ANALYST, JANUARY 1984, VOL. 109 43 Spectrophotometric and Analogue Derivative Spectrophotometric Determination of Cobalt with 2,2’-Dipyridyl-2-benzothiazolyl hydrazone Raj Bhushan Singh,” Tsugikatsu Odashima and Hajime lshiit Chemical Research Institute of Non-Aqueous Solutions, Tohoku University, Sendai, 980, Japan 2,2’-Dipyridyl-2-benzothiazolylhydrazone (DPBH) reacts with cobalt(l1) t o form a 1 : 3 (metal : ligand) complex having an absorption maximum at 530 nm in weakly acidic to alkaline media. The complex, once formed; remains stable even when its solution is acidified by addition of hydrochloric acid. The complex is extractable into chloroform, benzene and 4-methylpentan-2-one. The chloroform extract has an absorption maximum a t 504nm and gives a constant absorbance in the pH range 2.0-9.3.Apparent molar absorptivities and sensitivities for an absorbance of 0.001 in the proposed procedures without and with extraction are 2.50 x lo4 I mol-1 cm-l, 2.35 ng cm-2 and 3.43 x lo4 I mol-1 cm-l,l.72 ng cm-*, respectively. Both proposed methods are selective for cobalt and especially in the method without extraction none of the other ions usually encountered interfere in the cobalt determination. Application of one of the proposed methods to the determination of cobalt in standard iron and steel samples and further sensitisation of the methods by employing analogue derivative spectrophotometry are also described. Keywords : Cobalt determination; 2,2’-dip yridyl-2- benzothiazolylh ydrazone; spectrophotometry; extraction spectrop h otome tr y; analogue derivative spectrop hotom etr y The use of hydrazones in analytical chemistry is increasing because of their easy synthesis, high sensitivity and selectivity towards various metal ions.They are widely used as colori- metric, fluorimetric and gravimetric reagents and acid - base indicators. Their analytical applications have been reviewed. 1 Systematic studies have been carried out in our laboratory on the analytical use of hydrazones with special reference to benzothiazolylhydrazones.2-5 One such hydrazone, 2,2’- dipyridyl-2-benzothiazolylhydrazone (DPBH), has been reported as a spectrophotometric reagent for the determina- tion of iron in aqueous solutions containing a surfactant.6 In the work presented here the complex formation of DPBH with cobalt(I1) was studied spectrophotometrically in order to utilise for the determination of micro-amounts of cobalt.It was found that this reagent is very selective and sensitive for cobalt(I1) in aqueous acidic media. The method developed is further sensitised by extraction of the cobalt- DPBH complex and also by the introduction of an analogue derivative spectrophotometric technique. Experimental Reagents All reagents were of analytical-reagent grade and all solutions were prepared with distilled, de-ionised water, unless stated otherwise. DPBH solution, 2.5 x 1 0 - 3 M. Prepared by dissolving the required mass of DPBH in ethanol by heating on a water-bath. DPBH was synthesised as described earlier.6 Standard cobalt(ZI) solution. Prepared by dissolving 0.5 g of metallic cobalt (99.99% pure) in 10 ml of nitric acid (1 + 1) and diluting to 500ml with water. Working solutions were prepared by dilution of this solution with water. Apparatus The apparatus described earlier6 was used for measurements of the absorbance, the absorption spectrum and the derivative spectrum.* On study leave from Bareilly College, Bareilly-243005, India. 