2'-Hydroxy-4'-Methoxychalcone Oxime [HMCO] as A Gravimetric and Spectrophotometric Reagent for The Determination of Cu(II)

The ligand 2'-Hydroxy-4'-methoxychalcone oxime (HMCO) was developed as a gravimetric and spectrophotometric reagent for the determination of Cu(II). In the pH range of 3.0 to 6.0 this reagent gives brown complex with Cu(II). Job's method of continuous variation and Yoe and Jones mole ratio method revealed the stoichiometry of the complex to be 1:2 [M:L]. The obeyance of Beer's law was studied and the molar absorptivity and Sandell's sensitivity were calculated. The reagent and its complex have been characterized by elemental analysis and IR spectra. From TGA, the energy of activation was also calculated using Broido method for the first step decomposition. The reagent has been used for the determination of Copper content in Brass alloy. INTRODUCTION : In the current scenario, large number of organic reagents have been employed for the detection and quantitative determination of metal ions. They include o-hydroxy ketoximes1-2, phenyl hydrazones, thiosemicarbazones, chalcone oximes3-13 etc. These are generally used for spectrophotometric and gravimetric determination of transition metal ions. Here, we report the use of 2’-Hydroxy-4’-methoxychalcone oxime [HMCO] as an analytical reagent for Cu(II). EXPERIMENTAL : Instruments : Spectrophotometric measurements were done on a “Milton Roy” (Spectronic 20D+) Spectrophotometer and “Abotta make UV-1100, UV-Visible Spectrophotometer”. The IR spectra were recorded on “Perkin-Elmer” FTIR Spectrophotometer (RX-1) in KBr pallet. The NMR spectra were recorded on “Bruker Avance II 400 NMR Spectrometer. All pH measurements were done on Equip-Tronic pH meter (Model No.EQ 614). Stock solution : Stock solution of CuSO4.5H2O (0.05 M) was prepared by dissolving 3.121 gm of CuSO4.5H2O (A.R.) in minimum quantity of water and diluted to 250 ml with doubly distilled water. Concentrated sulphuric acid was added in little amount to prevent the hydrolysis of the salt. It was used after standardization14 with EDTA. Synthesis of Reagent [HMCO] : Resacetophenone was prepared from resorcinol, glacial acetic acid and anhydrous zinc chloride according to the method of R. Robinson and R. C. Shah15. Resacetophenone was treated with methyl iodide and anhydrous potassium carbonate in acetone on a water bath at 65-70°C for six hours. On acidification with dilute HCl, 2-hydroxy-4-methoxy acetophenone was obtained. The 2-hydroxy-4-methoxy acetophenone was converted to chalcone by its condensation with benzaldehyde in presence of aqueous KOH for 24 hours at room temperature. The 2’-hydroxy-4’-methoxychalcone was converted to its oxime using hydroxylamine hydrochloride and sodium acetate. On crystallization from alcohol pure oxime in the form of light yellow crystals with m.p.85°C was obtained. Stock solution of reagent (0.05 M) was prepared by dissolving in 70% aqueous ethanol. GRAVIMETRIC DETERMINATION OF CU(II) : Copper sulphate solution (0.05 M, 10 ml) was taken in a clean beaker and diluted to about 100 ml with distilled water. A little excess of reagent solution was added (0.05 M, 22 ml). The pH of the solution was adjusted between 3.0 to 6.0 using suitable acid buffer. A brown precipitate obtained were digested on water-bath for 60 minutes at 60°C. The precipitate were filtered through a previously weighed sintered glass crucible (G4) and washed with warm water followed by 70% aqueous ethanol to remove excess of the reagent. The chelate was dried to constant weight at 110°C in hot air oven, cooled and weighed as Cu(C16H15O3N). Duplicate experiments were performed in each case. The results are given in Table 1. The experiment was repeated at different pH of solution. The experiment was also repeated with different aliquots, keeping the optimum pH value to evaluate its applicability. The error in any case did not exceed 1.0%. INTERFERENCE FROM OTHER IONS : To study the effect of foreign ions on gravimetric determination of Cu(II), 8-10 mg of various cations were added to a solution containing 31.77 mg Cu(II) at pH 5.0 and gravimetric estimations were done. It was observed that Sr(II), Ca(II), Ni (II), Zn(II), Ba(II), Cd(II), Mn(II) and Mg(II) do not interfere at this pH but Fe(III) and Pd(II) interfered seriously. Interference of Fe(III) can be removed by masking it with H3PO4. Many common anions like nitrate, nitrite, sulphate, chloride, bromide, iodide were not found to interfere. TABLE 1 GRAVIMETRIC DETERMINATION OF CU(II) USING HMCO Cu(II) taken = 31.77 mg Drying temperature = 110-115oC Salt = CuSO4.5H2O


