The alkaline decomposition of hexamminocobalt(III)chloride

Anulytica

Chimicu Acta, 64 (1973)

107-l

12

@ Eiscvicr Scientific Publishing Company, Amsterdam - Printed in The Netherlands THE ALKALINE CHLORIDE

,DECOMPOSITION

CHARLES P. NASH and G. JAMES MIILLE Depnrtmetrt of Clrentisrry, Uuiuersitj~ of’ Culi]orr&, (Received

18th September

107

OF HEXAMMINOCOBALT(II1)

Duvis, CdiJ

95616 (U.S.A.)

1972)

It has become fashionable in recent years to have students in updergraduate laboratory courses prepare and analyze transition metal complexes, notably various chIoro-ammine complexes of cobalt(III) 14. In typical procedures the halide is determined gravimetrically, the ammonia is determined by a simplified Kjeldahi method, in which ammonia is evolved when the complex is decomposed in boiling strong base, and the cobalt is determined either spectrophotometri~ally as the tetrathiocyanato complex of cobalt(II), or titrimetrically by an iodimetric method. It has been known for more thana century that simple alkaline decompositions yield ammonia analyses for cobaIt( III) ammines which are systematically lows. Horan and Eppig6 suggested that the low values resulted from some uninvestigated reaction between ammonia and cobalt(II1). They also showed that if arsenic(II1) oqide was added to the distillation flask, ammonia could be recovered quantitatively. What seems not to have been appreciated, is that an iodimetric “cobalt” assay which is done on the alkaline reaction mixture which remains after the ammonia has been expelled, must be affected by the oxidation-reduction chemistry which has already taken place between cobalt(II1) and ammonia. The present work reports experiments designed to identify the principal reactions which occur during the alkaline decomposition and iodimetric analysis of hexammino~obalt(II1) chloride. EXPERIMENTAL

Analytical reagent-grade chemicals were used throughout. Sodium thiosulfate and potassium permanganate solutions were prepared and standardized by textbook methods’. Hexamminocobalt(II1) chloride was synthesized by the method of PaimerB. The pooled results of over 1200 student analyses give o/,Cl= 39.8 rtO.2; %NH3== 37.8 40.5 (theoretical: “/,el = 39.78 ; %NH:, = 38.20). When the oxidizing power of the compound was determined by a method similar to that described by Palmer (decompose in boil&g alkali, acidify, add excess of iodide, allow to stand, and titrate with standardized thiosulfate), it was found that the results based on cobalt(III) were cu. 5% low, and the starch end-point was very unstable. The blue color returned repeatedly, often within 10 s. An unstable starch end-point in an iodimetric analysis of a system open to the air suggests the presence of nitric oxide, the precursor of which is nitrous acidg.

108

C. P. NASH,

G. J. MIILLE

the existence of nitrite ion in a sample of the A Griess-Ilosvay test’ o confirmed complex which had been boiled in strong base in air and then was made strongly acidic. The oxidizing power of the system was then studied systematically. Identical 25.00-ml aliquots of a stock solution of Co(NH3),Cl,, each containing 0.4627 g (1.732 millimoles) of complex, were taken for analysis. The cobalt(II1) content of each aliquot is equivalent to 17.32 ml of 0. IO00 M sodium thiosulfate. Four procedures, which differ mainly in the point at which an argon atmosphere was introduced, were employed as follows. Procedure A To a 25-ml aliquot of complex stock solution add 10 ml of 6 M sodium hydroxide. Boil in an argon atmosphere for 10 min. All further operations are also performed under a blanket of argon. Remove the solid by filtration, preserving the filtrate. Transfer the solid and filter paper to a 300-ml Erlenmeyer flask. Add 25 ml of boiled, deionized water, 2 g of potassium iodide and 7 ml of 6 M hydrochloric acid. Allow the flask to stand for 5 min. Add 100 ml of boiled water and titrate with 0.1000 M thiosulfate to a starch end-point. To the alkaline filtrate, add 2 g of potassium iodide and 17 ml of 6 M hydrochloric acid. After 5 min add 100 ml of boiled water and titrate. Procedure B This is similar to Procedure A, except that the complex is decomposed in a ffask open to the atmosphere. After the filtration step has been done in air, every subsequent manipulation is performed under a blanket of argon.

Procedurec

This is similar 5 ml of 6% hydrogen

to Procedure B, except that the complex is decomposed peroxide and 10 ml of 6 M sodium hydroxide.

with

This is similar to Procedure C, except that the argon is not introduced until the reaction flasks are ready to be titrated. The llasks are stoppered during the 5-min waiting periods. RESiJLTS

AND

DISCUSSION

The results of experiments performed using Procedures A-D are summarized in Table I. The reproducibility of the titration volumes is r40.05 ml of 0.1000 M thiosulfatc. Procedure D is very similar to a procedure contrived in this laboratory to produce the “right” answer for students. They do not separate the phases, and the inert atmosphere is generated by adding a small piece of dry ice to the titration vessel. Table I shows that the success of this procedure is purely fortuitous. The entries for Procedure A in Table I show that nearly 25% of the cobalt(II1) is reduced by ammonia when the complex is decomposed in base in an inert atmosphere. Given the N:Co ratio of 6:l in the complex, together with the fact that 0.12 millimoles of nitrogen fail to appear as ammonia, it follows that 3.5 moles of

