IONIZATION R€$LA'I'IONS OF ALKALI HALIDES.
185
[CONTRIBUTION FROM THE CHEMISTRY DEPARTMENT OF THE UNIVERSITY OF'ILLINOIS. .I
HETEROGENEOUS EQUILIBRIA BETWEEN AQUEOUS AND METALLIC SOLUTIOEJS: THE INTERACTION OF MIXED SALT SOLUTIONS AND LIQUID AMALGAMS. VI. A STUDY OF THE IONIZATION RELATIONS OF SODIUM AND POTASSIUM CHLORIDES, BROMIDES AND IODIDES I N MIXTURES. BY L. S. WELLS AND G. McP. SMITH.^ Received August 5 , 1919.
A. Intcoduction. This investigation is a study of the equilibrium between mixed sodium and potassium salts and liquid amalgams, with free mercury present in such quantity that its active mass may be taken as constant, as represented by the equation, KHg, 4- NaX KX I- NaHg,, in which NaX and KX represent the dissolved halides, chloride, bromide, or iodide. The investigation is the sixth of a series on this subject,2 and its especial object is to study the influence exerted by the anions on the constitution of aqueous halide sol~tions.~ It has been shown in earlier papers of this series that in equivalent mixtures of sodium and potassium chlorides, the sodium ion fraction gains upon that of the potassium as the concentration is increased. Analogous observations have been macle, to an even greater degree, in the present investigation, which, in addition to the chlorides, includes the bromides and iodides of sodium and potassium. These ion-fraction changes may be due to any or all of the following causes: the existence of intermediate ions, of hydrated ions, and of complex ions.
:r
IQ. Theoretical. Since only the free atoms and ions of the metals are supposed to take a direct part in the reaction represented by the equation, KHg NaHgO NaHg MHO we must restate the equilibrium in terms of the equation,
+
:z
1 From a thesis submitted to the Graduate School of the University of Illinois by Lansing Sadler Wells in partial fulfillment of the requirements for the degree of Doctor of Philosophy in chemistry. * G. McP. Smith, THISJOURNAL, 32, 502 (1910); 35, 39 (1913);Smith and Ball, 39, 179 (1917);Smith and Braley, 39, 1545 (1917);Smith and Rees, 40, 1802 (1918). * The failure of the application of the different methods of determining the degree of ionization, namely the methods involving colligative propel ties, and also the conductivity method of Arrhenius, may be attributed to the occurrence of complex chemical processes between the different ionic and molecular species in solution, the nature, or, a t any rate, the extent of which is by no means well known (cf. Smith and Rees, LOG. Cit,).
I 86
L. S. WELLS
AND G . MCP. SMITSX.
+
KO Na+ Na" .-I-K t , (1) in which K O and Na" represent the free atoms of potassium and sodium in the mercurial phase, and Na+ and K+ the simple, free ions in the aqueous phase. And this equilibrium may be formulated into the fol-
lowing mass-law expression :
in which (KO) and (Na") are the mol. fractions of free, uncombined potassium and sodium in the mercurial solution, and (K+) and (Na+) the simple ion fractions of these metals in the aqueous solution. A liquid alkali amalgam contains, as the solute, a compound of the general formula MeHg,;2 but, even so, the alkali metal in such an amalgam possesses a solution tension, or tendency to enter into aqueous solution in the ionic condition. T i e solute in these amalgams must, therefore, be very slightly dissociated, in the sense of the equation, MeHg, f L Me" xHg, (3) for which the mass-law expression is
+
Or, in the very dilute amalgams, in which the active mass of free mercury is constant, we have
Assuming that these expressions hold true in amalgams containing both metals, we obtain
and, substituting in Equation
2,
for - the value (KHg), we obtain (l\Tao)' (Na€rg)
the expression
Now, in any individual mixed salt solution, the ratio oi the salt fractions must bear some numerical relationship to the ion-fraction ratio, or
1 It is thought advisable to return to the letter K (or IC), as in the original formulas] in place of the symbol C, which was used for a special reason in the third and fourth papers of the series (cf. Smith and Rees, LOG.cit., p. 1808,footnote). Cf. G. hlcP. Smith, 2.anorg. Allegem. Chem., 58, 381 (1908); 88, 161 (3914).
IONIZATION RELATIONS OP ALKALI HALIDES.
