Gravimetry

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Gravimetry

INTRODUCTION

Theodore W. Richard first proposed the technique of gravimetric analysis for the determination of chlorine and silver ions. This method is a quantitative analytical method which is done by a process of precipitation, isolation and weighing of the isolated product. It is mainly based on the measurement of the analyte.

PRINCIPLE

The principle involved in this method is that the sample is dissolved in a solvent and then the precipitating agent is added. The resulting precipitate is filtered dried and weighed.

The separation of the sample is carried out by the following principle methods:

• Precipitation method: In this method the sample is completely precipitated.
• Volatilization method: In this method the sample is completely volatilized.
• Electro-analytical methods: In this method the sample deposited on an electrode.
• Extraction and chromatographic methods: Here, the sample is separated from the matrix after precipitation.

THEORY

The procedure is nothing but the sample is dissolved in the appropriate solvent and then the excess amount of the precipitating agent is added. Followed by the formation of the precipitate, it is filtered through the filter paper. The obtained precipitate is commonly known as residue. This residue is dried and weighed.

HCl + AgNO3 AgCl (ppt.) + HNO3

Ag+ + Cl− AgCl (ppt.)

The principles involved in the gravimetric analysis are the following:

1. Law of mass action: The law of mass action states that the rate of reaction is directly proportional to the concentrations of the reacting substances.
3. BaCl2 + H2SO4 BaSO4 + 2HCl
5. Example:
6. Therefore,
8. Rate of reaction = [BaCl2] × [H2SO4] × k
10. where k1 is the constant for the concentrations.
11. Reversible reactions: This is same as that of the above-mentioned reaction. The reversible reaction is also having the rate of reaction which is given by the following equation:
13. Rate of reaction = [BaCl2] × [H2SO4] × k1
15. where k1 is another constant for the concentrations.
16. At the equilibrium, [BaCl2] × [H2SO4] × k = [BaSO4] × [HCl] × k1
18. 
20. where K is the equilibrium constant.
21. Solubility product: This is mainly useful for the difficultly soluble salts, precipitation and dissolution of the substances. The solubility product is defined as the product obtained by the dissociation of the practically insoluble salts.
23. BaSO4 Ba+2 + SO4−2
25. Then the solubility product is given by the following equation:
27. BaSO4 = [Ba+2] × [SO4−2]
29. Common ion effect: When BaCl2 is added to the H2SO4, the sulphate ion exceeds and the barium sulphate is precipitated.
31. Ba+2 + SO4−2 BaSO4
33. At the equilibrium, the barium ions concentration is equal to that of the sulphate ion concentration.
35. 

which is equal to the ionization constant.

The steps involved in the gravimetric analysis are the following:

1. Preparation of the sample solution.
2. Separation of the desired constituent as precipitate.
3. Weighing of the isolated constituent.
4. Calculating the amount of the particular constituent in sample from the observed weight.
6. 
8. Steps involved in Gravimetry
10. The Von-Weimar ratio or relative super saturation: RS = Q – S/S, where Q is the concentration of the solute and S is the equilibrium solubility.
11. The amount of the sample is given in the following expression of percentage of sample:
13. 

MECHANISM OF THE PRECIPITATION

The following are the steps involved in the mechanism of precipitation:

1. Induction period: The addition of precipitating agent to the sample solution is known as the induction period of time.
2. Nucleation: In this step, the nuclei of the atoms from the solution form clamps. For example, HCl is added slowly to the AgNO3 solution and the AgCl precipitate is formed where the Ag+ ions are attached around the Cl– ion. The primarily adsorbed Ag+ ions are again aggregated by the nitrate ions and form the AgCl nucleus.
3. Digestion: Growth of the larger nuclei or crystallites is increased by the digestion process. It involves the heating of the solid and mother liquor for a constant period of time. This method is also known as Ostwald's ripening.
5. 
7. Fig(2) Gravimetric curve

Approaches for the Homogenous Precipitation

There are four types of approaches for obtaining the homogenous precipitation.

1. Slow hydrolysis of the ester or other compounds: Anions are generated in this process.
2. Example: Phosphates generated from meta-phosphoric acid are readily precipitated to the insoluble phosphates.
4. R3PO4 + 3H2O ROH + 3H+ + PO4
6. By raising the pH of the solution
7. By synthesising the reagents: The main advantage is to improve the nature of the precipitates when the reagents are synthesised.
8. Example: Precipitation of nickel ion by synthesising dimethylglyoxime.
10. 
12. By changing the oxidation state of the complex.

Washing of the Precipitate

Washing of the precipitate is mainly carried out to remove the impurities contaminating the precipitate. Commonly water is used as the wash liquid.

Examples:

1. Calcium oxalate is washed with the aqueous solution of the ammonium oxalate.
2. Lead sulphate is washed with the dilute sulphuric acid.

Washing solution should posses the following properties:

• should able to remove the impurities easily,
• should not hydrolyze the precipitate,
• should be highly volatilisable at high temperatures, and
• should not carry any interfering substances.

