Accurate results cannot be obtained by the titration of tomato products
in the presence of the insoluble solids. If it be desired to determine
the acidity in the entire sample of tomatoes or tomato pulp rather than
in the expressed juice, the insoluble solids should first be removed
by the method given in the determination of insoluble solids or by
filtration through filter paper. The per cent of acid given in Tables
2 and 3 was obtained by titrating the liquor obtained by filtering the
pulp. In products of this nature, the addition of an alkali causes a
brownish color which has a tendency to obscure the end point shown
by the indicator. To obviate this, the sample should be diluted to
at least 200 cc. and a larger amount of indicator employed than is
necessary with a clear solution. The following details are suggested.
Dilute 20 grams of the filtrate under examination with over 200
cc. of water. Add at ½ cc. of phenolphthalein solution (prepared
by dissolving 1 gram of phenolphthalein in 100 cc. of 95 per cent
alcohol) and titrate with sodium hydroxide until the end point is
obtained. Add 1 cc. of tenth-normal hydrochloric acid, heat the
solution quickly to boiling and boil one minute to expel carbon
dioxide. Cool the solution quickly to about room temperature, and then
add tenth-normal sodium hydroxide until the end point is obtained.
The volume of hydrochloric acid added must, of course, be taken into
consideration in the final result. The filtrate may also be titrated
direct with tenth-normal sodium hydroxide solution with satisfactory
results.