INTRODUCTION :
In the current scenario, large number of organic reagents have been employed for the detection and quantitative determination of metal ions. They include o-hydroxy ketoximes 1-2 , phenyl hydrazones, thiosemicarbazones, chalcone oximes 3-13 etc. These are generally used for spectrophotometric and gravimetric determination of transition metal ions. Here, we report the use of 2'-Hydroxy-4'-methoxychalcone oxime [HMCO] as an analytical reagent for Cu(II).

Stock solution :
Stock solution of CuSO 4 .5H 2 O (0.05 M) was prepared by dissolving 3.121 gm of CuSO 4 .5H 2 O (A.R.) in minimum quantity of water and diluted to 250 ml with doubly distilled water. Concentrated sulphuric acid was added in little amount to prevent the hydrolysis of the salt. It was used after standardization 14 with EDTA.

Synthesis of Reagent [HMCO] :
Resacetophenone was prepared from resorcinol, glacial acetic acid and anhydrous zinc chloride according to the method of R. Robinson and R. C. Shah 15 . Resacetophenone was treated with methyl iodide and anhydrous potassium carbonate in acetone on a water bath at 65-70°C for six hours. On acidification with dilute HCl, 2-hydroxy-4-methoxy acetophenone was obtained. The 2-hydroxy-4-methoxy acetophenone was converted to chalcone by its condensation with benzaldehyde in presence of aqueous KOH for 24 hours at room temperature. The 2'-hydroxy-4'-methoxychalcone was converted to its oxime using hydroxylamine hydrochloride and sodium acetate. On crystallization from alcohol pure oxime in the form of light yellow crystals with m.p.85°C was obtained. Stock solution of reagent (0.05 M) was prepared by dissolving in 70% aqueous ethanol.

GRAVIMETRIC DETERMINATION OF CU(II) :
Copper sulphate solution (0.05 M, 10 ml) was taken in a clean beaker and diluted to about 100 ml with distilled water. A little excess of reagent solution was added (0.05 M, 22 ml). The pH of the solution was adjusted between 3.0 to 6.0 using suitable acid buffer. A brown precipitate obtained were digested on water-bath for 60 minutes at 60°C. The precipitate were filtered through a previously weighed sintered glass crucible (G 4 ) and washed with warm water followed by 70% aqueous ethanol to remove excess of the reagent. The chelate was dried to constant weight at 110°C in hot air oven, cooled and weighed as Cu(C 16 H 15 O 3 N). Duplicate experiments were performed in each case. The results are given in Table 1. The experiment was repeated at different pH of solution. The experiment was also repeated with different aliquots, keeping the optimum pH value to evaluate its applicability. The error in any case did not exceed 1.0%.

