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Specialty Chemistry Forums => Biochemistry and Chemical Biology Forum => Topic started by: vanessa on February 18, 2014, 07:20:20 PM

Title: Cauculating Ph of saturated saline suspension
Post by: vanessa on February 18, 2014, 07:20:20 PM

I have a suspension of bacteria producing Lactic Acid in a saline (NaCl) 2% solution.
I want to calculate the Ph of the solution, considering that the concentration of Lactic Acid reach 0.04M.
Which approach should I take in this case?

In one hand Na⁺ and CL^- are neutral ions and they don't influence the Ph, in another hand, the water on it is not pure H₂O.

Does the NaCl concentration in the solution influences the deprotonation of Lactic Acid to Lactate?
In case it does, how do I calculate the Ph of it?

Thanks!

Title: Re: Cauculating Ph of saturated saline suspension
Post by: l.alvarez on February 18, 2014, 11:42:34 PM

Hello,
i think that if you know the initial pH of the medium, and then the pH of the medium with NaCl, you can determine the pH of the lactic acid (assuming that is the only one component that can change it).

Title: Re: Cauculating Ph of saturated saline suspension
Post by: Borek on February 19, 2014, 02:43:30 AM

You should take into account ionic strength of the solution.

http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients

Unfortunately, it will be too high for a classic Debye-Hückel theory. You can try Davies equation.

Title: Re: Cauculating Ph of saturated saline suspension
Post by: vanessa on February 20, 2014, 02:46:41 PM

Thanks Borek!!!

"In fact whenever you put pH electrode into solution you are measuring not [H+] but activity of H+ ions. It is enough to add inert salt to the solution of known acid to observe pH change that confirms activity concept"
(from http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients (http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients))