Nov., 1939
THEAQUATION OF C A R B O N A T 0
of sucrose decomposed per photon absorbed by the system. The variation in 4 with the concentration of sucrose was given by 4 = a [l- exp( -g [sucrose])]. A t 254 mp, a = 0.3 and g = 0.6 when [sucrose] was expressed in moles per liter and solutions were buffered with acetate. 4 increased when the solutions were not buffered, but the pH then decreased slightly due to subsequent reactions. 4 was independent of (1) the light intensity over a 200-fold range, (2) the temperature between 10 and 35O, (3) the $H between
[CONTRIBUTION FROM TEE
PENTAMMINE COBALTIC
BROMIDE
3229
3 and 6, and (4)the concentration of uranyl sulfate between 0.0008 and 0.008 molar. The kinetics of the reaction have been discussed. Extinction coefficients of uranyl sulfate solutions were obtained a t intervals of 0.1 in the logarithm and are given for the dip in absorption about 366 mp. Beer’s law was obeyed and sucrose concentrations up to 100 times those of uranyl sulfate did not affect the measurements. CAMBRIDGE, MASSACHUSETTS
RECEIVEDAUGUST9, 1939
MALLINCKRODT CHEMICAL LABORATORY OF HARVARD UNIVERSITY ]
The Aquation of Carbonato Pentammine Cobaltic Bromide BY ARTHUR B. LAMBAND ROGERG. STEVENS Carbonate tetrammine cobaltic salts undergo no change in conductance a t 25’ by themselves, an little or no aquation in pure aqueous solution at increase in conductance took place when the soluordinary temperatures. In other words, the car- tions were left in contact with platinum electrodes. bonate group is not readily displaced from the Solely on the basis of this evidence he inferred that complex ion by molecules of water. This is ex- carbonato pentammine nitrate does not aquate plained by its firm attachment, presumably by spontaneously, but does so under the catalytic influence of the platinum. The latter part of this two covalent links. In carbonato pentammine salts the carbonato inference was and is quite unjustified, because no group appears to be held by one polar and one such catalytic effect on aquation has ever been obcovalent link. While the former would permit served by others, while it has long been recognized dissociation, the latter holds the carbonato group that the decomposition of cobalt ammines is firmly, so that it cannot function as a separate markedly accelerated by contact with platinum ion. In agreement with this, their freshly pre- electrodes.* In this situation it appeared of interest to expared solutions, like those of the corresponding amine solutions of these carbonato pentammines carbonato tetrammine salts, do not give the characteristic tests for the carbonate ion and exhibit more particularly for possible aquation, and this the conductance to be expected of uni-univalent we have now done by means of measurements of the conductance of dilute solutions of the bromide electrolytes. One might expect, however, that carbonato a t 25 and 0’. The carbonato pentammine cobaltic bromide pentammine salts with but a single covalent link holding the carbonato group, would undergo was prepared through the nitrate following the aquation more readily than the carbonato tetram- methods of Werner and Gosling.‘ The conductmine salts. Yet, Werner and Gosling,l to whom ances were measured using “dip” electrodes folwe owe most of our knowledge of the carbonato lowing the procedure previously described in anDuplicate readings were taken pentammines, appear never to have observed any other c~nnection.~ aquation of these substances, and Schwartz and over an interval of more than an hour with and Tede2state that dilute solutions of the nitrate are without the removal of the electrodes from the stable both in the dark and when exposed to ultra- solution. Identical changes in conductance took violet light, as shown by the constancy of their place in each instance indicating that the platielectrical conductance. Duffla to be sure, found num electrodes exerted no perceptible effect under that while dilute solutions of the nitrate showed the circumstances. (1) Werner and Goding, Be.,16, 2880 (1908). (2) Schwartz and Tede, ibid.. 60, 65 (19.27). (a) DUS, J . Chrm. SOC.,ma, 678 (1888).
(4) Lamb and Laraon, THISJOURNAL, U ,2625 (1920); Lamb and Yngve, ibid., 11,2864 (1921); Br6nsted, ibid., a), 438 (1927). (6) Lamb amd Btaven, ibid., 64, 8648 (1882).
