Etiket Arşivleri: GRAVIMETRIC ANALYSIS

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


Solution Stoichiometry

SOLUTION STOICHIOMETRY

– Previously, we dealt with comparing components of a chemical reaction by converting mass of substances to moles.

– Now we will consider solutions where we will need to convert volumes to moles to make comparisons.

GRAVIMETRIC ANALYSIS

Measure the concentration of solution by measuring the mass of a precipitate formed.

– Use grams of precipitate to find moles of solute.

– Divide by volume of solution to find concentration.

VOLUMETRIC ANALYSIS

Measure the concentration of solution by measuring the volume of solution need to find stoichiometric equivalence between reactants using a titration.

– Use volume of titrant to find moles of analyte.

– Divide moles of analyte by volume of analyte to find concentration.

– Acid/base reactions are commonly used but not always.

LIMITING REAGENT PROBLEMS

Recall the following about limiting reagent problems.

– To find limiting reactant, calculate number of moles of product formed from each number of moles of reactant.

– Limiting reactant will yield lowest number of moles produced.

….

Experiment-1 ( Gravimetric Analysis )

Gravimetric analysis : As the name suggests, gravimetric analysis involves very accurate weighing of a chemical as a means of working out something about that chemical. A common type of gravimetric analysis involves precipitation of a single component (usually an ion) from a known amount of a compound or mixture. This is accomplished by reacting a solution of the test compound with a solution containing an ion that forms an insoluble salt with the component you wish to test for.
…..

In this experiment the sample containing an alkali sulfate is dried, weighed and dissolved in dilute HCl.The precipitation was occur. Barium chloride solution is added in excess to precipitate barium sulfate, and the precipitate is digested in the hot solution. After the precipitate is filtered through a paper filter which is then ignited and completely ashed. From the weight of the sample and weight of the precipitate, the percentage of sulfate in the sample is calculated. The precipitation reaction is the following in below:

Ba2+ (aq) + SO42- (aq) —> BaSO4 (s)

Variations in the acidity, temperature, manner of addition of the precipitant and time of digestion markedly affect the filterability of the barium sulfate precipitate and the extent to which various foreign ions are coprecipitated. Foreign anions such as nitrate, chlorate and chloride are coprecipitated as their barium salts, and the ignited precipitate contains the salt or oxide as an additive impurity. The coprecipitation of chloride can be decreased by slow addition of the precipitant. Since nitrate and chlorate interfere even at low concentrations, they should be removed from the solution before precipitation. Foreign cations such as ferric iron, calcium and, to a lesser extent, the alkali metals are coprecipitated as the sulfates. These are substitutional impurities, and the magnitude of the error depends upon the differences between the weight of the foreign sulfate or oxide and the weight of an equivalent amount of barium sulfate. The presence of ferric iron can produce errors as high as 2% in the determination.