NITRATE-SELECTIVE ELECTRODE AND ITS USE FOR ANALYSIS OF VEGETABLES AND MINERAL WATERS

Abstract

The effect of the length of the radicals at the exchange center of the quaternary ammonium salts on the selectivity and the detection limit of the nitrate-selective electrode is investigated. The observed effects of improving the analytical characteristics while reducing the steric availability of the exchange center for quaternary ammonium salts are explained on the basis of the theory of ion association. The composition of the polyvinyl chloride (33 wt.%) electrode membrane for the quaternary ammonium salt (3,4,5-tris bromide (dodecyloxy)benzyltricethylammonium bromide, 5 wt.%) and the plasticizer (o-nitrophendecyl ether, 62 wt.%) was optimized. The developed electrode has a low detection limit (2.0∙10−7 mol/l) and a close to theoretical slope of the electrode function (56.8 mV/pNO3), and the potential of the nitrate-selective electrode is not affected by fluoride, nitrite, bicarbonate-, dihydrophosphate-, sulfate ions. The interfering action of chloride, bromide, and iodide ions can be eliminated by introducing 1·10–3 mol/l of a background solution of silver sulfate. Thiocyanate and perchlorate ions interfere with the operation of the electrode, but they are practically not found in real objects. The developed electrode was used to determine the nitrate ions in vegetables (white cabbage, cucumber, lettuce, tomato, onion, carrots, and potatoes), greens (dill, parsley) and mineral water. It was established that the maximum permissible concentration of nitrates is exceeded more than 4.5 times for cabbage (MAC = 500 mg/kg), almost 2 times for salad (maximum permissible concentration 1500 mg/kg), 1.5 times for potatoes (maximum permissible concentration 250 mg/kg) and 3 times for dill (maximum permissible concentration 1500 mg/kg).