This is the educational discussion.
What is pH?
What a pH meter measures?
Is the pKa (acidity) is constant? Does it depend on acid concentration?
I do not pretend that I know the answers.
I will not give here the definition of pH, which I know from the time of the technical high-school of chemistry . In the case of measurement using pH meter, its accuracy and the possibility of various types of corrections are important. The mixture composition and interactions between components will also be decisive. I wonder what the discussion will be like. Regards,
For this question: "Is the pKa (acidity) is constant? Does it depend on acid concentration?"
I think pKa is not constant because of the variation of temperature during the reactions (with aqua ligand for example), adsorptions/desorptions (if we have more than two phases)...
if we can have a system with no variation in temperature, then pKa could be constant and does not depend on acid concentration
pH is the negative log to the base 10 of the hydronium ion concentration of a solution i.e.
pH = -log10[H+]
Thus a neutral solution of water which has [H+] concentrations of 0.0000001 moles per litre has a pH of 7, which is a more compact expression.
However, using pH values has two disadvantages. First more acidic solutions (with higher [H+] concentrations) have lower pH values, which I find counter intuitive. Second, the pH values of two solutions cannot be averaged to find the pH of their mixture, whereas [H+] values can.
So pH values are useless if you are doing calculations. They are just a quick way of writing the acidity of a solution.
There is a page here "Learn What pH Stands For" https://www.thoughtco.com/what-does-ph-stand-for-608888 which explains the background of pH.
This problem has been discussed in J. Chem. Educ. 1994, 71, 117. I would like to know your opinion about this discussion.
There is an even greater problem with the acid dissociation constant (Ka). Its value is influenced by kind of acid, temperature, type of solvent (it can be one solvent, but it can be a mixture of different composition) and, of course, by the concentration, especially in the range of higher concentrations (activity coefficients, ionic strength). And there is also a measurement. Regards,
To all:
we assume that the temperature is constant.
To Alastair Bain McDonald
No, pH = -log10[H+] is wrong
To Nguyen LE PHUC
" pKa could be constant and does not depend on acid concentration" It depends how you define pKa
To Guy Schmitz
Excellent paper. I've read it a long time ago. I would recommend to read it to everyone dealing with pH. I absolutely agree with your conclusion:
"the pH scale is conventional. It is much more difficult than usually
realized to measure kinetic constants for acidity-dependent reactions!"
Yurii, So, like a boomerang, you return to your/our favorite topic - kinetic constants. But maybe not yet. Regards,
To Mirosław Grzesik
Yes, pH is related to kinetics, but kinetics is not the focus of this discussion. The focus is the pH.
Yurii (if I can still turn to you in this way), You assume here a constant temperature. But if will it not be constant? Of course, this is not a place for kinetics and I'm not going to write anything about it here. But I am interested how the further discussion wili be like. Regards,
To Mirosław Grzesik and others:
Are the pH and pKa temperature dependent?
Both are temperature dependent. But because of the logarithmic function, the variability is not too large. Regards,
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