Boffis
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Sodium phosphite from the corrosion products of white phosphorus
Has anybody else ever noticed how white phosphorus slow corrodes and disappears even when stored under water? Have you ever wondered what the products
of this corrosion are?
I have done a limited investigation before and found the "water" becomes extremely acidic and mildly reducing. Recently I acquired a small jar that
claimed, according to the label, to contain 50g of white phosphorous but the sticks were now almost hollow and deeply corroded.
I heated the jar in warm water until the phosphorus melted and coalesced into a single glob and the white material largely dissolved and then allowed
it to cool. The colourless solution was filtered off and neutralised with 40% (10M) sodium hydroxide to roughly pH 6.5-7. The solution became very hot
but remained clear and colourless and on cooling it slowly deposited large clear crystals. The crystals were filtered off using a Buchner funnel,
drained by drawing air through them for 10 minutes and then air dried at about 30-35° C. The filtrate was evaporated down further but it became
increasingly viscous and syrup like and would not crystallise even after a week or on treatment with ethanol; with which it was quite immiscible.
The crystals were exceedingly soluble in warm water but the solution tends to supersaturate and I have been unable to obtain the beautiful colourless
crystals I obtained from the original solution. The crystals I did obtain melted easily, lost water and re-solidified, clearly the salt is highly
hydrated.
I expected this material to contain much hypophosphite but tests with hot nickel sulphate do not deposit metallic nickel. Cold copper sulphate
solution produces a pale blue ppt and silver gives a creamy white ppt that only turns black when heated. Barium nitrate and calcium acetate solutions
both precipitate the whole of the reducing power of the salt. My conclusion is that this material is sodium phosphite pentahydate and is essentially
free of hypophosphite which reduces silver salts rapidly and gives no ppt with calcium salts. The syrup has the same properties but apart from the
precipitate with barium and calcium, less so than the crystals, suggesting that their reducing power is the result of residual phosphite. The high
solubility of the sodium salts in the syrup and their viscosity suggests a polymeric hydrophilic compound, perhaps a polyphosphate; further
investigation is required but I have neither the time nor the inclination.
The 18g of sodium phosphite I recovered I intend to subject to iodometric titration when time permits to confirm its identity and test its purity.
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Texium
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Thread Moved
28-4-2020 at 06:28 |
Texium
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Very interesting and good to know! I look forward to seeing the confirmation.
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draculic acid69
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So the white phosphorus slowly dissolves into the water forming h3po3.
I didn't know that happens and I've never heard anyone else suggest it.
Could it be caused by an impurity of synthesis or something in the water
causing it? How old is this stick of p4?
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Boffis
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@draculic acid; I don't know how old this recently acquired jar of P4 was but the suppliers name on the label I have never seen before so it could be
very old >50 years. However, I have found this process occurs quite quickly if the bottle is opened regularly. If you have some white phosphorus
try sticking some appropriate indicator paper into the water. There was no evidence that the jar I acquired recently had been opened recently so this
may be a slow spontaneous process perhaps liberating hydrogen. I have observed this with all of the P4 that I have ever had and have often wondered
what the cause of the acidity was.
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woelen
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I also have white phosphorus and indeed I know this phenomenon. But, it only occurs if oxygen can get into the jar. A bottle with a plastic cap slowly
allows oxygen to get in, even a thick cap is slightly porous. This is not an effect over days, but over many months or a few years.
I have white P in a glass bottle with a very tight plastic cap, with aluminium liner inside. This bottle is wrapped in multiple plastic bags and those
bags in turn are put in a secondary bottle with a tight cap. In this way, the phosphorus is not oxidized further.
I would expect that the following reaction occurs:
P4 + 3 O2 + 6 H2O --> 4 H3PO3
There may also be side reactions, in which other oxidized species are formed, such as H3PO4, H4P2O6, H4P2O4, and H3PO2. Apparently there only is
little H3PO2 and most of the phosphorus is oxidized to H3PO3.
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