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Author: Subject: Preparation of elemental phosphorus
blogfast25
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[*] posted on 7-2-2010 at 08:51


Quote: Originally posted by Myfanwy  
could phosphine and hydrogen peroxide yield some P4?

2PH3 + 3H2O2 -> 6H2O + 1/2 P4



Possibly but why go to the trouble of making a highly toxic, highly inflammable gas when you can make P from Calgon tablets, some fine sand and some Al powder?
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[*] posted on 7-2-2010 at 09:35


@blogfast25 - depending on where one buys it, Calgon(tm) may contain no phosphorous :( - some other products may - one has to look at the fine print on the package. Algal blooms & such are a p.i.t.a....
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[*] posted on 7-2-2010 at 11:44


i tested it today with small amounts in a test tube. Even without sand P4 is produced.

you ever smelled the vapours, when white P is stored under water. This odor is really nice and exotic.
Dont know how to describe it better, but its not phosphine.

some strange Phosphorus oxides/acids..




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[*] posted on 7-2-2010 at 11:49


Mmmmm! I can feel my jaw softening already! :D



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[*] posted on 7-2-2010 at 12:55


Quote: Originally posted by densest  
@blogfast25 - depending on where one buys it, Calgon(tm) may contain no phosphorous :( - some other products may - one has to look at the fine print on the package. Algal blooms & such are a p.i.t.a....


If you scroll up, you'll see I made my own metaphospate, as have others here. It can be made from just about any source of orthophosphate...

[Edited on 7-2-2010 by blogfast25]
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[*] posted on 10-2-2010 at 02:57


In the olden days when matches were made from white P4, the sort that could be struck on any rough surface, "phossy jaw" was common among the factory workers who were exposed to its vapor. Its symptoms were loss of bone density of the jaws and other soft bones in the head, causing loss of teeth eventually. Elemental arsenic is at least as bad in this regard, also being a cumulative poison.
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[*] posted on 28-2-2010 at 13:55
Attempted Reduction of AlPO4 with Carbon


Introduction

Intrigued by an article posted by SC Wack (Preparation of Elemental Phosphorus), concerning reduction of AlPO4 with carbon @ 1100 C yielding 72% of available P (IEC vol. 21, no. 11, page 1130) I did an experiment to verify the claim and explore an approach.

Methodology and Fixturing

I used a temperature controlled quartz tube furnace to heat the reactants, fed by a stream of inert or nearly inert gas to sweep the reaction products toward the exit portion of the tube. This is an improvement of a furnace design I’ve been using for about 5 years which utilizes a ni-chrome element from a cheap toaster oven re-formed over a ¾” (1.9 cm) threaded steel rod and heated to red-heat to anneal. After cooling, the element is “screwed off” the mandrel and fitted directly onto a 56 cm section of 18mm OD quartz tubing. The heating element portion of the tube (23 cm in length and near the input end) is then wrapped with 5 turns of ½” (1.25 cm) kaowool blanket and fitted into a section of 10 cm dia. galvanized pipe. The quartz tube is cut from a longer section with a dremel diamond dust cutoff wheel and its ends are fire-polished with a MAPP gas-oxygen torch to anneal the quartz and reduce the chance of cracks. The torch is barely able to fuse the quartz ends. Rubber stoppers are used in each end—2-hole at the entrance for temp probe and gas inlet tube and single hole at the other for gas exit. A 90 deg tube from this stopper terminates just under the surface of H2O in a 100 ml beaker. The reaction tube temperatures at each end do not go much beyond 100C when operating at full temperature. “00” stoppers fixed to the ends of dowel rods will fit snugly into the quartz tube and are used to position and contain the reactant mixture in the area of the heating coil during assembly. After assembly and final positioning of the furnace, the dowels and stoppers are removed and the tube ends fitted with the aforementioned 1 and 2 hole stoppers.

The temperature controller is an old design, originally used for an oil bath but now re-biased for control of much higher temperatures. The temperature probe is a k-type thermocouple (Omega TJ36-CAXL-18G-12) the leads of which terminate at the input to an AD 595. The 595 output is differenced with a reference voltage and that result input to a SG3524 variable proportional controller which is synchronized to the power line. The output of the 3524 triggers a triac via an opto-coupler. The triac drives the heating element.

