcentration and colorimetric determination of Co(II) in water using a digital image. The reaction is based on complexation of Co(II) by 4-(2- pyridylazo) resorcinol (PAR), which changes the detection zone from yellow to red. PAR is immobilized in a chitosan membrane to retain the complex in the detection zone. The designed μTAD makes it possible to pre- concentrate and determine cobalt between 25 and 600 µg·L−1 with a relative standard deviation of 4% (n = 5), and a detection limit of 6.5 µg·L−1. The device permits an enhancement factor of 11 by combining the use of a chitosan retention membrane and a sample volume of 50 µL. Recovery experiments were performed in tap water to evaluate the accuracy of the method, and the results obtained compared to a reference method presents an error no higher than 5.7%, This work was founded by Spanish “Ministerio de Economía y Competitividad” under Project CTQ2016-78754-C2-1-R and Junta de Andalucía under Projects B-FQM-243-UGR18 and P18-RT-2961. The project was partially supported by European Regional Development Funds (ERDF). Supporting Research in the State of Minas Gerais (Fapemig) (CEX-APQ-02436-15).

Chemometrics has proven to be a powerful tool for processing multivariate analytical data aimed at locating and extracting useful information relating to a particular analyte or material system in a complex sample from non-specific analytical signals that have been previously acquired and recorded by one or more analytical instruments or devices. In this paper, the basis for the application of both classification and quantitation multivariate methods is described, using a colorimetric twin sensor produced on a microfluidic paper-based analytical device (μPAD) as instructive example. The selected twin sensor provides a dual analytical signal for four anions (acetate, cyanide, fluoride and phosphate) in aqueous solution (concentration interval 0–0.1 M) from two reagent dyes (alizarin and 4-nitrophenol), which are immobilised in parallel on the same device containing four microfluidic channels designed in the form of an X. In this way, a data vector is obtained from each test whose elements are the colour coordinates obtained from the four responses, which is then used to build the chemometric models to be applied. Two multivariate classification methods (partial least squares discriminant analysis and support vector machine classification) are explored and the latter makes it possible to detect the presence or absence of each anion in an aqueous solution mixture. Single (each dye dataset separately) and fused (merging the two dye datasets) models were built and a support vector machine was shown to be the best classification method, obtaining sensitivity and precision values of 100% in almost all cases.

A neural network multivariate calibration is used to predict the pH of a solution in the full-range (0–14) from hue (H) values coming from imaging an optical pH sensor array based on 11 sensing elements with immobilized pH indicators. Different colorimetric acid-base indicators were tested for membrane preparation fulfilling the following conditions: 1) no leaching; 2) change in tonal coordinate by reaction and 3) covering the full pH range with overlapping between their pH responses. The sensor array was imaged after equilibration with a solution using a scanner working in transmission mode. Using software developed by us, the H coordinate of the colour space HSV was calculated from the RGB coordinates of each element. The neural network was trained with the calibration data set using the Levenberg–Marquardt training method. The network structure has 11 input neurons (each one matching the hue of a single element in the sensor array), 1 output (the pH approximation value) and 1 hidden layer with 10 hidden neurons. The network provides an MSE = 0.0098 in the training data, MSE = 0.0183 in the validation data and MSE = 0.0426 in the test data coming from a set of real water samples. The resulting correlation coefficient R obtained in the Pearson correlation test is R = 0.999.

The spectrophotometric study of the complexation reaction between 5,5′methylenedisalicylhydroxamic acid and V(V) shows that two complexes are formed, the 1:1 (ϵ = 5100 liters mol−1 cm−1 at 490 nm, log Kest = 5.8 ± 0.1) and the 1:2 (L:V) (ϵ = 6250 liters mol−1 cm−1 at 600 nm, log Kest = 6.1 ± 0.1). A spectrophotometric method is developed for the determination of vanadium (2–9 ppm) at 2 N HCl and 495 nm, which allows its determination in petroleum crude oils with a series of advantages over the ASTM D-1548-63 method.

A microdetermination method for titanium, at levels of micrograms per liter, based on ion-exchange spectrophotometry (IES) has been developed. Titanium reacts with chromotropic acid and oxalate and the ternary complex is sorbed on Dowex 1-X8 anionic resin. The resin phase absorbance at 440 and 800 nm is measured directly. The detection limit is 0.6 μg · liter−1 for a 500-ml sample method and 0.4 μg · liter−1 for a 1000-ml sample method. The RSD is about 4%. This method has been applied successfully to the determination of titanium in seawater samples from the Mediterranean coast.

A microdetermination method for vanadium by ion-exchanger colorimetry has been developed. Vanadium is sorbed on the anion-exchanger Dowex 1-X8 as vanadium-benzohydroxamic acid complex in 3 M HCl medium. The resin-phase absorbance at 516 and 800 nm is measured directly. The detection limit is 0.28 μg/liter and the distribution ratio is 340. It is possible to enhance the sensitivity by increasing the sample volume.

The iodide:tris(2-(phenyliminomethyl)pyridine) Fe(II) extraction system into nitrobenzene has been studied. The same method is proposed for the determination of iodide. The interference of foreign ions is also determined.

The thiocyanate: tris(2-(phenyliminomethyl)pyridine) Fe(II) extraction system into nitrobenzene was studied. The complex was isolated as solid and studied. The mixed-ligand iron(II)-thiocyanate-2-(phenyliminomethyl)pyridine complex is suitable for the determination of thiocyanate (4–20 ppm). The interference of foreign ions is determined and the most important of these are eliminated.