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Old 02-03-2013, 06:35 AM   #1
dbsmith
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Default pH and temperature

I have a question about pH 'correction'. I have seen it advocated to adjust the pH reading when it is being taken at mashing temperatures. But, I have to ask, why?

It seems that the %ionization for a weak acid will increase with more heat, due to the increase of available energy. Ka values are set for laboratory temperatures. As I understand it, there is a difference in the pH of two liquids of the same weak acid concentration at different temperatures. So is the correction really necessary? Basically, what I'm asking is the pH meter really reading more acidic because the heat is actually causing the pH to decrease because of the increased ionization, or is it the meter itself just 'truly off' by being affected by the temperature of the liquid?

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Old 02-03-2013, 12:32 PM   #2
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Don't trust the sources that want you to measure and adjust the mash temp pH. The best practice to test mash pH is to take a sample, cool it to 25 C and test pH.

You are correct that the pH of a substance generally falls as its temperature increases.

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Old 02-03-2013, 02:13 PM   #3
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Originally Posted by dbsmith View Post
I have a question about pH 'correction'. I have seen it advocated to adjust the pH reading when it is being taken at mashing temperatures. But, I have to ask, why?
Don't measure at mash temperature. It will shorten the life of your electrode and the pH measurements you will see discussed here and in the literature are all made at laboratory or room temperature. Exactly what that is is subject to some debate. Kai uses 25 °C, I use actual room temperature which is usually around 21 °C. I do all my calculations at 20 °C but note that the dissociation constant of water is 1E-14 at just below 25°C.

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It seems that the %ionization for a weak acid will increase with more heat, due to the increase of available energy. Ka values are set for laboratory temperatures.
Yes, most acids become slightly stronger at higher temperatures. For example, the two pK's of carbonic acid at room (20 °C) are 6.38 and 10.38 whereas at 50 °C they are 6.28 and 10.17.

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As I understand it, there is a difference in the pH of two liquids of the same weak acid concentration at different temperatures.
Yes. If for example you add 1 mmol of phosphoric acid to a liter of DI water the pH would be 3.057 at room temperature but 3.073 at 50 °C. Please understand that these are calculated, not measured, numbers.

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So is the correction really necessary? Basically, what I'm asking is the pH meter really reading more acidic because the heat is actually causing the pH to decrease because of the increased ionization, or is it the meter itself just 'truly off' by being affected by the temperature of the liquid?
There are two factors at work here. One is that the pH of the solution does change with temperature and the other is that the response of the meter (the voltage the electrode produces) is directly proportional to (Kelvin) temperature. It is the job of the meter to account for the second effect which it does through its ATC algorithm if the meter is digital (details at http://wetnewf.org/pdfs/ph-meter-calibration.html). If it is analog then you will have to manually implement this temperature correction by operating a temperature control both when calibrating and when taking readings unless you make sure that buffers and sample are at the same temperature.

If you measure at mash temperature using a meter that has ATC which you have verified then you are measuring 'truth'. You know what the true pH is at the actual temperature at which the process is taking place. There is no point in converting that to any other temperature except for comparison with other peoples' results and that is questionable because the slope (change in pH per degree temperature change) varies with grist and water. The penalty you pay for doing this is short electrode life.

If you do what most do you simply measure at room temperature and work with those numbers. You do not know truth because the actual pH of the reaction temperature can only be estimated. You can make the estimates if you want by applying the correction in the other direction. IOW if you measure 5.5 at room temperature you might estimate the mash pH at 5.4 if at a protein rest and at 5.3 if at a saccharification rest. What I have done on occasion is take a few measurements at different temperatures as the sample cools and calculate a slope from that. It doesn't expose my electrode to terribly damaging tempertures and I only have to do it once for Pils, once for Kölsch ...

I often use ATC (which means 'Air Traffic Control' here) as an analogy. When a controller tells a pilot: 'Climb to and maintain 10,000 feet - Dulles Altimeter 29.93') the pilot dials 29.93 into his altimeter and climbs until it reads 10,000 feet. He isn't really at 10,000 feet but everyone else who has set 29.93 is at the same altitude, whatever it is, when his altimeter reads 10,000 feet and that's what is important.
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Old 02-03-2013, 05:00 PM   #4
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Thanks for explaining this for me!

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