Why I Don't Have a pH Meter

As an analytical chemist you might expect that I have a lot of fancy gadgets to measure various parameters as I brew. Actually, I have the following: a bunch of thermometers, some of those glue-on liquid crystal temperature strips, a hydrometer, a refractometer, and some pH test strips. I could get rid of the refractometer, I only bought it because it was on sale and is useful as you are sparging to do a quick check on the gravity.

Yes, I just said I don't have a pH meter. This seems to astonish some people. Their reaction is usually to squeal, "how can you use strips, a meter is more accurate?!?" At which point I get to explain that, no, a pH meter is more precise than pH test strips, it is not more accurate. In fact, one could argue that pH strips are more accurate. Really? Oh, yes.

Many people fail to understand the difference between accuracy and precision. Accuracy is how close to the true value a measurement is. Precision is how large a spread the measurement has when it is repeated. There are data which have high accuracy but low precision and data with low accuracy and high precision. A pH meter (depending on how properly it is used) can be both accurate and precise, but is rarely as accurate as people think for various reasons. Test strips are graduated in about 0.25 unit increments (versus the 0.01 unit increments of a meter). They are less precise, by definition. However, they can be more accurate. To understand why, we need to look at the science behind both methods.

First, we need to think about what pH really is. Many people think that pH is a measurement of the concentration of the hydrogen ions in solution. It's actually a measurement of the hydrogen ion activity, which is a function of the hydrogen ion concentration, the temperature, and the ionic strength (the sum of all ions multiplied by their charge) of the solution.

A pH meter is a glass electrode that is measuring the potential drop across its surface. The drop is caused by hydrogen ions moving in and out of the glass as a function of their activity. This voltage potential is referenced to an internal reference electrode, usually a silver electrode in a saturated potassium chloride solution. The voltage generated is dependent upon the activity of the hydrogen ion and the temperature (again). Higher-end pH meters have a temperature sensor and will compensate. If you don't have one of those, you need to calibrate your pH meter at the temperature at which you will be taking your readings. For mash, this is around 150°F. Or, you can look at the little chart on the calibrant bottle and use that value to calibrate while you're at room temperature. Most people fail to do this, but at pH 4.0 the calibrant is off by 0.2 units at 150°F.

Now, the other thing about glass electrodes is that they depend upon a clean, well-hydrated surface so that the hydrogen ions can easily pass in and out of the glass. They can't do this if the surface is dirty (fouled). Nothing is better at fouling electrodes than solutions of proteins or sugars. Like mash. So, you need to clean the electrode with a special cleaning solution every once in a while. And store it in storage solution. And refill the internal standard. All of these activities are necessary, not negotiable.

I work in the pharmaceutical industry and we have specified ranges within which a product has to fall. These are something like 6.5-7.5 when the target pH is 7.0. Why so wide? Because even the high-end pH meters are considered to only be precise to about +/- 0.1 units. So, three standard deviations of that are +/- 0.3 units. What that means is that, if something has a true pH of 7.0 it will measure between 6.9 and 7.1 about 68% of the time. It will measure 6.8-6.9 or 7.1-7.2 a further 27% of the time.

So, let's put that in beer terms. Usually you are measuring the pH of your mash, and you want pH 5.2. Even my high end pH meters will tell me that it's 5.0-5.4. Most of the time it will be around 5.2. Does it really matter if you're more precise than that? Sure, the diastatic enzymes in your mash are balanced in activity between the alpha and beta amylases at 5.2, but that only helps you if the enzymes are in the same amount in each batch of grain. Which you can't know unless you're super anal retentive and measure the enzymatic activity in each new bag you get.

Strips, on the other hand, contain fixed mixtures of indicators; chemicals that change from one color to another as a function of pH. You dip the little piece of plastic into the mash and pull it out. You wait a couple of seconds for the color to develop and you read it. The temperature calibration is effectively self-correcting because the strip has a pretty small thermal mass, so you could assume it warms up to mash temp pretty quickly. As long as you don't mess around and let it cool off while you're trying to read it, you're set. The readout is defined by the mixture of indicators. For a given ratio of indicators, it's pre-calibrated! Just compare the color to the box. Store them so they don't get wet.

In the end, let's break it down in some easy math. Those little packets of pH meter calibration buffer cost around $1 each. You need two of them to calibrate the meter. They begin to change pH by absorbing carbon dioxide from the air as soon as you open the packet. Therefore, you can really only use them once. So, here you are. You can buy $2 worth of pH calibration solutions per brew day or you can spend about $0.40 per strip. That isn't including the initial $50-$100 outlay for a decent pH meter. That you have to calibrate at the proper temperature and make sure has a clean, properly maintained electrode. And which you will have to replace at least every 18 months.

So, strips are plenty accurate and precise for what you need, they don't require calibration, maintenance, or temperature compensation, and to top it all off, they're cheaper than a pH meter. It's up to you, but the next time one of your brew buddies gives you a hard time for dipping a strip in your mash, tell them an analytical chemist says they can stuff it.