Gravimetric factor :
Steps involved in gravimetric Analysis;
1.Preparation of the solution
- Precipitation using dilute solutions to decrease Q
- Slow addition of precipitating agent to keep Q as low as possible.
- Stirring the solution during the addition of precipitating agent to avoid concentration sites and keep Q low.
- Increase solubility by precipitation from a hot solution.
- Adjust the pH in order to increase s but not a too much increase as we do not want to lose. Precipitate by resolution
- Usually, add a little exercise of the precipitation agent for quantitative precipitation and check for completeness of the precipitation
3. Digestion of precipitate
4. Washing and filtering precipitate
5. Drying and ignition
6. Precipitation from Homogenius solution
- Precipitation is carrying down by a precipitate of substances normally soluble under the condition employed.
- The gravimetric analysis, which consists of precipitating the analyte. And measuring its mass to determine its concentration/purity, precipitation is a problem because undesired impurities often coprecipitate with the analyzing resulting in excess mass.
- This problem can obtain mitigated by digestion. or by dissolving the sample and precipitating it again.
There are three main mechanisms of co precipitating
Inclusion occurs when the impurity occupies a lattice site in The crystal structure of the carrier resulting in a crystallographic defect. This can happen when the Ionic radius and charge of the impurity are similar to those of the carrier.
If the precipitation medium contains ions of the same charge and size as fun forming The crystal structure of the precipitate, this extraneous ion can replace an ion from the precipitate. In The crystal structure for example
In the precipitation of NH4 mg po4 in presence of K+ ammonium leaves The crystal magnesium Ammonium Phosphate and is replaced by potassium since both have the same charge inside
Occlusion occurs when an absorbed impurity gets physically trapped inside the crystal as it grows.
some constituents of the precipitation medium may be Trapped In The crystal structure resulting in positive/negative errors. The Trap materials can be water, analyte, ions, precipitating agent ions for other constituents in the medium.
The addition of precipitating agent and stirring may avoid occlusion but does not occur, Dissolution off precipitate and Re precipitation may have to be done.
An adsorbate is an impurity that is weakly bound to the surface of the precipitate
This always results in positive errors in gravimetric procedure
B. Post precipitation
In cases where there are ions other than analyte ions that form a precipitate with the precipitating agent but at a much slower rate than the analyte and if the precipitate of the analyte is left for a long time without filtration then the other iron starts forming a precipitate over the original precipitate leading to positive errors
eg. Precipitation of copper as the sulfide in presence of zinc
Estimation of Barium sulfate
When dilute sulphuric acid H2so4 is added to the dilute solution of Barium Chloride Bacl2 to a white precipitate of Barium sulphate Baso4 is formed
Bacl2 + H2So4 → Baso4 + 2 to Hcl
- pipette out of 25 ml of a given solution of Barium Chloride bacl2 in 500 ml beaker. add 0.5 ml of concentration H2S o4 and a hundred ml of distilled water
- Heat the resulting solution to boiling to this hot solution add dilute H2 S o4 solution dropwise with constant stirring until the precipitation is not complete
- Allow the precipitate to settle down and taste the supernatant liquid for complete precipitation now, filter the precipitate by decanting method through Whatman filter.
- Wash the precipitate 3 to 4 times with hot water and dry it by placing the Funnel in an oven after trying, transfer the filter paper containing precipitate to the pre constantly weighed crucible and ignite it till all the carbonaceous matter is not burnt off
- Now, cool the crucible and add one drop of each concentration HCL and concentration H2 s o4. School the crucible by placing it in a desiccator and weigh it
- Repeat the process of heating, ignition, cooling, and weighing till the constant weight is obtained.
- Precipitation should be carried out in dilute hot solution and in the presence of 0.05 N HCL, which helps to increase the size of the precipitate particles.
Let the constant weight of Barium sulphate (Baso4) precipitate be X g
Baso4 is equal to ba
233.42 g of Baso4 contains (137.36 × x/233.42 )g of ba2+ ions
Advantages of gravimetric analysis
- No instrumental error
- Do not require calibration
- Cheap. Do not require expensive equipment
- Very precise method
- The filtrate can be examined for completion of the reaction
Application of gravimetric analysis
- An extensive number of inorganic ions can be easily determined
- It is the most widely applicable analytical procedure
- A variety of organic substances can be easily determined like lactose in a milk product, cholesterol in serial, etc
- To determine the atomic mass of many elements
- Elemental analysis of the organic compound can be done the composition of the elements of a compound can be done
- The analysis of rocks, ores, soils, etc, and other inorganic samples for their major components can be carried out.