Editing Gravimetric analysis (section)

==Solubility in the presence of diverse ions==

Diverse ions have a screening effect on dissociated ions which leads to extra dissociation. Solubility will show a clear increase in presence of diverse ions as the solubility product will increase. Look at the following example:

Find the solubility of AgCl (K<sub>sp</sub> = 1.0 x 10<sup>−10</sup>) in 0.1 M NaNO<sub>3</sub>. The activity coefficients for silver and chloride are 0.75 and 0.76, respectively.

:AgCl(s) = Ag<sup>+</sup> + Cl<sup>−</sup>
 
We can no longer use the thermodynamic equilibrium constant (i.e. in absence of diverse ions) and we have to consider the concentration equilibrium constant or use activities instead of concentration if we use Kth:

:K<sub>sp</sub> = aAg<sup>+</sup> aCl<sup>−</sup>
:K<sub>sp</sub> = [Ag<sup>+</sup>] fAg<sup>+</sup> [Cl<sup>−</sup>] fCl<sup>−</sup>
:1.0 x 10<sup>−10</sup> = s x 0.75 x s x 0.76
:s = 1.3 x 10<sup>−5</sup> M
 
We have calculated the solubility of AgCl in pure water to be 1.0 x 10<sup>−5</sup> M, if we compare this value to that obtained in presence of diverse ions we see % increase in solubility = {(1.3 x 10<sup>−5</sup> – 1.0 x 10<sup>−5</sup>) / 1.0 x 10<sup>−5</sup>} x 100 = 30%
Therefore, once again we have an evidence for an increase in dissociation or a shift of equilibrium to right in presence of diverse ions.