ELECTRICAL CONDUCTANCE OF CONCENTRATED SALTSOLUTIONS
Feb., 1956
189
.
1 -VrO -8
I 0
I
2 4 6 8 10 12 Moles/l. at 30" (25" for LiCI). Fig. 3.-Excess partial molal entropy of water in concentrated salt solutions.
of Frank and Robinson,'g helps to decide between these two possibilities. To calculate the partial molal entropy of the constituents of the solutions, vapor pressures and heats of dilution are needed a t high concentrations. Such data are available for sodium thiocyanate, sodium iodide10~20 and lithium chloride.20 For sodium perchlorate the vapor pressures are available'O but not the heats of dilution. Following Frank and Robinson, the excess partial molal entropy of water AS1*, and of salt, ASz*, in solutions of sodium thiocyanate, sodium iodide and lithium chloride up to concentrations near saturation have been computed. These values for AS2* are plotted in Fig. 4. From this plot we see that for concentrated sodium thiocyanate solutions (19) H. 8. Frank and A. L. Robinson, J . Chem. Phya., 8 , 933 (1 940). (20) F. D. Rosaini, D. D. Wagman, W. H. Evans, 5. Levine and I. Joffe, "Selected Values of Chemical Thermodynamic Properties," U. 8. Dept. of Commerce, National Bureau of Standards, 1952.
0
2 4 6 8 10 12 Moles/l. at 30" (25" for LiC1). Fig. 4.-Excess partial molal entropy of salts in concentrated solutions.
AS2+ is negative, indicating more order of the solute in concentrated solution than in the infinitely dilute reference solution. Sodium iodide behaves like sodium thiocyanate but to a lesser degree. This is consistent with its viscosity behavior. In contrast to these two salts, solutions of lithium chloride show a positive ASz* and a negative AS1*. At 11 N . where the viscosity behavior began to show up some arrangement in the solution, the A&* for water in the lithium chloride solution is rapidly becoming very negative. The little order in lithium chloride solutions at high concentrations is, therefore, associated with the solvent (hydrated lithium ion) while the more pronounced order in sodium perchlorate, sodium thiocyanate and sodium iodide solutions is associated for the most part with the solute. Acknowledgment.-The authors are indebted to G . Yates for carrying out many of the density and viscosity measurements reported here.
CONCENTRATED SALT SOLUTIONS. 111. ELECTRICAL CONDUCTANCE OF SOLUTIONS OF SODIUM THIOCYANATE, SODIUM IODIDE AND SODIUM PERCHLORATE BY M. L. MILLER Contribution from the Stamford Laboralories, Research Division, American Cyanamid Company, Stamford, Conn. Received April IO, 1066
The electrical conductance of solutions of sodium thiocyanate, sodium perchlorate and sodium iodide has been measured at 0, 30 and 50" from 1 N to saturation. A t higher concentrations, the conductances of all three salts (all sodium salts appear to be a roaching a common limit. This suggests that as the concentration increases more and more of the current is being carrieigy the sodium ions. Comparison of the energy of activation for conductance with the energy of activation of viscous flow is consistent with this view.
I. Introduction Preceding papers in this series's2 have reported measurements of the viscosity, density and vapor pressure of aqueous solutions of sodium thiocyanate, sodium perchlorate and sodium iodide from 1 N to saturation at 0, 30 and 50". The present paper deals with the electrical conductivity over the same concentration and temperature range. Although there is a very large amount of conductance data in the literature, there are very (1) M. L. Miller and C. L. Sheridan, THIEJOURNAL,80, 184 (1956). (2) M. L. Miller and M. Doran, ibid., 60, 186 (1956).
few measurements on aqueous 1-1 salt solutions in the region above 8 N and fewer still at these high concentrations a t two or more temperatures. There are the measurements of Campbell and Kartsmarka and Campbell, Gray and Kartzmark4 a t 25, 35 and 95" on silver nitrate and ammonium nitrate and some data on lithium chloride in "International Critical Tables." (3) A. N. Campbell and E. M. Kartzmark. Canadian J . C l e m . , SO, 128 (1952). (4) A. N. Campbell, A. P. Gray and E. M. Kartzmark. iWd.. 81, 617 (1953).
M. L. MILLER
190
VOl. 60
I 120
4 8 12 Concn., moles/l. Fig. 1.-Equivalent conductance at 29.9': 0,NaSCN; 0, NaCIO,; 0, NaI; OrLiCl(25').
4 8 Concn., moles/l. Fig. 2.-Equivalent conductance at 0 and 50': 0, NaClO,; 0, NaI.
