M . Kimura, I . M . Kolthoff,and E. J. Meehan
1262
Reduction of Mercuric Chloride to Mercurous Chloride Induced by the Oxidation of Oxalic Acidla M. Kimura,lb I. M. Kolthoff," and E. J. Meehan Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received December 5, 1972)
The oxidation of oxalic acid by cerium(1V) or manganese(VI1) in the presence of mercuric chloride induces the reduction of the latter to mercurous chloride (calomel). The amount of calomel formed is about the same in absence or presence of manganese(I1). The results are accounted for by a sequence of free-radical reactions which is initiated by the reaction C ~ 0 4 ~+- Ce(1V) C02 + C02.Ce(II1) leading to the overall induced reaction 2HgC12 + C ~ 0 4 ~ - HgzClz + 2C1- + 2C02. The amount of calomel formed increases with increasing temperature and with decreasing rate of addition of the oxidant. The induced reaction was also studied with cobalt(II1) oxalate, with iron(II1) oxalate, in the dark and light, and with the Fenton reaction mixture (iron(I1)-hydrogen peroxide), When chromium(V1) is the oxidant, no calomel is formed in its presence. Oxygen and chloride ions are powerful inhibitors; no calomel is formed in solutions saturated with oxygen, or in solutions more than 1M i n potassium chloride.
-
Introduction The slow addition of a trace of a strong oxidizing agent (Ce(IV), MnWII), or Cr(VI)) to dilute oxalic acid in dilute sulfuric acid in the presence of Mn(1I) causes the formation of free radicals which can bring about the reduction of oxygen to hydrogen peroxide.2-4 This reduction corresponds to the induced (thermodynamically favorable, but normally very slow) reaction 02
+
HZC20,
+
HZ02
+
2COz
and the amount of peroxide formed is much larger than the amount of oxidant added. The induced reduction of mercury(I1) chloride to mercury(1) chloride (calomel) initiated by the above free radicals is the subject of the present paper. It is shown that (in the absence of oxygen) the slow addition of a trace of the strong oxidants to oxalic acid containing Mn(I1) and sulfuric acid can bring about the induced reaction 2HgC1, + C,O,'Hg2Cl2 + 2C02 + 2C1-
-
As in the previously cited ~ o r k , 2 -it~ is found that the amount of mercury(I1) chloride reduced is much larger than the amount of oxidant added. The light-initiated reduction of mercury(I1) chloride by oxalate in neutral medium has long been known as Eder's reaction, and a mechanism accounting for the photochemical reaction was proposed first by Cartledge. In the present study the kinetics and mechanisms of the chain reactions involved in the induced reduction of mercury(I1) chloride have been studied with various oxidizing agents as initiators, in the dark and light, and in the presence of oxygen and of chloride ions. Both of the latter are retarders of the induced reduction. Experimental Section All chemicals were reagent grade. In all cases manganese(I1) was added as the sulfate. Experimental details and conditions and analytical methods were the same as described previously.2-4 Unless stated otherwise, oxygen-free nitrogen was bubbled through the reaction mixtures. Experiments in light were carried out by exposing the reaction mixture to a tungsten lamp (110 V). The amount of The Journal of Physical Chemistry, Voi. 77, No. IO, 1973
-
+
calomel formed was determined by weighing after filtering through a glass-sintered crucible, washing, and drying at 80". In order to compare conveniently the amount of calomel with the initial concentration of oxalic acid, the amount of calomel formed is expressed as molarity, which refers to the volume of the final reaction mixture. Results Cerium(IV) or Manganese(VII) as Oxidants. The effect of the amount of Ce(IV), expressed in equivalent percentage of the initial amount of oxalic acid, and added over a period of 2 or 4 hr, is presented in Table I. Unless stated otherwise the number of moles of calomel formed was almost equal to the number of moles of oxalic acid reacted minus twice the number of equivalents of oxidant added. Since the amount of calomel formed was much larger than corresponds to the amount of oxidant added, a chain reaction must occur which corresponds to the overall reaction Cz0,2- + 2HgC12 2COz + Hg2Ciz + 2C1- (I) +
(In this and subsequent equations, oxalate is written as C ~ 0 4 ~rather than as H2C204.) The relative efficiency of oxidant in initiating the reaction increases markedly with decreasing amount of oxidant added and also with increasing temperature. Table I1 illustrates this effect over the range 0-75" at varying rates of addition of oxidant, Variation of concentration of sulfuric acid from 0.1 to 0.5 M caused a small decrease in extent of reaction (from 2.3 X 10-3 to 1.7 X 10-3 M calomel formed, at 1 equiv % addition in 2 hr at 25") and further increase to 1 M caused a small further decrease (1 x 10-3 M ) . The amount of calomel formed increased greatly with increasing concentration of oxalic acid. For example, the addition of 1 equiv 70 of cerium(1V) in 2 hr a t 50" to 0.001, 0.005, or 0.01 M oxalic acid caused formation of 0.001, 0.0041, and 0.0082 M (a) This investigation was carried out under a grant from the National Science Foundation. ( b ) Present address: Department of Applied Chemistry, Yamagata University, Yonezawa-shi, Japan. I . M. Kolthoff, E. J. Meehan, and M. Kimura, J , Phys. Chem., 75, 3343 (1971). I. M. Kolthoff, E. J. Meehan, and M. Kimura, Taianta; 19, 1179 (1972). I. M. Kolthoff, E. J. Meehan, and M. Kimura, Talanta, 20, 81 (1973).
