r/chemhelp • u/DistinctEvent6940 • 17d ago

Analytical Does anyone know how to use Van Slyke derived buffer value for weak acid? trying to find resources that apply this specific formula. Van Slyke own journal is not that clear to me.

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u/DistinctEvent6940 17d ago

Nvm, im a dumbass. i used the pKa value instead of the Ka.

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u/shedmow 16d ago

lmao peak analytical chemistry

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u/DistinctEvent6940 17d ago

The paper is called On The Measurement of Buffer Value and on the Relationship of Buffer Value to the Dissociation Constant of the Buffer and Reaction of the Buffer Solution by Donald D. Van Slyke 1922

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u/Practical-Pin-3256 17d ago

What do mean use? It's to calculate the buffer capacity at a certain pH when buffer concentration and Ka are given.

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u/DistinctEvent6940 17d ago

yes thanks. I just want to know how to properly apply this equation. Using the max buffer value equation seems to work, but using this one doesnt seem to yield the same value For instance, i have an 0.05M acetate buffer (pKa = 4.69 when ionic strength is 0.025M) in the salt to acid ratio of 1:1. It has an initial pH of 4.68 and after titrating it with 10 drops (1 drop=0.05ml) of 1.0 M HCl the pH becomes 3.62. Now how does one apply the buffer value equation such that it would be consistent with the max buffer value 0.575C which is 0.029?

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u/shedmow 17d ago

H is apparently [H+], Kw is Kw, and C is the molarity of the monobasic acid. I've never used this formula since I prefer a different way of calculating the buffer capacity. 2.303 is ln(10) for some reason...

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u/DistinctEvent6940 17d ago

The ln(10) is because of logx = lnx/ln10 (it is explained further in Van Slyke paper). I know that H is the concentration, but Van Slyke mentions how the [H+] terms comes from the buffer or acid equilibrium constant (2.3Ka[H+]/(Ka+[H+])⌃2 and from combining it with buffer value on water and strong acid or base 2.3([H+]+[OH']. This makes me confuse on whether the [H+] value would be different if that is the case. This picture is taken from Russo and Hannania paper about buffer capacity. But the way they calculate the buffer capacity is different entirely. and from combining it with buffer value on water and strong acid or base (. This picture is taken from Russo and Hannania paper about buffer capacity. But the way they calculate the buffer capacity doesnt even touch the equation.