Signs of a failed/failing pH electrode?

[evoo4u]

I have a Hanna 98128, which is giving erratic readings.

I have just now calibrated it (2 point calibration). Then in sample rainwater from a plastic lined tank, it initially read around 6.2, the stability symbol disappeared, but the reading kept dropping for the next few minutes, down to around 5.8.
Rainwater from the other concrete tank initially read 6.8 but it too kept on dropping after the stability symbol disappeared, down to around 6.3.
Dipped the instrument back into the 7.01 solution, and it stabilised pretty well straight away at around 7.03.

If I rinse the electrode, then dip the instrument into the 4.01 solution, it reads pretty well close (4.04), but after rinsing again, the first rainwater sample now reads in the mid 5's. (To rinse, I'm letting running water from the tap I'm about to test from, run over the probe for 20 seconds or so)

So worse than useless (literally) as it stands at the moment.

I'm assuming this is not normal behaviour, so am I not rinsing the electrode well enough between readings, or could the electrode (glass bulb) be on the way out?

 

[Fat Bastard]

Sounds like it's on its way out. Eventually it will also read low in the buffer solutions and not be calibratable.

Do you clean the electrode at all? Or just rinse?

 

[evoo4u]

Do you clean the electrode at all? Or just rinse?

Just rinse - I'm hesitant to touch it with anything.

 

[Fat Bastard]

You could try soaking it in a mild detergent overnight, then sit it in ph7 buffer or storage solution overnight again and see if it works better. I managed to bring mine back that way, (same unit as yours) but be aware that once the coating on the electrode is gone, it's game over and you'll need a new one

 

[Danscraftbeer]

I got a call from the guys I bought mine from. They said they basically give them 1 year for accurate readings. I still bought it.

 

[mr_wibble]

Isn't 6.2 really high for rainwater ?

Teh internets say about 5.6 is average.

 

[evoo4u]

Isn't 6.2 really high for rainwater ?

Teh internets say about 5.6 is average.

"Assuming" my pH meter is accurate, albeit taking an awfully long time to give a steady reading (over 10 minutes), then the rainwater from the polymer lined (BHP Aquaplate) tank reads 5.84, but water from the concrete tank (which is over 30 years old and only ever gets topped up, so some of the water is that old as well!) reads 6.54.

But to resolve the uncertainty, I've ordered a replacement electrode, also some cleaning and storage solution which I don't currently have. I was just using the 7 calibration solution to keep the probe moist in storage.

On the up side, depending on the brew I'm making, I have a choice of water to start with.

 

[danestead]

To me if it is still reading your calibration solutions pretty accurately it should be fine. Have you tested the Ph of your calibration solutions say a month after last calibrating it? If it is still pretty close to the 4 or 7 it should be fine. To me, the reason you are getting erratic reading of your rainwater may be due to a low buffering capacity of your rqinwater much the same way that it is hard to get an accurate reading of PH for RO or RO/DI water.

 

[HBHB]

Biggest problem with all portable/hand held pH meters is that the bulb needs to be stored wet. If it dries out fully, then it will shorten the life to a fraction of it's potential lifespan.

To resolve that, store with the probe end in a bulb wetting solution always or else store it in the pH 7.0 testing solution, in a small cup and seal with plastic cling wrap.

 

[evoo4u]

To me if it is still reading your calibration solutions pretty accurately it should be fine. Have you tested the Ph of your calibration solutions say a month after last calibrating it? If it is still pretty close to the 4 or 7 it should be fine. To me, the reason you are getting erratic reading of your rainwater may be due to a low buffering capacity of your rqinwater much the same way that it is hard to get an accurate reading of PH for RO or RO/DI water.

Good advice - thanks. I'll be doing a brew tomorrow (if we don't get beaten up too badly in the storms rumbling around ATM) so I'll see how it goes with the mash.

Then I'll do what you suggest and see what it reads in the known solutions after a couple of weeks before the next brew.

 

[evoo4u]

...the reason you are getting erratic reading of your rainwater may be due to a low buffering capacity of your rqinwater much the same way that it is hard to get an accurate reading of PH for RO or RO/DI water.

Danestead - you're spot on! Tried it on an actual mash today and it settled very quickly, on 4.98, which was to be expected with the low pH rainwater from the polymer lined tank, and a fair amount of dark grain. After adding some chalk and a little baking soda, pH came up nicely to 5.3.

Anyway, I've taken out a mortgage on the house and ordered a spare probe, so it won't go astray...