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General Category => General Homebrew Discussion => Topic started by: jjflash on May 06, 2011, 01:37:13 AM

Title: Is pH stated at room temperature or operating temperature?
Post by: jjflash on May 06, 2011, 01:37:13 AM
In brewing I thought pH was expressed at room temperature.
As in pH 5.2 is that pH at room temperature of 68 degrees.
However, after reading Gordon Stong's book I now have the impression that pH is always expressed at the operating temperture.

So if I am aiming for a mash pH of 5.2, and at room temp of 68 degrees the pH meter reads 5.2,
I am really incorrectly mashing at a true pH of 4.9 or so?
What is the standard?
 ???
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: narvin on May 06, 2011, 02:03:05 AM
It's baffling because it seems to be inconsistent.  Most texts are referencing it at room temperature, and this is how you should be measuring it due to the fact that this is how equipment is calibrated; however, the oft-quoted 5.2 is at mash temperature, I think.  From what I gather, aim for 5.4 - 5.6 at room temp and you're good to go.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: tygo on May 06, 2011, 02:04:45 AM
From what I gather, aim for 5.4 - 5.6 at room temp and you're good to go.

This is my understanding as well. 
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: bluesman on May 06, 2011, 02:34:58 AM
From what I gather, aim for 5.4 - 5.6 at room temp and you're good to go.

This is my understanding as well. 

+1

The samples must be chilled down to room temp prior to taking a reading.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: tom on May 06, 2011, 03:17:22 AM
From what I gather, aim for 5.4 - 5.6 at room temp and you're good to go.

This is my understanding as well. 

+1

The samples must be chilled down to room temp prior to taking a reading.
No they don't.  pH meters have an operating temperature range wherein they are accurate.  The best would be testing at mash temperature, but a lot of pH meters can't be used at those temperatures.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: tygo on May 06, 2011, 03:49:00 AM
Right, testing at mash temperatures would be ideal but would burn most pH electrodes out pretty quickly.  So practically speaking we're usually taking about room temperature pH temps.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: malzig on May 06, 2011, 10:35:27 AM
I don't see any reason or advantage to testing at mash temperature, other than not having to cool the sample.  The target of 5.4 - 5.6 was established at room temp.

If you're using ColorpHast pH strips, they seem to give the same pH at mash temp as at room temp, so you still want to use the 5.4-5.6 range.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: tygo on May 06, 2011, 10:38:25 AM
The only advantage I see would be not having to cool the sample down.  It would be easier to take multiple readings throughout the course of the mash.  Colorphast strips probably cool down to room temperature pretty rapidly.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: narvin on May 06, 2011, 01:27:31 PM
I don't see any reason or advantage to testing at mash temperature, other than not having to cool the sample.  The target of 5.4 - 5.6 was established at room temp.

If you're using ColorpHast pH strips, they seem to give the same pH at mash temp as at room temp, so you still want to use the 5.4-5.6 range.

Yes, but to confuse things even more they also seem to skew a little bit low, so you can aim for 5.1 - 5.3  :)

http://braukaiser.com/wiki/index.php/An_Evaluation_of_the_suitability_of_colorpHast_strips_for_pH_measurements_in_home_brewing
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: colinhayes on May 06, 2011, 03:37:08 PM
So I'm a Physicist rather than a Chemist and don't know these things, but pH is the concentration of hydrogen ions in solution, and shouldn't that stay the same as temp increases?  Or does it respond inhomogeneously to temperature increase and effectively affect the concentration when heated?
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: SpanishCastleAle on May 06, 2011, 04:06:08 PM
So I'm a Physicist rather than a Chemist and don't know these things, but pH is the concentration of hydrogen ions in solution, and shouldn't that stay the same as temp increases?  Or does it respond inhomogeneously to temperature increase and effectively affect the concentration when heated?

Per Kaiser's Overview of pH article (http://braukaiser.com/wiki/index.php/An_Overview_of_pH):

Quote
When the pH of a sample is measured it is also important to know the temperature of the sample. This is important for 2 reasons. The temperature of the sample affects its H+ concentration and therefore its pH. This is the result of changing H+ and OH- dissociation balances in the sample and is substrate specific.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: tom on May 06, 2011, 05:56:43 PM
So I'm a Physicist rather than a Chemist and don't know these things, but pH is the concentration of hydrogen ions in solution, and shouldn't that stay the same as temp increases?  Or does it respond inhomogeneously to temperature increase and effectively affect the concentration when heated?
Increased heat provides more energy for dissociation I guess.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: bluesman on May 06, 2011, 06:11:46 PM
From what I gather, aim for 5.4 - 5.6 at room temp and you're good to go.

