Sciencemadness Discussion Board

Phosphates and electrolysis-safety

MeshPL - 25-5-2015 at 10:49

Hello everyone!

Assume I want to electrolyse solution containing PO4- and Cl-. My friend warned me that PH3 might form, which is really poisonous. Do I have to warry about any PH3, PClx or POCl3 forming?

You know this stuff I voltaile AND poisonous and is not something you can handle very safely without special equipment.

Pasrules - 25-5-2015 at 14:52

I doubt any Phospine would form as the hydrogen would be at the cathode and everything else at the anode. The phosphohalides would be more to worry about as uv can split the chlorine formed to react with the phosphorus.

When in doubt remember your ABCs Atropine, Bicarb, Calcium

Pumukli - 25-5-2015 at 20:49

In a solution (I assume aqueous solution) there is no danger of phosphorous halides or oxy-halides forming. If they formed in any way then they would be hydrolized instantly by water.

gatosgr - 25-5-2015 at 23:12

Can you explain this:
When in doubt remember your ABCs Atropine, Bicarb, Calcium

woelen - 26-5-2015 at 02:37

Phosphate is not easily reduced. PH3 will not form.

PCl3, PCl5 and POCl3 also will not be formed. These are very reactive and will be destroyed by water immediately.

Pasrules - 26-5-2015 at 02:42

@Pumukli
When the gas collects possible chlorine radicals could react with phosphorus if UV light was present however this is me just being pedantic.

@gatosgr
Firstly i'll correct myself and say it is AABC, this was the old protocol for cyanide poisoning which was Atropine to the heart, more Atropine to the heart just in case, Bicarb to reduce blood pH intravenous and a Calcium shot to cause the heart muscles to contract. Basically the atropine blocks the cyanide and in cause of phosphine or organophosphates it binds the acetyl choline nerve receptors to stop the nerve agent attaching thus it can be flushed from the system. The metabolic system when blocked by cyanide causes a large increase in blood pH due to lactic acid build up from anaerobic respiration (my father lost most of his leg muscles from cyanide poisoning overheating the muscle). And then the calcium shot was the start the heart up in case it stooped. The reason they stopped this protocol was for 2 reasons. The calcium would tighten the heart and stop it contracted if the dose was too high and they developed a new system to treat cyanide poisoning. However atropine i believe is still used as an anti nerve gas agent in the military.

If anybody has the notes for the extraction of atropine from nightshade i'd love to see the write up.

MeshPL - 26-5-2015 at 04:54

Ok. Thanks! I wanted to be sure. This phosphorous is terribly poisonous and it would be not worth taking risk if it would form.

gatosgr - 26-5-2015 at 12:28
Quote:

If anybody has the notes for the extraction of atropine from nightshade i'd love to see the write up.



I have no clue about that but you can put the leafs of a plant in a deep eutectic solvent and extract many compounds from them.

And an interesting story: http://www.rsc.org/chemistryworld/2013/09/atropine-belladonn...



[Edited on 26-5-2015 by gatosgr]

Pumukli - 26-5-2015 at 13:20

"@Pumukli
When the gas collects possible chlorine radicals could react with phosphorus if UV light was present however this is me just being pedantic."

I understand the "when the gas collects" part, I think. I also can imagine the presence of UV light (e.g. you do this outside on a sunny day), but where would the phosphorous come from?

You wrote that negative ions would go to the cathode. Cathode is positive. How could the cathode reduce the negative phosphate ion to phosphorous? No chance.
So there wouldn't be any phosphorous present to react with chlorine radicals.

Pasrules - 26-5-2015 at 14:42

I said I doubt any phosphine would form as the hydrogen would be at the cathode and the rest (phosphorus) at the anode. Hope that cleared things up.

Pumukli - 26-5-2015 at 22:26

I see. My thoughts were circling around PClx.

woelen - 26-5-2015 at 23:27

If I read this thread, then I get the feeling that a few persons over here had better read up more on phosphorus chemistry (or maybe even on chemistry in general).

First of all, some practical knowledge does help a lot. Do you really expect some UV-radiation to cause formation of PCl3 or PCl5? Where would the required phosphorus need to come from? In PO4(3-) ions the phosphorus already is in its maximum oxidation state. Only reduction can form phosphorus, not further oxidation. So, at the anode no phosphorus can form in any way. And then, if chlorine is present with phosphate, what would you expect to happen, even in the presence of UV? Think practically, there is a lot of water around, phosphorus already is at maximum oxidation state. The only plausible reaction is reaction of chlorine with water, forming HCl and O2 or forming HOCl and HCl.