ARTHUR B. LAMBAND ROGERG. S T E V E N S
3230
The resulting data are collected in Table I. TABLE1 OF CHANGEIN CONDUCTANCE OF A 0.004 M SOLUTION
COBALTIC BROMIDE CARBONATO PENTAMMINE AQUATION -0
Mol. Time, cond. min. l/ohm 0 (54.8) 4 55.5 5 55.7 7 56.1 10 56.6 11 5 6 . 8 15 57.5 16 57.6 25 58.9 50 61.7 101 6 6 . 0 145 6 8 . 8 205 71.6 (74.9)
-z l/ohm a
(19.7) 19.0 18.8 18.4 17.9 17.7 17.0 16.9 15.6 12.8 8.5 .5.7 2.9 0
+, l/min.
kl
k2
0.0089 .0094 ,0097 .0097 ,009i .0099
,0097 .0092 .0085 ,0083 .0085 .0084
-25' Mol. Time, cond. a r mm. l/ohm l/ohm 0 (102) (69.2) 1 108.9 62.2 2 115.8 5 5 . 3 4 127.2 4 3 . 9 5 131.4 39.7 (i 135..5 35.6 8 142.0 2 9 . 1 10 147.3 2 3 . 8 12 151.8 19.3 15 156.0 15.1 20 161.9 9.2 25 165.4 5.7 35 168.6 2.5 0.1 6 0 171.0 0 86 171.1 444 171.2 Average
-
hi 4- b
l/min.
0.106 ,113 .113 ,111 ,111 ,111 ,106 .lo6
.lo1 .lo1 ,099 .094
,105
Vol. 61
c o w the inference dram from the conductance measurementsthat aquation of the carbonato pentarnmine does indeed take place. From the data in Table I values of k in the formula for a unimolecular reaction have been computed taking a, the concentration of the reacting substance, as equal to the total change in conductance and a - x as equal to the change in conductance at the time t. Since the final state reached is, on the basis of all our previous studies of aquation, an equilibrium, the constants thus obtained represent the sum of the individual constants for the direct and the reverse reaction, i. e., kl k z . They are given in the last column of Table I and show a reasonable constancy a t each temperature.
+
The Mechanism of the Aquation
Aquation is usually looked upon as a direct replacement of a complexly held anion by a mole* .0004 cule of water. An alternative mechanism would It can be seen that the conductance increases be the primary addition of hydrogen ion to the slowly, indicating a slow aquation. A stationary complexly held anion with the formation of the state is reached after about an hour at 25' and undissociated acid which is then replaced by a molecule of water. It is, however, unlikely that after perhaps ten hours a t 0'. It was possible, to be sure, that the observed this latter is the sole mechanism of aquation, beincrease in conductance was caused by a decom- cause then the anions of weak acids should aquate position of the ammine. If this occurred, free more rapidly and completely than those of strong ammonia should be present. A current of air acids, and this does not appear to be the case. This mechanism is, however, presumably operawas therefore bubbled through a solution of the ammine and thence into Nessler reagent, blank tive in the case of the carbonato pentamtests having shown that minute amounts of am- mines. They react rapidly with acids to give monia in a solution can be detected in this way. carbonic acid and aquo pentammine s a h . Direct With the ammine solution at 0' no trace of am- replacement of the complexly attached carbonato monia was indicated until after an hour's bubbling. group by water certainly could not account for so A t 25' a trace was indicated in thirty minutes and rapid a reaction. There can, therefore, be little a marked test was obtained in seventy minutes. doubt that the hydrogen ion here combines diSince the change in conductance is nearly com- rectly with carbonato group to form the bicarplete before any decomposition is detectable, the bonato group. The resulting ion, for electrostatic observed change in conductance cannot be as- reasons, would aquate much more rapidly than the carbonato ion itself. Indeed, it might aquate scribed to decomposition. with sufEcient rapidity to account entirely for the Tests were also made to confirm the presence of the bicarbonate ions produced by the aquation. observed aquation of the carbonato ammine. Aquation and Anation of the Bicarbonato For this purpose a weakly ammoniacal solution ascertain how rapidly and of barium chloride was added to freshly prepared Pentammiaes.-To solutions of the bicarbonato ammine bromide at completely the aquation of the bicarbonato pent0'. At first no indication of carbonate or bicar- ammine takes place, we have measured the conbonate was obtained, but after the solution had ductance of dilute solutions of its nitrate a t 0' stood for an hour at this temperature a small pre- and a t successive intervals of time. The data are cipitate was formed. A similar sohrtion kept at given in the second column of Table 11. It cafl be seen that a relatively rapid rise in 2 5 O for seventy minutes and then cooled to Oo gave an immediate and copious precipitate when conductance takes place, indicating rapid aquathe barium hydroxide was added. These results tion, a stationary state being reached in about Average
0.0092
0.106 * .005
Nov., 1939
THEAQUATION OF CARBONATO PENTAMMINE COBALTIC BROMIDE TABLEI1
CHANGE IN CONDUCTANCE OF A 0.00280 hf SoLUTION OF
BICARBONATO PENTAMMINE COBALTIC NITRATEAND BROMIDE AT 0 AQUATION Molecular conductance Bromide (calcd.) l/ohm'
Time, min.
