This document discusses gravimetric analysis methods. It defines gravimetric analysis as isolating and weighing an element or compound in pure form to determine the quantity present. The main types discussed are precipitation gravimetry, electrogravimetry, and volatilization gravimetry. Precipitation gravimetry, the formation of an insoluble precipitate, is explained in detail including factors that influence successful precipitation and purity of the precipitate. Advantages include high precision and accuracy, while disadvantages include being time-consuming and requiring clean glassware and accurate weighing. An example of barium chloride estimation by precipitating and weighing barium sulfate is also provided.

1. Mr. SAJID HUSSAIN
(Asst. Professor)
S.R. Institute of Pharmacy
Bhuta, Bareilly (UP)

2. Definition
• It is the process of isolating and weighing an element or a
definite compound of the element in as pure form as
possible.
• It is concerned with the transformation of the element
or radical into a stable form.
• Pure compound can be readily converted into a suitable
form for weighing.
• It is time consuming but are reliable.
• Quantitative estimation by weight.

4. Types of Gravimetric Methods
Precipitation Gravimetry
A gravimetric method in which the signal is the mass of a precipitate. E.g.
direct determination of Cl– by precipitating AgCl
Electrogravimetry
A gravimetric method in which the signal is the mass of an electro deposit on
the cathode or anode in an electrochemical cell.
The oxidation of Pb2+, and its deposition as PbO2 on a Pt anode is one
example of electrogravimetry.
Volatilization Gravimetry
A gravimetric method in which the loss of a volatile species gives rise to the
signal. e.g. In determining the moisture content of food, thermal energy
vaporizes the H2O.

5. Precipitation Gravimetry
Precipitation gravimetry is based on the formation of an insoluble compound
following the addition of a precipitating reagent, or precipitant
[precipitant - A reagent that causes the precipitation of a soluble
species.], to a solution of the analyte.
Any reaction generating a precipitate can potentially serve as a
gravimetric method.
STEPS INVOLVED IN GRAVIMETRIC PRECIPITATION
• Preparation of the sample solution
• Precipitation process
• Digestion (or) Ostwald ripening
• Filtration
• Washing
• Drying
• Igniting
• Weighing
• Calculation
• Digestion is a process keeping the precipitate
within the mother liquor (or solution from which
it precipitated) for a certain period of time to
encourage densification of nuclei.
• During digestion, small particles dissolve and
larger ones grow (Ostwald ripening).
• This process helps produce larger crystals that
are more easily filtered from solution.

6. Factors determining successful Precipitation
• The precipitate must be of low solubility, high purity, and of known
composition.
• The precipitate must be in a form that is easy to separate from the reaction
mixture.
• Another important parameter that may affect a precipitate’s solubility is the
pH of the solution in which the precipitate forms
• Solubility can often be decreased by using a non-aqueous solvent.
• A precipitate’s solubility is generally greater in aqueous solutions because of
the ability of water molecules to stabilize ions through solvation.
• The poorer solvating ability of nonaqueous solvents, even those that are
polar, leads to a smaller solubility product.

8. • Saturation- The amount of salt dissolved is maximum under certain condition
of pressure and temperature.
• Supersaturation- The amount of salt is larger than at saturation step,
turbid solution. If the substance dissolved in excess. Super saturation is an
unstable state which is brought to a state of stable equilibrium by the addition of
crystal of solute usually termed as seeding.
Precipitate Formation Stages

9. Nucleation- When a small number of ions ,atoms, molecules initially unite either
spontaneous or induced resulting in very small aggregates of a solid during
precipitation Individual ions/atoms/molecules coalescence to form “nuclei (small
particles come together to form colloid particles)
Effects of Increase nucleation
• Initial nucleus will grow by the deposition of their precipitate particles
• To form a crystal of certain geometric shape.
• Again the greater super saturation
• More rapid crystal growth rate & colloidal precipitate formed
• Increase the growth rate increased chance of imperfection in the crystal &
surface area of precipitate increase this leads to easy trapping of impurities.

10. Nature of Precipitate
Physical nature of PPt will be determined by relative rates of nucleation and particle
growth
Particle growth: The three dimensional growth of particle nucleus converted
into a larger crystal

11. Post Precipitation- foreign compound precipitate on the top of
desired precipitate. E.g post precipitation of magnesium oxalate
occurs if a precipitate of calcium oxalate is allowed to stand too
long before being filtered.
• Deposition of impurity after ppt
Ex. Ca-oxalate ppt in presence of Mg ions
• After some time, Mg-oxalate formed, deposits on Ca-oxalate
surface
• Can be avoided by filtering within 1 – 2 hrs after ppt
Purity of the precipitate

12. Co-precipitation- is a process where the impurity is precipitated along with
the desired precipitate, even though the solubility of the impurity has been
not exceeded.
Inclusion of impurity during ppt formation
Adsorption:
• Adsorption of impurities at surface
• More for colloidal particles
• PPt have to be washed to remove adsorbed impurity
Mixed crystal contamination:
• Substitution of impurity ions in crystal lattice
• Impurity ions have similar crystallinity
• Ex, Ppt of Ba as BaSO4 in presence of Pb ions
• Separate analyte from contaminating ions before ppt
Occlusion: occurs when foreign ions get trapped in growing crystal
Mechanical Entrapment:
• several crystals growing together come close
• Traps portion of solution between pockets
Both occlusion and entrapment are at minimum when rate of ppt is low

13. Advantages
• Extensive numbers of inorganic ions are determined with excellent
precision and accuracy.
• Routine assays of metallurgical samples
• Relative precision 0.1 to 1%
• Good accuracy, relatively in-expensive
Disadvantages
• Careful and time consuming
• Scrupulously clean glassware
• Very accurate weighing
• Co-precipitation
Advantages & Disadvantages

14. The given barium chloride solution is made up to a definite volume. A measured volume
of it is then treated with dilute sulphuric acid and barium precipitated as barium
sulphate.
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.
Test Procedure:
• 100 ml of Barium chloride solution is taken in a conical flask and a few drops of methyl
red is added after adjustment of pH at 4.5-5.0 by HCl soln. (indicator colour changed to
orange).
• The solution is boiled and dilute sulphuric acid is added in excess slowly until
precipitation is complete.
• The precipitate is heated at 80-90°C for at least 2 hours or more.
• The precipitate is filtered through filter paper (What-man No. 42) and washed by warm
dist. water until the precipitate is free from chloride. This is determined by testing the
washed material with AgNO3 soln.
• The precipitate is dried on filter paper and ignited in a crucible at 800°C for 1 hr.- then
cooled and weighed.