Conductance measurements on the three salts, sodium thiocyanate, sodium perchlorate and sodium iodide, show that as the concentration increases the equivalent conductances of all three salts, all of them sodium salts, are approaching a common limit. This limit is lower than the limit approached by lithium salts (if such a limit exists for lithium salts) and suggests that as the concentration increases the sodium ion is taking over more and more of the conductance. This conclusion is consistent with measurements of selfdiffusion in sodium thiocyanate solutions made in this Laboratory.6 The equivalent conductance viscosity product is remarkably constant considering the wide concentration range covered (from 0.2 to 18.7 m for sodium thiocyanate). 11. Method
to better than four significant figures. A frequency of lo00 cyclea waa used in the measurements. b.-Because of the high conductance of these concentrated solutions, it was necessary to use capillary cells. Two cells with different capillary diameters were used. They consisted of a length of capillary tubing about 13 cm. long with a capillary diameter about 1 mm. Since the cells were calibrated against KCl, it was not necessary to measure these dimensions. The capillaries were attached to J-tubes containing smooth platinum electrodes, by rubber stoppers. Because substantially all of the effective resistance waa in the capillary, the positioning of the stopper WM unimportant. Rubber is not attacked by the solutions used. Measurements with the two capillary cells used for most of the work agree, in general, to within 0.03'%, A few of the more dilute preparations were measured in a conventional Washburn cell with platinized electrodes.
0
1. Solutions.-Solutions were made up and assayed by methods already described.'**. 2. Temperature.-Conductance measurements were made in an oil-bath regulated to f0.02'. The temperatures a t 29.92' and a t near 50" were checked against a thermometer calibrated by the National Bureau of Standards. A t 0" the cell WM immersed in a narrow rectangular trough of oil which had been precooled overnight in an ice-box. This oil trough was immersed in a large well-packed icebath. After the assembly came to temperature, 2-3 hours, the conductance remained constant indicating good temperature stability. 3. Conductance. a. Bridge.-Conductance was measured on the precision conductance bridge designed by E. E. Lineken.R It is a shielded, fked ratio arm bridge with a modified Wagner grounding device similar to the apparatus described by Shcdlovsky.' The precision of balancing was (5) To be reported. (6) Bound Brook Division of American Cyanamid Company, Bound Brook, New Jersey. (7) T. Shedlovsky, J . Am. Chum. Soc., 62, 1793, 1808 (1930).
0
12
0,NaSCN;
111. Data The results are summarized in Tables I through IV which are self-explanatory.* IV. Discussion 1. Specac Conductance.-The maximum in the specific conductance, K , as the salt concentration is raised, is characteristic of electrolytes and has long been known. It is rather remarkable that, if we start with a cube of 10 N NaSCN, and replace part of the NaSCN (a conductor) with water (a non-conductor) the conductance of the cube goes up. This calls to mind some of the modern work on crystals where adding an impurity loosens the structure (produces vacancies) and increases the conductance. 2. Equivalent Conductance.-Figures 1 and 2 plot the equivalent conductance, A. The most * Tables I, I1 I11 and IV referred to in thie paper may be obtained by ordering Doeurnent No. 4774. from the American Documentation Institute. Library of Congress, Washington 25, D. C., remitting in advance by cheok or money order $1.25 for microfilm or $1.25 for photoprint.
Feb., 1956
ELECTRICAL CONDUCTANCE OF CONCENTRATED SALTSOLU~IONS
191
interesting thing about these plots is the fact that at each temperature the equivalent conductance e OOC. I curves of the three salts, all of them sodium salts, 0.95 seem to be converging to a common limit. Examination of conductance data on lithium chloride in " International Critical Tables" indicates that the common limit to which the conductance of the sodium salts is converging is below that for the lithium salts (if such a limit exists for lithium salts). This suggests that in these concentrated sodium 0.85 salts more and more of the current is being carried by a common unit, the sodium ion. If we accept 2 the idea that these very concentrated aqueous salt solutions exhibit quasicrystalline local order, this is not unreasonable. Confirmation of this interpretation could be obtained by measuring trans0.75 ference numbers in very concentrated salt solutions. 3. Equivalent Conductance Viscosity Product. --Widden's rule states that the equivalent conductance viscosity product, hq, for an ion or salt is approximately constant and independent of solvent and temperature. This relationship follows 0 4 8 12 directly from Eyring's absolute rate theory: if mole/l. viscosity and conductance proceed by the same mech- Fig. 3.