The attitude of being careful and being prepared for formation of highly toxic compounds is a good one, but it is important to have some practical insight as well. Otherwise you can see snakes and bears everywhere with every reaction.

Another practical thing to keep in mind is that electrolysis does not easily make dangerous amounts of toxic chemicals. Keep in mind that for producing one mole of electrons, a current of 1 A needs to flow for nearly 100000 seconds. So, if you run an electrolysis for a minute or so, you only get millimole quantities of electrons and even less of new compounds. And finally, even in a small room, only a fraction of these small quantities of toxic compounds end up in your nose, unless you stick you nose into the vessel in which the electrolysis is performed. There are very few compounds which become really dangerous in sub-mg quantities (e.g. some wargases, neurotoxins), but you do not need to be afraid that you accidently make these from common chemicals like phosphates, chlorides or acids thereof.

chornedsnorkack - 27-5-2015 at 02:42

Marsh reaction is a classical test for As and Sb. And the "nascent hydrogen" is a legend - Zn electrode is nowhere near negative enough to produce real H atoms. No, As and Sb are reduced by surface of metal Zn acting as an electrode in acid.

Is PH3 really so much harder to reduce to than AsH3?

woelen - 27-5-2015 at 05:30

Phosphorus is quite different from As and Sb. It is remarkable how inert the highest oxidation state is for the last few elements elements in that row (compare also SiO3(2-), SO4(2-) and ClO4(-)). It is very hard to reduce any of these ions in aqueous solution, and phosphate is no exception. Things become different for non-dissociated concentrated acids.

I have experimented with this myself a few years ago, just out of curiousity. The only ion which showed noticeable reduction with zinc in acidic aqueous solution is sulfate. When you add finely powdered zinc to dilute sulphuric acid, then you obtain a rotten egg smell, albeit not particularly strong. With the other acids (H3PO4 and HClO4) I was not able to get any of the oxoanion reduced in dilute acidic solution. I could not test H2SiO3/SiO2, this is insoluble in water.

papaya - 27-5-2015 at 05:53

Look also into this thread: www.sciencemadness.org/whisper/viewthread.php?tid=31693#pid3...

woelen - 27-5-2015 at 23:55

The amounts of phosphine obtained in this way are VERY low, only trace amounts. But you might be able to smell it. Our nose is very sensitive to PH3 and especially the impurities associated with it (P2H4 and organic derivatives of it).

The only acid which gives more than trace amounts of reduced species is H2SO4. With that you can smell some H2S and the liquid becomes slightly cloudy (due to formation of elemental sulphur, which is formed when H2S is oxidized by air or by SO2, which also is formed in the reaction). But even with H2SO4, the amount of sulfate, which is reduced, is only a tiny fraction.

Complete reduction of phosphates, sulfates (and also perchlorates) requires dry chemistry at high temperatures. With carbon and metal powders it is possible to reduce these to phosphide, sulfide and chloride, and then these reactions can be very violent, or even flash-like. E.g. KClO4 with Al produces flash powder, but it also is possible to make flash powder with BaSO4 or CaSO4, although the stuff is not easily ignited.

j_sum1 - 28-5-2015 at 00:54

So woelen, are you suggesting that I might at high enough temp get a thermite going with something like iron phosphate and Al or Ti? (I have not looked up thermodynamic data yet.)

edit.
You know, I should think before posting. The suggestion above might give me a bit of Fe.
Question remains though. Is it possible to convert phosphate to phosphide through a thermite-type reaction.
(I have come across CaSO4 / TiO2 thermites. Is there a phosphate equivalent?)

[Edited on 28-5-2015 by j_sum1]

woelen - 28-5-2015 at 03:23

I have the impression that reduction of phosphate is even more difficult than reduction of sulfates. If you look at the phosphorus thread, then you see that making phosphorus requires strong and prolonged heating of phosphates with a reductor. So, I do not expect that you can find a thermite-like reaction which produces phosphides or phosphorus. Thermite-like reactions with phosphates are possible, but then the counterion works as oxidizer. E.g. if ypu prepare dry silver phosphate, then I can imagine that this can make quite a powerful mix with aluminium and that ignition of this mix produces metallic silver and mainly aluminium phosphate. Maybe a small fraction of the phosphorus is reduced if excess aluminium is used, but I never tried myself, so this is speculation only.

On the other hand, with sulfates, it is possible to have thermite-like reactions, or even flash-like reactions.

chornedsnorkack - 28-5-2015 at 07:36

Quote: Originally posted by woelen  
The amounts of phosphine obtained in this way are VERY low, only trace amounts. But you might be able to smell it. Our nose is very sensitive to PH3 and especially the impurities associated with it (P2H4 and organic derivatives of it).