1- To whom correspondence should be addressed. Procedure Ordinary spectrophotometry in aqueous acidic medium (procedure A ) To an aliquot containing less than 59 pg of cobalt(I1) in a 25-ml calibrated flask, add suitable masking agents if necessary, 2 ml of 2.5 x 10-3 M DPBH solution and 2 ml of 1 M acetate buffer (pH 4). After allowing the mixture to stand for a few minutes in order to complete the complexation, add 6ml of hydro- chloric acid (1 + 1) and dilute to the mark with water.Measure the absorbance of the resultant solution at 530 nm against a reagent blank prepared under the same conditions using 1-cm cells. Ordinary spectrophotometry with extraction (procedure B ) To an aliquot containing less than 17.2 pg of cobalt(I1) in a 50-ml separating funnel, add suitable masking agents if necessary, 2 ml of 2.5 x 10-3 M DPBH solution and 2 ml of 1 M monochloroacetate buffer (pH 2.5) and dilute to aproximately 20ml with water. Extract the cobalt complex into 10 ml of chloroform by shaking mechanically for a few minutes. Allow the phases to separate and transfer the organic layer into a flask containing about l g of anhydrous sodium sulphate in order to dehydrate it.Measure the absorbance of the extract at 504nm against a reagent blank prepared under the same conditions using 1-cm cells. Second-derivative spectrophotometry When the cobalt content of the coloured solution or the extract prepared by the procedures described above is too low to give a measurable absorbance, re-prepare them from the beginning using 2.5 x 10-4 instead of 2.5 x 1 0 - 3 ~ DPBH solution. Record the second-derivative spectrum from 650 to 400 nm against a reagent blank by using a combination of both first- and second-order differentiation circuits of No. 6 and a scan speed of 300nm min-1 and measure the second- derivative value (the vertical distance from a peak to a trough or that from the base line to a trough of the peak). Dissolution and pre-treatment of iron and steel samples To about 0.1 g of the sample add 10 ml of aqua regia and 10 ml of 60% perchloric acid, heat to decompose the sample and44 ANALYST, JANUARY 1984, VOL.109 evaporate the mixture to dryness. After cooling to room temperature dissolve the residue in 10 ml of hydrochloric acid (1 + l), filter to remove any undissolved portion (silica) and wash with a small amount of hydrochloric acid (1 + 1). Add 10 ml of 4-methylpentan-2-one to the filtrate and washings and shake for 5min to remove iron(II1). Separate the aqueous phase, heat it to remove most of the acid and dilute to 100 ml with water. Use 2ml or another appropriate aliquot of the resultant solution for the determination. Results and Discussion Properties of the Complex and Effect oi'pH Cobalt(I1) reacts with DPBH in weakly acidic to alkaline media to form a yellow - orange complex that is insoluble in water above pH 3, but soluble below pH 3 because of protonation at the pyridine nitrogen in the complex molecule.The complex, once formed, remains stable even when the solution is acidified and gives a maximum absorption at 530 nm against a reagent blank (curve A Fig. 1). Fig. 2 shows the effect of the amount of hydrochloric acid (1 + 1) added after the complexation was complete at pH 4. The absorbance increases gradually with increasing amount of the acid owing to the protonation of the complex and free DPBH exists; however, the variation in the absorbance is very sma!l and the absorbance can be considered to be almost constant when 6 k 0.5 ml of hydrochloric acid (1 + 1) are added per 25 ml of the final volume.Hence further studies were carried out by adding 6 ml of the acid in 25 ml of final solution after the complexation was complete, in order to avoid possible interferences from other ions. 0.4 al C m e g 0.2 n Q 0 1 2 0 0) > .- $ 1 4- .- 5 2 ? c 3 % -0 (7, 400 500 600 Wavelengthinm Fi . 1. (a) Absorption spectra of DPBH and its cobalt complex and (by second-derivative spectra of cobalt - DPBH complex. Cobalt(II), 720 p.p.b.; DPBH, 1 x M; A, C, E and F, cobalt - DPBH complex against reagent blank; B, reagent blank against water; and D, reagent blank against chloroform. Continuous lines, in 1.4 N hydrochloric acid solution; and broken lines, in chloroform - I L I I I n o 3 6 9 12 15 8 0.2 ' a HCI addedhl Fig.2. Effect of amount of hydrochloric aicd (1 + 1) added after complexation was complete. Cobalt(II), 823 p.p.b.; DPBH, 2 X M; reference, reagent blank The cobalt - DPBH complex