2526
A. A. NOYES,
F. W. HALL AND J. A. BEATTIE.
[CONTRIBUTIONS PROM THE RESEARCH LABORATORY O F PHYSICAL CHEMISTRY OF THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY, No. I IO.]
THE SOLUBILITY OF BISMUTH OXYCHLORIDE IN HYDROCHLORIC ACID AND ITS RELATION TO COMPLEX FORMATION. BY ARTHURA. Noms, FRANK W. HALLAND J A Received September 27, 1917.
CONTENTS: Determinations. 6 . Summary.
~ SA.
BEATTIE.
Introduction. 2 . Conductance Measurements. 3. The Solubility 4. The Solubility Data. 5. Discussion of the Solubility Values.
I.
Introduction. Although several investigations' have been made on the solubility of bismuth oxychloride in hydrochloric acid, no quantitatively verified explanation of the very large increase in solubility has been presented. Nor is any definite information available as to the form or forms in which bismuth chloride mainly exists when present in hydrochloric acid solutions. The purpose of this article is to offer a solution of these problems by presenting and discussing the results of measurements on the conductance of bismuth chloride dissolved in hydrochloric acid and on the solubility of bismuth oxychloride in that acid. This investigation has been carried on with the aid of a grant made t o A. A. Noyes by the Carnegie Institution of Washington, for which we wish to express our great indebtedness. We wish also to thank Mr. Roger Williams and Mr. A. W. Kenney for the valuable assistance which they gave us. 2. Conductance Measurements. I.
BY
JAMES
A. BEATTIE.
A study was made of the conductance of hydrochloric acid solutions of definite acid concentration to which varying quantities of bismuth chloride had been added. The solutions were prepared by dissolving in a I . 9079 formal hydrochloric acid solution enough accurately weighed bismuth oxide to give in one case a solution 0,2544 formal' in bismuth trichloride and I .1447 formal2 in hydrochloric acid and in another case a solution 0 , 1 2 7 2 formal in bismuth trichloride and I . 5263 formal in hydrochloric acid. These solutions were then diluted with multiple volumes of I .1447 formal hydrochloric acid and I .5263 formal hydrochloric acid, respectively. The bismuth oxide was prepared by igniting the carbonate precipitated by 1 Ostwald, J . p r a k f . Chem., [z] 12, 269 (1875); Herz and Bulla, 2. enorg. allgem. Chem., 61,387; 63, 59 (1909); Dubrisay, Compt. rend., 148,830 (1909). 2 That is, one containing 0.2544 formula weights of BiCls and I . 1447 formula weights of HCl per liter of solution at 2 5 ' . The small expansion of the solution on
dissolving the Biz08 in the HCI solution was taken into account.
BISMUTH OXYCHLORIDE
IN
2527
HYDROCHLORIC ACID.
ammonium carbonate out of a solution of the normal nitrate purified by recrystallization. The conductance of these solutions at 25' was measured by the usual method, a closed cylindrical cell 1 1 cm. long and I cm. in bore with the electrodes at the two ends being employed. The results are presented in the following table. The first aqd second columns show the concentration of the solution in formula weights of hydrochloric acid and of bismuth trichloride per liter, respectively; the third column gives the specific conductance of the solution a t 25'; the fourth column contains the difference between the specific conductance of the solution and that of the solution containing hydrochloric acid at the same concentration but no bismuth trichloride; and the last column shows the percentage decrease in specific conductance per formula weight of bismuth trichioride per liter. TABLEI.-CHANGE IN CONDUCTANCE PRODUCED BY DISSOLVING BISMUTHCHLORIDE IN HYDROCHLORIC ACIDSOLUTIONS. Mols HCI per liter solution. 1 . I447
.... ....
....
.... I .5263
.... .... ....
Mols BiCh per liter solution. 0.00000
0.03180 o .06360 0.1272 0.2544 0 .ooooo 0.03180 o .06360 0.1272
Specific condu$ance.
0.3622 0.3610 0.3593 0.3558 0.3503 0.4500 0.4477 0.4469 0.4445
Decrease in conductance.
