The pH meter is probably the least understood and most abused piece of equipment in the biological lab, probably because it is an undercover agent from chemistry and only a rare biologist likes chemistry. Look over there, it stands in the corner of the lab, covered in dust, glass probe encrusted with crystals.

In a nutshell, the glass probe of a pH meter contains an ionized solution. Placing a pH meter into another solution containing ions produces an electrical current, the strength of which allows calculation of the ionic strength of that solution.

The pH meter probe

1) The probe should not dry out – ever! Should the probe dry out, it will break or at least become unreliable. I once had a run of unsuccessful yeast transformations because the pH meter was wrong by 0.2.

2) To prevent the solution inside the probe leaking, the probe should be stored in a concentrated KCl solution, not in H₂O, and especially not distilled H₂O. Some probes are kept in a special solution with a neutral pH.

3) Some probes contain Ag+ solution (instead of KCl), which reacts with Tris. Before using a pH meter you don’t know, make sure the probe is Tris-compatible – the cheapest aren’t.

Calibrating the pH meter

You calibrate the spectrophotometer each time you measure the optical density, of a microbial culture for example, by using a blank. The same should be done with a pH meter. This is done by calibration, and should be performed every time you use the instrument or at least every morning using the provided standard solutions. The pH meter should be calibrated with at least two (pH4 and pH10), and better still three (pH4, pH7 and pH10), of the standard solutions.

Using, not abusing, the pH meter

The probe should always be wet. Therefore you have a choice between (i) keeping it in your solution during pH-ing, and (ii) taking it out, washing in deionized water, blotting with paper, putting in storing solution and washing again before putting it back in to your solution. The first choice is fine if you have quite a lot of solution, but care must be taken while using a magnetic stirrer: a medical student (they’re supposed to be super-smart) in our lab smashed a probe to pieces during their first day. The second choice is safer but very tedious.

pH-ing Solutions

1) Always pH your solutions at the temperature you’ll be using them; pH is temperature dependent.

2) Do not add concentrated acids to pH your solutions – they are corrosive and can change the pH of most the solutions suddenly, such that you can overshoot easily.

3) Add diluted acid to H₂O and not the other way around: because acid is heavier and heats up while dissolving you can end up with boiling acid droplets spraying on you.

4) Leave enough volume for adding acid/base to your solution, but not too much- no more than 25%; pH can change significantly after final dilution.

Further Reading

Thermo Scientific pH Electrode Handbook – Thermo Scientific Home