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XnSi,, = total number of moles of SiF,
titrated,
pSip4 = SiF, pressure at equilibrium
with the melt,
.?;(f'Sip4V//RTm) - correction term which in-
cludes the various dead vol-
umes at different iempera-
lures, Tm,
w : weight of the melt in kg.
IJsing Eqs. (10) and (11) the solubility was found
to be
K, : 0.032 t 0.005 mole kg-' atm-' at 499"C,
K, = 0.035 i0.005 mole kg-' atm-' at 540T.
These solubilities are of the same order of magni-
tudc as the solubility of HF in 2LiF * BeF, (-0.01
mole kg- atm '). ' At higher temperatures, in
the range 600 to 70OoC, the solubility was so low
that by the rather insensitive method of measure-
ment only an upper limit could be set: K, .:0.01
mole kg - atm- '. The data obtained indicate
that at 1000"K, using an SiF, pressure close to
the equilibrium value over 2LiF * BeF , the solu-
bility of SiF, is -0 003 mole %.
Another undesirable side reaction that should
bc considered in these systems is the formation
of silicon oxyfluorides; for example,
Six;,(&) t SiO,(s) 2SiOF ,(g) , (1 2)
3SiF 4(g) t SiO,(s) * 2Si ,OF ,(g) . (13)
Such silicon oxyfluorides have been identified by
Novoselova et d. during the synthesis of phenacite
from Si02, BeO, and Na,BeF, at ternperatures
in the range 700 to 800°C In one meas-
urement they report equilibrium partial pressures
of 0 162 atm of SiF, and 0.07 atm of SiOF, over
the reaction mixture at 813cC. In other experi-
mrnls wherein equilibrium was not reached, they
were able to identify Si,OF,. We performed tests
in which He aftel pnssage through molten 2LiF *
ReF, containing powdered Be0 and SiO, clt tern-
peratures up to 650°C was analyzed by means of
a mass spectrometer. No silicon oxyfluorides
were found to be present.
%. E. Field and J. H. Shaffer, J. P21ys. Chem., in
press.
139
Specfrophotometric Measurements
with Silica Cells
C. E. L. Bamberger J. P. Young
C. F. F3aes, Jr.
Behavior of u4+. - To estimate the extent to
which an SiF overpressure would prevent reaction
(3, UF,, a colored solute which would produce an
even less soluble oxide than does BeF,, was
added to LiF-BeF , melts in SiO, containers. 'retravalent
uranium thus should act as a sensitive
"internal indicator'' of the reaction of F- with
SO,. It has a known absorption spectrum of suitable
intensity, '' and hence its concentration can
be followed spectrophotometrically. For the react
ion
UF,(d) -t Si02(s) + SiF,(g) t I.JO,(s) , (14)
equilibrium partial pressures of SiF, were predicted
to he reasonably low; for example, with
0.002 mole fraction Up-, in 2LiF = BeF,, calculated
SiF, pressures varied from 3 mm at 700°K
to 115 mtri at 1000°K. Spectrophotometric monitoring
of the concentration of U4' provides a very
good means for studying the stability of the system.
Melts of 2LiF BeF containing variable concentrations
of UF, (0.0053 to 0.13 mole % OK 0.003
to 0.008 mole/liter) were held under Sip,# (--400
mm) at temperatures ranging from 490 to 70OoC in
sealed silica tubes. The tubes were placed directly
in a special furnace" located in the light
path of a Cary spectrophotometer, model 141, or
in a heated nickel metal block and later transferred
to the furnace inside the instrument. 'Temperatures
were controlled to floe by means of a Capacitrol
471 controller.
Constancy of the U4+ absorbance peaks at 1090
and 640 nm showed the uranium concentration in
the melt to be constant. within the precision of thc
spectral measurement (about 17%) for at least 43 hr.
It was, therefore, evident. that no significant attack
of the silica container Or significant contamination
of the melt occurred. During longer periods of time
V. Novoselova et ol., Proc. Acad. ~ci.
9 ~ .
USSR,
Chern. Sect. (Eriplish Transl.), 159, 1370 (1964); A. V.
Novoselova and Yu. V. Azhikina, Iziorg. Mater. USSR
(English Transl.), 1(9), 1375 (1966).
"J. P. Young, Inorg. Cherri. 6, 1486 (1967).
"J. P. Yourig, Anal. Chprn. 31, 1592 (1959).