INTERFERENCE FROM OTHER IONS :
To study the effect of foreign ions on gravimetric determination of Cu(II), 8-10 mg of various cations were added to a solution containing 31.77 mg Cu(II) at pH 5.0 and gravimetric estimations were done. It was observed that Sr(II), Ca(II), Ni (II), Zn(II), Ba(II), Cd(II), Mn(II) and Mg(II) do not interfere at this pH but Fe(III) and Pd(II) interfered seriously. Interference of Fe(III) can be removed by masking it with H 3 PO 4 . Many common anions like nitrate, nitrite, sulphate, chloride, bromide, iodide were not found to interfere. Conversion factor = 0.1055 SPECTROPHOTOMETRIC STUDY OF CU(II)-HMCO CHE-LATE : 5 mg of chelate was extracted in 25 ml of chloroform and the absorption spectra was recorded in the range of 300 to 800 nm. It was observed that the absorbance of the coloured solution of chelate increases continuously towards the shorter wavelength. A weak band is observed at 400 nm and hence all measurements were carried out at 400 nm.

VERIFICATION OF BEER'S LAW AND OPTIMUM CON-CENTRATION RANGE :
To 5 ml of solution (0.01 M) of the reagent HMCO, varying amount of the Cu(II) solution (0.005 M) was added and the pH was adjusted to 5.0, using [CH 3 COOH + CH 3 COONa ] buffer. The insoluble complex was extracted in chloroform using three 5.0 ml, portions of chloroform and final volume of chloroform extract was adjusted to 25.0 ml. The absorbances of these solutions were measured at 400 nm against chloroform as blank. Absorbance values were plotted against metal concentration expressed in ppm. A straight line passing through the origin, indicating obeyance of Beer's law is obtained up to 38.12 ppm of Cu(II). The molar absorptivity of the Cu(II)-HMCO complex was found to be 6.716 x 10 2 lit. mol -1 .cm -1 and the Sandell's sensitivity is found to be 0.0946 µg/cm 2 at 400 nm.

STOICHIOMETRY OF COMPLEX :
Job's method of continuous variation 16 and Yoe and Jones mole ratio method 17 were used to determine the stoichiometry of the Cu(II)-HMCO complex. From both the methods, it was found to be 1:2 [M:L] ratio. This is in agreement with the stoichiometry found from gravimetry. The average stability constant found from two methods is 4.065 x 10 8 . The Gibb's free energy change for complex formation reaction at 30°C was found to be -11.81 K.cal/mole.

IR SPECTRA :
Examination of the IR spectra of the chelates show that the band due to intramolecular hydrogen bonded O-H stretching of 2-hydroxy group disappears in the Cu(II)-HMCO complex while the band due to oximino -OH group at 3429 cm -1 in Cu(II) chelate.
The band due to the -C=N stretching which is observed at 1548 cm -1 in ligand is shifted to 1507-1608 cm -1 in complexes. This may be due to coordination of metal through nitrogen. This is further supported by slight downward shift of νNO from 1029 cm-1 in the ligand to 959 to 982 cm -1 in copper chelate. Thus, in the chelate, metal is covalently bonded with oxygen and coordinate bonded with nitrogen.

GRAVIMETRIC ESTIMATION OF CU(II) IN BRASS ALLOY USING HMCO :
Preanalysed sample of brass (0.5523g) was dissolved in 50% HNO 3 by heating for 30 minutes. The solution is evaporated to a volume of near about 5 ml but not to dryness and the bulk of nitric acid removed. The resulting solution was diluted to 100 ml with doubly distilled water in volumetric flask.
An aliquot of above diluted solution (10 ml) was taken in a clean beaker and copper was determined gravimetrically using 2'-hydroxy-4'-methoxychalcone oxime (HMCO) as per the procedure described previously. From the result of TGA, it is seen that the complex is stable upto temperature 150 0 C and so it can be dried safely without decomposition at 100-150 0 C. The loss in weight in the temperature range 150 0 C to 800 0 C is due to the removal of organic ligand molecule. The observed loss agrees well with the loss expected as per formula of the chelate in which M:L ratio is 1:2. The weight of residue agrees with the weight of residue expected as per the given formula. This confirms the metal:ligand ratio [1:2] which is also established from the results of gravimetric analysis, elemental analysis and spectrophotometric methods. Activation energy Ea was calculated using Broido method 18 . The authors are thankful to the Head, Chemistry Department, Sir P. T. Sarvajanik College of Science, Surat for providing the facilities to carry out the work.