Abstract
A violet coloured complex was developed when cobalt metal reacts with ninhydrin at pH 8.2, using sodium acetate buffer solution. Absorbance of the complex was measured at 395 nm. Various factors, such as volume of the ligand used, solution pH, stability of the complex with time and interference of other metals, which effect the complex formation have been studied in detail. Present developed method can be used for the spectrophotometric estimation of cobalt with ninhydrin complex. The method is simple, selective and cheap for the determination of cobalt in very less time.
Keywords: Spectrophotometric estimation, Cobalt, Ninhydrin
1. 1.Introduction
Coordinate complex is formed when ligand surrounds the metal atom through coordinate covalent or ionic bonds. Transition metals behave as lewis acids due to a partially filled d-orbital and therefore are capable of accepting electron pair/s. On the other hand ligands are capable of donating electron pair/s and referred to as lewis bases. Stability of a particular metal complex depends upon the nature of the metal and ligand being used (Kettle, 1979). Ligands having high pH usually form more stable complexes. A higher charge on smaller sized metals and smaller ligands with larger electron density also imparts stability to the metal complexes (Holme and Peck, 1998). These molecular interactions between the electron donors and acceptors are mostly associated with the formation of coloured charge-transfer complexes, which absorb in the visible region. The classical photometric methods based on molecular interactions are simple and suitable due to rapid formation of these coloured complexes (Foster, 1969).
In the recent literature, different spectrophotometric methods (Iqbal et al., 2006, 2007) have been developed with the aid of different complexing reagents (Zhao et al., 1999; Khedr et al., 2005; Bingol and Atalay, 2006). Both selective (Paleologos et al., 2002; Guzar and Jin, 2008) and simultaneous (Öztürk et al., 2000; Sözgen and Tütem, 2004; Niazi and Yazdanipour, 2008; Arain et al., 2009) spectrophotometric estimation of cobalt was carried out using different complexing ligands. To the best of the authors knowledge, the use of ninhydrin (2,2-dihydroxy-1,3-indanedione) as a complexing reagent (Singh, 1965) for the spectrophotometric estimation of cobalt has not yet been practised. In the present work, the possibility of ninhydrin (Fig. 1) to act as a ligand for the complexation with cobalt was studied. This work has provided some interesting and useful data for investigating the ability of ninhydrin (bidentate ligand) used to act as an analytical reagent. Here, ninhydrin was used as a chromogenic reagent for the spectrophotometric determination of cobalt.
Figure 1.
Ninhydrin (A colourless compound).
2. Materials and methods
2.1. Apparatus and reagents
Spectrophotometric measurements were performed on a Shimadzu (Japan), UV-1700, double beam spectrophotometer using matched 10 mm quartz cells. A ‘Horiba F.8’ (Spain), pH metre, calibrated with standard buffer solutions of pH 4 and 10, was used for pH measurements.
All the reagents such as ninhydrin (Fluka), cobalt chloride hexahydrate (Merck), ethanol (Merck), sodium hydroxide (Merck), sodium acetate (BDH), citric acid (BDH), boric acid (BDH), diethyl butyric acid (BDH), and potassium dihydrogen phosphate (Fluka) were of analytical reagent grade purity. Solutions were prepared with milliQ grade water of R = 18.2 Ω unless otherwise specified. Nitrogen was bubbled into each solution to release the interfering oxygen. Glass vessels were cleaned by soaking in acidified solutions of KMnO4 or K2Cr2O7, followed by washing with concentrated HNO3, and were rinsed several times with high purity de-ionized water.
Cobalt solution: 5 ppm working cobalt solution was prepared by diluting 0.5 ml of freshly prepared 1000 ppm stock cobalt solution with milliQ grade water to 100 ml in a volumetric flask.
Cobalt–ninhydrin complex: 10 ml of 5 ppm cobalt solution, containing less then 0.1 g of cobalt, was added into a 50 ml measuring flask followed by the addition of 10 ml of the saturated sodium acetate solution (10% in water). On 10 ml addition of ninhydrin solution (1% in dry ethanol) gives violet coloured cobalt–ninhydrin complex and the final volume was made up to 50 ml with milliQ grade water. Absorbance was noted at different wavelengths ranging from 360–440 nm using water as blank to find the λmax.
