Gravimetric Analysis and Precipitation Equilibria

Dr. A.K.M. Shafiqul Islam & Dr. Zarina Zakaria

Introduction

The term gravimetric pertains to a Weight Measurement.

Gravimetric method is one in which the analysis is completed by a weighing operation.

Gravimetric Analysis is a group of analytical methods in which the amount of analyte is determined by the measurement of the mass of a pure substance containing the analyte.

Gravimetric Methods can also be defined as quantitative methods based on the determining the mass of a pure compound to which the analyte is chemically related.

Example for Precipitation:-

Calcium can be determined gravimetrically by precipitation of calcium oxalate and ignition of the oxalate ion to calcium oxide.

  Ca2+  +  C2O42- →CaC2O4

  CaC2O4 → CaO  + CO2  + CO

The precipitate thus obtained are weighed and the mass of calcium oxide is determined.

Example for Volatilisation:-

The analyte or its decomposition products are volatilised at a suitable temperature. The volatile product is then collected and weighed, i.e. the mass of the product is indirectly determined from the loss in mass of the sample.

Example

Water can be separated from most inorganic compounds by ignition, the evolved water can then be absorbed on any one of several solid desiccants. The weight of water evolved may be calculated from the gain in weight of the absorbent.

Gravimetric Analysis

Gravimetric analysis is potentially more accurate and more precise than volumetric analysis.

Gravimetric analysis avoids problems with temperature fluctuations, calibration errors, and other problems associated with volumetric analysis.

But there are potential problems with gravimetric analysis that must be avoided to get good results.

Proper lab technique is critical

Steps in a Gravimetric Analysis

1.Preparation of the solution

2.Precipitation

3.Digestion

4.Filtration

5.Washing

6.Drying or ignition

7.Weighing

8.Calculation

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