Potassium Nitrate Darkening

[BlueComet24]

Earlier today I put 24g of finely powdered KNO3 in a toaster oven to dry at 232°C (450°F). After about 10 or 15 minutes, I noticed that it had partially caked together and had turned beige on top! I immediately removed it from the heat and let it cool, then mixed it 50/50 with sucrose and tested it. It burned just like it should, but I am still worried about why my KNO3 would have changed color.

One reason I thought of is that my KNO3 is impure. It's hydroponics grade with little dark flecks of what look like schist. The sample that I was drying was from the bottom of a somewhat old bag of this KNO3. It had been sealed in a ziplock bag, then transferred to a stump remover bottle for easy access and dispensing.

Before anyone asks, I am quite certain that my KNO3 actually is KNO3. It burns with a lilac flame and generally behaves like it should. I have used this same source of KNO3 for quite some time and am sure it's what it's supposed to be.

When I was heating the KNO3, I did have the oven quite hot and the KNO3 only a few inches from the top heating coils. However, KNO3 melts at 334°C (633°F) and decomposes at 400°C (752°F), far hotter than my oven should have been. I did notice a smell like burning oil, likely from the oil drips in the bottom of the oven, so there were probably oil vapors in the oven.

I do not know why my KNO3 would change color like this and am uncertain about whether or not it should be cause for concern. What do you guys think?

[Mumbles]

If it's pure, it shouldn't change color. It's not an organic material. It can't burn, char, toast, or caramelize like organic materials can. There is of course the chance that some dirt, grime, or organic material made it's way in to the bag during storage, shipping, or manufacture.

 

Are you sure it's turning beige, and not just an optical illusion. My nitrate was pretty wet (like a couple percent water). It turns from translucent crystals/prills into an opaque solid. If anything it honestly looks whiter.

[BlueComet24]

I'm pretty sure. This was several hours ago, so I could be wrong and I'll toast more nitrate and check the color soon. It started out white and opaque and turned to something like RGB 200,180,150 (C8B496) in the darkest areas (on the top). I did have it on a sheet of aluminum to bake, but transferred it to a white tub to mix with sucrose, and it still looked beige.

[MrB]

It's a used kitchen oven with contamination from "snacks past". It's likely to just be an effect of the burning of residue in the oven it self. Clean it out, and try it again?

B!

[Fulmen]

Fertilizers often contain coatings or anti-caking agents that could char or discolor. I doubt it's a problem unless the change is significant.

[Arthur]

100F to 100C is perfectly adequate for drying Pot Nitrate, and doesn't char any coating agents or transfer smoke from previous cooking. Get your own stove NOT the kitchen stove. Hell Hath no fury like.....

[WSM]

Earlier today I put 24g of finely powdered KNO3 in a toaster oven to dry at 232°C (450°F). After about 10 or 15 minutes, I noticed that it had partially caked together and had turned beige on top! I immediately removed it from the heat and let it cool, then mixed it 50/50 with sucrose and tested it. It burned just like it should, but I am still worried about why my KNO3 would have changed color.

One reason I thought of is that my KNO3 is impure. It's hydroponics grade with little dark flecks of what look like schist. The sample that I was drying was from the bottom of a somewhat old bag of this KNO3. It had been sealed in a ziplock bag, then transferred to a stump remover bottle for easy access and dispensing.

Before anyone asks, I am quite certain that my KNO3 actually is KNO3. It burns with a lilac flame and generally behaves like it should. I have used this same source of KNO3 for quite some time and am sure it's what it's supposed to be.

When I was heating the KNO3, I did have the oven quite hot and the KNO3 only a few inches from the top heating coils. However, KNO3 melts at 334°C (633°F) and decomposes at 400°C (752°F), far hotter than my oven should have been. I did notice a smell like burning oil, likely from the oil drips in the bottom of the oven, so there were probably oil vapors in the oven.

I do not know why my KNO3 would change color like this and am uncertain about whether or not it should be cause for concern. What do you guys think?

 

The simplest solution may not be the least expensive to arrange. If you dissolve the potassium nitrate in distilled water, vacuum filter it through slow, laboratory grade filter paper and recrystallize it; all such problems will probably disappear.

 

I'd use caution when heating the potassium nitrate, not to get things too hot or the nitrate may break down to nitrite.

