Colorimetric Estimation of Ligands via Effect of Their Cobalt(III) Complexes on Redox Indicators JACOB
S. HANKER, IRWIN
MASTER, LOUIS E. MATTISON, and BENJAMIN WITTEN
Chemical Research Division, Chemical Warfare Laboratories, Army Chemical Center, Md.
b Coordination of cobalt(l1) with certain ligands stabilizes the tripositive statei.e., increases the ease with which cobalt(l1) is oxidized to cobalt(ll1)-to a greater degree than does aquation. Many cobalt(ll1) complexes have characteristic ultraviolet or visible absorption spectra which can be used for analytical purposes. However, certain of these complexes behave as oxidizing agents because of the readiness with which cobalt(ll1) is reduced to the bipositive state. A sensitive measure of the amount of ligand present is obtained in these cases through the oxidizing effect of the complex on redox indicators such as benzidine, o-dianisidine, and o-tolidine. The theory, evolution, and application of this novel * method of analysis are discussed.
A
in oxidation potential that occurs in coordination has been recognized for some time ( 1 , 6, 8), the application of redox indicators to detect this change in oxidation potential has not been previously elucidated. The principle involved not only affords evidence of coordination in certain instances, but may be applied to the quantitative estimation of the ligands as described in this paper. The authors were involved, initially, in a search for a colorimetric method for the determination of a dithioether, of the type RSCH,CH,SR [a 1,2bis(alkylthio)ethane], in the presence of a monothioether or alkyl sulfide, RSR. Conditions were found under which cobaltous chloride would form a complex with the dithioether but not with the monothioether. This is not at all unexpected, considering the work of Tschugaeff (13-16), in which the tendency to complex formation of a series of dithioethers of general formula RS(CH2).SR with copper and nickel was investigated. Tschugaeff found that when n = 2, corresponding to the formation of a five-membered chelate ring, crystalline compounds were obtained with copper and nickel; where n = 0, 1, 3, or 5, complex formation as evidenced by precipitation did not occur. The present authors found that i t was necessary to use an alcoholether solution of cobaltous chloride and 1,2-bis(methylthio)ethane (9) in order to effect complex formation with microgram quantities of ligand. (It was LTHOUGH THE CHANGE
82
ANALYTICAL CHEMISTRY
further found that the presence of hydrogen peroxide in the solution was necessary to prepare the chelate.) When the solvent was evaporated to dryness in a water bath, a green complex was obtained which was found to be watersoluble, giving a yellow solution with a characteristic absorption spectrum (Figure 1). The absorption peak is a t 350 mp in the ultraviolet, and the base of the absorption band extends into the visible region, accounting for the yellow color. -4n analytical method based on the ultraviolet absorption of the aqueous solution of the complex was developed and the absorption was found to follow the Bouguer-Beer law; however, this method did not give the sensitivity required in the visible region. Chiarottino (3) and Spacu and Macarovici (12) have reported a method for cobalt using dimethylglyoxime and benzidine or dimethylglyoxime and o-tolidine as reagents. The latter authors (12) have also reported the isolation of brick-red complexes containing cobalt, dimethylglyoxime, and benzidine or o-tolidine. When redox indicators such as benzidine, o-dianisidine, and o-tolidine were used by the present authors in conjunction with cobaltous chloride for the estimation of the dithioether, a red color (absorption peak, 448 mp) was obtained, which was more desirable than the yellow for colorimetry (Figure 2,A). The method followed the Bouguer-Beer law in the concentration range from 15 to 90
I
230
I 255
I 2na
I
YIJ
