T H E CATALYTIC DECOPIIPOSITIOS OF HE-DROGES PEROXIDE BY CADhlIUJI IODIDE B Y R4LPH C. G O O D I S G A S D JAMES H. TVALTON
The catalytic decomposition of hydrogen peroxide by soluble iodides was first studied systematically by Waltonl who found that in neutral solutions of potassium iodide, sodium iodide and ammonium iodide the rate of decomposition of hydrogen peroxide is directly proportional to the concentration of the iodine ions present in the solution. This relationship may be quantitatively expressed as follows: velocity constant = 1.46 concentration of iodides X degree of dissociation The degree of dissociation was determined from conductivity measurements. I n the case of cadmium iodide the above expression gave a value of 1.91, which discrepancy was believed t o be due t o the complexity of the cadmium iodide molecules. In such a solution conductivity measurements would obviously give no data for iodine ion concentrations. McBain,2 who studied the properties of solutions of cadmium iodide, assumes very incomplete dissociation, and the presence of the complex anion Cd13- and the partially dissociated cation CdI+ formed as follows: CdIz & CdT+ CdIz $ CdIf 3Cd12 ~2Cd++
+
21-
+ I+ 2CdL-
Using the E . N . F . measurements of cadmium iodide in various concentrations in the cell Cd
I
1
Cdlz
~
KC1 KCl HgzClz Sat. 1 . I N Solid
1
Hg
McBain3 determined the concentration of the Cd++ ions and from these data together with the E.M.F.measurements made by Getman' on the cell Cd
1
Cd12
1
AgI
1
Ag
calculated the concentration of the iodine ions in the solution. Using the general formulation of migration of Laing and BIcBain5 the concentrations of the complex cation CdI+ and anion Cd1,- were determined. Walton: Z. physik. Chem., 47, 18j (1904).
* McBain: Z.Elektrochemie, 11, 21j (190j). J
6
McBain: J. Phys. Chem., 35, 999 (1931). Getman: J. Phys. Chem., 32, 940 (1928). Laing and McBain: J. Phys. Chem., 28,673 (1924).
CATALYTIC DECOMPOSITION OF HI-DROGEK PEROXIDE
3613
I t was hoped that by using hlcBain’s data3for the concectration of iodine ions in cadmium iodide solutions it would be possible to account for the difference in behavior between the catalytic action of cadmium iodide and the action of sodium, potassium and ammonium iodides on hydrogen peroxide. Experimental I n order to obtain data for the concentrations used by McBain a series of measurements were made on the catalytic effect of various concentrations of cadmium iodide solutions on the decomposition of hydrogen peroxide. The hydrogen peroxide used was Merck’s C.P. quality Superoxol which had been distilled from quartz under reduced pressure and preserved in quartz flasks. When the product thus purified was redistilled it gave results similar to that having but one distillation. In order to determine the effect of any volatile acids present on the rate of decomposition the distilled hydrogen peroxide was redistilled from barium carbonate. X sample of this hydrogen peroxide also showed the same rate of decomposition as that which had been distilled but once under reduced pressure. Merck’s C.P. quality cadmium iodide, twice recrystallized from conductivity water, was used. Concentrations are expressed in mols of solute per liter of solution. The rate of decomposition of hydrogen peroxide mas determined by measuring the volumes of oxygen evolved in various time intervals. The reaction was carried out in a pyrex flask of IOO cc. capacity connected by glass capillary tubing to a 50 cc. burette. The method of adding the catalyst to the solution and the device used to shake the flask, and thus avoid supersaturation, have been described by Wa1ton.l All measurements were made at 25’. The rates of decomposition of hydrogen peroxide by cadmium iodide were determined for several concentrations. The constants (unimolecular) agree within 3 per cent. Table I summarizes the data obtained with different concentrations of cadmium iodide. TABLE I The decomposition of hydrogen peroxide by various concentrations of cadmium iodide ,005
.01oqj
,0043
,00643
.I O 0
,010
,01287 ,01785
K ,061I ,0667
.I2j
,0764
,020
,0295
,200
,0932
. 0 2j 3 j
,0362 .Oj3I ,0535
,250
1079
. joo
1642
3101s CdI2
.0j o
,0547
K
M O ~CdI? S
Although the decomposition of hydrogen peroxide by potassium iodide has been repeatedly measured, it was thought best to repeat these measurements, and to extend the concentrations of potassium iodide used. The data are summarized in Table 11, columns one and four.
