The process involved benzoylation of 1-phenyl-3-methyl-pyrazolone-5 (HPMP) with p-nitrobenzoyl chloride, by dissolving HPMP (17.6 g, 0.100 mole) in dioxane (75 ml) with gentle warming in a 500 ml three-necked roundbuttom quick-fit flask equipped with a magnetic stirrer, separatory funnel, and a reflux condenser. Calcium hydroxide (7.5 g, 0.103 mole) was added dropwise within 2 - 5 minutes (which had been previously dissolved in 50 ml of dioxane). The mixture was continuously stirred, and gently refluxed for 90 minutes till the yellow calcium complex decomposed by pouring into 400 ml of chilled 3 M HCl, whereby cream nitro-benzoylpyrazolone-5 separates.

Re-crystallization of the crude product was from an ethanol-water mixture containing a little HCl to destroy any un-decomposed calcium complex. The crystals are orange-yellow; yield, 86%; and m.pt, 160.5˚C. This method has been adopted by Uzoukwu [18], Meckler [13] for the synthesis of other related acylpyrazolones. The reagent exhibits a keto-enol tautomerism and can be obtained in both forms. The orange-yellow enol form was obtained by re-crystallization from heptane or warm ethanol-water mixture, while the colorless keto form was obtained from hot water. No differences in behaviour of these two forms were observed. In the present work, the enol form was used. The HPMP was prepared as recorded by Okafor [19]. Microanalyses for N, H, and C were performed to confirm the purity of the reagent. All other reagents were of analytical grades, and were used without further purification.

Solutions of HPMNP were made by dissolving an appropriate amount of it in organic solvent, so that the stock solution was 0.02 M. A Stock solution of 1.7 × 10^{−3 M Co(II) was prepared by dissolving 0.48 g of CoSO}₄·7H₂O in 100 ml of 0.1 M H₂SO₄. Further dilutions were made from it as required.

To 5 ml of an aqueous buffer solution containing 2 ppm of Co(II) was added 5 ml of 0.02 M solution of HPMNP in chloroform. The two phases were mechanically shaken on a Stuart flask shaker for 60 minutes at 26˚C ± 0.5˚C. Preliminary experiments show that 30 minutes were sufficient for equilibration. For pH studies, the solutions of Co(II) were adjusted to different pH values using chlorides (KCl/HCl), actate (NaAc/HAc) and borate buffer solutions to cover pH 1 to 8. The equilibrated phases were allowed 30 minutes time, and the concentration of Co(II) remaining in the aqueous phase was determined spectrophotometrically using 1-nitroso-2-naphthol. Hydrogen peroxide was used as the oxidizing agent, followed by extraction with toluene and measuring at 410 nm. The aqueous phase in each case was maintained at a constant ionic strength of 0.3 M with sodium sulphate, Na₂SO₄·10H₂O. The pH values were measured with a digital pH meter Model HM-208 while a Pye Unicam SP8-100 spectrophotometer was used for spectrophotometric measurements.

5 ml aliquots of HPMNP of varying concentrations in chloroform were equilibrated with equal volumes (5 ml) of aqueous phase of fixed pH 5.5 (with NaAc/HAc buffer) and ionic strength of 0.3 M. The solutions were allowed to settle for half an hour in order to separate the phases after which suitable aliquots of the aqueous phase were taken for analysis of Co(II).