SULFATE OF ALUMINIUM.

Determine and report insoluble matter, aluminium oxide (Al₂O₃), ferric oxide (Fe₂O₃), ferrous oxide (FeO), basicity ratio, and, if present, free acid as H₂SO₄. If the material is what is known as “granular” sulfate mix it well before sampling. If it is in lump form crush it to ⅛ to ¼ inch size, mix, and sample it. It is unnecessary to grind the sample to a fine powder, but it is preferable to have the particles fairly uniform in size.

INSOLUBLE MATTER.

Treat 10 grams of the sample with 100 cc. of distilled water and digest one hour at boiling temperature. Filter through a weighed Gooch crucible and wash the insoluble matter with hot water freshly boiled to free it from carbon dioxide. Dry the crucible to constant weight at 100° C., cool, and weigh. Report the percentage of insoluble matter.

OXIDES OF IRON AND ALUMINIUM.

Dilute the filtrate from the determination of insoluble matter to 500 cc. with water free from carbon dioxide and thoroughly mix the solution. Transfer 50 cc. of the solution to a 250 cc. beaker, add about 150 cc. of water and 5 cc. of concentrated hydrochloric acid, and heat to boiling. Add ammonium hydroxide in slight excess; when the solution has been almost neutralized it is convenient to add a drop of methyl orange indicator and then to add about 0.5 cc. of ammonium hydroxide after the solution is neutral to the indicator. Digest at about 100° C. for a few minutes and filter. Some analysts prefer to wash this gelatinous precipitate with hot water by decantation, and some to wash it evenly distributed over the surface of a filter paper; either method may be used. It is difficult to free it completely from impurities and it is not necessary to do so unless unusual quantities of calcium, magnesium, sodium, or potassium are present. While the precipitate is being washed do not allow it to become dry, as it then packs and can not be washed clean. After most of the water has drained drying the filter may be hastened by placing it on a sheet of blotting paper. If much iron is present completely dry the precipitate, remove it from the paper, and ignite the paper separately. Finally, blast the precipitate, with free access of air to the crucible, for five or ten minutes, cool, and weigh as oxides of iron and aluminium (Fe₂O₃ + Al₂O₃).

Subtract the content of total iron, expressed as ferric oxide (Fe₂O₃), from the weight of the combined oxides and report the difference as aluminium oxide (Al₂O₃), in percentage.

TOTAL IRON.

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.

FERRIC IRON.

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 (Fe₂O₃) is equal to the number of cubic centimeters of stannous chloride used multiplied by 0.020.

FERROUS IRON.

The content of ferrous iron is the difference between total and ferric iron. The percentage of ferrous oxide (FeO) is, therefore, equal to 0.90 times the difference between the percentage of total iron expressed as ferric oxide and the percentage of ferric iron expressed as ferric oxide. Report the percentage of ferrous oxide (FeO).

BASICITY RATIO.

Transfer 50 cc. of the filtrate from the determination of insoluble matter to a 200 cc. casserole and dilute it to 100 cc. Boil the solution and titrate it at boiling temperature with N/1 sodium hydroxide in presence of phenolphthalein indicator. The percentage of acidity in equivalent of sulfuric acid (H₂SO₄) is equal to the number of cubic centimeters of sodium hydroxide used multiplied by 4.9. In this titration iron and aluminium are precipitated as hydroxides and any free acid is neutralized.

Calculate the percentage of sulfuric acid equivalent to the determined percentages of aluminium oxide, ferric oxide, and ferrous oxide by the following formula:

2.88 Al₂O₃ + 1.83 Fe₂O₃ + 1.36 FeO.

If this percentage of acid equivalent is less than that found by titration report the difference as percentage of free acid. If the percentage of acid equivalent is greater than that found by titration the difference divided by 2.88 is the percentage equivalent to the excess of aluminium oxide present. Divide this excess by the percentage of total aluminium oxide and report the quotient as the basicity ratio.