Why is there a minus sign in the definition of pH?

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We use the pH scale for two reasons.  The first is because we can talk about acids on the same scale instead of having to compare the concentrations of H+ and OH-.  The other reason is because the concentrations of H+ and OH- are typically very small and so it is easier to make math errors when working with such small numbers.

We take the negative in calculating pH out of convenience.  We tend to favor positive numbers in science.  Since the log of any number less than 1 will be negative, we change the sign by adding a negative into the definition so that the values we get out are positive.

log (0.001) = -3

- log (0.001) = 3

 

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