Sodium bicarbonate is the reaction product of sodium carbonate with acid. It
is a self limiting reaction that chemists refer to as a buffered solution. It
is actually benificial to the developing process because the developer
actually reaches a steady state with little change in pH as the sodium
carbonate is used up.

Suggest you pick up a first year college chemistry text and review the
chapter on chemical equilibria. It is usually the classical example used to
explain buffers.

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Leo Roos