Nitrate, l/ohm
0 6 7 8 9 10 11 12 13 15 18
(124.2) 138.1 139.4 141.1 141.9 142.5 143.2 143.8 144.2 144.5 144.8 (145.4)
(130.0) 143.9 145.2 146.9 147.7 148.3 149.0 149.6 150.0 150.3 150.6 (151.2)
a - x
l/oh&
(21.2) 7.3 6.0 4.3 3.5 2.9 2.2 1.6 1.2 0.9 0.6 0 Average 0.20
ki +,h, l/rmn.
3231
bicarbonato pentammine ion, i. e., anation. To compare these data with those obtained in the previous aquation experiment it was necessary to eliminate the conductance of the sodium bromide present here as shown by the equation (Co(N&)rSIaO)Bra
+(Co(NHa)dICOs)Brp NaHCOa + + NaBr + H20
and not present in the aquation experiment. This was accomplished by multiplying the observed specific conductance of the sodium bicarbonate solution before the addition of the ammine (0.0001334 l/ohm) by the ratio of the molecular conductances of sodium bromide and sodium bicarbonate a t the prevailing concentration, and subtracting the product from the observed specific conductances. The resulting corrected specific conductances were then converted to mo* 0.01 lecular conductances and were corrected for the eighteen minutes. The extrapolated initial molecu- diminished conductances, both of the ammine lar conductance, 124.2 l/ohm, is what would be ex- and of the sodium bicarbonate, in the mixture. pected from the previously observed' conductance These corrected molecular conductances are given of hydroxo pentammine chloride, 132 l/ohm. in the third column of Table I11 and are shown To ascertain whether this final stationary state graphically as the upper curve in Fig. 1. The represents an equilibrium, we have also carried r I I I I out the reverse process, i. e., the anation? of the aquo pentammine. For this purpose we added an equivalent weight of aquo pentammine bromide to a solution of sodium bicarbonate and followed the change of the conductance with the time. The resulting data are collected in Table 111. 0.177 .179 .198 .200 .198 ,205 .214 ,221 .209 * 198
TABLEI11 CHANGE OF CONDUCTANCE OF A 0.00286 M SOLUTION OF SODIUM BICARBONATE AND OF AQUOPENTAMMINE AT 0 COBALTIC BROMIDE ANATION Time,
mu.
0 1 2 3
4 5 7 10 18 38
Molecular conductance Observed, Corrected, l/ohm l/ohm
0.6624 .6393 .6261 .6166 .6107 .6029 ,5972 ,5937 ,5935
(183.3) 174.8 166.8 162.2 158.9 156.8 154.0 152.0 150.8 150.8 (150.8)
l/ohm
Al&2
(32.5) 24.0 16.0 11.4 8.1 6.0 3.2 1.2
0.304 .318 .346 .348 .337 .332 .330
a - x
0
10
20 30 40 Minutes. Fig. 1.-Change in conductance (corrected) of a solution of bicarbonatopentammine cobaltic bromide (aquation) and of a solution of aquopentammine cobaltic bromide and sodium bicarbonate (anation) at 0".
corresponding molecular conductances observed with the bicarbonato pentammine nitrate, corrected to bromide by addition of 5.8 l/ohm for ... the difference in mobility between two bromide ... and two nitrate ions, are given in the third column m Average 0.33 * 0.005 of Table 11 and are also shown as the lower curve in Fig. 1. It can be seen that these two curves The observed specific conductances are given approach each other and after about eighteen in the second column of the table. They exhibit minutes become nearly coincident. This india rapid decrease, indicating the formation of the cates that the stationary condition reached both (6) Lamb and Stevens, ref. 6, p. 2649. by aquation and anation is indeed an equilibrium, (7) A. B. Lamb, Tars JOURNAL, 61,808 (1WO).
