Nature of the Cobalt-Thiocyanate Reaction PHILIP W. WEST AND CHARLES G. DE VRIES’ Louisiana State University, Baton Rouge, La. The nature of the color produced by cobalt-thiocyanate complexesin the presence of certain organic solvents has been investigated, to provide a better understandingof this important color system. The complexes[Co(NCS)]+and [Co(NCS)e]---are formed in mixed solvents. The latter complex is probably responsible for the formation of the blue color produced in the Vogel test through its selective attraction of organic molecules. The thiocyanate addenda attract the less polar organic compounds, causing what is in effect a dehydration of the complex cobalt ion and giving rise to the familiar blue color of anhydrous cobalt compounds. This study should aid in providing an understanding of the nature of the Vogel reaction, which is of importance in the detection and determination of cobalt.
A
LTHOUGH the reaction of cobalt with thiocyanate ions in the presence of mixed solvents has long been used for the detection and determination of cobalt, the cause of the resulting change in color from a pink to a blue solution has remained unproved and a point of much speculation. This test, named the Vogel reaction after its originator (81 ), consists of adding thiocyanate ions to the solution to be analyzed and then adding a suitable organic solvent. The formation of a blue color indicates the presence of cobalt ions. Because of the widespread use of this reaction it is desirable t o gain more knowledge of its exact nature, thus enabling a logical approach for possible improvement of its application. Many modifications of Vogel’s original method of testing for cobalt have been made, including the choice of solvents for color developer (6-7, 11,20)and of ions for complexing addenda ( 9 , 1 7 ) . The use of this reaction as applied to quantitative measurements has not been overlooked. Not only has cobalt been determined colorimetrically, but small percentages of water in ethyl alcohol have been determined accurately employing this color system (3). According to Mellor (18), the reaction between a concentrated solution of ammonium thiocyanate and a solution of a cobalt(I1) salt first forms cobalt(I1) thiocyanate. This salt is converted to ammonium cobalt( 11)thiocyanate in the presence of excess thiocyanate ions. Although a number of theories have been postulated to account for the blue color, no definite explanation is available. Hill and Howell (IS)proposed that the color change in cobaltous solutions is due to the dehydration of a cobaltous-hexaquo complex ion. I n strongly acidic medium (concentrated mineral acids) they suggest that the color of the solution is transformed to blue conforming to the reaction,
This dehydration theory was not accepted by Bassett and Croucher (4). They claim that the reasoning of Hill and Howell, which was based on the comparison of magnesium and cobalt osides, is unjustified and that cobalt need not have a coordination number of 6. Rossi (19) bubbled dry hydrogen chloride through a 0.1 ill aqueous cobaltous chloride hexahydrate solution. The solution color changed from a pink to a blue. He suggested that the color is caused by a compound formation of the type,
In the present investigations a Beckman Model DU spectrophotometer was used for all absorptiometric work. The absorption cells were of Corex with a 1.00-cm. path length. Analytical reagent grade salts of cobalt nitrate and potassium thiocyanate were used in the preparation of standard solutions. The cobalt solutions were standardized by the method of electrolytic deposition, and the strengths of the thiocyanate solutions were found by the Volhard titration using standard solutions of silver nitrate. Absolute ethyl alcohol was used. Potassium selenocyanate was prepared following the method outlined in “Inorganic Syntheses” (12). EXPERIMENT4 L
To determine the complexes present in aqueous solutions and in mixed solvents, both Job’s method of continuous variation ( 2 2 ) and a method employing spectrophotometric titrations (25, 84) were applied. Job’s method of determining ionic species by noting the difference in optical density of a system while varying the mole fraction of components is well known. The coordination number of the central atom is found by applying his calculation formula,
2 1--2
