[jeffmoonchop]

Hi all, I'm trying to calculate the pH of a mixture of buffers. The solution includes a basic buffer Tris and an acidic buffer acetate. Each are at different concentrations. I'm pretty sure I calculated the pH of each component buffer correctly, but struggling to assign a pH weighting to each buffer to get a final pH.

So I guess I could just skip to that part, assuming my pH calculations are correct.

Tris Buffer pH = 7.61
Total Tris Buffer concentration = 0.02M

Acetate Buffer pH = 4.06
Total Acetate Buffer Concentration = 0.0044M

Because the acetate concentration is 4.6times lower, I'm assuming that the weight of the acetate on the pH is 4.6x lower.

So all I've done is reduce the combined buffer pH from 7.61, to 21.7% (1/4.6) of the difference between the pH of each calculated buffer. So the difference between the calculated pH is 3.55, so I reduced the pH by 21.7% of 3.55, or 0.77.

So the final pH I calculated as 7.61 - 0.77 = 6.84.

However, I know this pH cant be correct for other reasons, I don't think the pH is reduced so much but the final pH is more likely around 7.4 or 7.5. Anyone know where I'm going wrong?

thanks

[Borek]

pH of a mixture is not a weighted average, it doesn't work this way.

[jeffmoonchop]

Yes I gathered, any suggestions?

[AWK]

Calculate the stoichiometric concentrations of the components and use the WebCQ pHsolver
for example:
CH3COOH pKa=4.77 c=0.0036
CH3COONa pKb=9.23 c=0.0008
TRIS pKb=5.94 c=0.005
TRISH pKa=8.06 c=0.015
Use you preferred values of Ka and Kb

pH = 6.946

https://www.webqc.org/phsolver.php

[jeffmoonchop]

thanks very much, I got my expected result of 7.4