'87
in which the value of n is unknown and may, of course, vary with the conditions of salt concentration. But, whatever the value of n may be at any specific composition of the aqueous mixture, it follows from the preceding equations that, in the case of any particular equilibrium mixture,
The values of (KHg) and (NaHg) may readily be calculated from the analytical data of the equilibrium amalgams, and (NaX) and (KX) are known concentration fractions of sodium and potassium chloride, bromide, or iodide. Thus it is possible to calculate C,, the "equilibrium expression" a t any specific salt concentration. By definition, (Na') 3- (K+) = I. Combining this with Equation 5s and solving, we get
If, therefore, a means could be found of obtaining the value of Iz, we should be in a position to calculate the actual ion-fraction values, (Na+) and (K+), i;ll the various mixed solutions. C. Experimental Details. I. Materials and Apparatus.--In addition to the apparatus and materials used in the preceding investigations, and there described, the following are used for the first time in this investigation: (a) Sodium Bromide, NuBr.zHzO.-The pure commercial salt was recrystallized from hot distilled water. The mother liquor was removed as completely as possible by suction, after which the remainder was thrown off in a high speed electrical centrifuge. A second crystallization was made from pure water, and the mother liquor removed as before. After this the crystals were dried for sevteral days over calcium chloride, and the hydrated salt finally placed in tightly stoppered bottles. (b) Potassium Bromide.-A good grade of commercial salt was recrystallized twice from hot water, as described above. The salt was then dried in a platinum dish in an electric oven a t 150'. Just before use it wits heated in the dish in an electric muffle furnace for a t least two hours at a temperature just short of fusion. It was cooled in a desiccator over calcium chloride. (c> Sodiwn Iodide, NaI.zH~O.--l?he pure commercial salt was recrystallized three times from hot water, as described in the purification of ssdium bromide. After this the cxystals were dried for several days lover calcium chloride, and the hydrated salt finally placed in tightly stoppered bottles. C, then, represents the value of n.kp under specific experimental conditions.
I88
4. S. WELLS AND G. MCP. SMITH.
(d) Potassium Iodide.---A pure grade of commercial salt was recrystallized two or three times from hot water, The samples were then dried in an electric oven for several hours a t qo', and finally placed in tightly stoppered bottles. 2. Method ob Experimentation. (a)Solutiozzs.--Separate solutions of the NaX and KX (in which X represents 621, Br, or I) were made up, each equal in concentration to the total concentration of the mixed solution desired. By mixing these solutions in the proper volume relations, solutions of any salt-concentration ratio desired, and of the given total salt concentration, were prepared. In the case of the sodium chloride, potassium chloride, or potassium bromide, the pure dried salt was weighed out in the calculated quantity, dissolved in pure water, and the solution transferred to a calibrated volumetric flask, in which it was diluted to the mark a t 2.5'. In the ca6e of the potassium iodide, sodium iodide, or sodium bromide, the salt was weighed out in sufficient quantity to give a solution slightly more concentrated than desired. The normality of the solution so prepared was determined by the Volhard volumetric method, and it was then accurately diluted to the desired normality. ( b ) Equdibriww.-In order that, a t the start of each run, the amalgams should be as nearly as possible of the same concentration, the analyzed stock amalgams were diluted with mercury to the specific concentration desired. 'h'hese amalgams were then brought into equilibrium with the softition under investigation, as described in the previous paper. The amalgam mas in each case finally washed with water, and decomposed with hydrochloric acid, according to the procedure described in an earlier paper. Each run was made with 6 separate reaction mixtures, 3 of which were started with sodium amalgam and 3 with potassium amalgam; in all 6, identical mixed salt solutions were used. Equilibrium was, therefore, approached 3 times from each side. (c> Treatment of the Decomposition Products.---The hydrochloric acid solution containing the alkali metals from the equilibrium amalgam was in each case quantitatively removed from the mercury and evaporated to dryness on the steam bath. The mercury itself was dried, and weighed to within 0.1-0.2 g. The mixed alkali chlorides were weighed and analized as described in a previous paper.2 ID. Experimental Data. The data obtained in this investigation are recorded in the following tables: In Table I, which is given in full to show the data. in detail, as Smith and Ball, LOG.cit. Smith and Ball, Ibid. The above procedure differs from that of Smith and B d . in that the concentration of the equilibrium amalgams was accurately determined. 1
2
IONIZA’I%ON RELATIONS 05‘ ALKALI HALIDES.
