Perchlorate cleanup problem

[Swede]

There are a number of ways to eliminate residual chlorate from a batch of perchlorate. In the past, on a small scale, I was able to clean up some perc using sodium sulfite, but I took no notes. Today, I wanted to repeat the experiment, except this time, use potassium metabisulphite, and determine how much of the bisulphite was needed to reduce the chlorate in this particular, typical "crop."

 

First, the test for chlorate, a tincture of n-phenylanthranilic acid (3% in isopropanol), with the color "developed" by concentrated sulfuric acid, worked perfectly. The tests took less than 10 seconds each, and were reliable and repeatable.

 

Theory: In an acid environment, potassium metabisulphite dissociates according to the following:

 

K₂S₂O₅ + 2 H+ --> 2 K+ + H₂O + 2 SO₂

 

Of the products of the previous dissociation, the sulphur dioxide reduces the residual chlorate, according to this postulated process, not balanced.

 

2K+ + 2SO₂ + KClO₃- --> K₂SO₄ + KCl

 

I am unsure of the exact action. Likely products might include water, HCl, O₂, or others. Regardless, the chlorate is definitely preferentially reduced.

 

The procedure: 20 grams of dirty perc was dissolved in 100 ml boiling water. 5 drops of concentrated HCl was added to acidify. From a known mass, potassium metabisulphite was added in portions, with the NPAA test conducted between each, using a separate test tube. Hot water was added as needed to keep the volume of the perchlorate at 100 ml.

 

After 9 small additions, the perc tested clean. Total mass of potassium metabisulphite used was 4.23 grams, which seems like quite a lot for a 20 gram batch of perchlorate. One thing I noticed was that direct additions of the metabisulfite powder resulted in instant bubbling of SO₂ gas, with little chance at contact. About halfway through, I went with slow additions if aqueous metabisulfite using a pipette held near the bottom f the beaker, and added quite slowly.

 

Two things I am very interested in... the second equation in the "Theory" above - what products can I expect with this process? I'm strongly leaning towards KCl and K₂SO₄, both significantly more soluble than KClO₄, and both of which would be left behind during recovery of the cleaned perchlorate. Other desirable byproducts would be anything gaseous, or able to be boiled off.

 

At the end of the process, the sample was neutralized with a saturated KOH solution.

 

Here is a very interesting picture of the test samples as the cleanup progressed:

http://www.5bears.com/chem1/pcln008.jpg

 

Chemistry guys - comments or corrections are appreciated. Straight SO₂ gas, bubbled through the solution, would be a superiro method, IMO, but how to make an SO₂ generator?

 

The next step I'm going to try is just HCl, or perhaps ascorbic acid. Anyway, it appeared to work, but was a bit of a PITA.

[Miech]

SO₂ os obviously made the easiest and cheapest by just burning sulfur. I'd think about taking a length of duran glass tubing, putting a small amount of sulfur in the middle, and carefully blowing some air through it. Ignition would occur by a small electrically heated wire somewhere in the sulfur. I don't know if such a setup would work well in real life, but I think it is slightly unpractical to work with. This is because you need to keep the sulfur burning without blowing it away or extinguishing the flame.

 

More practical would be to gently burn sulfur using a not too strong oxidizer like potassium nitrate. Perhaps pressing it into pucks would make it more handable and controllable. I'd expect a mixture of sulfur and potassium nitrate to burn quite fast and hot. Pressing it into pucks would reduce the burning rate, as a loose powder will most likely burn fast.

 

Edit: Some experimentation on burning sulfur with oxidizers is obviously needed. The tendency of sulfur to polymerize can give some unexpected variables you'd not expect. I know you are a careful person (at least I think so), but I really don't want you to blow up yourself.

Edited April 17, 2009 by Miech

[Mumbles]

There are no easy ways to generate SO2 gas. Burning sulfur really isn't a viable option. The most controlable way is by the action of hot sulfuric acid and metallic copper. This of course isn't really suitable either. I'd stick with what you have, but I have two suggestions.

 

1) Like you said it's far more efficient if the sulfite is introduced as solution to the bottom of the liquid. Stick with that.

 

2) Add the sulfite, and seal it off in a container that can hold the pressure. The SO2 will be taken into solution as it is used up. Obviously you need something to contain the pressure.

 

You could always add acid drop-wise to a sulfite solution, and bubble the resulting gas into the perchlorate. That is probably one of the better ways to go about it actually. I agree with your equation of forming sulfate and chloride by the way.