y . It \Tas believed a t the time that ZI complex of cobalt, dithioether, and o-dianisidine was responsible for the color. Khen attempts were made to apply the method to a ligand other than the dithioether, an interesting phenomenon \vas noted. Dibutylamine coordinated with cobaltous chloride under the conditions used for the dithioether. An aqueous solution of the cobalt(II1)dibutylamine complex had absorption characteristics (Figure 3) different from an aqueous solution of the cobalt(II1)dithioether complex. However, upon the addition of o-dianisidine, identical absorption spectra were obtained (Figure 2,A and B). These results indicated that the visible absorption spectrum of the final solution was independent of the ligand, but characteristic of the redox indicator. This was confirmed by oxidizing o-dianisidine a t the same pH using Fenton’s reagent [ferrous sulfate and hydrogen peroxide ( I Y ) ] , whereupon a curve with the same visible absorption spectrum was obtained (Figure 2,C). Although attempts by Bricker and Loeffler (6) to find a stable cobalt(111) complex ion for use in oxidimetry have been unsuccessful, the oxidizing ability of cobalt(II1) complexes is not unknown. Shibata and Watanabe (11) have reported on the catalytic action of complex metallic compounds, including complexes of cobalt, which is similar to that of oxidation-reduction
,
330
I 315
I 380
I 401
I
430
W A V E LENGTH, M r
Figure 1. Absorption spectrum of cobalt(lll)-l,2bis(rnethy1thio)ethanecomplex
Aqueous solutions of the complex were found capable of oxidizing other reducing agents-e.g., iodide to iodine ( 2 1 - 8 Iz 2e-; E" = -0.5355 volt) and ferrous to ferric iron (Fe++ 8 Fe+++ e-; E" = -0.771 volt). It may be assumed, therefore, that the oxidation potential for the hypothetical half-reaction cobalt(I1)-dithioether 8 cobalt(II1)-dithioether e- is negative and that the cobalt(II1)-dithioether complex is a good oxidizing agent.
+
+
+
EXPERIMENTAL 520
460
400
580
WAVE LENGTH
Figure 2.
640
700
760
(MILLIMICRONS)
Visible absorption spectra a t pH 1.5
Ultraviolet and visible absorption spectrophotometry were performed on a Car. Model 10 recording spectrophotometer.
A. Cobalt(lll)-l,2-bis(methylthio)ethane; o-dianisid ine a d d e d 6. Cobalt(lll)-dibutylarnine; o-dianisidine a d d e d C. o-Dianisidine oxidized by Fenton's reagent
enzymes. I n particular, the formation of an indoaniline dye, indophenol hlue, from Nadi reagent (dimethyl-pphenylenediamine and 1-naphthol), an oxidation effected by various metallic complexes, seems closely related. The action of the cobalt(II1)-dithioether complex on the redox indicator does not appear to be catalytic, however. The theoretical basis for the oxidizing ability of certain complex compounds of cobalt(II1) has been amply reviered by Copley, Foster, and Bailar (5) and, more recently, by Moeller (8). That coordination of cobalt(I1) increases the ( w e with which i t may be oxidized to cobalt(II1) is evident from a consideration of the potentials (8) of some of the half-reactions of the cobalt(I1)cobalt(II1) couple using Latimer's ( 7 ) sign convention. ,"
I
= -1.229 volts). Hexacyano cobalt(II1) ion, on the other hand, is not a n oxidizing agent, because it is incapable of oxidizing hydrogen gas to hydrogen ion. It is believed that in this method of analysis, coordination of the ligand with cobalt(I1) increases the ease with which it undergoes oxidation to cobalt(II1). Furthermore, the cobalt(II1) complexes giving the reaction are good oxidizing agents and, therefore, oxidize the redox indicator. Some further qualitative evidence exists for the above postulated mechanism. The cobalt complex of the 1,2-bis(methy1thio)ethane could not be formed in the absence of hydrogen peroxide or other strong oxidizing agents such as a mixture of nitric oxide and nitrogen EO298
Hnlf-Reaction CO'+
E0*9*(volts)
(as.) F-=c'o++- (aq.)
+
E-
_3. From the potentials for these halfreactions, it is expected that cobaltic hydroxide and the hexammine cobalt(II1) ion are oxidizing agents having a tendency to oxidize hydrogen gas to hydrogen ion ( l / ~ HZ H+ e-; Eozg8= 0.000 volt). They are not so pom-erful as the aquo cobalt(II1) ion, however, which is capable of oxidizing 2H+ O2 2e-; water (H20
+
+
+
dioxide or chlorine. Oxygen passed through an ether-alcohol solution of the cobaltous chloride and ligand did not yield the complex. The disulfoxide of a similar dithioether, (HOCHzCHzS(O)CHz]z (IO), did not complex with cobaltous chloride under the conditions of the analysis, indicating that oxidation of the dithioether to the disulfoxide was not involved.