-E uivalent Concn., conductance viscosity product for anism. This can be seen if, in a purely f o m l N&CN: 0,29.92'; 0, 0.0"; (3, 52.25'. way we write, A for a 1-1 salt as9 It could mean that in this region viscosity and conductance are proceeding by different mechanisms. If as the curves in Fig. 1 lead us to suspect, this where F is the faraday, V8the volume of the mov- is the region where the sodium ion is taking over ing unit and AF.* the free energy of activation for more and more of the conductance as the concentraconductance. I n a similar way, viscosity can be tion increases this fact is sufficient to account for the rise in Aq since in viscous flow both ions must written as be moving together. The dependence of Aq on temperature is, of course, implicit in equation 3. where AFv* is the free energy of activation for Vis4. Energy of Activation of Conductance.-If cous flow, V , is the volume of the moving unit and viscous flow and conductance proceed by a difN is Avogadro's number. Thus, if the free energy ferent mechanism a t high concentrations, this fact and the moving units are the same in both processes should show up in differences in the energy of activation. qA = FZhN/RTV1/B (3) We have, therefore calculated A€€,* the heat of The values of Aq for sodium thiocyanate solution activation for the conductance of sodium thiocyat different concentrations are shown in Fig. 3. anate sodium perchlorate and sodium iodide a t 30" Plots for sodium iodide and sodium ,perchlorate are from the slopes of plots of In A against 1 / T . so similar that they have not been included. The Values of AHc a t 30"are given in Table V. This small spread in the Aq values considering the wide table also includes, for comparison, values of AHv* concentration range is surprising. Nevertheless, the heat of activation of viscous flow a t 30". This the observed variations are much greater than ex- last quantity was taken from a previous study.2 perimental error and follow consistent trends. This TABLE V could be a result of oversimplificationin the formalENERQP OF ACTIVATION FOR DIFFERENT SALTS ized treatment or it could mean that either the asNaSCN NaClO4 NaI sumptions that Vc = V , or AFc = AFv (or both) Salt/N AHo* AH^* AHo* AH"* AH,,* AHv* are breaking down. 2 3.64 3.63 3.64 3.54 3.64 3.40 The drop in Aq with concentration in the early 3 3.64 3.70 3.74 3.67 3.64 3.55 part of the curves in Fig. 2 can be attributed to an 4 2.64 3.70 3.84 3.67 3.64 3.55 increase in the volume of the moving unit as the 5 3 . 8 4 3.93 4.04 3.71 3.74 3.65 concentration of salt is increased since volume oc6 3.84 4.12 4.04 4.00 3.84 3.90 curs in the denominator of equation 3. 7 4.00 4.44 4.19 4.68 3.94 4.32 The rise in hq from 6 N ie less easily dealt with.
I
*
*
*
(8) 8. Glaastone, K. J. Laidler and H. Eyring. "The Theory of
Rate Processes," McGraw-Hill Book Co.. New York. N. Y.,1941. 1st Edition. (9) It is to be understood that the expressions in equation 1 and 2 are purely formal. Strictly speaking, the right-hand aide of each ehould be replaced by a s u m of terms referring to the anion,the cation and the eolvent, together with interaction terms.
8 9
10
4.55 4.88 5.10
4.78 5.70 8.18
4.56 4.77 5.20
5.25 8.70 9.85
4.04
...
...
4.42
... ...
It will be seen from this table that through 5 to 6 N AHc* = AH,+. I n this region conductance and viscosity may, therefore, be proceeding by the same
192
TRAMBAKLAL M. OZAAND VASANTRAI T. OZA
mechanism. At higher concentrations AHv* rises faster than AH,+. Previous work has shown2that at these concentrations there is a relatively high positive entropy of activation of viscous flow which we have interpreted as evidence for the presence of short range local order.
Vol. 60
Viscous flow would have to disturb the whole structure while conductance could avoid disturbing the arrangement of the big anions if it shifted more and more to cationic conductance. Therefore, conductance would not be expected to have as high an entropy of activation as viscosity.
THE DECOMPOSITION OF HYPONITRITES OF CALCIUM AND STRONTIUM BYTRAMBAKLAL MOHANLAL OZAAND VASANTRAI TRAMBAKLAL OZA The Inorganic and Physical Chemistry Laboratory, The M . R . Science Institute, Gujarat College, Ahmedabad, and The Institute of Science, Bombay, India Received July 6, 1066
The decomposition of SrNzOI and CaNzOZ.Hz0 is studied by the application of heat and by exposure to carbon dioxide. The thermal decomposition products are similar to thode of SrN202.5H20 and CaN2O24H20,respectively. The effect of carbon dioxide varies with the salt and the dryness of thc sample. The results suggest that reactions (1) MNzOZ= MO NzO (1) and 3MN2O2 = 2M0 f M(N0J2 2N2 (2), which appear to be simultaneous, are the characteristic reactions of the decomposition.
