Cold Weather & pH

I looked into this further and what you are seeing is normal. It turns out that there is an inverse relationship between temperature and pH.

It is also of interest that there are 2 different forms of temperature compensations to be considered. One, which we may be familiar with is commonly called ATC (automatic temperature compensation) of the pH probe junction. This addresses the temperature dependent flow of electrons from the pH probe junction (mentioned earlier in this thread). The 2nd form is the actual temperature of the liquid being measured. It turns out that this phenomenon is described by Le Chaterlier’s Principle. Science majors may remember hearing this in freshman inorganic chemistry.

The principle says that changes in temperature, pressure, volume, or concentration of a reacting system at equilibrium allow the system to change its state in order to nullify the effect of stress.

What that means here is, that when the temp in Justin’s system drops (even at the surface) the energy of atoms is decreased, thereby causing a corresponding increase in ionization to counteract the change. The effect is greater in alkaline conditions.

Conclusion: Does Temperature Affect pH?​

Temperature impacts the pH of samples in two ways, either by affecting the electrode or by changing the nature of samples. Most of these effects can be eliminated using pH meters with built-in ATC systems.

Not all ATC systems are reliable when working with temperature variant samples. Suitable pH electrodes for a particular temperature of the sample are to be employed.

* you may want to calibrate both pH & temp probes at the midrange temperature in your basement (ie ~69 deg F, between 60-78 F?)
I would also dose the alk earlier and perhaps more often. Maybe within an hour after the lights go off? Biochemistry isn’t like gunpowder. It usually takes time for effects to be felt. Adding the alkalinity supplement as soon as needed will help smooth out the curve & make things more stable.

Turning on the skimmer is going to increase both O2 & CO2 diffusion into the tank water. That increased CO2 may help drive down pH, especially if done at night when photosynthesis is not there to uptake it.

Some, me included, suggest running skimmers during the day, if you are going to choose, in order to minimize this effect. It is also when most wastes are being produced, as most fish are less active at night.
Well, I'm not one to revive an old post but I just had a new experience as it relates to this phenomenon. I was working at a homeless shelter last night and at 4:10 am I started getting pH warnings. I was surprised but the most shocking part was that it kept going up and up and up. If you look at the chart of the last few days you'll notice the Ph starts going up when the lights kick on and usually tops our around 8.5 or so by 6:00 pm. The data from today show it topping out at 8.62 at 5:20 a.m. It went from 8.28 - 8.62 in about an hour.

Needs less to say, I got a little freaked out and ran home. The culprit? When I got home the basement door had blown open with the high winds. As soon as I shut the door, I turned on a supplemental heater and the pH started going down immediately. I don't know if the pH swing did any damage but you can bet I'll be locking that door from now on. (Honestly, I thought it was locked but was quite obviously mistaken.)

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