This is my understanding as well.  

+1

The samples must be chilled down to room temp prior to taking a reading.
No they don't.  pH meters have an operating temperature range wherein they are accurate.  The best would be testing at mash temperature, but a lot of pH meters can't be used at those temperatures.

Perhaps must is not appropriate, but the actual values will be skewed at increased temps and manufacturers recommend sampling at room temp. So let me correct my statement by saying ideally one should measure mash pH at room temperature or 68F. I use a  Milwaukee MW101 and the manufacturer recommends measuring liquids at room temp and no higher that 130F. I recommend contacting the manufacturer of your specific unit and asking for the recommended range of operating temperatures.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: jeffy on May 06, 2011, 07:27:22 PM
I have observed my pH meter change values as the temp decreases.  Mash temp gives a pH reading about .2 or .3 lower than the same wort at 70F.  So if you measure the wort at mash temps, which I understand is bad for the probe, you need to adjust the reading higher.  ie a reading of 5.2 at 155F is probably good (but not for the probe).
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: gordonstrong on May 06, 2011, 07:27:41 PM
So I'm a Physicist rather than a Chemist and don't know these things, but pH is the concentration of hydrogen ions in solution, and shouldn't that stay the same as temp increases?  Or does it respond inhomogeneously to temperature increase and effectively affect the concentration when heated?


I'm neither, but I monitored the change in pH of a mash-range sample from 80C to 20C with a lab-quality bench pH meter. The pH change versus temperature was fairly linear.  I can't tell you why it happens, but my direct observations are that it is true.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: malzig on May 07, 2011, 03:00:39 PM
I don't see any reason or advantage to testing at mash temperature, other than not having to cool the sample.  The target of 5.4 - 5.6 was established at room temp.

If you're using ColorpHast pH strips, they seem to give the same pH at mash temp as at room temp, so you still want to use the 5.4-5.6 range.

Yes, but to confuse things even more they also seem to skew a little bit low, so you can aim for 5.1 - 5.3  :)

Kai's evaluation of ColorpHast pH strips. (http://braukaiser.com/wiki/index.php/An_Evaluation_of_the_suitability_of_colorpHast_strips_for_pH_measurements_in_home_brewing)
Very true and important, I should have mentioned that ColorpHast strips have been found to read about 0.3 low in the mash pH range.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: Tristan on May 18, 2011, 07:10:22 PM
So to take this one step further, if I test the pH of the mash at mash temp (~150) and obtain a reading of 5.0 it's roughly equivalent to a pH of 5.5 at room temp (.3 from the inherent error of the strip and .2 for the temp shift)?  So I'd shoot for a reading on the ColorPhast strip of 4.8 - 5.0 at mash temps or do I have this backwards?
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: malzig on May 19, 2011, 01:50:15 AM
So to take this one step further, if I test the pH of the mash at mash temp (~150) and obtain a reading of 5.0 it's roughly equivalent to a pH of 5.5 at room temp (.3 from the inherent error of the strip and .2 for the temp shift)?  So I'd shoot for a reading on the ColorPhast strip of 4.8 - 5.0 at mash temps or do I have this backwards?
No.  There's a temperature dependent change in the chemical reaction that produces color on these strips that causes them to read the same at mash temperature as they do at room temperature.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: Hokerer on May 19, 2011, 02:09:19 AM
yep, just look at is as .3 inherent error and .3 temp shift.  The .3's are in opposite directions so they cancel each other out.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: Tristan on May 19, 2011, 03:09:52 PM
Good deal!  So on the strips at mash temp I'm still looking for a value of 5.2 - 5.5?  I might be thinking a bit to hard on this one.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: nyakavt on May 20, 2011, 05:40:14 PM
Quote
Good deal!  So on the strips at mash temp I'm still looking for a value of 5.2 - 5.5?  I might be thinking a bit to hard on this one.

