Non-Instrumental Volumetric Analysis

S.Y. B.Sc. Semester - IV

• Indicators – Theory of indicators, Acid base indicators,Mixed and Universal indicators

Titrations -• Acid–Base titrations:

a. Strong acid–Strong base, b. Weak acid–strong base, c.Weak acid-Weak base titration d. Displacement titrations,e. Polybasic acid titrations.

• Redox titrations: Principle of redox titration, detection ofequivalence point using suitable indicators.

• Complexometric titrations: Principle, EDTA titrations,choice of indicators

• Iodometry and Iodimetry: Principle, detection of end point,difference between iodometry and iodimetry,

• Standardization of sodium thiosulphate solution usingpotassium dichromate and iodine method, Applications –estimation of Cu , estimation of Cl2.

• Instrumental techniques used for analysis of many substances,are quite expensive and not easily available in collegelaboratories.

• Analysis of some simple substance can be done by using non-instrumental volumetric analysis.

• Its very easy, inexpensive and can be carried out in ordinarylaboratory conditions.

• The volumetric method involves measurement of volumes ofthe reacting substances.

• The weight of the required constituents is indirectly obtainedby measuring the volume of a solution of known compositionrequired to react with a known volume of a solutioncontaining an unknown weight of the desired constituent.

Introduction

• Volumetric analysis is done by process known as titrationwhich is divided into four types depending upon thenature of the chemical reaction.

• In Volumetric analysis, concentration of substances arefound out by volume determination.

• The precision of volumetric work depends upon theaccuracy with which volumes of liquid can be measured.

Types of non-instrumental volumetric analysis

• Acid-base (neutralization) titrations

• Oxidation-reduction titrations

• Complexometric titrations

• Iodometry and iodimetry

• Argentimetry

Acid-base (Neutralization) titrations

• When the acid is added into a base or vice versa, formation of salt

and neutral water molecule take place.

• It is also called as Neutralization reaction as in this chemical

reaction there is formation of neutral molecule

• The amount of acid or base is found out by simple titration method

by using their standard solutions,

• To know exact neutralization point, it is necessary to use right

indicator which will change its colour very close to the equivalence

point.

• Equivalence point is the point at which equivalent amounts of acid

and base have been brought into reactions.

Neutralization Indicators

• When acid-base titration is to be carried out, then to

detect the end point of the titration, the use of

indicator is a must.

• For acid-base titrations, the neutralization or acid-base

indicators are used.

• According to Ostwald, the indicators are organic

compounds that are weakly acidic or basic in character.

• In solution, equilibrium is established between the

molecular and ionized forms.

For an indicator that is an acid, the equilibriumis given as

H In H+ + In-

acid form base form

It can be seen that the hydrogen ionconcentration will be the factor that determinesthe indicator colour of the form whichpredominates since if the hydrogen ionconcentration increases, the equilibrium will beshifted to the left and if it is decreased, theequilibrium will be shifted to the left

Theory of Indicators

https://www.youtube.com/watch?v=-MPHniw4SLs

https://www.youtube.com/watch?v=agTtcZoBKS4

• The colour of the indicator which depends upon the

value of ratio (basic/acid) can be determined by a

visual colour comparison. Both forms of the indicator

are present at any hydrogen ion concentration.

• The human eye has limited ability to detect either of

two colours when one of them predominates

• The solution will appear to have acidic colourwhen ratio [InA]/ [InB] is equal or greater than10

• The solution will appear to have acidic colourwhen ratio [InB]/ [InA] is equal or greater than10

• For acid colour pH = pK’In – 1. While foralkaline colour pH = pK’In + 1

• The colour change is gradual as it depends uponthe ratio of concentrations of acidic and basicform.

• When the pH of the solution is equal to theapparent dissociation constant of the indicatorpK’In the ratio becomes equal to one.

• The indicator will have a colour due to an equalmixture of the acid and alkaline forms. Thisbehaviour of the indicator is known as middle tintof the indicator.

pH Transition Range and Colour changes of Common Acid-Base Indicators

pH Transition Range and Colour changes of Acid-Base Indicators

• Preparation of indicator solutions, mixedindicator,

• Universal or Multiple Range indicator

Acid-base titrations - Neutralization curves

A plot of pH against the volume of alkali added (mL)is known as a neutralization or titration curve.The curve is generated by a 'potentiometric titration'in which pH is measured after each addition of alkali(or acid).The significant feature of the curve is the very sharpand sudden change in pH near to the equivalencepoint of the titration.For a strong acid and alkali this will occur at pH 7.If either the acid or base concentration is unknown,

a preliminary titration is necessary to find theapproximate equivalence point followed by a moreaccurate titration.The ideal pH range for an indicator is 4.5-9.5.