Induced Reduction of HgCI2 to Hg2CI2
1263
TABLE 111: Manganese(l1)-Hydrogen Peroxide as Oxidanta
TABLE I: Reduction of Mercury(l1) Chloride Induced by Addition of Cerium(lV)a
M x 103 Hg2CIz formed, M X I O 3 Ce(lV) added as equiv % of oxalic acid
Addition in 2 hr 25"
20 4.5 1 .oc 0.4 0.1
Addition in 4 hr
75Ob
3.6 2.2 1.7
0.01 0.01 0.1 0.1
3.6 2.9 2.0
a Reaction mixture 0.01 M in oxalic acid, 0.1 M in mercury(l1) chloride, and 0.5 M in sulfuric acid: dark: N2 saturated. 0.1 M H2SO4, Corresponds to 2 X M Ce(lV). Corresponds to 93% of oxalic acid used in reaction 1.
TABLE II: Effect of Temperature and Time of Addition of 1 Equiv % of Ceriurn(lV)a HgzClz formed, M X IO3, in addition time, hr
0.1
0.5
1
25
0.4
1.o
50
3.1b
1.5 7.3
75
6.0
Temp, "C
0
dark
light
H2C2O4 reacted
dark
light
H202
.dark
reacted light
25 75 25 75
0 1.5
20
6
6 12
8 26
19
9 27
0.1 1.1
0.2 1.50 0.3 1.6
a Reaction mixture 0.1 M in oxalic acid and sulfuric acid, 0.1 M in mercury(i1) chloride, and 2.1 X 1 0 - 3 M in hydrogen peroxide; 3 hr in M iron(ll1). a closed vessel In a nitrogen atmosphere. Added 1 X
0.0
-
HgzC12 formed
25'
9.3d 5.2 2.5
0
[Mn ( II) I, T e y , M C
2
4
0.1 2.3 8.2 9.3
3.4
Reaction mixture as Table I except 0.1 M sulfuric acid; same results in dark and light. 3.2 was found 2 hr after the addition.
calomel, respectively. Increase of the concentration of mercury(I1) also increases the extent of reaction 1. With conditions as in Table I, addition of 1 equiv 740 of cerium(1V) in 2 hr at 50" to solutions 0.01, 0.04, or 0.1 M in mercury(I1) chloride caused 32, 52, and 83% of the oxalic acid to react according to reaction 1. Presence of Mn(I1) up to 0.1 M in mixtures as in Table I increased the extent of reaction only to slight extent. Only a few experiments were carried out with manganese(VI1) as oxidant. Using the same reaction mixture and conditions as in Table I1 and adding 1 equiv % of manganese(VI1) in 2 hr at 50", the amount of calomel corresponded to 78% of the initial concentration of oxalic acid as compared to 82% with cerium (IV). Chromium(VI) as Oxidant. The essential difference from results described above is that no detectable reaction occurred in the absence of Mn(I1). At 50" and upon adding 1 equiw % of Cr(VI) in 2 hr to a mixture 0.01 M in oxalic acid, 0.1 M in sulfuric acid, and 0.1 M in mercury(I1) chloride, the concentrations of mercury(1) chloride formed were 0.001, 0.004, 0.005, and 0.005 M in the presence of 0.001,0.01,0.05, and 0.1 M Mn(II), respectively. Cobalt(III) as Oxidant. This was used as the trioxalate complex, prepared as previously described.2 A 0.01 M solution which was 0.1 M in sulfuric acid and 0.1 M in mercury(I1) chloride, but with no added oxalic acid, was kept oxygen free for 2 hr at 50" after which time considerable amounts of Co(II1) still remained. The amount of calomel formed was 0.0144 M , corresponding to 48% of the 0.03 M oxalate originally present in the complex. At 25" the decomposition of the complex is much slower than at 50", but the induced reduction of mercury(I1) chloride is still considerable. A mixture initially 1.2 X 10-3 M in the Co(II1) complex, 0.04 M in oxalic acid, and 0.1 M in mercury(I1) chloride and sulfuric acid after 3 hr (oxygen free)
