A direct photometric method for chloride in biological fluids, employing mercuric thiocyanate and perchloric acid.

The reagent described is 5.5 M in perchioric acid and 3.3 M in urea. It contains ferric ions, mercuric thiocyanate, mercuric ions (from mercuric perchiorate), and mercuric chloride. Serum dissolves directly in this reagent to yield a clear, reddish solution. When chloride ions are added, they combine first with the free mercuric ions, and then with some of the mercuric ions from the mercuric thiocyanate. Liberated thiocyanate combines with ferric ions to yield red ferric thiocyanate. The color is much more intense in the presence of strong perchioric acid than in other aqueous acid mixtures, Its intensity can be regulated at will by changing the concentration of the ferric iron. The presence of mercuric chloride in the reagent improves linearity between absorbance and chloride concentration. After the total absorbance is determined, compensation for absorbance by other substances is secured by adding mercuric ions to the photometer tube to reverse the color-producing reaction of chloride, reading the residual absorbance, and subtracting it from the total absorbance, to give a net absorbance produced by chloride alone.