DECOMPOSITION TABLE

109

OF Co(NH,),Cl,

I

TITRATION

VOLUMES

Prorcclrr rr

A B C D Theoretical

0.02 0.32 0.40

13.12 14.60 15.80 15.87 17.32 --

1.63 0.00

---

13.14 14.92 16.20 17.50 17.32 ~_I--~--____l_--________*..

cobalt(W) are reduced to cobaIt(I1) per mole of unrecovered nitrogen. The results of Procedure B show that atmospheric oxygen serves both to reoxidize somecobalt(I1) to cobalt(W) and to cause l/4 of the unrecovered nitrogen to appear as nitrite, In Procedure C the added hydrogen peroxide acts principally on the hydroxide of cobalt(II), but the reaction is incomplete. Procedure D shows that almost 10% of the total oxidizing power of the system found when everything but the titration is done in air, may be attributed to nitrogen chemistry, according to the reactions 2W’-f-NO;-t-INO+3 2 HffN02i-2

0,

-+&+NOfH20

(1)

-+NO,

(2)

I--,Iz+NOfI-120

(3)

According to the results of Procedures A and B, the presence of oxygen is essential to the production of nitrite in the system. The results of Procedure A are most readily interpreted by postulating that the oxidation of ammonia by cobalt(III) occurs in two successive ‘-electron steps. It is suggested that the initial oxidation product is hydroxylamine, which forms by a reaction which may be written schematically as zCo3”-NH,

t--HO-Co3+f-

H,NOH-t-2

Co2+

(4)

There is ample precedent for suggesting the deprotonation of ‘ammonia molecules bound to cobalt(II1) in strongly basic solution”, and the precipitation of the hydrated cobaIt(II1) oxide almost certainly occurs by a series of stepwise substitutions of hydroxide for ammonia. Strong evidence for the initial production of hydroxylamine comes, from a series of experiments in which 0.12 millimoles of hydroxyammonium chloride in 35 ml of 1.7 M sodium hydroxide was boiled in air for 10 min, with and without the presence of solid hydrated cobalt(1 II) oxide. This amount of hydroxylamine is equivalent to the ammonia loss from an aliquot of the Co(NH,)&l, stock solution. In the first set of experiments, the boiled solutions were cooled and filtered, and the volumes were readjusted to 35.00 ml by adding water. A 25.00-ml portion was withdrawn, to it was added 25.00 ml of 0.020 M potassium permanganate and, after 10 min, 11 ml of 3.6 M sulfuric acid and 2 g of potassium iodide were added.

110

C. P. NASH, G. J. MIILLE

After 5 min, the liberated iodine was titrated with standard thiosulfate solution. It was found that when cobalt(II1) oxide was absent, 25 ml of the boiled reaction mixture reduced an amount of permanganate equivalent to 2.7 kO.1 ml of 0.1000 M thiosulfate. This may be compared with the value of 3.7 +O.l ml found when 25 ml of the reaction mixture was not boiled, but rather was assayed immediately after it had been prepared. When cobalt(III) oxide was added and *the mixture was boiled, the permanganate consumption was equivalent to 1.5 + 0.1 ml of 0.1000 M thiosulfate. In a second set of experiments, in which the sulfuric acid was added to the 25.00-ml portions of boiled, reconstituted solutions before the permanganate was added, the permanganate‘losses were equivalent to 3.1 +O.l ml of thiosulfate when cobalt(I11) oxide was absent, and 0.5+0.1 ml when it was present. Cobalt-free reaction mixtures assayed in acidic solution without boiling, consumed permanganate equivalent to 4.3 +O. 1 ml of thiosulfate. The two different titration volumes which were found when cold hydroxylamine was reacted with permanganate in acidic or basic solution both agree well with the permanganate consumptions found by Kurtenacker and Neusser” under similar conditions. Since the same ratio of permanganate consumptions in acidic and basic media was found for both the cold and the boiled (cobalt-free) reaction mixtures, it can be concluded that in boiling alkaline solution, hydroxylamine partially decomposes to form volatile products, possibly nitrous oxide and ammonia13. When hydroxylamine and cobalt(II1) oxide were boiled together in alkali, not only were the amounts of permanganate consumed by the reaction mixtures greatly reduced, but more importantly, the consumption in acidic solution was only one-third of the consumption in basic solution. Thus the hydroxylamine must have reacted essentially completely with the trivalent cobalt oxide. Nitrite could be detected in the acidified reaction mixture, but the marked nonequivalence of the titration volumes found after permanganate treatments in acidic and basic media showed that nitrite cannot be the sole non-volatile oxidation product of the reaction, since permanganate and nitrite do not react in alkaline solutiong. This system obviously merits further study. In the present study, 25-ml aliquots of Co(NH,),Cl, were next decomposed according to Procedure B. The solids were filtered away, the solutions were reconstituted to 35 ml, and 25-ml portions were subjected to permanganate oxidations under both acidic and basic conditions as above. The permanganate consumptions were equivalent to 0.5fO.l and 1.5 +O.l ml of 0.1000 M thiosulfate, respectively. Since these volumes are identical to those found for the corresponding cobalt oxidehydroxylamine systems, hydroxylamine is almost certainly the initial oxidation product in the alkaline oxidation of ammonia by cobalt(M). Since the results of Procedure A show that a total of 3.5 electrons are involved in nitrogen-cobalt redox chemistry, and since nitrite must derive from at least one non-volatile oxidation product of hydroxylamine, it can be presumed that much of the hydroxylamine then reacts with cobalt(II1) in another 2-electron process. The most probable reaction product is nitroxyl (NOH), or its anion (NO-), for the reason that many repeated attempts to detect hyponitrite (N,Of-) in the alkaline filtrates from Procedure A all produced negative results, The hyponitrite ion has an intense ultraviolet absorption maximum (&,,,, = 247 nm, E= 3980)14. According to the kinetic data of Buchholz and Powell l 5, at pH 14 and 25” the half-life for the