AKTHUR B. LAMBAND ROGERG. STEVENS
3232
Vol. 61
constants for a uniniolecular reaction of this sort, kl k ~ the , sum of the unimolecular velocity constants of the simple aquation and anation reactions, respectively, are given in the final columns of Tables I1 and III.' I t can he seen that the individual values for kl kz are in each series reasonably constant. The average of the values obtained in the anation measurements is, however, considerably larger (CO(NH~)&ICOS)++ (Co(NHs)&I~O)+++ HCOsthan the average obtained in the aquation meas@fat) if(fast) urements. This may well be due to a lack of rigor (Co(NHs)sCOa)+++ (Co(NHs)t,OH)+' 4- HzCOs in our assumption of direct proportionality between the change in conductance and the concenCalling Aq+++ and Hy++ the concentrations of trations of the ions undergoing either aquation aquo and hydroxo pentammine ions, respectively, or anation, or to the failure of the approximate and considering solely the above equilibria on the unimolecular formula to represent adequately the right bimolecular anation reaction when corrected for the kinetic salt e f f e ~ t . ~ Aquation in Mixtures of Carbonato and BiThat is, Aq+++ = 2Hy+f a t the start.8 Taking carbonato Pentammine.-We have also made the molecular conductances of the aquo and hy- measurements of the change in conductance of droxo pentammine bromide, sodium carbonate and solutions of carbonato pentammine bromide to sodium bromide a t Oo, and a t the prevailing con- which aliquot amounts of hydrochloric acid have centration, as 222, 130, 47.5 and 62.6 l/ohm, re- been added. This has afforded a convenient spectively, the initial conductances of the system means of obtaining mixtures of the carbonato and on the right, before any bicarbonato had formed, bicarbonato ammines. The observed conductwould be 222 47.5 - 62.6, or 206.9 l/ohm, if ances of the ammine immediately upon the addithe pentammine were exclusively aquo, and 130 tion of the acid and a t intervals thereafter are l/ohm, if exclusively hydroxo. Since, however, given in Table IV. Velocity constants computed Aq+*+ = 2Hy++, the initial conductance should in the same way as for the pure compounds are be 206.9 - 1/3(206.9 - 130) = 181.3 l/ohm. also given. This value corresponds closely with the extrapoIt can be seen that the solution to which an lated value shown in Fig. 1. equivalent amount of acid was added showed The concentrations of the various ions in these substantially the same initial conductance and solutions a t any time during the aquation or ana- the same rate of aquation as a solution of pure bition could likewise be calculated from the observed carbonato pentammine. Solutions containing conductances and the known dissociation con- lesser amounts of acid showed progressively stants of aquo pentammine ion and carbonic acid smaller velocities of aquation. Indeed, the veif the dissociation constant for the bicarbonato locity constants for the aquation of the several pentammine ion were also known. From these solutions followed closely the formula concentrations velocity constants for aquation ki + kz = 0.214(Co(NHa)dfC08)++ and anation could then be directly evaluated with 0.00286 a good deal of certainty. Lacking this informa- the calculated values for K1 kz when 1 / 1 ~ and tion as to the bicarbonato ammine, we have equivalents of hydrochloric acid were added, simply assumed, as usual, that the extent of the being 0.0214 and 0.0535, respectively, whereas reaction is proportional to the change in conduct- 0.0233 'and 0.0515 were actually observed. ance, and since an equilibrium is attained we have The application of this formula to the 0.00286 taken the total concentration of reacting sub- M solution of pure carbonato pentammine brostance as equal to the amount transformed a t mide where the velocity constant of aquation is equilibrium. The resulting apparent velocity 0.0092 l/min. would indicate a concentration of (8) From the data of Br6aatcd. 2.ghysik. Chrm., 184,188 (1528); bicarbonato pentammine = 0.0092 X 0.00286/ and Moclnnrs, THISJOURNAL, 6 4 2 8 a 8 (1888); 17, 1064 (108S), by 0.214, or 0.00012 M , provided aquation takes application of the obacrved temperature d d r n t m .
