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Various UV spectrophotometric Simultaneous

estimation methods

The quantitative estimation is the method to determine how much of each constituent is in the

sample1-2. Estimation of a given drug or medicine in the dosage forms needs the quantitative analysis of

that drug or medicinal in it. The first quantitative analyses were gravimetric, made possible by the

invention of a precise balance. It was soon found that carefully calibrated glassware made considerable

saving of time through the volumetric measurement of gravimetrically standardized solutions.

In contrast to above two techniques namely gravimetric and volumetric analysis, methods of drug

analysis today utilize rather sophisticated instruments involving simple physico-chemical properties.

In instrumental analysis, a physical property of a drug is utilized to determine its chemical composition.

A study of the physical properties of drug molecules is a prerequisite for product formulation and often

leads to a better understanding of the inter-relationship between molecular structure and drug action.

These properties may be thought of as either additive or constitutive such as mass is an additive

property. Many physical properties are constitutive and yet have some measure of additivity such as

specific gravity, surface tension and viscosity. Molar refraction of a compound is the sum of the

refraction of the atoms and groups making up the compound. The arrangements of atoms in each group

are different however and so the refractive index of two molecules will be different; that is the

individual groups in two different molecules contribute different amounts to the overall refraction of the

molecules. In the instrumental analysis some physical properties of molecules such as absorption of

radiation, scattering of radiation, Raman Effect, emission of radiation, rotation of the plane of the

polarized light and diffraction phenomenon are involving interaction with the radiant energy. Physicalproperties encompass specific relations between the molecules and well defined forms of energy e.g.

Half-cell potential, current voltage, electric conductivity, dielectric constant, heat of reaction, thermal

conductivity or other yardsticks of measurements. By carefully associating specific physical properties

with the chemical nature of closely related molecules. Conclusions can be drawn that (1) Describe the

spatial arrangement of drug molecules (2) Provide evidence for the relative chemical or physical

behavior of a molecule and (3) Suggest methods for quantitative and qualitative analysis of a particular

pharmaceutical agents3,4.

Analytical methods, in a broad sense, can be classified into chemical methods and instrumental

methods. Chemical methods are defined as those that depend on chemical operations in combination

with the manipulation of simple instruments. In general, the measurement of mass, i.e. gravimetric and

of volume, i.e., volumetric analysis falls in this class. An instrumental method encompasses the use of

more complicated instrumentation based on analytical methods:

Although in recent years, spectrophotometric methods are extensively used, but it would be wrong to

conclude that instrumental methods have totally replaced chemical methods. In fact, chemical steps are

often an integral part of an instrumental method. The sampling, dissolution, change in oxidation state,

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removal of excess reagent, pH adjustment, addition of complexing agent, precipitation, concentration

and the removal of interferences are the various chemical steps which are part of an instrumental

method. In recent years HPLC5 (High Performance Liquid Chromatography) is extensively used, because

HPLC is not limited by sample volatility or thermal stability. HPLC is able to separate macromolecules

and ionic species, labile natural products, polymeric material and a wide variety of other high molecular

weight poly-functional group because of the relatively high pressure necessary to perform this type of

chromatography; a more elaborate experimental setup is required.

Because of the high cost of the instrument and costly analytical process, small-scale industries cannot

afford to procure and use HPLC. So, in spite of other advantages of HPLC, we often select the

spectrophotometric method of analysis. The variation of the color of a system with change in

concentration of some component forms the basis of what the chemists commonly term as colorimetric

analysis. The color is usually due to the formation of a colored compound by the addition of an

appropriate reagent, or it may be inherent in the constituent itself. Colorimetry is concerned with the

determination of the concentration of a substance by measurement of the relative absorption of light

with respect to a known concentration of the substance. Colorimetric determinations are usually madewith a simple instrument termed a colorimeter.

In spectrophotometric analysis a source of radiation is used that extend into the ultraviolet region of the

spectrum. From this, definite wavelengths of radiation are chosen possessing a bandwidth of less than 1

nm. This process necessitates the use of a more complicated and consequently more expensive

instrument. All atoms and molecules are capable of absorbing energy in accordance with certain

restrictions; these limitations depend upon the structure of the substance. Energy may be furnished in

the form of electromagnetic radiation (light). The kind and amount of radiation absorbed by a molecule

depend upon the structure of the molecule, the amount of radiation absorbed also depend upon the

number of molecules interacting with the radiation. The study of these dependencies is calledabsorption spectroscopy.