Calibrating a pH meter

Purpose

To put the meter into a state where the number it shows can be believed, and to find out at the end how far it moved while you were using it.

When to use it

Before every session in which a pH reading will be written down — not weekly, and not when the readings start looking odd. The constant the meter needs in order to turn millivolts into pH gathers up several contributions that change from day to day and from electrode to electrode, so a calibration is what makes today’s reading mean anything.

Before you start

You should already have run the pH measurement lab, which is where every step here comes from, and read your own electrode’s manual — electrodes differ, and where the manual disagrees with this page, follow the manual and note the disagreement in the log.

On the bench: the electrode, which should have been standing in its storage solution; fresh pH 7.00 and pH 4.01 standards; a third, unopened pH 4.01 sachet as a check standard; distilled water in a wash bottle; lint-free tissue; a waste beaker; a thermometer; the calibration log.

Procedure

1. Take the electrode out of its storage solution and confirm the bulb has been wet.
2. Bring the standards and the samples to the same temperature, and write that temperature down.
3. Rinse the electrode with distilled water, into the waste beaker.
4. Blot it dry against lint-free tissue. Do not wipe or rub it: rubbing builds a static charge on the glass that takes minutes to leak away and makes the reading wander meanwhile.
5. Stand it in the pH 7.00 standard, bulb fully covered and clear of the bottom, and swirl gently.
6. Wait until the reading stops moving — stops, rather than slows — then set the meter to 7.00. That sets the offset.
7. Rinse, blot, stand it in the pH 4.01 standard, wait again, and set the meter to 4.01. That sets the slope.
8. Write down the slope, if your meter shows it, in millivolts per pH unit or as a percentage of theoretical.
9. Verify against a standard the calibration has not seen: open the third sachet and measure it. Record the reading.
10. Measure the samples, in order of increasing pH, rinsing and blotting between each, and record the pH, the temperature and the time for every one.
11. Measure the pH 7.00 standard again at the end, as though it were a sample. The difference from 7.00 is the session’s drift.
12. Rinse the electrode thoroughly and return it to its storage solution before you write anything up. The electrode comes first; the notebook can wait five minutes.
13. Write the calibration log line.

How you know it worked

• The check standard in step 9 read within your meter’s stated accuracy of 4.01.
• The slope is written down and sits alongside the last few entries in the log, so that a decline is visible as a trend rather than as a surprise.
• The closing reading in step 11 is close to 7.00, and how close is recorded rather than judged.
• The electrode is in its storage solution — not in distilled water, which draws the reference salt out through the junction, and not in air.
• No standard that has had an electrode in it went back into a stock bottle.

If a step fails

Step 1, the electrode has dried out. Soak it for at least eight hours in the storage solution its own manufacturer specifies, then calibrate. A dry electrode can be calibrated and its readings believed, which is the worst of the available outcomes.

Step 6, the reading will not settle. Give it two minutes. If it is still moving, rinse, soak in pH 4 buffer for an hour and try again. A slow response is the classic first sign of an electrode that has been stored dry or is near the end of its life.

Step 7, the slope is far below 59 mV per unit. Slope falls as an electrode ages. This course has not verified an acceptable window for any particular product and does not invent one, so use your meter’s manual — but note the pattern: a falling slope with a good offset usually means an ageing bulb, while a large offset with a good slope usually means a blocked reference junction or a diluted reference solution.

Step 9, the check standard is outside the stated accuracy. Do not go on to the samples. Recalibrate once, with fresh standards. If it fails again, suspect the standards first — a bottle that has had an electrode in it is no longer a standard — and the electrode second.

Step 10, the meter reads one number everywhere. Work through it in this order, because each step rules something out for nothing: is the bulb actually immersed; is the protective cap still on; does the pH 4.01 standard also read that number; is the connector loose or damp; does the manufacturer’s revival procedure change anything. If it still reads one number everywhere, the electrode is dead: a cracked bulb short-circuits the two half-cells and gives a steady, plausible, mid-scale value.

Step 11, the closing drift is large. Every reading in the session carries it. Write the drift in the log, and report the session’s numbers as a range rather than as values.

A sample of distilled water will not settle. That is documented behaviour rather than a fault: with almost no ions present the junction potential is large and unstable, and dissolved carbon dioxide genuinely acidifies the water. Record the range you saw and move on.

What to record

One line in the calibration log per session: the date, the temperature, the offset and the slope, the check-standard reading from step 9, the closing drift from step 11, the standards’ lot and the date each was opened, and how old the electrode is. Kept in one place at the front of the notebook, those lines show an electrode dying months before it fails outright — which is the only warning you are going to get.