Study of Cobalt(ll1) Complexes by Atomic Absorption Spectrometry Effect of Ligands and Distribution of Atomic Cobalt in the Flame Kitao Fujiwara, Hiroki Haraguchi, and Keiichiro Fuwa Department of Agricultural Chemistry, Unicersity of Tokyo, Tokyo, Japan 113
CHEMICAL INTERFERENCES in atomic absorption spectrometry have been studied by numerous workers. These studies have included the effects of ligands of a complex which are added to the solution of the metal ions to be analyzed (1-8). Hartlage observed that concomitant amines in a solution of cobalt (11) ion depressed the signal of cobalt atomic absorption (9). Very little attention has been given, however, to the various species of complexes in terms of the atomic absorption intensity changes of their coordinated metals (10-13). Sastri el al. (11,12) compared metallocene complexes with acetylacetonate complexes of highly oxidizable metals such as Ti, Zr, Hf, Ta, and Nb. The sensitivities of the atomic absorptions for the former complexes are several times higher than those of the latter. Some of the fluoro and ammine complexes of the same highly oxidizable metals show better analytical sensitivities than simple salts of those metals (2, 4. 798). These studies seem to suggest that the kinds of atoms coordinating to the central metals may have a role in determining the effects, as well as the conditions, of the flame, such as the acetylene flow rate and the flame height, and the distribution of the atoms in the flame (14-17). The present authors have, therefore, carried out a more detailed investigation of interference effects of ligands on cobalt, after preparing representative cobalt(II1) complexes. EXPERIMENTAL
A Hifachi 207 atomic absorption spectrophotometer was used with the attachment of the water-cooled burner for the nitrous oxide-acetylene flame and the adjusting screws for the vertical and horizontal movements of the burner. The cobalt hollow cathode discharge tube was also obtained from Hitachi Co. Ltd. Reagents. An accurately weighed sample of cobalt metal (99.999%, Mitsuwa Chemical Ind. Co.) was dissolved in a Apparatus.
(1) C. T.J. Alkernade, ANAL.C H E M . , 1252(1966). ~~, (2) A. M. Bond and T. A. O'Donnel, ibid., 40,560(1968). (3) M. Yanagisawa, H . Kihara, M. Suzuki, and T. Takeuchi, Talnnta, 17,888(1970). (4) A. M.Bond, ANAL.CHEM.,42,932(1970). (5) K. Govindaraju, Appl. Spectrosc., 24,81(1970). (6) B. Fleet, K.V. Liverty, and T. S. West, Talanta, 17,203 (1970). (7) A. M. Bond and J. B. Willis, ANAL.CHEM..40.2087 (1968). (8)A. M. Bond and J. B. Willis, Spectrocliim.' Acta,~22B; 1325, 2128(1966). (9) F.R. Hartlage, Jr., Anal. Chim. Acra. 39.273 (1967). (10) C. L. Chakrabarti and S. P. Singhal, Spectrocliim. Acta, 24B, 663 (1 969). (11) V. S. Sastri, C. L. Chakrabarti, and D. E. Willis, Talanta, 16, 1093 (1969). (12) V.S.Sa& C. L. Chakrabarti, and D. E. Willis, Can. J. Chem., 47,583 (1969). (13) M. Freeland and R. M. Hoskinson, Andyst (London), 95, 578 (1970). (14) R. E. Popharn and W. G. Schrenk, Spectrochim. Acta, 24B, 223 (1969). (15) J . Y. Marks and G . G. Welcher, ANAL.CHEM.,42,1033(1970). (16) V.A. Fassel, J. 0.Rasrnuson, and R. N. Kniseley, Spectrocliim. Acta, 25B,559 (1970). (17) C.S.Rann and A. N. Harnbly, ANAL.CHEM.,37,879 (1965). .
I
u 0.21 0
Z
a m a
g 0.12 m U
i0
2.5 3.0 3.5 4.0 4.5 ACETYLENE FLOW RATE, Ilmin.
