Experiment 3 Standardisation of hydrochloric acid with borax
Theory
Borax (Na2B4O7 .10H2O) is a stable compound which
is readily obtained in a very pure form (99.999% pure). It is
therefore an ideal substance to use as a primary standard. In
water, the salt (derived from a very weak acid) is extensively
hydrolysed thus:
B4O72- + 7H2O --------> 4H3BO3 + 2OH-
The boric acid formed is such a weak acid that the pH of a
decimolar solution is about 6. Hence the liberated hydroxide ions
may be titrated against a strong acid quantitatively using an
indicator which changes colour at a pH less than 5.
Method
Weigh out accurately in duplicate (on an analytical
balance) about 0.3 g of borax. Dissolve each sample in
about 25 cm3 of water, warming gently, if necessary, and titrate
against the hydrochloric acid `D' using screened methyl orange
indicator. For accurate results, only one drop of indicator
should be used, in which case, the end point will be almost
colourless against a white background. Thus calculate the
molarity of the acid.
Questions
1. What would you expect to observe if phenolphthalein was used
as the indicator in the above titration?
2. Name one other indicator suitable for the above titration.
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