Twenty-five milliliters of the
K4Fe(CN)6 solution is pipetted into a 250-mL Erlenmeyer
flask and 5 mL of 3 M sulfuric acid, 4 drops of diphenylamine
indicator, and 3 drops of freshly prepared 1% potassium
ferricyanide (which oxidizes the diphenylamine to its colored
form in the absence of ferrocyanide) are added.