where S = coordination number, and z = mole fraction of complexing ion present a t maximum difference in optical density. The second method, that of spectrophotometric titration, also gives indication as t o the nature of complexes formed in solution. -4 number of solutions are prepared containing the 8ame concentration of the central atom in the coordination sphere. Using these, a series of solutions is made by adding the complexer in amounts ranging from 0 to 15 times the concentration of the central atom present. Optical density measurements are then made a t a distinguishing wave length and the coordination combinations are determined from breaks in the slope of the curve of opti-
HCoCla .nH,O
Co[CoClr] Blue
APPARATUS AND REAGENTS
Ay
One possible explanation for the change of color upon dehydration may be shown by the equation (8) 2[Co(Hz0)6]Ch Pink
Feigl (10) has suggested (very significantly) that the blue color is probably due to solvation of complex cobalt-thiocyanates such as Kz[Co(NCS)r], inasmuch as upon dilution the color returns to pink. Young and Hall (25)pointed out that the capacity of a solvent for preventing decomposition of a complex varies inversely as its dielectric constant and that the complex formed with cobalt and ammonium thiocyanate is extracted from aqueous solutions by organic solvents. Absorptancy curves have been run on solutions of cobalt thiocyanate in nonaqueous solvents. I n a nonaqueous solvent, L, the complex C O ( ~ T S ) ~is Lformed ~ (16). The complex [Co( N C S ) ] + is present in aqueous solutions containing an excess of cobalt, while an excess of thiocyanate produces [Co(NCS)a]-according to Kiss and Csokan (16).
+ 12HzO
1 Present address, Department of Chemistry, University of Oklahoma, Norman, Okla.
334
335 2.
2
I-
0.2-
W
n 510 M r >
J
4
2 I-
n 0
--*-_ .-
*--
/
0.1*--/
/ ' ,, 0
I
I
I
I
I
I
RATIO
Figure 1.
I
OF
I
1
I
I
c w NCS'
Spectrophotometric Titration
Water solution, 0.00885 4 2 Co(I1)
0,91 0.8
0 .. 77-1
0 ..66- i 0.5-
0.4V
z
W
0
0.3-
W
L
4 0.2a
0.II-
o*2* 0 -0
I
1
I
I
0
Figure 2.
0 I. 5 1 0.5 FRACTION
I
I
I
I1.o I .o
MOLE NCS' Complexation Shown Using Job's Method 0.200 M water solutions of Co(I1). NCS1. A t 520 mp 2. A t 500mp
3. At 480 mp 4. At 460mp 5 . At 560mp Slit width a t 0.12 mm.
cal density versus ratio of compleser concentration to concentration of central atom. RESULTS
The complexes of cobalt-thiocyanate formed in aqueous media were found to consist of two species-[Co(SCS)]+ and [Co( NCS)s]-----with the number of possible water molecules in the coordination spheres undetermined. Spectrophotometric titration results, shown in Figure 1, dictate this conclusion. Although the breaks in the curve in Figure 1 are not great, repeated experiments have given the same results. Each gave definite breaks a t ratios of 1 to 1 and 1 to 6. (Precision is indicated by the size of the points.) Further proof was obtained by continuous variation studies: A complex of the type [Co(NCS)] is clearly indicated, as shown by Figure 2. hforeover, the curve is tilted slightly to the right, thereby indicating the evistence of another complev, one richer in thiocyanate.
I
The curve variation procedure of Job was studied a t wave lengths of both 520 and 620 mp. At the concentrations i m posed by the conditions of the esperinient i t was impossible to perform a spectrometric titration a t 620 mp in I I I uqueous media. Spectral absorptancy c u r v e s of o p t i c a l density versus wave lengths for cobaltt h io c y a n a t e -water solutions were found t o have only one peak, at 510 m p , with no sign of absorption in the blue region of the spectrum. Results of the investigations of cobalt and thiocyanate ions in mixed media (water-ethyl alcohol) are shown in Figures 3 and 4. .1spectrophotometric titration and continuous variation studies, using a wave length of 510 mp, gave the same results as those obtained with aqueous solutions. However, when these same observations mere made using a wave length of 620 mp, a new species was indicated, for both methods gave irrefutable evidence of the formation of a new type of [Co(XCS)6]---- ion. That the organic solvent is not involved stoichiometrically was proved by the application of both methods, keeping the ratio of cobalt-thiocyanate constant and varying the amount of solvent. Ethyl alcohol a t more than forty times t,he molar concentration of the cobult was required before t,he blue color was produced. Because the organic solvent neither is present in the coordination sphere nor enters into the reaction stoichiometrically, investigations were made as t o the effect of change in dielectric constant on optical density in the blue region of the spectrum. A study of a series of alcohols used as developers for the Vogel reaction showed that the blue color increased as the dielectric constant of the developer decreased. Sufficient data could not be found in the literature to correlate the change in optical density with other physical constants of the developers.
