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ajdelange 02-08-2012 02:16 PM

pH Meter Calibration and Use
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I frequently get asked about how to use and calibrate a pH meter. Let's start with use and then move on to calibration.

Measurement of mash pH is the use to which meters are most often put by brewers. Assuming the meter is calibrated (see below) here is how that is done.

1. Stir the mash thoroughly. This is especially important if the measurement is to check on the effects of an acid or alkali addition. Withdraw a small sample of the liquid. It doesn't matter if some grain is included.
2. Cool the sample to room temperature, ideally the same temperature as the buffers you used for calibration. This prolongs electrode life and reduces the burden on ATC (more on ATC below). If you use a small metal saucepan you can achieve the cooling quickly by immersing it in cool/cold water.
3. Rinse the electrode with DI water, shake off and blot (see calibration below) and insert the electrode into the sample. Move the electrode around for a few seconds (sample rinses any water off bulb and junction) then stop and wait until the reading is stable. Some meters will decide when the reading is stable for you and beep to signal this. Record both the reading and the temperature.
4. Repeat the process every 5 minutes or so until the readings stop changing. This usually 15 - 30 minutes after strike.
5. Rinse the electrode with DI water and return to storage solution or just tap water for short term (i.e. between readings) storage.

For calibration the overall instructions are simple: follow the manufacturer's instructions. For those who don't have a meter in hand and want to have an idea as to what is involved or for whom the supplied instructions are less than adequate the following is offered. Buffers and samples should be at room temperature.

1. Store the electrode in a storage solution recommended by the manufacturer. This will often be a saturated or nearly saturated solution of potassium chloride (see below for more on storage solution).
2. Prepare fresh 4 and 7 buffer solutions using deionized water. Several manufacturers sell capsules of powder which contain the buffers’ chemical components. These are simply added to a specified amount of DI water (50 or 100 mL) just before use. Premixed buffers are also sold in sealed packages (similar to the ketchup packages from fast food restaurants). These work as well as the buffers one mixes on the spot and are obviously more convenient but tend to be, because of the packaging, more expensive. Premixed buffers are also sold in bulk i.e. 1 L bottles or 4 L jugs or cubitainers. If buffers in this form are being used check that they are not beyond their expiration dates and pour small amounts of each into a clean beaker or preferably, sealed container, at the beginning of each brew day. Do not return used buffer to the bulk storage.
3. Remove the storage cap from the electrode. If the electrode is the refillable type, insure that it contains adequate fill solution, top up if neecessary and, whether you top up or not, open the fill hole so air can enter the electrode body allowing fill solution to freely flow out through the reference junction.
4. Rinse the electrode with a stream of DI water from a wash bottle. Blot dry with clean tissue or paper towel. Don’t touch the actual electrode bulb when you do this. You don’t need to get all the adhering water, just the bulk of it. Wicking of water into the paper is adequate.
5. Turn the meter on, allow it to stabilize for a few minutes, and then lower the electrode into the first buffer solution. With most modern meters it does not matter which one you go into first as these meters have automatic buffer recognition. Following the manufacturer’s instructions put the meter in calibration mode and initiate calibration if necessary (e.g. press the ‘read’ or ‘Cal’ button).
6. Move the electrode around in the buffer a little to rinse any adhering DI water off the bulb and away from the reference junction.
7. Wait until the reading stabilizes. Modern instruments tend to have stability indicators which beep or otherwise alert the operator when the reading is stable (hasn’t changed by more than a threshold amount in a given period of time). These often also instruct the operator to move on to the next buffer when stability is detected. In others you may have to determine when the reading is stable yourself and indicate this to the meter by pressing a button. Follow the manufacturers instructions and/or prompts on the meter’s display.
8. When instructed to move to the second buffer, remove the electrode from the first buffer, shake adhering buffer off and rinse with a stream of DI water. Blot away as above and insert the electrode in the second buffer. Move electrode around in second buffer.
9. When the second reading is stable, take whatever action is necessary to complete the measurement as above. In some meters there will be an option for a third buffer. In those meters you will have to do something (e.g. press an ‘exit’ button) to indicate to the meter that calibration is complete if you are doing a 2 buffer calibration.
10. The instrument will now calculate the calibration parameters (slope and offset) and, in some cases, display these to you in the case of slope either as a percentage (should be near 100) or a number like 57.3 which is the number of millivolts change per unit change in pH at some reference temperature. The offset will be a millivolt number which should be small i.e. a few millivolts (it can be negative). If the meter presents those numbers, write them in your log book. They represent a record of the rate at which your electrode is aging. Fancy meters will automatically store the calibration data, tagged with time and date, in the meter’s memory.
11. Take whatever action is necessary to indicate that the calibration is to be accepted (e.g. press a ‘store’ or ‘exit’ or other button as directed by the manual).
12. Remove the electrode from the second buffer. Shake, rinse and blot as before. Place in sample.
13. Press ‘read’ button if necessary. Otherwise monitor display. Move electrode around in sample.
14. When reading is stable (as determined by you or meter electronics) record pH and temperature. Fancy meters will automatically store these in memory and some will even transmit them to an external computer.
15. Remove from sample, rinse and blot dry as before. Move to next sample. If finished, rinse extra thoroughly. After shaking and blotting dry insure that cap contains sufficient storage solution to cover bulb and replace cap. Turn meter off if finished for the day. If not finished for the day the probe can be left in the last sample.

