HEDP Overdose and Clouding

[chem geek]

In one of the tip pages Ben Powell wrote here he said the following:

A small overdose of the most popular stain and scale control chemical ingredient, HEDP, will precipitate unfilterable colloidal calcium phosphonate. In English, your pool will look like milk till you drain it.

I would presume that this mostly happens if you use more sequestrant than there is metal to sequester so it starts to sequester more calcium and this apparently diffracts/reflects light in a way that makes the water look cloudy.

Since we haven't heard many reports of this, I wonder how much overdosing is required. I also wonder if one can fix this problem via any sort of clarifier or flocculant (I somewhat doubt it) or if ironically one can add more metals to the pool (such as copper) to tie up the sequestrant. Of course, dilution via partial drain/refill would work.

Richard

 

[JasonLion]

I have wondered about this one before. This clearly doesn't happen very frequently or we would hear about it regularly. Cloudiness does occur often enough, but it goes away without any special effort after a couple of days.

I have a couple of different theories:
1) It actually takes a very large overdose to see this effect.
2) The manufacturers have changed the formulation of their products to avoid this problem.
3) Some additional factor, formerly common, now rare, is required to transform the cloudiness that does occur into something that doesn't go away.
4) It was never true as written.

My favorite guess is #2. If this was a common problem at one time, manufacturers would be motivated to find a way to fix it.

 

[chem geek]

This paper describes the calcium phosphonate precipitate in more detail, but it's still not clear exactly when this would occur in pools.