constants for the heavy metal-EDTA complexes so that they do not react to any significant extent.
Meanwhile magnesium forms only a very weak cyanide complex and can be titrated without
interference. For example, in the case of zinc, the stepwise formation constants for its cyanide
complexes are K 1 = 3 × 105 , K 2 = 1.3 × 105 , K 3 = 4.3 × 104 , K 4 = 3.5 × 103 , whence from an expression
similar to equation (5.22) β ≈ 10 –^19 when [CN–] = 1 mol dm–^3. then becomes 1.1 × 10 –^3 , indicating
that the presence of cyanide effectively prevents the formation of the zinc-EDTA complex. Demasking
at a subsequent stage in order that the zinc may be determined is effected by a formaldehyde-acetic acid
mixture, i.e.
Chloral hydrate (Cl 3 CCHO) may also be used in demasking reactions.
End-point Detection for EDTA Titrations
The now familiar alternatives of visual and potentiometric detection are available. A number of organic
dyes form coloured chelates with many metal ions. These coloured chelates are often discernible to the
eye at concentrations of 10–^6 – 10 –^7 mol dm–^3 and can function as visual indicators. Most metal ion
indicators will also undergo parallel reactions with protons bringing about similar colour changes.
Hence, a careful consideration of pH is prudent when selecting an indicator. Some typical indicators
appear in Table 5.9. Of these, eriochrome black T, which forms red complexes with over twenty metal
ions, is amongst the most widely used. Its behaviour will serve as a general example of indicator
function.
Firstly, an acid–base equilibrium is established in solution. When a metal cation is added, complexation
equilibria are established concurrently.
(Note: H+ ions are omitted for simplicity.)
A pH in the range 8–10 conveniently maximizes the indicator form HIn^2 – and enables the associated
complexing reaction to go to effective completion by facilitating the removal of hydrogen ions. Thus,
the pH dependent, conditional constant for the indicator is given by
for the reaction MIn– = M' + In' (see equation (5.1)).
Here [M'] and [In'] represent the unreacted metal ion and indicator respectively. If the midpoint of the
colour change occurs when