11b. As a check on the calibration you can measure the 4 and 7 buffers again at room temperature. You may wish to do this after some time has passed or even after you have finished measuring samples for the day. When specifically checking a meter for stability as in the case of a new meter or one that is suspected of degraded performance you should run a fresh calibration and then remeasure the pH 4 buffer over time every couple of minutes up to about 20 minutes and then every 10 minutes or so for couple of hours or more. Record the values and if possible plot them against time. Readings all over the graph (e.g. 4.00, 4.05, 4.07, 3.98, 3.89, 4.10, 3.90...) are indicative of general instability. Readings that cluster around a straight line with a slope are indicative of drift (e.g. 4.00, 4.04, 4.06, 4.08, 4.11, 4.15...).

pH values are often printed on the buffer package. Sometimes they are not. If not and assuming you are using NIST traceable pH 4 and 7 technical buffers the pH values of the buffers are:

pH 7: 1911.4/K -5.5538 + 0.022635*K - 6.8146E-6*K*K

pH 4: 1617.3/K -9.2852 + 0.033311*K - 2.3211E-5*K*K

where K = °C + 273.15 (i.e. K is the temperature in Kelvins).
The values you read should be close to those given by the formulas or on the buffer package. If they are not then your meter is drifting.

Cool the 4 buffer to about 40 °F and measure its pH. Do this right after completing calibration. If your meter reads off by more than a few hundredths then its isoelectric point is not equal to 7 and you must be careful to measure buffers and samples at close to the same temperature (ATC won't work well).

ATC - What Is It?

This is a subject that has confused many a brewer encountering pH meters for the first time. A pH meter has an electrode which produces a voltage that depends on the pH and the temperature of the sample being measured. The voltage is

V = Vo - s*(R/F)*K*(pH - pHi)

Vo is a small 'offset' voltage. It is determined during calibration. s is the 'slope', close to 1, also determined during calibration. R and F are physical constants. K is the temperature in Kelvins (K = °C + 273.15). pHi is the 'isoelectric pH', i.e. the pH at which the electrode response in insensitive to temperature. It is 7, or very close to it in modern meters and is always assumed to be 7 by such instruments. The pH meter calculates and dispalys

pH = pHi - (V-Vo)/(s*R*K/F)

Thus, as the temperature increases, the magnitude of the electrode voltage increases but the reported pH does not because the meter divides by the temperature in calculating its interpretation of the electrode voltage. This is ATC - Automatic Temperature Compensation. Since almost all meters are now digital which implies a calculation like that of the last formula being done in a microprocessor nearly all modern meters are ATC capable. ATC only compensates for the electrode response to temperature. It does not respond to changes in the pH of a sample as temperature changes. As noted in 11b above, even buffers designed for meter calibration have pH values which change with temperature as the formulas given there indicate. To repeat: ATC does not compensate for those changes. The actual pH of mash, for example, decreases by about 0.0055 pH/°C. Even if your meter is equipped with properly functioning ATC you need to allow for actual mash temperature if it is more than a few degrees above room temperature. All pH measurements in brewing are referred to room temperature. If measured at other than room temperature they should be corrected by 0.0055 pH per centigrade degree of difference.

Measure at room temperature. You won't have to do the corrections that way, room temperature avoids thermal stresses on your electrode and it will last longer and, as your buffers are at room temperature, ATC will work well even if your meter's pHi is not exactly 7.

