I would like to thank AJ deLange for permission to re-post his pages.
pH Meter Calibration Procedure
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. 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.
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 ro 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 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. 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 isolectric 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).