In brewing I thought pH was expressed at room temperature.
As in pH 5.2 is that pH at room temperature of 68 degrees.
However, after reading Gordon Stong’s book I now have the impression that pH is always expressed at the operating temperture.
So if I am aiming for a mash pH of 5.2, and at room temp of 68 degrees the pH meter reads 5.2,
I am really incorrectly mashing at a true pH of 4.9 or so?
What is the standard?
???
It’s baffling because it seems to be inconsistent. Most texts are referencing it at room temperature, and this is how you should be measuring it due to the fact that this is how equipment is calibrated; however, the oft-quoted 5.2 is at mash temperature, I think. From what I gather, aim for 5.4 - 5.6 at room temp and you’re good to go.
This is my understanding as well.
+1
The samples must be chilled down to room temp prior to taking a reading.
No they don’t. pH meters have an operating temperature range wherein they are accurate. The best would be testing at mash temperature, but a lot of pH meters can’t be used at those temperatures.
Right, testing at mash temperatures would be ideal but would burn most pH electrodes out pretty quickly. So practically speaking we’re usually taking about room temperature pH temps.
I don’t see any reason or advantage to testing at mash temperature, other than not having to cool the sample. The target of 5.4 - 5.6 was established at room temp.
If you’re using ColorpHast pH strips, they seem to give the same pH at mash temp as at room temp, so you still want to use the 5.4-5.6 range.
The only advantage I see would be not having to cool the sample down. It would be easier to take multiple readings throughout the course of the mash. Colorphast strips probably cool down to room temperature pretty rapidly.
Yes, but to confuse things even more they also seem to skew a little bit low, so you can aim for 5.1 - 5.3 
So I’m a Physicist rather than a Chemist and don’t know these things, but pH is the concentration of hydrogen ions in solution, and shouldn’t that stay the same as temp increases? Or does it respond inhomogeneously to temperature increase and effectively affect the concentration when heated?
Per Kaiser’s Overview of pH article:
[quote]When the pH of a sample is measured it is also important to know the temperature of the sample. This is important for 2 reasons. The temperature of the sample affects its H+ concentration and therefore its pH. This is the result of changing H+ and OH- dissociation balances in the sample and is substrate specific.
[/quote]
Increased heat provides more energy for dissociation I guess.
Perhaps must is not appropriate, but the actual values will be skewed at increased temps and manufacturers recommend sampling at room temp. So let me correct my statement by saying ideally one should measure mash pH at room temperature or 68F. I use a Milwaukee MW101 and the manufacturer recommends measuring liquids at room temp and no higher that 130F. I recommend contacting the manufacturer of your specific unit and asking for the recommended range of operating temperatures.
I have observed my pH meter change values as the temp decreases. Mash temp gives a pH reading about .2 or .3 lower than the same wort at 70F. So if you measure the wort at mash temps, which I understand is bad for the probe, you need to adjust the reading higher. ie a reading of 5.2 at 155F is probably good (but not for the probe).
I’m neither, but I monitored the change in pH of a mash-range sample from 80C to 20C with a lab-quality bench pH meter. The pH change versus temperature was fairly linear. I can’t tell you why it happens, but my direct observations are that it is true.
Very true and important, I should have mentioned that ColorpHast strips have been found to read about 0.3 low in the mash pH range.
So to take this one step further, if I test the pH of the mash at mash temp (~150) and obtain a reading of 5.0 it’s roughly equivalent to a pH of 5.5 at room temp (.3 from the inherent error of the strip and .2 for the temp shift)? So I’d shoot for a reading on the ColorPhast strip of 4.8 - 5.0 at mash temps or do I have this backwards?
No. There’s a temperature dependent change in the chemical reaction that produces color on these strips that causes them to read the same at mash temperature as they do at room temperature.
yep, just look at is as .3 inherent error and .3 temp shift. The .3’s are in opposite directions so they cancel each other out.
Good deal! So on the strips at mash temp I’m still looking for a value of 5.2 - 5.5? I might be thinking a bit to hard on this one.