....
% decrease per mol BiCt.
... .4
0.0012
IO
o .002g 0.0064
IZ .6
0.0119
12.9
....
...
0.0023 0.0031
16 .o 10.8 9.6
O.OOj.5
13.8
In considering the significance of these results it is to be noted that the conductance of the hydrochloric acid solution would be much increased if the bismuth chloride existed as Bi+++and 3Cl-, or as BiCl++ and 2C1-, or as BiO+, 2H+ and 3Cl-; that it would be little changed if it existed as un-ionized S i c & ; and that it would be considerably decreased if it existed as Sic&- or BiC15', since these ions replace the doubtless faster moving C1- ion. The fact that a pronounced decrease in conductance takes place shows that complex anions are present in large proportion. Another more accurate series of conductance measurements at 25 O was made, mainly to determine the relative conductances of the complex chlorobismuthic acid and of pure hydrochloric acid. In this series a conductance cell was used consisting of two ellipsoidal electrode compartments joined by a narrow vertical tube about 12 cm. in length and 0 . 8 mm. in bore. The specific conductance of pure hydrochloric acid was first determined a t a series of concentrations between I . 1226 and 2.2452 normal. The values found are given in the following table:
2528
A. A. NOYES,
3. W. HALL AND J.
A. BEATTIE.
TABLEIL-SPECIFIC CONDUCTANCE OF HYDROCHLORIC ACID SOLUTIONS AT 2 5 ”. Form. wts. HC1 per liter.. . . . . . I .1226 I ,4033 I ,6839 I .9646 2.2452 Specific conductance.. . . . . . . . . . o ,3650 o ,4369 o ,5000 o ,5571 o ,6054
The ‘following table contains the results of the measurements with the bismuth solutions, and of the calculations based upon them. In the column headpd ZH is given the total concentration of the acid in equivalents per liter of solution a t 2.5’; in that headed BBi is given the corresponding concentration of the bismuth in atomic weights per liter. The calculations are made on two bases, indicated by the letters A and B in the table, A being on the basis that all the bismuth is present as H+BiC14- and B on the basis that it is all present as H+zBiC16’. The “specific conductance of the free hydrochloric acid” in the mixture was found by plotting the above given specific conductances of hydrochloric acid, interpolating for the concentration ZH, and multiplying this value by the ratio (A) of ZH - ZBi to ZH, or (B) of ZH - 2BBi to BH. This calculation is based on the assumption that the ionization of the HC1 of the mixture is that which would prevail in a pure hydrochloric acid solution of a concentration equal to the sum of that ofethe hydrochloric acid and HBiC14 or HtBiClb in the mixture. The “specific conductance of the chlorobismuthic acid” in the mixture is found by subtracting that of the free hydrochloric acid from that of the mixture. And the equivalent conductances of hydrochloric acid and of the chlorobismuthic acid in the mixture are computed by dividing the two specific conductances by the concentration of the free hydrochloric acid and by (A) ZBi or (B) zZBi, respectively. TABLEIII.-EQUIVALENTCONDUCTANCE
OF CHLOROBISMUTHIC ACID IN SOLUTIONS OF HYDROCHLORIC ACID AT 25’. Equivalent conductance of Specific conductance of
I
Free HCl. ZH.
ZBS.
I . 1401
0.01754 0.03508 1 . I577 I . 1928 0.07016 I .2629 0.1403 I .4032 0.2806 I .6839 0 . 5 6 1 3 2 . 2 4 5 2 I . 1226
I
Mixture.
(A).
0.3683 0.3716 0.3781 0.3906 0.4143 0’4540 0.5069
0.3640 0.3630 0.3611 0.3572 0,3495 0.3333 0.3027
Chlorobismuthic acid. p__?
7 -
(B). 0.3583 0.3516 0.3385 0.3126 0.2621 0.1667 0.00
(B). (A). 0.0043 0.0100 0.0086 0.02oo 0 . 0 1 7 0 0.0396 0.0334 0.0780 0.0648 0 . 1 5 2 2 0.1207 0.2873 0.2042 0.5069
Hydrochloric acid.
