EDTA Titration Procedures
If the analyte metal ion forms a stable EDTA complex rapidly, and an end point can be readily detected,
a direct titration procedure may be employed. More than thirty metal ions may be so determined. Where
the analyte is partially precipitated under the reaction conditions thereby leading to a slow reaction, or
where a suitable indicator cannot be found, back titration procedures are used. A measured excess of
EDTA is added and the unreacted EDTA titrated with a standard magnesium or calcium solution.
Provided the analyte complex is stronger than the Ca-EDTA or Mg-EDTA complex a satisfactory end
point may be obtained with eriochrome black T as indicator. An alternative procedure, where end points
are difficult to observe, is to use a displacement reaction. In this case, a measured excess of EDTA is
added as its zinc or magnesium complex. Provided the analyte complex is the stronger, the analyte will
displace the zinc or magnesium.
The magnesium will be liberated quantitatively and may then be titrated with a standard EDTA
solution. Where mixtures of metal ions are analysed, the masking procedures already discussed can be
utilized or the pH effect exploited. A mixture containing bismuth, cadmium and calcium might be
analysed by first titrating the bismuth at pH = 1–2 followed by the titration of cadmium at an adjusted
pH = 4 and finally calcium at pH = 8. Titrations of this complexity would be most conveniently carried
out potentiometrically using the mercury pool electrode.
Applications of EDTA Titrations
Table 5.8 gives an indication of the range of elements that may be determined. Most procedures will
require an analyte concentration of 10–^3 mol dm–^3 or more, although with special conditions, notably
potentiometric end-point detection, the sensitivity may be extended to 10–^4 mol dm–^3. The analysis of
mixtures of metal ions necessitates masking and demasking, pH adjustments and selective separation
procedures. Areas of application are spread throughout the chemical field from water treatment and the
analysis of refined food and petroleum products to the assay of minerals and alloys. Table 5.10 gives
some selected examples.
Titrations with Complexing Agents Other Than EDTA
A number of other reagents containing substituted amino and carboxylic acid groupings are used to a
limited extent. Their general behaviour is very similar to that of EDTA, but the metal ion complexes
formed may be stronger or weaker than parallel ones formed by EDTA. Stronger complexing agents
(DETPA, DCTA) can extend the scope of complexometric titrations to elements not adequately
complexed by EDTA and weaker ones (NTA) may show a better selectivity. Some reagents of these
types are