Perc Cleanup Attempt 1
Entry posted by Swede
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All of the work in the last few months has gone into the hopeful production of pyrotechnic grade potassium perchlorate. It is not difficult to make perchlorate... contaminated with chlorate and perhaps other unwanted hitchhikers. It is apparently difficult to make it pure. In a previous blog, I mentioned how well n-phenylanthranilic acid and sulfuric acid work in tandem to reveal chlorate contamination down to very low levels. I finally got to put these chemicals to use, and the results, as we shall see, were not pretty.
The first step in any purification process of a crystalline material (if possible) is recrystallization. Not all salts are candidates, but potassium perchlorate is an excellent one. Essentially, you want a salt that has a very low solubility at a low temperature, and a high(er) solubility at near boiling. The solvent does not have to be water, but most often is. In rare cases, solubilities are reversed and the solute exhibits a higher solubility at low temperatures, but these are mostly gasses and are brought up purely for curiosities sake here. The greater the spread between the two solubility values, the better the yield. I started with 294.50 grams of potassium perchlorate, as harvested amd dried from the cell.. The material was washed during collection with cold water, but that was it. Consistency was odd; mostly powdery, but also with a tendency to form hard clumps. Along with the bulk being perchlorate, there were also salts of chlorate and chloride in unknown quantities. Both have higher solubilities than potassium perchlorate, which is as we want it!
The "hot plate" is actually a simple, two-burner stove with good ceramic surfaces, purchased for $18 at a local hardware store; a fine substitute for a more expensive lab heater. If you try something like this, be sure the heater surface is a chemically resistant ceramic and NOT a coil of nichrome metal, which will shatter and destroy borosilicate beakers, and heat very unevenly.
The recrystallization process begins by heating to near boiling approximately 90% of the calculated water needed to dissolve the 294.5 grams of perc, which was approximately 1 liter of distilled water. Both burners were used, the second one to boil additional distilled water, to be added as needed.
With much manual stirring, the perc mass is dissolved (be patient!), and additional boiling water from the right-hand beaker was added until all of the solids are dissolved. I overshot a bit, probably by 300 ml or so... no big deal, just boil it down until the first crystals begin to form. At this moment, the solution is saturated with your product. In the meantime, we can begin to execute some of the cleanup processes. The first step is to neutralize the solution. As dissolved, it was very slightly acidic, somewhere between 4 and 5. I have a pH meter but decided to use simple strips for this easy task, rather than "pollute" my pristine pH probe.
Since I started with potassium salts from the very beginning (KCl) I decided to keep it that way as much as possible, and minimize added sodium, so I used concentrated KOH to neutralize. When you are this close to neutral, it does not take much, and using the concentrated KOH, exactly three drops took the solution (1.5 liters) to slightly above neutral, which is fine.
The next step was simply a shot in the dark. Somewhere in my huge collection of chlorate and perchlorate literature, I had read that the industry uses reducing agents (sulfites) to remove chlorate from their perchlorate batches. Not knowing how much chlorate was in there, I took a wild guess and added 4.0 grams of Sodium Sulfite (Potassium would have been a better choice) to the hot solution.
Important retrospective: I added my sulfite to a hot, slightly basic solution. Preferably, I should have left the solution weakly acidic, yielding:
Na2SO3 + 2 H+ → 2 Na+ + H2O + SO2
The SO2 is the reducing agent desired, dissolves beautifully in water, and is oxidized to SO3, yielding sulfate ion.
There is an excellent chance my sodium sulfite did NOTHING to reduce traces of chlorate due to the solution being basic.
Anyway, at this point, I should have tested for chlorate after attempting to let the sulfite do its thing. Instead, I was excited and impatient, thinking everything would work, so I threw the beaker in the lab fridge AFTER letting it cool to room temperature. Crystals begin to form immediately as the heat comes off, and the slower the heat is removed, generally, the larger the crystals, and the better the process works.
The next morning, I set up for harvesting of the recrystallized (and hopefully purified) potassium perchlorate.
We had gone through a cold snap and the solution ws found to be ~ 50% ice. I could see the perc at the bottom. No problem, I simply broke the ice up a bit, transfered it to another beaker, and allowed it to melt. The vast majority of perc is in the solution, NOT the ice.
The crystalline perchlorate was collected using a variety of means. As much as possible, it is best to gather the perc from the bottom with a scoop or spoon, and place the crystals into whatever filtration system you may have set up. In my case, I used the much-hated coffee filters in a funnel. Point-blank, coffee filters suck. They clog quickly, are fragile, and for chemistry in general, may I recommend to try to find a real filtration system, especially a vacuum system. My only Buchner funnel is too small, so I'm still looking for a better method to filter and wash large quantities of crystals.
Once the bulk of the crystal mass (let's say 95%) is gathered mechanically, you now have two options... you can pour everything in the beaker over the crystals, and collect the 0.003% of the perc that is physically suspended in the liquor as nearly invisible particles; OR you can decant, and simply dump the clear liquid into your "Used Perc Cell" container. I foolishly filtered all of it when I should have decanted. This took a LONG time for little additional yield.