All solutions used in the dielectric constant studies had the same concentration of cobalt and thiocyanate ions present and the molarity of the alcohol was kept constant. To 10-ml. volumetric flasks I ml. of 0.02 X cobalt nitrate and 5 ml. of 1 M potassium thiocyanate were added. Enough developer was added to yield a molarity of,4.28 M and the solutions were made to volume with distilled water. Optical density measurements were taken a t 620 mp. The results obtained are shown in Table I. Absorptancy curves of the cobalt-selenocyanate-water system were made and a peak was found to be present a t 510 mp. I n miled media a spectral absorptancy peak a t 620 m+ was observed. Table I. Solution
Developer
+
Isopropyl alcohol sec-Butvl alcohol terl-BuCyl alcohol
Optical Density Dielectric Constant
Optical Density
;1 20.8 (ii
0.015
1 3 . 7 (14) 1.5 5 ( 1 4 ) 3 76 ( I )
n " . n" "m"
0.303 0.783
o.9m 1 16
ANALYTICAL CHEMISTRY
336 0.5-
44
-
>.
-I-
n Z w
n
0.3-
J
U
-IV
0.2-
n 0
0
5 RATIO
Figure 3.
OF
Wn) NCS'
IO
15
Spectrophotometric Titration
Water-alcohol solutions, 0.00885 M Co(I1) DISCUSSIOh constant betveen isopropyl alcohol and see-butyl alcohol is 0.2, It has been shown that the complexes, [Co(SCS)]+ and the difference in optical density is proportionally much higher in [co(lu'Cs)6]----, exist iii both aqueous andmixedsolvent systems. comparison with the other members of the series. Also, when In mixed solvents a blue color forms due, it is believed, to the considering acetone [e = 19.5 ( 1 ) ; optical density = 1.101 and selective attraction of nonaqueous molecules by the thiocyanate dioxane [e = 2.1 ( 2 ) ; optical density = 0.8851 not in the series, of the [ce(scs)6]---complex. This preferential attraction of thew seems to be no relation between their nhility as developers the nonaqueous molecules causes, in effect, a dehydration of the coand their dielectric constants. balt complex and gives rise to the formation of the familiar hlue tert-Butyl alcohol is as effective as acrltoiic ab a developer of thc color of anhydrou cobalt salts. blue color Because the vapor prcssure of fert-butyl alcohol is so A considerable amount of evidence, other than that presented much lower than that of acetone, it seems de7irabk to investigate above, can be cited in support of the theory. Local investigait for use in quantitative tests. tions have shown that a majority of metal thiocyanates can be Dielectric constant docs play :i role in the formation of the extracted from aqueous solutions by means of nonpolar organic colored complex, but these data suggest that perhaps other facsolvents. Such extractions have been utilized in a number of tors, including molecular size, also influence a solvent's ability to ways in the isolation of thiocyanate complexes of such metals as produce the Vogel blue. iron, uranium, and molybdenum. Their importance to the present That this blue color may br associated with the configurations discussion lies in the emphasis they add to the solubility phrnomof thr elrctrons in the central atom and not directly viith the pna attributable to the thiocyanate group effect, Because the color-developing solvrnts used in the Vogel reaction are miscible with water, no phase separation occurs in this case. The attraction between the thiocyanates arid the organic solvent is still in