[mr.v.]

Hi,

I work in a lab. I'm making a 10x tris buffer pH 7.4 but when I dilute it to 1x the pH changes to 7.8. I know that tris buffers change their pKa depending on temperature but the water i'm diluting the tris-buffer in is at the same temperature as the 10x stock. And also I measured the temperature before and after dilution, and they were identical (22oC)

Unless I'm mistaken, buffers from weak acids/bases should not be affected by concentration since the pH is determined by the relative concentrations of acid and conjugate base (log [HA]/[A-]) and not absolute concentration. Since the relative concentrations don't change...shouldn't the buffer remain the same pH regardless of whether you dilute it 10x? Just for absolute numbers, the 10x buffer is 0.5M Trizma base pH'd with HCl to 7.4 and measured with a pH probe (corning gel combo on a VWR pH meter). Both pH measurements are using the same calibration on the same probe side by side. Also, I haven't added any NaCl or other agent. It's just plain Trizma base and water (and HCl).

Any ideas what the trouble is?

Thanks!
Vik

[Borek]

0.5M solution is heavily concentrated so I would expect your problems come from the activity coefficients. Rule of thumb is - for ionic strength under 0.01M use concentrations, for ionic strength under 0.1M use Debye-Huckel theory, for ionic strentght above 0.1M don't bother with calculations, measure.

Check if further dilution (to say 0.1x) changes pH - it should, but to much lesser extent.