+
+
I n a previous study we investigated the thermal decomposition of calcium hyponitrite tetrahydrate’ ; this work was undertaken to determine the role of water in the decomposition. Sodium hyponitrite which decomposes in vacuo a t about 300°,produces no nitrous o ~ i d e ~whereas -~ the pentahydrate under the same condifions produces nitrous oxide.’ Carbon dioxide displaces nitrous oxide almost quantitatively from sodium hyponitrite3 and in the presence of water nitrous oxide is formed from the anhydrous salt.4 The fact that carbon dioxide decomposes barium hyponitrite has been reported by Kirschner.6 The present paper describes (a) the decomposition of anhydrous strontium hyponitrite and of calcium hyponitrite monohydrate and (b) the action of carbon dioxide on the hydrates of these salts. Our results indicate that the state of hydration of the hyponitrite has little influence on the nature of the decomposition products, but that the relative amounts of these products are affected by the conditions of the decomposition. Carbon dioxide acts on the hyponitrites to a variable extent, but it may have no effect, if the salt has been intensively dried. Experimental Materials.-Strontium hyponitrite pentahydrate was prepared according t o Partington and Shah4; dehydration in vacuo at 150’ for 0.5 hour gave the anhydrous salt. Anal. Calcd. for SrNzOr5HzO: HzO, 37.8; Sr, 59.4; N. 18.97. Found: HzO. 38.6; Sr, 59.47; N , 19.08. Calcium hyponitrite monohydrate precipitated when calcium nitrate dissolved in absolute alcohol was mixed with Na2N10r5H20in a drop of water in the cold; the precipitate was washed with alcohol and ether, and dried in a vacuum desiccator. Anal. Calcd. for CaNzOz.HzO: Ca, 35.28; N, 23.73. Found: Ca, 35.4; N, 23.75. Like the tetrahydrate, the substance could not be dehydrated without decomposition. Analyses.-The gases from all the experiments and the residue from the calcium salt were analyzed as described in Oza and Oza.1 The residue of the strontium salt was -__I___
( 1 ) T. M. Oza and V. T. O m , J . Chem. Soc., 909 (1953). (2) E. Divers, ibid., 47, 97 (1899). (3) T.M. Oza, J . Ind. Chem. Soc., 91, 71 (1944). (4) J. R. Partington and C. C. Shah, J . Chem. SOC.,2071 (1931). (5) A. Kirachner, 2.onorg. Chem., 16, 424 (1898).
tested only for nitrate by tlic phenolsulfoiiic acid reagent; nitrate was found absent in all experiments. Table I shows that the dehydration of strontium hyponitrite for various periods of time has no influence on the product8 of the decompositioii. The gas evolved consisted of a fairly constant mixture of nitrous oxide and nitrogen; no nitric oxide was present. The hydrated salt8gave similar results. From the last column of the table it is evident that the decomposition residue contained little hyponitrite .I
TABLE I DECOMPOSITION OF SrN20z AT 300 IN 0.5 HR. AFTER DEHYDRATION BY EXPOSURE TO PzOa i n VaCUO A T 150’ FOR VARYING PERIODS
Expt.
no.
I 2 3 4 a
Mass, g.
Gas evolved Time on treatof exment of residue posure Evolved gas, nil. toPzOr, w-- withw:ter, hr. Total NIO Na ml.
0.1050 1 .I075 2 .lo89 3 .I200 4 The gas was absorbed
6.4 6.6 7.0
62.5 37.5 59.1 40.9 6 0 . 0 40.0 8 . 9 66.3 33.7 in cold alcohol.
0.4 0.5 0.5 0.7
As in the case of the tetrahydrate, the quantity of the calcium hyponitrite monohydrate decomposed has a similar effect on the gas evolved (Table 11). In the experiments summarized in Table 111, the salts, which were coated on glass beads to increase the surface, were exposed to carbon dioxide in an evacuated apparatus. The gas, which was pumped off through potassium hydroxide, was analyzed after removal of the carbon dioxide. The quantity of gas evolved, which was low with the ordinary dry calcium salt, was appreciable with the moistened salt. Under identical conditions exposure to carbon dioxide caused almost quantitative displacement of nitrous oxide from the dry sodium salt; when, however, the sodium salt was intensively dried in vacuo over P206 in the system for 24 hours prior to exposure to dry carbon dioxide, the effect was slight. I n general it has been reported that dry carbon d i o ~ i d e ~ * ~ * ~ . or dry aire has little effect upon dry hyponitrite salts, but that they are RIOWIY converted to the carbonate upon exposure to (tir.2~4Alo (6) T. M. Oza and S. A. Patel, J . I n d . Chem. Soc., 81, 523 (1954). (7) T. M. O m , V. T. O m and N. L. Dipali, ibid., 28, 15 (1951). (8) A. T h u m , Inaug. Diss. Prague, 1893. (9) J. R. Partington and C. C. Shah, J . Chem. Soc., 2589 (1932). (IO) W. Zorn, “Die Untersalpctrigensaure und deren Organinchen Derivate,” Heidelberg, 1879.
