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the article was fetched, scanned and brought to my
attention by lugh. i translated only the first part,
the second part is about the molecular size and the
conductivity of hypophosphoric acid.
316 Arthur Rosenheim and Jacob Pinsker: On the synthesis and the molecule
size of hypophosphoric acid
(Received June 21th, 1910)
Salzer, discoverer of hypophosphoric acid, at first synthesised (1) the acid
by air oxidation of phosphorus rods with limited air influx and low
temperature over water. The acidic sodium salt NaHPO3 was precipitated from
the mix of acids by partial neutralisation with sodium acetate or sodium
carbonate. This procedure is used until today whenever bigger amounts of
hypophosphates are to be produced in basically the same manner with only
marginal modifications like for example doing the oxidation directly over a
25% sodium hydroxide solution instead of water or changing the suspension of
the phosphorus rods. It suffers major nuisances though, mainly that the very
slow oxidation is bound to a limited temperature range from 5 to 10° and very
low yields. Below 5° there is nearly no notable oxidation of the phosphorus,
above 10° the oxidation is so fast, that mainly the metaphosphoric and
phosphoric acids are created and the reaction is so exothermic that usually
the phosphorus rods burn down.
Two other known hypophosphoric acid syntheses have not been used for the
preparation of bigger amounts and we tried if they could be used for this
purpose.
Corne (1b) observed that yellow phosphorus, when melted with limited air
influx under a solution of Cu(NO3)2, is coated with a mixture of metallic
copper and Cu2P2, that there is NO2 development and that the discoloured
solution contains, besides phosphorous acid and phosphoric acid, also
hypophosphoric acid, which can be isolated as acidic sodium salt by
neutralising half of the solution with Na2CO3. Philipp (2b) discovered that
silver nitrate reacts with phosphorus in an analogous way and gives the
following instructions: 6g AgNO3 are dissolved in 100 ccm nitric acid (1.2)
and 100 ccm water. 8-9g yellow phosphorus is added to the solution, which is
strongly heated on water bath. When the vigorous reaction subsides, let the
solution cool down and decant it from the unreacted phosphorus. From the
solution, which contains besides phosphorous acid and phosphoric acid also
hypophosphoric acid, a part of the hypophosphoric silver salt precipitates.
The remaining solution is treated dropwise with ammonia, which makes more
silver hypophosphate precipitate until finally yellow silver phosphate starts
to form.
When testing these methods, we could confirm the observations of Philipp in
all points. But the second procedure seems not to be suitable for a
convenient preparation of bigger amounts of hypophosphate, because the amount
of hypophosphoric acid produced is dependent on the amount of silver salt and
the silver is removed from the reaction as insoluble silver hypophosphate. To
bring the silver back into solution, the silver hypophosphate must first be
treated with sodium carbonate and the carbonate turned into the nitrate.
The procedure by Corne is considerably more convenient because it can be
easily transformed into a continuous process with some modifications. If one
adds, according to the specifications of Corne, yellow phosphorus to an
aqueous copper nitrate solution and heats the mixture to 70°, at first a
violent reaction sets in, during which the molten phosphorus is covered with
a layer of red, metallic copper and black copper phosphide and the flask is
filled with nitrous oxide vapours. But the reaction subsides very fast and it
is not possible to achieve a complete discolouration of the blue solution.
The reaction stops before all the nitric acid, which, as can easily be shown,
is also transformed partly into ammonia, is used.
When the concentration of the nitric acid solution is increased, the reaction
proceeds considerably more violent, but with a better yield. One preferably
proceeds in the following manner: in a 3l round bottom flask 100g copper
turnings are covered with 100 ccm water and 200 ccm nitric acid (1.4). After
the violent reaction has slowed down, one carefully adds little pieces of
phosphorous rods to the 50-70° warm solution, which are covered by a layer of
red phosphorus after being exposed to sunlight for several days. In order to
reduce air influx, the flask is loosely closed with a funnel. Evidently, upon
the addition of the phosphorus, the reaction speeds up and the nitrous gases
coming from the solvation of the copper are replaced by colourless nitrous
oxide gases. If the reaction is too fast and phosphorus vapours escape,
little portions of water are added in order to cool it down. If the reaction
proceeds to slow, the flask is immersed in warm water to revive it. When all
the copper is precipitated as copper phosphide or metallic copper, and the
solution is thus colourless, it is decanted, half of it neutralised with
sodium carbonate and the other half added to obtain pure NaHPO2 + 2H2O. By
adding nitric acid to the residue the copper nitrate solution is regenerated
and can be reused for the same reaction.
The problem with this reaction is that it needs constant monitoring because
the very exothermic reaction can lead to burning or big losses of phosphorus
vapours. The yield of hypophosphate is at most 10% of the theoretical amount.
At least the reaction is easily controlled and multiple kgs of sodium
hypophosphate were produced.
If one wants to understand the reaction mechanism, one has to consider that
the action of nitric acid on phosphorus alone does not produce hypophosphoric
acid. When silver or copper nitrate in the two mentioned methods were
replaced by nitrates of other metals, no hypophosphoric acid could be
isolated. We tried the nitrates of zinc, manganese, nickel, cobalt, mercury
(2+) and iron (3+). One has to conclude that in this reaction copper and
silver have a specific, probably catalytical effect. We will continue to
explore this reaction. There have been experiments on the action of copper
sulfate on yellow phosphorus, but they could not explain this reaction in a satisfying
way.
