Gravimetry

1. Gravimetry

2. Jons Jacob Berzelius (1779 - 1848),
considered the leading chemist of his time,
developed much of the apparatus and many
of the techniques of 19th century analytical
chemistry. Examples include the use of
ashless filter paper in gravimetry, the
use of hydrofluoric acid to decompose
silicates, and the use of the metric system
in weight determinations. He performed
thousands of analyses of pure compounds
to determine the atomic weights of most of
the elements known then. Berzelius also
developed our present system of symbols
for elements and compounds.
A History of Gravimetric Analysis

3. A History of Gravimetric Analysis
Theodore W. Richards (1868 - 1928)
…and his graduate students at Harvard
developed or refined many of the techniques
of gravimetric analysis of silver and
chlorine. These techniques were used to
determine the atomic weights of 25 of the
elements by preparing pure samples of
the chlorides of the elements,
decomposing known weights of the
compounds, and determining the chloride
content by gravimetric methods. From this
work Richards became the first American to
receive the Nobel Prize in Chemistry in
1914.

4. Analytes: The constituents of interest in a sample.
Matrix: All other constituents in a sample except for the analytes.

5. Types of Gravimetry
Physical Gravimetry: Physically separated and categorised on the
basis of volatility and particle size; Example: Environmental
samples
Thermogravimetry: A substance is kept at a controlled
temperature and its mass is measured with respect to
temperature. Example: Pharmaceuticals, food, volatile solids
Electrodeposition: Electrochemical reduction and simultaneous
deposition of metal ion occurs at cathode; cathode is weighed
before and after the process; difference in the weight of
corresponds to the mass of the analyte; Example: Environmental
samples
Precipitative Gravimetry: Involves chemical precipitation of an
analyte

6. How to Perform a Successful Gravimetric Analysis
• Steps involved
1. Sampling
2. Preparation of the solution
3. Precipitation
4. Digestion or Ostwald’s ripening
5. Filtration
6. Washing
7. Drying or igniting
8. Weighing
9. Calculation

7. Gravimetric Analysis
• Gravimetric Analysis – one of the most accurate and
precise methods of macro-quantitative analysis.
• Analyte selectively converted to an insoluble form.
• Measurement of mass of material
• Correlate with chemical composition
• Simple
• Often required for high precision

9. Common Desiccants
Mechanism of Action
Hydration ANHYDRONE® (Magnesium
Perchlorate anhydrous),
CaCl2, MgO, MgSO4, K2CO3,
KOH, Drierite, Na2SO4
(anhydrous), H2SO4, ZnCl2
Absorption and/ or
Adsorption
BaO, CaSO4, Molecular
Sieve, H3PO4, NaOH Pellets
Chemisorption CaO, P2O5

10. • Silica gel goes from blue to
pink as it absorbs moisture
Can be regenerated in oven
• Anhydrous sodium sulfate
gets clumpy as it absorbs
water

12. GRAVIMETRIIC ANALYSIS (OR)
QUANTITATIVE ANALYSIS BY WEIGHT
The substance to be determined by converting
in to pure chemical compound then weighed.
For conversion of compound to pure compound
to follow various technique.
Eg: Precipitation methods by precipitating
agent. Volatilisation methods or ignition
Electro gravimetry
Thermo gravimetry.
The substance to be measured at the pure from
without chemical change by gravimetry.

13. ADVANTAGE
It is accurate and precise.
Possible source of error and readily checked.
To involve direct measurement of substance.
Need only inexpensive apparatus.

14. PRINCIPLE
It is process to isolating and weighing an pure
compound from the element. The separation
of the element or the compound by following
ways:
1.Precipitation methods
2.Volatilisation - ignition or drying.
3.Electro gravimetry.

15. PRECIPITATION
Weighed quality of sample dissolved in suitable
solvent to solution form.
Precipitated by precipitation agent
Filter –dried – weighed – calculate the % of
content.

