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Topic: The relationship between pH and alkalinity  (Read 4946 times)

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Offline agworr

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The relationship between pH and alkalinity
« on: July 09, 2012, 05:36:20 AM »
I have water samples which have a pH of ~11. The manual for the titration method I used to measure the alkalinity states that over a pH of 8.3 there is no HCO3 present. However other books have stated that there is HCO present over a pH of 8.3 and it is the dominant anion up until ~pH 10. Which is correct?

Also how do I calculate the pH of a solution when I have the total, bicarbonate, carbonate and hydroxide alkalinities. I found the following webpage which explains how to calculate the HCO3, CO3 and OH alkalinities from the pH so I back-calculated but I have not found any peer reviewed literature which uses this method. http://comp.uark.edu/~ksteele/hc2004/ALKALINITY-%20CALCULATION.htm
Would you know if this is a valid method or of any peer reviewed literature/books which include similar equations?

Thanks

Offline Borek

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Re: The relationship between pH and alkalinity
« Reply #1 on: July 09, 2012, 06:28:17 AM »
I have water samples which have a pH of ~11. The manual for the titration method I used to measure the alkalinity states that over a pH of 8.3 there is no HCO3 present.

That's incorrect.

Quote
However other books have stated that there is HCO present over a pH of 8.3 and it is the dominant anion up until ~pH 10. Which is correct?

Ratio of acid and its conjugate base concentration is a function of pH and pKa and can be easily calculated just by rearranging dissociation constant:

[tex]K_a = \frac {[H^+][A^-]}{[HA]}[/tex]

changes to

[tex]\frac {[A^-]}{[HA]} = \frac {K_a}{[H^+]}[/tex]

or (with some further tricks):

[tex]\frac {[A^-]}{[HA]} = 10^{pH-pK_a}[/tex]

Carbonic acid has pKa2=10.3, so at pH=10.3 ratio of concentrations of HCO3-/CO32- is exactly 1.

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Also how do I calculate the pH of a solution when I have the total, bicarbonate, carbonate and hydroxide alkalinities.

Technically just by knowing ratio of concentrations of carbonate and bicarbonate you can calculate pH plugging them into Henderson-Hasselbalch equation (which is yet another rearranged form of the same dissociation constant). I have a gut feeling it should be possible to solve it this way - but I also have a gut feeling it is a waste of time, as pH of such a system depends on the presence of other substances. There is a reason why we sometimes prefer pH and sometimes alkalinity - they are not easily interchangeable.
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