PERC!
Entry posted by Swede
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Houston, we have perchlorate. 
I've been finding this blog, for me, has become a source for easily accessible notes. For example, a while back I made an entry where I did a bit of math to determine expected yields, and investigated costs and efficiencies... THAT particular entry, I have referred to a few times for data and information. This is probably going to be another one like that. Online note-taking.
The situation in the War Room - the perchlorate cell has amazed me with its cleanliness and ease of operation. After an initial shock over the generation of what I am convinced is ozone, the cell settled down (albeit with continued ozone production) and the perc begain to fall like rain. The crystals are much finer than the chlorate variety. The cell chemistry seems exceptionally straightforward. There is no chloride, chlorite, or hypochlorite. Just a simple mechanism at the anode whereby the chlorate ions are getting one more oxygen molecule jammed down their throats, and as they oxidize, they wave goodbye to their more soluble brothers, and sink to the bottom.
The ozone is unmistakeable... I have worked with ozone outside any lab for years, first with reef aquaria, then with pool ozonators, so I do know what it smells like. It's pretty unique. But certainly I may be wrong.
Somewhere in that cell, just a guess:
ClO3- + 2O2 ----> ClO4- + O3
This seems extraordinarily inefficient. There are a number of references to ozone generation on the WWW using platinum anodes; for example:
Foller, Peter C., et al. Electrolytic Process for the Production of Ozone.
This patent discusses an electrolytic cell for production of ozone with current efficiencies of up to 52%. The cell uses a solution of highly electronegative anions, preferably hexafluoroanions of phosphorus, arsenic, or silicon. The anode is made of either platinum or lead dioxide, and the cathode is made or platinum, nickel, or carbon. A DC current is applied across the electrodes, ozone and oxygen are produced at the anode, and hydrogen gas is produced at the cathode.
Regardless of the mechanism, ozone is an extremely potent oxidizer, and may be involved heavily in the production of perchlorate ions, much more so than being a simple byproduct.
My chemistry theoretical knowledge is nowhere near what it used to be. It is probably at a college freshman level, rather than at a graduate level. My college senior paper in p-chem (yuk) is titled An Investigation into microwave-excited gaseous emissions spectra, with considerations of Doppler and Lifetime-broadenings, and dealt with the hash of atomic species produced when low-pressure gasses are chopped with microwaves. It used an Oriel 7240 grating monochrometer and a chart recorder, Ar, Xe, and CO2 gasses, and a military-grade uWave generator; a pretty cool setup. The handwriting is mine, but the language is foreign! 
Anyway, with beaucoup perch in my cell, I am at a loss as to how to identify the endpoint. Wouter suggests some titrations that would work, but are more complex that your average titration, and in a practical sense, day to day, would be too much work.
Some options:
1) Cell resistance: Everyone's setup is different, but with good notes, one might be able to determine endpoint with a good ohm meter.
2) Specific Gravity: Since the chlorate in a perc cell is dissolved, the SG will be well above 1.0, and as it converts to perchlorate, the SG will steadily drop due to the ultra-low solubility of the perch. This would be VERY temperature-dependant, and perc solubility varies greatly with heat, so a sample would have to be chilled in a refrigerator to a specific temp. It's possible, and one way to do it would be to create a series of standards, known samples with X chlorate and Y perchlorate, saturated at a higher temp and allowed to cool to a temperature of Z. This would drive a proportion of both salts out of solution, leaving you with a sample at a certain SG.
In use, pull a sample with a turkey baster, cool to temperature Z, measure SG, and compare. With good hydrometers, it just might work.
3) Voltage, current, and time plots.
Wouter states that 50 ampere-hours is required (at 100% efficiency) to convert 100 grams of sodium chlorate to sodium perchlorate. The MW of sodium chlorate is 106.44, thus 53.25 AH is needed to convert one mole of chlorate ion to perchlorate ion.
My cell contained 600 grams of potassium chlorate, or 4.894 moles. 4.894 X 53.25 = 260 AH for this particular cell, at 100% efficiency, to convert ALL of the chlorate. Assumptions... the cell is 70% efficient (assumption), so 371.4 AH is needed. But converting ALL of the chlorate is a bad idea - stresses the anode. Let's assume the goal is to convert 80% of the chlorate present to perchlorate. 0.80 X 371.4 = 297 AH.
Another way to look at it, in reference to the stop-point... no one's homemade cell is 100% efficient, so if you halt production at the theoretical 100% conversion point, you will leave behind a percentage of chlorate roughly equal to the inefficiency of your cell.
As of this moment, I have pumped 307 AH into this system, somewhat higher than the theoretical 100% level of 260 AH. I will go to my lab and pull the plug, dry and weigh the product. The comes purification.
Postscript: The yield, while it didn't quite suck, was less than I thought. Even worse, these crystals are super-fine, of the sort sometimes referred to as "mashed potatoes" crystals. You spoon a gelatinous blob into your filter, and wait... and wait... for the liquid to drain. Then you have to wash the stupid things, which makes it worse.
Right now they're drying, I'm guessing the yield at maybe 150 grams. No big deal, all the chlorate is still there in the electrolyte, which I saved. I simply need to learn how long (and how hard) I can run a perc cell. This is the first step.
Double Secret Postscript: The yield was better than I thought, at 320 grams. Still, it is not acceptable given 600 grams of chlorate in the cell. Better this than a fried Pt anode.
At 20 degrees C and 4 liters, 100 grams of perchlorate remains in solution. Total perchlorate yield: 420 grams or 3.03 moles.
Summary...
Start: 600g / 4.89 moles KClO3
End: 420g / 3.03 moles KClO4
Dry Yield: 320g / 2.31 moles KClO4
In solution: 228g / 1.86 moles KClO3, and 100g / 0.72 moles KClO4
307 AH ---> 3.03 moles ClO4-, efficiency 52.6%
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