1 Gm. of zinc dust and 4.00 gms. sodium bichromate are dissolved up, and the solution made up to a litre with the addition of 20 c.cs. of 20 % sulphuric acid.

250 C.cs. of this solution are taken and diluted up with 900 c.cs. water. 150 C.cs. of 20 % sulphuric acid are added and 100 c.cs. of 10 % potassium iodide. This solution is allowed to stand in the dark for half an hour and the excess of iodine titrated back with N/10 thiosulphate.

In order to determine the strength of the bichromate exactly 0.800 gm. is treated in a similar manner.

Calculation: If B = c.cs. thiosulphate for 0.800 gm. bichromate,

A = c.cs. for 4 gms. bichromate plus zinc dust, then % metallic zinc = (B X 1.25 - A) X 1.308.