# Gravimetric Analysis

OBJECTIVES

By the end of this lecture, students will be able to:

• Explain Gravimetric analysis

• Explain the principle involved in gravimetric analysis

• Discuss various co-precipitation techniques in Gravimetric analysis

• Explain drying and ignition of precipitate in Gravimetric analysis

• Brief the applications of gravimetric analysis

## Gravimetric Analysis

Quantitative method that is based on determining the mass of a pure compound to which the analyte is chemically related.

Gravimetric methods are of two types: Precipitation gravimetry Volatilization gravimetry

Precipitation gravimetry – Analyte is separated from a solution of the sample as a precipitate and is converted to a compound of known composition that can be weighed

Volatilization gravimetry – Analyte is separated from other constituents of a sample by converting it to a gas of known chemical composition Mass of the gas then serves as a measure of the analyte concentration

## Precipitation Gravimetry

• Analyte is converted to a sparingly soluble precipitate

• Precipitate is then filtered, washed free of impurities

• Converted to a product of known composition by suitable heat treatment and weighed

• For example, determination of calcium in water is one of official methods of Association of Official Analytical Chemists

• Excess of oxalic acid is added to an aqueous solution of the sample

• Ammonia is added to neutralizes the acid

• Also aid in the precipitation of calcium oxalate

• 2NH3 + H2C2O4 ———à 2NH4+ + C2O4

• Ca+2 + C2O4 ———à CaC2O4

• Calcium oxalate precipitate is filtered using a weighed filtering crucible

• Precipitate is then dried and ignited

• Ignition converts the precipitate to calcium oxide

• CaC2O4 ———à  CaO + CO + CO2

• After cooling, the crucible and precipitate are weighed

• Mass of calcium oxide is determined by subtracting the known mass of crucible

• Calcium content of sample can be determined by gravimetric analysis

Properties of precipitates and precipitating reagents

• Ideally gravimetrically precipitating reagent should react specifically or at least selectively with the analyte

• Specific reagents are rare, react only with a single chemical species

• Selective reagents which are more common react with limited number of species

Ideal precipitating reagent would react with the analyte to give a product that is

1) Easily filtered and washed free of contamination

2) Sufficiently low solubility that no significant loss of analyte occurs during filtration and washing

3) Unreactive with the constituents of the atmosphere

4) Known composition after it is dried or if necessary ignited

Few reagents produce precipitates that have all these desirable properties

Particle size and filterability of precipitates

• Precipitates consisting of large particles are generally desirable for gravimetric work

• Because these particles are easy to filter and wash free of impurities

• Precipitates of large particles are usually purer than precipitates made up of fine particles

Factors that determine the particle size of precipitates

• Particle size of solids formed by precipitation varies enormously

• One extreme are colloidal suspensions

• Tiny particles and are invisible to naked eye (10-7 to 10-14 cm in diameter)

• Colloidal particles show no tendency to settle from solution and are difficult to filtrate

• Other extreme are particles with dimensions on the order of tenths of a millimeter or greater

• Temporary dispersion of such particles in the liquid phase is called a crystalline suspension

• Tend to settle spontaneously and are easily filtered

• Mechanism of precipitate formation is not fully understood

• Particle size of precipitate is influenced by

• Precipitate solubility,

• Temperature,

• Reactant concentrations and

• Rate at which reactants are mixed

• By this, particle size is related to single property called relative supersaturation

• Relative supersaturation = Q-S/S

• Where Q is concentration of solute at any instant

• S is its equilibrium solubility

• Precipitation reactions are slow

• Even precipitating reagent is added drop wise to solution of an analyte, some supersaturation is likely

• Experimental evidence indicates that particle size of a precipitate is inversely proportional to relative supersaturation

• Q-S/S is large, precipitate tends to be colloidal

• Q-S/S is small, crystalline solid is more likely

• A supersaturated solution is an unstable solution that contains higher solute concentration

• Excess solute precipitates out with time, supersaturation decreases to zero

• Precipitates form by nucleation and particle growth

• If nucleation predominates large number of very fine particles is produced

• If particle growth predominates, a small number of larger particles is obtained

