To determine the alkali in the sample, 10 cubic centimeters are pipetted into a beaker, a little distilled water added, then 3 or 4 drops of phenolphthalein indicator. From a burette, quarter normal (N/4) sulfuric acid is added until the pink color is just discharged. When this point is reached 4 to 5 c. c. more of acid are added and the solution is boiled to expel the carbon dioxide. Should the solution turn pink, it is necessary to add more acid. After having boiled for 3 to 4 minutes, N/4 caustic soda is added until the pink color just returns and the amount of caustic soda used is read on the burette. The difference between the number of cubic centimeters of N/4 sulfuric acid and N/4 caustic soda gives the amount of alkali in the sample. By using a 10 c. c. sample and N/4 sulfuric acid and N/4 caustic soda each c. c. obtained by the difference of these two solutions is equal to one-tenth of one per cent. (0.1%) of the total alkali in the lye. As an example, say we first used 7.7 c. c. of N/4 sulfuric acid to just discharge the pink, then added 4 c. c. more, or 11.7 c. c. in total. After boiling it required 5.3 c. c. to bring back a slight pink, the total alkalinity would be 11.7 c. c. - 5.3 c. c. = 6.4 c. c., or 0.64% total alkali in the lye in terms of caustic soda. If there were 40,000 pounds of lye to be treated then we should have to neutralize:
40,000 × .0064 = 256 lbs. alkali. Since sulfate of alumina neutralizes one-third of its weight in caustic, and there are say 9 lbs. of this added per thousand pounds of lye we would add
40,000 × 9 = 360 lbs. of sulfate of alumina. This would neutralize 360 × 1/3 = 120 lbs of alkali. There are then 256 - 120 = 136 lbs. of alkali still to be neutralized. If 60° B. sulfuric acid is used it requires about 1.54 lbs. of acid to one pound of caustic. Therefore to neutralize the caustic soda remaining it requires:
136 × 1.54 = 209.44 lbs. 60° B. sulfuric acid to neutralize the total alkali in the 40,000 pounds of spent lye.
The acid is added and the lye well stirred, after which another sample is taken and again titrated as before. From this titration the amount of acid to be added is again calculated and more acid is added if necessary. Should too much acid have been added, caustic soda solution is added until the lye is between exactly neutral and 0.02% alkaline. The filtered lyes at this stage have a slight yellowish cast.