Sciencemadness Discussion Board

Yield of KClO₃ from boiled bleach

Keras - 18-6-2023 at 11:39


for those who have tried to synthesise potassium chlorate from bleach by boiling the latter and then adding KCl, I'd be interested to know your final yield.

I have tried a quick online search, but apart from qualitative remarks such as ‘This process is not really efficient’, I never came across even a rough percentage. If you have one to share, it'd help me eyeball my own results.


violet sin - 18-6-2023 at 12:09

3 NaOCl (aq) → 2 NaCl (aq) + NaClO3
From Wikipedia.

But that means you get 1/3 product and 2/3 byproduct + separation time. You can do the math based on your starting percentage of your bleach. And that's only going from NaOCl content, discounting any NaOH or carbonate, surfactants etc in there. Some of which may consume bleach at temp? I'm not sure, never done it. Just the best case scenario seems lossy, but easyish.

Your adding the KCl after the decomposition to get KClO3 but the first parts the same.

[Edited on 18-6-2023 by violet sin]

Keras - 18-6-2023 at 12:14

No, I mean, I know that. But, out of the theoretical maximum, which is one third of the hypochlorite initially present, how much do you get?
For example, my last batch gave me about 36 g of potassium chlorate out of a 1 l bottle of 9.6% concentrated bleach.

violet sin - 18-6-2023 at 12:18

Right, that's where you would do math. Figure mol NaClO in sol, from concentration and volume. Think 100% effective use and 1/3 mol % success, adjust molecular wt. to K cation, and compare that to you yield. Find out how you did.

Sorry, I'm enjoying father's day with my youngest child curled up asleep on me right now, so I'm not going to crunch those numbers. Good luck

B(a)P - 18-6-2023 at 16:42

I have tried this a couple of times and just looked back over my notes. For my two attempts I started with 1 L of what was advertised as 12.5% sodium hypochlorite solution (pool chemical). My final yield was 22.6 and 24.1 g of potassium chlorate from what I assumed was 125 g of sodium hypochlorite. So around 10%, or on the basis that no more than 1/3 of the starting material can be converted, about a 30% yield.

Keras - 18-6-2023 at 21:33

Thanks for sharing!

Actually, your yield might be even worse because concentrated bleach has a density well above 1 (about 1.2 for 12% hypochlorite) so you must’ve started with more NaClO than what you think.

I guess my overall yield of 60% of the maximal theoretical amount is far from ridiculous, then, given the figures you mention.

metalresearcher - 19-6-2023 at 09:08

Electrolyzing diet salt (2 parts KCl and 1 part NaCl) and then precipitating KClO3 after cooling is much more effective. Even an old computer power supply suffices.

Boffis - 19-6-2023 at 10:16

Hi Keras, I did this a couple of months back and from 1270g (circa 1.03 L) of 14-15% sodium hypochlorite you should obtain about 95g of KClO3. My crude yield was 80.8g from a single batch.

I ran 3 batches, each of roughly 1 L. I then recrystallised 148.8g from 400ml of de-ionised water to yield 107.7g of excellent quality potassium chlorate. I then evaporated the filtrate to 150ml and obtained a further 20.7g. The second filtrate was evaporated down until crystals started to form and cooled to room temperature and then chill overnight to 4 C and the crystals filtered off to give a further 12.3g. This final 33g was recrystallised with the remainder of the primary stuff.

This is equivalent to a recovery of about 60-63% (depending on the true strength of the starting hypochlorite) without recycling the 2nd and 3rd crops during recrystallisation. With the recycling the recovery is about 72-75% which given the cost of the starting materials is very good. I didn't weigh the fractions from the 2nd recrystallisation but I have 31g of crude KClO3 to carry over to some future batch.

Would you like me to post my detailed methodology as I seem to have done rather better than most?

B(a)P - 19-6-2023 at 13:52

I would be interested to hear your method. I suspect my yield was so low because my starting material had substantially decomposed already. I didn't measure it's concentration before starting and used it to make chlorate because it was old.

Keras - 20-6-2023 at 03:04

I don't do anything fancy, I suppose. I start from 1L of 9.6% bleach (10 % NaClO w/w, d = 1.15) that I boil down to around 75 ml, filtering the precipitated salt once or twice along the way. 75 ml corresponds to about the minimal quantity of water in which the theoretical amount of produced sodium chlorate is soluble into at 25 °C. Matter of fact, I think I should allow more water because of the common salt phenomenon. In my last batch I could swear some sodium chlorate started to crystallise out of the solution on cooling (the crystals are very different from common table salt, they look like minute needles).

I then dissolve potassium chloride into the amount of water necessary to keep the produced salt (KCl → NaCl) in solution, and mix both flasks together. KClO₃ precipitates immediately. I put the resulting slurry in the freezer until the water just starts freezing and I filter anything one last time.

I suppose trying to minimise the amount of water and getting down to as low a temperature as possible help pushing the yield up.

Finally, I'd say you wouldn't care about bleach decomposition, since that’s precisely what you do when boiling it. In other words, when bleach degrades, it precisely forms sodium chloride and sodium chlorate, so that shouldn't affect your yield.