• To increase the particle size of precipitate, minimize the relative supersaturation during precipitate formation

• Experimentally methods to minimize supersaturation and produce crystalline precipitates include:

• Elevated temperatures to increase the solubility of the precipitate

• Dilute solutions

• Slow addition of precipitating agent with good stirring

## Colloidal Precipitates

• It is very difficult to filter the particles of a colloidal suspension

• To trap these particles, the pore size of the filtering medium must be so small that filtration takes a very long time

• With suitable treatment, individual colloidal particles Can be made stick together or coagulate to produce larger particles that are easy to filter

Coagulation of colloids

• Colloidal suspensions are stable

• Because all of the particles are either positive or negative charge and repel

• Can be observed by placing electrodes

• Process by which ions are retained on the surface of a solid is known as adsorption

• Can be hastened by heating, stirring and by adding an electrolyte to the medium

• Adsorption of ions on ionic solid originates from the normal bonding forces

• For example, a silver ion at the surface of a silver chloride particle has partially unsatisfied bonding capacity for anions

• Chloride ions at the surface of solid exert an analogous attraction for cations dissolved in the solvent

Peptization of colloids

• Peptization is a process by which a coagulated colloid returns to its dispersed state

• Coagulated colloid on washing- some of the electrolyte responsible for its coagulation is leached

• Repulsive forces will be reestablished and particles detach themselves

• Practical treatment of colloidal precipitates

• Best precipitated from hot, stirred solutions containing sufficient electrolyte to ensure coagulation

• Filterability of coagulated colloid often improves if allowed to stand for an hour or more in contact with hot solution

• Process is known as digestion

• Result is denser mass that is easy to filter

Crystalline precipitates

• More easily filtered and purified than coagulated colloids

• Particle size of crystalline solid can be improved by

• Using dilute solutions

• Adding precipitating reagent slowly with good mixing

• Adjusting pH of the solution

Digestion of crystalline precipitates often yields a purer and more filterable product

## Coprecipitation

Process in which otherwise soluble compounds are carried out of solution by a precipitate

Four types of coprecipitation:

ii) Mixed crystal formation

iii) Occlusion

iv) Mechanical entrapment

(i) And (ii) are equilibrium processes

(iii) and (iv) arise from kinetics of crystal growth

• Common source of coprecipitation

• Major source of contamination in coagulated colloids with large specific surface areas

• Effects on purity and are usually undetectable

• In adsorption, a normally soluble compound is carried out of the solution on the surface of a coagulated colloid

• Net effect of surface adsorption is carrying down of an otherwise soluble compound as a surface contaminant

• For example, coagulated silver chloride formed in gravimetric determination of chloride ion is contaminated with

• Primarily adsorbed silver ions and nitrate or other ions in counter-ion layer

• Result is silver nitrate, a normally soluble compound is coprecipitated with silver chloride

• Purity of coagulated colloids is improved by digestion

• In digestion, water is expelled from the solid to give a denser mass that has a smaller specific surface area for adsorption

• Washing a coagulated colloid with electrolyte solution improves purity

• For example, determination of silver by precipitation with chloride ion- the primarily adsorbed species is chloride

Washing with an acidic solution converts the counter-ion layer largely to hydrogen ions

• So that both chloride and hydrogen ions are retained by the solid

• Volatile HCl is then given off when the precipitate is dried

Reprecipitation

• Effective way to minimize the effects of adsorption

• Filtered solid is redissolved and reprecipitated

• First precipitate usually carries a fraction of contaminant

Mixed-crystal formation

• Is a type of co precipitation in which contaminant ion replaces an ion in the lattice of a crystal

For this exchange to occur, both the ions should have same charge and difference in size should be not more than 5%

• Both the salts should belong to same crystal class

• For example, barium sulfate formed by adding barium chloride to a solution containing sulfate, lead and acetate ions

• Can be contaminated by lead sulfate

• This contamination occurs even though acetate ions prevent precipitation of lead sulfate by complexing with lead

• In this case, lead ions replace some of the barium ions in the barium sulfate crystals

• It is a troublesome type of coprecipitation

• Because little can be done about it when certain combinations of ions present in a sample matrix

