Temp / pH Correlation

[Colione]

Hi Everyone,

I am starting up my second reef. I have an Apex and I am running the seasonal temperature. Since it's Jan, the tank has been running fairly cold at about 75 degrees. My pH has been consistently a bit high, hovering at about 8.5, and barely fluctuating in the evenings.

After a few weeks of that, I figured I should heat the tank up, and run things at about 78 degrees to get everything going, give the bacteria a good environment to grow, and get everything settled.

As I raised the temp, I noticed the pH drastically dropped down to a normal level. Below is my Apex graph.

I tried to do some research on why this happened. But nothing was too conclusive.

Do you think that it was the temperature calibration of the probe? And the probe was reading high because the water was cold? Or do you think there was a process that changed as I heated the water up?

Thoughts anyone??

 

[disc1]

As I raised the temp, I noticed the pH drastically dropped down to a normal level. Below is my Apex graph.

The pH only dropped by 0.1 pH units. That is not drastic in any sense of the word.

The scale on your graph just makes it look that way. Take your probe out of the tank and stick it in some pH 7 buffer for a little bit and let it collect one data point there. That will cause the scale on the graph to change so it can fit that point in. You probably won't even be able to make out this little shift after that.


The actual cause of that little shift could be any number of things. It is quite likely that the increased temperature led to an increased amount of respiration going on in the tank as the animals get more active. The added CO2 dropped the pH a little.

It is also very likely that the difference is related to temperature and the pH meter. Most are temperature compensated but it's not always exact, especially if the temperature probe and pH probe are far apart.

 

[Colione]

disc1..

hahaha... yeah I guess 0.1 isn't really that much. But in my mind it went from 8.5 which is too high to 8.4 which is acceptable.

The probe is brand new, and I calibrated it when I put it in the tank, I suppose I could do it again, which I might, but then I feel like if I need to calibrate it again, it may be dysfunctional.

The temp probe and pH are a few inches away in the same body of water ... I suppose it could of been higher production of c02. Though, there only is 3 chromis in the tank!

 

[Randy Holmes-Farley]

There are several reasons that pH measurement is temperature sensitive. I cover that in one of my pH articles, but I do not have the link handy.

IMO, pH 8.5 is fine.

 

[Colione]

Cool. Good to know. Thanks Randy!

 

[Randy Holmes-Farley]

FWIW, I discuss those issues here:

Measuring pH with a Meter
http://www.advancedaquarist.com/2004/2/chemistry

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.