There are several nuances about pH that a chemistry teacher should know.
We've defined pH as:
Actually, we should define pH in terms of the activity of the hydrogen ion. Activity is related to concentration, but takes into account other factors such as ionic strength (the present of dissolved ionic salts.) Read about Sorenson, and see the section on activity in that article.
Better definition of pH:
pH = -log10(aH+)
pH can be measured using colored dyes called indicators. Usually, however, pH is measured using electrodes such as the glass pH electrode.
When expressing pH, the number of significant digits is an issue. The digits to the left of the decimal really just express a power of ten and should not be counted. Only the digits to the right of a decimal are counted. Also, pH measurements do not go past the third decimal place; two, or at most three decimal places are available.
