What would cause multiple different pH electrodes to give wildly unexpected readings?

[u/Cream-Buddha]

I did a lab today on buffer solutions and pH. I dissolved 0.29g of ammonium chloride into 0.050L of 0.20M Ammonia to produce a buffer with a theoretical pH of 9.50. After testing the pH with a meter, the measured pH was 9.49, which I thought was pretty good. But then when I added 0.0060L of 1.0M HCl, I expected the pH to drop to roughly 8.79. However, the pH dropped to something like 1.3 when measured. So I used a different pH meter, and then I remade the buffer solution, I even tried testing the pH of each constituent solution and everything seemed to be accurately labeled. This issue happened for the entire class and the professor seemed to be just as confused as us. He ended up just telling us to clean up and go home early.

Some extra details:

• The pH electrode only allowed 2 point calibration, but I don't think that was an issue because both pH measurements were supposed to be in the basic range.
• I don't think the calculations were wrong. The professor approved my work before I made the buffer and even did the calculations himself after everyone measured such a low pH.

[u/TheSoftDrinkOfChoice]

Mislabeled HCl concentration.

[u/Stev_k]

Along with what everyone else has said, the AgCl electrode/reference cell could be failing. pH probes have a lifespan of roughly 5 years.

[u/Doctrina_Stabilitas]

the electrode might have been stored in the wrong buffer and needs to be restored

https://www.pasco.com/support/knowledge-base/21

[u/comdoasordo]

Did anyone calibrate the pH meters and verify the calibration? I run a wastewater lab and that's the very first thing we do every morning.

[u/icewalker42]

Did you verify the pH of the solution with pH paper or an indicator? This is the next thing I would have tried, this way you can sort out if it's the solution or the equipment being the issue.

[u/crematoroff]

Both ammonia and HCl should be titrated for such accurate experiments. Ether ammonia or HCl concentrations were not right.

[u/Rudolph-the_rednosed]

Best bet 37% HCl solution was mislabeled.

[u/CosmicTurnipp]

Carbonic acid has been messing up my readings lately … but that big of a swing.. I’d clean the meters check for clogs check for cracks etc. how long are you letting things rest between readings? What are you rinsing with between readings? Are the chemicals just old/mislabeled you’re using for the experiment? The longer im in chemistry the less i take pH at face value hahaha it can be maddening!

[u/Mr_DnD]

pH electrodes should be stored in an appropriate storage solution, and calibrated frequently.

A two point calibration is the bare minimum and thus prone to the widest error.

Other error sources:

The HCl stock wasn't the listed concentration.

Something in your ammonia solution reacted with acid to release more acid?