'environmental pH is the activity of its oxonium cations expressed on a negative logarithmic scale. It is thus defined as:
Since the activity is very dependent on concentration, and outside very concentrated solutions, it is possible to consider the activity coefficient can be written:
where is in mol · dm-3 and pH is then dimensionless.
In an aqueous environment, oxonium cations are de facto hydrated hydrogen cations, hydrons, i.e. particles identical to protons, so pH is often stated as
pH scale in aqueous environment
In almost all real situations, only the pH of the aqueous environment is considered. Water undergoes autoprotolysis according to the equilibrium reaction
with the equilibrium constant
The concentration of water in usual systems can be considered practically constant, so it is advantageous to work with the ionic product of water From the above equation, the relationship for it follows
The ionic product of water is proportional to the equilibrium constant and therefore, like it, strongly depends on temperature. So the pH scale will be different at different temperatures. At 25 °C, laboratory temperature, Kw corresponds to 1.008 · 10-14, so the pH scale will have a center, the so-called neutral pH, almost exactly 7. This is because
At other temperatures, the center of the scale is different (0 °C corresponds to 7.47; 10 °C 7.27; 20 °C 7.08; 30 °C 6.92; 40 °C 6.77; 50  ;°C 6.63; 100 °C 6.14). The pH can take on an infinite number of values, at 25 °C it can even be negative with the use of very strong acids, and under suitable conditions it can even exceed the value of 14.
Reason for introduction of pH
The concentration of oxonium ions takes on values in the range of many orders of magnitude, most commonly from to . When specifying such numbers, it is advantageous to specify only the exponent, which is why a logarithmic scale was introduced, since applies. At the same time, using logarithms converts multiplication to addition and division to subtraction (log (A · B) = log A + log B, which is a direct consequence of the fact that ). This fact was of general importance mainly at a time when computers and calculators were not commonly available and technical calculations were performed using a logarithmic ruler.
When calculating pH, it is always necessary to consider what is the source of oxonium cations in a given environment.
Strong monosaturated acids
For strong monosaturated acids the dissociation follows the equation
For the calculation we assume:
- the substance quantity of according to the above equation will be the same as , which, given an identical volume, is also true for the concentration, i.e. ;
- all acid - because it is a strong acid - is converted into a , therefore we will mark its concentration, i.e.
and to calculate the pH we get the formula
Strong monosaturated bases
For strong monosaturated bases the dissociation follows the equation
We assume, as in the case of strong monosaturates, that:
- the amount, or concentration, of hydroxide ions and the resulting is the same according to the above chemical equation, i.e. ;
- dissociation occurs completely, i.e.
The calculation is therefore analogous, we just have to remember that unlike acids, the base is not a source of oxonium cations, but takes oxonium cations from the environment (see the theory of acids and bases), so we add from the equation for the ionic product of water:
and from these assumptions, we deduce
Calculate the pH at 25 °C using the formula
Strong dibasic acids
Strong dibasic acids dissociate according to the equation
we assume, then:
- complete dissociation, i.e.
- however, the amount of oxonium cations and the amount of formed is - in contrast to monosaturated acids - in a ratio of 1:2, i.e.
From this we derive
and the pH is calculated according to the formula
Strong dibasic bases
Strong dibasic bases dissociate according to the equation
as we assume for monosaturated bases and dibasic acids:
- complete dissociation, i.e.
- concentration of the formed and the concentration of hydroxide anions is in the ratio 1:2, i.e., in addition, according to the previous assumption
- hydroxide anions drain oxonium cations from the environment, .
Then we derive
and the pH at 25 °C is calculated according to the formula
PH of weak acids and bases
pH of medium strength acids and bases
Moderately strong acids and bases defy both assumptions from the previous models – complete dissociation cannot be assumed, but the amount of undissociated acid or base cannot be completely neglected either. If we use any of the previous procedures, which are in themselves idealized, i.e. burdened with a certain error, we will move even further away from the real situation, so it is necessary to realize that the error will be several times larger.
- BERKA, Antonín – FETL, Ladislav, et al. Practitioner's Guide to Quantitative Analytical Chemistry. 1. edition. Bratislava : SNTL, 1985. 228 pp. pp. 56–66.