The Journal of Physical Chemistry B
ARTICLE
+
+
MBO2 to monomeric species and M BO (M = Li or Na) to a
still valid - that the second-law enthalpy of sublimation differed
significantly from the third-law values in all vaporization studies
performed so far on sodium metaborate.
2
2
more complex molecule (possibly M BO ). These authors did
not report vapor pressures or any other thermodynamic data. In
963, B €u chler and Berkowitz-Mattuck performed the first
detailed KEMS study of vaporization of sodium metaborate,
3
3
6
1
The vaporization of solid sodium metaborate, practically to
1,13,14
single vapor species (monomers), renders TTG
a conve-
the sample prepared by dehydration of Na B O 8H O. They
nient means of obtaining reliable vapor pressures and sublima-
tion enthalpy values. With no TTG measurements reported so
2
2
4
3
2
+
+
+
identified in the mass spectrum the ions Na , NaBO , Na BO ,
2
2
2
+
Na (BO ) in the ratio 2.7:1.0:0.37:0.001. From duplicate
far on NaBO (s), we conducted a systematic study for the first
3
2 2
2
sublimation runs (990 to 1140 K), these authors gave the
enthalpy values at T = 1070 for the sublimation reaction NaBO2-
time to obtain vapor pressures over a temperature range of 150 K.
A reasonably good consistency was obtained between second-
and third-law values. We also conducted a KEMS reinvestigation
not only to confirm whether the vapor phase is predominantly of
monomers but also to resolve the discrepancy that existed within
and among the results of previous KEMS studies. In conformity
with the identification of alkali metaborates as pseudohalides by
(
s) = NaBO (g) and for the dimerization reaction 2NaBO (g) =
2
2
Na (BO ) (g). Employing silver for pressure calibration, they
2
2 2
also gave the partial pressure values again at T = 1070 K for
ꢀ
2
ꢀ3
NaBO (g) (5 ꢁ 10 Pa) and Na (BO ) (g) (2 ꢁ 10 Pa).
2
2
2 2
These results showed unambiguously for the first time that the
vapor phase over sodium metaborate is composed of monomeric
and polymeric species (with the latter, dimer and trimer together,
being only e4% of the monomer). The authors identified alkali
metaborates as pseudohalides based on the resemblance in the
composition and dimerization energies of metaborate vapors
with those of alkali halide vapors. It is not clear as to why B €u chler
6
B €u chler and Berkowitz-Mattuck, the electron-impact ionization
behavior of vapor species also appeared to be similar to that of
alkali halides ꢀ dissociative ionization dominating over simple
ionization, favoring the formation of positive ions with the loss of
+
BO from NaBO (g) and (NaBO ) (g). The intensity of Na
2
2
2 2
+
was 3 to 4 times greater than that of NaBO , and the ion
Na (BO ) was not detected at all. Very disconcerting results
2
6
+
and Berkowitz-Mattuck chose not to report pꢀT relations for
2
2 2
NaBO (g) and Na (BO ) (g).
were obtained in the first two series of KEMS measurements
though: the temperature dependence of ion intensities was often
inconsistent between the runs, and they also corresponded to
relatively lower enthalpy of sublimation than the TTG results.
However, KEMS measurements in the last two series, performed
with the samples that were fully predehydrated in the TTG
apparatus, yielded results that were consistent among themselves
as well as with those from the TTG experiments in terms of the
2
2
2 2
7
In 1971, Gorokhov et al. studied the vaporization of sodium
metaborate by KEMS as well as by the mass-loss Knudsen effusion
+
+
method. They listed the ion intensities of Na , NaBO , and
2
+
Na BO (in the ratio 3.30:1.00:0.25), the values of total vapor
2
2
pressure, and the dimer-to-monomer ratio at 1270 K. They also
deduced the second-law enthalpies of sublimation for the mono-
mer and the dimer from the temperature dependence of ion
intensities (1080ꢀ1230 K). It is not clear as to why these authors
did not report vapor pressures at temperatures other than 1270 K.
vapor pressure and the sublimation enthalpy for NaBO (g).
