DETERMINATION OF COBALT IN PURE COBALT METAL BY COMPLEXOMETRIC ANALYSIS

Introduction:

Cobalt is a hard, silver-gray metal that melts at 1495°C. As a pure metal, cobalt is used in electroplating applications to create hard, oxidation resistant surfaces. Due to its magnetic, corrosion-resistant and wear-resistant properties, along with its strength at relatively high temperatures, cobalt is predominantly used as an alloyed metal in the production of wear-resistant super-alloys, industrial catalysts, gas turbine blades, jet aircraft engines and prosthetic parts for joint replacement. There are several methods for the determination of cobalt namely gravimetric estimation with 1-nitroso-2-naphthol and also by potentiometric method. In addition to that, the same can be estimated as cobalt tetrathiocyanatomercurate. 1-nitroso-2-naphthol gives a red precipitate with solution of cobalt salts. There is some doubt as to the exact composition of the precipitate, the formula Co(C10H6O2N) has been assigned to it, but probably it is not pure. The complex is best converted into cobalt sulphate, CoSO4 or into metallic cobalt and weighed in either of these forms. Careful ignition in presence of oxalic acid gives cobalt oxide with the formula Co3O4. But the method is time consuming. Moreover, complete precipitation to be ensured very cautiously since the precipitate is too much bulky, furthermore there is a probability of co-precipitation which may yield slight high result.

In the present work, An attempt has been made to determine cobalt by EDTA complexometric method in pure cobalt metal having a very high purity ( > 99.99- %). The wet chemical results have been compared with Inductively Coupled Plasma (ICP) Optical Emission Spectrometric results after determining all the impurities which may be present in the material as per Chinese Chemical Letters Vol. 17, No.8, pp 1077-1080, 2006.

Theory behind the Experiment:

The quantitative determination of many metal ions in solution can be achieved by titrating a standard solution of a lewis base (ligand). A necessary requirement is that the ligand should combine (complexes) quantitatively with a particular metal ion under the solution condition. The most common ligand is the anion of ethylene diamine tetra acetic acid (EDTA, H4Y). It is usually prepared by dissolving the disodium salt of this acid, Na2H2Y, since the acid is only slightly soluble in water. The reaction of cobalt ion with fully ionized EDTA (Y4-) can be represented by the equation:

Co2+(aq)  +  Y4-(aq) = CoY2-(aq)

The end point of the titration can be obtained by observing the colour change of a suitable metal ion indicator.

Experimental & Results:

The pure cobalt standard sample of Sigma Aldrich (Cas No. 7440-48-4, Certified purity ≧ 99.99- %) has been choosen for this study. 0.2 gm sample taken for the quantitative wet chemical analysis. 10 ml dilute hydrochloric acid added to make the solution by placing the beaker on a hot plate. After complete dissolution of the sample, volume made up to 250 ml. 50 ml aliquote equivalent to 0.04 gm sample taken for titration with 0.01 M EDTA which has been further standardised against CRM grade calcium carbonate (CaCO3). The pH of the entire solution durig titration was maintained at 10 by adding NH4OH. The metal ion indicator used in this study was Eriochrome Black-T (EBT). The end point is from blue to red.

On the other hand, for ICP analysis, Approximately 3 g of the test sample is weighed and taken in the teflon vessel of the Digestion Bomb. 2 ml of HF, 2 ml HCl and 6 ml of conc HNO3 are added to them and the containers are then sealed using the Teflon cap of the vessels. A reagent blank is also prepared using the above acids without the sample. The bomb is then placed inside an air oven which is capable of rising at 200°C. The solution is heated inside the bomb for 2 hours for complete digestion of the material. After complete digestion, the final volume of each solution is made upto the mark using HPLC grade water in a 100 ml volumetric flask. The calibration curve for each of the single elements measured are made using the 0.1, 0.5 and 1 mg/L standard solution prepared from the ICP multielement standard solution. The concentration of trace elementals (wt.%) in the sample are measured against the calibration curve made using the reagent blank and the standard samples as prepared above. The summary of all results as derived from wet chemical analysis and ICP-OES is shown as follows:

cobalt1

 

Conclusion:

The complexometric EDTA method for the determination of cobalt is less time consuming, selective and inexpensive as compared to other methods. At the same time, the accuracy of the method is well established when the cobalt content is derived by analyzing the trace impurities from ICP.

*The work has been carried out jointly by Mr. Kajal Ray and Ms. Priya Das at MSK Central Laboratory, Kolkata.

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