Element Collection

Element Collection

Wednesday, September 18, 2013

Potassium Chlorate from Bleach

This post serves as the Video Companion to this video.

This experiment was also featured on Hack a Day!

In this simple experiment you can create potassium chlorate, a powerful oxidizer that finds use in amateur rocketry, a convenient source of oxygen, and the famous "screaming gummy bear" demo (among other things), from common household items with a minimum of effort. The tradeoff is that it's a very inefficient process and yields tend to be very low. Electrolysis is a far superior method, and is something I plan on trying out in the future.

To begin, I measured out 500mL of household bleach. This is about the minimum for this method to produce reasonable (but still quite low) yields. The active ingredient in bleach is sodium hypochlorite, and the higher the concentration in yours the better. Mine was listed as 8.25%.
The good news for chemists is that when shopping for household chemicals to use in experiments, the cheaper the better! The cheapest brands generally do not include extra fancy chemicals like detergents and fragrances, which is desirable for chemistry use.

This solution was then boiled on a hotplate to get rid of most of the water, and to drive the conversion of hypochlorite to chlorate (via a disproportionation reaction, with table salt as a byproduct):
3NaClO == 2NaCl + NaClO3
This only occurs on boiling, so slow evaporation will not work in this experiment. Once crystals start to appear in the liquid, it is time to remove from heat and let cool.
These crystals are the sodium chloride byproduct, and need to be filtered off and can be discarded. The remaining solution contains sodium chlorate, as well as more sodium chloride.
Sodium chlorate is quite soluble, so to recover it and separate it from the sodium chloride it must be converted to potassium chlorate. This can be done with another common household chemical: sodium-free salt alternative. Mine was No Salt brand, which is almost entirely KCl.
For this experiment, we need enough KCl to convert all the sodium chlorate to potassium chlorate. This happens via a simple metathesis reaction:
NaClO3 + KCl == KClO3 + NaCl
Using 500mL of an 8.25% solution of sodium hypochlorite and the two equations above, I calculated that this reaction should only require 13.3g of KCl to go to completion. Other experimenters have recommended using a saturated solution of KCl that is equal in volume to the boiled bleach solution, so that is what I went with this time. Since my bleach ended up at about 200mL, I needed about 75g of KCl to make a saturated solution. The extra ingredients generally leave a solution of No Salt rather cloudy, so I always filter it before use.

Next I combined the solutions together. This should precipitate KClO3 crystals immediately, but I did not see any at all! I think the culprit was using that huge excess of KCl solution. This means that there is a lot of extra water around for the products to dissolve in, so even though potassium chlorate is much less soluble there was still enough solution for it to dissolve in. I believe that if I had used the stoichiometric amount of KCl in the minimum amount of water, 13.3g in about 39mL of water at room temperature, I think I would have seen immediate crystallization. Certainly there is room for future experimentation here.
To get crystallization, I put the solution in the lab fridge over night. When I took the beaker out, there was a nice layer of white, plate-like crystals covering the bottom. I was able to recover 9 grams of product this way.
 22 grams of KClO3 was the theoretical yield here, meaning I had only a 41% recovery. In an attempt to get more yield, I went ahead and boiled the remaining solution down to half its volume and placed it back in the fridge. I ended up with 23.5g of additional product, which by itself is greater than the maximum possible yield! That means that this second crop is very impure; perhaps I shouldn't have reduced the volume quite as much. These crystals looked much different too - they were much more compact, and some were needle-like rather than the nice plates from the first batch. Here is a comparison of the two, with the impure product on the right:
There's still a chance to recover good product from this second batch, and it would require a recrystallization or two. I have yet to try this, but if I do I will update the post. It honestly may not be worth the effort, considering how cheap the reagents were and how simple the process is.

Finally, I wanted to test the product to make sure it was in fact potassium chlorate. There is a simple and exciting test for this - combine some chlorate with half its weight of sugar, and add a drop or two of concentrated sulfuric acid. The mixture crackles and quickly bursts into beautiful lilac flames, making a lot of smoke as well. I tested 1 gram of my pure product combined with 0.5g of sugar, and two drops of acid.
See the video link at the top to see this reaction in action. It appears near the end.

20 comments:

  1. I enjoyed watching this experiment. I am inspired to learn more about chemistry. I think it's fascinating how chemicals react.

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  2. Could a solution of 12% sodium dichloro-s-triazinetrione be used instead of bleach?

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  3. Is it possible to use calcium hypochorite as found in pool shock? Some products have 75% calcium hypochlorite?

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    1. Good question! There's a chance this might be possible, but calcium hypochlorite is unstable in solution. Boiling will likely speed up this decomposition. The solubilities do look promising, though, so if this is your only source of hypochlorite it may be worth a try. I'm leaning towards no, though.

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    2. It's a different procedure but yes, though I don't recommend it due to chlorine gas being produced and hydrochloric acid being one of the reagents.

      Dissolve 16g of potassium hydroxide and 44g of sodium hydroxide in 70 ml of distilled water. Fill the
      flask with 85g of calcium hypochlorite, secure the PVC tube in the stopper hole. Bring the hydroxide
      solution to 70 to 90 degree Celsius and depose the outlet of the tube in the solution, add 200g of
      hydrochloric acid to the flask in small increment and close it each time with the stopper. Chlorine gas
      will be produced in the flask and will react with the hot alkaline solution to make Chlorate and chloride
      anion. After a few minutes all the calcium hypochlorite will have reacted to make chlorine gas and the
      reaction will be considered as completed. Cool to 0 degree Celsius. On cooling Chlorate anion will
      condense with the potassium cation to make crystals of potassium chlorate. Filter the solution to get the
      crystals and Dry the potassium chlorate on a glass or polypropylene sheet. If the intended purpose is
      pyrotechnics there is no need for recrystallization. Purification is done by recrystallization with three
      times the weight of the crystals of distilled water.
      Yield: 14-15g

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  4. Lowes sells a drain cleaner that is 93% H2SO4 and has some metal inhibitors to help protect plumbing from the acid. Is this form of sulfuric acid strong enough to react with the potassium chlorate? Will the inhibitors interfere with the reaction?

