Experiment 28 Determination of the Effective Ingredient in
A. Standardisation of Sodium Hydroxide `A'.
Standardisation of sodium hydroxide requires a pure solid acid.
Amongst the most commonly used reagents for this purpose is
potassium hydrogen phthalate,
KHC8H4O4 , which is obtainable
in 99.99% purity and is readily soluble in water. Phthalic acid
is a dibasic acid but KHC8H4O4
acts as a monobasic acid.
Weigh accurately (on an analytical balance)
using the method of weighing by difference, between 0.50 and 0.55
g of potassium hydrogen phthalate into a 250 cm3
conical flask and dissolve it in about 25 cm3 of
water. Titrate with sodium hydroxide using phenolphthalein as
indicator. Repeat the standardisation using a second sample of
potassium hydrogen phthalate. For each titration, calculate the
molarity of the sodium hydroxide solution and decide whether or
not a third titration is necessary.
B. Determination of the hydroxide ion content of the
The tablet is dissolved in an accurately known volume of
standard acid. The unreacted (excess) acid is then determined by
back titration using standard alkali.
Add by pipette, 25.00 cm3 of the hydrochloric acid
`B' to one of the tablets in a conical flask.
Swirl gently until the tablet has dissolved and then titrate the
excess acid using screened methyl orange as indicator. Carry out
the experiment in duplicate. Use the concentration of the
standardised hydrochloric acid provided to determine the
hydroxide ion content of the tablet. Check if your value is
consistent with that on the label.
1. Give three reasons why potassium hydrogen phthalate is used
as a primary standard.
2. Explain why phenolphthalein, rather than methyl orange, was
used as indicator for the titration of the potassium hydrogen phthalate
with sodium hydroxide.
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