The sweep gas used was “balloon grade” helium stated to be 94-96% He. The tank is a low pressure disposable type pressurized to 18 atmospheres (260psig) when new. I’m sure these tanks are neither back flushed nor evacuated prior to filling, so I’ll assume they contain 1 atmosphere of air and 18 of He. This would make the residual O2 content ~ 1%. Because I could not effectively control the flow from the tank using only a needle valve, I adapted a single stage acetylene regulator to the tank fitting. This involved removing the CGA (Compressed Gas Association) 510 nut and nipple from the regulator body and replacing with a ¼” flare fitting. All connections to the regulator body are ¼” NPT so commonly available plumbing hardware was used. The photo shows the tank pressure to be about 40 psig and the output gage pointer is just off the stop at about 2 psig. With the output needle valve, this arrangement maintains a uniform flow throughout the experiment- set it and forget it. Flow rate used was about 1.5cc/sec.

During a test run, I placed a layer of aluminum oxide (assumed to be inert) throughout the heated length of the tube so that it filled the tube about halfway (half of tube diameter) and then passed a current of air (aquarium air pump) through the tube. I stepped the heating up incrementally to check performance. It appeared to work fine up to and including 1250 deg. C was as far as I needed or wanted to go as I was concerned with burning out the element. Also, although the probe is calibrated to 1335 C, it is not recommended to use it over about 1150 C continuously. The flow rate of air through the tube was varied to check out the effect on the temperature controller duty cycle. There was not a perceptible change for the modest flow rates used.

Preparation of AlPO4

I prepared AlPO4 (intended yield 5 g) from 200cc solutions of Al2(SO4)3 (15.5 g) and Na3PO4.12H2O (14g):

Al2(SO4)3 + Na3PO4.12H2O ==> Na2SO4 + AlPO4


When these solutions are mixed, the insoluble AlPO4 immediately precipitates. As I could not be sure of the exact water of hydration in the Al2(SO4)3, I heated it first to drive off most of the H2O, and for purposes of calculating a weight to use, assumed an anhydrous product. I added an additional 200 ml of H2O to reduce the concentration of the remaining Na2SO4 solution to .1M. The resultant solution pH was highly basic so I incrementally added Al2(SO4)3 until the pH was lowered to 6.1, the published pH of a .1 M sol. of Na2SO4. The excess liquid was poured off and the remaining liquid and precipitate were filtered through a Buchner funnel. Yield after drying was 4.8 g out of 5g theoretical.

Reduction

The reaction to be tried was: 2AlPO4 + C ==> 2Al2O3 + CO2 + 2P. The authors used equal weights of AlPO4 and C although this results in a large excess of C by stoichiometry, possibly to ensure that all of the AlPO4 is reduced.

I added 4.5 g of dried AlPO4 to 4.5 g of carbon black and mixed this in a coffee grinder for two minutes. I was only able to get 4.7 g (of the 9g total) of this mix into the reaction tube as I wanted it no more than half full. More could have been added by reducing the headspace above the mix and/or by tightly packing the mix.

4.7 g of reactants would represent .6g available P at 100% yield. With a yield of 70% this amount would be reduced to about .4g.

Experimental

When finally assembled and ready, I started the gas flow and let it run about 10 minutes before ramping the temperature to 650 C. After allowing the initial transient to settle, I continued to ramp the temperature in 100 deg increments every 5 minutes. As the temp transitioned from 1050 to 1150 C, the exit portion of the tube darkened and the exhaust gas bubbles burst into flame as they surfaced in the beaker. I continued the temp ramp to 1250 C. After a few minutes at this temp, the probe readings (monitored at the 595 output) became erratic, dropping to as low as 600 and to as high as 1400. I suspected a poor connection in the circuitry or a failure of the 595. After a few minutes, I shut the power off to the controller but left the gas flowing and allowed the tube to cool. As it cooled the readings became steady again and at 250 C I removed the insulation and quartz tube from the galvanized pipe section and unwrapped the kaowool. As I unwrapped the tube it broke into two pieces. (Fig 7) The area under the heating element was extensively cracked, and through handling, another section broke off. I removed the tc probe and found that the Ni-Chrome sheathing had melted and the melt had largely gathered into three globules. One of these had a vitreous solid adhering to it –this appeared to be a piece of melted quartz.

Some phosphorus was evident in the exit section of the tube (Fig 9) but this was not recovered. About 2.7 g of the original 4.7 g of reactants was recovered (Fig 8) and this had not fused but was still a loose powder as described in the article.

