What’s the relationship between temp and pH?

[Fisherman Joe]

Interestingly my pH seems to rise with temp, I would think that the metabolism of the fish is higher at warmer temps and therefore there would be more waste and more acidic compounds to lower the pH.

 

[Randy Holmes-Farley]

Interestingly my pH seems to rise with temp, I would think that the metabolism of the fish is higher at warmer temps and therefore there would be more waste and more acidic compounds to lower the pH.

While there is a tiny relationship, what you are seeing is both responding to the light cycle, not to each other.

 

[Fisherman Joe]

Normally I’d ask if you were sure. But given that you literally wrote the book here, I’ll take it as a fact. Ha ha.

 

[Randy Holmes-Farley]

Normally I’d ask if you were sure. But given that you literally wrote the book here, I’ll take it as a fact. Ha ha.

lol

The true pH measurement change with temperature is about 0.003 pH/deg C/pH unit away from pH7. Thus at pH 8.0, the change is 0.003 pH units per degree C. Your tank is not changing by more than a few degrees C, so the pH change from temp alone is less than 0.01 pH unit.

Daily photosynthesis using CO2 durign teh day and respiration producing CO2 24/7 can cause changes of several tenths of a pH unit daily, with the peak pH in early to late afternoon.

I discuss the temperature relationship here:

Aquarium Chemistry: Measuring pH with a Meter

This article describes how pH meters work, how to select one to use, and how to best use it to measure the pH of a reef aquarium.

reefs.com

from it:

Temperature Effects on pH Measurement
There are two different ways that temperature impacts pH measurement. The first involves actual chemical changes in the solution that you are measuring. Acids can, for example, become stronger or weaker as the temperature is changed. This is how calibration standards change their pH as a function of temperature (which is discussed in more detail in the calibration section below). If the solution has solids in contact with it (as is the case with saturated limewater in the presence of excess solids), the temperature can also impact how much acid or base is in solution impacting pH, and how much is just solid sitting on the bottom of the container. These effects are specific for every solution that you will encounter, and there is nothing general that one can or should do about this, except be aware that it happens.

The second impact of temperature is on the pH electrode itself. pH electrodes change their response in a very clear way as temperature is changes. They respond more strongly to pH changes at higher temperature than at lower ones. At 100 ºC, they change their output potential by 74 mV/pH unit, and at 0 ºC, they change by 54 mv/pH unit. Because pH meters are typically standardized at pH 7 (that is, zero mv = pH 7), the error from temperature differences gets greater and greater as the pH being measured gets further from 7. So it may be trivial when measuring something with a pH of 7.1, but very important when measuring something with a pH of 10 (or when calibrating with a pH 10 buffer).

There are usually three different ways of taking temperature into account. One is to make measurements close to the temperature at which you calibrated the meter (say, within a few degrees). The second is to “tell’ the meter what the temperature is (digitally or with a dial). The third is that some meters have a temperature probe, usually called an ATC, which you stick into the measuring solution. This probe reports the temperature back to the meter, and the meter makes any necessary corrections (for this type of temperature effect).

As long as you use one of these three ways of dealing with temperature issues, you will get reasonably accurate readings.