Reagents. Ceric sulfate (ammonium tetrasulfatocerate) 0.1 A'

in 0.05 h' sulfuric acid. Potassium iodide solution, 10%. Sodium

thiosulfate, 0.1 A'. Ferroin indicator (0-phenanthroline-ferrous

ion).

Procedure. The sample of per acid is accurately weighed and

placed in a 500-ml. Erlenmeyer flask containing 150 ml. of 570

sulfuric acid and sufficient cracked ice to maintain a temperature

of 0 ' to 10' C. An adequate sample is chosen, when possible,

to give approximately 40 ml. of thiosulfate titration. Three

drops of ferroin indicator are added and the flask contents are

titrated with 0.1 A- ceric sulfate to the disappearance of the

salmon color of the indicator. Ten milliliters of the 10% potassium

iodide solution are then added and the liberated iodine is

tirated with 0.1 N sodium thiosulfate. Starch indicator is added

near the end point for the thiosulfate titration

Calculations.

% H2O2 = ml. of ceric sulfate X N X 17

10 X sample weight