Influence of Impregnation of a Mixture of Silica Gel and Kieselguhr with Copper (II) Sulphate (VI) on Profile Change of the Spectrodensitograms and the RF Values of Nicotinic Acid and its Derivatives

Abstract

Nicotinic acid and its derivatives were separated using adsorption TLC on aluminum plates precoated with a mixture of silica gel 60 and kieselguhr F254 non impregnated and impregnated with 2.5% and 5% aqueous solutions of CuSO4. The mixture of an acetone‐n‐hexane in different volume compositions was used as mobile phase. The obtained results of the investigations concerning the evaluation of chromatographic separations were considered in three groups, namely, 1st group: nicotinic acid (1), methyl nicotinate (2), ethyl nicotinate (3), isopropyl nicotinate (4), butyl nicotinate (5), hexyl nicotinate (6), benzyl nicotinate (7); 2nd group: nicotinic acid (1), nicotinamide (8), N‐methylnicotinamide (9), N,N‐diethylnicotinamide (10); and 3rd group: nicotinic acid (1), nicotinamide (8), 3‐pyridinecarbaldehyde (11), 3‐pyridinecarbonitrile (12), 3‐pyridylmethanol (13), methyl 3‐pyridyl ketone (14). Impregnation of the mixture of silica gel and kieselguhr with 2.5% and 5% aqueous solutions of CuSO4 causes a decrease of the RF values of the 1st, 2nd, and 3rd groups of the compounds in relation to the RF values of these substances obtained on non impregnated solid support. Impregnation with a water solution of copper (II) sulphate (VI) of the mixture of silica gel and kieselguhr worsens the separation of the 2nd and 3rd groups of the investigated compounds. Improvement of the chromatographic separations is observed only in the case of esters of nicotinic acid (1st group of compounds). It was found that the spectrodensitograms of the studied compounds on impregnated plates and by use of Rhodamine B as visualizing reagent were different than the spectrodensitograms obtained on the plates without use of a visualizing reagent. Spectra of the investigated compounds on non impregnated plates and impregnated with aqueous solution of CuSO4 and without the use of a visualizing reagent have two or three absorption bands. Investigated compounds on the plates impregnated with the aqueous solution of CuSO4 and by use of Rhodamine B as visualizing reagent have one additional absorption band at λ≈560 nm. This fact has analytical significance in the identification of the investigated substances.