May 31, 2020, 03:02:17 AM
Forum Rules: Read This Before Posting

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

0 Members and 1 Guest are viewing this topic.

#### agworr

• Very New Member
• Posts: 1
• Mole Snacks: +0/-0
##### 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

#### Borek

• Mr. pH
• Deity Member
• Posts: 25773
• Mole Snacks: +1686/-400
• Gender:
• I am known to be occasionally wrong.
##### 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:

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

changes to

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

or (with some further tricks):

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

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

Quote
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.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info