Sciencemadness Discussion Board

KN03 ---- or not???

jgourlay - 8-9-2008 at 03:46

I bought some "potassium nitrate" off of ebay. Now I'm wondering if I got pranked.

A million years ago my 7'th grade science teacher did the demonstration where you put a match to sugar (nothing), put a match to kno3 (nothing), mix them together and put a match to it (slow burning pile making massive smoke cloud).

I was passing that onto my son yesterday, and it was nothing, nothing, nothing. This make me think it's not KNO3. Is there any way to allay suspicion short of the extreme sports of making gunpowder or boiling it in sulfuric acid to make nitric?

Will (attempted) crystallization or putting some on a toothpick over a flame tell me anything?

ScienceSquirrel - 8-9-2008 at 03:58

You can try the flame test for potassium followed by the brown ring test for nitrate.
It will not tell you how pure it is though
Recrystallisation is very easy and potassium nitrate forms distinctive needles.
The crystals are not hydrated so taking a few grams of the dry recrystallised salt and heating it gently will cause little or no loss of mass.

Saber - 8-9-2008 at 05:23

how fine was the sugar you mixed it with?
Normally you either mix it with icing sugar or melt granulated sugar with the nitrate carefully.
If this still doesnt work hold a few crystals of it in a blue flame (of a bunsen burner), If it gives a purple/lilac colour you have a potassium salt, if not, if its a orangey colour, then you have been tricked.
Next make a solotion of the salt you are testing. Add a soloution of FeSO4 and then drop conc sulphuric acid into the two soloutions... brown rings should appear if there is a nitrate anion present.

jgourlay - 8-9-2008 at 06:02

Thanks Guys, I'll try that. I could also be due to using "suganot" which is some unrefined, brown, large crystal crap my wife buys instead of sugar.

I say "crap", actually it's got a much better, richer flavor that Imperial Pure Cane Sugar.

jgourlay - 9-9-2008 at 03:59

Okay, I'm now more certain I was pranked. I put a little pile of "the stuff" next to a little pile of Alum on a stainless steel plate and played over them them a propane flame.

The alum has a color I wouldn't call "lilac", at least not the flaming Liberace purple I would expect, sort of a yellowy orange with just a hint of blue. "The Stuff", otoh, has a definite sodium orange. I'll do it against salt tonight just to confirm, but I'm pretty sure it's a sodium compound. I don't have any ferric sulfate for the nitrate test.

I have some black phospated deck screws. Could I dissolve one in some H2SO4 to get close enough? Unfortunately I just cleaned up the garage over the weekend, so I don't have any of the typical steel chips laying around next to the lathe.

Also, "the stuff" melted immediately and turned to a glassy substance. This makes me think it's not NaCl. I took the glass and dropped it in a little dish of hot water on the way out the door to work. When I come back this afternoon I'll see if any kind of crystallization has occured--enough at least to see needles vs cubes or dendrites vs crust.

If this stuff turns out to be sodium nitrate, could I take equal weights of "the stuff" and road salt (KCl), dissolve in water and recrystallize to get KNO3 and NaCl?

ScienceSquirrel - 9-9-2008 at 04:05

Alum will not give a good potassium colour in a flame test as it is a hydrated salt and has aluminium in it as well.
Potassium sulphate or potassium chloride give an excellent lilac shade.
You need ferrous sulphate (iron II sulphate ) not ferric sulphate ( iron III sulphate) for the brown ring test.
The easiest way to make it fresh is to add a little fine steel wool to dilute sulphuric acid, warm and filter.
I can't see you getting very far with a black phosphated deck screw :(

You reallly have to make a hot saturated solution and let it cool to do the crystallisation test properly.

[Edited on 9-9-2008 by ScienceSquirrel]

Why didn't I think of that?

jgourlay - 9-9-2008 at 04:10

Geez...steel wool. I'm such an idiot. I have packs and packs of that that I bought for furniture refining before I discovered abralon pads.

"dilute" H2SO4: I really need to go back and review molarity and the definitions of "dilute" and "strong" and "holy crap!" with respect to acids/bases.