Conclusions

It appears that AlPO4 is reduced by carbon in the vicinity of 1100-1150 C as described, although I was not abIe to confirm the % released (claimed as 72-83% after 1 hr) as the apparatus itself was also reduced to junk. It also appears that a carbon reaction, presumably with O2, is responsible for the extreme temperature excursion experienced. I had not thought of it at the time, but the He tank I used had been sitting in the garage for over a year since it’s purchase. In that time the pressure had dropped from 255 psig to 50 psig. If that represented He preferentially escaping vs air, the O2 content could have re-enriched by a factor of 5 to ~ 5%. That could have been enough for a low-level charcoal fire to have taken place in the tube.

I will probably try this again after I have replaced the quartz tube, perhaps with CO2 as the sweep gas. CO2 will nearly certainly be reduced to CO at temp in the excess carbon environment and suitable precautions must be taken to ensure safety. Also, this gives me an idea for a very hot tube furnace, with temperature modulated by air flow. Materials (alumina tubes, type R thermocouple) would be expensive.



Fig. 1 - Reaction loaded and captured under heating element



Fig. 2 - Assembled, ready to begin



Fig. 3 - Temperature controller



Fig. 4 - He tank and gas regulator



Fig. 5 - Inlet end of furnace at temperature



Fig. 6 Exit end of furnace and gas beaker at temperature



Fig. 7 - Broken tube, temp probe and residual reactants



Fig. 8 - Carbon glow



Fig. 9 - Some P condensed in exit section of tube



[Edited on 28-2-2010 by Strepta]

[Edited on 28-2-2010 by Strepta]
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[*] posted on 28-2-2010 at 14:55


Strepta, I think this is very nice work for a first attempt, especially if that is really P condensed in the exit section.

Might it perhaps be better to embed or otherwise surround the heating element with some type of refractory that would give a more uniform temperature distribution in the quartz and avoid hot spots from direct contact with the hot wire? I guess refractories would have low thermal conductivities and might not work as I am thinking.

Better luck on your next try, which I hope is soon!
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[*] posted on 28-2-2010 at 15:09


Very interesting experiment. I'm intrigued by the gas bursting in flame after passing through water in the beaker. Surely this is CO, right? I never saw this with my ZnO reduction with carbon.

I suggest you look into buying an argon cylinder: not too expensive, refillable, and readily available at your compressed gas dealer.

I agree with entropy that it would be better to wrap your heating element around a ceramic tube such as one made of Al2O3 or mullite. I presume you have seen the designs of garage chemist and myself. I have taken my Al2O3 tube up to 1300C many times with no apparent problems.




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[*] posted on 28-2-2010 at 15:40


The spontaneously flammable gas may have been phosphine but that would beg the question - where did it get the hydrogen?
The breaking element could have produced a hot arc which would melt quartz and possibly account for the fluctuating temperature.
I'm guessing though!
But you sure came close - next time (or the one after that?) could be entirely successful if you decide to go through it one more time.

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[*] posted on 1-3-2010 at 00:43


There could be moisture in the tank, but not very much. The charcoal could adsorb a lot, though. What source of charcoal, was it calcined before use?

Tim




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[*] posted on 1-3-2010 at 01:43


Strepta

"The sweep gas used was “balloon grade” helium stated to be 94-96% He. The tank is a low pressure disposable type pressurized to 18 atmospheres (260psig) when new. I’m sure these tanks are neither back flushed nor evacuated prior to filling, so I’ll assume they contain 1 atmosphere of air and 18 of He. This would make the residual O2 content ~ 1%"

I think you have much more O2 than you realize. To keep children enticed by the high pitched voice from suffocating if they inhale the gas every balloon grade He I have seen contains much oxygen. I ran into this trying to build a lifting device where I was experimenting with carrying a wire high into the air in an experiment measuring the voltage on the wire. To save money I tried balloon gas from more than one vendor eventually having to buy a tank of pure He. The amount of oxygen was so high it was negating the lift I needed without resorting to filling an unrealistic number of surplus weather balloons joined together. Meaning of course the entire 5 or 6 percent is oxygen not the 1 percent you think it is.


[Edited on 3-1-2010 by IrC]




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[*] posted on 1-3-2010 at 04:53


Quote: Originally posted by Magpie  
I suggest you look into buying an argon cylinder: not too expensive, refillable, and readily available at your compressed gas dealer.

