ammonium chloride, triethylamine and triethanolamine. Conveniently, the same reagents can provide
the basis for a buffer system as well. These selected reagents will retain a range of metal ions in
solution without seriously competing with the metal-EDTA equilibrium. The conditional constant
concept again proves valuable in the assessment of practical situations. For example, when zinc is held
in solution by an ammonia-ammonium chloride buffer at pH = 10, the fraction of zinc unreacted with
EDTA which remains as Zn2+ ions can be expressed by
where CM is the total zinc uncomplexed with EDTA, and is given by
It is apparent that β will depend in part on the concentration of ammonia and in part on the formation
constants, K 1 , K 2 , K 3 and K 4 for the various ammonia complexes, where
Substitution into (5.20) gives
whence β = 8.0 × 10 –^6 if the solution is 0.1 mol dm–^3 with respect to ammonia. At pH = 10, the
conditional constant for zinc-EDTA can then be computed (Chapter 3).
It is thus concluded that the zinc-EDTA complex will be formed quantitatively at pH = 10 in a solution
of ammonia (0.1 mol dm–^3 ).
The second reason for employing an auxiliary complexing agent is to mask the effect of an interfering
ion. Both zinc and magnesium form stable EDTA complexes at pH = 10 and may be titrated in
solutions buffered to that pH. If they are present during the titration of magnesium, zinc and many other
heavy metals will interfere. However, heavy metals in general form stable cyanide complexes (K 4 ≈
1020 ) and the addition of sodium or potassium cyanide to the titration mixture will reduce the
conditional