To work properly, a pH meter must be calibrated. You do this by preparing (or buying) buffered solutions of known pH and testing the meter against them. My meter uses a two point calibration. It works by immersing the meter in the first buffer solution (pH 6.86 in this case) then reading the pH and temperature values.
You turn a calibration screw until the meter shows the correct pH for the given temperature (the bottle of buffer solution has a table of temperature and pH values). The pH of the “6.86” buffer solution that I’m using is 6.85 at 30 Celsius, the closest temperature on the correction table to 31.4, so I turned the calibration screw to 6.85. That’s the first “point” of the two point calibration process. The second step is exactly the same but with a different buffer solution (ph 4).
I’ll use my new toy – um, important scientific instrument – along with my simple acid titration kit to analyze my oregano wine. Fermentation has been very slow and I’m afraid the pH of the wine has fallen so low that it’s inhibiting the yeast.
In the case of wine you don’t expect high pH, as all wines are at least slightly acidic, so two points calibration is enough. Some other hobbyists – like aquarium owners, especially those with reef aquariums – need to measure higher pH, about 8 – so they either use three points calibration, or two points, but with the use of pH 7.00 and pH 10.00 buffers. Your 6.85 buffer stands for 7.00 🙂
If you want to know more about pH meters, their usage, calibration, theory behind, history and so on, and so on, and so on – visit my site, http://www.ph-meter.info