Hard Green Deposit on pH Meter Probe - What Is It?

With that lot, iron sulphates in a calcium sulphate matrix are the only possibility, Boss. Crumbs! If you are hammering it off there's something seriously wrong. No wonder you're breaking pH probes!

You need to get a Process Engineer involved to sort out what is going on. There may be other ways of doing it that don't involve pH probes in the reaction mix, you see, and a sludge settling step may be needed before you monitor pH. After all, the pH is just a polooceman before it goes to the sewer, n'est-ce pas?

Have it analyzed, I can't see it well enough from here to tell you what it is.

Or, start soaking it in various chemicals until one works.

Green sounds like copper to me.

Have it analyzed!

Your green scale is calcium sulphate tinted with copper. Calcium sulphate is gypsum, a construction material and is almost completely insoluble in water. What acid are you using to dissolve it off, sulphuric? If you are, it won't work. Try using hot concentrated hydrochloric acid instead. Alternately, you can use sodium hydroxide to neutralize your acid waste instead. This will not result in the formation of a hard insoluble scale.

I would suggest that you immerse your probe in a strong solution of EDTA after each use. This is a powerful chelant that will take most metals into solution as a metal-EDTA complex. Works with all Group 2 metals (magnesium, calcium etc.) and all the first transition metals (Sc,Ti,V,Cr,Mn,Co,Fe,Ni,Cu,Zn). EDTA is usually obtained as the disodium salt and is weakly alkaline in solution. It is readily available, non-toxic and is used in such diverse applications as boiler water treatment and in the human/animal body for removing poisonous heavy metals.

Thanks folks. We are having the deposit analyzed. Lots of trace metals present, including copper, so it may be a lot of things. Switched (on a trial basis) to another means of pH control - working well so far and the deposit (and other deposits as well) have gone away. Formerly hydrochloric acid would not dissolve the green scale on the probe, but never tried hot acid or EDTA. Yes I know it's hard to see scale from there ... just look harder!!