pH meter reading a constant value

Lifted from measuring pH, whose troubleshooting section gives the ordered checks below.

What you see

The meter reads 7.00 in the pH 7 standard, 7.00 in the tap water, and 7.00 in the stop-bath sample. Every reading is stable, plausible and the same.

A related and more insidious presentation is a reading that will not settle at all in a standard, drifting for minutes without arriving anywhere.

Likely causes

In the order they turn out to be the answer.

1. The bulb is not actually in the liquid, or is only partly immersed.
2. The protective storage cap is still on the electrode.
3. The connector is loose or damp. A poor BNC connection gives a dead, stable, mid-scale reading that looks entirely reasonable.
4. The electrode has been stored dry and is responding sluggishly.
5. The bulb is cracked.

What is happening: the chemistry and physics

A combination glass electrode measures a potential difference across a very thin glass membrane between two half-cells — the internal reference inside the bulb and the external reference in the sample. The reading is that potential difference, translated into pH units.

A crack in the bulb short-circuits the two half-cells through the crack. With the two sides electrically joined, there is no membrane potential left to vary with the sample, so the meter reports a constant value — and it reports it steadily and confidently, which is why it does not look like a fault. The same reasoning explains the dead connector — an open or leaky connection presents the meter with a signal that does not change, and mid-scale is what a signal that does not change looks like in pH units.

An electrode that has been stored dry has a different problem. The hydrated gel layer on the glass surface is what responds to hydrogen ion activity, and it takes time to re-form; a dry electrode is slow rather than dead, which is why it drifts instead of sticking.

Diagnostic questions

Work these in order. Each rules out something specific and the first two are free.

1. Is the bulb fully immersed? This rules out the commonest cause and costs five seconds.
2. Is the protective cap still on? That is the second commonest.
3. Measure the pH 4.01 standard. If it also reads 7.00, the electrode is not responding at all and the fault is in the electrode or its connection. If it reads 4.0-something, the electrode is fine and your samples really are all near neutral — which would be a sample-preparation error rather than an instrument fault.
4. Check the connector. Loose, or damp?
5. Try the manufacturer’s shock treatment for a sensor stored dry or responding sluggishly, then recalibrate.
6. If it still reads one number everywhere, the electrode is dead.

!A related fault with a different signature

A falling slope — much less than 59 mV per pH unit — is ageing rather than death, and a large offset with a good slope usually means a blocked reference junction or a diluted reference solution. Neither produces the constant reading this entry describes. Manufacturers publish an acceptable slope window for their own products; this course has not verified a specific percentage and will not invent one, so use your meter’s manual.

Corrective action

Immersion and cap first, then the connector. Then the shock treatment and a fresh two-point calibration at 7 and 10, per the pH meter calibration procedure.

If the electrode still reads one number everywhere after all of that, replace it. The average lifetime of a typical glass electrode is several years, so an old one is a plausible diagnosis rather than a counsel of despair.

Discard every reading taken since the last verification that worked. This is the part that costs something. A meter that reads one number everywhere has been reporting that number for however long the crack has existed, and a pH written into a notebook from a dead electrode is worse than no pH at all, because it will be trusted later.

Prevention

Store the electrode wet, in the storage solution the manufacturer specifies, and never in distilled water.

Calibrate at two points that bracket the range you are working in, and then verify against a standard you did not calibrate with. A two-point calibration will always pass its own two points; only a third solution tests whether the electrode is responding.

Write the electrode’s stated accuracy beside every reading in the notebook. It is the difference between a measurement and a number, and it is what tells you later whether a difference you are looking at was ever visible to the instrument.