189
well as to afford an indication of the degree of accuracy attained,’ the figures in the first column refer to the number of the experinienk; the second mlumn Indicates the amalgam used a t the start; the third, fourth and fifth columns contain the data, in grams, obtained by the analysis of the amalgam after the establishment of equilibrium between it and a solution of the composition given at the top of the table. Thus Col. 3 gives the weight of the mixed alkali chlorides; Col. 4 the weight of potassium chloroplatinate obtained in the analysis of the mixed chlorides; and Col. 5 the weight of the mercury in the equilibrium amalgam. Col. 6 shows the concentvation of the equilibrium anatgam in total milli-equivalents of alkali metals per IO g. of mercury,, as calculated from the data in Cols. sp4 and 5; Gols. 7 and 8 give the mol. fractions of the amalgamated metals in the mercurial phase at eyuilibrium, as calculated from Cols. 3 and 4; and Col. 9 gives the value of the expression
in which (ICH,) and (NaHg) are the respective mol €ractions in the mercurial phase, from Cols. 7 and 8, and (NaX) and (KX) are the known mol fractions of the halides in the aqLueousphase. In the light of the above description the subsequent tables, which contain the results in abbreviated iorm, wily be self-explanatory. I, Effect of Varying the Concentration of the Mercurial Phase at a Fixed (Equivalent) Salt-Concentration Ratio and a Fixed Normal Concentration o€ the Aqueous Phaae.-l n the sodium-potassium equilibrium, the sodium-strontium equilibrium, and the potassium-strontium equilibrlum, it has previously been observed that the total concentration of the mercurial phase exerts a maiked effect on the equilibrium value C,. In the case of the sodium-strontium equilibrium the C, value was found to be a linear function of the amalgam concentration.2 In the potassiumstrontium equilibrium it was found that the C, value increases directly with the amalgam concentration tip to about 0.3 milli-equivalent of metals per IO g. of mercury.3 In all 3 equilibria studied it has been obwmed that when highly concentrated liquid amalgams are used the values of C, obtained axe apt to be erratic. When it was first realized that the concentration of the liquid amalgams was a factor of moment, a study was made of the question by diluting the original amalgams with varying amounts of mercury, in a series of determinations, and then bringing them to equilibrium, a t 2 5 O , with a 0 . 2 N 2 The most consistent values are obtained with amalgams which contain 0 . 1 5 a . z o milli-equivalents of metals per I O g. of Hg; for this reason, beginning with Table IV, the amalgams used were all of this approximate concentration 8 Smith and Braky, LOG. cit, a Smjth itnd Rees, Ibid.
.
.
.
I,. S WELLS AND G McP SMIm
190
.
equivalent mixture of sodium and potassium chlorides;' but. owing to the incomplete nature of this first study. it has seemed desirable to investigate the question further.
.
TABLE: I The Effect of Varying the Concentration of the Mercurial Phase a t a Fixed (Equivalent) Salt-Concentration Ratio NaCl : KCl Temperature. 25
.
..
Expt . No
Amalgam at
Analysis of the equilibrium amalgam
. start . NaCl+KC: . 0.2118 I ......... Na
......... Na 3......... Na 4 ......... K 5 ......... K 6......... K 2
I
&PtCls
.
0.2353
0.1860
0.2090
0.2180
0.2407 o .2300 o .2368
0.2091 0.2137 0.1915
0.2III
. Hg
.
-
.
.
MilliMol fractions of equiv . of amalgamated metals metals per 7 10 g Hg Na R
. .
.
57.3
0.586 Q.713 50.7 0.581 0.709 57.6 0.600 0.715 55.2 0.601 0.715 56.3 0.602 0.713 51.0 0.596 0.715
-
.
.
0.287
cc. 0.403
0.291
0.411
0.285
0.397 0.398
0.285
0.287 0.285
0.2018
109.5
0.2055
0.2367 0.2403 0.2181 0.2544 0.2148 0.2515 0.2299 o .2698 0.2133 0.2491
107.7
0.291 0.301 113.2 0.304 115.9 0 . 2 8 5 1 1 8 . r 0.307 108.8 0.309
0.401 0.695 0.305 0.696 0.303 0.698 0.302 0.696 0.304 0.695 0 305 0.697 0 * 303 3
14. . . . . . . . . Na
25 ......... Na
16., ....... K I7. . . . . . . . . K 18. . . . . . . . . K
. I843
0.2210
198.2 197.4 197.4 199.0
0.436
0.688 0.1990 0.2379 0.159 0.689 0.2069 0.2476 0.165 0.689 0.177 0.684 0.2240 0.2720 0.2265 0.2715 201.0 0.177 0.688 0.2310 0.2773 199.6 0.182 0.688
0
0.146
0.312 0.311 0.311 0.316 0.312 0.312
23
......... K
24. . . . . . . . . K
.2091
261.8 256.8 0.1650 0 .I951 0.1868 a61.z 0 . I575 0.2176 0 .2648 262.9 0.2311 0.2790 279.2 0.1694 0 2039 253.6 0
0.2551
~
Na 26. . . . . . . . . K 27......... K 28 . . . . . . . . . 'K
1
0.2546 0.2152 0.2627 0 .2033 0 .2463 0.2171 0.2594 0.2101
. .
