The method is based on weighing a test portion, dissolution in water, addition of ammonium oxalate solution, diammonium hydrogen phosphate solution and ammonium hydroxide solution, allowing to stand for at least 8 h, filtering out the precipitate through filter paper, transfer of the filter paper with precipitate to a prepared crucible, burning it over a gas burner, incineration of the residue in a furnace, cooling in a desiccator, and reweighing.