In most cases, the pH of a solution will change as temperature changes. However, there is not a general relationship between temperature and pH: it depends on the composition of the solution.
pH is the negative common (base 10) logarithm of the hydrogen ion (H+) activity in a solution. The hydrogen ion activity depends on all the possible chemical reactions that produce or consume hydrogen ions. The extent of a chemical reaction depends on its equilibrium constant, and the equilibrium constant of any reaction depends on the temperature. Some reactions become more likely as temperature increases, some less.
The most basic chemical reaction affecting pH is the dissociation of water:
H2O --> H+ + OH-
Over the temperature range 0 to 60 degrees Celsius, this reaction becomes more favorable. This means that neutral water will contain a higher concentration of H+ as the temperature increases, so the pH will decrease.
The reactions by which acids dissociate to create H+ and bases consume H+ depend on temperature in a variety of ways. Some increase with temperature, some decrease, and some increase or decrease away from a maximum or minimum value at a specific temperature. To predict the pH change of a specific solution with temperature, you would need to know all the components of the solution, all the reactions involving those components that produce or consume H+, and how the extent of these reactions varies with temperature.