One could assume that the nitrous gases, developed by the action of nitric
acid on copper oxidise the yellow acid to hypophosphoric acid. Would this be
so, then this reaction would basically be the same as the oxidation of
phosphorus rods with restricted air influx at 5-10°. One would have to use a
nitrogen-oxygen compound or a mixture of those, whose oxidation ability
corresponds approximately to the one of air at the given temperature.
For the time being all our experiments trying to prove this assumption were
failures. N2O was, as was to be expected, without action on yellow
phosphorus; NO reacted only at the melting point of phosphorus, NO2 and N2O3
already at room temperature with different intensity. In all cases where a
reaction took place, only phosphoric acid could be isolated. But these
experiments too have to be continued.
Whatever the results of the final clarification of the reaction mechanism may
be, one thing seems to be certain: The development of hypophosphoric acid is
enabled only by oxidation agents that are weaker than concentrated nitric
acid. One thus should be able to obtain the same effect by electrolytical
anodic oxidation of phosphorus, provided that the voltage can be kept whithin
the bounds favourable for the development of hypophosphoric acid. Because
phosphorus is not suitable as anode, metal phosphides had to be used, and
indeed, when electrolysing a slightly acidic solution between an metal
phosphide anode and a cathode of the metal of the phosphide at room temperature
and a voltage between 3 and 10 Volt, hypophosphoric acid is obtained in good,
until now still fluctuating, yield (up to 60% of theoretical yield).
Different metal phosphides were used and it appears that, already by
considering the given voltage range, only phosphides of weakly
electronegative metals are suited. Especially copper phosphide with 14% P,
which is technically produced for the manufacture of copper alloys and
commercially available in medium sized plates is a very utilisable anode. Other
metal phosphides, like probably nickel phosphide and also silver phosphide
behave similarly to copper phosphide, whereas the usage of iron phosphide
leads exclusively to phosphoric acid, but no hypophosphoric acid.
The oxidation must be performed in acidic solution, because in basic
solution, besides other mischiefs, the reaction is hindered by the formation
of insoluble metal phosphates and hypophosphates. The choice of acid is
limited by the fact that oxidising acids like nitric acid or hydro halogenic acids,
through the formation of free halogens during the electrolysis, oxidise in a
second step the hypophosphoric acid and thus reduce the yield, but on the
other hand most organic acids create a solution with too much resistance. The
best results were obtained with solutions of 1-2% sulfuric acid or 3-5%
formic acid. During the electrolysis, the bath was kept at 15° using water
cooling.
The experiments were performed in the following way: The weighed and analysed
anode was suspended in the solution and the electrolysis performed for 24-144
h with a regulation of the voltage. After completion of the electrolysis, the
electrolyte is decanted, resp. filtered from the metal that deposited on the
cathode, its volume measured and the content of the different phosphoric
acids determined according to A. Rosenheim and J. Pinsker (1b). The anode was
weighed and the electricity yield calculated using the weight loss and the
analysis of the electrolyte (1c).
The quantitative course of the experiments will be described later elsewhere,
likewise the modification of the reaction by adding catalysts to the
electrolyte or by using membranes during the electrolysis. It has to be
mentioned that, when using the given voltage, hypophosphoric acid is obtained
in good yields besides phosphoric acid, whereas hypophosphorous acid or
phosphorous acid are detectable only in trace amounts. When increasing
voltage though, the amount on phosphoric acid increases, whereas decreasing
voltage favours the development of phosphorous and hypophosphorous acid. The
little amount of solvated metal in the the electrolyte was precipitated on a
platin cathode by short electrolysis; then half of the solution was
neutralised and the NaHPO3 + 2H2O precipitated by boiling down after adding
the other half of the solution.
Besides other methods, the precipitation of the only very slightly soluble,
very characteristic guanidinium hypophosphate was used for the detection of
the hypophosphoric acid in the solution. This salt can be obtained by adding
a solution of guanidinium carbonate to a solution of free hypophosphoric acid
or a soluble hypophosphate. The salt precipitates immediately, giving white
shiny needles when recrystallised from water.
This compound is of special interest because according to analysis it
pertains to the class of abnormal ammonia salts (2c) and is, as opposed to
other analogous compounds, very stable. It contains 4 mol guanidine per mol
H2PO3: (CN3H5)4,H2PO3 + 5H2O.
(CN3H5)4,H2PO3 + 5H2O calculated: C 11.80 H 7.86 N 41.28 P 7.63
found: C 11.77 H 7.95 N 41.26 P 7.58
The aqueous solution of the salt is strongly alkaline, in fact at 28.5° 1.038
g (CN3H5)4,H2PO3 corresponding to 0.265 g H2PO3 are soluble in 100 ccm water.
With lower temperature, the solubility drastically decreases. Because the
formation of such a nearly insoluble guanidinium salt is a particular
characteristic of the hypophosphoric acid, this reaction is very well suited
for the detection of the latter in mixtures of different phosphoric acids.
[...]
Notes:
1) Ann. d. Chem. 187, 322 (1877).
1b) Journ. Pharm. Chim. (5) 6, 123 (1882).
2b) These Berichte 16, 749 (1883).
1c) Ztschr. f. anorg. Chm. 64, 327 (1909).
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