RALPH U. GOODING AND JAMES H. WALTOS
3614
A. T h e catalytic eflect of the iodine i o n The data obtained are shown in Table 11, where the relation between the velocity constant and the concentration of iodine ions has been calculated. The values used for the dissociation of potassium iodide were taken from the International Critical Tables 5'01. TI, page 2 3 j. TABLE I1
Mols K I per liter
Relation between the speed of decomposition of hydrogen peroxide and iodine ion concentration K I (Ae = 149.8)
.005 ,010 ,020
,030 050
. IO0
hv
a
K
WCI-
144.2 141.9 139. I 137.1 134.1 130.5
96.3 94.7 92.9 91.5 89.5 87.1
.0072.;
I , 50
,01439 ,02920 '04499
1.52
.07 18
. I410
1.57 1.64 I .60 I
.61
Avg. 1.57 The average value in the last column differs from that determined by Walton' but agrees with that determined by ilbeL6 Walton found it to be I 46 while the average of Abel's determinations give a value of 1.57. From the data of McBain and the results of our experiments given in Tables I and 11, it is now possible to determine whether or not the catalytic decomposition of hydrogen peroxide by cadmium iodide is directly proportional to the iodine ion concentration as in the case of the other iodides.
TABLE I11 The composition of cadmium iodide solutions according to McBain and the catalytic decomposition of hydrogen peroxide by cadmium iodide solutions. Mols CdI,
I-
,005
,0063
,010
,050
,0103 ,0148 ,0228
,100
.02j2
.ZOO
. joo
.020
CdI'
,0029
Cd13-
,0011
,0285
,0079 ,0196 ,0237 ,0359
,0181
,0004
,1769
,0064 ,0153 .03jO
K(0bs.) KI-(Calc.)
,01045 ,01785 ,0295 ,0530 ,0667 ,0932 ,1642
,00989 ,01617 ,0232 ,0358 ,0396 ,0447 ,0284
K(0bs.)-KI-(Calc.) , 0 0 0j 6 , 00 I 68 ,00627 ,01728 . 0 27 I ,0485 ,1358
PvlcBain's data for the concentration of the various ions in cadmium iodide solutions are given in the first four columns of Table 111. To calculate the velocity of decomposition of the hydrogen peroxide by the iodine ion in a given Abel: Z. Elektrochemie., 14, 598 (1908)
CATALYTIC DECOMPOSITION O F HYDROGEN PEROXIDE
3615
concentration of cadmium iodide it is simply necessary to multiply the iodine ion concentration by the value 1.57. The results, listed in column 6, plainly do not agree with the observed velocity constants given in column j. The fact that K observed is greater than K calculated shows that in the decomposition of hydrogen peroxide by cadmium iodide other factors beside the iodine ions are operating to decompose the peroxide. Other ions present in the solution which might act as catalysts in the reaction are Cd++, CdI+ and CdI3-. In order to determine the effect of the Cd++ ion the rates of decomposition of hydrogen peroxide by solutions of cadmium iodide with the addition of various concentrations of Cd++ ions, in the form of cadmium chloride, were determined. The addition of .s gram and I gram of cadmium chloride to a .IO molar solution of cadmium iodide had practically no effect on the rate of decomposition, the value K being ,0649,.0641 and .0667 respectively for the three solutions, consequently the cadmium ions can be eliminated. The effect of CdI+ and Cd13- will now be considered. From McBain's data in Table I11 it will be seen that in 0.01molar cadmium iodide, iodine ions and CdI- ions are present while the concentration of 0.j molar cadmium iodide contains iodine ions and Cd13- ions, with a negligible concentration of CdI' ions. Since the catalytic effect of the iodine ion is known it is possible to calculate the effect of the CdI+ ion for the .OI molar solution: This gives a value of .j8for the effect of the CdI+ ion. Similarly the effect of the Cd13ion, calculated from the data for a 0.j molar solution, gives . 7 7 for the effect of that ion, a value very close to one half that for the iodine ion. Using these values it is now possible to calculate the catalytic effect of each ion in the various solutions of cadmium iodide. The data are given in columns 4 and 5 of Table IT. The assumption that, the catalytic effect of these ions is directly proportional to the concentration of each ion is seen to be justified. The sum of the values in columns 4 and 5 , listed in column 6, which give the combined effects of CdIf and Cd13- show a very good agreement with the data in column 7. TABLE IV The catalytic effect of CdI+ and CdI
-
ions '
CdI-
,010
,020
,0029 ,0079
,0011
,004j8
,00085
,050
,0196
,0064
,0113;
,00493
.ooj43 ,0163
. O Ij
,100
,0237
.01j3
,01178
.02jj
,0271
,200
.03j9 ,0004
.03j0
.0137j .0269j
,02695
,0477
,048j
. j00
CdIa-
K
Mols Cd12
-
KCdI-
. 00 I 68
CdIa-
-
Calc.
Obs.