Smith and Ball. LOGcit
557.1
573.5 554.0
560.5
0.451
0.462 0.453 0 a453 0.454 0 .462 0 -445
0.126
0.684
0.316
0.101
0.692
0 308
0.09.5 0.130
0.689 0.685 0.686 0.686
0.311 0 *452 0.315 0.459 0.314 0.457 0.314 0.456
0.130 0.105
-
.
Mean. 0.115 25 .........
0.453 0.451
.__
~
Mean. 0.168
r9. . . . . . . . . Na 20 ......... Na 2 1 . . . . . . . . Na 22. . . . . . . . . K
0.438 0.435 0.433 0.436 0.439 0.435 I I
I _
Mean. 0.299 13 . . . . . . . . . Na
0.398 I__
Mean, o 394
7.......... Na 8......... Na g ......... Na IO ......... K 1 1 ......... K 1 2. . . . . . . . . K
0.401
0.0593 0.684 0.0586 0.683 0.0573 0.688 0.0617 0.687
-
Mean. 0.0592
0.316 0.317 0.311 0.313
0.455 0.461 0.463 0.453 0.455 0.458
IONIZA'i'ION RBLATIONS OF ALKALI HALIDES.
191
'The data €or this study are given in Table I. In this series of experiments, with equivalent sodium and potassium chloride mixtures at a total concentration of 0.2 N, the concentration of the amalgams was decreased in each successive run. 1%is to be noted that the mean values of the "equilibrium expression, ' C,, obtained in t5e different runs iwcrease with decwasing amalgam concentration. In any one r 1~ n , t h e equilibrium amalgams of the individual mixtures were always found to differ somewhat in concentration; because, in the first place, the stock amalgams were weighed out more or less roughly for dilution, and, in the second place, evolution of hydrogen (always in evience to a very slight extent, at best) varied somewhat in the individual e x p e r i m e n t s . However, in the case of 1, b i equilibrium, t i1 e amalgam coaicentration, at different salt concentrations, Temperature, 2 5 '. values of C, are not in- KaC1 : KC1. fluenced enough by the slight differences of amalgam concentration in the individual experiments of a run t u warrant a correction, as in the case of the sodium-strontium, or of the potassium-strontium equilibrium. Table I1 contains data showing the effect of varying the amalgam concentration, but with equivalent sodium and potassium chloride mixtures of 0.2, 0.5, 1.0, 2.0, and 4.0 N concentration, respectively. In Fig. I , the values of C, as ordinates, are plotted against the total amalgam concentration in milli-equivalents of metals per IO g. of mercury. Each point indicated in the plot is the average of about 6 separate determinations.
I,. S. WELLS AND G. MCP. SMITH.
192
TABLE11. The Effect of Varying the Concentration of the Mercurial Phase at Various Fixed (Equivalent) Concentrations of the Aqueous Phase. NaCL : KCI. Temperature 2 5 '. Mean values of Expts. NO.
I-
3
4- 6 7- 9 10-12
13-1 5 16-18 19-21 22-24 25-27 28-30
3 1-33 34-36 37-39 4-42 43-44 45-47
48-50 51-53 54-56 57-59
Amalgam a t start.
Milli-equivalents of metals per 10 g. Hg.
Mol. fractions of amalgamated metals. r
Na.
I _
Sodiuni 0.162 0.701 Potassium 0,166 0.700 __ Mean, 0,164 Mixed Aqueous P:llase 2.0 N . Sodium 0.537 0.735 0.735 0 2 1 Potassium Mean, 0.539 Sodium 0.362 0.728 Potassium 0x4 0.729 Mean, o .368 Sodium 0.119 0.710 Potassium 0 3 9 0.715 Mean, 0.143 Mixed Aqueous Phase 4.0 N . Sodium 0.412 0.757 Potassium o .420 0.756 Mean, 0.416
-
Sodium 0.178 Potassium 0 3 Mean, 0.175
0.745 0.746
cc.
K.
