The amount of iron present in the supplied sample can be estimated according to the following procedure:
Prepare approximately 0.0167 M solution of K2Cr2O7 .
Dilute the supplied sample to 250 mL in a volumetric flask and shake well (A). Take 25.00 mL of the diluted sample (A) with a pipette into a 500 mL conical flask, add 5 mL of concentrate HCl to the sample and heat for a minute or two. Add concentrate SnCl2 to the solution drop by drop till the solution becomes greenish or colorless. Add one or two drops more of dilute SnCl2. Cool the solution rapidly under tap water and add 10 mL of HgCl2 solution at a time. A silky white precipitate should appear. If the mixture becomes grey or the precipitate becomes heavy, discard the mixture and repeat with another aliquot of the sample. About 2-3 minutes after the addition of HgCl2, introduce 25 mL 2M H2SO4, 2-3 mL 85% H3PO4, 50 mL distilled water and 3-4 drops of diphenyl amine indicator to the solution. Titrate it with standard K2Cr2O7 solution. Transition of color from green to violet will indicate the end-point.
1 mL 1M K2Cr2O7 ≡ (0.05585 × 6) g Fe