effect, however, and the coordinated thiocyanate consequently attracts the organic d molecules to provide an anhydrous atmosphere 0 0. :%roundthe complex. The observation of Young and Hall ( 2 5 ) that the blue complex is soluble in ether lends support t o the dehydration theory. Studies of dielectric effects also support this view. Q 0.1 The evidence submitted here indicates that the 0 dielectric constant of the developing agent of a O f I I I I I I I I I I chemical series is related to its strength as a color0 0.5 1.0 producing agent in the Vogel test. In the alcohol MOLE F R A C T I O N NCS' series which was studied this strength is roughly inFigure 4. Complexation Shown Using Job's Method versely proportional to its dielectric constant. Water-alcohol solutions The observations indicated that other physical Slit width at 0.04 mm. Co(I1) NCSfactors of a solvent also affect its value as a de1. At620mp veloper. Although the difference in dielectric 2. At600mp ~
o.21
V O L U M E 23, NO. 2, F E B R U A R Y 1 9 5 1 addenda in the coordination sphere is indicated by the absorption peak at 620 mp in the spectral absorptancy curves for both the thiocyanate and selenocyanate mixed solvent solutions. Other addenda such as thiosulfate and cyanate also give bluc colors with cobaltous ions in mixed solvents. Finally, a wide variety of mixed solvents can be used to develop the blue color, tvnding to give credence t o the dehydration theory.
337 Engelder, Dunkelberger, and Shiller, ”Seniimicro Qualitative Analysis,” p. 159, Kew York, John \Tiley & Sons, 1936. Feigl, F., “Qualitative ilnalysis by Spot Tests,” 3rd ed., p. 112, New York, Elsevier Publishing Co., 1946. Feisl. F.., and Stern. H.. 2.aiinl. Chon.., 60.. 1-43 11921). . , Fernelius, IT. C., “Inorganic Syntheses,” Vol. 11, pp. 186-8, S e w York, h‘fcGraw-Hill Book Co., 1946. Hill, H., and Howell, 0. R., Phil. Mug.. 48, 833 (1924). International Critical Tahles, Sew Ilork, McGraw-Hill Book I ,
.
I
co.
Kiss, .i., and Csokan, P., 2. p h y s i k .
ACKNOWLEDGMENT
( y / ~ c v A186, ,, 2 3 9 4 7 (1940).
The authors acknowledge the aid of the Officcl of Suva1 Rrsrarch, under whow program this research &-asc o n d u c t d
Ibid., A188, 27-40 (1941). Korenman, I. hi., and Ashhel, E. AI.. Z a i u d s k o y a Lab., 10, 493 (1941 ). Mellor, J. W., “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” 1-01. S I V , p. 516. Kew York. Long-
LITERATURE CITED
mans, Green and Co., 1935. Rossi, Luis, Rea. asoc. bioquim. coyentinu. 9 , S o . 33, 11-13
Akerloff, Gasta, J . A m . Chem. Soc., 54, 4123 (1932). Akerloff, Gasta, and Short, 0. A , , Ibid., 58, 1241 (1936). Ayres, G. H., and Glanville, R. V., - 4 s . t ~ CHEni., . 21, 930-34 (1949).
Bassett, H., and Croucher, H. H., J . Cheiri. SOC., 1930, 1784-819. Charlot, G., and Benzier, D., A m . chin!.anal., 25, 90-4 (1943). Danzier, J. L., J . d i n . Chem. Soc., 24, 578-80 (1902). Dwyer, F. P., Austmlinii Chem. Inst. J . a n d Proc., 3, 2 3 9 4 4 (1936).
Emeleus and Anderson, “hIodern Aspects of Inorganic Chemistry,” p. 144, Sew York. D. Van Sostrand Co., 1938.
(1943).
Ryazanov, I. P., A b h n d . S t u u t u t i i ~ , Sarrifor Cheni.. 1. 113 (1936).
Vdgel, H. JV., Ber., 8, 1533 (1875). Vosburgh, IT. C., and Cooper, G. H.,J . A n i . (‘hem
Soc.. 63,
437-42 (1941).
Test. P. 17..and Amis. E. S., IYD. ENG.CHEM.,.IN En., 18,
