pH

This unit appears in the water quality processes section of the 1D Water Quality datafile. The unit models the pH of water.

Description

pH is a measure of acidity or alkalinity. It is defined as the negative logarithm of the concentration of hydrogen ions. Within the 1D Water Quality Solver, pH is used primarily to determine the proportion of ammonia that is present in a unionised form, which is toxic to fish at fairly low concentrations. It exists in equilibrium with ammonia ion and hydroxide ion, as follows:

                    (1)

Thus, unionised ammonia is favoured by high pH (i.e. alkaline conditions). The unionised proportion also increases with temperature.

The 1D Water Quality Solver simulates pH by converting to total hydrogen ions and transporting them as a conservative pollutant with no decay. On output the total hydrogen ions are converted back to pH. The unionised ammonia concentration is calculated from the total ammonia concentration as a function of pH and water temperature.

Equations

The pH of water is defined by the following equation.

                                                           (2)

Where:
H = concentration of hydrogen ions (mol/l)

concentration of unionised ammonia (UA, mg/l) is calculated using the following function of total ammonia concentration (ionised and unionised) (A, mg/l)

                                                                 (3)

Where:

x = a function of pH and temperature (T, ° C):

                                               (4)

General

The pH module must be run in conjunction with the DISSOLVED OXYGEN module. The pH module simulates the transported variable name:

• pH

The pH module also simulates the following variable:

• Unionised ammoniacal nitrogen (mg/l)