Etiket Arşivleri: Precipitation Equilibria

Gravimetric Analysis and Precipitation Equilibria

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


Gravimetric Analysis and Precipitation Equilibria

GRAVIMETRIC METHODS OF ANALYSIS TYPES

1. Precipitation gravimetry (oldest)

2. Electrogravimetry

3. Volatilization gravimetry and Thermogravimetry

4. Gravimetric titrimetry

5. Particulate gravimetry

When signal is mass of a precipitate, the method is called precipitation gravimetry. For example, determination of Cl– by precipitating it as AgCl.

Electrogravimetry: the analyte is deposited on one electrode in an electrochemical cell. For example

• oxidation of Pb2+, and its deposition as PbO2 on a Pt anode or

• reduction of Cu2+ to Cu and its electrodeposition on a Pt cathode, for direct analysis for Cu2+.

Volatilization gravimetry: when thermal or chemical energy is used to remove a volatile species. For example, determining moisture content using thermal energy to vaporize H2O. Also carbon and hydrogen in an organic compound may be determined by combustion with O2 to CO2 and H2O.

Gravimetric titrimetry, mass of titrant intead of its volume is measured. (Mass measurements are much more accurate and precise)

Finally, in particulate gravimetry the analyte is determined following its re-moval from the sample matrix by filtration or extraction. The determination of sus-pended solids is oneexample of particulate gravimetry.

Precipitation Gravimetric Analysis

• Gravimetric Analysis – one of the most accurate and precise methods of macro-quantitative analysis.

• Analyte is selectively converted to an insoluble form and precipitated quantitatively from its solution.

• Precipitate is treated to make it easily filterable and then filtered, dried and finally its mass is measured.

• Analyte mass is then calculated on the basis of the chemical composition of ppt and its mass.

Why use gravimetric analysis?

– Conducted with simple apparatus.

– Interpretation of results is easy – readings are directly related to analyte amount.

– Provides very accurate and precise results – in fact gravimetric results are used to check the accuracy of other methods.


Source: http://www.emu.edu.tr/mugarip/CHEM247/LECTUREPPT/C12%20GRAVIMETRIC%20ANALYSIS.pdf