is extractable into chloroform, benzene, 4-methylpentan-2-one and 1 ,Zdichloroethane and gave the largest absorbance in chloroform; hence chloroform was used as the extraction solvent in the procedure with extraction. The complex is quantitatively extracted into chloroform in the pH range 2.0-9.3 as shown in Fig. 3 and has a maximum absorption at 504nm against a reagent blank (curve C Fig. 1). Effect of DPBH Concentration A 4-fold molar excess (in procedure A) and a 5-fold molar excess (in procedure B) of DPBH were required in order to obtain a constant absorbance.The excess of DPBH was not critical in either instance and did not interfere. Effect of Shaking Time Variation of the shaking time from 1 to 15min revealed that 1 min was sufficient for complete extraction of the complex. The extraction of cobalt was reproducible; 99.7% of that present in the aqueous phase was extracted by a single extraction, according to the results obtained by a single extraction followed by the determination of cobalt in the organic and aqueous phases. One extraction, therefore, seems to be sufficient for the determination of cobalt. Effect of Other Variables in Extraction The absorbance of the extract remained constant even when the organic to aqueous phase ratio was varied from 1 : 1 to 1 : 5 and ethanol concentrations up to 15% in the aqueous phase did not affect the absorbance of the organic phase. Stability of the Complex The cobalt-DPBH complex in aqueous acidic medium is stable and gave a constant absorbance even after 10h.The extracted complex is also stable and gave a constant absorb- ance even after 13 h. Composition of the Complex The molar composition of the complex formed under the conditions for the determination of cobalt was ascertained by Job's method of continuous variations and the molar-ratio method. Both methods indicated the formation of a 1 : 3 (metal : ligand) complex under either of the conditions em- ployed for the determination of cobalt. Calibration Graph, Sensitivity and Precision Straight-line calibration graphs passing through the origin were obtained using the recommended procedures.The equations of the lines obtained by a least-squares treatment were Co(p.p.m.) = 2.3514 . . . . . . (1) Co (p.p.m.) = 1.72A . . , . . . (2) where A is the absorbance and equations (1) and (2) L g 0.2 I 2 I I I I I 0 2 4 6 8 10 PH Fig. 3. Effect of pH on the extraction of cobalt - DPBH complex. Cobalt(II), 720 p.p.b.; DPBH, 2.5 x l o - 4 ~ ; reference, reagent blankANALYST, JANUARY 1984, VOL. 109 45 correspond to the calibration graphs in procedures A and B, respectively. The optimum ranges for the determination of cobalt, the sensitivities for an absorbance of 0.001 and the molar absorptivities calculated from equations (1) and (2) were 2.7-58.9 pg, 2.35 ng cm-2 and 2.50 X 1041 mol-1 cm-1 in procedure A and 0.8-17.2 pg, 1.72 ng cm-2 and 3.43 x 104 1 mol-l cm-1 in procedure B, respectively.Two series of ten standard solutions each containing 20.6 and 7.2pg of cobalt were analysed by the recommended procedures. The results gave relative standard deviations of 0.71 and 0.46% in procedures A and B, respectively. Effect of Diverse Ions The effects of foreign ions on the cobalt determination by procedures A and B are summarised in Tables 1 and 2, respectively, from which it can be seen that as a rule both procedures are selective for cobalt, except that nickel(I1) interferes seriously in procedure B because of complex formation with DPBH, thiosulphate interferes in procedure A because of precipitation and the tolerance limit for palladium- (11) is low in both procedures because the palladium complex is equally stable in acidic medium.In addition, in procedure A Table 1. Tolerance limits for various ions in the determination of 20.6 pgof cobalt by procedure A. Tolerable error: +3% Ion Tolerance limit C1-, Br-, NO3-, C104-, S042-, P043-, tartrate, I- . . . . . . . . . . . . . . . . . . 130mg Thiocyanate . . . . . . . . . . . . . . 