Etiket Arşivleri: Gravimetric method

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


Determination of Calcium

PURPOSE:

             In this experiment, we will determinate the  value of SO4-2 in the sample solution. By this way we will become familiar with basic processes of gravimetric analysis.

THEORY:

             Gravimetric method is one of the three subgroups of quantitative analytical method. It is based on measuring the weight of a known substance which is related to the analyte and, by using the relation, determining the quantity.

          Gravimetric analyses divides into three subgroups; precipitation, volatilization, electro deposition. In this experiment we are going to use precipitation method.

          As we all know solubility varies for all compounds. There are such compounds that nearly insoluble. Precipitation method is based on this principle that precipitating a soluble analyte as insoluble compound of it and weighing the precipitation.

           In this experiment we are going to determine the SO4– quantity by precipitating it as BaSO4 for that BaSO4 has a solubility of 0.4 mg/100 g water. We are going to use BaCl2 and as
barium source.

                          Ba+2+SO4-2 à BaSO4(s)

We will separate BaSO4 crystalline precipitate by filtering and wash it. After drying and ignition we’ll weigh the pure BaSO4.

MATERIALS:

v BaCl2 solution (5  g / L).

v HCl solution.

v AgNO3 solution.

v Ashless filtering paper.

v Flask.

v Porcelain crucible.

v Beaker.

v Water.

v Bunsen burner.

PROCEDURE:

v 10 ml of sample was taken in a beaker.

v 1 – 2 drops or 1 ml HCI solution was added.

v This solution was heated until boiling.

v And 100 ml of (1,3 w / v) BaCI2 was heated until boiling an other beaker.

v Then BaCI2  was added quickly  into hot sample.

v Solution was waited and cooled. Until the top of solution became clean.

v After that solution was decanted.

v Solution was filtered and washed with hot water three times.

v Filteted solution was tested with AgNO3 solution. If color does not change.solution was waited one week to dry.

v One week later, filter paper and solid substance were put in  porcelain crucible. and they were chared off about one hour in bunsen burner.

v  Then porcelain crucible was transfered at oven and ignated at 800  0C

v finally,we had only solid (Ba2SO4 (s) ) and weighted.

v Results were recorded.

RESULTS:

Weight of sample and porcelain crucible =    12,7010 g

Weight of porcelain crucible                     =   11,4836 g

Weight of (Ba2SO4 (s) )                              =    1,2174 g

DISCUSSION:

            In this experiment we studied a gravimetrical method; precipitation. This method is so vital for ones who carry out analysis on food sector. It has many advantages as it is easy to carry out, cheap and needs only a little apparatus and reagents. But it has disadvantages as results are very vulnerable to personal or methodic mistakes so less trustable. And it takes nearly one day to get results. So it can be said to be time consuming.

         This method is easy method on the other hand, this method takes so much time.but in food factories, production rate is very important and personal mistakes change results very quickly.