Dr Ashwini Wadegaonkar

Gravimetric Analysis

Unit 1: Gravimetric Analysis

Common ion effect and solubility product principles

Conditions for good precipitation

Factors affecting precipitation like acid, temperature, nature of solvent

Super saturation and precipitation formation

Precipitation from homogeneous solution and examples

Co-precipitation, post-precipitation and remedies for their minimization

Washing of precipitate and ignition of precipitate

Brief idea about method of filtration and drying of precipitate

Introduction to electrogravimetry: principle, applications, electrolytic separations of Cu and Ni

Numerical problems only on gravimetric analysis

Introduction

Gravimetric analysis is a technique through which the amount of ananalyte (the ion being analyzed) can be determined through themeasurement of mass.

Gravimetric analyses depend on comparing the masses of twocompounds containing the analyte.

The principle behind gravimetric analysis is that the mass of an ion in apure compound can be determined and then used to find the masspercent of the same ion in a known quantity of an impure compound.

In order for the analysis to be accurate, certain conditions must be met:

-The ion being analyzed must be completely precipitated.

-The precipitate must be a pure compound.

-The precipitate must be easily filtered.

Common ion effect and solubility product principles

Common Ion Effect

The role that the common ion effect plays in solutions is mostly visible inthe decrease of solubility of solids. Through the addition of common ions,the solubility of a compound generally decreases due to a shift inequilibrium.

The common ion effect also plays a role in the regulation of buffers.Buffering solutions contain either an acid or base, accompanied by itsconjugate counterpart. Addition of more like conjugate ions willultimately shift the pH of the solution.

The common ion effect must be taken into consideration whendetermining solution equilibrium upon addition of ions that are alreadypresent in the solution.

To a weak acid, when another electrolyte containing a

common ion is added the degree of dissociation of the weak

electrolyte is suppressed.

Common ion effect gives valuable method to control the

concentration of the desired ion given by the weak

electrolyte.

Solubility

Solubility refers to the amount of material that is able to bedissolved in a particular solvent.

For example, table salt (NaCl) placed in water eventuallydissolves.

However, if more table salt is continuously added, the solutionwill reach a point at which no more can be dissolved; in otherwords, the solution is saturated, and the table salt haseffectively reached its solubility limit.

https://opentextbc.ca/chemistry/chapter/11-3-solubility/

http://bioprofe.com/en/solubility-and-precipitation/

Chemical equilibrium is the chemical state where there are no netphysical or chemical changes between the reactant and theproducts of a reaction.

This is because the rate of the forward (reactant to product) andreverse (product to reactant) reactions are equal.

Solubility equilibrium refers to the state of chemical equilibriumbetween a chemical compound in the solid state and a solutioncomposed of that dissolved compound.

This equilibrium is established when the rates of migrationbetween the solid and aqueous phases of the molecules (or ions)are equal.

Common Ion Effect on Solubility

Adding a common ion decreases solubility, as the reaction

shifts toward the left to relieve the stress of the excess

product.

Adding a common ion to a dissociation reaction causes the

equilibrium to shift left, toward the reactants, causing

precipitation.

https://chem.libretexts.org/Bookshelves/Physical_and_Th

eoretical_Chemistry_Textbook_Maps/Supplemental_Modul

es_(Physical_and_Theoretical_Chemistry)/Equilibria/Solub

ilty/Common_Ion_Effect

Solubility product

In a saturated solution of a sparingly soluble electrolyte (salt), theproduct of ionic concentrations when raised to proper powers isconstant at a given temperature, is called solubility product,denoted by Ksp or S.

It is when compared to ionic product, the conclusions are asfollows –

1. When Ionic product = Ksp, the solution is saturated, noprecipitation

2. When Ionic product < Ksp, the solution is unsaturated, noprecipitation

3. When Ionic product > Ksp, the solution is supersaturated,precipitation

Solubility product

The solubility products Ksp's are equilibrium constants in

hetergeneous equilibria (i.e. between two different phases).

If several salts are present in a system, they all ionize in the

solution.