In the end, I had nothing but a smattering of crystals in the bottom of the 4L beaker. Into the beaker went ice water. This was swirled to gather those crystals, and then dumped onto the crystal mass in the coffee filter. This process was repeated two or three more times, and each time, the crystal mass in the filter was manipulated so as to expose 100% of the crystals to the wash water. The last washing is with a 50/50 mix of ice cold water and ethanol, which aids wetting and speeds drying.
I went through about six coffee filters to gather the perc. The filters hang onto a lot of the crystals... judicious scraping with a plastic spoon helps gather them into a plastic drying tray.
While the crystals themselves were drying, the best way to process the wash water and used liquor is to re-heat. Into my giant 4L beaker went all the liquor, which by definition has 8 grams per liter perchlorate, AND all the filters. This was warmed and stirred. The crystals clinging to the coffee filters dissolved, and these filters were then removed from the beaker. The heat was increased, and the alcohol, plus perhaps 1/2 of the water, was boiled off. What remained went into my "Used Perc Cell container."
Caution: Filters saturated with perchlorate, once dry, are quite flammable and can combust. If you toss these into the trash and they dry out, you are asking for trouble. My technique is to have a dedicated plastic trash bag, quite small, into which I add about 4 ounces of water. Used filters, plastic spoons, any debris from this process goes in there, additional water is added so that the contents are soaking wet, and then the bag is knotted so everything stays that way.
After two days, the perchlorate crystals were dry. It massed 250.76. I lost a minimum of 11 to 12 grams in the liquor itself, with a further unknown quantity lost during washing. Why the remainder of the perchlorate lost stayed in solution, I don't know. None of it is truly wasted, as it will be added to the next perchlorate cell, once re-saturated with chlorate. Time to test!
Sorry for the crappy image quality here. I bought a few dropper bottles (they are cheap) and created a chlorate test kit. It might be best to put the NPAA in the opaque bottle rather than the clear, as I am unsure of its sensitivity to light. The NPAA is a 3% tincture in 99% isopropyl alcohol.
A miniscule amount of perchlorate crystal was added to a test tube:
Four or five drops of water was added next, and swirled. The first step with the NPAA test is to shake the NPAA tincture, and add 3 or 4 drops to the sample. In all previous cases, a white precipitate formed, and I was not disappointed:
The big moment: the sample is acidified with concentrated sulfuric acid, 3 to 6 drops. A sharp magenta color immediately formed, and I knew this process, as I executed it, was a bust.
Some thoughts on the failure... recrystallization is a time-honored way to purify, and I have no doubt I did, in fact, remove a lot of chlorate... but not enough. The sensitivity of the test is daunting, but given that commercial perc remains white tells me that is what I need to strive for, and nothing less will do. Without cleanup, you don't have perchlorate, you have a chlorate/perchlorate mixture that retains the sensitivities and danger chlorate alone exhibits.
After whining to Tentacles that my perc was dirty, he made a wise suggestion; actually, more than one. The next time, rather than trying to clean 300 grams, I am going to subdivide, and do the attempts using maybe 50 grams. He suggested Potassium Metabisulfite, available from Midwest Brewing Supplies as a reducing agent, inexpensive at $4.50 per pound. (Off Topic - these guys also have benzoates, ascorbic acid, and other pyro chems.) This will be added while the solution is mildly acidic, liberting the SO2. Slowly add the reducing agent to the hot liquor, and test with NPAA. If it fails, add more. Track the result. Find what works, on a smaller scale. About the only problem I can think of is that redox products are generated, and these may pollute or contaminate your perchlorate cell if recycled. It might be entirely acceptable to trash (in an environmentally sound way) the liquor plus washings, and accept the losses; OR there may be a chemical method to precipitate unwanted salts, decant or filter, and retain dissolved perc for recycling.
This is nothing more than a tiny bump in the road... I've come to far not to see it through. The test of the lead dioxide anode is forthcoming, as are more adventures in lead dioxide plating. In a fit of paranoia, and after a bit of research, I ordered chelating agents from a vitamin shop... just in case. Chelators like DMSA and EDTA are used to remove toxic heavy metals from your body. If you decide to pursue this for whatever reason, do your research, and select the better chelator, DMSA: meso 2,3 dimercaptosuccinic acid. This, combined with alpha-lipoic acid, taken on an empty stomach, will do the job, and do it well. In fact, if done under the supervision of a physician, the regimen will be the same. Only in cases of acute poisonings will you be hospitalized and the chelators IV'ed... otherwise, with DMSA especially, it is orally active, although at a lower percentage. The process also includes vitamin C and a mineral supplement, to replace the beneficial minerals stripped from your body by the chelator. Something like this is not to be undertaken lightly, but the use of DMSA as a highly successful chelator has a decades-long track record of safety and success. So if you think you've gotten into a bit too much mercury, arsenic, or lead, consider it.
Stay Green,
Swede
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