The method is based on weighing a test portion, dissolving it in water, transfer of the solution to each of two Nessler colour-comparison cylinders, preparing a test solution by adding freshly prepared ammoniacal silver nitrate solution to one of the Nessler cylinders and mixing well, preparing a blank solution by adding ammonium hydroxide solution and water to the other Nessler cylinder and mixing well, allowing the solutions to stand for 2 min, and comparing the colour and turbidity of the two solutions.