ELECTROLYTIC FORMATION OF PHOSPHINE FROM RED PHOSPHORUS
237
EXPERIMENTAL
The unreacted iodine was titrated with a 0.1 N Na2S2O3
solution.
The completeness of phosphine absorption during
the experiment was checked by bubbling the gas
through a copper sulfate solution. If the gas contained
phosphine, the solution changed color to black owing to
the formation of copper phosphides.
The concentration of hypophosphite ions in the
solution was determined by iodometric titration in a
1−2 M HCl solution. The total concentration of phos-
phite and hypophosphite ions was determined at
pH 8.5.
Before and after each experiment, the gas space over
the catholyte and the gas removal line were purged with
hydrogen or an inert gas.
We used samples of commercial red phosphorus dif-
fering in color and purity (97 to 99%). To remove pos-
sible white phosphorus impurities, the samples were
boiled in a 5% alkali solution, washed with water, and
then vacuum-dried at room temperature.
In I–V measurements, we used both rotating and sta-
tionary electrodes and an IPC-2000 programmed
potentiostat. The potential was scanned from 0 to
−2.0 V at a rate of 0.2 V/s and was measured with
respect to a silver/silver chloride reference electrode.
Electrolysis was performed by two procedures:
using a phosphorus suspension in an aqueous solution
of the electrolyte and using pressed electrodes contain-
ing red phosphorus.
Experiments with a red phosphorus suspension
were carried out in a cylindrical glass electrolyzer fitted
with a ceramic membrane. As the cathode, we used a
perforated cylinder from a sheet of the material to be
tested, and the anode had the form of a 10-mm-diame-
ter tube of N-1 nickel. The volume of the cathode cham-
ber was 250 ml. The catholyte was agitated by a propel-
ler stirrer.
Experiments with pressed electrodes were carried
out in a filter-press electrolyzer. The anode and cathode
chambers were separated by a Nafion cation-exchange
membrane. The catholyte and anolyte were circulated
on a gas lift principle, owing to the gas released during
electrolysis. The cathode chamber measured 1 × 2 cm in
dimensions. The cathode–membrane spacing was
about 2 mm.
The experiments were carried out as follows.
Red phosphorus suspension. The cathode chamber
of the electrolyzer contained a 15% (3.7 M) NaOH
solution (170 ml), red phosphorus (3.0 g), and zinc ace-
tate (1.0 g). The cathode was of lead, and the anolyte
was a 30% NaOH solution. The process was run at a
catholyte temperature of 75–80°C and a current of 4 A
(current density of 0.062A/cm2). To absorb the released
phosphine, the forming cathode gases were bubbled
through a 10% HgCl2 solution. The remaining hydro-
gen was collected in a Mariotte vessel. The hydrogen
volume evolved was measured every 30 min. Table 2
illustrates the dynamics of hydrogen release. Note that
the hydrogen yield in the first run is somewhat underes-
timated because of the zinc deposition on the cathode.
Subsequently, the hydrogen yield was relatively stable
and approached a quantitative yield ( 5% with allow-
ance made for the uncertainty in gas volume measure-
ments). After electrolysis, 2.6 g of unreacted red phos-
phorus was filtered off from the catholyte (i.e., 0.4 g
was spent). According to chemical analysis data, the fil-
trate contained 0.21 g of phosphorus in phosphite and
hypophosphite forms. The absorber, filled with an
HgCl2 solution, contained 0.19 g of phosphorus. The
phosphine yield was 47.5% in terms of phosphorus.
Assuming that phosphine resulted from the electro-
chemical reduction of red phosphorus by reaction (1),
we find that the current efficiency for phosphine
was 4.6%.
The following electrode materials were tested (wt %):
Red
phosphorus
Poly(vinyl difluoride)
binder
Graphitized soot
80
60
60
15
35
35
5
5
5
The materials were pressed at ~20 MPa at room tem-
1
perature.
In a number of experiments, a red phosphorus sus-
pension was sonicated during electrolysis, and an H-
shaped glass electrolyzer with a porous glass mem-
brane, a lead plate as the cathode, and platinum wire as
the anode was used. Sonication ensured vigorous stir-
ring. The volumes of the cathode and anode chambers
were 75 ml, and the cathode area was 2 cm2. The elec-
trolyzer was mounted in the bath (1.3 l) of an ULTRA-
SONIC 1.3LH processor, filled with water.
Pressed electrodes. The cathode chamber of the
electrolyzer (cathode: 80% P, 15% soot, 5% poly(vinyl
difluoride)) contained 40 ml of 4.2 M KOH, and the
anode chamber contained 35 ml of 4.2 M KOH. The
process was run at a current of 0.8 A (current density of
0.4 A/cm2) and catholyte temperature of 64–65°C. The
quantity of electricity passed was 2.7A h, and the current
efficiency for hydrogen was 92%. Under these condi-
tions we obtained 0.0816 g PH3 (0.03 g/(A h)). The for-
mal current efficiency of reaction (1) was 7.1%.
The forming phosphine was absorbed by a mer-
cury(II) chloride solution. The resulting insoluble com-
plex (HgCl)3P was dissolved in a mixture of I2 and KI.
Sonication. The cathode chamber contained a
4.4 M NaOH solution (35 ml) and red phosphorus
(2.0 g). The electrolyzer was mounted in the bath of the
1
Pressing was performed by T.L. Kulova, Frumkin Institute of
Electrochemistry, Russian Academy of Sciences.
INORGANIC MATERIALS Vol. 42 No. 3 2006