TOTAL ALKALI.
In the filtrate remaining after having washed the fatty acids in the determination of total fatty and resin acids all the alkali present as soap, as carbonate and as hydroxide remains in solution as sulfate. Upon titrating this solution with half normal alkali the difference between the half normal acid used in decomposing the soap and alkali used in titrating the excess of acid gives the amount of total alkali in the soap. By deducting the amount of free alkali present as carbonate or hydroxide previously found the amount of combined alkali in the soap may be calculated.
To quickly determine total alkali in soap a weighed portion of the soap may be ignited to a white ash and the ash titrated for alkalinity using methyl orange as an indicator.