## How to compute pH of a mixture?

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alicanter99
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Joined: Wed Feb 01, 2023 11:00 am

### How to compute pH of a mixture?

I know about the tool at [https://www.webqc.org/phsolver.php], but I was wondering how the values are computed.
Is there any generic formula to compute the pH of a mixture of different acids and bases that are not conjugate?
Last edited by alicanter99 on Wed Feb 01, 2023 11:48 am, edited 1 time in total.
ChenBeier
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### Re: How to compute pH of a mixture?

A pH solver is nonsense.
pH is depending on the value of the negative logarithm of concentration of H+ or more accurate of the activities, but its different if it is a strong/weak acid or base and mixtures of it and also if buffer behaviour takes place.
alicanter99
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Posts: 4
Joined: Wed Feb 01, 2023 11:00 am

### Re: How to compute pH of a mixture?

But knowing the component concentration (and its constants) seems possible to compute the pH.
This seems how the webqc's solver works, but I don't know of any method to build the equations needed.
Approximations like the Henderson-Hasselbalch exist for conjugate buffers, but maybe anything similar exists for generic solutions, or maybe you can just compute the pH separately and then compose the values somehow.
At least to get some theoretical approximation.
ChenBeier
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### Re: How to compute pH of a mixture?

Do you have a link for this.
alicanter99
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Posts: 4
Joined: Wed Feb 01, 2023 11:00 am

### Re: How to compute pH of a mixture?

ChenBeier wrote: Wed Feb 01, 2023 11:23 am Do you have a link for this.
I'm refering to: [https://www.webqc.org/phsolver.php]
ChenBeier
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Location: Berlin, Germany

### Re: How to compute pH of a mixture?

Beside the given 4 examples it doesn't work.
pH = -log cH+ strong acids
pH = 0,5 *(pKa-logcAcid), middle strong to weak acids
pH = pks- log(cBase/cAcid) buffersolutions
The same for bases pH = 14-pOH

Depends also on dissociation degree. A mixture of hydrochloric and acetic acid will expell the acetic acid by protonation.
If I add 0.1 HCl and 0.1 CH3COONa it says pH 7 that is wrong. Result should be 2,88.
alicanter99
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Posts: 4
Joined: Wed Feb 01, 2023 11:00 am

### Re: How to compute pH of a mixture?

ChenBeier wrote: Wed Feb 01, 2023 11:48 am Depends also on dissociation degree. A mixture of hydrochloric and acetic acid will expell the acetic acid by protonation.
If I add 0.1 HCl and 0.1 CH3COONa it says pH 7 that is wrong. Result should be 2,88.
If you input:

HCl c=0.1 pKa=-5.9
sodium_acetate c=0.1 pKb=9.25

You get 2.9

But how did you calculate the value then?
ChenBeier
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Posts: 1265
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

### Re: How to compute pH of a mixture?

HCl reaction with sodium acetate to acetic acid according
CH3COONa + HCl => CH3COOH + NaCl equilibrium is on right side, so we have only acetic acid and sodium chloride

Formula for weak acid pH = 0.5*(pKa-logcHAc)

pH = 0,5*(4,75-log0.1) = 2,88