MadHatter - 20-6-2023 at 16:05

My experimentation with this yielded less than what I would have obtained with a
straight electrolysis of KCl.

1st the reaction 3NaOCl --> NaClO3 + 2 NaCl gives at most a 46.6%
yield of NaClO3. I know it's less because I could smell the chlorine
during the boiling which means a loss of the crucial chlorine needed.

As others have pointed out there is a loss, depending on the age and
decomposition of the input material(NaOCl) in the form of the
carbonate(from atmospheric absorption of CO2) and any surfactants
already in the mix.

This really depends on the % of hypochlorite in the solution. How much are you
willing to spend ?

You can boil down the hypochlorite to a level that boots out the NaCl given its
narrow range of solubility(35.65 @ 0C to 38.99 @ 100C). Boiled down to that
level should with the addition of KCl precipitate out KClO3 in a "rod", or
needle, as you described it, form. Tip: filter out any NaCl while it's hot.

The basic problem is the concentration of NaOCl to start with. If you're just
interested in the synthesis and not production that's a good start. However, if
you need more KClO3 then I suggest the electrolysis route or just buy it
online if possible.

violet sin - 20-6-2023 at 19:16

Is there any convenient way to monitor pH during the main heat induced reordering? I noticed in the wiki article, that pH around 6-6.5 seems to favor the chlorate formation, while pH approaching 10 led to oxygen evolution and NaCl byproduct. They said up to 95%. When I was making bleach water by electrysis, I had a hard time finding pH, it just bleached the litmus

"These two decomposition reactions of NaClO solutions are maximized at pH around 6. The chlorate-producing reaction predominates at pH above 6, while the oxygen one becomes significant below that. For example, at 80 °C, with NaOCl and NaCl concentrations of 80 mM, and pH 6–6.5, the chlorate is produced with ∼95% efficiency."

I can't have been the only one to see that. So, what about pH controll with HCl to make nothing new in solution, just more salt. The MSDS for liquid chlorinator has salt contamination in the 5-11% with a decent NaOH concentration in 0.5-2.5% range with a 7-15% for the hypochlorite. It would be nice to see a calculated possible amount of product for an as labeled concentration, stated 3->1 usage, and 95% conversion mentioned. B/c it could only go downhill from there.

I lean in the direction of buying vs. doing all that. Though I don't get much time to DO barely any experimental work. I also don't make explosive comps or need the chlorate. If your goal is to do it cheaply, the calculation would tell if it's in your price range. If your looking to acquire it without tying your name to an online purchase,.. this may be for you. Small experimental quantities sure, this could do. But the route is of limited use.

Busted out the pen and an envelope.
Assume 10% pool bleach, 1 gal, no decomp, no mech losses

Source has 10% pool chlorinator as 9.21% wt NaClO and a specific gravity of 1.14.
Math, hopefully right
(1.14 g/ml * 3785.41ml/gal) = 4,315.36 g/gal

4,315.36 g/gal * 0.0921 = 397.44 g NaClO/gal (considering 1 gal)

397.44 gram NaClO / 74.442 g/mol NaClO = 5.338 mol NaClO

5.338 mol NaClO * (1 mol NaClO3 / 3 mol NaClO)= 1.779 mol NaClO3 possible

95% conversion realized 1.69 mol NaClO3

1.69 mol NaClO3 1:1 metathesis with KCl = 1.69 mol KClO3 ppt.

1.69 mol KClO3 * 122.55 g/mol KClO3 = 207.19 g KClO3

For me a gal can be 6-10$. With a perfect conversion, no loss, no accounting time, ideally getting max return, would be just over 200g as seen above. Seems kinda rough, but I could have failed basic math/reasoning somewhere, it's 2 am now.

I see your point about the pH controll, thanks for the head start on testing that. I can check my pool but I don't think the red/yellow drop system would work for a concentrated bleach sol. Guess I could try.

Perhaps diluted bleach could be more effectively pH adjustable, sulfuric sounds like a bad idea by adding to the mess. Then your just adding work to the back side with heating the extra water out if the diluted sol works.

[Edited on 21-6-2023 by violet sin]

[Edited on 21-6-2023 by violet sin]

Keras - 20-6-2023 at 22:55

I don’t think you can control the pH with HCl, because hypochlorite will oxidise it to chlorine. NaClO + 2 HCl → NaCl + Cl₂ + H₂O. You'd have to use sulphuric acid, which is not oxydable or maybe phosphoric acid.

Apparently, the best method to assess the pH is to neutralise the hypochlorite using hydrogen peroxide, then go on either measuring directly using pH paper or doing a titration using HCl.

violet sin - 21-6-2023 at 13:46
This has some data that seems interesting. Chlorate formation rises untill pH 7.2. read the chlorate section beginning on pg 18 of linked paper.

I did crunch numbers, but edited last night's efforts into my post two comments up, if anyone wanted to check my math.

Hope anything helps. I'm still looking into if it's smart/efficient to bother pH adjustment, and how it might be done industrially.

