What is pH and
what does it measure?

The answer to the question "What is pH", is that pH is a measure of the hydrogen ion activity in a water solution. Whether a solution is perceived as acidic or alkaline depends on the activity of the H+ ions.

The reason we use the term activity is that actual what we measure with a pH sensor is the activity of H+ ions, not the concentration. The H+ concentration and the H+ activity are not the same, except for very diluted solutions. The activity is an effective concentration of hydrogen ions, rather than the true concentration.


Activity versus concentration

In a soccer game there are 22 players on the field. The number ("concentration") of players are the same for every game, but as we know, the activity level can be very different. If, for some reason, the spectators are entering the field, the activity of the players will decrease. The field will be too crowded.

Actually, this is what will happen when, for example, sodium, sulfate, calcium, chloride, potassium and nitrate ions are present in a solution. These ions tend to limit the mobility of the hydrogen ion (H+), and as a result the activity of the hydrogen ion will decrease.


Equilibrium principle

In any water solution, the relation between H+ and OH- ion activities are constant for a given temperature. Therefore, if the activity of one ion increases by a factor of 10, then the activity of the other ion must decrease by a factor of 10.


Logarithm

The dimensionless activity of H+ ions can change in very wide range, most often it has values lying somewhere between 0.001 or 0.0000000000001. However, using numbers differing by many orders of magnitude is impractical. That is why we use a logarithmic scale. I guess you remember from school what a logarithm is?

The logarithm of a number y with respect to base 10 is the exponent to which we have to raise 10 to obtain y.


For example, log 100 = 2 and log 0.01= -2 .