Electrolysis : Ionisation In Stages

Molecules capable of giving more than two ions often dissociate in stages, but this is not always the case. Potassium ferrocyanide, K₄Fe(CN)₆, ionises directly according to the equation: K₄Fe(CN)₆ = 4K˙+Fe(CN)₆'''', whilst sulphuric acid ionises in two stages: H₂SO₄ = H˙ + HSO₄', followed by HSO₄' = H˙ + SO₄''. The second stage of the dissociation occurs only to a very limited extent, except in very dilute solutions.

At moderate dilutions, therefore, sulphuric acid should behave as a monobasic acid. The conductivity shows that this is the case. But if The add is neutralised with a base, the hydrogen ion is completely eliminated, with the hydroxide ion of the base, in the form pf water: H₂SO₄ + 2OH' = SO₄'' + 2H₂O. The reason for this is the ionisation of the HSO₄' ion into H˙ and SO₄''. As soon as the hydrogen ion corresponding with the first stage of the ionisation: H₂SO₄ = H˙ + HSO₄' has been removed, the HSO₄' ion begins to dissociate to a slight extent. The trace of hydrogen ion so produced, however, is at once removed by the base added, and further ionisation of HSO₄' results This goes on until all the HSO₄' has been ionised, and finally only SO₄'' ions remain. This, however, corresponds with the formation of the normal salt, and the acid, therefore, behaves as if it were dibasic. Water is principally ionised, to a slight extent only, into H˙ and OH' (or perhaps into H₃O˙ and OH'). A further dissociation of OH’ probably occurs to a very minute extent: OH' <=> H˙ + O''.

It is almost always the case that the later stages of ionisation of polybasic acids are small; e.g., with phosphoric acid

H₃PO₄ <=> H˙ + H₂PO₄' <=> 2H˙ + HPO₄'' <=> 3H˙ + PO₄''',

the last stage is so slight that the PO₄''' ion in a phosphate such as Na₃PO₄ acts as a strong base:

PO₄''' + H₂O <=> HPO₄'' + OH'.

Protein crystallography
Chemistry guide
Electrolysis
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