100 mg F-, oxalate . . . . . . . . . . . . . . 40 mg Thiourea . . . . . . . . . . . . . . 30 mg Thiosulphate . . . . . . . . . . . . . . Precipitated Mn(I1) . . . . . . . . . . . . . . . . 3 000 pg* Ca(II), Mg(I1) . . . . . . . . . . . . . . 2 000 pg* Al(III), Cd(II), Fe(III)t, Pb(II), Zn(I1) . . . . 2000 pg Cu(II)$,$ . . . . . . . . . . . . . . 1 500 pg Ag(I), Hg(II)§, V(IV, V)q . . . . . . . . 1000 pg citrate, thioglycollate, ascorbate . .. . . . 200 mg* . . . . . . . . . . . . . . Cr(II1, VI) 500 Pg Ni(II)§ 100 Pg Pd(I1) 2 Pg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . * Maximum tested. t 2 ml of 1 M sodium fluoride solution were added. $ 2 ml of 1% thiourea solution were added. § 4 ml of DPBH solution were added. n2 ml of 0.1 M sodium oxalate solution were added. Table 2. Tolerance limits for other ions in the determination of 7.2 pg of cobalt by procedure B. Tolerable error: k3Y0 Ion Tolerance limit C1-, Br-, I-, NO3-, C104-, S042-, PO4-, Tartrate, citrate . . . . . . . . . . . . 100 rng Thiocyanate . . . . . . . . . . . . . . 60 mg Oxalate . . . . . . . . . . . . . . . . 30mg F-, thiourea . . . . . . . . . . . . . . 20 mg Ca(II), Mg(I1) . . . . .. . . . . . . . . Al(III), Cd(II), Mn(II), Pb(II), Zn(1I) . . . . 1000 pg thioglycollate, thiosulphate, ascorbate . . . . 200 mg* 2 000 pg* Cr(II1,VI) 200 Ag(I), Fe(II1)t 100 I.I-8 50 Yg V(IV,V)$ . . . . . . . . . . . . . . Cu(II)§ 30 Pg Hg(I1) 15 Pg Pd(I1) 2 Pg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ni(I1) . . . . . . . . . . . . . . . . <1pg * Maximum tested. t 1 ml of 1 M sodium fluoride solution was added. $ 1 ml of 0.1 M sodium oxalate solution was added. § 1 ml of 1% thiourea solution was added. with acidification the amounts of foreign ions tolerated are larger than those in procedure B with extraction. In the absence of masking agent amounts of copper(1I) and iron(II1) tolerated are 100 and 30 pg, but these limits can be increased to 1500 and 2000pg by masking them with thiourea and fluoride, respectively.Nickel(II), which interferes seriously in procedure B, can be tolerated up to 1OOpg provided that sufficient DPBH is present in the solution to react completely with cobalt(I1) before the addition of hydrochloric acid (1 + 1). Application to Actual Samples In order to confirm the usefulness of the proposed methods, procedure A was successfully applied to the determination of cobalt in standard iron and steel samples. The results are summarised in Table 3. Sensitisation by Employing Analogue Derivative Spectrophoto- metry Ishii and co-workers have previously reported that derivative spectrophotometry using an analogue differentiation circuit is extremely effective for the sensitisation of ordinary spectro- photometry.7.8 As an example of sensitisation, the second- derivative spectrophotometric determination of cobalt is described here.Selection of conditions for measurement of the second- derivative value As both the time constant of the analogue differentiation circuit and the scan speed of the spectrophotometer affect the second-derivative value (the vertical distance from a peak to a trough or that from the base line to a trough of the spectrum) in second-derivative spectrophotometry, these need to be selected to give a well resolved large peak (to give good selectivity and higher sensitivity in the determination). This is done on the basis of the breadth of the band in the ordinary absorption.spectrum. Each differentiation circuit in our apparatus has six additional circuits with different time constants, which can be varied easily by turning a kn0b.7.