I feel strongly acids have tendency to dissociate complexes. Thus, the more a given complex persists in an acidic media the more stability it is said to have and vice versa. Corollary, preparation of complexes is often favored in media. So that is the reason. And that is the way I understand it. Thanks.

Hi, I agree with Asemave and, if I follow his answer, I would say that the ligand is, in lots of cases, the conjugate base of an acid (for example Citrate vs. citric acid) so the acidic medium destabilize the complex by forming the acid form that is not able to coordinate with the metal anymore. Asemave, I think your Corollary was "preparation of complexes is often favored in BASIC media", isn´t it?

Hope it helps

Gilles

Well explained Gilles. Coordination compounds may even decompose in high acid concentration.

I think the importance of acid medium when study stability constant of coordination compound to prevent the hydrolysis of metal ions in water

When you determine stability constants, you need to find the equilibrium concentration of all the relevant species. Since you usually cannot do that, you obtain the concentration of some of the species at many slightly different compositions, like during a titration. You then rely on some numerical method to find all the unknowns.

If your ligand is a Brönsted base, adding different amount of acid will produce such collection of solution, one for each volumen of titrant added.. In this case, you can monitor the change in composition (i.e. equilibrium concentration of all the relevant species) by measuring pH.

If your ligand does not have Brönsted base character, the complex formation would (in principle) be independent of pH; like Cl- for instance, you would need some other measurement. (uv-vis, conductivity,etc.). Also, if the affinity of the ligand towards the metal is much larger than towards the proton, (like Cu2+ and o-phenanthroline) the metal complex will be formed even at low pH values and there will be no appreciable change in composition during a pHmetric titration and no reliable data can be obtained. (the best values for this system were obtained from solvent extraction equilibria and monitored spectrophotometrically.)