Why should distilled water be boiled and cooled when determining its pH?

1 Answer
anor277
Dec 12, 2015

An accurate determination of #pH# should made from a sample of distilled and COOLED water, i.e. distilled water at room temperature.

Explanation:

When we measure #pH# we measure the extent of the following equilibrium:

#H_2O rightleftharpoons H^+ + OH^-#

(alternatively from, #2H_2O rightleftharpoons H_3O^+ + OH^-#).

If we want accuracy in our measurement, we should use water that contains ONLY the ions that result from this equilibrium. Water from the tap would contain many ions that would slightly affect this equilibrium. We specify a given temperature because as written above this is a bond breaking reaction, i.e. an #O-H# bond is broken.

Given all this at room temperature we find that under standard conditions that for pure distilled water #pH# #=# #pOH# #=# #7# at #298K#. At temperatures greater than #298# #K# would you expect #pH# to increase or decrease or stay the same? Remember that we deal with a bond breaking reaction.

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