The Esk
Player Valuation: £70m
How does refrigeration affect ph levels Mr. Esk?
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pH can indeed vary with temperature. The reasons why depend on the context, but even a simple solution of a weak acid (HA) will exhibit a (weak) temperature dependence. The pH is given by the Henderson-Hasselbalch equation:
pH = pKa + log { [A-]/[HA] }
where Ka is the equilibrium constant for the reaction
HA ---> H+ + A-
( ka = [H+][A-] / [HA] )
and pKa = - log Ka .
Ka is itself a function of temperature, since it is related to the Gibbs free energy of reaction (delta G) by the equation
delta G = - RT ln Ka = -2.303 RT log Ka = 2.303 RT * pKa
so we have
pkA = delta G / (2.303 RT)
delta G is itself given by
delta G = delta H - T * delta S
where delta H is the enthalpy of reaction and delta S is the entropy of reaction. Combining these, we get
pKa = (delta H / (2.303 RT)) - (delta S / (2.303 R))
If we assume for the sake of simplicity that delta H and delta S are approximately independent of temperature T (constant), then the variation with temperature is determined by the sign of delta H. For example, if delta H is positive (endothermic dissociation), pKa gets smaller as the temperature gets larger. A decrease in pKa amounts to an increase in Ka, which means that the reaction favors dissociation more as temperature increases (in agreement with LeChatelier's principle). This increases [H+] and decreases the pH. If the reaction is exothermic the opposite effect will be observed.
My point was that if sauce is kept in the fridge to reduce contamination then it is a pointless exercise as most micro-organisms cannot grow in an environment with the same pH as tomato ketchup.