As filter alum usually contains 0.02 to 0.04 per cent of ferric iron use
a 20 gram sample. Boil 50 cc. of distilled water to expel oxygen, add 50
cc. of concentrated hydrochloric acid, and add the sample while the
solution is boiling. Keep it boiling till the sample is dissolved. The
flask should be kept filled with carbon dioxide during this process by
dropping in occasionally small amounts of sodium carbonate. When
solution of the sample is complete titrate it hot immediately with N/20
stannous chloride.
If a 20 gram sample is used the percentage of ferric oxide (Fe_{2}O_{3})
is equal to the number of cubic centimeters of stannous chloride used
multiplied by 0.020.