Effect of Varying Bulk.—The following experiments show that considerable variations in bulk have no practical effect. In each case 20 c.c. of ferrous sulphate solution and 10 c.c. of dilute acid were used.

Bulk of assay30c.c.100c.c.500c.c.1000c.c.
Permanganate required20.0"20.0"20.2"20.5"

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 acid1.0c.c.5.0c.c.10.0c.c.20.0c.c.50.0c.c.100.0c.c.
Permanganate required19.3"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 present0 gram1 gram10 grams50 grams
Permanganate required19.3 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 present0 gram1 gram10 grams
Permanganate required19.3 c.c.19.2 c.c.19.3 c.c.
Sodic sulphate present0 gram1 gram10 grams
Permanganate required19.3 c.c.19.3 c.c.19.3 c.c.
Magnesic sulphate present0 gram1 gram10 grams
Permanganate required19.3 c.c.19.3 c.c.19.3 c.c.