Calculation of pH

[maki]

Hello.

 

I am conducting basic practice for GWB.

Now I want to calculate the pH of the solution comprised of some aqueous species using SpecE8.

 

Composition:

Na+: 1000 mg/L

K+: 30 mg/L

Mg++: 300 mg/L

Ca++: 2000 mg/L

Cl-: 500 mg/L

SO4--: 500 mg/L

HCO3-: 3000 mg/L

 

Although I gained the result of calculation regarding almost all of the System parameters, the value of pH wasn't be shown.

How can I gain the value of pH in this case?

 

Many thanks,

 

maki

[Brian Farrell]

Hello Maki,

 

There is no H+ component selected in the current basis, so there is no way that pH can be calculated. If you look at the results, without a pH value all carbon is present as HCO3-/CaHCO3+/MgHCO3+/NaHCO3 but not as CO2(aq) or CO3--. To consider pH in your calculation, you'll need to add H+ to the basis. If you don't have a measured pH value to enter, you might assume equilibrium with some species, gas or mineral in place of H+. If your solution is open to the atmosphere, for example, you could use the following commands to calculate a pH value based on the reaction CO2(g) + H2O = HCO3- + H+:

 

HCO3- = 3000 mg/L

swap CO2(g) for H+

log fugacity CO2(g) = -3.5

 

On the other hand, if you believe your fluid is in equilibrium with Calcite, the reaction Calcite + H+ = HCO3- + Ca++ might fix pH:

 

HCO3- = 3000 mg/L

swap Calcite for H+

1 free gram Calcite

 

These are two different assumptions that are commonly used. You'll need to decide what sort of assumptions are appropriate for your particular system.

 

Hope this helps,

Brian Farrell

Aqueous Solutions LLC