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I'm from Colombia and I joined this site to ask a question that I could not solve yet. I am calculating the amounts to prepare a phosphate buffer with two salts. I want to prepare a buffer system with pH = 7.0 and a total concentration of Ct = 0.1 M. All my calculations are in the image attached. When I mix the calculated amounts, the obtained pH = 6.5 in the lab. I thought it was my mistake in calculations or an error when I weighed the amounts, but I repeated everything several times and still not get the pH = 7, and the value always was 6.4 or 6.5.

I don't know the reason but I would like to know this. Please help me. I think that If the concentration of the monobasic salt is greather than the dibasic, the pH tends to decrease, but in the calculations the relation [dibasic]/[monobasic] is always less than 1 and this again causes that [monobasic] > [dibasic]. Thanks, I really need a help.

Calculations

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  • $\begingroup$ Hi and welcome to Chemistry.stackexchange.com. Feel free to take the tour and go to the help center for questions related to how this site works. My first assumption is that your sodium salts may be hydrates and that that could skew the amounts you weighed in. $\endgroup$ – Jan Oct 15 '15 at 17:23
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The flaw in your calculations is the assumption that there exists a pK that is a constant independent of concentration.

In reality, pK is a function of ionic strength.

At infinite dilution (zero ionic strength) the pK is 7.199 as you assumed.

At 0.1M phosphate concentration, the pK is 6.81.

See Phosphate Buffers which cites to A. A. Green, "The Preparation of Acetate and Phosphate Buffer Solutions of Known PH and Ionic Strength", J. Am. Chem. Soc. 55, 2331, (1933).

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    $\begingroup$ Thank you very much. Now I understand what happens, you're absolutely right. Regards, Camilo. $\endgroup$ – camibuidi Oct 15 '15 at 18:52

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