Zetapotential of untreated pnc-Si varies with pH

EDIT – 2/10/14 – Adding 1 mM phosphate buffer fixes the problems outlined in this post.

I’ve been having some trouble taking consistent pH measurements with the new pH probe. Prior to Thanksgiving I made a spectrum of 10 mM KCl buffers (at 4, 6, 7, 8, and 10 pH units) for use in my streaming potential device, with the goal of getting a feel for the relationship between pH and surface charge. The problem was that even with the new beckman-coulter pH probe I was unable to reproduce my pH measurements of the standards. Some of this I was able to fix by improving my immersing technique (the new probe is prone to getting bubbles around the bulb, which makes for fluctuating values). The pH cleaning and immersing techniques are detailed in the following two videos:

“Cleaning the electrode”

“Removing bubbles from the electrode”

Once I switched over to this technique I was able to get reasonable repeatable pH’s (+/- 0.3 pH units) over a series of calibrations for all except for one of my solutions – the pH 10 solution, which fluctuated wildly (from 9.93 to 7.31 pH units) with no apparent trend. There was some crud in the vial that was not in the other tubes – I’m hoping that’s the reason for the inconsistencies. Ignoring the problem solution, the pH v. Apparent Zetapotential relationship looks like the following:

Such a narrow range of pH’s and such small sample sizes (one chip per data point) detract from the usefulness of the data, but the chart is still important as a proof of concept. Charge v. pH graphs for untreated pnc-Si & SiN as well as UV-ozone cleaned pnc-Si & SiN are next.