Heats of d i l u tion of sodium hydroxide solutions were measured a n d corrected to a temperature of 68’ F. The experimental results cover a range of 48 per cent to 13.6 per cent by weight of sodium hydroxide. By combining these data with earlier r e s u l t s , the enthalpies of sodium hydroxide solutions at 68’ F. from 0 to 48 per cent by w e i g h t were calculated.
SODIUM HYDROXIDE
SOLUTIONS JOHN W. BERTETTI‘ AND WARREN L. McCABE
Heat of Dilution
University of Michigan, Ann Arbor, Mich.
at N A PREVIOUS paper data were re-
68”F.
brought to approximately 20” C. by use of the auxiliary heater. A preexperimental drift period, as in the heat capacity experiments, was then observed until a constanttemperature drift of reasonable magnitude occurred. The stirrer inside the cup was then stopped, and, by a slight pull on the proper thread, the disk of valve a was removed. Valve a had an opening 3 mm. in diameter and was directly opposite the large stirrer and near the middle of the bomb. Dilution was thus started and proceeded slowly. Although, because of a high initial rate of temperature rise, adiabatic conditions could not be maintained as satisfactorily as in the heat capacity experiments, deviations from adiabatic conditions were small a n d short-lived, and attempts were made to make the positive and negative deviations c a n c e l each other. W i t h i n a few minutes after opening valve a , t h e dilution rate diminished considerably. By 8 slow manual o p e r a t i o n of t h e s t i r r e r i n s i d e the cup, it was possible to cause the dilution to proceed a t an appreciable rate. Valve b in t h e bottom of the dilution cup was opened when the rate was againvery low, and, when the d i l u t i o n FIGURE 1. DILUTION CUP IN PLACE was a l m o s t comA . Dilution cup pleted, valves c and a, b, c, d . Valves d were opened. 8 . Rubber gasket
ported on the specific heats of sodium hydroxide solutions (1). The apparatus used in this work was, with slight modifications, also used to measure heats of dilution. The method was the same as that described by Richards and Rowe (6). Starting with a concentrated solution, successive dilutions were carried out by adding to the solution varying amounts of freshly boiled, distilled water from nickel dilution cups immersed in the solution and in temperature equilibrium with it.
Apparatus The modifications necessary for the calorimeter are shown in Figure 1: The quantitative heating coil was removed, since temperature adjustments could be made with the auxiliary heating coil. In any given dilution one of three dilution cups, of 20, 75, and 120 cc. capacity, was used; in all cases the water for dilution was contained in the cup, and the solution was in the calorimeter. The method of fastening the cups to the lid of the bomb is evident from Figure 1. The short-threaded nickel rods were fastened to the dilution cups with a small amount of silver solder. A rubber gasket, g, was used to prevent distillation of water before starting an experiment. The gasket and silver solder were at 311 times above the level of the solution. Four openings, a, b, c, andd, were closed by small nickel disks which were made fast with paraffin wax. Each disk was removable individually by means of a small nickel wire, and this permitted the solutions to be mixed when desired. The wires were terminated just above the lid of the bomb and connected to lengths of waxed silk thread which passed through one of the chimney tubes of the “submarine” cover. In order to provide maximum stirring efficiency and promote rapid attainment of thermal equilibrium between the diluting water and the solution, a different stirrer was used for the inside of each cup. The changes in the water equivalent brought about by these changes in the calorimeter were calculated from the heat capacities and weights of the parts concerned.
Experimental Procedure
a_lliL
Carbon-dioxide-free distilled water was used for t,he dilutions. With weighed amounts of slightly cooled liquids, the calorimeter was assembled and the temperature of the solution Present address, Standard Oil Company of Indiana, Whiting, Ind.