Figure 1. Atomic absorption of cobalt -0-:
-E-. -X-: -0-: -A-:
.
co2+ [Co(en)slCl3 K~[CO(CZO~)~] '2Hz0 Ka[Co(CN)e]
(NH~)[CO(NH~),(SO~)Z].~HZO
Measured at a point 25 m m above the burner, 20 pg Co/ml, and air flow rate 13 I./min
slightly excess portion of concentrated hydrochloric acid and diluted into a solution of 1000 p g of Co(I1) per ml with deionized water. A dilution series of 5 to 30 pg of Co(I1) per mi was prepared from the stock solution. The following cobalt(II1) complexes were prepared from reagent grade chemicals according to the ordinary methods, and their purities were examined by elementary analyses for the ligands and by iodometric and EDTA titrations for cobalt; for all complexes the purities were 95 or better: [Co(NH&IC13, [C0(en)~]C1~,[Co(trien)ClZ]C1, [Co(tetren)Cl](ZnClJ, [Co(dip)d(C104)3, KdCo(CN)61, NadCo(NOd61, K E o (CA3&]. 2Hz0, (NH4)[Co(NHo)c(SOs)zI.3 H 0 , ("a)[(Co(en)(NH3)2(S03)2]. 3Hz0, K[Co(edta)]. 2Hz0, [ C O ( N H ~ ) ~ C O ~ I C ~ , [Co(NH3)5CN]Clz,K,[Co(CN)sCl]. Aqueous solutions of these complexes were prepared in the same range of concentraticns as the Co(I1) solutions. Procedure. The 2432 A analytical line of cobalt was used for all measurements for the air-acetylene flame. The dependence of atomic absorption on the acetylene flow rate in the air-acetylene and nitrous oxide-acetylene flames was measured by varying the acetylene flow rate with constant flow rates of air (13 l./min) and nitrous oxide (6 l./min). The distributions of cobalt atoms in the flame were obtained by measuring absorbances at thirty different positions in the flame, successively moving the burner with the adjusting screw. The cobalt(I1) solutions were used as the reference in all experiments. RESULTS AND DISCUSSION
The dependence of absorbances on the acetylene flow rates was first investigated for the cobalt complexes. The cobalt concentrations were 20 pg per ml and the height of the flame was 25 mm above the surface of the burner head. Selected results are shown in Figure 1 and Acetylene Flow Rate.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
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2.0
2.5 3.0 3.5 4.0 4.5 ACETYLENE FLOW RATE, Ilmin.
Figure 2. Atomic absorption of cobalt. Interfering effect of CN as a ligand of cobaltQII) complex co2+ [Co(NH&]C13 --t: [Co(NH&CN]CIz --IxI-: Ka[Co(CN),CI] -0-: K~[CO(CN)B] -0-:
-V-:
Measured at a point 25 mm above the burner, 20 pg Co/ml, and air flow rate 13 I./min
I
3
0
18
28
38
HEIGHT OVER THE BURNER, mm Figure 3. Dependence of absorbance CoZf [Co(NJ33),CN]CIz -x-: [CO(NH,),C08]CI Concentrations of complexes 30 p g Co/ml, acetylene flow rate 4.0 I./min -0-:
-o-.
.
Figure 2. In Figure 1, absorbances of cobalt(II1) complexes decrease in the order of Co I+, [Co(en)313+, [Co(C~04)3] [Co(CN),13-, and [CO(NH~)~(SO&]over 3.5 l./min of acetylene flow rate. There is a small change at a flow rate less than 3.0 l./min. Figure 2 shows the similar dependence of absorption of the complexes with mixed ligands of NH3 and C N on acetylene flow rate. The absorbances of these complexes decrease in an acetylene-rich flame as the number of CN ions increases. The depressing effects of all ligands investigated in this experiment are summarized in the following order: Co2+< [CO(”Q)B] 3+, [Co(en)31a+,[Co(trien)Cl,]+, 1896
[Co(tetren)C1I2+, and [ C ~ ( d i p )*+ ~ ] < [ C O ( N H ~ ) ~ C N ]