Universal buffer solution: A mixture of 6.00 g of citric acid, 3.893 g of KH2PO4, and 1.769 g of boric acid and 5.66 g of pure diethyl butyric acid was dissolved in water and diluted to one litre. Various volumes of 0.2 M NaOH solution were added in this universal buffer solution to maintain different pH values.
3. Results and discussion
In the present work ninhydrin is used as chromogenic reagent that forms a violet coloured complex with cobalt in slightly basic medium (pH 8.2). The absorption maximum (λmax = 395 nm) of the complex was found experimentally, whereas the ninhydrin reagent did not show any absorbance. In order to obtain high sensitivity, a wavelength of 395 nm was chosen for the spectrophotometric measurement of the cobalt complex against a reagent blank. The proposed reaction of ninhydrin with cobalt ion in alkaline solution is given in Fig. 2.
Figure 2.
Proposed reaction of ninhydrin with cobalt ion in an alkaline solution.
Different molar excesses of ninhydrin were added to fixed cobalt ion concentration and absorbance was measured according to the standard Procedure. Effect of ligand concentration on cobalt–ninhydrin complex is given in Fig. 3. The results indicate that absorbance increases with the increasing concentrations of ninhydrin solution. After 10 ml addition of ninhydrin (1%), the absorption remained constant indicating ninhydrin was sufficient enough to form a complex with 5 ppm cobalt solution. Greater excesses of ninhydrin up to 22 ml addition do not show a change in the absorption.
Figure 3.
Effect of ligand concentration on cobalt–ninhydrin complex.
Stability of the cobalt–ninhydrin complex with respect to time is given in Fig. 4. The cobalt–ninhydrin complex was formed immediately on mixing the reactants giving violet colour. This instantaneous reaction reveals that the complex remains stable for a period of 30 min as the absorbance remained almost constant for this time interval and then decreased rapidly showing decomposition of the cobalt–ninhydrin complex.
Figure 4.
Stability of the cobalt–ninhydrin complex with respect to time.
pH is always considered as quite important and critical for complex formation and its stability. It is always inevitable to find the optimum pH at which the complex is most stable (Säbel et al., 2010). Effects of pH and buffer solution on cobalt–ninhydrin complex are given in Fig. 5. The results show that pH 8.2 is most suitable for the complexation of cobalt with ninhydrin. It was also seen that in acidic media, the complex was less stale. The universal buffer solution used for pH maintenance did not interfere in actual analysis and provided the desired pH for complex formation.
Figure 5.
Effect of pH on cobalt–ninhydrin complex.
The effect of six different cations in the determination of cobalt has been studied and results are tabulated in Table 1. These data describe changes in the absorbance of cobalt–ninhydrin complex in the presence of various interferants added in equal quantity. The results indicate that the cations like cadmium, zinc, and sodium do not undergo too many significant changes in absorbance. However barium caused the highest change in absorbance followed by calcium and nickel, respectively.
Table 1.
Interference study of different metal cations.
| Sr. # | Absorbance without metal interference | Effect of different metals | Absorbance |
|---|---|---|---|
| 1 | 2.044 | Ni | 2.031 |
| 2 | 2.044 | Ba | 2.024 |
| 3 | 2.044 | Cu | 2.026 |
| 4 | 2.044 | Cd | 2.046 |
| 5 | 2.044 | Zn | 2.041 |
| 6 | 2.044 | Na | 2.040 |
4. Conclusion
The present method is very simple, selective and cheaper for the spectrophotometric determination of cobalt in very less time. Ninhydrin is a very common and easily available reagent. The method does not demand ultra conditions to be maintained. Very low concentrations of cobalt can be determined with good accuracy at an optimum pH of 8.2. However the complex is stable for 30 minutes only and decomposition starts afterwards.
