Quantificação dos nutrientes minerais Na, K e Mg por espectrometria atômica e dos ânions Cl- , SO4 2- , HCO3 - , NO3 - e NO2 - por eletroforese capilar de zona em amostras de agua de carcinicultura

Abstract

In this work, analytical methods were proposed for the determination of Na+ , K+ and Mg2+ contents and the simultaneous determination of Cl- , SO4 2- , HCO3 - , NO3 - e NO2 - in shrimp farming water samples, using atomic absorption and emission spectroscopy and capillary electrophoresis with UV detection. The methods were applied in 6 (six) samples coming from different shrimp farming. The precision, measured by the RSD (%) (n=10), was below 4.0 % for cations and below 5.0 % for anions, thus presenting low data dispersion. Regarding accuracy, the methods obtained mean recoveries (n=3) of 102 % for Na+ , 96 % for K+ and 95 % for Mg2+ , 98 % for Cl- , 100 % for SO4 2- and 99 % for HCO3 - and NO3 - . The quantification limits of the methods varied between 85 and 814 mg/L for cations and between 09 and 326 mg/L for anions. In the development of the capillary electrophoresis method, an optimized electrolyte consisting of K2CrO4 60 mM, CTAB 2.5 mM, acetonitrile 0.88% and methanol 2.6% (v/v) and a pH range between 8.0 and 8.5 was used. Its optimization was performed from a mixture design, considering 6 (six) different electrolyte composition conditions, and having as parameters the chromatographic resolution statistics (CRS) and repeatability tests (n=6) for migration time and peak area. The hydrated radii of the anions were also determined in each of the 6 (six) conditions evaluated in the experimental mixture design. The addition of the organic solvents methanol and acetonitrile reduced the ion-analyte interaction in the solution, causing a decrease in the anion radii, between 4 and 12 %; methanol showed the greatest decreases in the anion-solvent interaction. As for the quantification of the cations in the samples, the determined contents of Na+ , K+ and Mg2+ varied up to 244, 300 and 330 % among the samples, respectively; and for the ions, the concentration of Clvaried up to 262 %, while SO4 - , HCO3 - e NO3 - varied up to 4000, 341 and 2900 %, respectively. Viability studies were carried out for the development of a wet based kit for potassium quantification, which results indicated the possibility of a semi-quantitative approach for this analyte determination, from the mass of the precipitate formed in solution after reacting with tetraphenylborate. The developed methods proved to be reliable and applicable in routine analysis, either due their relative simplicity or the moderate cost of the operational equipment used.