Today I ran some more pH experiments. In particular a test against the colorpHast strips that I did about 1 1/2 years ago when I got the pH meter (results were posted here). For a while now I cannot calibrate the meter (a Milwaukee pH53) anymore as it will not recognize accept the calibration solutions. It has never been easy to calibrate that meter. That's why my next meter will have little knobs for calibration.
So I had to do the 2-point calibration by myself. The 4.01 buffer read at 4.29 and the 7.01 buffer read at 6.98 (all temps at 62F). Based on that I calculated the slope and offset to get the corresponding pH for a measurement. I then used vinegar and baking soda to mix buffer solutions of various pH which I tested with the meter and pH strips:
| meter pH | strip (pH) |
| 3.94 | 4 |
| 4.46 | 4.7 |
| 4.77 | 4.7 |
| 4.87 | 4.7 |
| 4.96 | 4.7 |
| 5.12 | 4.7 |
| 5.35 | 4.85 |
| 5.94 | 5.8 |
| 6.24 | 6.1 |
| 6.78 | 7 |
and the graph
Very odd was the measured range of 4.5 .. 5.3, where the strips alwasys read 4.7 (read in tungsten light) I ran the test a few times and I also had to add baking soda to increase the pH of the solution. But even though the reading of the pH meter went up, the reading of the strips remained at ~4.7 pH. I may have to rerun this experiment with mixing the 4.01 and 7.01 buffers as I did it initially, just to take the different types of test solution out of the mix. But pH should be pH regardless of the medium.
I then went on to test the temperature sensitivity of the pH meter, and there is actually one. The same solution at different temperatures, showed these different readings (not corrected for calibration):
61F -> 5.27
90F -> 5.31
100F -> 5.36
I was also curious of the temperature dependency of the strips and found this result:
60F -> the strip matched the color for 4.7
140F -> the strip matched the color for 5.0
The odd thing is, that the reading at the higher temperature is higher even though it is commonly assumed that the pH is lowered with increased temperature. The one submerged in cold solution was definately more yellow than the one in the hot solution. Based on that I will now always make pH readings with the strips on cooled samples. When I pulled the strips out, which allowed the one immersed in the 140F solution to cool, both strips colors seemed to settle at 5.0. This was unexpected. I would have thought that they settle at 4.7 or keep their color.
Looks like as if these experiments seem to bring up more questions than answers.
