Gravimetric analysis is based on the conversion of
ion(s)/elements(4) radicals (s) into a pure and stable compound after precipitation reaction; which can be directly weighed and quantified.
1. Analyte Solution will be taken.
2. Analyte will be precipitated by using a suitable precipitating agent.
3. Precipitate will be pure and stable
4. Precipitated compounds will be filtered dried and weighed accurately.
% of analyte = wt. of precipitate Χ Gravimetric factor Χ 100
weight of sample
Gravimetric factor :
Atomic mass of BaSO4 = 233.42G/mol
Atomic mass of ba2+ ion= 137.36g/mol
Gravimetric factor = 137.36g/mol / 233.62g/mol = 0.5885
Analysis can be done accurately A precisely by using sensitive and modern analytic balance.
Filtrate can be analyzed for completion of the precipitation reaction and errors can be reduced to a great extent.
Direct measurement (weighing) o the compound is done in gravimetric analysis So calibration is not required and it an absolute method.
– it is less expensive as a compound than another analytical method.
Steps involved in gravimetric Analysis;
In this technique, the analyte is converted to an insoluble which can be washed, dried, and weighed in order to determine the concentration of the analyte in the original solution.
gravimetry is applied to sample where the good precipitating agent is available
e precipitate should be quantitative, easily wash and filter, and is off in suitable quantity for an accurate weight.
Therefore gravimetry is regarded as a macro analytical technique.
Gravimetry is one of the few analytical methods that do not require a standard solution as the weight of precipitate it is the only important parameter in analyte determination.
After the appropriate dissolution of the sample, the following steps should be followed for the successful gravimetric procedure.
1.Preparation of the solution
This may involve several steps including adjustment of the pH of the solution in order for the precipitate to oca quantitatively and get a precipitate of desired properties removing interference adjusting the volume of the sample to suit the amount of precipitating agent is to be added.
This requires the addition of a precipitating agent solution to the sample solution upon addition of the first drops of the precipitating agent supersaturation occurs and then nucleation starts to occur where every few molecules of the precipitate.
Aggregate together forming A nucleus at this point, the addition of an extra precipitating agent will either form of your nuclei or are we’ll build upon existing nuclei to give a precipitate.
This can be predicted by the nonweimarn ratio where according to this relation the particle sizes inversely proportional to a quantity called the relative supersaturation
Relative supersaturation is equal to (Q – S)/S
Q= concentration of reactant before precipitation
S= solubility of a precipitate in the medium from which it is being precipitated.
In order to get particle growth instead of further nucleation, we need to make. the relatives’ supersaturation ratio as small as possible
The optimum condition for precipitation which makes the supersaturation are;
- 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
The precipitate is left hot (below boiling) for 30 minutes to 1 hour in order for the particles to be digested. Digestion involves the dissolution of small particles and Re-precipitation on larger ones resulting in a particle. Growth and better participation. Characteristics, this process is called colled ‘ostwald repeniing.
An important advantage of digestion is observed for colloidal precipitates where a large number of absorbed ions cover the huge area of the precipitate. Digestion forces the small colloidal particles to agglomerates which decrease their surface area and absorption.
You should know that absorption is a major problem in gravimetric in the case of colloidal precipitate since a precipitated tends to absorb it’s over ions present in excess; therefore forming what is called a primary iron layer which attracts ions from solution forming at 2% counter iron layer individual particles repel each other keeping the colloidal properties of the precipitate.
Particle coagulation can be forced by either digestion or the addition of a high concentration of diverse ions strong electrolytic solution in order to Shield the charges on colloidal particles and force agglomeration. Usually, angulated particles return to the colloidal state if washed with a water process called peptization.
4. Washing and filtering precipitate
It is crucial to wash the precipitate very well in order to remove it. all absorb bird species which will add to the weight of precipitate.
One should be careful not to use it. Too much water it seems part of the precipitate Mabel lost.
Also in the case of colloidal, we should not use water as a washing since precipitation would occur.
In such situations dilute nitric acid, Ammonium Nitrate, or Dilute Acetic Acid may be used.
Usually, it is a good practice to check for the president of the precipitating agent in the filtrate. Of the final washing solution. The presence, of precipitating Agent, means that extra washing is required.
Filtration should be done using ignition filter paper.
5. Drying and ignition
The purpose of cheating at about 120 to 150 ° C [ in an oven] ignition in a muffle furnace. at the temperature ranging from 800 to 1200 degrees, celsius is to get material with exactly known chemical structure so that the amount of analyte. Can be accurately determined.
6. Precipitation from Homogenius solution
In order to make Q minimum use can in some situations generate the precipitating agent in the precipitation medium rather than adding it.
For eg. In order to precipitate iron as the hydroxide, we dissolve. Urea in Hydroxide ions from the hydrolysis of Urea hydroxide ions are generated at all points in solution and thus there are no sides of concentration.
We can also adjust the rate of urea hydrolysis and thus controlled the Hydroxide generation rate. This type of procedure can be very advantageous in the case of coal ideal precipitates.
**Purity of precipitate
- 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.