at 25" yielded 2.3 X 10-3 M calomel. 90% of the original Co(II1) remained. Manganese(II)-Hydrogen Peroxide Couple as Oxidant. No calomel was formed in the absence of Mn(I1). In its presence calomel formation occurred (Table 111); as was the case with Ce(IV), the amount formed was greater at 75" than at 25", and increased with increasing concentration of Mn(I1). A t 75" the same results were found in dark and in light, but at 25" there was much more extensive light than dark reaction. Since oxygen is a pronounced inhibitor of the chain reaction (vide infra) it is evident that no oxygen can be formed in the decomposition of the peroxide. Iron(III) as Oxidant in Light. Fen$on Reaction. In a reaction mixture 0.1 M in oxalic acid, mercuric chloride, and sulfuric acid and 10-4 M in iron(II1) sulfate no calomel was formed in the dark in 3 hr at 25 or 75". However, when exposed to tungsten light the induced reaction occurred. At 25" the amount of calomel formed corresponded to 2.5 X M , and to 3.3 X M in the additional presence of 0.1 M manganese(I1). It is well known that oxalatoferrate slowly decomposes in light with formation of iron(I1). It was therefore expected that the calomel formation would be increased in the presence of hydrogen peroxide which not only oxidizes the iron(II), but also yields the OH. free radical. Indeed, when the above reaction mixture was made 2 X 10-3 M in hydrogen peroxide, a trace of calomel was formed in the dark after 3 hr at 25", corresponding to 0.7 X M , but to 14 X 10-3 M in light, the same amount being formed at 75" in light. In the latter experiments in light all hydrogen peroxide had disappeared after 3 hr, while practically none had reacted in the dark. Even as little as 10-5 M iron(II1) in the presence of the peroxide greatly enhances induced reaction 1, the amount of calomel formed after 3 hr at 25" corresponding to 9 X M . It is well known that the combination of hydrogen peroxide with a little iron(I1) induces the oxidation of many reducing agents (so-called Fenton reaction) whereas no reaction occurs in the absence of iron(I1). Experiments were carried out with the above reaction mixture which was made 2.1 X 10-3 in hydrogen peroxide and 1 X M in iron(I1) instead of iron(II1). After 3 hr in the dark both at 25 and 75" the amounts of calomel formed corresponded to 6 X 10-3 M , while the amounts of peroxide which disappeared corresponded to 0.5s X 10-3 M . In light the corresponding values at 25 and 75" were 13 X (calomel) and 2.1 X M (peroxide), respectively. Note that again at both temperatures all peroxide had reacted. Even with 10-5 M iron(I1) in the M calomel was formed in initial reaction mixture 11 X light after 3 hr at 25", while the amount of peroxide deThe Journal of Physical Chemistry, Vol. 77, No. 10, 1973
M. Kimura, I . M. Kolthoff, and E. J. Meehan
1264
composed corresponded to only 0.5 X M . In all these experiments the number of moles of oxalic acid disappeared was almost equal to the number of moles of calomel formed. Effect of Retarders. Oxygen. As in the Eder reaction oxygen is a powerful retarder of the induced reduction of mercury(I1) chloride by oxalic acid. For example, in a reaction mixture the same as in Table I, to which 1 equiv % of Ce(1V) was added in 4 hr and through which oxygen was passed, no calomel was formed either at 25 or 75";the hydrogen peroxide concentration formed was 0.12 x 10-3 M after 4 hr. In the presence of 0.1 M Mn(I1) and a t 25" a trace of calomel (corresponding to 0.1 X 10-3 M ) was formed, but peroxide concentration was 2.8 X 10-3 M (formed). Chloride (in nitrbgen atmosphere). In a reaction mixture as in Table I to which 1 equiv % of Ce(1V) was added in 2 hr the amounts of calomel formed at 25" corresponded to 2.3, 0.44, 0.15, 0.06, and 0 X M and a t 50" to 8.2, 4.2, 1.2, 0.8, and 0 X 10-3 M in the presence of 0, 0.05, 0.1, 0.2, and 1 M potassium chloride, respectively. In all experiments the number of moles of oxalic acid which disappeared in the reaction period was almost equal to the number of moles of calomel formed.
