3395The stability constants of the binary complex of cobalt with pyridine, log Kl (Copy2 +) = 1·16 ± ± 0·13 (1·30), log K2(COPy~ +) = 0·61 ± 0·35 (0·70), log K3(COPy~ +) = (0·58), were determined by potentiometry and by an extraction method (the values in brackets). Also determined were the dissociation constant of pyridine PKa(CsHsNH+) = 5·31 ± 0·03 and its partition constant in the system benzene-water, KD = 2·83 ± 0·07. All the equilibrium constants are valid for 25°C and 10·1 (NaCI0 4 ) and were measured under conditions corresponding to the extractionphotometric determination of cobalt with IJ(-furilmonoxime.
A method is described for the determination of nitrates in cow milk, human milk, milk powder or milk-based infant formulae using liquid chromatography on Spheron DEAE and a direct photometric detection (205 nm). The influence of removing proteins by precipitation with Carrez reagent on the accuracy of determination was studied. The proposed method gives identical results with the reference method (photometry after reduction of nitrate to nitrite) but is more rapid. Its limit of determination is 0.5 mg NO3-/l of milk, its reproducibility is 4% (relative standard deviation).
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