Proving that ATC Does Not Work To Correct PH Temperature

[Brewmegoodbeer]

Hello all,

I have been researching a lot about ph and the effect temperature can have on ph. I wanted to put my ph meter's ATC to the test to see if "automatic temperature compensation" compensates for the differing temp of ph. I measured water with lactic acid in it at room temp and got 2.15. I then microwaved it to 146 degrees and I got 1.74. Boom! No compensation. Moral of the story is: always measure any ph at room temperature and always assume that any ph reading or range given for beer is given at room temperature or atleast 70-80 degrees farenheit to get it close enough.

 

[jmrybak]

Hello all,

I have been researching a lot about ph and the effect temperature can have on ph. I wanted to put my ph meter's ATC to the test to see if "automatic temperature compensation" compensates for the differing temp of ph. I measured water with lactic acid in it at room temp and got 2.15. I then microwaved it to 146 degrees and I got 1.74. Boom! No compensation. Moral of the story is: always measure any ph at room temperature and always assume that any ph reading or range given for beer is given at room temperature or atleast 70-80 degrees farenheit to get it close enough.

Sooo... pH is a measure of free H+ ions present. When you microwaved the sample to heat it up, that heat actually increases the amount of free H+ ions. So the meter isn't necessarily wrong. The pH actually does go down when you heat a sample.

 

[Brewmegoodbeer]

What does atc actually compensate in regard to temperature?

 

[jmrybak]

automatic temperature compensation (ATC)?

A: The Solution Temperature Effect
When there is an increase or decrease in the temperature of a solution, the pH of the solution can change. This change is not an error caused by the variation in temperature; it is the true pH of the solution at the new temperature. Since this is not an error, there is no need to correct or compensate for the solution temperature effect.

The pH Electrode Temperature Effect
There is only one major temperature effect in pH measurement that can cause errors in readings. This is the change in the electrode's response (or sensitivity) to pH which results from changes in the samples temperature. It is the only reasonably predictable error due to changes in temperature, and is the only temperature related factor that pH instruments with temperature compensation can correct for. This temperature error is very close to 0.003 pH/°C/pH unit away from pH 7. If a sample is measured without using an automatic temperature probe, the solutions temperature needs to be entered into the meter manually to allow it to account for this error.

 

[ajdelange]

Boom! No compensation.

Given that, in your opinion, Automatic Temperature Compensation does no compensation the obvious question for us to direct to you is "What does it do?" and why is it called that?

Just a few lines up in this Forum is a Sticky, a place where commonly sought information about brewing science subjects are stuck because they get asked about over and over again. ATC is discussed in this Sticky (https://www.homebrewtalk.com/showthread.php?t=302256) in quite a bit of detail. What it does do (automatically compensate for the temperature response of the elcctrode) and what it doesn't do (account for changes in the pH of solutions caused by the changes in the pKa's and pKb's of, respectively, the acids and bases in the solution.

For the majority of acids pK decreases with increased temperature so the pH of a solution containing the acid decreases as temperature is raised. Carbonic acid is an example. Lactic acid is one of several exceptions (with phosphoric being another). The pH of solutions of those acids increases with an increase in temperature and not by as much as you observed. Thus there is some problem with your measurement.

 

[estricklin]

Just wanted to add, many probes are only rated to 140F max.

 

[ajdelange]

One of the nuances of pH measurement that is mentioned in the Sticky is that the modern pH electrode has an isoelectric pH (the pH at which the electrode response is not influenced by temperature) assumed to be close to 7, so close in fact that no modern pH meter (that I have ever seen or heard of) allows the user to set it to any other value. In fact the isoelectric pH (pHi) of most electrodes is udually specified to be 6 < pHi < 8. As the equations in the Sticky make clear the estimated pH is not terribly heavily influenced if the electrode's actual pHi isn't exactly 7 (hence the wide tolerance) but an error is introduced if pHi &#8800; 7 and the temperature of the sample is not the same as the temperature of the buffers when the calibration was done. For the most accurate results the calibration buffers and the samples should all be at as close to the same temperature as it is possible to get them. Because the pH sensing bulb is made of very thin glass thermal stresses should be avoided to the extent possible. This means that the chosen temperature should be as close to the storage temperature, room temperature, as possible. Furthermore, in the literature pH values for beers and worts are measured on cooled samples (unless specifically otherwise stated). Thus the advice in #1 to take all measurements at room temperature is sound advice (though not for the reason given) and the fact that the electrode's specified temperature range may be limited is not a problem.