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 (H_{2}SO_{4}) 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 Al_{2}O_{3} + 1.83 Fe_{2}O_{3} + 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.