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9.gravimetric analysis

This document provides an overview of gravimetric analysis. It describes gravimetric analysis as a quantitative method that involves separating a substance into an insoluble compound and weighing it to determine the amount of the original constituent. The key steps are precipitation, filtration, washing, drying, ignition, weighing, and calculation. Gravimetric analysis is accurate and precise but also time-consuming and requires careful technique.

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GRAVIMETRIC ANALYSIS G. Nikitha, M.Pharmacy Assistant Professor Department of Pharmaceutical Chemistry Sree Dattha Institute of Pharmacy Hyderabad
CONTENTS  Introduction  Advantages  Disadvantages  Experimental Technique of Gravimetric Analysis  Reference
INTRODUCTION  Similar to the volumetric method, gravimetric analysis is used in the quantitative determination of substances.  Gravimetric analysis involves the separation of the constituents to be estimated in the form of an insoluble compound of known composition.  The insoluble compound is washed to free it from all impurities dried and weighed either as such or ignited to leave a residue of some other comound which is then weighed.  Now from its weight and known composition, the amount of the constituent in the given sample is calculated.  From this the purity of the original compound is found out.  Depending upon the nature of the sample, the results are expressed as percentage weight (w/w) or weight in volume (w/v)
ADVANTAGES  Accurate and precise: Gravimetric analysis is potentially more accurate and more precise than volumetric analysis.  Possible sources of errors can be checked: Gravimetric analysis avoids problems with temperature fluctuations, calibration errors, and other problems associated with volumetric analysis.  It is an ABSOLUTE method.  Relatively inexpensive
DISADVANTAGES  But there are potential problems with gravimetric analysis that must be avoided to get good results.  Proper lab technique is critical.  Careful and time consuming.  Scrupulously clean glassware.  Very accurate weighing.  Coprecipitation.
EXPERIMENTAL TECHNIQUE OF GRAVIMETRIC ANALYSIS The technique of gravimetric involves the following steps 1. Precipitation 2. Filtration 3. Washing 4. Drying 5.Ignition 6. Weighing 7. Calculation
PRECIPITATION In the gravimetric analysis the anion or Cation to be determined from a solution of the substance is in a form that is having a definite itself or which leaves a residue of a definite composition on ignitation NaCl+ AgNO3 AgCl + NaNO3 ppt
FILTRATION  The separation of the precipitate from the solution by filtration is carried out either filter paper, sintered crucible.  Different grades of filter paper have to be used for different precipitates. Ash- less filter papers are used routinely. The size of the filter is selected according to the bulk of the precipitate. The filter paper must be selected in such a way that the pores of filter paper must be smaller than the size of the particles of the precipitate.  The precipitate is separated by filtration either through Whatman’s filter papers because they are having very small ash valves. The diameter of filter paper may vary according to the bulkiness of precipitate. The bulky precipitate like aluminum hydroxide needs a large filter paper than dense precipitate ie barium sulphate. The Folded filter paper is kept in a funnel moistened with the water and pressed to expel the air. Then precipitate is transferred in a usual way and is separated.
WASHING AND DRYING Washing: The precipitate must be washed free of all soluble matter otherwise impurities will remain sticking to the precipitate. An important consideration in this connection is the choice of the washing liquid. Drying: After washing precipitates which are collected in filtering crucibles may be dried directly in a drying oven at 110-120C. The adsorbed water or solvent can be easily removed from the crystalline ppt by heating. Precipitates which are collected on filter papers are dried by placing the funnel on a metallic chimney kept on tripos stand.
IGNITION The air dried precipitates are then made to ignite in crucibles of platinum, silica, nickel, in a muffle furnace at a desired temperature. The temperature of ignition and period of ignition have been found to depend upon the nature of the precipitate. During ignition the filter paper is first of all carbonized and the temperature is slowly raised. If there exists risk of reduction of the precipitate by carbon a low temperature for incineration would be used. For Instance for barium sulphate, after ignition of precipitate it gets cooled and a drop of sulphuric acid is added. The residue is again ignited to a high temperature. This is called ash treatment.
WEIGHING The ignited samples in the crucible are cooled for a few minutes and then they are kept in a desicator, so that the moisture in atmosphere does not get absorbed by the residue. Then it is accurately weighed on balance.
CALCULATIONS In gravimetric determinations the result is generally calculated in terms of constituent in the sample. If the constituent is weighed in the form in which percentage is estimated the result is calculated by applying the following relation: P= Percentage of the constituent required w= weight of the constituent W= weight of the sample taken
REFERENCE  Pharmaceutical Chemistry -Inorganic Volume-1 by G. R. Chatwal.  Essentials of Inorganic Chemistry by Katja A. Strohfeldt.  Indian Pharmacopoeia.  M.L Schroff, Inorganic Pharmaceutical Chemistry.  P. GunduRao, Inorganic Pharmaceutical Chemistry, 3rd Edition  A.I. Vogel, Text Book of Quantitative Inorganic analysis.  Bentley and Driver's Textbook of Pharmaceutical Chemistry.
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