Calculatinh pH value from Major ion concnetrations

[kbrohith]

Hello all,
I am very new to both the software and the community. I am currently working with a dataset in which 6 samples do not possess any pH values. I intend to calculate pH values for further analysis. Despite being a beginner, I have attempted several scripts, but they have produced extremely outlier values that are not suitable for the dataset. I would greatly appreciate it if someone could share a sample script for pH calculation.

[dlparkhurst]

There is not a reliable way to determine the pH of a solution. It is much better to have a measured  value. pH is usually best described as the ratio of HCO3-:CO2(aq)--the higher this ratio, the higher the pH. Usually, you do not have a measurement of CO2(aq), so, without a measurement of pH or CO2, it is hard to assess an appropriate pH.

Here are three ways that you can approach the problem with PHREEQC; you can decide if any are adequate for your situation.

I think best is if you can assume your solution is in equilibrium with calcite:

Code: [Select]

SOLUTION
pH 7 calcite 0 10
Ca 10
Alkalinity 20
END

You could use charge balance to calculate the pH, but then all analytical errors and missing analytes are attributed to pH:

Code: [Select]

SOLUTION
pH 7 charge
Ca 10
C(4) 20
END

The final method is almost never used and usually gives unreasonable results, but if you have an analysis of Alkalinity and TDIC (total dissolved inorganic carbon), PHREEQC will calculate a pH.

Code: [Select]

SOLUTION
pH 7
Ca         10
Alkalinity 20
C(4)       21
END