Making your own pH control solution?

[TrojanAnteater]

Hey all, I have a crappy little pH pen meter (well, it seems to work ok for me), but I want to check its calibration since I haven't checked in a few months. Before I have to run out and buy actual test solutions/salts, or order online, is there a way to use DI water and either StarSan or 88% Lactic Acid to make control solutions?

I have a chemistry degree but haven't been in that field since leaving undergrad and now I pretty much know next to nothing about stuff like this. Searched a little online, but couldn't find anything. Thanks for any help.

 

[And1129]

I did this a while back when I got a new meter. If you make a 1.0 molar solution of a strong monoprotic acid, the pH should, by definition, be 0. If you have a strong acid you can dilute to appropriate concentrations, that would work, or else, you could find some household acid, say vinegar, note the concentration of the acid (household vinegars range from 3-6%) and look up (via math) what the pH for that acid in that concentration theoretically should be. Then you can test it with your meter to see what you read and compare it to what the math expects it to be.

 

[ajdelange]

If you think back you will probably remember that pH depends not on the concentration of H+ ions in a solution but on their activity (which approaches concentration as the solution becomes dilute). Using data on hydrochloric acid from the Wikepedia article and extrapolating the pH of 1 N HCl would be about -0.27. The problem with using acids (any acid including lactic) is that you would have to obtain high grade acid and then assay it in order to prepare a solution of a given normality.

You could, conversely, buy standardized 0.1 N HCl which would have a pH of a bit less than 1 from a laboratory supply house. This acid is held to close tolerance because it is used for measuring the alkalinity of water samples. You wouldn't want to do the test with full strength acid because you are interested in checking at mash pH levels so a 1:99 dilution of 0.1 N down to 0.001 N with a pH of 3 (activity is going to be closer to concentration for this weaker dilution). But why spend money on acid when you can just buy some buffers intended for checking pH meters?

If you want to gin something up you could of course make your own buffers. pH 4 buffer is potassium bipthalate based and the 7 buffer is a phosphate buffer. There are lots of calculators on the net that will tell you how much of the salts are needed for a particular pH.

If you want to use something around the house a solution of sodium bicarbonate will have a pH close to 8.38 (half way between the two pK's of carbonic acid).

 

[TrojanAnteater]

thanks for the replys, will just be picking up the buffer solutions next time I'm at the homebrew shop