Solutions for Maintaining Constant Relative Humidity D. S. CARR AND B. L. HARRIS The Johns Hopkins L'niversity, Baltimore 18, M d . T h e aqueous tensions maintained bj twelve saturated aqueous salt solutions in an enclosed space from room temperature to 90" C. are given and the relative humidities calculated therefrom. The salts include the halides of potassium and sodium, the nitrate, nitrite, chromate, and dichromate of sodium, and chromic oxide. The data were obtained by a static method using a modification of the isoteniscope of Smith and Menzies.
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N COYKECTIOK n l t h studies ot adsorption oi mater vapor on silica gel it was desired to equilibrate samples of gel in air a t various relative humidities a t a constant temperature. Inasmuch as the static equilibration was to take months, the simplest method was thought to be that of exposing the adsorbents in a desiccator to air in contact Jqith saturated aqueous salt solutions a t eonstant temperature. Lange's handbook (8) gives a list of such salt solutions, but this list is not very complete and values for various salts arr given a t single temperatures or over very restricted temperature ranges. Reference to the literature showed that the data are fragmentary and widely scattered, nor could data be found on several systems that the authors wished to use. Some few studies were found for several salts (1, 5). Accordingly, it was undertaken to nieabure the vapor pressure of water over saturated salt solutions from room temperature to 90" C. The salts chosen were the chlorides, bromides, and iodides of potassium and sodium, potassium fluoride, sodium nitrate, nitrite, chromatp, and dichromate, and chromic olidr
cock lubricant (silicone) was used on all submerged lubricatrti surfaces. The operational procedure was as follows. A solution was prepared from C.P. chemicals and redistilled water, containing sufficient salt, so that some would remain undissolved a t 90" C. The Bask containing the solution was connected to the ap aratus and cooled with dry ice until the solution was frozen. $hen all air was evacuated from the entire apparatus and the stopcocks were closed. The solution was allowed t,o melt and was stirred vigorously to free dissolved gases. This procedure was repeated twict, more, after which the t,hermostat was placed in position and set.rtt a desired temperature. As t'he pressure of the water vapor increased] the differential manometer was kept roughly balanced by allowing air to flow slowly into the pressure-measuring syst,ein. When vapor-liquid equilibrium was reached, t>he pressure was
TABLD I. AQUEOCXTIWSIOX AKD PERC E ~ IZELATIVI~ T HUNIDITY O F SATURaTED SALT SVLETIoNS .Igueoiis Relative Tension, Humidity Mm. Hg % Sodium Chloride, SRCI 15.5 10.03 75.Y 20.7 13.85 75.7 30.2 24.10 74.9 $0.0 41.30 74.7 00.0 68 30 74 9 60.0 111 9 74 9 70.0 175.4 75.1 80.0 271.0 76.4
kinuerature,
C.
Sodirini Erornide hJaBr.2HsO 4 N a B r a t b0.6' C. 20.3 10.35 58.0 40.9 30.37 52.4 50.0 46.02 49.7 60.0 74.50 49.9 70.0 118.4 R0.7 80.0 181.0 50.9
EXPERIM EhT 4 L
Sodium Iodide, NaI.2HsO -+ N a I a t 68. l o C . 30.2 11.71 36.4 40.0 17.94 32.3 60.0 26.28 28.4 60.0 37.80 25.3 52.28 42.6 70.0 81.37 23.2 80.0 90.0 123.4 23.5
The apparatus constructed was a modification of the isoteniscope of Smith and Menzies (10).