34 323 322 318 311 297 270
Chlorobismuthic acid. c--h_---.
(A).
246 245 242 238 231 215 182
(B). 285 285 282
278 271
256 226
It will be seen from Table I11 that the equivalent conductance of the chlorobismuthic acid under the assumption that it consists of H+BiC14is in the five most dilute mixtures about 7570 of that of the hydrochloric acid, and in the most concentrated solution only 67yo of it. Since transference experiments have shown that the conductance of the hydrogen ion in I . 2 N hydrochloric acid is 83% of the whole conductance, it would be necessary to assume that H+BiCL- is a much less ionized acid than
BISMUTH OXYCHLORIDE IN HYDROCHLORIC ACID.
2529
H+C1-. On the other hand, under the assumption that the acid is H+aBiC15-, its equivalent conductance in the six most dilute solutions is 86 to 88% of that of the hydrochloric acid. This last result seems the more probable, and affords some evidence that the acid is H-tzBiCls’, so long as a considerable excess of free hydrochloric acid is present. 3. The Solubility Determinations. BY F. W. HAL+.
The bismuth oxychloride used was prepared by precipitating with water a solution of bismuth nitrate in hydrochloric acid; and it was purified by successive reprecipitations from its solutions in hydrochloric acid. The hydrochloric acid used was purified by fractional distillation of the “chemically pure” commercial product. Two series of solubility determinations were made. In one of these the equilibrium was approached from the undersaturated side, and in the other from the supersaturated side. The equilibrium from the undersaturated side was attained by rotating in a thermostat a t 2 5 O solutions of various concentrations of hydrochloric acid with an excess of bismuth oxychloride and analyzing the resulting solutions for their chloride and bismuth content. To avoid possible variation in the condition of the solid salt, it was freshly prepared each time by precipitating the oxychloride from a concentrated solution of bismuth chloride in hydrochloric acid by adding water a t a definite rate and a t the same time stirring the solution a t a fixed rate. The equilibrium from the supersaturated side was attained as a follows: A 2070 hydrochloric acid solution was nearly saturated with bismuth oxychloride, and the resulting solution was diluted with water until the acid was about 3 normal. This was then filtered; the filtrate was placed in a wide-mouth jar 5 cm. in diameter and 20 cm. high in a thermostat a t 2 5 O, and the bismuth oxychloride was precipitated by adding water (through a tube passing through the stopper of the jar) a t the rate of 5 cc. per minute; at the same time stirring a t a fixed rate by means of a. motor-driven centrifugal stirrer. The resulting solution was then stirred until equilibrium was reached, this being determined by drawing off successive samples a t definite intervals and determining the chloride content. Equilibrium was found to be established within twelve hours, but the solutions were in fact stirred twenty-four hours or more. The samples were withdrawn through a tube provided with an expansion at its lower end containing asbestos-felt and glass-wool. Weighed portions were analyzed for their total chloride content by precipitating the bismuth as sulfide with hydrogen sulfide, filtering, heating the filtrate t o expel the hydrogen sulfide, and titrating the acid in it with standard alkali solution. Separate weighed portions were analyzed for their bismuth content by evaporating them to dryness, moistening with nitric
2530
A. A. NOYES, F. W. HALL AND J. A. BEATTIE.
acid and evaporating repeatedly to convert the chloride into nitrate, dissolving in dilute nitric acid, precipitating with ammonium carbonate, igniting, and weighing as Biz03. The density of each sample was determined, in order to enable the concentration to be expressed on a volume basis, if this should prove advisable. The densities were measured in most cases with a Westphal balance, but in some cases (those to which a p is attached in the table below) by a 12 cc. pycnometer. 4. The Solubility Data. The results of the solubility determinations are given in the following table. In the experiments to whose numbers asterisks are attached the equilibrium was approached from the oversaturated side; in all other experiments, from the supersaturated side. The densities, given in the second column, are those of the solutions a t 25' referred to that of water ' . The directly determined chloride content (ZCl) and bismuth at 4 TABLE I~.+OLUBILITY OF BISMUTHOXYCHLORIDE I N HYDROCHLORIC ACID AT No.
