Pehlic, E; Nuhanovic, M; Sapcanin, A; Banjanin, B; Nanic, H; Redzic, S; Poljakovic, M
Aim: Characterization of acetylsalicylic acid with thin-layer chromatography and hot-stage microscopy depending to solvent system.
Material and methods: Synthesis of aspirin is classified as esterification reaction. In this reaction, to salicylic acid as reactant is added acetate anhydrate, a derivative of acetic acid, and in presence of 85% phosphoric acid (sometime sulphuric acid is used) as catalyst gives acetylsalicylic acid and as by-product acetic acid [1]. Obtained quantity of raw aspirin is 15 g. Precrystalisation of raw aspirin is done with these solvents: ethanol, methanol and petroleum ether. For synthesised aspirin identification following instrumental methods is used: Thin Layer Chromatography (TLC) and Hot-stage microscopy (HSM). As stationary phase: HPTLC plate F 254 MERC, silica gel G 20x20, thickness 0.25 mm [13]. Detection of compounds is performed with UV/VIS detector-CAMAG. Mobile phase: benzene-ethanol (8: 2). Thermal properties are observed on OLYMPUS BX 51 thermal microscope with polarised light. Sample is mounted and heated on 10 degrees C/min in temperature range 40 degrees C-150 degrees C. On given visible terminal crossovers pictures are made (1 picture/3 sec) [12].
Results: Identification of given compounds is studied by TLC method in different systems. By identification of acetylsalicylic acid (precrystalised acetylsalicylic acid in ethanol, methanol and petroleum ether) benzene and ethanol are used, but in different ratios:
benzene-ethanol (8 : 2)
benzene-ethanol (9 : 1)
benzene-ethanol (7 : 3)
Sample precrystalised in ethanol is solved in range of 0.5-1 degrees C, what is proof that acetylsalicylic acid precrystalised in ethanol shows superb purity, thus ethanol could be used as suitable solvent for acetylsalicylic acid precrystalisation. Sample precrystalised in methanol is also solved in range 0.5-1 degrees C. In petroleum ether precrysalised sample is melting on 99.8 degrees C, where is visible that in sample which is precrystalised in petroleum ether some impurities remains, so it cannot be suitable solvent for purification of acetylsalicylic-acid.
Discussion: Drugs which contain derivates of salicylic acid, but with structure similar to aspirin, are used in medicine since ancient times. Extracts from willow bark are rich in salicylates which are known by specific effects on elevated temperature, pain and inflammation, since mid 20th century. Aspirin also have a property of inhibiting blood coagulation, because of thromboxane prostaglandins inhibition, which normally bind molecules of platelets for making thromboses on open or damaged blood vessel or injured tissue [2]. The aim is evaluation of the influence of different precrystalisation solvents on acetylsalicylic acid purity assessed by thermal microscopy and the influence of different mobile phase composition on acetylsalicylic acid identification in the presence of salicylic acid by thin-layer chromatography.
Conclusions: Most suitable solvents for purification of raw aspirin are ethanol and methanol, thus petroleum ether is not, because after petroleum ether precrystalisation remains impurities. Sample precrystalised in ethanol and methanol is solved in range of 0.5-1 degrees C, what gives conclusion acetylsalicylic acid precrystalised in ethanol shows high purity so ethanol is suitable solvent for acetylsalicylic acid precrystalisation. R-f value by mobile phase benzene - ethanol in ratio (9:1, 7: 3) is such that spots are on the same position what means there were no separation, i.e. this system isn't appropriate for aspirin identification by thin-layer chromatography method.