Occlusion and mechanical entrapment

• Occlusion is a type of coprecipitation in which a compound is trapped within a pocket formed during rapid crystal growth

• Foreign ions may get trapped or occluded in the rapidly growing crystal

• Amount of occluded material is greatest in the part of a crystal that forms first

• Mechanical entrapment occurs when crystal lie close together during growth

• Crystals grow together and trap a portion of solution in a tiny pocket

• Both occlusion and mechanical entrapment are minimum at low supersaturation

### Precipitation from homogenous solution

• Homogenous precipitation is a process in which a precipitate is formed by slow generation of a precipitating reagent homogenously throughout a solution

• Solids formed by homogenous precipitation are generally purer

• More easily filtered than precipitates generated by direct addition of reagent to the analyte solution

• Because precipitating agent appears gradually and homogenously throughout the solution and reacts immediately with the analyte

• Relative supersaturation is kept low during entire precipitation

• Urea is often used for the homogenous generation of hydroxide ion

• NH2CONH2 + 3H2O ————à CO2 + 2NH4+ + 2OH

• This hydrolysis process proceeds slowly at temperatures just 100 0C

• 1 to 2 hours needed to complete a typical precipitation

• Urea is particularly valuable for the precipitation of hydrous oxides or basic salts

•For example, hydrous oxides of iron (III) and aluminium formed by direct addition of base are bulky and gelatinous masses

• Heavily contaminated and difficult to filter

• In contrast, same products produced by homogenous generation of hydroxide ion are dense and easily filtered, considerably high purity

• Results better crystal size as well as improvements in purity

Drying and Ignition of Precipitate

• After filtration, a gravimetric precipitate is heated until its mass becomes constant

• Heating removes the solvent and any volatile species carried down with the precipitate

• Some precipitates are also ignited to decompose the solid and form a compound of known composition

• New compound is often called weighing form

• Temperature required to produce a suitable weighing form varies from precipitate to precipitate

• Can be done in automatic thermobalance, an instrument that records the mass of a substance continuously as its temperature is increased at a constant rate

• Moisture is completely removed from silver chloride at a temperature higher than 110 0C

• Dehydration of aluminium oxide is not complete until a temperature greater than 1000 0C is achieved

• Aluminium oxide formed homogenously with urea can be completely dehydrated at about 650 0C

• For calcium oxalate, about 1350C unbound water is eliminated to give monohydrate

• About 4500C signals the decomposition of calcium carbonate and carbon monoxide

• Final step depicts the conversion of calcium carbonate to calcium oxide and carbon dioxide after reaching about 1000 0C

## Applications of Gravimetric Methods

• Developed for most inorganic anions and cations

• For neutral species as water, sulfur dioxide, carbon dioxide and iodine

• Organic substances can also be determined gravimetrically

• Like lactose in milk products, salicylates in drug preparations, nicotine in pesticides, cholesterol in cereals

• Gravimetric analysis do not require a calibration or standardization step

• Because results are calculated directly from the experimental data and atomic masses

• Requires less time and effort

SUMMARY Gravimetric analysis

• Quantitative methods that are based on determining the mass of a pure compound to which the analyte is chemically related.

• Gravimetric methods are of two types: Precipitation gravimetry and volatilization gravimetry

• Analyte is converted to a sparingly soluble precipitate

• Ideally gravimetrically precipitating reagent should react specifically or at least selectively with the analyte

• Precipitates consisting of large particles are generally desirable for gravimetric work

• A supersaturated solution is an unstable solution that contains higher solute concentration

• Major source of contamination in coagulated colloids with large specific surface areas

• Reprecipitation- Effective way to minimize the effects of adsorption

• Mixed crystal formation- Is a type of coprecipitation in which contaminant ion replaces an ion in the lattice of a crystal

• Occlusion is a type of coprecipitation in which a compound is trapped within a pocket formed during rapid crystal growth

• Homogenous precipitation is a process in which a precipitate is formed by slow generation of a precipitating reagent homogenously throughout a solution

• Urea is often used for the homogenous generation of hydroxide ion

• After filtration, a gravimetric precipitate is heated until its mass becomes constant

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