 

SO2 + H2O ---> 3 H2SO3 + ClO3 ---> Cl- + 3 SO4(2-)

 

The electrons balance, it's probably something like that.

[Swede]

Thanks guys, that's given me some ideas. SO2 gas is far more soluble in colder water than boiling water. The solubility of K-perc is on the order of 20g/100ml at 100 C. A surplus of water would allow 20g to dissolve in 200ml, for example, at 60 degrees.

 

With the perc dissolved in a non-boiling solution, the aqueous bisulfite can be added quite slowly, with stirring, using a burette or similar. When the batch tests clean, it can then be neutralized, and boiled down until xtals first appear. From there, slowly chill and filter, saving the clean perc. The much more soluble KCl and slightly more soluble K2SO4 stay aqueous and are discarded.

 

There is some web info on the construction of an SO2 generator using acid slowly added to a sulfite solution, and burning S will also do it. I like the idea of bubbling SO2 more than adding additional salts to a batch... less contamination that way. And even a simple lid would keep the SO2 in contact above the solution, giving it a better chance to dissolve.

 

Added: I should have looked at Dann2's web site earlier. Here is his postulated mechanism:

First, the Metabisulphite dissociates:

 

Na2S2O5 + H2O <----> 2NaH- + SO32-

The Sulphite then reacts with Chlorate:

 

3SO32- + ClO3 - => 3SO42- + Cl-

The overall reaction is:

 

3Na2S2O5 + H2O + 2NaClO3 => 3Na2SO4 + 3H2SO4 + 2NaCl

There is more info. here on dissociations.

 

You can then neutralise the solution using Sodium Hydroxide which will give you more Sodium Sulphate and water.

 

H2SO4 + 2NaOH ==> Na2SO4 + 2H2O

 

Interestingly, I did notice that it took a healthy dose of KOH to neutralize, far more than I expected, indicating that acid was formed after the reduction of the chlorate ions.

[Mumbles]

Oh yeah, when sulfur dioxide dissolves in water it forms sulfurous acid. I showed it above in my reaction, I just didn't keep the protons all the way through. It's the same reaction as when SO3 dissolves in water to form sulfuric acid, just a lot less exothermic.

 

The only problem I have with the quote from the website is that top reaction. First off, there is no such thing as NaH(-). Secondly NaH would instantly react with water forming sodium hydroxide and hydrogen gas. I like the following:

 

NaS2O5 + H2O ---> 2 Na(+) + 2 SO3(2-) + 2 H(+)

 

You might want to stick with potassium metabisulfite and potassium hydroxide too. They're both relatively cheap from homebrew and soap supply shops and similarly priced to their sodium counterparts.

[Swede]

I agree. My very first cleanup attempt used sodium sulfite, adding Na to the batch; not good. Since then I did pick up potassium metabisulfite and KOH (cheaply) to keep the Na down.

 

I am going to test this cleaned batch for chloride content, and by adding surplus lead nitrate and weighing the precipitated lead sulfate, I can determine how much sulfate salt remained behind. Do you think this will work? The chloride and sulfate contamination may be small enough to ignore. The idea is pyrotechnic grade perc, no chlorate, not 99.999% pure ACS perchlorate.

 

Sulfate, sulphate. I need to pick a spelling and be consistent!

[crazyboy25]

Sulfur dioxide gas can be generated by adding HCl to sodium bisulfite and gently heating.

[Arthur]

For a gas generator wiki for Kipp's apparatus.

[Swede]

Thank you Arthur, I will look into it. I like the idea of bubbling SO2 gas vs. adding unneeded salts to gain the same reduction.

 

I made a recent blog entry with a few more photos and an outline of the procedure from beginning to end. It definitely works. A check of chloride and sulfate showed nil... the perchlorate is clean, at last.

 

Slightly OT question - I know ammonium perchlorate is fairly cheap and available from Firefox and other suppliers. I'd like to try the AP blue... Can anyone think of a way to substitute ammonium for potassium? I don't think it can be done, due to the low solubility of the potassium salt. I'll probably just end up ordering some, rather than starting from scratch with sodium.

 

Thank you all for the suggestions and encouragement.

[Mumbles]

The only thing I know of that might work is ammonium tartrate. It's quite soluble, while the potassium salt is not. I don't know if it's less soluble than perchlorate though.