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Juan Jesús Piña (3)María de los ánGeles arada Pérez (4)

The minimal instrumentation voltammetry with manual staircase voltage scan allows to obtain electrochemical results with non-commercial equipment. Several researches have demonstrated its analytical and metrological validity; however, some variables have not been studied in this kind of methods, such as the effect of the supporting electrolyte on the repeatability of the voltammograms. In this paper, a study is presented about the effect of KNO3, KCl, KClO4, K2SO4 and Na2SO3 as supporting electrolytes on the voltammograms obtained for the oxidation of KI and succinic acid, both at 0,1 mol/L; ascorbic acid 0,01 mol/L and potassium hexacyanoferrate (II) 0,08 mol/L, all of them as an analytical model. A graphite microdisc was used as working electrode; a bar of the same material was used as the auxiliary electrode. A silver electrode was used as the reference electrode. The Analysis of Variance was the statistical tool for the comparison of the voltammetric currents. A Multiple Range Contrast by a Fisher's LSD Test was used to identify the groups with statistical differences. There were statistical differences between the currents obtained with different supporting electrolytes for all the electroactive species studied. The best voltammetric results were obtained using KCl and KNO3 as supporting electrolytes.

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This paper presents a voltammetric method for Chemical Oxygen Demand (COD) quantification in a dairy wastewater using homemade minimal electrochemical instrumentation. The working electrode was a carbon microdisc, the auxiliary electrode was a carbon rod, and a silver electrode was used as reference. A low-cost non-commercial potentiostat was employed. The linear working range was 1-20 g/L. Repeatability was evaluated on 10 sample quantifications, the coefficient of variation was 3.6%, lower than that obtained for the reference spectrophotometric method (8.2%). The voltammetric method did not require treatment of the sample. The trueness was evaluated by comparing the voltammetric results with the spectrophotometric method. Mean COD of 10 quantifications using low-cost voltammetry was 10.2 g/L, whereas for the spectrophotometric method it was 11.0 g/L. A Paired Sample t-Test did not show statistically significant differences between both. Detection limit was 0.6 g/L, and quantification limit was 1.8 g/L. Both are acceptable for the sample in which there is a high organic content.