The principle of the method consists in the hot extraction of the material by following the procedure specified in ISO 787/3, to the stage of obtaining a perfectly clear filtrate. The determination is carried out with indicator solution (method A). If the solution with the methyl red indicator is yellow (alkaline), titrate it with the hydrochloric acid solution to an orange end-point; if the solution with the methyl red indicator is red (acid), titrate it with the sodium or potassium hydroxide solution to an orange end-point. The other method (method B) is the potentiometric determination. Take 100 ml of the test solution, insert the electrodes of the pH measuring device and read the pH value.