You still want to shoot for 5.1-5.3 on the colorpHast strips regardless of the temp reading (I shoot for 5.0-5.3, which are the strip markings).  The decrease in pH of the solution due to increased temp is offset by an increase in apparent reading on the strip due to the chemical reaction of the litmus paper being affected by temperature.  The -0.3 systematic error inherent to the strips is still present and must be accounted for.  Also be aware that with darker beers the wort color can affect the color of the strip, so make sure to dab it with a towel before trying to read.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: mabrungard on May 20, 2011, 05:54:12 PM
yep, just look at is as .3 inherent error and .3 temp shift.  The .3's are in opposite directions so they cancel each other out.

Uh...I don't use the strips since I have a meter and the fact that they have proven to be inaccurate in the mash, but I don't think the temp shift enters into this issue.
 
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: Tristan on May 22, 2011, 02:42:05 PM
You still want to shoot for 5.1-5.3 on the colorpHast strips regardless of the temp reading (I shoot for 5.0-5.3, which are the strip markings).  The decrease in pH of the solution due to increased temp is offset by an increase in apparent reading on the strip due to the chemical reaction of the litmus paper being affected by temperature.  The -0.3 systematic error inherent to the strips is still present and must be accounted for.  Also be aware that with darker beers the wort color can affect the color of the strip, so make sure to dab it with a towel before trying to read.

Thanks!  This is helpful info!
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: malzig on May 23, 2011, 12:50:29 AM
yep, just look at is as .3 inherent error and .3 temp shift.  The .3's are in opposite directions so they cancel each other out.
Uh...I don't use the strips since I have a meter and the fact that they have proven to be inaccurate in the mash, but I don't think the temp shift enters into this issue.
That's quite an oversimplification.  If you correct for the pH shift at room temperature, ColorpHast strips are both accurate and precise.

If you don't calibrate a pH meter you'll get the wrong pH measurement, but that doesn't make it inaccurate.

Like any tool, you need to know how to use it.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: euge on May 23, 2011, 02:33:24 AM
My Brown ale's mash was 5.2 pH @ 80F on Friday. I'm assuming it was in range.  Just check the pH to make sure the mash isn't off. If it was off I don't have anything food-grade to drop it but I could raise it with some bicarb. 
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: johnf on May 23, 2011, 04:45:09 AM
yep, just look at is as .3 inherent error and .3 temp shift.  The .3's are in opposite directions so they cancel each other out.
Uh...I don't use the strips since I have a meter and the fact that they have proven to be inaccurate in the mash, but I don't think the temp shift enters into this issue.
That's quite an oversimplification.  If you correct for the pH shift at room temperature, ColorpHast strips are both accurate and precise.

If you don't calibrate a pH meter you'll get the wrong pH measurement, but that doesn't make it inaccurate.

Like any tool, you need to know how to use it.

How precise? The design precision is .3 pH. That is not as precise as even a very inexpensive meter.

As for accuracy, they measure .3 low at reference temperature, compared to a calibrated meter at reference temperature.

This isn't a temperature issue, they are just wrong in mash, for whatever reason.
Title: Re: Is pH stated at room temperature or operating temperature?
Post by: malzig on May 23, 2011, 11:32:50 AM
How precise? The design precision is .3 pH. That is not as precise as even a very inexpensive meter.

As for accuracy, they measure .3 low at reference temperature, compared to a calibrated meter at reference temperature.

This isn't a temperature issue, they are just wrong in mash, for whatever reason.
So the pH meter makes an electronic correction for it's inaccuracy and you have to make a minor mental correction.  After that correction they are both accurate.  Besides, it's pretty easy to get a bad reading from a pH meter in complex solutions like a mash.  Some pH meters don't handle ions as simple as sodium very well, let alone complex species like proteins.  Two calibrated pH meters can easily differ by 0.3-0.6 when measuring the same solution.  Don't be fooled into thinking that, just because you get a digital readout that you know the exact pH of the mash.  You really have no idea if the pH is 5.3 when the meter says 5.3.

The strips will tell you that the mash pH is between 5.3 and 5.6.  When the color is between those two spots you can assume it's closer to 5.4-5.5.  I think that's an appropriate precision for brewing, which you may not.