Gravimetric analysis is a subject that involves the separation of the constituents to be estimated in the form of an insoluble compound of known composition. The insoluble compound is washed to free it from all impurities, dried, and weighed either as such or ignited to leave a residue of some other compound which is then weighed. Now, from its weight and known composition, the amount of the constituent in the given sample is calculated.
Experimental technique of Gravimetric analysis
The different steps in the gravimetric analysis are summarised below:
1. Preparation of solution:
A small amount of substance whose gravimetric analysis is to be done is weighed accurately and is dissolved in a suitable solvent to get the solution of the sample.
2. Precipitation
The solution so prepared is diluted with distilled water and is heated gently. To the hot solution, a suitable precipitating reagent is added. The precipitate in gravimetric estimation should have the following characteristics:
- It must be highly soluble.
- The precipitate should be filterable.
- It must have a known stable composition itself,
- It should be of high purity.
Co-precipitation: The precipitate is generally found to be contaminated with substances that are soluble during the precipitation. This occurs due to absorption. This is called co-precipitation.
Post precipitation:
The phenomenon of precipitation of a foreign substance by delayed action after the primary precipitate is formed is known as post-precipitation. Actually, it is the phenomenon in which the impurities in the form of insoluble substances are precipitated on the surface of the first precipitate after its formation. The precipitate is often carried out in Pyrex or Borosil beaker free from scratches and provided with a watch glass cover and a stirring rod. The precipitate is added from a pipette.
Optimum conditions for precipitate
Following conditions tend to yield large particles and minimum contamination of precipitate:
- Dilute solution of reagent: Precipitation should be carried out from dilute solution. This removes the error due to co-precipitation.
- Slow addition of reagent : A dilute solution of precipitating reagent should be added slowly with constant stirring of the solution.
- Precipitation from hot solution : Precipitation is usually made in hot solution. This checks post precipitation and helps in coagulation and better growth of crystals of the precipitate.
- An excess of the reagent : A slight excess of the reagent is required to ensure complete precipitation.
- However, a large excess is to be avoided as it may lead to increase solubility of the precipitate.
- Re-precipitation : It is sometimes necessary to obviate post-precipitation. The precipitate first formed is filtered and dissolved in some acid and then precipitated again.
- Digestion of precipitate : It is always recommended to digest the precipitate before filtration by keeping it on the steam bath overnight.
3. Filtration
The separation of the precipitate from the solution by filtration is carried out either by filter paper, sintered glass crucible, or Gooch crucible.
- Filtration by filter paper : The precipitate is washed in the beaker and the wash liquid is transferred to the filter paper and the precipitate is washed again.
- Filtration by crucible : Sintered glass and Gooch crucible are unique type of crucibles which have porous base. The crucible is attached to the mouth of the filtration flask by the help of rubber padding. The flask has one outlet which is attached with the lower mouth of pump by rubber tube. The main mouth of filter pump is attached with the top. When fast current of water passes through pump, it takes some air of the filtration flask reducing its pressure which affect the quick filtration.
4. Washing
The precipitate must be washed to remove the undesirable ions sticking on the precipitate that may give a higher yield. An ideal wash liquid should have the following features:
- A common ion to reduce the solubility of precipitate.
- A volatile electrolyte to prevent peptisization of the precipitate.
- Ion to prevent hydrolysis of the precipitate.
Washing should be done several times. The technique involved in washing is very simple and the apparatus used are wash bottle, glass rod, beaker, funnel, etc.
5. Drying of the precipitate
The precipitate collected in filtering crucibles may be dried directly in a drying oven at 110-120°C.
6. Ignition and weighing
In the case of precipitates that have only to be dried before weighing or which do not require strong ignition, the ideal crucible is the Gooch or Sintered glass. Their constant weight is determined before filtration at the same temperature as it is to be used for the final treatment of the precipitate. After drying, crucibles are allowed to cool in a desiccator and then weighed with the precipitate. Heating, cooling, and weighing are repeated until the weight becomes constant. In this case, when the precipitate has to be ignited subsequently, filter papers are employed as filtering media and silica crucibles are used for igniting and weighing the precipitate. For this, the filter paper with dried precipitate is taken out from the funnel, and the precipitate as much as possible is collected on glazed paper and covered with an inverted funnel. White glazed paper is used for colored precipitates and black for white precipitates. Some portion of the precipitates remains to stick on the filter paper. This paper is ignited to remove carbon completely and then the precipitate is converted back with the help of a suitable reagent as it is mostly reduced in doing so. The whole operation is called ash treatment which is done as follows: The filter paper is folded and is held by means of tong and burnt completely in non-sooty flame. The ash obtained so is collected in silica crucible which is then heated strongly until the ash becomes white. After this, the affected precipitate is converted to its original form. For eg., some of the BaSO4 precipitate sticking to filter paper is converted to BaS after ash treatment. It is then treated with one drop of conc. HCl and finally 1-2 drops of conc. H2SO4 to convert it into BaSO4. Now, the whole precipitate is transferred from the gazed paper to the crucible and finally, it is weighed.
