ajdelange

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I frequently get asked about how to calibrate a pH meter. My answer is always the same "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).

Hermit

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You bucking for your own forum section? Nice write up.

jholen

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Thanks for this.

Biobrewer

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I work at Thermo Fisher Scientific in Fremont, CA as a particle scientist, and we have to calibrate our pH meters every time we use them. The unfortunate thing is that every meter is different (buttons, setup, platform), and that makes it a pain sometimes. The fortunate thing is that they should all come with extremely detailed instructions, usually pasted to the instrument as to make sure you do not lose them.

No two pH meters will have the same protocol for calibration, so just make sure you**FOLLOW THE INSTRUCTIONS**!! If you do not do this correctly, your calibration curve will be wrong, and then you can end up with some seriously incorrect sample measurements. If you get an incorrect reading, you will be making additions under false pretenses, which may or may not adversely affect your beer.

Just follow the calibration procedure and always use fresh calibration buffer (bacterial contamination in the buffers will quickly change the pH), and you will be fine.

ajdelange

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That really shouldn't be necessary (unless, of course, your boss says it is). Modern electrodes are incredibly stable. I just went and stuck one in 4 and 7 buffers and read pH values using the cal in the computer which is 2-1/2 days old and the cal in the meter which is a couple of weeks old. For 4 buffer at 21.5 °C the pH is 4.0036. The 2 day old cal gave me 4.008. The weeks old cal, 3.96. For 7 buffer the pH at 21.5 °C is 7.0110. The 2 day old cal gave me 7.015 and the 2 week old cal 7.02. This is an old electrode so maybe it's aging has stabilized but I am just amazed at how stable these things seem to be.

Now I'm not advocating skipping cals but certainly once a day with a check half way through ought to be good enough for most applications. If you are using ±0.02 buffers the amount of drift I'm observing is less than the uncertainty in the buffer.

I think where people get in trouble WRT calibration is not waiting long enough i.e. the meter has it's own algorithm for determining when to measure the buffer voltage. For a good cal you really need to wait 5 - 10 minutes - especially with an older electrode.

I think the other place people get in trouble is relying on ATC. You have no way of knowing what the isoelectric pH of your meter is (well you do but it takes an extensive set of measurements and some hairy math to get a good estimate) and even if you do you can't punch that into any meter I've seen (that's why I do cal and measurement in the computer and, of course, for the recorded data). So it is imperative that sample and buffers be at the same temperature. I'll note that the electrode I spoke of earlier has an isoelectric pH of 8.6 - way out of spec.

Biobrewer

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In fact it is very necessary. Because we are ISO 13485 certified, we have very strict standard operating procedures (SOPs) we have to follow. The branch of Thermo Fisher Scientific I work for makes clinical diagnostics, and if we don't follow the SOP and the FDA or some OEM customer does an audit, we are in serious trouble. We're talkin' corrective actions, fines, and potentially losing your job for negligence. In a highly regulated facility, the thought is: how do you know that someone else didn't mess with it? How do you know the calibration is still good? In a good manufacturing procedure (GMP) lab, the only way to know is to calibrate before every use.

I'm sure fly-by-night labs or academic labs can do what ever they want. In fact, I know they can because I just completed my graduate studies a year ago, and we only calibrated when it gave an electrode error. However, being an ISO 13485 facility, there are rules, and if they are not followed, there is hell to pay.

DeafSmith

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I have a question about step 5.

5. "Turn the meter on, allow it to stabilize for a few minutes, and then lower the electrode into the first buffer solution."
This implies it is OK to have power applied to the electrode without the electrode being submerged.

There was an article by Chris Bible in the September, 2010 issue of BYO magazine "How a pH Meter Works", in which he said to place the electrode in the sample before turning on the meter, and stated under the subheading "Practical tips for using a pH meter":
"6. Turn on the power to the electrode. The power to the electrode should never be on unless the electrode is submerged. Applying power to an electrode when it is not submerged will decrease electrode life."

Is this really a bad thing?

Woodbury419

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I pre and post cal all of my equipment when i brew, from my pH meter to my thermometer. although on the pH buffer solutions I just cheat and buy the solutions instead. But yeah, I do industrial hygiene and environmental work..so all of my equipment I operate must be pre-calibrated or ops checked before use and post-cal'd afterwards. besides, there are some equipment that "drift" when in use. The Sam-935 (radiation meter) drifts every 30 minutes and has to be re-calibrated. thankfully my pH pen does conductivity, pH, temp, and turbidity!

ajdelange

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Figured it was some deal like that.

So I am prompted to ask do you really calibrate the meter i.e. measure slope, offset and isoelectric pH or just slope and offset? And if not the former are all measurements, buffer and sample, made at the same temperature?

ajdelange

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With all due respect to the gentleman apparently he doesn't understand "How a pH Meter Works". The electrode is a completely passive device (unless it's an ISFET type and those still aren't really competing with glass). IOW no power is applied to it. It is, in fact, a feeble battery that supplies power to the meter (though precious little).

Now modern electrodes may put some of the electronics in the electrode body. I have one which apparently contains the preamp, A/D and microprocessor in the electrode as once you calibrate it you can move it to any other meter in that family and resume using it without recalibration. The cal is stored in the electrode itself. In this case power is supplied to the head of the electrode to run all the electronics in it but no power is supplied to the actual electrode.

So not to worry about this. You can safely take the electrode in and out of solutions while the power is on or off. What you should not do, however, is allow the bulb to dry.