# Estimation

The estimation of stability constants is a necessary first step to establishing the experimental conditions required for a stability constant determination. To illustrate this let us consider a very simple case: the determination of a protonation constant of a base, B, which can also be considered as the conjugate base of the acid BH.

 B + H = BH; [BH]=K[B][H] (1)

[BH], [B] and [H] signify the concentrations of the species BH, B and H. (Electrical charges are omitted from generic formulae such as these). Note: pKa refers to the dissociation of the acid and in this case is numerically equal to logK.

Now, it is well known that at half neutralization the pH of a solution of an acid is numerically equal to its pK value. This can easily be seen by rewriting equation (1) as (2).

 log[H] = log[BH] - log[B] - logK (2)

When [BH]=[B], pH =logK. It is not so well known that equation (2) can also be used to define the region in which the equilibrium mixture contains appreciable concentrations of both the base and its conjugate acid. Let us assert that this region is defined by the expression 0.99 < [BH]/[B] < 0.01. When this ratio is substituted into equation (2) it is found that the designated region is in the range pH=pK±2.

So, to determine the protonation constant the experimental measurements should lie in the region approximately 2 pH units above and below the pK value. If the pK value is known then the experimental conditions can be specified exactly. Conversely, measurements outside this region are of very little use for determining the protonation constant since the equilibrium lies effectively to one side.

This simple example illustrates the relationship between stability constants and the range of conditions over which measurements need to be made in order to determine them. In more complicated situations the algebra is more complicated but the principle is the same.

For organic acids and bases there are two databases in the public domain.

These databases should allow fairly good estimates to be obtained.

For metal complexes SC-Database (http://www.acadsoft.co.uk/scdbase/scdbase.htm) is fully comprehensive and is being regularly updated.

Contents > Planning: Preliminary | HQD file | Estimation | Simulation