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CaSO4 added to Water > Hach Calcium test reads 4 times higher than expected?

2075 Views 5 Replies 3 Participants Last post by  cliffclof
I added CaSO4 in the amount of 0.1g/L to deionized water. I mixed tested and let sit over night then tested with same results. My calculations by atomic weight would show that I would have ~23.5 ppm Ca and ~56.5 ppm Sulfate based on the Chemical formula CaSO4·2(H20). When I do a Calcium test using HACH Total Calcium + Calcium Hardness both results show a hardness of ~80 ppm. When I test the Sulfate with a DR890 colorimeter using Acuvac Ampuls I get a reading of ~37 ppm SO4. Why? What is happening?

Thank You.


I forgot that the values are probably reported as CaCO3 so I need to multiply by 0.4 to get Ca content. This would still be weird because I would have ~32 ppm Ca and still ~37 ppm SO4. There must be something I am missing.



Plaster of paris is made from calcium sulfate (CaSO4), not calcium chloride (CaCl2). Some brands do contain calcium carbonate (CaCO3). Technically you can use it but make sure you find a product that doesn't have extra additives in it.

About the hydrate part, some compounds like CaSO4 and MgSO4 attract water to themselves so you'll have to take that into consideration when you make your measurements.
Maybe my CaSO4 has some CaCO3 in it or something else impure.
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So the test results pretty much work if I consider that the additive is ((CaSO4)·2H2O + CaCO3)

Is that possible?
Possible. How pure was the starting reagent?
You must be in the water/wastewater treatment field :)
I don't know the purity. It came from or .net with or without s not sure.

I did another control mixture and it added up perfect. 150 mg CaSO4 2H2O in 1 L

Tested at ~5 gpg or 85.5 ppm Ca as CaCO3. It was possibly a solubility issue? I dunno?
This is just because for some reason I loop back and find my own posts on the net when searching things. I found this post while gathering information on impurities of CaCl2 2(H2O). The humidity is up where I'm at and my CaCl2 left out turned to liquid and my test results were off when measuring the dihydrate version so I'm having to assume it is in transitory 4(h2o) to almost 6(h2o) state. It was melting while I was measuring it. :(

Okay, I see what you are after. I what back in time to my Chemistry books,and my CRC handbook and that CaCl2 is most stable in its CaCl2•6H20, but this form has a melting temp of -55ºC. So like you said if you leave it out in the air it turns to a liquid, and that is the CaCl2•6H20 form, I could not find any data for the 4H2O, so I'd assume that that these are rare transitory forms.

If your CaCl2 is still crystallized, it is dihydride . if it's a liquid its CaCl2•6H20.

Hope that helps
Found here:
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