Drying of the Precipitate

The precipitates are dried at 103–110 °C in hot air oven for 1 h. Then allow the precipitate to cool to the room temperature in desiccators. Weigh the residue and repeat the procedure until a constant weight is obtained.

TYPES OF GRAVIMETRY

1. Precipitation gravimetry: In this method, the sample is determined by the formation of the precipitate.
2. Example: (a) Determination of the calcium in neutral water.
4. 
6. (b) Dimethylglyoxime for nickel determination.
7. Volatilization method: This is the method for the separation of the volatile samples by the thermal or chemical energy.
8. Example: Determination of the sodium bicarbonate in antacid tablets.
10. 
12. Electrogravimetry: In this method the sample is deposited as a solid film on the electrode in the electrochemical cell.
13. Example: Deposition of the copper ions on the platinum cathode.
14. Particulate gravimetry: In this method the sample is determined by the filtration or by the extraction.
15. Example: Determination of total suspended solids.

IMPURITIES PRESENT IN THE PRECIPITATES

1. Coprecipitation: Along with the main precipitate, it also forms the coprecipitate. Washing and filtration avoid the coprecipitation. It occurs when unwanted ion or molecule is trapped in the precipitate. This coprecipitation is generally occurred by the precipitate of substance under the specific conditions. There are main procedures for the coprecipitation. They are as follows:
• Surface adsorption
• Mixed crystal formation
• Occlusion
• Entrapment
2. The coprecipitation of salts has low solubility and increases the quantity of the precipitation. This can be removed by removing the impurities.
3. Super saturation: In this precipitation, the solution contains the excess concentration of the solute. This is an unstable state. This error can be minimised by the addition of other solute or by mechanical stirring.
4. Post-precipitation: This can occur by the contamination of mother liquor. The post-precipitation makes the precipitate impure. Post-precipitation is caused by the super saturation of the supernatant liquid.
5. Example: Precipitation of calcium as oxalate in the presence of magnesium. The magnesium is coprecipitated with the calcium oxalate. The remaining magnesium is post-precipitated as magnesium oxalate.
6. The magnitude of contamination by post-precipitation is greater than the coprecipitation.
7. Surface adsorption: This can occur by the unwanted materials adsorbed to the surface of the main precipitate. Digestion decreases the surface area.

Criteria for the Gravimetric Analysis

1. The sample must be completely precipitated.
3. Ag+ + Cl− AgCl (s)↓
5. The weighed substance should be of known composition.
6. The product should be pure and easily filtered.
8. HCl + AgNO3 AgCl ↓ + HNO3

ADVANTAGES

• Simple
• Cheap
• Specific
• Less time
• Accurate
• Precise
• Sensitive

DISADVANTAGES

• Coprecipitation
• Surface adsorption
• Occlusion
• Post-precipitation
• Not applicable for qualitative determinations
• Less selective

APPLICATIONS

• Used in the analysis of the ores.
• Example: Iron ores
• Method: The sample is treated with the excess of sodium carbonate. Then the residue is dissolved in the dilute HCl. Then the solution is evaporated to dryness for 1 h at 110 °C. Then the acid soluble substances are removed by the addition of the 10:1 ratio of water and HCl. Then filter through the filter paper. Wash the residue with the hot 2% v/v HCl followed by the hot water. The filtrate is evaporated and to this 50% v/v H2SO4 and 10 ml of hydrogen fluoride are added. Then evaporate the filtrate to get the constant weight. The weight directly represents the amount of the sample.
• Used in the analysis of industrial materials.
• Example: Paintings
• The thickness of the paintings is measured by the gravimetry. In this method, the part is weighed before and after painting which gives the thickness of the painting. Then, the thickness is calculated by the following equation:
• T = m × 10/d × A
• where m is the mg of the coating material, d is the density, A is the area.
• Used in the calibration instruments.
• Example: Balances calibration
• Used in the elemental analysis of the inorganic compounds.
• Examples: Ni, Cr, Al etc.
• Nickel is precipitated as the nickel dimethylglyoxime by the addition of alcoholic solution of glyoxime and the aqueous ammonia. Then the precipitate is filtered and the residue is dried to the constant weight.
• Used in the measurement of the essential elements in the plant foods.
• Used in the estimation of the pollutants in the air.
• Example: SO2.
• Used in the estimation of the SO2 in the soft drinks.
• The sample is initially titrated with the standard NaOH solution using methyl red as indicator and the end point is the yellow colour formation. To this add 10% BaCl2 solution is added to form white colour precipitate and filter the precipitate to collect the residue. This residue is dried at 110 °C to obtain the constant weight.
• Used in the estimation of the chloride ion in the water supplies.

REVIEW QUESTIONS

1. What are the different principles involved in the gravimetry?
2. Explain the steps involved in the gravimetry.
3. What is Von-Weimar ratio?
4. What is the mechanism involved in the gravimetry?
5. What are the different types of gravimetry?
6. What are the advantages of gravimetry?
7. List out the applications of gravimetry.