General Category > Equipment and Software
How to use a pH meter
mabrungard:
Calibration within the range of interest is an important quality assurance measure when using a pH meter. Since mashs are acidic, calibrating at the pH 4 and 7 points is most useful.
Calibration solutions are composed primarily of water with buffering ions that stabilize the solution pH at the desired reference value. Interaction with the atmosphere introduces CO2 and other contaminants that will cause the pH of the solution to drift. Most calibration solutions are date stamped and they do have a finite lifespan after opening. The time to pH drift is not well defined since it would depend on a lot of external factors. I've heard that the solutions should be changed every year or two. We aren't working with critical issues in mashing, so I feel that the longer timeframe is probably OK.
Of course, keep the unused calibration solutions capped as much as possible and avoid cross contamination. DO NOT immerse your pH probe into the bottle of calibration solution. You want to avoid contaminating that supply. Always pour a small amount into another small container for the calibration use. Use separate small containers for each solution.
This is not completely good advice, but I use the bottle caps from my pH solutions as the small containers for pH calibration. I fill each cap with their respective solution and dip the probe into the solutions. The probe is rinsed with DI or RO water after each immersion and I blow off the excess liquid by mouth prior to moving the probe from one solution or rinse to the other. Don't touch the glass probe. Its fragile and oils on skin can contaminate the probe. The solution in the caps is discarded after each calibration event. Never pour the used solution back into the bottle. I shake out the bottle cap after discarding the spent solution, but clearly the opportunity for a little bit of contamination exists because that cap goes back on the bottle.
I don't think that pH calibration solutions should be used as probe storage solution. They do not have the proper high Potassium concentration needed to keep the probe healthy. You need to keep the probe in commercial storage solution or you need to create some. I did use pH 4 solution as the starting point for storage solution I created. You have to add a certain amount of potassium chloride to the pH 4 solution to create storage solution. I don't remember what the ratio of KCl to solution was. You'll have to google it.
Always rinse the probe with distilled or RO water between every immersion in wort or water since there are plenty of contaminants or dissolved solids that could coat the probe. I also suggest that occassionally dipping the probe in a strong acid and a strong base can help dissolve stubborn deposits.
Enjoy.
denny:
Excellent info as always, Martin! Thank you!
bluesman:
--- Quote from: mabrungard on October 11, 2011, 07:43:54 AM ---Calibration within the range of interest is an important quality assurance measure when using a pH meter. Since mashs are acidic, calibrating at the pH 4 and 7 points is most useful.
--- End quote ---
I believe it is also important to correct for temperature. Measuring the wort pH and compensating for it can be a significant factor in pH measurement accuracy.
Thoughts Martin?
mabrungard:
Indicated pH is dependent upon the temperature of the liquid. In addition, there are other reasons why the user should standardize the temperature at which they perform their pH measurements.
pH meters will register differently as the temperature of the liquid varies. There are a couple of reasons why this occurs. The first is the difference in how the meter and it's electrochemistry operate with temperature variation. Manufacturers have incorporated temperature correction circuitry into higher-end meters to take care of that problem (temperature compensation). That feature is helpful, but it DOES NOT correct for the change in mash pH due to differing chemical activity related temperature change. The dissociation of ions in the mash varies with temperature and that creates an actual change in the mash pH. That change is reported to be up to 0.35 standard units, but both Kai Troester and AJ DeLange have reported that they have only observed about a 0.2 standard unit difference. In any case, the pH at mash temp will be lower than at room temperature.
So even with a temperature-compensated pH meter, you may not be reading the correct mash pH or getting a value that is useful.
The other reason that brewers should avoid measuring mash pH at mashing temperature is that the high thermal stress placed on the probe's thin glass bulb. Going from room temperature to mash temperature by plunging the probe into the mash will shorten the probe's life.
For the reasons above, pH measurement for the mash should be done at room temperature. There are fewer side effects in performing pH measurement at room temperature and it also means that you don't have to buy the more expensive temperature-compensated meter!
James Lorden:
Since the mash pH at room temperature should be higher then mash pH at mash temps do you know at what temperature level the common preferred mash pH levels are quoted. (e.g are we looking for 5.3 at room temp or 5.3 at mash temp). I've always assumed 5.3 at mash temps so I look for about 5.5 for room temperature measurements.
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