Calibrating A pH Meter: Buffer Solution

What is a buffer solution?

You need to calibrate your pH meter for it to work properly, but to do that, you need to immerse it in a solution of known pH. Buffer solutions are the way out of that little chicken and egg problem. These are made of precisely measured ingredients that combine to form a solution of known, and highly stable, pH. It would be pretty tough for most home winemakers to make their own buffer solution, so it’s a good thing that they’re widely available at homebrew shops.

Not all buffers are created equal

I was out of pH 4 buffer, and bought some more from a local shop. The first difference I noticed, between the new and old solutions, was the color – the new one was pink and the old one was colorless. There were two other differences, though, that were more significant. Each solution came with a temperature table that indicated the precise pH, to two decimal places, at a given temperature. The old solution listed the pH from 0C to 95C in 5 degree increments, and over that wide range the pH varied from 4.00 to 4.22. The new solution listed the pH at 20C (pH = 4.02) and 30C (pH = 4.99). The detail and temperature stability of the old solution gave me a lot of confidence. The new one is … pink.

How to shop for buffer solution

My pH meter had been pretty steady, often needing no adjustment at all between uses. When it had drifted, it was only by 0.02 or 0.03. So I was taken aback to see my pH meter read 3.74 when I first put it in the new solution. Maybe the meter drifted by that much since the last time I used it, but maybe the pH of these two “pH 4” buffers differed by 0.26. I was out of the old solution, so I couldn’t check this. The new solution is fresher, so it’s possible that the old one drifted over time, but I’ve got my doubts about the new one. Live and learn. The next time I buy buffer solution, and that’s going to be pretty soon, I’ll try to find out when it was made and how much detail is in the temperature table.

Update 5/13/2012 – Hanna buffers are my favorite

I’ve been calibrating my pH meter for five years now, and I’ve bought various brands of buffer solution. I keep going back to Hanna. I’m not a chemist, but the detailed temperature corrections and the small drift give me a sense that theirs is a cut above. They’re readily available at good prices too, so for whatever it’s worth this is what I buy.

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