W A V E LENGTH, M r
Figure 3. Absorption spectrum of cobalt(ll1)-dibutylamine complex
The preparation of gram quantities of the cobalt(III)-1,2-bis(methylthio)ethane complex, its structure, and magnetic properties will be reported later. Although complex formation between the dithioether and cobaltous chloride could not be effected in aqueous solution, the amines tried (dibutylamine and 2-diethylaminoethanol) were readily complexed in aqueous solution. Two procedures are therefore described, one for the estimation of ligands dissolved in ether and one for ligands in aqueous solution. Ligands in Ether Solution. REAGEKTS. Cobaltous chloride hexahydrate (0.2 gram) is dissolved in 5 ml. of absolute ethyl alcohol, and the solution is diluted to 100 ml. with anhydrous ether, For estimation of the dithioether, 2 drops of 3% hydrogen peroxide solution are added to the reagent solution. For estimation of the amines, 2 drops of 30% hydrogen peroxide solution are added to the reagent solution. o-Dianisidine (0.04 gram) is dissolved in 100 ml. of absolute ethyl alcohol. Clark and Lubs p H 1.5 buffer solution (4) is diluted 1 to 3 with water. ESTIMATION O F 1,2-BIS(METHYLTHIO)ETHANE. A standard curve is prepared by using known concentrations of the dithioether in ethyl ether in the range from 15 to 90 y per ml. To 1 ml. of the sample solution in a 15-ml. test tube is added 1 ml. of the cobaltous chloride reagent. The test tube is heated in a water bath ( S O 0 to 80" C.) until the solvent has evaporated (about 10 minVOL. 29, NO. 1, JANUARY 1957
83
Utes). After cooling, 4 ml. of buffer solution and 0.2 ml. of the o-dianisidine reagent are added and the color is measured after 4 minutes with a KlettSummerson colorimeter using a KO.41 filter. ESTIMATION OF I)IBUTTLAJIIKE. A standard curve is prepared by using known concentrations from 0 to 400 y of dibutylamine per ml. of ethyl ether. To 1 ml. of the sample solution in a 15-ml. test tube is added 1 ml. of the cobaltous chloride reagent solution. The test tube is heated in a w t e r bath (60” to 80” C.) until the solvent has evaporated. After cooling, 0.2 ml. of the o-dianisidine reagent and 4 nil. of the buffer solution are added and the color is measured inimediately with a Klett-Summerson colorimetei using a KO.44 filter. Results. Different concentrations of hydrogen peroxide are used in the cobaltous chloride reagent for the estimation of the dithioether and the amine. The order of addition of the redox indicator also differs in the t1I-o procedures. Qualitative experiments indicated that excess peroxide x i s undesirable in the estimation of the dithioether, popsibly because of a competitive osidation of the dithioether. Ligands in Aqueous Solution. REAGENTS. Cobaltous chloride hexahydrate (0.2 gram) is dissolved in 100 ml. of water to which 2 drops of 307, hydrogen peroxide have been added. o-Dianisidine, 0.04% in ethyl alcohol. Clark and Lubs p H 2 buffer solution
seconds in a Klett-Summerson colorimeter using a No. 44 filter.
spectrophotometry in connection with this investigation.