21. PRECIPITATION
Precipitation are usually made by using beakers.
Beakers is covered by clock glass.
To add precipitating reagent (by using pipette or burette) by
slowly with sufficient stirring.
To avoid the splashing by adding precipitating at moduate speed.
The stirring rod should not touch the surface or walls of the
beaker.
Then allow the precipitate settle overnight or by heating the
precipitate.
To reduce the coarseness of the precipitate.
Filter
Collect the precipitate

22. CO-PRECIPITATION
The soluble compounds from solution
removed during precipitation.
The soluble substance present in mother
liquor in solution form.
TYPES ( 4 TYPES ):
Surface Adsorption
Mixed Crystal Formation
Occlusion
Mechanical Entrapment.

23. SURFACE ADSORPTION
By using surface adsorption process to get the co-precipitation.
eg: coagulated colloids.
The reaction between AgNo3 in determination chloride ion to get Agcl as colloids and also
the silver ions adsorbed with nitrate or other anions.

24. 2.MIXED – CRYSTAL FORMATION:
It is type of co-precipitation. The contaminate ion replaced by ion containing crystal.
Eg: Barium sulphate formed by adding BaCl2 to a solution containing sulphate, lead,
and acetate.

25. 3.OCCLUSION OR MECHANICAL
ENTRAPMENT:
When crystal is growing rapidly during precipitation the formation is
affected by mechanical entrapment or pressure disturbance.
Digestion process is helpful to avoid the co-precipitation process.

26. POST PRECIPITATION
The precipitation and occurs on the surface of the first precipitation after its
formation.
Eg: Formation of calcium as calcium oxalate in the presence of magnesium, but it
produce presence of magnesium oxalate gradually, but it produce error in determine.

27. Difference between co-precipitation
and post precipitation:
1. In Post precipitation, contamination or impurities
increases as longer the precipitate is in contact with
the mother liquor in post precipitate; in
coprecipitation contamination is less
2. The contamination increases in post precipitate by
(mechanical or thermal process.) digestion but in co-
precipitate-contamination decreases
3. The magnitude of contamination by post precipitate
much be greater than the co-precipitate.

29. FILTRATION
To separate the precipitate by using
filteration process.
Filter paper, sintered crucible or gooch
crucible.
FILTER PAPER:
Ashless filter paper
Whatmann filter paper.

30. FILTRATION BY APPARATUS:
1.Gooch crucible either porcelaim or silicon in
avaliable market.
2.Sintered glass or silica crucibles.
GOOCH CRUCIBLES:
Asbestos pad
Perfoated
plate
Particular pore
Size plate
flask
Gooch crucibles

35. Objective:
To estimate the amount of barium in the whole of the given solution
of barium chloride.
The Gravimetric Estimation of Barium:
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 then treated with dilute sulphuric acid and barium precipitated
as 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.
Estimation of Barium Sulphate using Gravimetry

36. Procedure:
•The given barium chloride solution is made up to 100mL in a standard flask.
•20mL of solution is pipetted into a 250 mL beaker.
•About 5mL 2N HCl is added and diluted to 150mL with distilled water.
•The solution is heated to boiling and a hot solution of 4N H2SO4 (10 - 15mL) is added
drop by drop with constant stirring, till the precipitation is complete.
•The solution containing the precipitate is heated in a water bath for 5 minutes.
•The precipitate is allowed to stand for an hour.
•The clear solution is decanted through an ashless filter paper (whatman No. 40).
•The precipitate is washed with hot distilled water to free sulphate ions.
•The particles adhering to the sides of the beaker and glass rod are removed by a
policeman.
•Finally the precipitate is washed once again.
•The dried filter paper is folded and placed in a crucible which has been previously
weighed.
•The filter paper with the precipitate is first incinerated on a Bunsen burner by a low
flame and then transferred to an electric burner.
•The crucible is transferred to desiccator and cooled.
•When cold, the crucible is weighed.
•Heating, cooling and weighing are repeated till concordant values are obtained.

37. Calculation:
Mass of crucible + lid = a g
Mass of crucible + lid + Bariumsulphate = b g
Mass of Barium sulphate = (b-a) g.
233.36 of barium sulphate contain 137.36 g of barium.
Mass of barium in (b-a) g of Bariumsulphate =
Therefore, Mass of Barium in the whole of the given solution =
Result:
Mass of Barium in the whole of the given solution = ------------ g