Trichloroacetic acid (TCA) solution— Dissolve a quantity of TCA in water to obtain a 20% (w/v) solution.

Gold chloride solution— Dissolve a quantity of gold chloride in water to obtain a 0.5% (w/v) solution.

Blank— Use water.

Bromide standard stock solution— Dissolve an accurately weighed quantity of USP Sodium Bromide RS in water to obtain a solution having a known concentration equivalent to about 20 mg of bromide per mL.

Standard preparations— Prepare four Standard preparation solutions of known concentrations of about 2.0 mg of bromide per mL, 1.0 mg of bromide per mL, 0.5 mg of bromide per mL, and 0.25 mg of bromide per mL, respectively, by diluting quantitatively the Bromide standard stock solution with the appropriate volumes of water.

Assay preparation— Dilute Injection, Veterinary quantitatively with water (1:19).

Procedure (see Spectrophotometry and Light Scattering 851)— To 750-µL aliquots each of Blank, the four Standard preparations, and the Assay preparation, add 500 µL of Trichloroacetic acid (TCA) solution and 250 µL of Gold chloride solution. Mix on a vortex mixer, and immediately read the absorbance at 440 nm. Generate a standard curve of absorbance versus bromide concentration, correcting for the blank, and calculate the regression line and the regression coefficient. The test is considered valid if the square of the correlation coefficient of the regression curve (r2) is not less than 0.99. Calculate the concentration, in mg per mL, of bromide (Br ) in the portion of Injection, Veterinary taken by the formula: in which C is the concentration, in mg per mL, of bromide in the Assay preparation as calculated from the regression line.USP32