It consisted essentially of a differential manometei filled with dibutyl phthalate] connected on one side t o the sample tube and on the other to a pressure-measuring system. The entire apparatus was connected t o a high vacuum system by suitable stopcocks through cold traps, The ressure-measuring system was filled with dry air at pressure sugcient to balance roughly the differential manometer a t any point, Being thus isolated from any condensable vapor, the pressure could be measured by McLeod gages Two such gages, one measuring from 0.01 to 10 mm. and one from 10 to 123 mm. of mercury, were provided. Pressures above 125 mm. were measured on a large-bore manometer. The difference between the levels of the dibutyl phthalate in the two legs of the differential manometer was measured by cathetometer and translated to millimeters of mercury a t 0" C. using the known density of the fluid a t the operating temperature. The reading of the gage was corrected by this amount. The pressure-measuring system was provided with a large-volume float (2 liters) t o eliminate corrections due to compression when the McLeod gages were operated. The flask containing the solution was attached to the apparatus by a standard-taper joint for ease in changing solutions. The flask contained a glass-enclosed iron rod by which the contents could be stirred magnetically from the outside during degassing evacuations. All parts of the system which came in contact with water vapor, from the differential manometer, including the flask and over to the stopcock used for evacuation, were immersed in a water thermostat maintained to i-0.1' C. over a long period of time. Values were read only when the temperature was constant to better than ~ 0 . 0 . C5 ~for 0.5 hour A high temperature Stop-
Potasbium Fluoride, KF.2HzO 3 K F a t -50' C. 8.91 27.4 30.4 12.62 22.8 40.0 18.88 20.4 60.0 31.36 21.0 60.0 51.34 22.0 70.0 80,25 22.8 80.0 122.8 23.4 90.0 Potassium Chloride, KC1 14.91 85.0 20.2 27.36 84.5 30.3 45.19 81.7 40.0 81.2 74.20 50.0 120.3 80.7 60.0 80.0 187.0 70.0 79.5 382.4 80.0 78.3 411.5 90.0 Potassium Bromide, KBr 44.0 79.6 40.0 50.0 73.26 . 79.2 117.0 79.0 60.0 185.0 79.2 70.0 80.0 281.5 19.3
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TernAqueous ltelativr UeLature, Tension, Humidity. C. hlin. Hg '% Potassium Iodide, K I 40.0 36.94 66.8 50.0 60.11 65.0 60.0 94.30 63.1 70.0 144.0 61.7 80.0 916.0 fin 8 90.0 817.3 60 1
Sodium Nitmte, NaKU, 30.1 20.16 63.0 40.0 33.97 61.5 30.0 55,30 59.8 60.0 88.61 ,59.3 70.0 137.6 5 8 !4
Sodiiini Nitrate, NaNOi
:10.0 40.0 50.0 60.0 70.0 80 0 90.0
23.14 39.58 63.46 100.7 153.5 232.5 341.8
72.8 71.5
68.8 67.6 65 .i 65 5
65,O
Sodium Chromate NasCr0a.4Hz0 -+ NapCrOa kt 64.8' C 30.2 20.80 64.6 40.0 34.19 61.8 50.0 54.40 58.8 60.0 83.06 55.6 70.0 127.6 54.7 80.0 56.2 199,s 90.0 302.9 57.6 Sodium Dichromate, Na~Cr207. 2HzO -+ NazCrzOi a t 74 8' C . 17.26 54.2 30.0 29.61 53.6 40.0 50.0 50.30 54.4 60.0 82.49 55.2 80,O 190.0 56.0
Chromic Oxide, CrOa 15.0 5,81 45.4 44.6 14 18 30 0 40 0 25.00 45.0 .iO.O 42.01 45.4 60.0 68.41 45 8 70.0 108.0 46.3
September 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
read, as was the difference in levels of the legs of the differential manometer. The procedure was then repeated a t a higher temperature. RESULTS
The data are given in Table I. The per cent relative humidity was calculated from the aqueous tension by dividing by the vapor pressure of water at the same temperature, as given in International Critical Tables (6). Clausius-Clapeyron plots (log p vs. 1/T) were linear over the temperature ranges studied, and existence of phase transitions was characterized on such plots by abrupt changes of slope. Such transitions are discussed below. SODIUM CHLORIDE.The aqueous tension of saturated solutions of sodium chloride increased uniformly over the range studied
(15.5' to 80.0"C.). No transition was observed, as was expected. SODIUM BROMIDE.A break in the plot of log p us. l / T was observed a t 50" C. This agrees fairly well with the transition temperature of 50.6' C. re orted by Dingemans ( 8 ) . Below this temperature the form o f the compound is NaBr.2H20 and the transition is to NaBr. SODIUM IODIDE. This compound was observed to have a break in the plot at about 70" C., in agreement with the transition temperature from NaI.2Hz0 t o NaI a t 68.08' C. reported by Dingemans (2). A value of 68.9' C. is listed in Perry's handbook (9). This solution was studied from 30.4" POTASSIUM FLUORIDE. to 90.0" C. and a transition temperature was found to occur in the neighborhood of 50" C. The transition from the dihydrate to the anhydrous form is reported by de Forcrand ( 4 ) to occur a t 41 O C. POTASSIUM CHLOBIDE. The aqueous tension of this solution was found to increase linearly over the temperature range studied (30.3" to 90.0" C.)with no evidence of transition. POTASSIUM BROMIDE.The a y e o u s tension of this solution was studied only over the range 40.0 to 90.0" C. The relative humidity was found to be nearly constant over this range. POTASSIUM IODIDE. This solution exhibited no transition, the aqueous tension increasing to give a linear plot. SODIUM NITRITE. This compound has the greatest variation of solubility with temperature of any salt investigated here. No phase transitions were observed, and the relative humidity decreased uniformly. SODIUMNITRATE. The aqueous tension was measured over a
