[suneyna]

Hi,

I am a doctorate student and was recently working with phosphate buffer solution(1M).
To my surprise, when I started adding NaCl in increasing order of concentration i.e. from 0.1M, 0.2M, 0.3M,
0.4M to 0.5M solutions. I started reading decrease in the pH value of the buffer.

While being NaCl neutral in nature, I was expecting no change in the pH of buffer solution on addition of NaCl. So, I started searching for any documented research article for the same but didn't able to find them.

Please if anyone can explain what is going on to the buffer capacity on addition of NaCl, It would be of great help to me. I request if someone have any idea about any research article for the same, please forward me the link.

Thanks
Suneyna

[UG]

Because you are increasing the ionic strength of the solution

[Babcock_Hall]

Others here might be able to give a more comprehensive answer, but I will add my two cents.  Thermodynamic pK_a values are tabulated as if the process were happening with an ionic strength of zero.  However, practical pK_a values are sometimes also reported at various ionic strengths.  Ionic strength affects the activities of the chemical species.  Robert Scopes' book "Protein Purification" discusses this effect on pp. 238-241 of the second edition.  Phosphate and citrate buffers are strongly affected by ionic strength.  Tris is less affected by ionic strength, but is more affected by temperature, for example.

[Borek]

Phosphate and citrate buffers are strongly affected by ionic strength.  Tris is less affected by ionic strength

That's because ionic strength effects are more important for ions with higher charge. Around pH 7 Tris is either +1 or 0, phosphoric acid is dominated by -1 and -2 forms, citric acid is already completely dissociated at -3.

[Babcock_Hall]

Borek, That is interesting.  Good's buffers (MOPS, PIPES, AMPSO, CHES) are zwitterionic in one protonation state and -1 in the other.  How would they behave?

[Borek]

Borek, That is interesting.  Good's buffers (MOPS, PIPES, AMPSO, CHES) are zwitterionic in one protonation state and -1 in the other.  How would they behave?

Good question, and I am not convinced I know the answer.

First of all - in Debye-Hückel theory activity coefficient for an X^z± ion is calculated as

[tex]\log f_z = \frac {0.51z^2 \sqrt I} {1+\sqrt I}[/tex]*

so the higher the charge, the more different from 1 the activity coefficient is.

Technically zwitterion has charge of zero, so - if we were to apply the DH theory blindly - it should have activity coefficient of 1. But whole theory is based on the assumption that electric field of the ion charge arranges all other charges around the ion - and zwitterion for sure creates an electric field around itself. My bet is that the activity coefficients for zwitterions will be somewhere between activity coefficients for single charge ions and 1, and the exact value would be a function of charge separation. Something like the electric field of charge and dipole.

*This is just one of several possible variants, they differ by accuracy and used approximations, but they are all similar enough to support everything I wrote.