Potassium stock solution— Dissolve 190.7 mg of potassium chloride, previously dried at 105 for 2 hours, in water. Transfer to a 1000-mL volumetric flask, dilute with water to volume, and mix. Transfer 100.0 mL of this solution to a 1000-mL volumetric flask, dilute with water to volume, and mix. This solution contains 10 µg of potassium (equivalent to 19.07 µg of potassium chloride) per mL.

Standard preparations— To separate 100-mL volumetric flasks transfer 10.0, 15.0, and 20.0 mL, respectively, of Potassium stock solution. To each flask add 2.0 mL of sodium chloride solution (1 in 5) and 1.0 mL of hydrochloric acid, dilute with water to volume, and mix. The Standard preparations contain, respectively, 1.0, 1.5, and 2.0 µg of potassium per mL.

Assay preparation— Transfer an accurately measured volume of Oral Solution, equivalent to about 600 mg of potassium chloride, to a 500-mL volumetric flask, dilute with water to volume, and mix. Transfer 5.0 mL of the solution to a 100-mL volumetric flask, dilute with water to volume, and mix. Transfer 5.0 mL of the resulting solution to a 100-mL volumetric flask, add 2.0 mL of sodium chloride solution (1 in 5) and 1.0 mL of hydrochloric acid, dilute with water to volume, and mix.

Procedure— Concomitantly determine the absorbances of the Standard preparations and the Assay preparation at the potassium emission line of 766.5 nm, with a suitable atomic absorption spectrophotometer (see Spectrophotometry and Light-scattering 851) equipped with a potassium hollow-cathode lamp and an air–acetylene flame, using water as the blank. Plot the absorbance of the Standard preparation versus concentration, in µg per mL, of potassium, and draw the straight line best fitting the three plotted points. From the graph so obtained, determine the concentration, in µg per mL, of potassium in the Assay preparation. Calculate the quantity, in mg, of KCl in the portion of Oral Solution taken by the formula: in which C is the concentration, in µg per mL, of potassium in the Assay preparation, and 1.907 is the ratio of the molecular weight of potassium chloride to the atomic weight of potassium.