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. Bibliography Haur, Z . Elektrochem., 16, 301 (1910). Briggs, J. SOC.Chem. I n d . , 22, 1275 (1903). Brlinsted, Z . physik. Chem., 68, 707 (1910); 64, 641 (1908). Burt, J . Chem. Soc. (London), 86, 1339 (1904). Daudt, Z . physik. Chem., 106, 226 (1923). Dieterici, W i e d . Ann., 42, 613 (1891); SO, 47 (1893); 62, 616 (1897); 67, 865 (1899). Ferguson, J . SOC.Chem. Ind., 24, 781 (1905). Hacker, A n n . P k y s i k . , [41 39, 1342 (1912). Helmholtz, Wied. Ann., 27, 532 (1886). Lunge, Ber., 11, 370 (1878). Regnault, A n n . chem. phys., [3] 16, 129 (1845).
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Roscoe, J . Chem. SOC. (London), 13, 146 (1860) Thomas and Ramsay, I b i d . , 123, 3256 (1923). Raleigh, Phil. Mag., [SI 4, 521 (1902). Smits, Z . physik. Chem., 39, 385 (1902); 61, 33 (1905); Arch. iVbeuland. sci., 121 1, 97 (1898). Sorel, Bull. Soc. I n d . Mulhouse, 59, 240 (1889); J . SOC.Chem. I n d . , 9, 175 (1890); 2. angew. Chem., 2 , 272 (1889). Tammann, 2. physik. Chem., 2, 42 (189s); Memo. acud. Petersbourg, [71 36 (1887). Wilson, J , Ind Eng. Chem., 13, 326 (1921). Porter, Trans. Faraday Soc., 13, 373 (1918).
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.