I agree with entropy that it would be better to wrap your heating element around a ceramic tube such as one made of Al2O3 or mullite. I presume you have seen the designs of garage chemist and myself. I have taken my Al2O3 tube up to 1300C many times with no apparent problems.
I'll second both of these suggestions. Argon is a good choice of carrier gas. It's cheaper that helium and doesn't leak out of every nanoscopic pore.

The reason to use a ceramic tube is to mechanically support the heating element. The strength of heating wire goes down at high temperature, eventually enough that it can't support itself. My guess is that you had a hot spot on your reaction tube that made a weak spot in the wire. That in itself wouldn't have been a problem, unless the wire were subject to mechanical forces. As a wire loosely wrapped around a tube, it was free to move. The hot spot would have caused differential thermal expansion at that point, which is enough to generate such forces. By cementing heating wire to a fixed ceramic tube, it constrains the motion of the wire, greatly reducing the need to rely on the mechanical strength of the wire itself.
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[*] posted on 1-3-2010 at 05:49


Balloon grade helium is 94% He and the rest being O2 to stop suffocation of poeple who breathe it soley to get a high voice :O
Maybe use N2 carrier gas? Cheap and more effective!
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[*] posted on 1-3-2010 at 05:50


Edit: deleted double post.

[Edited on 1-3-2010 by Picric-A]
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[*] posted on 1-3-2010 at 06:13


Frankly, He is too rare (on Earth) and valuable to be used just for party balloons and unmanned advertizing blimps (for which H2 could be used), or for inert-gas applications for which the much more common Ar can be used. Because the only economic source of it on Earth is from certain deposits (as in Texas) of natural gas contained in sedimentary rocks derived from granite, which contains U-238 and U-235 and K-40, the alpha-decay of which (and of daughter isotopes) gives rise to He nuclei, its availability (by separation from these natural gas deposits which are likely to be exhausted by 2100 at the present rate of consumption) is quite finite. While there is a small amount of it in the atmosphere, it is too small for economic extraction by fractionation of liquid air; and once in the atmosphere it is slowly lost by diffusion (like gaseous H2 unless oxidized) into space due to its low atomic weight.
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[*] posted on 1-3-2010 at 17:57


I should clarify my objectives a bit. In the above experiment with AlPO4 I wanted to verify that P was reduced with carbon at or near 1100 C in the yields described (72%) and stimulate some discussion of this as an approach. My longer term interest is in exploring paths for the preparation of elemental P at the amateur level. This assumes no extreme temperature capability such as carbon resistance tubes, MoSi2 heating elements, arc furnaces, etc, but limited to the temperatures attainable in resistance wire tube furnaces such as those nicely documented by garage_chemist and Magpie or other readily available means. The chemistries exlored to date include:

1) Reduction of Pb3(PO4)2 with H2 @ 700 C

2) Reduction of NaPO3 with pyro Al

3) Reduction of AlPO4 with C @ 1100 C

I've also conducted some experiments atempting to reduce H3PO4 by adsorbing it onto activated charcoal and heating in a microwave but had no success with that approach.

@ Magpie: The flames may well be hot CO oxidizing to CO2 as it emerges from the water. I do not recall seeing any "smoke".

As far as the constuction of the apparatus goes, I've used this before to 1200 C without incident. I don't believe it was an issue with the resistance wire on the quartz (which did not burn out by the way) so much as a "charcoal fire" in the tube. The inconel sheathing of the thermocouple probe melted off implying temps on the order of 1400 C. The vitreous deposit on one nodule of sheathing would likely have been from either the quartz tube or the AlPO4 --either would imply temps > 1500 C.

@12AX7: I preheated the C to 450C but it was not truly "calcined" and did contain some moisture as was evidenced in the exit portion of the tube when first heated to 650 C. I allowed this to dry out with the sweep gas before further ramping the temperature.

[Edited on 2-3-2010 by Strepta]
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[*] posted on 1-3-2010 at 18:38


@strepa - Your careful description of your experiment is a model of clarity - congratulations!

If you get Ar from a welding supply house (usually by far the least expensive place to get it!) be sure to ask for pure Ar since welding Ar usually has 1% O2 in it because it improves the weld quality.

For such low pressures and low rates of flow from a high pressure (140 bar/2000 PSI or so), a "two stage" regulator would perhaps be beneficial. They are quite expensive new but can be had for $40 or so on EBay or other surplus sources.

