How to calculate pH of the solution in GWB

[Yousaf Ashfaq]

Hi,

 

I am calculating the saturation indices and Ionic strength of a solution. Temperature is already specified. How can I calculate the pH of the solution? I tried to do it but did not get any answer. File is attached as an example.

01. 25C.gss

[Guest Melika Sharifi]

Dear Yousaf,

 

Thanks for sharing your GSS file with us. You’ll need some way to constrain pH in your system in order for GSS to be able to calculate it for you. Your GSS file does not include any constrains on pH. Do you have any other information on pH? Is your system is in equilibrium with the atmosphere, or calcite or any other minerals?

 

Bests,

Melika Sharifi

Aqueous Solutions LLC

[Yousaf Ashfaq]

Yes, the solution will be allowed to equilibrate in the atmospheric CO2 for few days. And no other minerals will be added.

[Guest Melika Sharifi]

Dear Yousaf,

 

Thanks for providing the additional info on your system. Unfortunately, knowing about the equilibrium between your system and the atmosphere by itself is not going to help you calculate pH. You would need another constrain (need some lab data or assumptions) on the amount of carbonate present in your system, too. That way, you are putting some constrains on the H+, and then GSS/SpecE8 can calculate pH of your system.

 

Hope this helps.

 

Bests,

Melika Sharifi

Aqueous Solutions LLC

[Yousaf Ashfaq]

Dear Mr. Melika,

 

Thanks for your response. Actually, I don't have in-depth knowledge about chemistry. So, I am also learning. In addition to what I have specified earlier, I would like to add that during these experiments; we have to ensure that the solution is undersaturated with respect to polymorphs of CaCO3 to achieve only super-saturation and precipitation of polymorphs of CaSO4.

 

Hope that will help?

 

Regards

[Guest Melika Sharifi]

Dear Yousaf,

 

If you are able to measure the pH after equilibrium with the atmosphere, you’ll be able to calculate the saturation indices of various calcium carbonates and sulfates. Or, if you know how much inorganic carbon is dissolved in the fluid (you might know the carbonate alkalinity) then you can assume equilibrium with the atmosphere to calculate pH.

 

In the attached script, for example, I've set your sample 3 to be in equilibrium with the atmosphere and picked a random pH to calculate saturation indices of minerals. I would recommend you wait until you are comfortable with the SpecE8 calculation before setting up a batch job in GSS.

 

Bests,

Melika Sharifi

Aqueous Solutions LLC

spece8-Yousaf.sp8

[Yousaf Ashfaq]

Hi,

 

Thanks for your response. I went through the file you have attached and I also got the tutorial for SepcE8.

 

However, from my collaborators, I had the attached table for one sample in which saturation indices (& pH probably) were calculated using GWB. Using GWB, I was able to get similar SI values but could not calculate pH values. What do you think?

GWB.docx

[Guest Melika Sharifi]

Dear Yousaf,

 

Thanks for providing more info on your system.

 

You can not calculate the pH of your system with the known concentration of Na, Ca, S, Cl, and no info on carbonates. You would need at least two constraints on carbon to be able to calculate the pH of your system. One can be equilibrium of H+ with CO2 fugacity of the air, and the other one can be, for example, charge balance on HCO3-. That is most probably what your collaborators have made an assumption about to calculate the pH, ionic strength, and saturation indices.

 

Hope this helps.

 

Thanks,

Melika

Aqueous Solutions LLC

[Yousaf Ashfaq]

Thanks for your response. I got your point.

 

Could you please help me in adding these constraints in the GWB file? I would be very thankful to you. Since temperature is same, our results should match with the collaborator's table. Then I will calculate for different temperatures.

 

Many thanks,

GWB.docx

01. 25C.gss

[Guest Melika Sharifi]

Dear Yousaf,

I'm not sure what assumptions your collaborators have made to get those result, but I'm attaching a script for you for sample 2, for example, that I get the same results for pH, ionic strength, and saturation indices as your attached table using the assumptions mentioned in my previous comment. You can set the same script in GSS using the Command option. For more information on Command option in GSS please read section 3.5 Launching SpecE8 and React from the GWB Essentials guide that can be accessed from the Docs pane in the GWB dashboard.

Also, there is no downside to measure the pH of your solution in your experiment before or after mixing that way you do not have to make assumptions to calculate the pH.

Hope this helps.

Bests,

Melika Sharifi

Aqueous Solutions LLC

sample2.sp8

[Yousaf Ashfaq]

Dear Mr. Melika,

 

Thanks a lot for your help.

 

Alternatively, I am just changing the temperature values from the Basis window to calculate the pH, ionic strength, and saturation indices for different samples.

 

Hope this will work too.

 

Many thanks again.

 

Regards,

 

Yousaf