Purifying Potassium Nitrate (KNO3)




About: I love explosives and the chemistry behind them.

Song: Battle Without Honor or Humanity by Tomoyasu Hotei

   Procedures for purifying chemicals are being invented and re-invented all the time. Purification depends very much on the chemicals that are considered the “contamination”, and their properties compare to the chemical you are trying to purify. The mixture that you are trying to purify will dictate how easy the purification process will be. There are no– "one fit all solutions", and indeed, some chemicals are so hard to purify that the finished purified product will fetch amazing prices.

   Recrystallization is a physical method targeting the purification of compounds. This procedure involves dissolving a compound in a hot solvent to form the saturated solution followed by cooling down the solution. The dissolved compound would crystallize from the solution due to the difference in solubility at high and low temperatures. This method is especially effective for potassium nitrate (KNO3) since its solubility in 158g of water is 247g at 100 degrees C, and only 13.3g at 0 degrees C.



Stump remover that contains Saltpeter (KNO3)

Water (H2O)

Paint stick or spoon

Pot or pan


Funnel (optional)

Hot plate

Safety glasses

Step 1: Measurements

   First, weigh out 247g of stump remover and set it to the side. Now measure 175ml of H2O (which is about 158g in weight). You can multiply these numbers if you would like.

Step 2: Dissolving the Salt

   Pour the water into a pot or pan and bring the water to a boil on a hot plate. When the H2O starts to boil, lower the hot plate’s temperature and empty the stump remover into the pot.

   After that, stir the mixture with a spoon or paint stick, preferably with a paint stick, until the stump remover has completely dissolved.

   Continue to heat and not boil the solution as you stir it for a few minutes. Add a little more water to the solution if you notice that plenty of the H2O has evaporated. The amount of water is not critical however; you do not want too much or too little water in your solution.

Step 3: Cooling Down for Recrystallization

   Now turn off the hot plate and pour the solution into a glass jar after heating it for about 3-4 minutes. If needed to so you don’t make a mess, funnel the solution. When you’re done pouring the solution into your jar you may have to remove some of the saltpeter that has already begun to recrystallize inside the pot by scraping it out with your paint stick. If you remove any saltpeter from inside of the pot, put it into the solution.

   Allow the solution to cool down to room temperatureand you should see formation of salt crystals. Place the jar in the refrigerator for 24 hours once the solution has cooled down to room temperature. This will allow the KNO3 crystals to fully grow.

Step 4: Filtering & Absorbing the Solution

   When 24 hours have passed and the crystals have fully grown, empty the remaining solution out of the jar. Place your paint stick at the rim of the jar to insure that none of the saltpeter will flow out as you dispose of the unwanted solution. You could instead place a coffee filter over your jar and filter the solution. You may not have to worry about doing any of that because the salt crystals may have just formed into one solid mass sticking to the inside wall of the jar.

   Once you are finally done filtering the contaminant(s) extract the KNO3 from the jar using your paint stick or spoon. Place your product on two pieces of paper towel that are folded in half together. Fold the paper towels in half again so that one half is overlapping the KNO3. Press down on your product so that both paper towels absorb the moister. You may want to grasp your paper towels with your saltpeter still in it, in a way as to ring out the contaminant(s) but be careful not to tear the paper. Repeat the process until the saltpeter is no longer sopping wet.


Step 5: Drying Your Product

   After you have absorbed most of the moister set your final product in a plastic container or on a wooden board and allow it to sundry if possible. If you place a lid on the container leave an opening so the remaining moister can evaporate into the air.

   After about 24 hours your potassium nitrate should be completely dry and ready for use. Depending on the environment your product is drying in it may take longer than 24 hours for it to dry.

Step 6: Note:

-Take notice that the solution has a discoloration when compared to clean H2O. This is due to the dissolved contaminants in the potassium nitrate. The stump remover brand I used is called Grow More Stump Remover & Potassium Nitrate and its ingredients are derived from KNO3.

Potassium Nitrate (KNO3)………………..40%

Total Nitrogen (N)………………..13% (3% Nitrate Nitrogen)

Soluble Potash (K2O)………………..44%

My saltpeter stump remover had an off white color and an odor that I can’t explain rather than a snow white color with no odor like KNO3 should be but when I performed the purification procedure my extract had all of the right characteristics saltpeter should have.

