# Reacting Calcium nitrate w/ potassium sulfate



## luckymoney (Dec 31, 2014)

Based on my chem knowledge from highschool, CaSO4 is fairly insoluble so you may be able to do it. Calculate the appropriate amount, mix them, and get the CaSO4 out?

However, be prepare that it will raise your GH because you'll miss some CaSO4 anyway.


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## ChemGuyEthan (Apr 13, 2014)

Yeah, they should react upon mixing. Both of the starting salts are pretty soluble in water, but then the CaSO4 is not really soluble, so it will form a solid upon stirring. Then you will need to filter out the insoluble CaSO4. The solubility of K2SO4 in water is 11 grams per 100 mL, the solubility of CaSO4 in water is only 0.24 grams per 100 mL. This means if you dissolve 10 grams of K2SO4 in 100 mL of water at room temperature, you can add 13 grams of Ca(NO3)2 for a 1:1 mix, then you should be able to collect about 9 grams of CaSO4 (leaving about half a gram behind in solution) and you should have about 11 grams of KNO3 still dissolved. Don't add the Ca(NO3)2 until all the K2SO4 has dissolved, the Ca(NO3)2 is VERY soluble in water so it should dissolve immediately, meaning any solids should be the CaSO4 you want to get rid of. These can obviously be scaled up with volume, and your filter could just be a coffee filter.

This whole thing works on the concept of equilibrium. The nitrate salts of either calcium or potassium are perfectly happy being dissolved in water. The sulfate salts are not so happy. Potassium sulfate is ok, as you can see from the solubility above, but the calcium sulfate really wants to be a solid, not dissolved. So as the calcium ions in solution bump into the sulfate ions, they hold onto each other tightly and start to form solid particles because they like each other more than water. So as the sulfate is removed by the calcium, more of the sulfate will be liberated from potassium as it is pulled out of the solution so now potassium associates itself with the nitrate ions instead.

After initially filtering out the CaSO4, you can probably get more out by adding some alcohol (either isopropyl or ethyl) to the solution to remove just a little more of the CaSO4 since KNO3 is still slightly soluble in ethanol but the CaSO4 is not. Though this might be over-complicating things.

Another option would be to try and crystallize the potassium nitrate. This would be fairly involved though as you'll need to remove all the water, then redissolve in hot water, probably filter out more solids, then let this slowly cool to room temperature and then finally in the refrigerator for more solids. You can probably find a procedure online. Just be careful heating this solution since nitrate salts are technically unstable, though potassium nitrate is not really testy at all.

Or you can, as luckymoney suggested, just use the solid as is but be aware that you'll be adding some amount of calcium to your aquarium. 

Good luck if you go this route!


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## Darkblade48 (Jan 4, 2008)

I am a bit confused; if you live in New York (USA), potassium nitrate should be readily available.


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## tangy_fish (Mar 1, 2014)

ChemGuyEthan your name says it all! Awesome post! Im going to probably go follow the coffee filter method as it is the most simple and just get my GH tested (probably dont even have to) but just incase and for peace of mind.

Darkblade48 I was living in New York for the past year and am now back in the Philippines. Makes me wish I was back in NYC just for the KNO3 LOL


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