Isn't the carbonate part only that is active? That is, pH is controlled only by cabonate, not the counter-cation.
I did know K+ salts are only a little soluble than Na+ salts. Maybe is this the difference?
At the strengths usually employed in photo graphic developers the hydrolysis of potassium carbonate (separation into caustic potash and bicarbonate) liberates a much greater amount of caustic alkali than does the hydrolysis of sodium carbonate, so that potassium carbonate gives developing solutions of greater energy.
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