Sciencemadness Discussion Board

Ca(NO3)2, K2SO4 to KNO3 - sulfur impurity?

janger - 20-7-2012 at 22:35

Haven't been here in a while so I hope I'm posting in the correct section.

I needed some reasonably pure KNO3 to make aquarium plant fertilizer and not having a source until recently was making it as per:

Ca(NO3)2.4H20 + K2SO4 -> 2KNO3 + CaSO4 + 4H2O

This has another plus - I can use the CaSO4 as a water hardness booster (mixed with epsom salt).

Anyway, once I crudely filter it, boil it down until crystals begin to form, throw it in the freezer and cool to 0 deg C I get fairly clear KNO3 crystals, with a bit of CaSO4 mixed in. I pour the liquid out, and proceed to dry the product before purifying it via recrystallization. This is where things get a bit funky. Upon drying over a steam bath the product turns slightly yellow, and gives off a sulfur dioxide smell. Once dry I dissolve it in as small amount of boiling water as possible. The liquid is a pale yellow color. After filtering it to remove the rest of the CaSO4 and recrystallizing it there's nice clear crystals in this yellow liquid.

What I'm wondering about is where the yellow color and the SO2 smell is coming from. Is it from impurities in the agricultural grade of K2SO4 I'm using? The only other thing I can think of is the calcium nitrate is actually the Ca, ammonium double salt. And the ammonium sulfate produced is decomposing when heated. But I thought that requires temperatures above 250 deg, not on a steam bath. Any ideas? Because it usually takes me a few recrystallization steps to get a product that doesn't smell "sulfury" upon drying and turns into nice white powder.

ldanielrosa - 21-7-2012 at 00:44

That would be my first guess too- for agriculture a little extra sulfur in a sulfate salt does not harm

I did something similar with (NH4)2SO4, which had spilled and been swept up. I purified the sulfate salt by recrystallization before the metathesis RXN so I would minimize my error.

janger - 21-7-2012 at 03:02

Quote: Originally posted by ldanielrosa  
That would be my first guess too- for agriculture a little extra sulfur in a sulfate salt does not harm

I did something similar with (NH4)2SO4, which had spilled and been swept up. I purified the sulfate salt by recrystallization before the metathesis RXN so I would minimize my error.


But the K2SO4 is nice white powder. I dissolved some in water and heated the crap out of it until dry and never got the yellowing or SO2 smell. IIRC it says 41.5% K. Not sure if that's K2O equivalent as used in NPK ratio or not. The recent batch I recrystallized from distilled H2O to make sure there wasn't any strange catalytic/side reactions going on with our town water additives. But the strength of smell and yellow color leads me to believe there's a significant amount of sulfur or unstable sulfurous compounds in there.

watson.fawkes - 23-7-2012 at 07:37

Quote: Originally posted by janger  
Upon drying over a steam bath the product turns slightly yellow, and gives off a sulfur dioxide smell.
I would guess sulfite contamination. The solubility of SO2 in hot water is pretty small, so most of any residual SO2 in the mixture is going to come off as a gas until all the water is gone.

There's a bit of industrial chemistry behind this guess. If you run a contact process plant with a fast input gas stream, you won't get as complete a conversion from SO2 to SO3, but you will increase total sulfur throughput. This mode of operation would be just fine for agricultural sulfur compounds. In this mode, you wouldn't dissolve in water, but in an alkali solution to form the salt directly. Disclaimer: Don't have direct knowledge that this is how it's actually done.