IONOMETRIC DETERMINATION OF ZINC IN MILK

Abstract

The possibility of ionometric determination of zinc in the rhodanide complex form in milk is represented. The indicator electrode was ion-selective electrode (ISE) with solution of tetradecylammonium bromide in nitrobenzene as a membrane.  As a complexing agent the potassium thiocyanate with 1.0 mol/l optimal concentration in investigated solutions was selected. The calibration dependence E = f(pCZn(II)) were typical anionic functions. The steepness of the electrode function (29±2 mV/pC), close to the theoretical value for the doubly−charged ions, suggests that the electrochemically active anions are [Zn(NCS)4]2-. The interval of linearity of calibration curves is 1.0 to 5.7 pC, the detection limit is equal to 1∙10-6 mol/l. The selectivity of determination of zinc in the rhodanide complex form was investigated with “The mixed solutions” method in the presence of potassium, calcium, aluminum, iron (III), manganese (II), copper (II) chlorides potassium iodide and nitric acid solutions. The background concentration of extraneous (j) substances in investigated solutions was calculated with usage of published data and corresponds to their maximum content in milk. It was founded that the ionometric determination of zinc is possible in range from 1.0 to 5.0 pC for all investigated j-ions. The technique for different types of milk was developed including preliminary sample preparation, used in special certified laboratories and based on dry mineralization of sample and dissolving in nitric acid solution. The concentrations of zinc-ions in analyzed samples were determined with “limiting solutions” method. The correctness of obtained data was confirmed with “entered-found” scheme. The relative standard deviation was not higher then 0.08.