As filter alum usually contains 0.2 to 0.3 per cent of iron use a 10
gram sample for the determination of total iron. Treat the sample with
50 cc. of freshly boiled distilled water and add 5 cc. of concentrated
hydrochloric acid and 1 cc. of bromine water. Evaporate the solution to
dryness, dissolve the residue in water, and wash it into a flask with
sufficient water to make the volume about 50 cc. Add 50 cc. of
concentrated hydrochloric acid, boil to expel oxygen, and titrate, as
hot as possible, with N/20 stannous chloride.
If a 10 gram sample is used the percentage of iron (Fe) is equal to the
number of cubic centimeters of stannous chloride used multiplied by
0.028, and the percentage of iron expressed as ferric oxide is equal to
the number of cubic centimeters of stannous chloride used multiplied by
0.040.