Boffis - 22-6-2023 at 12:40

You can adjust the pH and to better control the formation of chlorate but its a question of whether the extract chemicals used for this process (say with mineral acid or sodium dihydrogen phosphate) are worth expending when bleach is so cheap. From the farm supply shop down the road its £30 for 25L of 10-11% sodium hypochlorite. There is a good deal of free hydroxide in most bleach in order to stabilise it, it is this you are neutralising at first. As soon as significant chlorine starts to be evolved the solution is "neutralised" regardless of its pH. Once most of the free hydroxide has been removed

I have just purchased some new 10-11% hypochlorite and am in the process of running an experiment with this fresh though more dilute hypochlorite. It is a repeat of the process described below.

Preparation of potassium chlorate from sodium hypochlorite bleach

I had 3 litres of 14-15% bleach left over from earlier experiments that I had no particular use for and gave a poor yield of chloroform with acetone so I decided to convert it into a chlorate salt. The reaction to convert the sodium hypochlorite is roughly:

3NaClO + KCl -> 3NaCl + KClO3

Strictly this reaction is carried out in 2 stages, the conversion of hypochlorite to chlorate and chloride ions and then the precipitation of the K salt. The reactions occurs spontaneously over time but is significantly accelerated by light, heat and lowering of the pH. Rendering the hypochlorite solution acid results in a competing reaction that liberates chlorine. Since this reaction occurs spontaneously old bleach often has a significant chlorate content and it is my experience that old bleach often gives a better yield that new bleach in spite of the apparent lower hypochlorite content. The bleach used for this experiment was not analysed prior to use for hypochlorite but was about 2 years old.

1270g (roughly 1030ml) of old commercial 14-15% bleach was placed in a large glass saucepan and evaporated down to about 700ml and then cooled to about 45°, filtered to remove a little salt and mixed immediately with 300ml of saturated potassium chloride brine. The filter cake was discarded.

The solution, which was still slightly yellow no longer smelled of chlorine, was allowed to cool to room temperature and then chilled overnight to 4° C. Thin diamond shaped plates of potassium chloride slowly formed and settled to the bottom. The following day the crystalline ppt was filtered off, washed with a very small amount of ice cold water and sucked as dry as possible then dried at 40-45° C on a large watch glass for about 4 hours. The dried crystals of crude potassium chlorate weighed 60.73g.

The washings and the filtrate were evaporated down to about 700ml, cooled to about 45° C and filtered to remove the copious salt deposits (note 1) and mixed with 200ml of saturated potassium chloride brine. The mixture was cooled to room temperature and then chilled to 4° C again. The crystals of crude potassium chlorate were filtered off, washed as before and such as dry as possible before being removed from the filter funnel and then dried at 40-45° C for several hours to give 20.12g of crude potassium chlorate.

Further evaporation failed to produce any worthwhile additional material only vast amounts of salt.

The combined crops weighed 80.84g or about 80% of theory, they are however, very likely contaminated with significant salt.

These crystals and material from another batch were combined and 148.84g were recrystallised by dissolving in 400ml of boiling water and filtered hot. The filtrate was cooled with gentle stirring until almost at room temperature and then chilled in the fridge to 4° C overnight. The crystals were filtered off using a Buchner, washed with a little ice-cold water and sucked as dry as possible. After air drying gave 107.68g off fairly pure potassium chlorate. A dilute solution gave only a slight cloudiness with silver nitrate (note 2).

The filtrate was evaporated down to about 150ml to give a further 20.71g of less pure product. The filtrate was again evaporated, this time on a water bath to a small volume of liquid containing abundant crystals of potassium chlorate. This was allowed to cool to room temperature and then pressed into a small Buchner funnel and vacuum drained for 15minutes, washed with a little cold water and dried to give a final 12.28g of crude potassium chlorate. The filtrate was discarded. The combined 2nd and 3rd crops (32.99g), was kept separate from the initial crop was will be added to a future batch.

Note 1; the crystalline solid that forms during evaporation was found to be almost entirely composed of sodium chloride salt and can be filtered off and discarded before the final cooling of the liquid.

Note 2; silver chlorate is sparingly soluble in cold water so strong solutions of alkaline chlorate will give a precipitate with silver nitrate but the ppt tends to be crystalline in contrast to the very fine milky ppt obtained with chloride ions.

Current Work
I am currently repeating this experiment with roughly 1000ml of 10-11% sodium hypochlorite solution and have made the following observations. On evaporation to 700ml there is no separation of salt. Salt doesn't start to precipitate until more than half has been evaporated.

I evaporated the first 1030ml to 700ml and added 220ml of KCl brine without filtration and cooled to room temperature and then chilled overnight to 4 C to obtained 12.04g. The solution was evaporated down to 700ml again at which point salt began to crystallised on the surface. The solution was cooled without further KCl addition, chilled in the fridge and the crystals filtered off and they are currently drying. Edit: gave 35.63g

The filtrate was evaporated down to 500ml, the precipitated salt filtered off and 100ml of saturated KCl brine added. Current cooling down. Gave 15.81. This is a total yield of 63.50g of crude K chlorate which represents a yield of about 90% of theory assuming the hypochlorite is still 11% since its brand new.

[Edited on 23-6-2023 by Boffis]