The dissociation of water increases as the temperature is raised: H20 --> H+ + OH-. If you adjust a buffer to pH 7.4 at room temperature (25 degree Celsius), then cool it down to 4 degree Celsius for an enzyme assay, will the pH increase or decrease?
This effect can be explained by the common ion effect, which is an application of LeChatelier's Principle. If the pH is adjusted to 7.4 ...
It explains how the pH of the buffer solution changes with temperature.