Nichrome heating element wire is surprisingly(?) inexpensive from companies who deal with it in quantity. Arklay S. Richards www.asrichards.com sells standard thermocouples and bulk resistance wire at reasonable prices. I had them fabricate 25 simple type K thermocouples for me at about $1 each - 100 wouldn't have cost much more. They sell bulk nichrome and if I remember their web site, they will wind it into heating element form for a not excessive cost. I don't know if they sell Kanthal or other super high temperature elements.

Mullite/Al2O3 tubes show up as surplus or overstock occasionally at industrial or scientific suppliers - I got two 1" x 36" x 3/16" wall mullite tubes for $12 each a while back.

I -think- the AD595 has a "broken thermocouple" indication on some output. I've always bought complete temperature controllers. The Omron 1/16 DIN size (universal temp sensor input, relay outputs for 1A or so) used to go for $40 or $50 on EBay. They come calibrated and have a lot of features like proportional-integral-differential control which make operating the furnace more consistent.

Perhaps some of the furnace details should be archived for reference?

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[*] posted on 1-3-2010 at 20:10


@strepa: In looking at your pictures again there appears to be a considerable amount of yellow flame within the quartz tube. GC says that P burns with a characteristic yellow flame. Perhaps that is P burning. But I'm not sure where the O2 would be coming from other than the He tank or a leak of some kind. If it was just a small amount of O2 coming in with the He you would think that your large excess of carbon would take that up as CO.



[Edited on 2-3-2010 by Magpie]




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[*] posted on 2-3-2010 at 13:28


Quote: Originally posted by Magpie  
@strepa: But I'm not sure where the O2 would be coming from other than the He tank or a leak of some kind.
[Edited on 2-3-2010 by Magpie]


Commercial He used for filling balloons is not pure He but is cut with air which is the source if thats where he got his He from.





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[*] posted on 3-3-2010 at 17:31


Sorry about that last post (removed). I'll edit the photos down in size and re-post later.
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[*] posted on 4-3-2010 at 13:46


Not quite sure why you didn't go with the NaPO3/Al/SiO2 reaction. With this very interesting set up (but using Argon welding gas instead of He), you should have had some nice phosphorus by now.

At those temperatures, consider also the original method: bonemeal (calcium orthophosphate), sand and carbon...
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[*] posted on 4-3-2010 at 17:22


I took some photos of the quartz tube from my AlPO4 experiment near the point where the tube broke. There is a white, porcelain-like circumferential deposit inside the tube which is fuzed to the quartz wall. Everywhere this deposit is present, the quartz is shattered. It's likely that the tube failure was due to the difference in coefficients of thermal expansion between the coating and the quartz as the tube cooled. This white coating is probably either Al2O3 or AlPO4. I measured the thickness of a sliver of the coating and found it to be .05 mm; the tube wall is 1.5 mm. The last photo is of the temp probe beside the same model of a new probe. Things got pretty toasty in there. I will probably repeat the experiment in the near future using CO2 as the sweep gas.











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[*] posted on 5-3-2010 at 07:50


@ Strepta:

The coating must be alumina, possibly with some unreacted phosphate. I've been fooling around with trying to coat things with alumina as a by-product of reactions, you've inadvertently succeeded!

CO2 is likely to be an oxidiser for Al in these 'toasty' conditions but whether the Al will favour being oxidised by the CO2 or by the metaphosphate depends on the chemical equilibria reigning.

Cheap argon welding gas is still your best bet as an inert atmosphere but have you considered not using an inert atmosphere at all? The amount of oxygen contained in your assembly is really quite small: at STD conditions 22.4 L of O2 is still only about 32 g (1 mole). Without inert gas, some P would be lost to oxidation by O2 but once the small amount of O2 has been scavenged, your P is good to go!

Which type of thermocouple did you use and where did you get them?
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[*] posted on 5-3-2010 at 10:51


@blogfast:

I've used the CO2 before even with elemental Al in the tube. While some may have been consumed, the amount did not appear to be significant.

The gas is used solely for sweeping the product away from the reactants and making recovery/harvesting of any P4 easier.

I favor CO2 because of ease of fixturing, availability and cost. A reducing gas such as H2 could also be used, and I did use it (produced from an electrolysis cell)in a reaction at 700 C described somewhat upthread. Some are concerned with using flammable gas at elevated temps but the volume in these experiments is so small that even a tube failure should not produce more than a loud pop.

The probe is an Omega TJ36-CAXL-18G-12.
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