[CarlJF]

I'm wondering if there's a way of calculate the effect of sodium chloride on phosphate buffer.

More to the point, I've made the following buffer:

6.09 mM NaH2PO4
43.96 mM Na2HPO4
175 mM NaCl

Now, now matter how I calculate the pH (HH, activity coefficient, online table, online calculator...), I get a result of 7.7. But when I measure the pH of the solution, it's rather 7.55. Not very far, but slightly off... The only reason I can see for this difference between the calculated and observed pH is that NaCl affects the pH. But, I don't understand how. The only thing I can see is that Na+ ions coming from the NaCl somehow affects the equilibrium of the buffer, which also has Na+ ions...

Any ideas or explanations ?

[Borek]

Now, now matter how I calculate the pH (HH, activity coefficient, online table, online calculator...), I get a result of 7.7.

So you are doing something wrong, but it is hard to say what not seeing neither of your approaches.

Note that the solution ionic strength will be over 0.3 - that's way too much for the Debye-Huckel theory usually used is such cases.

Using Buffer Maker and Davies equation I got 7.45, just 0.1 pH unit off - not bad.

[Babcock_Hall]

Thermodynamic pKa values assume that the ionic strength is zero, but sometimes "practical" pKa values can be found for cases in which the ionic strength is greater than that.  Phosphate buffer is commonly encountered, and I know I have seen such values, even though I don't have a citation off the top of my head.  In addition, some ions form complexes with other ions, and that should be taken into consideration also.  This effect depends on charge and type of ion.  I would predict there would be more complexation of sodium ions by pyrophosphate ion than phosphate ion, for example.

[CarlJF]

Now, now matter how I calculate the pH (HH, activity coefficient, online table, online calculator...), I get a result of 7.7.

So you are doing something wrong, but it is hard to say what not seeing neither of your approaches.

I simply used the classical HH equation:

pH = 6.82 + log(6.09/43.96) = 7.7

I've also used the equation by using activity coefficients (sorry, I don't have the numbers on hand right now) instead of the concentration and the usual 7.21 pKa with the same result.

[Borek]

I simply used the classical HH equation:

pH = 6.82 + log(6.09/43.96) = 7.7

I've also used the equation by using activity coefficients (sorry, I don't have the numbers on hand right now) instead of the concentration and the usual 7.21 pKa with the same result.

Why did you use 6.92 for pK_a2 instead of 7.21 (or 7.21 instead of 6.82, depending on which way to look)?

[CarlJF]

I simply used the classical HH equation:

pH = 6.82 + log(6.09/43.96) = 7.7

I've also used the equation by using activity coefficients (sorry, I don't have the numbers on hand right now) instead of the concentration and the usual 7.21 pKa with the same result.

Why did you use 6.92 for pK_a2 instead of 7.21 (or 7.21 instead of 6.82, depending on which way to look)?

6.82 is the pKa from Sigma-Aldrich. Other manufacturer are also using this value (see http://www4.mpbio.com/ecom/docs/proddata.nsf/%28webtds2%29/191442 for another one). I suppose this is a value corrected for activity coefficient, so that you can work directly with concentrations.

The 7.21 value is from my Physical and Chemical handbook, and thus what I suppose is the official one. Thus, this is the one I used when calculating with activity coefficients instead of concentrations.

BTW, I've just noticed I inverted the two concentrations value in the equation above (working from memory on my laptop, my spreadsheet being on my desktop...). It should read:

ph = 6.82 + log (43.96/6.09)

Sorry if this caused any confusion...

[Borek]

If you are changing pKa values at random, you can produce any set of results. Be consistent if you want to get consistent results.

Corrected values should have the ionic strength listed, as activity coefficients are a function of ionic strength. My sources give:

IS	pKa2
0	7.2
0.1	6.72
1.0	6.46

Note that your solution has IS somewhere between 0.1 and 1.0, so neither of these values fits exactly.

[CarlJF]

Using Buffer Maker and Davies equation I got 7.45, just 0.1 pH unit off - not bad.

I've just try Buffer Maker and got 7.64. How do you get 7.45 with this software ?

[Borek]

Honestly - I don't remember. Could be I used some modified value for the Davies equation, I was playing with the defaults lately. Now I got 7.51.

[CarlJF]

Many thanks!

Looking at your printout, I found out the problem. I've made a mistake entering the NaCl formula. I entered Na+,Cl- instead of Na^+,Cl^-. Thus, in my printout, these ions were identified as "spectator ions" that doesn't contribute to anything in the buffer calculation.

I have one last question. When looking at your pH calculation, at the bottom, the calculated osmolality and buffer capacity are shown. How do you get to see these results ? I don't see them in my pH calculation report and don't see them mention in the help either... It would be extremely useful for me to have these number as well.

[Borek]

I am using a working copy of the program, not published yet - it is possible these things were added after the latest release was made public. I can't say when they will become officially available (hopefully later this year), but if you will buy the program, I can send you an unofficial upgrade.