2-Hydroxy-5-methylbenzophenone oxime ( HMBO ) as an Analytical Reagent for Gravimetric Determination of Cu ( II )

2-hydroxy-5-methylbenzophenone oxime (HMBO) was developed as a new analytical reagent for the gravimetric determination of divalent copper ion. In pH rang of 3.0 to 6.0, the reagent gives a buff colored precipitate with Cu(II). Job’s method and mole ratio method revealed that the stoichiometry of the complex is 1:2 (metal: ligand). Beer’s law is obeyed up to 61.25 ppm of Cu(II). Molar absorptivity and Sandell’s sensitivity at 400 nm were found to be 6.32 x 10 L mol cm and 0.359 μg/cm respectively. The stability constant of Cu(II)-HMBO complex is found to be 2.43 x 10. Gibb’s free energy change for complex formation reaction was found to be -12.88 Kcal/mol. The reagent can be used for the analysis of brass and alloy.


Introduction
In the current scenario of analytical chemistry, many reagents are widely available.They include o-hydroxy ketoximes [1][2][3] , phenyl hydrazones, thiosemicarbazone [4][5][6] , chalcone oximes 7 etc.These are generally used for spectrophotometric and gravimetric determination of transition metal ions.In this work, we report the use of 2-hydroxy-5-methylbenzophenone oxime (HMBO) as a gravimetric reagent for Cu(II).Spectrophotometric methods have been used to confirm the stoichiometry of complex and to determine the stability constants of the complex.The reagent is used to determine copper in brass and alloy.

Instruments
Schimadzu UV-visible spectrophotometer (160-A) was used for all absorbance and pH of solutions were measured on Elico-pH meter (LI-10T) and buffer solution of required pH were obtained using (sodium acetate-acetic acid) and (hydrochloric acid-sodium acetate) buffers of suitable concentration.
The elemental analysis of the reagent was found to be:

Gravimetric procedure
A 0.05 M solution of the reagent in 70 % aqueous ethanol was used.Copper sulphate solution (0.05 M, 10.0 mL) taken in a clean beaker was diluted to about 100.0 mL with distilled water and pH of the solution was adjusted to 3.0-6.0with suitable buffer.The solution was warmed at 60 °C and small excess of reagent solution was added (0.05 M, 22.0 mL).A buff precipitate obtained was digested on water bath for 1 hour at 60 °C.The precipitate was filtered through a previously weighed sintered glass crucible (G 4 ).The precipitate was washed with hot water followed by 70 % aqueous ethanol to remove excess of the reagent which might have precipitated on dilution.The chelate was dried to constant weigh at 110-115 °C in hot air oven, cooled and weighed.

Gravimetric determination of Cu(II)
To establish the applicability of the reagent for gravimetric estimation of Cu(II), the metal ion was determinate in the pH range 3.0-6.0.The maximum error being ±1 %.Estimation were done at pH 5.0 using different aliquots of Cu(II).In all cases the error in Cu(II) content did not exceed ± 0.84 % (Table -1).

Spectrophotometric study of Cu(II)-HMBO complex
For taking the absorbance spectra 5.0 mg chelate was dissolved in 25.0 mL chloroform and the absorbance was measured in the range of 350 -800 nm.It was observed that absorbance of the solution increased continuously towards the shorter wavelength.The absorbance spectrum showed a shoulder at 400 nm and hence all measurement was carried out at 400 nm.The Cu(II)-HMBO complex is insoluble in methanol and ethanol.It is soluble in solvent like chloroform, MIBK, 1:4 dioxane etc. Hence the complex was extracted in chloroform.For spectrophotometric studies varying amount of Cu(II) solution was taken and pH was adjusted to 5.0 with (sodium acetate + acetic acid) buffer and HMBO solution was extracted with three 5.0 mL portion of chloroform and the volume of chloroform extract was made up to 25.0 mL the absorbance of chloroform extract was measured against reagent blank.

Validity of Beer's law
The Cu(II)-HMBO complex in chloroform obeys law up to 61.25 ppm of Cu(II).Beyond this concentration the absorbance plot showed deviation from linearity (Figure 1).Standard graph thus obtained may be used for the determination of copper in an unknown solution using HMBO.Molar absorptivity calculated from Beer's law plot was found to be 6.32 x 10 2 lit.mol -1 cm -1 for Cu(II)-HMBO complex at 400 nm and Sandell's sensitivity was calculated and found to be 0.359 g/cm 2 of Cu(II) at 400 nm.

Stoichiometry of complex
The stoichiometry of Cu(II)-HMBO complex was determined by (1) Job's method of continuous variation 8 , and (2) Yoe and Jone's mole ratio method 9 (Figure 2-3).Both the methods gave the metal: ligand ratio of the complex 1:2.This is in agreement with the stoichiometry found from gravimetry.From this data the structure of complex can be written as follows:

Stability constants of the complex
The stability constants of the complex were calculated using the formula: Where, = (E m -E s ) / (E m ) E m =maximum absorbance obtained at the intersect of the two lines.E s =absorbance at the stoichiometric ratio of the metal to reagent in complex The average stability constant found from two methods is 2.43 x 10 9 from Ks value.Gibb's free energy change for complex formation reaction was calculated and its value was found -12.88 K.cal/mole at 27 °C.

Table -
InterferenceTo study the effect of foreign ions on gravimetric determination of Cu(II), 8-10 mg of various cations were added to solution containing 31.77 mg Cu(II) at pH 5.0 and gravimetric estimations were done.It was observed that Mg(II), Ca(II), Mn(II), Ni(II), Sr(II), Zn(II), Ba(II), Cd(II) and Al(III) do not interfere at this pH but Pd(II), Co(II) and Fe(III) interfere seriously.Many common anions like chlorides, bromide, iodide, nitrate, nitrite, sulphate were not found to interfere.