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tained. By adding to this value the heat contents of the stronger solutions relative to a 33.59 per cent solution as determined in the second series of dilutions, the heat contents of all solutions relative to that of an infinitely dilute solution were obtained. The same procedure was followed in connecting the first series of experiments to the second. Heat contents a t c o n c e n t r a t i o n s other than those of the experiments were obtained by finding an equation approximating the data over a short range and plotting a deviation curve showing the difference, a t any given concentration, b e tween the heat content as given WEIGHT PER CENT NaOH. bv the eauation and the actual FIGURE 2. COMPARISON OF DILUTION DATA value. Thus, by use of the equation and the d e v i a t i o n curve, The small stirrer was then allowed to operate a t its normal it was possible to make interpolations without sacrificing rate, and the pumping action of the stirrers moving in opposite accuracy. Table I1 gives the interpolated results in terms of directions insured the attainment of complete homogeneity in enthalpies at 68" F. relative to an infinitely dilute solution. The values below a concentration of 14 per cent were obthe solution. tained from the data of Richards and co-workers. As in the heat capacity experiments, a postexperimental temperature drift was observed, and initial and final temperatures of the experiment were established. As soon as an experiment was completed, the calorimeter TABLE11. ENTHALPY OF SOLUTIONS AT 68" F. RELATIVE TO was dismantled, some of the solution was removed from INFINITELY DILUTDSOLUTION the calorimeter, and all parts, except the bomb itself, were Weight €3. t. u./Lb. B. t. u./Lb. Weight B.t. u./Lb. R . t. u./Lb. thoroughly washed and dried in preparation for the next run. % NaOH NaOH Soh. % NaOH NaOH Soh. The product of one dilution became the starting point for the 0 0 0 26 17.14 2 +1.18 +O. -0,0808 0236 28 26.43 next. 4 -2.04 30 37.34 -4.78 6 -0.287 32 49.97 A larger cup was used as the solution became more dilute 8 -7.15 -0.572 34 64.05 and the temperature rise for a given amount of added water -8.60 -0.860 10 79.63 36 -9.13 12 -1.09 38 96.50 diminished. Two series of dilutions were made. In the first -8.65 -1.21 14 40 114.2 -7.34 -1.17 16 132.8 42 series the concentration range covered was from 48.14 to 17.16 -4.99 -0,897 18 151.7 44 per cent, and in the second series the range was from 26.14 to -1.50 20 -0.301 46 170.7 f3.28 f0.721 22 48 189.7 13.59 per cent. The solutions were analyzed gravimetrically 24 9.47 2.27 a t the beginning and end of each series by the method suggested by Richards and Hall (4). From the data of an experiment, the heat of dilution was Comparison with Previous Work readily calculated and then corrected to exactly 20" C. by use of specific heats already determined. Such calculations have In Figure 2 the experimental data are compared with those been given quite often in the literature and will not be reof Fricke (a) and of Tucker (7). Fricke diluted comparapeated here. The experimental results are given in Table I. tively small amounts of concentrated solution with large From a large-scale plot of the data of Richards and coquantities of water. His results lie above those obtained in workers (3,6),the heat content of a 13.59 per cent solution rethe present work, and the reason for the discrepancy is not ferred to an infinitely dilute solution a t 68" F. (namely, apparent. Tucker followed the same general procedure as -8.790 B. t. u. per pound of sodium hydroxide) was obwas used in the present work, and his results show a trend opposite to those of Fricke in comparison with the new data. If, however, the new specific heats (1) are used in reducing TABLE ' I. SUMMARY OF EXPERIMENTAL HEATSOF DILUTION Tucker's observations, his results are in excellent agreement with those of the present investigation. -First Series-Second SeriesConcn weight % daOH Initial Final 48.14 47.12 47.12 46.09 46.09 45.10 46.10 41.77 41.77 38.80 38.80 35.88 35.88 33.11 33.11 30.44 28.00 30.44 28.00 25.71 25.71 23.44 23.44 21.30 21.30 19.36 19.35 17.66 F
Heat evolved B. t. u./lb. NaOH 9.564 9.811 9.413 31.54 27.11 24.82 21.11 17.60 13.58 10.45 8.360 6.157 4.277 2.858
Concn. weight NaOH Initial Final 26.14 22.46 22.46 19.15 19.15 16.14 16.14 13.59
Heat evolved B. t. u./lb. NaOH 13.09 7.64 4.08 1.59
Literature Cited Bertetti and McCabe, IND.ENG.CHEM.,28, to be published (1936). Fricke, 2. Elektrochem., 35, 638 (1929). Richards and Gucker, J. Am. Chem. Soc., 51,712 (1929). Richards and Hall, Ibid., 51,707 (1929). (6) Richards and Hall, Ibid., 51, 731 (1929). (6) Richards and Rowe, Ibid., 43, 770 (1921). (7) Tucker, Phil. Trans., 205A, 334 (1916).
I
R E C ~ I V EAugust D 3, 1935. Abatracted from a diseertation submitted by John W. Bertetti in partial fulfillment of the requirements for the degree of doctor of philosophy, University of Michigan.