Mixed Aqueous Phase 0.2 N (see Table I). 0.713 0.287 Mean, o ,594 Mean, 0.299 o ,696 0.314 Mean, 0.168 0.688 0.312 Mean, 0.115 0.687 0.313 Mean, 0.0592 0.685 0.314 Mixed Aqueous Phase 0.5 N . Sodium 0.435 0.708 0.292 Potassium o .426 0..707 0.293 Mean, 0.430 Sodium 0.170 0.695 0.305 0.694 0.305 024 Potassium Mean, 0 .I 72 Mixed Aqueous Phase 1.0 N . Sodium 0.504 0.720 0 .a80 Potassium 0 2 7 0.722 0.278 Mean, 0.515 Sodium 0.352 0.710 o .290 Potassium 0.357 0.710 0.289 Mean, 0.354 0.299 0.300
o .40i 0.436 0.454 0.455 0.458 0.412 0.414 .__ 0.413 0.439 0
0.439 0.388 0.386 I _
0.387
0.408 0.408 0.408 0.427 0 x 7 0 $427
0.265 0.265
0.360 0.360 0.360
0.272
0.373
-
0.271
0 3 2
o .290
0.373 0.406
o .285
0.398 0.401
0.243 0.244
0.320 o .322 0.321
0.255 0.254
0.343
-
0
2
0.342
IONIZATION WLATIONS OF ALKALI HALIDES. 3,
I93
Effect of Varying the Normal Concentration of the Aqueous Phase
at a Fixed (Equivalent) Salt-Concentration Ratio and at Various §pecific
I11 contains data showing the effect ~ ~ a l g aConcentrations.-Table m of increasing the total s 8 I t concentration in stages from o.2N to 4.0 N,the salts (sodium and po tassium chlorides) being present in equivalent proportions, at various k e d a,malgam concentrations. The values of C,in this table were obtained by simply reading them Erom the plots in Fig. I , a t the indicated amalgam Concentrations. Fig. 2 illustrates the ef-. fecit upon the C, value, at different specific amalgam concentrations, of varying the normality of the aqueous phase in the case QE equivalent SOd i u m a n d potassium 5"4M ~ ~ OF AgbP ~ chloride mixtures. From this series of plots it appears that a change in Fie. - z.--Showim- the value of C, as a function of total c 0 11 c E: n t r a t i o n of 0.1 salt concentration, a t different amalgam concentratim. Temperature, 25 '. milli-equivalentof metals NaC' : wccl. per IO g. of mei-cury produces a change of about 0.012 in the Value of
c,.
111. The Value of E, as a Function of Total Salt Concentration a t a Fixed Equivalent SaltConcentration Ratio and a t Various Specific Amalgam Concentrations. TABLE
Tem$erature 25 '.
Normality of aqueous phase. 0.2
0.5 T .o 2 .0
4.0
x
-
NaC1:K CC.
-
C, values a t 1: milli-equivalents of metals per 10 g. of mercury. 0.1.
0.458 0.449 0.43s o .408 0.354
-
- .
0.2.
x = 0.3.
x = 0.4.
x
0.447 0.438 0.423 0.396 0.34
0.435 a .426 o .412 0.384 0.331
0.424 0.415
o ,412
2
0.401
0.373 0.319
0.5.
0.403 0.389 0.360 0.308
3. Effect of Varying the Concentration Ratio of the Salts at a Fixed the Normal Concentration and a Fixed Amalgam Concentration.-If
~
L. S. WELLS AND G. McP. SMITH.
I94
reaction between alkali metal amalgams and alkali salt solutions is ionic, the mass-law expression, (KHI)(Na+)- k , (NaHg)(K+) demands that the value of k should remain constant, regardless of the relative concentrations of the simple sodium and potassium ions; on the other hand, however, constancy is not necessarily demanded in the value of C, in the equation, ( K N g ) (N a x > = P5.k C,. ~
“Hg)
(I=)
The results o€ previous experiments carried out in this laboratory with undiluted liquid sodium and potassium amalgams show “that if the total concentration of the solution remains constant, the value of C, is constant even though the mol fractions of the two salts in the mixtures vary over wide limits. . . . . When diluted amalgams and more concentrated aqueous solutions were employed, a slight change in the value of c, could be observed as the ratio of the salt concentrations was varied.”l TABLEIV. The Effect of Varying the Concentration Ratio of the Salts a t a Fixed Normal Concentration and a Fixed Amalgam Concentration. Temperature 2 j ’. Mixed Aqueous Phase 0.2 1v. Mean values
of Zxpts. No.
Milli-equivalents of metals per 10 g Hg.
Amalgam a t start.
Mol. fractions of amalgamated metals
--
’
R.
cc.
NaCk:I
