Colorimetric determination of chloride in biological samples by using mercuric nitrate and diphenylcarbazone.

A colorimetic method is outlined for the determination of the chloride ion in biological samples (blood serum, plasma, and urine). The present method is based on the quantitative reduction of free mercuric ions by chloride ions. Chloride ions form an indissociable complex with mercuric ions. The remaining free mercuric ions form a purple complex with diphenylcarbazone with an absorption maximum at 550 nm. The reduction of color intensity at 550 nm is directly proportional to chloride concentration in the sample. The linear concentration range in the final reaction mixture was 0-100 microM with a correlation coefficient of -0.9997. The coefficient of variation for the 50 microM chloride ion in the final reaction mixture was 0.9% (n = 6). The analyzed value of chloride concentration in the human control serum Accutrol Normal (Sigma) was 101+/-4 mM (mean+/-SD, n = 12). The certified value of chloride in Accutrol Normal by Sigma is 102 mM, with a mean in the range 91-113 mM. This method was applied to the measurement of urinary chloride excretion in experimental rats. During 16-h urine collection, no food was given and rats had free access to purified water. The urinary excretion rate of chloride was 23.6+/-9.3 micromol/h (mean+/-SD, n = 8) and 126.2+/-28.0 micromol/h (n = 8) for rats fed a normal diet (2.6 g NaCl/kg diet) and a high-salt diet (82.6 g NaCl/kg diet) for 70 d prior to urine collection, respectively. This method is appropriate for low concentrations of chloride in samples or when sample volume is limiting, as in many animal studies such as metabolic urine collection from rats.