-You could do this indoors but I suggest outside since the steaming solution gives off somewhat of an odor.

-If you would like to, ball mill your potassium nitrate using steel or high-alumina media.

-50% Magnesium/50% KNO3 makes a pretty sweet flash powder. Ball mill the Mg and KNO3 separately for excellent results.



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    22 Discussions


    5 years ago on Introduction

    I think you are wasting your time with this procedure. According to the label, the stump remover is 40 % potassium nitrate and 60 % something else. All you are doing with this procedure is dissolving and recrystallising the water soluble salts, and what you finished with is probably STILL only 40 % potassium nitrate. I tend to concur with corradini that someone who knows as little chemistry as yourself should not be messing about with pyrotechnics, nor giving advice to others about making these products. Do you know what happens if you steel ball mill magnesium and potassium nitrate together ? DON'T try it !!!!

    4 replies

    Reply 4 years ago on Introduction

    Kymyst, the author didn't say to mill them together.
    "Ball mill the Mg and KNO3 separately for excellent results."

    As to the rest of the chemistry related specifics, I wouldn't know. But that's why people read Instructables, to learn, right? The comments are often a source of wisdom for others.


    Reply 2 years ago

    yea I kind of agree, work smart not hard


    Reply 4 years ago on Introduction

    The author has only added that sentence ( "Ball mill the Mg and KNO3 separately for excellent results.") AFTER my comments 12 months ago. It was not there initially. Instructables like this one are NOT a source of wisdom, just a source of often dangerous misinformation.


    9 years ago on Step 1

    Something ain't right... Water is pretty close to 1g/ml so 100g of water should be 100ml not 175ml...

    5 replies

    Reply 9 years ago on Introduction

    Okay you just about right. I measured and weighed the H2O again and it turned out that 175ml weighs 158g.


    Reply 9 years ago on Introduction

    Even that's a bit of a worry. At 20 degrees C (approx room temperature) the density of water is 998.2 kg/m^3 that means it should be 0.998g/ml. That would put 175ml of water at 174.6g If your scales are showing 158g they may not be as accurate as needed for this task. But hey, if you always use the same scales and the same measurements, you should get the same resuts.


    Reply 9 years ago on Introduction

    I don’t know much about water density, but I suppose you’re right. Also, when I measured the H2O for a second time I used a more accurate scale.


    Reply 6 years ago on Introduction

    Dude. 'I don't know much about water density' -- SERIOUSLY? Let me clue you in - as others have sort of tried here:


    And to get all complete, here: a cubic centimeter of water is a milliliter. which weighs a gram. Kind of why the metric system is all nice and neat. (And, to Mr. '998.2' kg/m^3' -- sometimes being 'super-accurate' isn't 'super-helpful'. A gram is a cc is a ml is just fine, here.;-)

    Back to you, YoungPyro19: if you started out with solubility of 247 g in 158g of water -- but don't even know that 158g of water is NOT 175ml -- I'd bet a kidney you cut-and-pasted your recipe. Which is fine for peach pie -- BUT I DON"T THINK YOU OUGHTA BE POSTING CHEMISTRY PROCEDURES.

    Ummm...especially seeing as how - guessing from 'YoungPyro19'? And the fact that we're dealing w/ nitrates here? I SO don't want to be negative, per Instructables 'be nice' policy, but, dude, it's not Sherlock to suppose that (a) your advice is intended for people who want things that go kapow or boom, and (b) you don't even have a CLUE about -- jeezus -- about pretty much THE most fundamental measurement concept in chemistry.

    Y'know, when you're making things that go boom or kapow (again, your handle is 'YoungPyro19', so I'm not stretching it much to assume that), things like 'oops!' (or, to quote you directly: 'I don't know much about water density', and thus being off by, oh, tens of percents) -- cost people fingers and hands and eyes.


    Reply 8 years ago on Introduction

    This procedure is just for purifying KNO3 from stump remover.

    I think this is what you’re looking for==> http://www.youtube.com/watch?v=g4FpKqG93rY

    The process is similar. Study the video and improvise.


    Reply 9 years ago on Introduction

    Weight is not the same as volume. Volume is how much three-dimensional space a substance (solid, liquid, gas, or plasma) occupies. Weight is a measure of the heaviness of an object.