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Thermal analysis in Ayurvedic drugs
The success of preparing a genuine ayurvedic bhasma depends on the technique of heat treatment (called as putas) to the homogeneous paste prepared by mortaring microfined metal thoroughly mixed with appropriate plant juices. Normally a large number of calcinations cycles are necessary and for each cycle the process of mortaring the microfined metal and plant juice is to be freshly repeated. Therefore synthesis of these bhasmas is a complicated and tedious procedure. Due to increasing popularity of ayurvedic medicines, the number of ayurvedic pharmacies are rapidly expanding for commercialization of the reputed ayurvedic drugs. Therefore, an examination of different commercial samples using modern analytical techniques is a current need. Thermogravimetry is expected to be one of useful techniques, because thermograms of the bhasmas may throw light on (a) thermal stability of the samples as a whole as well as of its major constituents (b) Nature of decomposable materials, if present (c) difference or similarity of different samples under study. Five representative samples of tamra bhasma prepared by reputed ayurvedic pharmacies from India are selected for thermal analysis in the temperature range (30-900) 0C. The results are very interesting to understand the current status of the commercial samples.
Tamra bhasma, prepared from metallic copper is a famous ayurvedic preparation. It is a versatile drug and specially recommended for all sorts of complaints related to liver and spleen, tumors, dropsy etc. It is synthesized by a variety of methods developed by ayurved pharmacists which may be classified into three main categories. These categories are based on methods involving use of (a) mercury and sulfur (traditionally called as kajjali) (b) Sulfur purified according to ayurvedic procedure (c) plant materials. Pure copper in the form of powder, small pieces, or very thin sheets is the starting material for all methods. This copper is subjected to a number of processes which include repeated calcinations in combination with specific plant juices. The end product of each method is assumed to be tamra bhasma. As a consequence of huge diversity in synthetic procedures it is difficult to know whether tamra bhasma prepared by different methods is identical or variable because no systematic work has been carried out from this point of view. In view of this the various techniques generally used for thermal analysis are briefly described in this article.
Thermogravimetry (TG) :
Thermo gravimetric analysis (TGA) is the most widely used thermal method. It is based on the measurement of mass loss of material as a function of temperature. In Thermogravimetry a continuous graph of mass change against temperature is obtained when a substance is heated at a uniform rate or kept at constant temperature. A plot of mass change versus temperature (T) is referred to as the thermo gravimetric curve (TG curve). For the TG curve, we generally plot mass (m) decreasing downwards on the y axis (ordinate), and temperature (T ) increasing to the right on the x axis (abscissa). Sometime we may plot time (t) in place of T. TG Curve helps in revealing the extent of purity of analytical samples and in determining the mode of their transformations within specified range of temperature. In Thermogravimetry, the term âdecomposition temperatureâ is a complete misnomer. In a TG curve of single stage decomposition, there are two characteristic temperatures; the initial Ti and the final temperature Tf ,Ti is defined as the lowest temperature at which the onset of a mass change can be detected by thermo balance operating under particular conditions and Tf as the final temperature at which the particular decomposition appear to be complete. Although Ti has no fundamental significance, it can still be a useful characteristic of a TG curve and the term procedural decomposition temperature has been suggested. The difference Tf â Ti is termed as reaction interval. In a dynamic thermogravimetry a sample is subjected to continuous increase in temperature usually linear with time whereas in isothermal or static thermogravimetry the sample is maintained at a constant temperature for a period of time during which any change in mass is noted. Beside these there are many other applications of thermo gravimetric analysis. Some are listed below:
i) Purity and thermal stability.
ii) Solid state reactions.
iii) Decomposition of inorganic and organic compounds.
iv) Determining composition of the mixture.
v) Corrosion of metals in various atmospheres.
vi) Pyrolysis of coal, petroleum and wood.
vii) Roasting and calcinations of minerals.
viii) Reaction kinetics studies.
ix) Evaluation of gravimetric precipitates.
x) Oxidative and reductive stability.
xi) Determining moisture, volatile and ash contents.
xii) Desolvation, sublimation, vaporisations, sorption, dÃ©sorption, chemisorptions.
Differential Thermal Analysis (DTA):
In Differential Thermal Analysis, the temperature difference that develops between a sample and an inert reference material is measured, when both are subjected to identical heat â treatments. The related technique of Differential Scanning Calorimetry relies on differences in energy required to maintain the sample and reference at an identical temperature. Analytical method for recording the difference in temperature (?T) of a substance and an inert reference material as a function of temperature or time any transformation change in specific heat or an enthalpy of transition can be detected by DTA. In DTA both test sample & an inert reference material (alumina) controlled heating or cooling programming if zero temperature difference of sample & reference material sample does not undergo any chemical or physical change. If any reaction takes place temperature difference (?T) will occur of sample & Reference material. A DTA curve can be used as a finger print for identification purposes, for example, in the study of clays where the structural similarity of different forms renders diffraction experiments difficult to interpret. DTA detects the release or absorption of heat, which is associated with chemical and physical changes in materials as they are heated or cooled. Such information is essential for understanding thermal properties of materials. Analysis of decomposition of glass batch materials, crystalline phase changes, chemical reactions and glass transition temperature. DTA shows two types of heat changes in which one is endothermic and the other is exothermic. It is classified as,
Sharp Endothermic: changes in crystallanity or fusion
Physical changes: usually result in endothermic curves
Chemical reactions: exothermic or endothermic
A DTA curve can be used only as a finger print for identification purposes but usually the applications of this method are the determination of phase diagrams, heat change measurements and decomposition in various atmospheres. DTA is widely used in the pharmaceutical and food industries. DTA may be used in cement chemistry, mineralogical research and in environmental studies. DTA curves may also be used to date bone remains or to study archaeological materials.
In recent years, plant derived products are increasingly being sought out as medicinal products, nutraceuticals and cosmetics and are available in health food shops and pharmacies over the counter as self medication or also as drugs prescribed in the non-allopathic systems . Herbal medicines widely used in health-care in both developed and developing countries are complex chemical mixtures prepared from plants and are limited in their effectiveness because they are poorly absorbed when taken orally. According to an estimate of the World Health Organization (WHO), about 80% of the world population still uses herbs and other traditional medicines for their primary health care needs Thermal analysis is used to characterization of the materials,Physical or chemical changes in various products including herbal drugs and also used to study pre formulation or drug excipient compatibilityThermogravimetric analysis (TGA/ DTG),
TGA may be operated under subambient conditions to analyse ethanol in herbal formulations such as asavas and arista Differential thermal analysis (DTA / DDTA)
DTA analysis of mercury based Indian traditional metallic herbal drug Ras-sindoor indicated the presence of mercury sulphide based on a sharp peak at 354o C which corresponded to melting temperature of mercury sulphide. The optimized extraction obtained by distillation showed the presence of volatile oil in dry ginger as a component of volatile oil-beta-cyclodextrin inclusion compound using DTA .
Differential scanning calorimetry (DSC / DDSC)
DSC thermograms data confirmed the formation of phospholipid complex with emodin (an anthraquinone) and naringen .