References
- Arain M.A., Wattoo F.H., Ghanghro A.B., Wattoo M.H.S., Tirmizi S.A., Iqbal J., Arain S.A. Simultaneous determination of metal ions as complexes of pentamethylene dithiocarbamate in Indus river water-Pakistan. Arabian J. Chem. 2009;2:25–29. [Google Scholar]
- Bingol H., Atalay T. Study of kinetics of complexation reaction of Co2+ with 2-benzoylpyridine-4-phenyl-3-thiosemicarbazone and kinetic-spectrophotometric determination of cobalt. Acta Physico-Chim. Sin. 2006;22:1484–1488. [Google Scholar]
- Foster R. Academic Press; London, UK: 1969. Organic Charge Transfer Complexes. [Google Scholar]
- Guzar S.H., Jin Q. Simple, selective, and sensitive spectrophotometric method for determination of trace amounts of nickel(II), copper(II), cobalt(II), and iron(III) with a novel reagent 2-pyridine carboxaldehyde isonicotinyl hydrazone. Chem. Res. Chin. U. 2008;24:143–147. [Google Scholar]
- Holme D.J., Peck H. 3rd ed. Addison Wesley Longman; NY, USA: 1998. Analytical Biochemistry. [Google Scholar]
- Iqbal J., Tirmizi S.A., Wattoo F.H., Imran M., Wattoo M.H.S., Latif S., Sharfuddin S. Biological properties of chloro-salicylidene aniline and its complexes with Co(II) and Cu(II) Turk. J. Biol. 2006;30:1–4. [Google Scholar]
- Iqbal J., Wattoo F.H., Tirmizi S.A., Wattoo M.H.S. Dehydroacetic acid oxime as a new ligand for spectrophotometric determination of cobalt. J. Chem. Soc. Pak. 2007;29:136–139. [Google Scholar]
- Kettle S.F.A. Thomas Nelson and Sons Ltd.; UK: 1979. Coordination Compounds. [Google Scholar]
- Khedr A.M., Gaber M., Issa R.M., Erten H. Synthesis and spectral studies of 5-[3-(1,2,4-triazolyl-azo]-2,4-dihydroxybenzaldehyde (TA) and its Schiff bases with 1,3-diaminopropane (TAAP) and 1,6-diaminohexane (TAAH). Their analytical application for spectrophotometric microdetermination of cobalt(II). Application in some radiochemical studies. Dyes Pigments. 2005;67:117–126. [Google Scholar]
- Niazi A., Yazdanipour A. Simultaneous spectrophotometric determination of cobalt, copper and nickel using 1-(2-thiazolylazo)-2-naphthol by chemometrics methods. Chin. Chem. Lett. 2008;19:860–864. [Google Scholar]
- Öztürk B.D., Filik H., Tütem E., Apak R. Simultaneous derivative spectrophotometric determination of cobalt(II) and nickel(II) by dithizone without extraction. Talanta. 2000;53:263–269. [PubMed] [Google Scholar]
- Paleologos E.K., Prodromidis M.I., Giokas D.L., Pappas A.C., Karayannis M.I. Highly selective spectrophotometric determination of trace cobalt and development of a reagentless fiber-optic sensor. Anal. Chim. Acta. 2002;467:205–215. [Google Scholar]
- Säbel C.E., Neureuther J.M., Siemann S. A spectrophotometric method for the determination of zinc, copper, and cobalt ions in metalloproteins using Zincon. Anal. Biochem. 2010;397:218–226. doi: 10.1016/j.ab.2009.10.037. [DOI] [PubMed] [Google Scholar]
- Singh C. The use of ninhydrin in the detection of cationic complexes of cobalt, nickel and chromium. J. Chromatogr. A. 1965;18:194–196. doi: 10.1016/s0021-9673(01)80350-2. [DOI] [PubMed] [Google Scholar]
- Sözgen K., Tütem E. Second derivative spectrophotometric method for simultaneous determination of cobalt, nickel and iron using 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol. Talanta. 2004;62:971–976. doi: 10.1016/j.talanta.2003.10.020. [DOI] [PubMed] [Google Scholar]
- Zhao S., Xia X., Hu Q. Complex formation of the new reagent 5-(6-methoxy-2-benzothiazoleazo)-8-aminoquinoline with cobalt and nickel for their sensitive spectrophotometric detection. Anal. Chim. Acta. 1999;391:365–371. [Google Scholar]