DISCUSSION
The ligands, ethylenediamine and cyanide ion, which form very stable complexes with cobalt, could not be detected or estimated by either of the procedures described here. This would be expected for cyanide from the value for the couple
[co(cs)S]-4
[Co(CN)G]---
+ e-
which is +OB3 volt. The fact that ethylenediamine does not give the reaction may be interpreted as meaning that the oxidation potential for the couple [Co(e~i)~] * + ~t [ C ~ ( e n ) ~ ] +++
+ e-
is too positive for the oxidation of o-dianisidine. The cobalt(I1)-ethylenediamine complex is, in fact, a good reducing agent (6). Inasmuch as complex formation has an effect on oxidation potential, and oxidation potential is related to dissociation or stability constant through the Kernst equation, it is possible that information on the oxidizing effect of coordination compounds on redox indicators having different oxidation potentials may lead t o a semiquantitative approximation of dissociation constants in certain cases. The scope and limitations of this (4). method with respect to ligands and ESTIMATION OF 2-DIETHYLBMINOETHmetals have not yet been determined. AKOL. A standard curve is prepared by using known concentrations of 2-diethylACKNOWLEDGMENT aminoethanol in water. T o 1 nil. of the sample solution is added 1 nil. of cobalThe authors wish to thank William tous chloride reagent, 0.2 nil. of o-dianisiD. Ludemann, Jr., and Arturo L. dine reagent, and 2 ml. of buffer soluCardenas for performing some of the tion. The color developed is read in 30
LITERATURE CITED
Bailar, J. C., Jr., J . Cheni. Edirr. 21, 523 (1944). Bricker, C . E., Loeffler, L. J., ANAL. CHEU.27, 1419 (1955). Chiarottino, A, Industria chiniica 8, 32-3 (1933). Clark, W. M., “Determination of Hydrogen Ions,” 3rd ed., Williams & Wilkins, Baltimore, )Id., 1928. (5) Copley, M.J., Foster, L. S., Bailar, J. C., Jr., Chem. Revs. 30, 227 (1942). ,(6) Diehl, H., Butler, J. P., SNAI.. CHm. 27, 777 (1955). (7) Latimer, W.M., “Oxidation Potentials,” 2nd ed., pp. 2-5, PrenticeHall, New York, 1952. (8) hIoeller, J., “Inorganic Chemistry,” pp. 300-305, W’iley, Xew Tork, 1952. (9) SIorgan, G. J., Ledbury, IT., J . Chern. SOC.121, 2886 (1922). (10) Price, C. C., Roberts, R. &I,,J . Org. Chena. 12, 261 (1947). (11) Shibata, K., Watanabe, ,4,, Izoata Inst. Plant Biochena. Pub. 2,97-128 (1936). (12) Spacu, G., Macarovici, C. Gh., Bul. SOC.Stiinte Cluj 8, 245-56 (1935). (13) Tschugaeff, L., Ber. 41, 2222 (1908). (14) Tschugaeff, L., Compt. lend. 154, 33 (1912). (15) Tschugaeff, L., Kobljanski, h.,Z. anorg. Chena. 83, 8 (1913). (16) Tschugaeff, L., Subbotin, K., Ber. 43, 1200 (1910). (17) Waters, W., A., “The Chemistry of Free Radicals,” pp. 247-252, Osford University Press, London, I
~
1946.
RECEIVED for review June 15, 1956. Accepted September 14, 1956. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., February 27, 1956.
Isotopic Analysis of Tetramethyllead G. L. BATE‘, D. S. MILLER, and J. L. KULP lamont Geological Observatory, Columbia University, Palisades, N.
b The method of isotopic assay of lead by the analysis of tetramethyllead has been studied. The average reproducibility for duplicate runs is 0.6,0.4,0.4, and 0.2% for lead-204, -206, -207, and -208, respectively. Within these limits thereis no evidence of fractionation or contamination during chemical preparation. Two 6-inch mass spectrometers were used with slightly differing sources. The lead spectra in both the Pbf and Pb(CH&+ regions were analyzed. Within the experimental 1 Present address, Department of Physics, iJ-heaton College, Kheatori, Ill.
84
ANALYTICAL CHEMISTRY
Y
errors the analyses from both mass spectrometers were in agreement. An interlaboratory comparison shows agreement generally to within 0.5% for the heavier isotopes, and to about 1 for lead-204.
yo
T
HE USE of tetramethyl vapor for the isotopic determination of lead has been reported by Dibeler and Mohler (3) and Collins, Farquhar, and Russell ( 2 ) . The great convenience and accuracy of this method for macroscopic lead samples have led to its adoption in this laboratory for geologic
studies involving large numbers of samples. Many geological interpretations, including age determinations by the lead method, need an accurate isotopic determination in order to be meaningful. This paper reports experinients on the reproducibility of the method using different spectrometer tubes, sources, and spectra. Interlaboratory comparisons \vere also made. MASS SPECTROMETER
Two mass spectrometers were employed in the course of this work, both of the direction-focusing, &inch sector
