Chemistry Reference
In-Depth Information
purity is obtained by dividing this calculated weight by the mass of sample
weighed out.
Using the figures above:
Volume required
23.95 mL of 1
M
(0.998) NaOH
23.95 mL 0.998
M
NaOH
(23.95/1000)
0.998 moles of NaOH
0.0239 moles NaOH.
Since 1 mole of citric acid
3 moles of NaOH,
Number of moles of citric acid reacted
1
(0.0239/3)
0.007967 moles citric acid
Since
M
r
192.1 g,
Mass of citric acid reacted
192.1
0.007967
1.53053 g
But 1.5268 g was weighed, so the content of citric acid is given by
1.53053
———
100
100.2% w/w
1.5268
which is the same answer as obtained above.
Sources of error can be introduced in each conversion from volume to
moles and back to weight, although for simple examples such as the one
above it does not really matter which method of calculation is employed as
long as the correct answer for the purity of citric acid is obtained. However,
for more complicated calculations, involving the use of back and blank
titrations, this author believes that factors and equivalents simplify volu-
metric analysis and they will be used for that reason (rather than any reason
of dogma) in the remainder of this topic.
Choice of indicators
The end point of the titration is detected by the use of a suitable indicator.
These indicators are themselves weak acids or bases whose colour in solu-
tion depends on their degree of ionisation. In practice, the endpoint pH is
estimated (see Chapter 1, p. 22), and an indicator that changes colour at this
pH chosen. For convenience, a table of common indicators and their pH
ranges is shown in Figure 6.3.