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    1. That's actually exactly the type I used! The brand I found is 'Liquid Fire'.

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  5. THANKS MAN!!!...I had a "?" moment and was wondering who came up with the idea of "matches"...long story short(I wrote the long story but lost it by going back to this page from the google sign up page before posting it =P), I decided I wanted to make some myself.
    Would you say is more cost-efficient-effective to buy the KClO3 already made, or to make my own?
    How do you know when you have KClO3 in "pure" form?
    What quantity (of chlorine, salt substitute...final yield) would you say is "safe" to begin with?
    I went to school for ME and only had to take CHM for engineers (one class only) and I hated it...THANK YOU for making it interesting my man!!!...lol...I am getting all into home made experiments now...hahaha...Thanks man.

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    Replies
    1. It's probably cheaper to make it yourself, with household bleach and no-salt being cheap (and if you're clever, you can find better versions of both of these). It just takes time. Without a spectrometer you can't really know for sure how pure your chemicals are, but after a few recrystallizations they can be very high purity. Glad you enjoyed it! (and sorry for the slow response!)

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  6. sk:

    1. Therefore, the liquid turns pink when it is warm?

    2. it's okay if I do not have 8.25 percent? (2.7)

    3. is what the bleach concentrated in a carton 9.6 percent this is good?

    4. is that a stainless pot of kitchen is good?

    Thank you man good video :D

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    1. 1) It shouldn't. Does yours? Sounds like additives - try buying the cheapest brand available.
      2-3) Concentration doesn't matter as long as you account for it in your calculations. Lower concentration just means more water to boil off.
      4) Stainless steel may corrode in the presence of strong oxidizers. Glass is best!

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  7. How u calculated the 13.3g of KCl?

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    1. Well I started with 500mL of 8.25% bleach, which means it contains roughly 41.25g NaClO (assuming 500mL weighs 500g, which might not be correct but should be pretty close). Then you can use the first orange equation above to calculate how much sodium chlorate will be produced upon boiling. Finally, use the second orange equation to calculate how much KCl needs to be added to fully react this all to potassium chlorate. Stoichiometry!

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    2. I have one more question .... I'm gonna explain to you.... I used 500ml of bleach but I don't now how much sodium hypochlorite I have in %. And in the end I have 19.5g of KClO3..... How can I calculate the theoretical mass of KClO3 because I need to calculate the yield.

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    3. The theoretical yield is calculated with some stoichiometry.
      Store bought bleach is usually 10-12% by w/w or w/v NaOCl, it should say on the bottle. So 10% bleach will have 10g per 100g solution or 10g per 100 mL solution. If you used 500 mL of a 10% solution you have 50g NaOCl. The mole ratio for the first step is 3:1 NaOCl/NaClo3. So using moles and molecular weight, 50g/74.44g(1M NaOCl)= 0.67M/3(the mole ratio)=0.22M. Now for the 2nd reaction it's all a 1:1 molar ratio so you don't have to convert to grams further. 0.22M of KClO3(1M 122.55g) = 27.36g. This is all assuming your KCl addition was in the right quantity and everything precipitated.

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  8. n (NaClO3)=1/3n (NaClO)
    But I dont have the mass of sodium hypochlorite ..... n (NaClO)=m/M (NaClO)..... next step m (NaClO)=w (NaClO)×p (NaClO)×V (NaClO).... is this corect .... did you calculate this way

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  9. Instead of using a salt substitute which costs more & has impurities in it. I go to home depot or any hardware store & buy a 50lb bag of Potassium Chloride used for snow melt. Way cheaper,$15-$20 for 50lb bag & the msds has it at 98%-99%

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  10. Instead of using a salt substitute which costs more & has impurities in it. I go to home depot or any hardware store & buy a 50lb bag of Potassium Chloride used for snow melt. Way cheaper,$15-$20 for 50lb bag & the msds has it at 98%-99%

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  11. Did you ever try the experiment with your calculated 13.3g of KCl? If so how much water did you use for the solution?

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  12. I thought I should add some figures I came up with when researching the equation and process so it's up for people to see, I had to look around for it. The 1st reaction is simply to decompose the bleach into sodium chlorate, heat does this, and remove water. You can let it boil down to 1/9th of the original volume if you want. Solubility of NaCl is 36g/100g of water and solubility of NaClO3 is 100g/100g of water (doesn't precipitate upon cooling. So after you boil down and cool (cooling decreases solubility=more NaCl crystals), you filter off the solids and have a solution left with concentrated NaClO3. You want a super saturated solution of KCl to add to the NaClO3, solubility being 35.5g per 100g of water. So depending on how much NaClO3 you think you have in the solution, adjust accordingly. Rule of thumb I found is about 10g KCl per 250ml bleach. Lastly the solubility of KCl03 (potassium chlorate) is 8.61g per 100g water, meaning it'll readily precipitate out of the cooled solution. You basically don't want to over due it on the water because you want stuff to crystallize out and throw it in the fridge to get more crystal formations.

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