2
There was good agreement between second- and third-law values
for the sublimation reaction as well. All these aspects of the
present study and comparison with previously reported results
are presented and discussed in detail in the paper.
8
In 1985, Yasue and Asano published a brief report (of Kyoto
University - in Japanese) of mass spectrometric study of vapor-
ization of NaBO (s). They gave pꢀT relations for NaBO (g)
2
2
(
(
914 to 1129 K), Na (BO ) (g) (1005 to 1127 K), and Na -
2 2 2 3
BO ) (g) (1149ꢀ1201 K). These results, while confirming the
2
3
6
2. EXPERIMENTAL SECTION
finding of B €u chler and Berkowitz-Mattuck that polymeric species
constitute no greater than 4% of the vapor phase, yield at T= 1070 K
a value of partial pressure for the monomeric species that is
relatively 3.5 times higher (0.18 Pa). The enthalpy values for
sublimation reactions, iNaBO (s) = (NaBO ) (g) (i = 1 to 3) as
NaBO (s) was prepared by dehydration from NaBO 4H O
2
2
3
2
(Sigma-Aldrich, USA, purity: 99 mass percent) in two steps. The
first step involved heating ∼12 g of the sample (taken in a quartz
beaker) at 423 K for 24 h in a vacuum oven. From the mass loss, it
was inferred that 0.85 mass fraction of water of hydration was
removed from the original sample. The residue was stored in a
plastic container, sealed with cellophane paper, and designated as
‘bulk sample’. The second step of removal of remaining fraction
of water of hydration from the bulk sample was performed in situ
at higher temperatures ꢀ by heating the known aliquots of the
bulk sample in a platinum crucible for TTG study (as well for the
last two series of KEMS measurements) and in a platinum
Knudsen cell for KEMS studies (in the first two series).
2
2 i
well as for polymerization reactions, iNaBO (g) = (NaBO ) (g)
2
2 i
9
(
i = 2, 3) were tabulated by Asano and Yasue at selected tem-
peratures of 1071 K (i = 1), 1066 K (i = 2), and 1175 K (i = 3).
The enthalpy of sublimation to NaBO (g) and that of dimeriza-
2
9
tion of NaBO (g) reported by these authors are in reasonable
2
accord (within error limits) with the values reported by B €u chler
6
and Berkowitz-Mattuck, although it is not clear as to why the
sublimation enthalpy for NaBO (g) reported in ref 9 (Asano and
2
ꢀ1
Yasue) is about 9 kJ mol lower than that deducible from the
3
pꢀT relation given in ref 8 (Yasue and Asano).
The transpiration measurements were performed by using the
commercial thermogravimetric apparatus (Mettler Toledo TG/
Analysis of results of all three mass spectrometric studies
revealed one common disconcerting feature - the third-law en-
thalpy of sublimation of NaBO (s) to NaBO (g) being distinctly
1,13,14
SDTA 851),
byusingamagneticsector massspectrometer(VGMM 30BK).
and the KEMS measurements were performed
1,15
2
2
ꢀ
1
higher than the second-law value: the difference is 28 kJ mol
The details of both instruments are given in the references cited,
and only the salient features pertinent to the present study are
given here.
6
ꢀ1
(
B €u chler and Berkowitz-Mattuck ), 34 kJ mol (Gorokhov
7 ꢀ1 8,9
et al. ), and 32 kJ mol (Yasue and Asano). On the other
hand, the vapor pressures obtained in the studies of Cole and
For TTG studies, the aliquot from the bulk sample was trans-
ferred to a cylindrical Pt-crucible (dia: 6 mm and depth: 4 mm),
nearly up to its brim, and placed on the pan of the micro-
balance, positioned in the center of the horizontal furnace.
1
0
11
Taylor and Kr €o ger and S €o rstr €o m yield higher second-
ꢀ
1
law enthalpies than the third-law values by 132 kJ mol and 54
ꢀ
1
7
kJ mol , respectively. Thus, the remark by Gorokhov et al. is
1
3262
dx.doi.org/10.1021/jp206586u |J. Phys. Chem. B 2011, 115, 13261–13270