Limit Test for Potassium and Sulphates

Potassium

To 10 ml of the prescribed solution add 2 ml of a freshly prepared 1 percent w/v solution of sodium tetraphenyl borate. Prepare a standard solution, in the same manner, using a mixture of potassium standard solution (20 ppm K) and 5 ml of water. After 5 minutes, any opalescence in the test solution is not more intense than in the standard solution.

Sulfates

NOTE - The solutions used for this test should be prepared with distilled water.
To 1.0 ml of a 25.0 per cent w/v solution of barium chloride in a Nessler cylinder add 1.5 ml of ethanolic sulfate standard solution (10 ppm SO4 ), mix and allow to stand for 1 minute. Add 15 ml of the solution prepared as directed in the monograph or a solution of the specified quantity of the substance under examination in 15 ml of water and 0.15 ml of 5 M acetic acid. Add sufficient water to produce 50 ml, stir immediately with a glass rod and allow standing for 5 minutes.

When viewed transversely against a black background any opalescence produced is not more intense than that obtained by treating in the same manner 15 ml of sulfate standard solution (10 ppm SO4) in place of the solution under test.