The only acid which gives more than trace amounts of reduced species is H2SO4. With that you can smell some H2S and the liquid becomes slightly cloudy (due to formation of elemental sulphur, which is formed when H2S is oxidized by air or by SO2, which also is formed in the reaction). But even with H2SO4, the amount of sulfate, which is reduced, is only a tiny fraction.

Depending on concentration, presumably?
How conductive are concentrated solutions of H3PO4 and H4P2O7? What happens on cathode when H4P2O7 is electrolyzed?

woelen - 29-5-2015 at 01:35

Solutions of H3PO4 and condensed variations of it are not very conductive. H3PO4 is not a really strong acid and most of the acid simply exists as molecule.

You can of course make a solution of H3PO4 much more conductive by adding some HCl, but this has the disadvantage that at the anode, there is formation of Cl2, which interferes with reduction at the cathode.
If you have HClO4, then you can use that to make the solution more conductive. At the anode, this will lead to formation of O2 and acid if you have a suitable anode, so no extra contaminants are produced, and at the cathode, the ClO4(-) will not be reduced.

Maybe (but I explicitly "maybe") you get a small amount of PH3 at the cathode. It does not really matter whether you have H3PO4 or a more condensed species. Phosphorus in oxidation state +5 simply is very hard to reduce to a lower oxidation state.

MeshPL - 29-5-2015 at 12:06

Ok, I pretty much knew nothing bad would happen.

I just asked, because a friend, who I supposed is even better at chemistry warned that PH3 or something may form.

chornedsnorkack - 29-5-2015 at 12:29

Quote: Originally posted by woelen  
Solutions of H3PO4 and condensed variations of it are not very conductive. H3PO4 is not a really strong acid and most of the acid simply exists as molecule.

H3PO4 is not a strong acid compared to H2SO4, but H2SO4 is not a very strong base, either. Concentrated H2SO4 has a conductivity minimum near - not exactly at - 100 %: H2S2O7 is strong enough to be a (weak) acid capable of protonating H2SO4 and making oleum more conductive than sulphuric acid.
Quote: Originally posted by woelen  

You can of course make a solution of H3PO4 much more conductive by adding some HCl, but this has the disadvantage that at the anode, there is formation of Cl2, which interferes with reduction at the cathode.
If you have HClO4, then you can use that to make the solution more conductive. At the anode, this will lead to formation of O2 and acid if you have a suitable anode, so no extra contaminants are produced, and at the cathode, the ClO4(-) will not be reduced.

Does addition of H2SO4 also protonate H3PO4 and make it more conductive than pure phosphoric acid? Of course, then reduction of sulphur might become competitive...

woelen - 29-5-2015 at 13:19

From your last post it is not clear what circumstances you are referring to. My posts are about aqueous solutions, I am not talking about concentrated acids. With pure acids, both H3PO4 and H2SO4 only will be slightly conductive.

chornedsnorkack - 29-5-2015 at 23:15

Quote: Originally posted by woelen  
From your last post it is not clear what circumstances you are referring to. My posts are about aqueous solutions, I am not talking about concentrated acids. With pure acids, both H3PO4 and H2SO4 only will be slightly conductive.


The original post was not specific about the concentration of H3PO4. So the only requirements were that it should be liquid (rules out the exact compositions of H3PO4, which freezes at 35 degrees, and H3PO4.H2O, which freezes at 28 degrees, but leaves in the eutectics between them, including the eutectic between H3PO4 and H4P2O7, which freezes at 16 degrees) and the requirement that some chloride be dissolved in it.

Concentrated H2SO4 may have low conductivity compared to dilute battery acids, but the ion product of pure H2SO4 is still estimated to be in the region of 10-4, compared to pure water that has just 10-14. It seems to me that the possibility of electrolyzing concentrated sulphuric acid and getting various side reactions is not impractical.

Regarding quantity of chemicals produced by electrolysis: is it a correct range, that lead batteries by electrolysis increase concentration of sulphuric acid from 20 % to 40 %?

Electric devices to produce large amounts of sulphuric acid, lead and lead dioxide, and the devices to produce such a current, are therefore widely available consumer devices. These can also be, and are, used to hydrolyze water. And commonly explode due to buildup of hydrogen.

Is electrolysis of 40 % sulphuric acid on lead cathode accompanied by formation of smellable amounts of hydrogen sulphide in the resulting hydrogen?

[Edited on 30-5-2015 by chornedsnorkack]

[Edited on 30-5-2015 by chornedsnorkack]