 

WSM

Edited February 19, 2016 by WSM

[Mike]

One other question, if you know that your kno3 is kno3 and pure enough (i assume you are using something like krista-k, multi-k?) And just need to dry it, without diszurbing the additives, why you don' t just use a normak convection oven at 150ºC?

 

A toaster oven even if set only 230ºC temperatur will normally heat the elements up to a lot higher temperature and has direct heat radiation too. The heat radiation can cause higher temperatures then the 230ºC.

[Fulmen]

I'd use caution when heating the potassium nitrate, not to get things too hot or the nitrate may break down to nitrite.

That's not going to happen unless your oven can go past 500°C.

[lloyd]

In any case, Fulmen, you must agree that the temperature need not be that high to properly dry nitrate!

 

We're not trying to drive off any waters of hydration, only to remove any excess absorbed by its hygroscopicity.

 

I find that 125F at low humidity is plenty. Yeah... it takes longer, but if you just store it that way, then there's no treatment period after the first dry-down.

 

LLoyd

[Fulmen]

No argument there, unless you're dealing with water of hydration there really isn't any reason to go above 105°C.

And for potassium nitrate there are good reasons for not going that high either, at least not at first. It's increased solubility at elevated temperatures means that even small amounts of moisture will be able to dissolve a noticeable amount of salt, so the end result can quickly become a solid block. If one starts low, say 50°C (125F) and then ramps up to 105°C (220F) one should get a perfectly dry product with a minimum of clumping.

[lloyd]

And that too! <G>

Lloyd

[Mumbles]

It's turning beige on the top surface, not where it's in contact with the metal, right? I just noticed you mentioned you were using an aluminum pan. I've always used glass personally. Did you notice any corrosion or marring of the aluminum? I doubt they'd really react much in the solid state, but with heat and moisture involved it's not impossible.

[BlueComet24]

I was only heating it so high because I had limited time to dry it and was impatient. I looked at the Wikipedia page on KNO3 and saw that it says its decomposition and melting points are much higher than I would be heating it. However, in the future I will only heat it to around 100°C.

Yes Mumbles, it was only really beige on the top surface. If I recall correctly, it was also very slightly darkened throughout the KNO3. I thought of a reaction between the nitrate and the aluminum and examined the sheet of foil, but it appeared unmarred.

I'm thinking that this darkening must have had something to do with oil vapors. Is it possible that vaporized oil contacted the surface of the KNO3 and burned from the direct heat of the infrared, creating tiny spots of carbon that would darken the previously-white KNO3?

[lloyd]

Blue,

Your last question has to do with the condition and past working temps of your toaster.

 

1) how dirty with old cooking fats was it?

2) was the temperature run high enough in the past to volatilize and/or burn off those fats.

 

If it was dirty, and if you hadn't run it that hot for that long before, there's a good chance that it might be oils being deposited on the material. But the fact that it was amber beneath the surface also says that's probably not the case. It's more likely there was an organic (flow agent or other) added to the KNO3 that charred under that much heat.

 

Don't ever get impatient with pyro processes. They take as long as they take. Pushing them almost always costs more trouble than the time it saves.

 

Lloyd

[BlueComet24]

There was some oil dripped in the bottom. I don't think it had been that hot since the oil was in the oven. As far as impatience; every time I rush something in pyro I learn that it is an easy way to ruin things and create safety hazards. This time I figured it's one chemical and well below its decomposition temp, so what could go wrong? Lesson learned.

[Fulmen]

Don't ever get impatient with pyro processes.

You beat me to it, but I'll say it as well. Impatience will kill you.

[WSM]

When once asked, "What is the most dangerous thing in your pyrotechnics workshop?", someone answered, "Impatience!".

 

WSM

Edited February 19, 2016 by WSM

[BlueComet24]

I know that impatience can be very dangerous. I only thought it might be okay to try to speed up the drying process because it was one chemical, not a composition, and I had checked the decomposition and melting temperatures of KNO3 and they are well above the temperature I heated it to. I think I learned my lesson here, and that's that impatience will almost always cause problems. There is often something that isn't thought of, like that the KNO3 could have organic flow agents that might not take the heat too well. Thank you all for your input.

[Fulmen]

Well, at least you used sound reasoning and did the basic research. And you were right, there wasn't much that could go horribly wrong in this little experiment. Your conclusion is also spot on, impatience will almost always cause problems.