FEEDti

153

condenser must be thoroughly rinsed before each distillation, to insure
freedom from basic solutions.

Pour 50 cc of saturated sodium hydroxide solution (/) down the inclined
flask in such a way that mixing does not occur. Immediately connect with
the condenser, carefully mix the contents of the flask by shaking gently,
then distill into the standard acid until about 150 ee of distillate has been
collected. It sometimes happens that a considerable excess of sulphuric acid
has been used in order to hasten a difficult digestion, or that the Bodium

Fro. 44. ...... "Appuruturi for ammonia

hydroxide solution is not saturated. The consequence ie that the solution
still contains an cxc<!ss of acid when ready for distillation. This will not be
the case if the directions have been carefully followed but the addition of a
drop of phenolphthalein to the solution will nerve to indicate the fnet.
(It should be remembered, however, that a concentrated Bolution of a base
soon decolorizes phenolphthalein and this action may be mistaken for an
indication of an excess of acid.) If copper sulphate httH been used an an
accelerator a deep blue color will indicate the presence of sufficient sodium
hydroxide,
When the distillation is finished lower the receiving flank until the delivery
tube is above the liquid, then remove the flame. Disconnect the delivery
tube from the condenser and rinse inside and outside, allowing the rinsings
to run into the flask. Add enough methyl red to tint the solution then
titrate with standard base. Subtract the excess of add thus indicated and