I*
Density. 1.002
p
2
1.007
3* 4 5 6* 7
.009 p I .009
8
I
9*
I
IO I1
IZ* I3 14* IS* 16 17* I8 I9 20
21* 22
23 24
I
I .OIO I .OII
p
I .012
.013 .or5 p I .OI5 I .or8 I .020
I
p
,025
1.028 p
.036 p 1.044 , I .048 p 1 .os5 I .061 I ,066 I .066 p 1.075 I .083 I
1:122
26
.I57 I . I85
27
I .221
28
1.237 P I .288 p 1.329 P
25
29 30
I
p
Chloride content
Bismuth Hydrogen Activcontent content ity coeffi. ZBi. ZH. cient a.
0.3477 0.4350 0.4414 0.4892 0.5221 0.5276 0.5796 o .6244 0.6299 0.7038 0.7375 0.7579 0.8824 0.9125 I ,0760
0.00130 0.00376 0.00396 o .co646 0.00869 0.00899 0.01323 0.01767 0.01856
ZCl.
1.2277
1.2724 1.4348 1.5321 I ,6235 I ,6350 I ,7706 I .go21 2.5578 3.1865 3.6366 4.2552 4.5056 5.325 6.066
o .3438 0.4237 0.4295 o .4698 0.4960 0.5006 0.5399 0.5714 o .5742 0 . 0 2 7 2 0 0.6222 0.03138 0.6434 0.03473 0.6537 0 .os338 0.7223 0.05936 0.7343 0.08937 0 .a079 0.1177 0.8746 0.1324 0.8752 0.1620 0.9488 0.1810 0.9891 0.2025 1.016 0.2050
1.020
0.2352 0.2657 0.4216 0.5685 0.6792 0.8324
1.065 1.105 1.293 1.481 1.599 1.758 1.799
0.9022
0.800
0.794 0.795 0.793 0.793 0.793 0.795 0.797 0,797 0.802 0.805 0.806 0.815 0.817 0.828 0.836 0.839 0.849 0.856 0.859 0.861 0.866 0.873 0.905 0.941 0.970 I ,013 I .022
I .I00
2.025
I .Io0
1.317
2.115
I.
150
25'
Mass action constants. ~~
K. 0.228* 0.294 0.291* 0.335 0.363 0.362* 0.390 0.412 0.423* 0.439 0.436 0,45I* 0.445 0.458* 0.446* 0.412 0.455* 0.385 0.352 0.349 0.345* 0.325 0.307 0.225
0.151 0.117
0.083 0.079 0.045 0.038
0.700*
K;, 2.48*
0.712
2.21
0.697* 0.744
2.14
Ki.
2.14*
0.772
2.15
0.763~ 0.778 0.790 o.814* 0.806
2.17* 2.06 2.04 2.10* 2.04 I .97
0.788
0.812* 2 .os* I .98 0.757 0.804* 2 . 1 7 * 0.768* 2.25* 2.26 0.727 0.851* 2.99* 2.69 0.711 2.60 0.654 3 .OI 0,669 0.662* 3.01* 3.32 0.645 0.503 3.95 10.13 0.568 0.435 25. 0.386 92 * 0.324 3375. .... 0.354 .... 0.232 .... 0.331
BISMUTH OXYCHLORIDE I N HYDROCHLORIC ACID.
