preface to fifteenth edition

PRACTICAL LABORATORY INFORMATION 11.91
11.6.6.1.3 Substitution Titrations. Upon the introduction of a substantial or equivalent amount
of the chelonate of a metal that is less stable than that of the metal being determined, a substitution
occurs, and the metal ion displaced can be titrated by the chelon in the same solution. This is a
direct titration with regard to its performance, but in terms of the mechanism it can be considered
as a substitution titration (or replacement titration).
In principle any ion can be used if it forms a weaker EDTA complex than the metal ion being
determined. Still weaker metal-EDTA complexes would not interfere. Exchange reactions are also
possible with other metal complexes to permit application of the chelometric titration to non-titrable
cations and anions. The exchange reagent can be added and the titration performed in the sample
solution without prior removal of the excess reagent. A most important example is the exchange of
silver ion with an excess of the tetracyanonickelate ion according to the equation:
2 2
2Ag Ni(CN) 4 N 2 Ag(CN) 2 Ni
The nickel ion freed may then be determined by an EDTA titration. Note that two moles of silver
are equivalent to one mole of nickel and thus to one mole of EDTA.
11.6.6.1.4 Redox Titrations. Redox titrations can be carried out in the presence of excess
EDTA. Here EDTA acts to change the oxidation potential by forming a more stable complex with
one oxidation state than with the other. Generally the oxidized form of the metal forms a more stable
complex than the reduced form, and the couple becomes a stronger reducing agent in the presence
of excess EDTA. For example, the Co(III)–Co(II) couple is shifted about 1.2 volts, so that Co(II)
can be titrated with Ce(IV). Alternatively, Co(III) can be titrated to Co(II), with Cr(II) as a reducing
agent.
Manganese(II) can be titrated directly to Mn(III) using hexacyanoferrate(III) as the oxidant.
Alternatively, Mn(III), prepared by oxidation of the Mn(II)–EDTA complex with lead dioxide, can
be determined by titration with standard iron(II) sulfate.
11.6.6.1.5 Indirect Procedures. Numerous inorganic anions that do not form complexes with
a complexing agent are accessible to a chelatometric titration by indirect procedures. Frequently the
anion can be precipitated as a compound containing a stoichiometric amount of a titrable cation.
Another indirect approach employing replacement mechanism is the reduction of a species with the
liquid amalgam of a metal that can be determined by a chelometric titration after removal of excess
amalgam. For example:
2
2Ag Cd(Hg) Cd 2 Ag(Hg)
The equivalent amount of cadmium ion exchanged for the silver ion can readily be determined by
EDTA titration procedures.
11.6.6.2 Preparation of Standard Solutions
11.6.6.2.1 Standard EDTA Solutions. Disodium dihydrogen ethylenediaminetetraacetate dihydrate
is available commercially of analytical reagent purity. After drying at 80C for at least 24 hr,
its composition agrees exactly with the dihydrate formula (molecular weight 372.25). It may be
weighed directly. If an additional check on the concentration is required, it may be standardized by
titration with nearly neutralized zinc chloride or zinc sulfate solution.
11.6.6.2.2 Standard Magnesium Solution. Dissolve 24.647 g of magnesium sulfate heptahydrate
in water and dilute to 1L for 0.1M solution.
11.6.6.2.3 Standard Manganese(II) Solution. Dissolve exactly 16.901 g ACS reagent grade
manganese(II) sulfate hydrate in water and dilute to 1L.
11.6.6.2.4 Standard Zinc Solution. Dissolve exactly 13.629 g of zinc chloride, ACS reagent
grade, or 28.754 g of zinc sulfate heptahydrate, and dilute to 1L for 0.1000M solution.