The composition of the solution of aquo pentammine bromide and sodium bicarbonate a t the start, before any bicarbonato pentammine has been formed by anation, can be computed from the known dissociation constants of aquo pentammine ion and carbonic acid. The reactions involved in this equilibrium can be represented as follows
-
+
+
+
+
+
THEAQUAT [ON
Nov., 1939
OF
CARBONAT0 PENTAMMINE COBALTIC BROMIDE
3233
TABLEIV CHANGE IN CONDUCTANCE OF A 0.00283 M SOLUTION OF CARBONATO PENTAMMINE COBALTIC CHLORIDE CONTAINING VARIOUS CONCENTRATIONS OF HCl AT 0' HCI = 0.000286 M
Time, min.
0 1 2 3 4 5 6 7 8 10 12 15 20 25 30 35 45 55 70 90 125 160 195
Mol. cond obsd., l/ohm
(64.4) 65.0 65.7 66.3 67.0 67.5 68.1 68.7 69.3 70.3 71.2 72.7 74.7 76.6 78.2 79.7 82.1 84.0 86.2 88.4 90.5 91.6 91.9 (92.2)
- I, l/ohm
a
(27.8) 27.2 26.5 25.9 25.2 24.7 24.1 23.5 22.9 21.9 21.0 19.5 17.5 15.6 14.0 12.5 10.1 8.2 6.0 3.8 1.7 0.6 .3
Average
+ l/min.
k1
kl,
0.0240 .0235 ,0244 ,0237 ,0237 ,0240 .0242 ,0240 .0235 ,0237 ,0230 ,0230 .0228 .0228 .0226 .0221 ,0219 ,0221 ,0223 ,0240 ,0233
*
Time, min.
0 2 3 4 5 6 7 9 11 13 16 19 22 26 31 36 41 51 66
HCI = 0.000716 M Mol. cond. obsd., a x, l/ohm l/ohm
-
(74.6) 77.2 78.5 79.6 80.7 81.7 82.7 84.5 86.1 87.5 89.4 91.0 92.5 94.1 95.7 96.9 97.8 99.3 100.4 (101.2)
0.0233 .0006
place exclusively via the bicarbonato ammine, that is, with no direct aquation of the carbonato. This concentration of bicarbonato in equilibrium with the carbonato in pure water would correspond to a dissociation constant of about 4 X 10-lo. A direct determination of this constant would show the actual concentration of bicarbonato in equilibrium with the carbonato and thus permit a decision as to what part of the aquation of the carbonato takes place directly and what part via the bicarbonato pentammine.
(26.6) 24.0 22.7 21.6 20.5 19.5 18.5 16.7 15.1 13.7 11.8 10.2 8.7 7.1 6.5 4.3 3.4 1.9 0.8
kl
+,kz,
l/min.
0.0516 ,0528 ,0521 ,0521 ,0518 ,0518 ,0518 ,0516 .0512 ,0510 .0506 .0510 .0510 ,0510 ,0507 ,0502 ,0516 ,0530
Time, min.
0 3 4 5 6 7 8 9 10 11 12 14 17
HC1 = 0.00286 M Mol. cond. obsd., a x l/ohm l/ohm
-
(130.0) 142.0 144.9 147.3 149.0 150.5 151.5 152.4 153.0 153.3 153.8 154.2 154.5 (155.3)
(25.3) 13.3 10.4 8.0
6.3 4.8 3.8 2.9 2.3 2.0 1.5 1.1 0.8 0
k1 -, k2,
l/min.
0.215 ,217 ,280 ,232 ,237 ,237 ,240 ,240 ,230 ,235 ,224 ,206
0
Average
*
0.0515 .0006
Average
0.228
* ,007
equilibrium with the opposing anation reaction by which aquo pentammine is converted into bicarbonato pentammine. The velocities of these aquation and anation reactions have been measured and velocity constants have been computed. 2 . Evidence is presented to show that in the equation of the carbonato pentammine salts a rapid, reversible addition of hydrogen ion to the complexly held carbonato group takes place, followed by a relatively slow, but reversible, aquation of the bicarbonato group. 3. It has been pointed out that these measureSummary ments, coupled with measurements of the disso1. It has been found from conductance meas- ciation constant of the bicarbonato pentammine urements that carbonato pentammine bromide ion would permit an estimate of how much of the undergoes a slow aquation to give an equilibrium aquation of the carbonato pentammine takes place mixture of bicarbonato, aquo and hydroxo pent- directly and how much through the intermediate ammines. Bicarbonato pentammine salts un- formation of the bicarbonato pentammine. dergo a much more rapid aquation, reaching an CAMBRIDGE, MASSACHUSETTS RECEIVED JUNE27, 1939
