ARTICLES
Micro-scale precipitation titration (continued)
Before titration
AgCl forming
Close to end-point
End-point
Overrun
This experiment also provides an opportunity to review the use of
solubility products to predict the formation of precipitates to show
why chromate (K sp Ag 2 CrO 4 = 1.1 x 10 −12 ) is a suitable indicator
for this experiment and why it is unsuitable for measuring sulfate
ion concentration. (K sp Ag 2 SO 4 = 1.2 x 10 −5 ).
The experiment yields consistent results at the normal pH of
seawater. At pH values greater than 9, silver hydroxide may
precipitate. Below pH 6, the concentration of the chromate ion
will not be sufficient to create a precipitate at the equivalence
point.
The errors in this experiment are predictable. The equivalence
point must be reached before the end-point because for the
coloured precipitate to be visible, excess silver must be present.
This can be accounted for in one of two ways:
• Firstly, you can produce a calibration curve by performing
titrations with a range of standardised solutions of chloride
ions.
• A second technique is to perform a blank titration with no
chloride ions and a white insoluble powder such as calcium
carbonate to replicate the masking effect of the silver nitrate.
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SCIENCE EDUCATIONAL NEWS VOL 68 NO 3