Diclofenac sodium is a synthetic compound and therapeutically belongs to the group of non-steroidal anti-inflammatory drugs. Although the drug is well known and commercially available on the market in various pharmaceutical dosage forms (enteric coated oral tablets, sustained release oral tablets, topical preparations, injections and others), literature reports on thermal characterization and melting behaviour of the drug substance are contradictory. Even the melting point (about 280°C) of diclofenac sodium cited in some reference literature (e.g., European Pharmacopoeia, 3rd edition) does not correspond to the melting point reported in most scientific literature (285-289°C). 1-3)A thorough investigation of the available literature data on thermal behaviour and melting characteristics of diclofenac sodium in our laboratories suggests that the anomalies observed in reported data are due to the fact that most investigators used types of equipment which are not sensitive enough to detect all the phenomena that occur during melting of the substance, and most experiments were not well designed to make comprehensive conclusions possible. It is possible that most of the investigators did not have access to all facilities required for studying the melting and decomposition characteristics of diclofenac sodium. In fact, no differential scanning calorimetric data are available on the influence of scanning rate on the thermal behaviour of diclofenac sodium, and none of the investigators performed differential scanning calorimetry (DSC) on this drug under helium. Therefore, the main objective of our study reported here was to investigate the thermal behaviour of diclofenac sodium under various conditions using a number of techniques-DSC, thermogravimetric analysis (TGA), Fourier-transform infrared (FT-IR) spectroscopy and thin layer chromatography (TLC). ExperimentalMaterials Diclofenac sodium (lot 9911010) of British Pharmacopoeia quality was purchased from Sekhsaria Chemicals Ltd. (Mumbai, India). Gases-helium (He), nitrogen (N 2 ) and synthetic air, were purchased from SOL SPA (Monza, Italy) with stated purity of 99.9996%, 99.999% and 99.995%, respectively. A reference standard of 1-(2,6-dichlorophenyl)-indolin-2-one (Cat. No. 18881; Lot G) was purchased from the USP (Rockville, U.S.A.).DSC Analysis Perkin Elmer's Pyris 1 equipment was used to analyse the samples under various conditions. Samples weighing 4-6 mg were placed in aluminium pan, sealed and scanned (within the range 50-320°C) at various rates (1, 5, 10 and 20°C/min) to see the effect of scanning rate on the thermal behaviour of the studied substance. The scanning runs were repeated under dynamic flow of either helium (He), or nitrogen (N 2 ) or synthetic air to see the effect of environmental atmospheric condition on the thermal characteristics of the drug substance. The data were analysed using the software package "Pyris for Windows" (version 3.52). Onset temperature was considered as the temperature of the peak (T o ). The equipment was calibrated by scann...

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