Discussion As stated previously, the induced reduction of mercury(I1) chloride bp oxalic acid, eq 1, is a chain reaction. With Ce(1V) the initiating reaction is
-
+
Cz0,2- + Ce(IV) C O ~+ CO,-Ce(II1) (2) The sequence which follows is analogous to that proposed by Cartledge5 for the Eder reaction C02.- + HgClZ Hg(1) + 2C1CO, (3)
+
Hg(1)
+
CZO,'HgCl,
Hg
-.c
+
Hg
+
C02
+
COz--
Hg,C&
(4) (5)
The sequence of eq 3-5 corresponds to chain reaction 1. Termination presumably occurs mainly via COz*- + Ce(1V) with some termination uia
-
CO,
+
Ce(II1)
(6)
Hg(1) + CO,*Hg + CO, (7) The assumption of steady-state concentrations for COzs-, Hg(I), and Hg, together with the assumptions that the rate constant of eq 2 is smaller than the rate constant of the subsequent reactions involving free radicals, and that the consumption of Ce(1V) occurs mainly in eq 2, leads to the result [Hg2Clzlformed=
where ha = ka/2kzk3, k b = k3k4/2k7, and co indicates the total concentration of oxidant added over tEe time interval 7 . When the initial concentrations of oxalic acid and mercury(I1) chloride are much larger than the concentration of calomel formed, the former concentrations will remain almost constant during the reaction period. Under these circumstances eq 8 predicts a linear relation between { ~ / [ H g & l ~ ] f and ~ ~ 7/co. ~ ~ d This } ~ is found experimentally as shown in Figure 1. It is no longer true a t large extents of reaction 1. The Journal of Physical Chemistry, Vol. 77, No. 10, 7973
0.5
C
1.5
1
\ T/Cc
2
x 1GWh, h r / P
Figure 1. Plot of ~ ~ / [ H g 2 Table 1 1 ) : co = 2 X 10-4 M , 25".
C 1 2 ]vs. ~ ~T /~C ~ O ~ (cf. ~ deq 8 and
It is interesting that Cr(V1) in 0.1-0.5 M sulfuric acid does not initiate the reaction unless Mn(I1) is present. Presumably the chromium(V1) oxalate complex, written as Cr(VI)Ox, reacts as follows Cr(V1)Ox Cr(1V) + 2C02 (9) Cr(V1) Cr(V)
+
+ Cr(1V)
C,O,~-
4
2Cr(V)
(10)
+
(11)
Cr(III)
2C02
Since there is no formation of calomel, this must mean that Cr(V1) does not react with CzO42- to yield Cr(V), COz, and COz.-, in analogy to initiating reaction 2 with Ce(1V). Alternatively, the absence of induced reaction might mean a very rapid reaction of C0z.- with Cr(V1). However, since induced reaction is observed in the presence of Mn(II), this possibility is ruled out. The effect of Mn(I1) can be accounted for by the reactions Cr(V1)Ox + Mn(I1) -e Mn(II1)Ox + Cr(V) (12)
+
Mn(I1) CO, + COz*- (13) Mn(II1)Ox followed by reaction 11. The initiating reaction with the Co(II1) complex is analogous to eq 13. The reaction in light and dark between hydrogen peroxide and oxalic acid in the presence of Mn(I1) and oxygen has been discussed previously.3 In the presence of mercury(I1) chloride and absence of oxygen, the reaction sequence appears to be (where Ox denotes oxalate in any form) HzOz + Mn(I1) Ox -w Mn(II1)Ox + OH- + OH. (14) . )
+
-
OH. + Mn(I1) + Ox Mn(II1)Ox + OH- (15) followed by eq 13. From the effects of light at 25 and 75" (Table 111) it seems that dark reaction 13 is much faster a t 75" than a t 25", whereas the rate of the analogous photoreaction is little affected by temperature. The effect of (5) G H Cartledge, J. Amer Chem. SOC.,63,906 (1941).
