~Effect of Free Sulphuric Acid.~--Free acid is necessary for these assays; if there is an insufficiency, the assay solution, instead of immediately decolorising the permanganate, assumes a brown colour. The addition of 10 c.c. of dilute sulphuric acid suffices to meet requirements and keep the assay clear throughout. The following experiments show that a considerable excess of acid may be used without in the least affecting the results. In each case 20 c.c. of ferrous sulphate were used. Dilute sulphuric acid 1.0 c.c. 5.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c. Permanganate required 19.3 " 19.3 " 19.3 " 19.3 " 19.3 " 19.3 " ~Effect of Foreign Salts.~--When the assay has been reduced with zinc varying quantities of salts of this metal pass into solution, the amount depending on the quantity of acid and iron present. Salts of sodium or ammonium may similarly be introduced. It is essential to know by experiment that these salts do not exert any effect on the titration. The following series of experiments show that as much as 50 grams of zinc sulphate may be present without interfering. Zinc sulphate present 0 gram 1 gram 10 grams 50 grams Permanganate required 19.3 c.c. 19.3 c.c. 19.3 c.c. 19.3 c.c. Magnesium, sodium, and ammonium salts, are equally without effect. Ammonic sulphate present 0 gram 1 gram 10 grams Permanganate required 19.3 c.c. 19.2 c.c. 19.3 c.c. Sodic sulphate present 0 gram 1 gram 10 grams Permanganate required 19.3 c.c. 19.3 c.c. 19.3 c.c. Magnesic sulphate present 0 gram 1 gram 10 grams Permanganate required 19.3 c.c. 19.3 c.c. 19.3 c.c. ~Effect of Varying Amounts of Iron.~--It is important to know within what limits the quantity of iron in an assay may safely vary from that used in standardising. In the following experiments the conditions as to bulk, acidity, and mode of working were the same as before:-- Ferrous sulphate solution taken 1 c.c. 10 c.c. 20 c.c. 50 c.c. 100 c.c. Permanganate required 1.0 " 9.7 " 19.6 " 48.9 " 97.7 " The ferrous sulphate solution is here a little weaker than that of the permanganate of potassium, but the results show that the permanganate required is proportional to the iron present.