The alkalinity of a solution is defined as the amount of acid required to bring this solution to a pH of 4.5.

ORCHESTRA does not include alkalinity calculations by default, however it is straightforward to include these in the chemistry input file by explicity defining an “alkalinity” variable and accompanying expression:

@Var:   alkalinity  0
@Calc: (5, "alkalinity = {OH-.con} + 2 * {CO3-2.con} + {HCO3-.con}")

Note:

• Each variable has to be defined with a default value (in this case 0)
• The number 5 in the expression indicates that this calculation will be performed during stage 5, which is at the end of all calculations, when all species concentrations are known.
• The {curly brackets} around the variable names are necessary for names that contain characters such as “- .”

In the output file you can simply put the “alkalinity” variable in the row of variable names, re-run and you will get the required output.