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range from 30.0" to 90.0' C. The relative humidity decreased with rising temperature; no transition was observed. SODIUM CHROMATE.This solution was found t o undergo transition from Na2CrOa.4H20 t o the anhydrous form at about 65 O C., agreeing with Dingemans' value of 64.8" C. for the transition temperature (2). SODIUM DICHROMATE. A transition was observed for this solution a t about 75" C. A value of 74.8" C. for the transition from the dihydrate to the anhydrous form has been reported by Hartford ( 5 ) ,whereas the value reported by both International Critical Tables (6) and Perry's handbook (9) is 84' c. The aqueous tension of this solution was not measured above 80" C., but because International Critical Tables does not list a transition at 75' C.., i t is believed that the value is in accidental error and that this has been copied into the handbook without checking. CHROMIC OXIDE. No transition was observed in the case of this solution, in agreement with a study by Kremann ( 7 ) . Several of the salts exhibit equilibrium relative humidity values which first decrease to a minimum value and then increase slightly. These variations are due to slight deviations from absolute linearity on the Clausius-Clapeyron plot, possibly due t o associations in the solution. LITERATURE CITED (1) Am. Paper and Pulp Assoc., Instrumentation Program, Rept. 40 (Feb. 15,1945). (2) Dingemans, P., Rec. traw. chim., 57,144-51, 703-9 (1938). (3) Edgar, Graham, and Swan, W. O., J . Am. Chem. SOC.,44, 570-7 (1922). (4) Forcrand, R. de, Compt. rend., 152,1073-7 (1911). (5) Hartford, W. H., J . Am. Chem. SOC.,63, 1473-4 (1941). (6) International Critical Tables, Vol. I, p. 68; Vol. 111, p. 211-12. New York, McGraw-Hill Book Co., 1926. (7) Kremann, R., Monatsh., 32,619-22 (1911). (8) Lange, N. A., "Handbook of Chemistry," 5th ed., p. 1412. Sandusky, Ohio, Handbook Publishers, 1945. (9) Perry, J. H., "Chemical Engineers' Handbook," 2nd ed., p p . 456-72, New York, McGraw-Hill Book Co., 1934. (10) Smith, A., and Menzies, A. W. C., J . Am. Chem. SOC.,32,907-14
(1910). RECEIVED August 26, 1948.
Solubilities of Aliphatic Dicarboxylic Acids in Water J
APPLICATION OF DUHRING5 RULE
c. ATTANE
AND THOMAS F. DOUMANI U n i o n Oil Company of California, Wilmington, Calif.
EDWARD
'rhe solubilities of glutaric, adipic, and 6-methyladipic acids in water have been determined at several temperatures. Solubility data for oxalic, malonic, maleic, fumaric, malic, tartaric, glutaric, adipic, and p-methyladipic acids have been correlated by means of a modification of Diihring's rule. With the exception of glutaric and @-methyl-
adipic acids, a straight line is produced when the t e m perature at which the acid has a given solubility is plotted against the temperature at which succinic acid has the same solubility. The danger of employing Diihring's rule when only two or three values for the solubility of a particular compound are known is stressed.
A
corresponding density data are given. The solubility of adipic acid at 40" C. has been reported by Bancroft and Butler ( 1 ) and a t 100" by Schrauth ( 1 2 ) . It was decided to determine the solubilities of glutaric, adipic, and B-methyladipic acids a t several different temperatures and to test the applicability of Duhring's rule t o the solubilities of several dicarboxylic acids. For the latter purpose, solubility data were taken from the literature for oxalic (7), malonic ( 6 ) , maleic, fumaric, and i-malic ( 9 , 16), d- or 2-tartaric acid ( g ) , and succinic
iK EXAMINATION of the literature reveals that although
the solubilities of the three lower dicarboxylic acidsoxalic, malonic, and succinic-have been reported by various investigators over a rather wide range of temperatures, only a few solubility data have been reported for glutaric and adipic acids. Henry(5) reported the solubility of glutaric acid a t 14" and t h a t of adipic at 15" C. Lamouroux (8) has given the solubility of glutaric acid in water from 0 to 65 ' C., but unfortunately his results were expressed in grams of acid per 100 cc. of solution and no