If the salts contain a common cation or anion, these salts

contribute to the concentration of the common ion.

Contributions from all salts must be included in the calculation

of concentration of the common ion.

Consideration of charge balance or mass balance or both leads to

the same conclusion.

When NaCl and KCl are dissolved in the same solution, the Cl− ionsare common to both salts.

In a system containing NaCl and KCl, the Cl− ions are common ions.

NaCl⇌ Na+ + Cl−

KCl⇌ K++ Cl−

CaCl2 ⇌ Ca2++ 2Cl−

AlCl3 ⇌Al3++ 3Cl−

AgCl⇌Ag++ Cl−

Precipitation

•To precipitate is to form an insoluble

compound, either by decreasing the

solubility of a compound or by reacting two

salt solutions.

•The solid that forms via a precipitation

reaction is called the precipitate.

• Precipitation reactions serve important

functions. They are used for purification,

removing or recovering salts, for making

pigments, and to identify substances in

qualitative analysis.

https://www.chemicool.com/definition/precipitate.html

https://media.iupac.org/publications/analytical_compendiu

m/Cha09sec50.pdf

Conditions for good precipitation

What is precipitation gravimetry?

Precipitation gravimetry is an analytical technique that uses a

precipitation reaction to separate ions from a solution.

The chemical that is added to cause the precipitation is called

the precipitant or precipitating agent.

The solid precipitate can be separated from the liquid

components using filtration, and the mass of the solid can be

used along with the balanced chemical equation to calculate

the amount or concentration of ionic compounds in solution.

Ideally, an analytical precipitate for gravimetric analysisshould consist of perfect crystals large enough to be easilywashed and filtered.

The perfect crystal would be free from impurities and belarge enough so that it presented a minimum surface areaonto which foreign ions could be adsorbed.

The precipitate should also be "insoluble”

It has been shown (Von Weimarn) that the particle size ofprecipitates is inversely proportional to the relativesupersaturation of the solution during precipitation

The following methods are used to approach these criteria -

Precipitation from hot solution. The solubility S of precipitatesincreases with temperature and so an increase in S decreases thesupersaturation.

Precipitation from dilute solution. This keeps Q low. Slow addition ofprecipitating reagent with effective stirring. This also keeps Q low,stirring prevents local high concentrations of the precipitating agent.

Precipitation at a pH near the acidic end of the pH range in whichthe precipitate is quantitative. Many precipitates are more soluble atthe lower (more acidic) pH values and so the rate of precipitation isslower.

Digestion of the precipitate. Also the digestion period results in someimprovement in the internal perfection of the crystal structure[sometimes called ripening], here some internal foreign atoms may beexpelled.

Coprecipitation

This is anything unwanted which precipitates with the thing youdo want. Coprecipitation occurs to some degree in everygravimetric analysis (especially barium sulfate and thoseinvolving hydrous oxides). It can be minimized by carefulprecipitation and thorough washing.

Surface adsorption

Here unwanted material is adsorbed onto the surface of theprecipitate. Digestion of a precipitate reduces the amount ofsurface area and hence the area available for surface adsorption.Washing can also remove surface material.

Occlusion

This is a type of coprecipitation in which impurities are

trapped within the growing crystal.

Postprecipitation

Sometimes a precipitate standing in contact with the mother

liquor becomes contaminated by the precipitation of an

impurity on top of the desired precipitate.

Factors affecting precipitation

1. Effect of Acids on the Solubility of a Precipitate

The solubility of a sparingly soluble salt of strong acid

increases upon addition of strong acid. Addition of strong

acid increases the ionic strength of the solution and hence

decreases the activity coefficients of both ions of sparingly

soluble salt.

2. Effect of Temperature on the Solubility of a

Precipitate

The solubility of a precipitate increases with rise in

temperature. The dissolution of a solute is mostly an

endothermic process, so the solubility product constant

increases as the temperature is increased.

3. Effect of the nature of Solvent on the Solubility of a Precipitate

A solvent has a large effect upon solubility of a solute. Generally polarsolutes are more soluble in polar solvents and non polar solutes aresoluble in non polar solvents. Solubility of inorganic compounds isreduced when an organic solvents is added to water.

