Solutions of sparingly soluble salts

Studies on such saturated solutions of sparingly soluble salts have provided evidence for our ideas on dynamic equilibria. Radioactive labelling experiments with lead(ii) chloride solution have provided evidence for the exchange of ions in an equilibrium situation (Figure 17.14). Solid lead(ii) chloride, PbCl2, is only slightly soluble in cold water. Some solid lead(ii) chloride is placed in a saturated solution of radioactive lead(ii) chloride. The solution contains radioactive Pb (aq) ions. Although the solution is saturated and no more lead chloride can dissolve overall, the solid takes up some of the radioactivity. This shows that some of the radioactive lead ions in the solution have been precipitated into the solid, and an equal number of non-radioactive lead ions from the solid must have dissolved to keep the solution saturated. 

The activity of a pure solid is 1, and, for dilute solutions of sparingly soluble salts, the activity of a solute species can be replaced by its molarity. 

The commonest type of reference electrode is of the following form metal/saturated solution of sparingly soluble salt of metal + additional strongly ionized salt with a common anion. The calomel electrode is a case in point, represented by... 

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