DECOMPOSITION

OF Co(NH&,CIJ

111

decomposition of hyponitrite is cu. 15 days: If the activation energy for the decomposition is ca. 25 kcal, then at 100” its half-life would be ca. 5 min, and the species should be detectable in the present reaction mixtures. In no instance was absorption found at wavelengths above 225 nm. It is noteworthy that nitroxyl has also been postulated as an intermediate in the alkaline oxidation of hydroxylamine by either CU(OH)~‘~ or pertungstate” at room temperature. In these systems in the presence of oxygen, nitrite yields of 30-50% based on hydroxylamine were obtained. SUMMARY

When hexamminocobalt(II1) chloride is boiled in 1.7 M sodium hydroxide, 3.5 moles of cobalt(II1) are reduced to cobalt(I1) per mole of unrecovered ammonia. In air, nitrite is produced. The oxidation of ammonia probably proceeds by two 2-electron steps to produce first hydroxylamine, and then nitroxyl. RESUME

On examine la decomposition du chlorure d’hexamminocobalt( III), en milieu hydroxyde de sodium 1.7 M, & Cbullition: 3.5 moles de cobalt(III) sont reduites en cobalt(B) par mole d’ammoniac. A l’air il y a formation de nitrite. L’oxydation de l’ammoniac se fait probablement en deux stades (de 2 electrons): d’abord formation d’hydroxylamine, puis de nitroxyle. ZUSAMMENFASSUNG

Wenn Hexamminkobalt(III)-chlorid mit 1.7 M Natronlauge zum Sieden erhitzt wird, werden pro Mol umgewandelten Ammoniaks 3.5 Mol Kobalt(II1) zu Kobalt(I1) reduziert. In Luft wird Nitrit gebildet. Die Oxidation von Ammoniak verlauft wahrscheinlich tiber mei 2-Elektronenstufen unter Bildung von zuntichst Hydroxylamin und dann Nitroxyl. REFERENCES 1 2 3 4 5 6 7 8 9

D. K. Scbera, J. Chen~. E&c.. 40 ( 1963) 476. H. B. Jonassen, J. Client. Ednc., 42 (1965) 495. H. B. Jonassen, J. Chenr. E&c., 46 (1969) 47. L. R. Wilson, J. C/rent. Ecluc., 46 (1969) 447. W. Gibbs and F. A. Genth, Amer. J. Sci., 73 (1857) 234. H. A. Horan and H. J. Eppig, J. Amer. C/tent. Sot., 7 1 (1949) 58 1. E. H. Swift, A System of Chemical Anulysis, Prentice-Hall, New York, 1939. W. G. Palmer. Experimetltul Iwrgadc Chemktry, Cambridge Press, Cambridge, 1954, p. 530. C. A. Streuli and P. R. Avercll. The Amdyticol Chemistry of Nitrogen am1 Its Compotrt~tls. WileyInterscience, New York, 1970. p. 119. 10 F. D. Snell and C. T. Snell, Calorimetric Met/wtls of Analysis, Vol. 11, D. Van Nostrand, New York, 1949. p. 803. 11 F. Errs010 and R. G. Pearson, Meclta~~isms of Inoruarzic Reuctions, John Wiley, New York, 2nd Ed., 1967, p. 183. 12 A. Kurtenacker and R. Neusser, Z. Attorg. Ally. C/rem., 131 (1923) 27.

112

C. I’. NASH,

Cli%, N. J., 2nd Ed.. 1952, p. 97. 14 M. N. Hughes and G. Stcdmun. J. Cl~ou. Sot.. (1963) 1239. IS J. R. Buchholz and R. E. Powell. J. Au~cr. Clfeur. Sot.. 85 (1963) 509. 16 J. l-l. Anderson. ~~~ctl_~:sr.91 ( 1966) 532. 17 0. L. Lebcdcv and S. N. Kazarnovskii. RKW. .I. Itrorg. Chow., 6 ( 1961) 400.

G. J. MIILLE