Storage Solution

It is important that the electrode be stored in a manner consistent with the manufacturers instructions. These can vary somewhat but the most usual recommendation is that one buy a storage solution made by the manufacturer for his electrode. These solutions are, as noted above, usually saturated or near saturated solutions of potassium chloride which may include mold inhibitors and a buffer. The reason most electrodes use saturated KCl is because the electrode's reference half cell contains saturated KCl. The interior of the reference half cell communicates with the sample through a 'junction' which is often a small piece of fritted glass though on the electrode I am looking at as I write this it is more like a wick that passes between the body of the electrode and the face through which the temperature sensor and pH bulb protrudes.

When the electrode is placed in sample the concentration of potassium and chloride ions is lower in the sample than in the interior of the reference half cell and ions flow to equalize those concentrations thus conducting the electric current necessary to complete the circuit: Interior of pH bulb glass -> bulb electrolyte (inside bulb) -> pH electrode (platinum wire inside bulb) -> pH meter -> reference cell electrode (platinum wire inside reference half cell) -> reference cell electrolyte (KCl), through the reference junction into the sample -> outside of pH bulb, through bulb glass to bulb interior. When the electrode is placed in storage solution the concentrations of K+, Cl- and water are the same at the outside of the reference junction as at the interior of the reference half cell and, as no concentration gradient exists, no ions or water flow. The electrolyte within the reference cell is thus preserved.

Buffering the pH to the proper value maintains the proper ion distribution within the active layer of the outer surface of the pH glass (bulb) membrane and, of course, the water keeps this layer hydrated.

Meter Recommendation

I am always being asked for recommendations for pH meters and this has, in the past, been difficult because good meters have been priced out of the reach of most home brewers. The inexpensive ones seem to suffer primarily from instability and this is why we gave the stability check instructions earlier. A hint that the meter may not be stable is an indication that the precision is 0.01 but the accuracy is appreciably worse that the precision. A meter is accurate to its precision at calibration. If the calibration does not drift then it will stay accurate. In the fairly recent past tremendous improvements have been made in electrode technology which is as much art as it is science. I believe most of this to be in the area of junctions but I think getting the impedance of the sense glass down may be part of it too. This leads to greater electrode stability. These are what I think the requirements for a good meter are:

1) Stability.
2) 2 Buffer calibration
3) Electrode longevity
4) Junction resistant to fouling by sugars, proteins
5) Lets the user decide when to accept a buffer calibration reading
6) ATC
7) Automatic buffer recognition

1) is the most important. 2) is also very important as one cannot get good accuracy half way between standard buffer values (i.e. mash pH) with single buffer calibration. Conversely with 2 buffer calibration one can get slightly better accuracy that the accuracy of the buffers themselves. 3)Electrodes tend to be the most expensive component in an inexpensive meter. You want an electrode that lasts 2 or more years. 4)This used to mean a double junction electrode but I think some of the new single junction designs are as good, in this regard, as double junction ones. 5)The Hanna pHep meter is quite stable but decides when to accept the calibration reading on its own. It grabs it too early. This throws the calibration off. The operator should make the decision as to when to accept the buffer reading and should only do so when he is positive it is stable. 6)ATC is important more because it signals that the processing is digital than because of a real need for temperature compensation. ATC isn't perfect at doing what it is intended to do unless the electrode is perfect but it is fairly easy to determine the relevant perfection and correct the ATC where very fine work (correction to 0.01 - 0.02 pH) is desired. 7)This just makes calibration a bit easier.

The only inexpensive meter I have met so far that meets these requirements is the new Hach PocketPro+. As it is new I can't of course, comment on the longevity of the electrode nor whether it will plug up with protein. Watch this space.

I hope that this unit will prove over time to be the answer so many have been looking for. Beyond that I hope it represents an advancement in the art that will result in many stable meters in this price range.

The only other inexpensive meter I have any experience with the the Hanna pHEp. It appears to meet all the requirements above except 5 and I'm not sure about 4. This is really too bad because it seems to be quite stable. One can easily overcome this problem by calibrating, then doing the cal check and finding where the meter stabilzes during the check. If, for example, the buffer pH is 4 and the meter stabilizes at 3.98 then just add 0.02 to each meter reading.

Also keep firmly in mind that I have checked only one meter of each type.

The Milwaukee MW102 is getting good grades here on HBT.

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