4. Supersaturation and precipitate formation

Smaller the particle size more is the solubility of a substance.

A supersaturated solution the one that contains a greaterconcentrations of solute than expected equillibrium conditions at agiven temperature.

Supersaturation plays important role in determining particle size ofthe precipitate.

particle size of the precipitate decreases with increasing concentrationof the reactants.

Super saturation and precipitation formation

A supersaturated solution is an unstable solution that

contains more solutes than a saturated solution, with time,

supersaturation is relieved by precipitation of the excess

solute.

To increase the particle size of a precipitate, minimize the

relative supersaturation during the precipitate formation.

Relative supersaturation = (Q-S)/S

where Q is the concentration at any instant and S is the

equilibrium solubility.

5. The Purity of the Precipitate

Co-precipitation – the contamination of the precipitate by substances which are normally soluble in the mother liquor is termed as co-precipitation.

Two types of co-precipitation –

a. Due to surface adsorption

b. Due to formation of solid solution – Occlusion

Co-precipitation can be minimized by –

Taking dilute and pure solution

Use double precipitation

Operations like digestion or aging

Use of organic solvent

Slow addition of reagent

Stirring

Proper temperature

Post precipitation

The process in which precipitation of some component(impurity) occurs on the surface after its formation is calledprecipitation.

It occurs with sparingly soluble substances

The longer thee time for precipitate to stand, the greater is theerror due to due to this cause

Post-precipitation can be minimized by –

Complex formation

Change in oxidation state

Rapid filtration of precipitate

Concentration of impurities

Distinction between Co-Precipitation and Post-Precipitation

https://image.slidesharecdn.com/gravimetry-

160407142410/95/gravimetry-21-638.jpg?cb=1460039131

Introduction to Electrogravimetry

Electrogravimetry is a method in which the element / metal to be

determined is deposited electrolytically upon a suitable electrode.

From the gain in the weight of electrode the amount of the metal

in the sample is calculated.

The method of electrogravimetry is rapid and accurate.

Electrogravimetry is a method for the separation of the metal

ions by using the electrodes.

The deposition takes place on the one electrode. The weight of this

electrode is determined before and after deposition.

This gives the amount of the metal present in the given sample

solution.

Principle of Electrogravimetry

The main principle

involved in this method is

the deposition of the

solid on an electrode

from the analyte solution.

Electrogravimeter

The material is deposited by means of potential application. The

electrons are transported to electrode by the following

mechanisms:

Diffusion

Migration

Convection

THEORY

A metal is electrolytically deposited on the electrode by increasing the mass of the electrode.

M+2 + 2e− M(S)

Therefore,

Eelectrolysis = Ecathode − Eanode

The electrons deposition is governed by Ohm's and Faraday's laws of

electrolysis which states that the amount of the electrons deposited on the

electrode is directly proportional to the amount of the current passed

through the solution and the amount of different substances deposited is

directly proportional to the molar masses divided by the number of electrons

involved in the electrolysis process.

That is the current (I) is directly proportional to the electromotive force (E)and is indirectly proportional to the resistance (R).

E = IR

From the above equation, we get the following:

Eelectrolysis = Ecell − IR

Ecell = Ecathode − Eanode

where

Therefore,

Eapplied = Ecathode − Eanode − IR

I = (−Eapplied/R) +1/R(Ecathode − Eanode)

I = (Ecell − Eapplied/R)

I = (−Eapplied/R) + K

where K is the constant.

A plot of the current of the applied potential in an electrolytic cell should be straight line with a slope equal to negative reciprocal of the resistance.

Electrogravimetric plot

http://rxpharmaworld.blogspot.com/2016/12/electrogravi

metry.html

Electrolytic separation of Copper and Nickel

Constant current electrolysis is used in the separation and

determination of metals in alloys.

Some alloys, mainly contain copper and nickel as - Monel

metal, coinage alloys, cupro-nickel alloys etc. they contain

impurity like iron.

The impurity is removed and the two metals are electro-

deposited at different applied potentials.