Reaction between Oxalate Ion and Peroxodisulfate
1265
iron(II1) on the calomel formation in the light is accounted for by the photodecomposition of oxalatoferrates which has been studied in detail by Cooper and DeGraffG Fe(II1)Ox
+
hv
-
Fe(I1)
+
COz
+ COz*-
(16)
the C02.- free radical initiating the chain reaction. In the additional presence of hydrogen peroxide the iron(I1) formed reacts as Fe(I1) + HzOz+ Ox Fe(II1)Ox + OH- + OH. (measurable rate)
-
-
the OH- reacting with C Z O ~ - ~
+
OH- + COz + COz*- (fast) (17) followed by reactions 3-5. Under our experimental conditions all hydrogen peroxide had disappeared at the end of the reaction period and the number of moles disappeared was considerably smaller than the number of moles of calomel formed, which is accounted for by reactions 16, 3, 4, and 5. From the experimental results it is clear that the rate of the photodecomposition of oxalatoferrate in the presence of peroxide determines to a great extent the amount of calomel formed. The reactions involving peroxide, iron(I1) and (111), oxalate, and oxygen in the dark and light have been discussed in a previous paper.4 The retarding effect of oxygen is easily accounted for by the reaction OH.
C204?-
The OzCOz- reacts further by a chain mechanism which yields hydrogen peroxide.2 Apparently the reaction of CO2.- with 0 2 is much faster than that with mercuric chloride (eq 3). Actually hydrogen peroxide formation in an oxygen atmosphere under conditions described previously213is not affected by the presence of mercuric chloride. The retarding effect by the chloride ion is accounted for by several reactions. Mercuric chloride with chloride ion forms complexes HgC13- and HgCl2- which may react with C02.- with a slower rate than HgC12 (eq 3). Also the rate of reaction 5 may be decreased by the complex formation. Furthermore chloride promotes the reaction HgZClz(solid) + C1- * + HgC1,which has bqen studied by Sillen, et a2.7 The solubility of calomel is increased by chloride. In a reaction mixture such as in Table I, but which was 2 M in potassium chloride the solubility a t 50" was 1 X 10-6 M . Probably the main cause of the retarding effect of chloride is that the stability of Hg(1) formed in reaction 3 greatly decreases 2Hg'
t Hgzz+
since [Hgz2+] becomes extremely small in the presence of excess chloride ion.8 (6) G. D. Cooper and B. A. DeGraff, J. Phys. Chem., 75,2897 (1971). (7) B. Lindgren, A. Jenssen, and L. G. Siiien, Acta Chem. Scand., 1, 479 (1947). (8) i. M. Kolthoff and C P. Barnum, J. Amer. Chern. SOC., 62, 3061 (1940).
Catalyzed Reaction between Oxalate Ion and Peroxodisulfate. I. Copper( I I) as Catalyst Masaru Kimura Department of Applied Chemistry, Yamagata University, Yonezawa 992 Japan (Received October 27, 1972)
According to the composition of the reaction mixture, two separate mechanisms of the reaction are presented to account for the copper-catalyzed reaction between oxalate and peroxodisulfate. (1)In an excess of oxalate the catalyzed reaction kinetics are half order in catalyst, first order in peroxodisulfate, and zero order in the oxalate concentration. The reaction is completely inhibited by the presence of 1%acrylonitrile, and remarkably inhibited by the molecular oxygen. (2) In a low concentration of oxalic acid, in the relatively high concentrations of catalyst, and in 0.1 M sulfuric acid, the catalyzed reaction is almost zero order in the catalyst, first order in peroxodisulfate, and half order in the oxalic acid concentration. The reaction is not subject to the inhibition by the molecular oxygen but is completely inhibited by 1% acrylonitrile. Iron(1II) and manganese(I1) are considerably strong inhibitors of the reaction, and the former is much more effective inhibitor than the latter. The effects of the various factors on the copper-catalyzed reaction between oxalate and peroxodisulfate are reported. Mechanisms and kinetic treatments are presented to account for these facts.
Introduction Recently, cobalt(I1) catalysis on the reaction between oxalic acid and peroxodisulfate was investigatedl. The cobalt-catalyzed reaction was not subject to inhibition by molecular oxygen, but was c o m p ~ e t e ~inhibited y by 1% acrylonitrile or 1%allyl acetate. Ben-Zvi and Allen2 have
studied the copper-catalyzed reaction of oxalate with peroxodisulfate and found that molecular oxygen is a strong inhibitor Of the catalyzed reaction. However, only one de(1) M. Kimura, J. Phys. Chern., submitted for Publication.
(2) (a) T. L. Allen, J. Amer Chem. Soc., 73, 3589 (1951); (b) E. BenZvi and T.L. Ailen, !bid, 83, 4352 (1961) The Journal of Physical Chemistry, Vol. 77, No. 10, 1973
