Instant Notes: Analytical Chemistry

The major application of complexation titrations is for the determination of

the concentrations or amounts of metallic elements in water, food and other

industrial samples.

Example

The calcium and magnesium ions in hard water may be determined. The solution

is adjusted to pH 12 with NaOH, when Mg(OH) 2 is precipitated. The calcium is

then titrated with EDTA using calcon carboxylic acid as indicator. Both calcium

and magnesium are then determined in by titrating a sample with EDTA at pH 10

using eriochrome black T, and finding the magnesium by difference.

For precipitation reactions, the change in the concentration of either ion

forming the precipitate may be considered. Since the changes often involve

many orders of magnitude of concentration, it is again convenient to use the pX

notation. For example, for the reaction of silver ions with chloride to form an

insoluble silver chloride precipitate

Ag++Cl-=AgCl (s)

the concentration may be expressed as:

pAg =-log (a(Ag+)) ~ -log (c (Ag+))

Figure 1shows the pAg values in the titration of sodium chloride by silver

nitrate as a function of the volume of silver nitrate added. This figure shows

that, before the end point pAg is very high (that is, the concentration of silver

ions is small) and changes little, because there is still an excess of chloride and

the silver is almost completely removed as precipitate. After the end point, there

is an excess of silver ions, the concentration increases and pAg decreases. In the

region around the end point, where the amounts are nearly equal, the change in

pAg with volume added is very large. If a mixture of iodide and chloride ions is

titrated (dashed line), the iodide, which is less soluble, precipitates first and pAg

is even higher than for chloride. Then the chloride precipitates. Both end points

can be found.

As noted in Table 1of Topic A5, silver nitrate, sodium chloride and potassium

chloride are primary standards for silver halide precipitation reactions. Other

Precipitation
titrations

92 Section C – Analytical reactions in solution

14

12

10

8

6

4

2

0

0 5 10 15 20 25 30 35

Volume AgNO 3 added

pAg

(a)

(b)

Fig. 1. Silver-halide titrations. (a) Chloride alone (solid line); (b) iodide plus chloride (dashed

line).