~ They are represented by circuit numbers from 1 to 6 and an increase in the circuit number means an increase in the time constant. In Fig. 4 the second-derivative spectra of the cobalt - DPBH complex solution measured with varying cir- cuit number or scan speed are shown; circuit No. 6 and a scan speed of 300 nm min-1 are seen to be preferred for the cobalt determination. The second-derivative spectra of the complex solution measured under the recommended conditions have already been shown in Fig. 1 in comparison with the corresponding absorption (zero-derivative) spectra.Table 3. Determination of cobalt in standard iron and steel samples by procedure A Cobalt found, YO Sample Proposed method Certified value Iron and steel, JSS-611-5 . 0.364 0.36 0.362 0.366 0. 368 Average: 0.3& Iron and steel, JSS-606-5 . 0.47, 0.468 0.468 0.471 Average: 0.47() 0.4646 ANALYST, JANUARY 1984, VOL. 109 0.2 0 0) .- +4 .- 9 0.2 8 L Q) 5 * 0.4 0.6 0.8 4 240 300 600 U 1 500 600 400 500 600 Wavelengthhm Fig. 4. Influence of ( a ) scan s eed (with circuits all No. 6) and (b) circuit number (with scan spee8300 nm min- 1) on second-derivative s ectra of cobalt - DPBH complex in 1.4 N hydrochloric acid solution. &balt(II), 115 p.p.b.; DPBH, 1.5 x 1 0 - - 5 ~ ; reference, reagent blank; slit width, 1 nm. Numerical values indicate scan speeds in ( a ) and first- and second-differentiation circuits in ( b ) Calibration Graph The calibration graph, prepared by plotting the second- derivative value of coloured solutions prepared in aqueous acidic medium versus the cobalt concentration, was a straight line passing through the origin when either the peak-to-trough values or the base line-to-trough values were plotted. The equation was Co (p.p.b.) = 1370 .. . . . . (3) for the peak-to-trough measurements and Co (p.p.b.) = 159D . . . . . . (4) for the base line-to-trough measurements, where D is the second-derivative value represented by the conversion of the value into absorbance. Fig. 5 shows a calibration graph obtained under the recommended conditions, from which it can be seen that cobalt down to the 7 p.p.b.level can be easily determined by the proposed method. A similar calibration graph was obtained in the extraction method. Comparison with Other Methods Several hydrazones have been proposed for the determination of cobalt, e.g., pyridine-2-aldehyde-2-quinolylhydrazone,~ 2,2’-dipyridyl-2-pyrimidylhydrazone,1() 2-benzoylpyridine-2- pyridyl hydrazone, 1 I benzil mono( 2-pyridylhydrazone)I2 and 0.E 0.6 - m 0) .- 4- > f 0.4 .- L U 8 v) 0.2 0 Fig. 5. 20 40 60 80 100 Co, p.p.b. An examde of the calibration graDh for second-derivative spectrophotometri. A, peak-to-trough Galies and B, base line-to- trough values. Circuits, all No. 6; scan speed, 300 nm min-I; slit width, 1 nm; recorder sensitivity, x 1 ; reference, reagent blank; DPBH, 1.5 x 10-5 M ; hydrochloric acid concentration, 1.4 N ; cobalt(I1) concentra- tions (a) 7.2, (b) 14.4, (c) 28.8, (d) 57.6and (e) 115 p.p.b. most recently biacetylmonoxime-2-pyridylhydrazone. l 3 In comparison with methods using these hydrazones the pro- posed methods seem to have similar sensitivity but higher selectivity. In addition, extremely high sensitivity is obtained in the proposed methods by introducing analogue derivative spectrop ho tome try. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. References Singh, R. B., Jain, P., and Singh, R. P . , Talanta, 1982,29,77. Odashima, T., and Ishii, H., Anal. Chim. Acta, 1975,74,61. Odashima, T., Anzai, F., and Ishii, H., Anal. Chim. Acta, 1976, 86,231. Odashima, T., Satoh, S., and Ishii, H., Nippon Kagaku Kaishi, 1982,1322. Ishii, H., Odashima, T., and Imamura, T., Analyst, 1982,107, 885. Singh, R. B., Odashima, T., and Ishii, H., Analyst, 1983,108, 1120. Ishii, H., and Koh, H., Nippon Kagaku Kaishi, 1980,203. Ishii, H., and Satoh, K., Fresenius 2. Anal. Chem., 1982,312, 114. Singhal, S. P., and Ryan, D. E . , Anal. Chim. Acta, 1967,37,91. Singh, R. B., Jain, P., Garg, B. S . , and Singh, R. P . , Anal. Chim. Acta, 1979,104,191. Going, J. E., and Pflaum, R. T., Anal. Chem., 1970,42,1098. Pflaum, R. T., and Tucker, E. C., Anal. Chem., 1971,43,458. Asuero, A. G., Rosales, D., and Rodriguez, M. M., Analyst, 1982,107,1065. Paper A31245 Received August5th, 1983 Accepted August22nd, 1983