Spectrophotometric Methods for the Determination of Lipoic Acid by using 1,10-Phenanthrene and Ferric Chloride

Spectrophotometric Methods for the Determination of Lipoic Acid by using 1,10-Phenanthrene and Ferric Chloride

1. Department of S & H, Amrita Sai Institute of Science & Technology, Paritala, A.P.

The oxidative coupling of the proposed method is simple, rapid and sensitive with reasonable precision and accuracy. The precision of the method was found by analyzing a set of eight solutions, each containing a final concentration value approximately in the middle of the Beers law range. The percent relative standard deviation in this method is presented in table-2. The accuracy of the method was determined by taking different known amounts (with in Beers law limits) of the drug and analyzing them by proposed method. The results are given in table 3. In the determination of Lipoic acid the excipients usually present in formulations (glucose, starch, sodium hexa phosphate and some vitamins) and the other antioxidants and antidiabetics did not interfere.

A very few physico-chemical methods have appeared in the literature for the determination of Alpha-Lipoic Acid (-LA) in bulk and pharmaceutical formulations. The literature suggested and reported only a few chromatographic techniques like Gas chromatographic methods with flame ionization detection and flame photometric detection, High-performance liquid chromatographic methods with ultraviolet, fluorescence and electrochemical detection, spectrophotometric using palladium(II) chloride, Reversed phase liquid chromatography, LC-MS/MS with atmospheric pressure chemical ionization and electro spray ionization interfaces, Tandem mass spectrometry, Electrophoresis, Extractive spectrophotometry and potentiometric techniques. The analytically important functional groups of -LA are not fully exploited for designing suitable spectrophotometric methods for the determination of -LA. Hence the need arises to develop certain sensitive, precise,

accurate and flexible visible spectrophotometric methods, which prompted the authors to choose -LA for the investigation based on various chemical reactions, by exploiting various functional groups from the structure.

These methods are based on the reaction of -LA with 1,10- phenanthrene and compounds like ferric chloride and o-phosphoric acid to produce colored species of reasonable stability, paving the possibility for spectrophotometric determination of -LA in its bulk from and pharmaceutical formulations.

I) Lipoic acid (0.25mg/ml): The stock solution (0.25mg/ml) of Lipoic Acid was prepared by dissolving 100mg of the drug in 400ml of distilled water to get a clear solution. A portion of this stock solution was diluted to get the working standard solutions of concentration 100 g/ml.

1. Ferric chloride (0.27%): 270mg of analytical grade Fecl3 is dissolved in 100 ml of distilled water.

2. orthophosphoric acid(0.2M)

   All the other chemical reagents were of analytical grade.

   Procedure : An aliquotes of standard lipoicacid solution (100 g/ml) ranging from 0.4 to 2.0ml are taken into a series of 10ml graduated test tubes and aqueous solutions of 1,10 phenanthrene (1.0-1.5ml), Fecl3 (1.0 1.5ml) and orthophosphoric acid (2.0-2.5ml) were added. The solution was kept aside for 10minuites and solutions were stir occasionally. The

   solutions were finally made up to the mark with distilled water and absorbance of the green colored solution was measured at 515nm against the corresponding reagent blank.

   Comparison of the results incorporated in Tables 1 4 reveal that the proposed method is simple, rapid and sensitive with reasonable precision and accuracy.

   Fig 5.1: absorption spectra of

   Lipoic acid with 1,10phenanthrene and Ferric chloride

   Concentration range(µg/ml) (or) Beers law limit	4 – 20
   * Regression equation	A= 1.605 0.0193C
   Correlation coefficient	998.7
   Molar absorptivity (1-mole -1 cm-1)	3.01*104
   Sandells sensitivity (µg / cm2 / 0.001 absorbance unit)	0.0068
   Optimum photometric range (µg/ml)	10.1 31.6

   *Found in this work; it must be determined independently by users of the method.

   Compound	% RSD**	% Range of errors confidence limit
   		0.05 level	0.01 level
   Lipoic acid	1.02	1.087	1.608

   Amount of Lipoic acid ( µg)		% Error
   Taken	Found
   450	439.7	2.28

   Sample	Labeled amount	**Amount found(µg) in method		*Recovery proposed method
   		Proposed	Reported
   tablet	150	148.2	147.2	1.2
   Capsule	150	147.4	146.9	1.73
   Capsule	300	296.3	290.1	1.23
   Capsule	450	443.7	440.1	1.4

   * Each result is an average of three determinations ** After adding 5 mg of drug.

   The proposed method is simple, rapid and sensitive with reasonable precision and accuracy and it is useful for the determination of Lipoic acid in bulk samples, pharmaceutical preparations and biological fluids.

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