Recrystallization: I'll do that. matter of fact, I should go through all this from the beginning with my son. What a great lesson!

ScienceSquirrel - 9-9-2008 at 04:21

If you have a look at the solubility table here.

You will see that you can dissolve ca 15g in 25mL of water at 60C. Try that using a warm water bath, then you want to get it a little hotter than that to stop it dropping out in the filter, say 80 - 90 C. Filter and cool in ice to produce a crop of needles ca 12g.
This extreme solubility curve of potassium nitrate was used to purify it in the old days when it was made from urine etc and also to estimate its purity.
A mass of good saltpetre would disssolve in a certain volume of water at a certain temperature.
If it did not and you were selling to the king then 'a taste of the lash' awaited you.

The concept of dilute sulphuric acid is a bit nebulous, but 2 -5M is about right

[Edited on 9-9-2008 by ScienceSquirrel]

MagicJigPipe - 9-9-2008 at 12:16

If you have even technical grade KNO3 mixing it w/ normal granulated sugar will cause it to be sensitive to flame (i.e. it will burn). Period. If it doesn't burn it's either not KNO3 or it's so impure it might as well not be KNO3.

Yes, there are other ways to test but, like I said, if it doesn't burn you got jipped.

jgourlay - 9-9-2008 at 13:04

Right, I got jipped. Now I need to find out what the stuff really is. Maybe it will be useful, or maybe it will laundry detergent.

Magpie - 9-9-2008 at 14:41

Maybe you can turn this into a real learning experience for you and your son - identifying an unknown! :D

1. Is it soluble in water, dilute H2SO4, alcohol, etc ?
2. Is it flammable? What kind of flame?
3. If an inorganic do flame testing for cations.
4. Then if you can get the test reagents do some standard qualitative analyses for cation and anions.
5. Determine melting point if organic.

kilowatt - 9-9-2008 at 17:22


Also, "the stuff" melted immediately and turned to a glassy substance.

Sodium tetraborate? See if you can acidify it to precipitate boric acid, identified by a green flame test.

[Edited on 9-9-2008 by kilowatt]

jgourlay - 10-9-2008 at 06:16

<<See if you can acidify it to precipitate boric acid, identified by a green flame test.>>

Can I drop some H2SO4 in there and achieve that?

ScienceSquirrel - 10-9-2008 at 06:48

If you have sulphuric acid then adding dilute and then concentrated sulphuric acid to small portions of the material and then heating the samples would be interesting.

It would certainly give you a good indication of the anion.

not_important - 10-9-2008 at 06:54

borates and phosphates come to mind for making glassy melts. Dissolve a small amount of the stuff in water, add a drop of hydrochloric acid and mix, let sit and pour the liquid into another container. Add some concentrated calcium chloride solution, a precipitate could be phosphate, borate, sulfate, or a few others. Adding a little HCL to make it strongly acid will dissolve calcium borates and phosphates. The green flame test for borates can distinguish the two. If adding HCl gives bubbles the likely carbonate or bicarbonate, a drop of lime water on a glass rod held in the off-gas will confirm.

A little of the stuff in a strong warm solution with some strong H2SO4 added would form nitric acid if the material contains a nitrate, dropping in some bits of copper wire would result in the wire quickly dissolving with the formation of brown NO2. If adding the H2SO4 gave a precipitate and copper indicates a nitrate, it could be a fertilizer containing calcium and nitrates, some of these mixtures aren't very energetic when heated with fuels and will 'melt' when heated.

A strong heating - red heat - will fully decompose such mixes, when cooled the solids likely will not be very soluble and the solution will test alkaline. Phosphates and borates and halides will melt, but don't change much and their water extract will not be strongly alkaline.

Almost any organic will char or vapourise, salts or organic acids will burn to carbonates and oxides, and will have an alkaline water extract, or sulfates from sulfonic acid salts (detergents) with near neutral water extract; most of these won't melt but char instead.

Phosphor-ing - 10-9-2008 at 07:43

Please say you gave the seller a negative feedback on ebay!

jgourlay - 10-9-2008 at 08:18

Unfortunately no. I bought this over a year ago, and just now got into the bag. I don't have a record of the seller.