About 3-5 gms. of a good average sample of the paste is weighed out accurately between two watch glasses. 5 Gms. of Mohr's salt is then added, and the whole rinsed into a 200 c.c. flask. The mixture is then heated up on a boiling water-bath for half an hour and 25 c.cs. concentrated sulphuric acid are added.

It is boiled up once, and, after cooling, the excess of Mohr's salt is titrated back by means of potassium permanganate.