Solubility diagram

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Solubility diagrams give an indication of the conditions needed to separate different elements.

How to use solubility diagrams

In the figure, a solubility diagram for some of the common metal hydroxides is shown.

Solubility diagram for some metal hydroxides

The line for each metal ion represents the equilibrium between a 1 M solution of a metal ion and its hydroxide, respectively. On the y-axis, the metal ion concentration is given and pH is on the x-axis. At low pH-values the hydroxides are soluble and at higher pH-values the metal ion concentration decreases as the hydroxides are formed. It can be seen that trivalent metal ions can be precipitated at lower pH values compared with the divalent metal ions. Metal ions located close to each other like Fe2+ and Zn2+ are difficult to separate and give mixed hydroxide precipitates if both are present during neutralisation. The most difficult metals to precipitate (in the diagram) are the alkaline earth metals calcium and magnesium.

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