Reef Chemistry Question of the Day #9

[Randy Holmes-Farley]

Since the last question was kind of hard, and its a weekend, this one is a bit easier.

What is the scum that forms on the surface of limewater (kalkwasser)?

Good luck!

 

[briany66]

Its Lime right? The crust at the water line from exposure to air.

 

[DRThompson]

Calcium carbonate. The high pH kalkwasser solution is reacting with CO2 in the air; that's why it forms on the surface.

 

[DFW]

I never have seen any scum forming there. And I have been using limewater for going on 6 years. Is it because I mix a small batch each day and then close it up in the drip jug right away that I am not seeing this? All I see is a clear solution above sediment on the bottom of container. Thank you!

 

[fishroomlady]

I think all of your questions are hard randy

 

[Randy Holmes-Farley]

Here's a picture of what I'm referring to (on top of the water)

 

[Randy Holmes-Farley]

Yes, our responders are exactly correct: the scum or crust is calcium carbonate that precipitates when carbon dioxide enters the high calcium, high pH solution.

Preventing CO2 from getting to the limewater will prevent the crust. The crust is not a problem in most cases, however, and it can greatly reduce the entry of more CO2 into the water.