I N D U S T R I A L A N D ENGINEERING CHEMISTRY
522
Vol. 17. No. 5
Partial Pressure of Water Out of Aqueous Solutions
of Sulfuric Acid'
By Crawford H. Greenewalt
BIPERIMEWCAL S T A T I O N ,
E. 1.
DU P O N T DE
H E data on the partial pressures of water out of sulfuric
acid solutions may be divided into two groups, those
for solutions up to 50 per cent strength and those for
solutions over 50 per cent strength. Many investigators
have determined the vapor pressures of the weaker solutions
and their data are remarkably concordant for all temperatures and concentrations. For acids stronger than 50 per
cent, however, the data are very sparse and the agreement
between various investigators is very poor. This lack of
agreement may be seen in the chart in which the logarithms
of the vapor pressures for a given concentration have been
plotted against the reciprocals of the absolute temperature.
The general form for the vapor pressure equation may be
derived from the Clausius-Clapeyron equation
T
e=--..L
(1)
T(v - u )
where T is absolute temperature in O C., L the total heat of
vaporization, V and v the specific volumes of vapor and liquid,
respectively. Assuming that the vapor obeys the perfect
gas laws, and that the volume of the liquid is negligible,
we have
dt
Logp = A -
L
1
2.3.R
T
(2)
It is obvious, then, that the slopes of the curves obtained
by plotting log p against 1/T will be dependent on the total
heat of vaporization of the vapor out of the solution.
Porter gives values for L a t various concentrations and
temperatures. An examination of these values shows that
the latent heat increases with increase in concentration, and
for any given concentration decreaseswithincrease in temperature. Applying these generalities to Equation 2, it will
be seen that the slopes of the vapor pressure curves should
increase with increase in concentration, that instead of being
straight lines they will all show a slight concave downward
curvature, and that this curvature will become more pronounced as the concentration decreases. These facts afford
a basis for a critical examination of the available vapor
pressure data.
The investigators who worked with strong sulfuric acids are
Burt, Briggs, Regnault, Sorel, and Daudt. Of these Burt
is by far the most outstanding. He used a dynamic method
which consisted in determining the boiling points of
acids of various concentrations a t reduced pressures. He
seems to have worked with extreme care, which is reflected
in the remarkable concordance of his very large mass of data,
but his method limited him to vapor pressures not lower than
35 mm., which a t high Concentrations represent temperatures of 100" to 200" C.
Briggs worked over the same range as Burt, using an airsteaming method in which a known volume of air is passed
through solutions of sulfuric acid, and the water vapor in the
effluent air absorbed in pumice and sulfuric acid. Unfortunately, Briggs used an incorrect formula for calculating
his vapor pressures, which introduced errors in his final
results amounting to from 20 to 100 per cent. His original
data, when recalculated according to the correct formula,
give results which are fairly concordant with those of Burt.
1
Received January 13, 1925.
NEMOURS
82 C O . , W I L M I N G I O N , DEL.
Regnault, although quite consistent with other investigators
a t low concentrations, deviates greatly a t the higher ones.
This is in all probability due to his use of the static method,
which is not very satisfactory for vapor pressures of less than
1 mm., owing to the extreme difficulty of freeing the measuring tubes from all traces of residual air. The presence of air
equivalent to a few hundredths of a millimeter, although introducing but a negligible error a t the higher vapor pressures,
would cause errors of as much as 100 per cent a t very low pressures.
Sorel used an air-steaming method similar to that of Briggs.
He says nothing as to the probable accuracy of his various
measurements and does not give his original data. His table
is simply a tabulation of values obtained by smoothing curves
obtained from the data of Regnault and himself. The results for weak acids are consistent with those of other investigators, but those for high strength acids show very serious
deviations.
Daudt determined the vapor pressures of strong acids a t
extremely low temperatures. He used an electrical method
which entailed equalizing the thermal conductivity of the
vapors of ice and the acid to be measured in two gas conductivity cells by varying the temperature of the ice element while
keeping that of the acid constant. At equal thermal conductivities he recorded his two temperatures and assumed the
vapor pressure of the acid to be equal to that of the ice a t the
observed temperature. His results, considering their magnitude (0.003 to 0.754 mm.) are remarkably consistent.
Fortunately, the data of Burt and Daudt a t the two temperature extremes agree moderately well as to slope and position of the log p vs. 1 / T curves, which furnishes sufficient
justification for the extrapolation of Burt's values to the lower
temperatures.
I n preparing the final chart Burt's values were taken a- a
basis for the higher concentrations, and the mean of the
values of all the investigators for the lower concentrations.
of t h e Vapor Pressure Equations
Table I-Parameters
Log9 - A
Per cent
HnSOc
0
10
20
30
35
40
45
50
33
A
8.946
8.925
8,922
8.864
8,873
8.844
8.so9
8.832
8.827
B
2260
2259
2268
2271
2286
2299
2322
2357
2400
-T
Per cent
HnSOi
60
65
I!
(3
80
85
90
95
A
B
8.841
8.853
9.032
9.034
9.293
9.239
9.256
9.790
2458
2533
2688
2810
3040
3175
3390
3888
Vapor pressures were plotted against concentrations for even
temperatures, and from these curves log p vs. 1/T curves
taken for even concentrations. The parameters for the
equations of these curves are given in Table I.
The heats of vaporization as calculated from the slopes
of these curves were compared with those taken from Porter's
data and found to agree remarkably well. The boiling points
as obtained by extrapolation to 760 mm. were compared with
Ferguson's values and found to agree.
Table I gives the parameters of the eGuation of these
curves a t 5 per cent intervals in concentration. The values
.
May, 1925
19Db-STRIAL A1YD E S G I S E E R I S G CHEJIISTRY
obtainable from these equations are accurate to =tZ per cent
from 0" C. to the boiling point.
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523
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Swedish Tariff Revision-"Artificial
tanning materials, not
specially mentioned, wholly or partly organic," have been embodied in an item in the Swedish tariff, and will be free of duty
by an Act of March 8, 1925, effective on a date to be determined
by the King.