Voltage drift over time, pH

[Perreman]

Hi all,
One of my colleagues asked about a problem with the pH-meter. Apparently a Micro-electrode we are using, which was taken into use in May 2015, have constantly been drifting (dropping voltage) over time. Now it had the pH 4.0 buffer outside of set limits.

For now I don't have more information about this electrode, but could it be considered normal aging for an electrode to be worn out in approximately half a year? We are not a large scale lab where pH-measurements is used on a daily basis, the manufacturer however seem to think of this issue as normal pretty much.

My limited knowledge of electrodes still tells me this is weird to wear out an electrode in such a short period of time. What are your input on this issue? Could it be anything else affecting the electrode to over time drop in voltage? Other electrodes seem to work as normal, so it should be electrode specific.

Thank you

[bunnahabhain]

Did you store the electrode appropriatly (usually in 3M KCl)?
Try to replace the liquid in the electrode (usually this is also 3M KCl)
Refer to the manual befor doing this

Good luck
Jög

[ScottHorn]

What is the geometry of the electrode? (spherical bulb, flat surface, spear-type?)
What type of samples has it been used with?

I've read some protocols for "regenerating" old electrodes that have been abused, but depending on the cost of the electrode, it may be better to buy a new one, maybe from a different manufacturer.

[Perreman]

Additional info and response:
The electrode we're using is Orion 8220 BNWP (http://www.thermoscientific.com/en/product/orion-perphect-ross-combination-ph-micro-electrode.html). We have been storing it in 3M KCl and it is changed when the level in the electrode drops somewhat. We are using it to measure saponin products in PBS from what I heard. I don't think these sample types should be that harsh on the electrode, do you?
Regeneration has been tried according to manual, but without success.

A regular non-Micro electrode is also used, but seem to work well for a long duration of time. Is micro electrodes really that much sensitive?

[ScottHorn]

When you say "drifting" and "dropping voltage", do you mean that for any given measurement you take, the voltage is lower (more negative) than it should be? For example does pH 7, which is ideally 0 mV read as -10 mv? Or do you mean than the absolute magnitude of the voltage is lower. For example pH 9, which should be about -120 mv reads as -100 mv instead?

Basically, is it the offset, or the slope that is outside spec?

[Perreman]

The offset is below set specification (voltage more negative than it should be all over the entire range). I checked a logbook quick too see some values.
We do a 3-point calibration from pH 4-9.
Date 2015-05-18:
4: 175.5 mV
7: 1.7 mV
9: -115.4 mV
Slope: 99.9 %

Date 2016-01-11:
4: 147.7 mV
7: -21.5 mV
9: -135.8 mV
Slope: 97.9 % (considered to be within normal variation range I.E. not drifting)

We've ordered new electrodes now, but if you have any suggestions on how to solve this or prevent it in the future I would be very thankful.

[ScottHorn]

Strange, you've got that negative offset across the whole range.

The first thing I'd do would be to look around your lab and find a good working electrode, plug it in to the meter you're using the micro electrode on. You mentioned you have a non-micro electrode that seems to work fine. Try swapping it in. If it show's the same error, the problem is in the meter. Maybe there's an offset knob somewhere that got bumped. Another possibility is that there's a pre-amplifier in line between the meter and electrode, which may also have an offset knob that could get mis-adjusted.

Otherwise, I'm stumped. I don't know enough about electrodes to know what would cause an offset like that across the entire range. I'd be inclined to suspect the meter.