Indirect determination of chinoform in drug preparation by atomic absorption spectrophotometry

Abstract

Chinoform, being a derivative of oxine, forms metal chelate with various metal ions. It is known that chinoform reacts with zinc to produce a yellow chelate of 2:1 ratio and it is extractable with methyl isobutyl ketone (MIBK). Therefore, if zinc in MIBK layer is measured by atomic absorption spectrophotometry, then chinoform may be determined quantitatively. Chinoform zinc chelate can be extracted easily with chloroform, but MIBK is considered to be more suitable than chloroform as a solvent for atomic absorption spectrophotometry.
In this experiment, a Hitachi 207 Atomic Absorption Spectrophotometer was used, and the 2138 Å analytical line of zinc was used for the measurement using airacetylene flame. The procedure for the determination is described below. Ten ml of 4.1 × 10^-5 M chinoform MIBK solution was taken in a 100 ml separatory funnel, and then 10 ml of 2.0× 10^-4 M aqueous zinc sulfate solution and 10 ml of sodium borate-boric acid-sodium chloride buffer solution, adjusted at pH 7.9, were added. The mixture was shaken well for 10 minutes and kept standing for 10 minutes. The lower aqueous layer was firstly separated off and secondly MIBK layer was transfered into a test tube. After MIBK layer was filtrated through filter paper, 2 ml of the filtrate was diluted to 10 ml with MIBK, and then the absorbance was measured by atomic absorption spectrophotometry against a reagent blank. From the above investigation, optimum conditions for the extraction have been found.
Most suitable pH range was found to be 7.4 9.0, the concentration of zinc sulfate was 0.5 20 times larger than that of chinoform and the shaking time was more than 10 minutes. The authors examined how the volume ratio (R) between aqueous layer of the metal ion and the buffer, and MIBK layer influences upon the intensity of the absorbance. Consequently, the intensity was found to be higher as R increased. Throughout this measurement, R=2 was used.
Under the optimum conditions mentioned above, a good linearity was obtained between absorbance in organic layer and chinoform content in the range of 6.030.0μg chinoform per ml. When the repeat, ability of this method was examined, the standard deviation was calculated to be 0.495. The composition of the chelate extracted was found to be 2 : 1 ratio according to the continuous variation method.
This method was applied to drug preparations. Chinoform preparation was dissolved in MIBK and insoluble substances in the solution were removed by filtration. MIBK layer was washed with sulfuric acid and then chinoform was extracted as a zinc chelate. Zinc in MIBK layer was measured by atomic absorption spectrophotometry, and this preparation was determined to contain 18.2% chinoform. Recovery was found to be 98.5%, according to the standard addition method.