K C d I - f K C d I s - KCdI'fKCdII-
,00627 3
,1769
These data, therefore, substantiate the work of McBain in that the concentration of each ion he postulates as being present in solutions of cadmium iodide agrees with its catalytic decomposition of hydrogen peroxide. Each specific ion has a definite rate of catalytic action and the remarkable agreement between the Concentration of each type of ion in solution, as calculated
3616
RALPH C . GOODING AND JAMES H. WALTOS
by McBain from conductance measurements and transport numbers, with the results of the present study can not be attributed to chance. While it has not' been proven that the ions present are those postulated by McBain, nevertheless, it has been shown that the effect of the ions in this decomposition can be attributed to the same ions which influence conductance and transference. B. T h e e$ect of the addition of iodine to solutions of c a d m i u m iodide The influence of the addition of iodine on the catalysis of hydrogen peroxide by potassium iodide has been determined by Walton.l He found that as the concentration of iodine increased the rate of decomposition decreased. This effect was explained on the theory that potassium iodide is practically completely dissociated and with the addition of iodine, 13- ions are formed. K+ IIs = Kf 1s-
+ +
+
The 1 3 - ions were found to have practically no effect on the decomposition of hydrogen peroxide. This effect was quantitative since the number of mols of potassium iodide in a solution could be calculated by kinetic measurements as well as by the method of J a k ~ w k i n . ~ Experiments were performed to determine the effect on the decomposition of hydrogen peroxide of solutions of cadmium iodide to which iodine had been added. These results are recorded in Table V.
TABLE V The decomposition of hydrogen peroxide by solutions of cadmium iodide containing various concentrations of iodine M O ~CdIz S
Idols
12
K
MOISCdI?
3101s I?
K
. 0000
.0178j
. 2j
.OI
'0045
,01316
,250
,00338
,1024
.OI
,007
j
.OIIZj
. 2 jo
,02040
,1051
,03624
, 1 0 3j
.OI
.2
.I25 .I 2 5 . I2j
. 0000 . O Ij
,0703 ,0765
j
,0740
.02
o
jo
0000
,1079
These data, Table V, indicate that in solutions of cadmium iodide the addition of iodine has a greater effect on the rate of decomposition of hydrogen peroxide in dilute solutions than it does in concentrated solutions. The velocity constant decreases from .0178j , for a solution of .OI molar cadmium iodide to .01316 for a solution of the same concentration of cadmium iodide to which ,004; mol of iodine has been added. I n contrast to this, for a . z j molar solution of cadmium iodide the velocity constant is .1079 as compared to . I O Z ~ for the same concentration of cadmium iodide to which 0.00338 mol of iodine has been added and .ro3 j for .03624 mol of iodine added. Jakowkin: 2. physik. Chem., 20, 19 i1896,.
CATALYTIC DECOMPOSITION OF HYDROGEN PEROXIDE
3417
Bruns8 has shown that when iodine is added to solutions of cadmium iodide the conductance of the solution is increased. According to his theory this increase is due to the shift in equilibrium between the associated molecules and the simple molecules of cadmium iodide according to the following equation : (Cd12), F’t nCdIp G nCd++ 2nI-
+
This explanation agrees with our results since in a dilute solution there are practically no associated molecules while in a concentrated solution there is a considerable percentage of them. When iodine is added to a dilute solution of cadmium iodide 13-ions are formed. When, however, the iodine is sdded t o concentrated solutions, as the 11- ions are formed the complex molecules are broken down into simple molecules which can in turn dissociate to give iodine ions. These results also agree with the work of Van Name and Browng on the distribution of iodine between aqueous solutions of cadmium iodide and carbon bisulfide. Summary I n the catalytic action of cadmium iodide the Cd++ ions have been (I) nilown to be without appreciable catalytic effect on the hydrogen peroxide. The active agents in the catalytic decomposition of hydrogen peroxide are Iions and the CdI+ ions and CdI3- ions, postulated by McBain. Cd13- ions decompose hydrogen peroxide a t a rate which is nearly 50% that of the iodine ions. I n each case the velocity of decomposition is directly proportional to the concentration of the active ions. (2) The ionic complexity of cadmium iodide solutions, as determined quantitatively by hfcBain, has been confirmed by kinetic measurements. (3) The effect on the decomposition of hydrogen peroxide of solutions of cadmium iodide to which various concentrations of iodine have been added has been determined. The addition of iodine to dilute solutions of cadmium iodide showed large decreases in the velocity constants. The addition of iodine to concentrated solutions of cadmium iodide showed very small decreases in the velocity constants. Madzson, Wzsconszn.
Bruns: Z. Physik, 34, 751 (1925). Van Name and Brown: Am. J. Sci., 44, 105 (1907).
