Chemical interaction- is this a known problem?

I usually shock my in ground 18000 gal pool with a couple pounds of shock, but having some trouble getting it cleared up this year, so I decided to use 3 pounds. I had some shock from last year and a new container I just bought so, to use up the old, I put in 2 scoops of it and one scoop of the new one in a large bucket (I regularly have done this in the past with a single brand of shock, no problems). Added water by scooping in the pool, immediately the contents started boiling up bright yellow and orange, threatening to overflow the bucket. I was not sure why this was happening so didn't look like something I wanted in my pool and I quickly set it out in the yard. I appeared to be producing large quantities of chlorine gas which spread out onto the ground and killed a circle 20 feet across in the yard. Looking at the shock containers I had 2 scoops of sodium dichloroisocyanurate dihydrate (99%) and one of Calcium hypochlorite (78%). Got a whiff of this in the process and can say it was life threatening to breath (no joke). So, is mixing these a known bad thing to do? I looked around the net and see sights that say no problem using both in the same pool, but no mention of the mixing part.

Ben needs to address this post. I'll alert him. Welcome to the Pool Forum, by the way!

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Originally Posted by rdkelley

Looking at the shock containers I had 2 scoops of sodium dichloroisocyanurate dihydrate (99%) and one of Calcium hypochlorite (78%). Got a whiff of this in the process and can say it was life threatening to breath (no joke). So, is mixing these a known bad thing to do?

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Yeah. It's known. It could have been worse, actually.

What we tell people -- consistently -- is that they should NEVER mix chemicals outside the pool. You could have added both of those to your pool with no problems IF you'd done so separately.

There are more pool chemicals that can NOT be mixed safely, than can be mixed. Borax is one of the exceptions: it can be mixed with most things -- but the ONLY time you'd have a reason to do so, is when you want to add borax and have tabs in the skimmers. With borax, you don't have to remove the tabs first: with all other chemicals, you do.

Fortunately, some of the pool gases (chlorine, nitrogen trichloride, hydrogen chloride (muriatic acid)) are seriously IRRITATING before they are dangerous. I think ozone is an exception, since I believe it can be dangerous before it really bothers you.

But, the general rule is:

Never mix chemicals outside the pool!

You never want to mix Cal-Hypo with either Trichlor or Dichlor. See the video on this page to see the result of Trichlor with Cal-Hypo. There are many other bad combinations so Ben's rule is the general one.

Thank you for your prompt replies. I am indeed one of those who thought that all shock was the same and had not read the label until after the incident.
Glad I escaped with only a large circle in my yard.

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Originally Posted by rdkelley

Glad I escaped with only a large circle in my yard.

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We are, too!!