A: To 2 mL of a dilution of Oral Solution (1 in 40) add 5 mL of sodium cobaltinitrite TS: a yellow precipitate is formed immediately (presence of potassium).

B: It responds to the test for Citrate 191, 3 to 5 drops of Oral Solution and 20 mL of the mixture of pyridine and acetic anhydride being used.

C: It responds to the tests for Ammonium 191 and for Chloride 191.

(Official January 1, 2007)

Potassium stock solution— Dissolve in water 0.9535 g of potassium chloride, previously dried at 105 for 2 hours. Transfer to a 500-mL volumetric flask, dilute with water to volume, and mix. This solution contains 1000 µg of potassium per mL.

Standard preparations— To separate 200-mL volumetric flasks transfer 19.0 mL and 25.0 mL, respectively, of the Potassium stock solution, dilute with water to volume, and mix. The Standard preparations contain 95.0 µg and 125.0 µg of potassium per mL, respectively.

Assay preparation— Transfer an accurately measured volume of Oral Solution, equivalent to about 782 mg (20 mEq) of potassium, to a 100-mL volumetric flask, dilute with water to volume, and mix. Transfer 7.0 mL of this solution to a 500-mL volumetric flask, dilute with water to volume, and mix.

Procedure— Concomitantly determine the absorbances of the Standard preparations and the Assay preparation at the resonance 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 absorbances of the Standard preparations versus concentration, in µg per mL, of potassium. From the graph so obtained, determine the concentration, C, in µg per mL, of potassium in the Assay preparation. Calculate the quantity, in mg, of potassium in each mL of the Oral Solution taken by the formula: in which V is the volume, in mL, of Oral Solution taken. Each mg of potassium is equivalent to 0.02558 mEq.

Ionic strength adjusting solution— Use 5 M sodium nitrate.

Procedure— Transfer an accurately measured volume of Oral Solution, equivalent to about 100 mg (2.8 mEq) of chloride, to a suitable beaker. Add 2.0 mL of Ionic strength adjusting solution and water to make about 100 mL, and titrate with 0.1 N silver nitrate VS, determining the endpoint potentiometrically, using a silver-sulfide specific ion-selective electrode and a double-junction reference electrode containing potassium nitrate solution (1 in 10). Perform a blank determination, and make any necessary correction. Each mL of 0.1 N silver nitrate is equivalent to 3.545 mg of chloride (Cl). Each mg of chloride is equivalent to 0.0282 mEq of Cl.