I don't remember the details from my A-level chemistry half a century ago but suspect it is to do with the use of high purity rare elements on the electrodes, perhaps plated on, that dissolve during the measurement which involves an electrochemical reaction. Possibly a less pure version is adequate for home use.

The last fourteen years of my working life I was using portable pH meters daily, yes the probes dont have a long life and yes they are not cheap, as stated they are immersed in distilled water and need washing after use. We used Hanna Instruments Leighton Buzzard.

Martin P

Lots of inexpensive ph meters on ebay, advertised for testing swimming pool water. Also used for testing boiler water instead of using litmus paper to determe amount of treatment required.

The pH meters I used years ago had a glassbulb electrode that was stored in potassiumchloride solution after use (and cleaning).

Thor

Hi, I've been a water chemist for more than 40 years. You need to store the electrode in KCl solution and if dead they can be revived with 2m HCl. Ideally don't store them in distilled water. I think it's 2m KCl but will check. You calibrate the meter using buffer solutions at pH 4 and 9 or 4 and 7. These can be purchased cheap off ebay. They are called buffer solutions because they are designed to resist pH change and hence allow an accurate calibration. Between measurements wash the bulb with distilled water and wipe with a tissue. The bulbs are made of thin glass and easy to break, then you need to replace them. Like most things the more you pay the better the electrode.

Typically only accurate between pH 3 and 10. They are a combination of a silver/silver chloride reference electrode and a glass membrane electrode.

The voltage produced between the reference electrode and the glass bulb is proportional to the log10 of the hydrogen ion concentration acording to the nernst equation. Hope this helps

Barry