Gravimetry

Chapter 9

 Gravimetry

 Introduction

• Gravimetric analysis involves measurement of weight of the substance to analyses from a solution after isolating by precipitating the component as an insoluble compound of known chemical composition. The method is a quantitative analysis method by weight.

 There are different ways through which the separation can be achieved which are given below:

1. Precipitation method

2. Volatilization or evolution method  

3. Electro - analytical method  

4. Miscellaneous physical methods 

• In precipitation method, the analyte is suitably converted into precipitate which is insoluble and separated from solution by filtration. This product is then weighed and used for calculation. In this, the loss in mass of the sample can also act as a parameter for calculation. 

Principle and Steps Involved in Gravimetry

Principle: As already described, the principle involved in gravimetry is the quantitative estimation of component on the basis of measurement of mass. More precisely, the mass of an ion in a pure compound can be determined using gravimetry which is then applied to determine the mass percent of the same ion in a known quantity of a sample or impure compound.

• Preparation of a solution containing a known weight of the sample
• Separation of the desired constituent
• Weighing the isolated constituent 
• Computation of the amount of the particular constituent in the sample from the observed weight of the isolated substance

Steps Involved in Gravimetry: 

The steps involved in practice of Gravimetric analysis are explained below:

1. Sample preparation.

2. Preparation of solution or dissolution

3.Precipitation

4. Testing the completeness of precipitation

5. Digestion or Ageing of precipitate

Purity of Precipitate 

Coprecipitation

• In Gravimetric analysis, using precipitation method, it is generally observed that the precipitate which is formed is always not in pure state. It will be accompanied by some impurities or extraneous matters even after certain treatments such as thorough washing. The presence of impurities in the precipitate is dependent upon the condition of experiment adopted, nature of the analyte used and precipitate. In co-precipitation, an impurity or impurities which are precipitated along with analyte are normally not expected to be precipitated in the absence of the analyte or in general conditions. That means those impurities precipitate or co-precipitate only in presence of the analyte. This process is said to be co-precipitation.

There are four types of co-precipitations:

1. Surface adsorption

2. Occlusion

3. Mixed-crystal formation Mechanical Entrapment

 Post Precipitation

• Sometimes when the precipitate is allowed to stand in presence of mother liquor, the second substance will form the precipitate with precipitating reagent, this is called post precipitation. It generally occurs with sparingly soluble substances which form super- saturated solutions. They usually have an ion in common with primary precipitate, e.g., when calcium oxalate is precipitated in presence of magnesium ions, magnesium oxalate does not precipitate immediately because it tends to form super-saturated solution, but if this precipitate is allowed to stand for longer time before filtration, magnesium oxalate separates out on precipitate of calcium oxalate. Longer the precipitate remains in the solution, greater will be the error due to post precipitation.

Post precipitation differs from that of co-precipitation in the following aspects:

(a) The extent of contamination increases with the time of contact with mother liquor in case of post precipitation, while in co-precipitation it decreases contamination.

(b) The extent of contamination increases with faster agitation by mechanical or thermal means in case of post precipitation while reverse is the case in co-precipitation.

(C) Magnitude of contamination by post precipitation is much greater than that by co-precipitation.

• Digestion: When a precipitate is allowed to stand in the presence of mother liquor, i.e., the solution from which it is precipitated, the large crystals grow at the expense of small ones, and this process is termed as digestion or Ostwald’s ripening. In this, small particles tend to dissolve and re-precipitate on the surface of larger crystals in addition to individual particles agglomerate to share a common counter-ion layer and finally cement together to form large crystals.

 Estimation of Barium Sulphate

The precipitated barium sulphate is separated and weighed. The mass of Barium in the whole of the given solution is calculated knowing that 233.36 g of barium sulphate contains 137.36 g of barium

Procedure: 

• The given barium chloride solution is made up to 100 ml in a standard flask. From this20 ml of solution is pipetted into a 250 ml betake.
•  To this solution, about 5 ml of 2 N HCI is added and diluted to 150 ml with distilled water.

• The solution is heated to boiling and a hot solution of 4 N H2SO4 (10 — 15 ml) is added drop by drop with constant stirring, till the precipitation is complete.

Calculation:

Mass of crucible + Lid = a g

Mass of crucible + Lid + Barium sulphate = b g

Mass of Barium sulphate = (b — a) g.

233.36 of barium sulphate contain 137.36 g of barium.