2531
content (ZBi), given in the third and fourth columns, are expressed in atomic weights per 1000g. of water. The hydrogen content (ZH), given in the fifth column, was calculated by subtracting g(ZBi) from (ZC1); it therefore represents the total acid in the solution. The activity coefficients CY, given in the sixth column, were obtained by interpolation a t the concentration (ZH) on a plot of the values given by Ellis' for hydrochloric acid a t 18'. The values of K , K l , and K P ,given in the last three columns, are those of the expression which correspond, as will be described below, to the mass-action equations, for the quantitative formation in the solution of un-ionized BiC13 and of the complex acids H+BiC14- and H+zBiCls', respectively. 5 . Discussion of .the Solubility Values. The increase in the solubility of bismuth oxychloride caused by hydrochloric acid will depend on the form in which the bismuth exists mainly in the solution. We need consider only the three reactions expressed by the following equations : BiOCl(so1id) 2HfC1- = HzO BiC13 gH+Cl- = HzO H+BiCI4BiOCl(so1id) BiOCl(so1id) 4H'CI- = HzO Hz+BiC16" The mass-action expressions for these reactions are evidently
+ + +
+ + +
If we assume now that substantially all the bismuth in the solution is in the form indicated by one of the above given equations and that the complex acids H+BiC14- and H+2BiCI5' have in any mixture the same ionization y as the HC1, these mass-action equations may be expressed in terms of the total bismuth content ZBi and total hydrogen content ZH, as follows : ZBi ZBi = K1; and = K; (ZH) 4Y4 (ZH)2(2H- ZBi)3y4 ZBi = Kz (ZH)2(2H- 2ZBi)4y6 These expressions may probably be made more exact by substituting for the ionization y the activity coefficient LY which has recently been determined by Ellis in this laboratory. This was done in calculating the values of K,K1 and Kz, given in Table IV. It will be seen from the table that the values of K increase rapidly, pass through a maximum value, and then rapidly decrease; that the values of K1 show a variation similar in direction, but much smaller in magnitude; and that the values of KZ show a behavior exactly opposite to that 1
THISJOURNAL, 38, 759 (1916).
ARTHUR A . NOYES AND JAMES H .
2532
ELLIS.
of K I , in that they first decrease slowly, pass through a minimum, and then increase rapidly. This result shows that the actual conditions are intermediate between those assumed in computing the constant K1 and the constant Kz,and makes it probable that the two complex acids H+BiCtand H+zBiC16’ are both present in the solution. The HzBiC15doubtless predominates in the more dilute solutions where there is a large excess of free hydrochloric acid,’ and the HBiC14 in the more concentrated solutions, where ultimately (in the last three solutions) there is not even enough chloride present to convert all the bismuth into the BiCl5’ ion (since ZC1 is less than 5ZBi). Alkali salts of both of these complex acids have previously been separated from their solutions.2 6. Summgry. In this article have been presented measurements on the electrical conductance at 25’ of solutions of bismuth chloride in aqueous hydrochloric acid. The results show that the conductance of this acid is considerably reduced by dissolving bismuth chloride in it, and indicate the formation of a complex acid of the form HBiCt or HzBiC16. Experiments on the solubility at 25 O of bismuth oxychloride in hydrochloric acid through a wide range of concentration have also been presented. The results considered from the mass-action standpoint are shown to be intermediate between those required by the assumption that the complex acid is HBiC14 and the assumption that it is H2BiC1S. The latter doubtless predominates in solutions containing a considerable excess of hydrochloric acid; and the former, in those in which this is not the case. CAMBRIDGE, MASS.
[CONTRIBUTIONS FROM THE RESEARCH LABORATORY OF PHYSICAL CHEMISTRY OF THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY. No. I I I.]
T H E FREE ENERGY OF HYDROCHLORIC ACID IN AQUEOUS SOLUTION. 11. BY ARTHURA. NOYES AID
JAMBS
H.ELLIS.
Received Octobir 15, 1917.
CONTENTS.-I. Introduction. 2. Description of the Apparatus. 3. The Effect of Light on the Silver Chloride Electrodes. 4. The Experimental Results. 5. Decreases in Free Energy and in Heat Content Calculated from the Electromotive Forces. 6 . Discussion of the Free-Energy Values. 7. Computation of the Specific Electrode Potentials. 8. Relative Activities of the Ions of the Acid a t Various Concentrations. 9 . Summary.
Introduction. In a previous articleS were presented the results of measurements I.
1
It will be noted that, while ZBi increases from 0.0038 to 0.053, the constant
K z decreases only 2
*
IO%, a decrease that might be accounted for by ionization effects. Abegg, “Handbuch d. anorg. Chem.,” j, 111, 653. Ellis, THISJOURNAL, 38, 737-762 (1916).
