Why does pH change with temperature? I recently read up on some chemistry notes, and found out that the higher the temperature of distilled water, the lower the pH. Why? Does this apply to other fluids too? Does this also mean dipping a litmus paper into 2 beakers of water of different temperature would yield different results?
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$\begingroup$ The equilibrium $\ce{H2O <-> H+ + OH-}$ changes with temperature and hence so does the pH. As to 'other fluids', they may not dissociate at all in any meaningful sense, e..g benzene. (The equilibrium constant for any reaction changes with temperature to a greater of lesser extent, look up the Van't Hoff isochore) $\endgroup$– porphyrinCommented Jan 18, 2018 at 9:16
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2 Answers
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Water always contains a certain amount of $\ce{OH-}$ and $\ce{H3O+}$ ions. The pH is the negative decadic logarithm of the $\ce{H3O+}$ concentration (pH = - log(c($\ce{H3O+}$)). As chemical equilibria might change with temperature so does the concentration of $\ce{H3O+}$ and therefore the pH.
Although I never tried, I suppose this could affect measurements using pH paper. On the other hand the pH changes might very well be to small to be detectable using this method.
How this affects other fluids / equilibria strongly depends on how each equilibrium is affected by temperature. At higher temperatures equilibria are shifted in the endothermic direction and at lower temperatures they are shifted in the exothermic direction. You will need to look up the details for each equilibrium I suppose.
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$\begingroup$ What about for other substances? $\endgroup$ Commented Jan 18, 2018 at 9:34
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$\begingroup$ Hmm... that is very very brief $\endgroup$ Commented Jan 18, 2018 at 10:11
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...the higher the temperature of distilled water, the lower the pH. Why?
$$\ce{2H2O(l)<=> H3O+(aq) + OH-(aq)}$$ ΔH = +ve
The autoionization of water is endothermic therefore the increase in temperature will cause the equilibrium to shift to the endothermic direction, to oppose the increase in temperature.
This increases the amount of H3O+ ions in the system. since pH=-log [H3O] this decreases the pH.
Does this also mean dipping a litmus paper into 2 beakers of water of different temperature would yield different results?
Yes it does, if you have very good litmus paper and a sharp eye!, if one beaker was 273K (pH 7.47) and another at 373K (pH 6.14) the difference in pH would be 1.33.
For this very reason, a worker would have to take temperature into account when doing pH measurements.
A common oversight is when you take a sample from a process tank and make the pH measurement in the laboratory. At that time, you are probably not measuring pH at the same temperature as the temperature in the process tank, this means that you will not have the correct pH value for the sample. Thus a pH value without a temperature value is meaningless. This can be simply overcome by testing pH on site and at the source of the sample.