Determination of the equivalence point

From the neutralization curve, the initial and final slopes are drawn and a parallel line is drawn such that the mid-point is on the curve. This is the equivalence point, producing a titration value.

Acid-base titrations - Neutralization curves

https://www.youtube.com/watch?v=l5fk7HPmo5g

https://www.youtube.com/watch?v=WuBPy4eur0k

https://www.youtube.com/watch?v=uO8DWyRJMHo

https://www.khanacademy.org/test-prep/mcat/chemical-processes/titrations-and-solubility-equilibria/a/acid-base-titration-curves

Neutralization curves

• The mechanism of neutralization process canbe understood by studying the changes inhydrogen ion concentration during the courseof titration.

• The plot of the change in concentration of H+

or pH as a function of the titrant (burettesolution) added is known as neutralizationcurve or titration curve.

• End Point : The volumetric analysis is done, usually bysimple titration method. The point at which thereaction is observed to be complete is called the endpoint. The end point is noticed with the help ofsuitable indicator. The indicator changes colour when itpasses through its transition colour.

• Equivalence Point : It is the point attained duringtitration when exactly equivalent quantities of acid andbase are added to one another.

• Neutralization Point : In the acid-base titration whenthe pH of the solution becomes exactly equal to 7 atthat time the process is said to be reached atneutralization point.

• Suitable Indicators : The indicator which showscolour change at the equivalence point is calledsuitable indicator. At equivalence point, thechange in pH of the solution also takes place.Usually the indicators have transition rangebetween 3 and 10 pH units in which colourchange occurs.

• Best Indicator : A suitable indicator which has itstransition range around the pH at the equivalencepoint is called the best indicator. E.g.phenolphthalin in titration of NaOH andCH3COOH

• The nature of titration curves depends upon the natureof acid and the base used in titration.

• The titration curves for monobasic acid andmonoacidic bases are distinctly different than those forpolybasic acids and polyacidic bases.

• Four titration curves for monobasic acids andmonoacidic bases are

– A strong acid and strong base– A weak acid and strong base– A strong acid and weak base– A weak acid and weak base

Type Analyte Titrant

SA/SB Strong Acid Strong Base

WA/SB Weak Acid Strong Base

SB/SA Strong Base Strong Acid

WB/SA Weak Base Strong Acid

• In an acid-base titration, a known volume of either the acid or thebase (of unknown concentration) is placed in a conical flask.

• The second reagent (of known concentration) is placed in a burette.• The reagent from the burette is slowly added to the reagent in the

conical flask.• A titration curve is a plot showing the change in pH of the solution

in the conical flask as the reagent is added from the burette.

• A titration curve can be used to determine:1) The equivalence point of an acid-base reaction (the point at which

the amounts of acid and of base are just sufficient to cause complete neutralization).

2) The pH of the solution at equivalence point is dependent on the strength of the acid and strength of the base used in the titration.

-- For strong acid-strong base titration, pH = 7 at equivalence point-- For weak acid-strong base titration, pH > 7 at equivalence point-- For strong acid-weak base titration, pH < 7 at equivalence point

Redox titration

• A redox titration is a titration of a reducing agent byan oxidizing agent or titration of an oxidizing agent by areducing agent.

• Typically, this type of titration involves a redox indicator ora potentiometer.

Example Set-Up• For example, a redox titration may be set up by treating an

iodine solution with a reducing agent to form the iodide.• A starch solution can then be used as a color-change

indicator to detect the titration endpoint.• In this case, the solution begins blue and disappears at the

endpoint when the iodine is all reacted.

Complexometric titrations

• Complexometric Titration or chelatometry is a type ofvolumetric analysis wherein the coloured complex is usedto determine the endpoint of the titration.

• Titration is one of the common method used in laboratorieswhich determines the unknown concentration of ananalyte that has been identified.

• It is a method used in quantitative chemical analysis.• It is sometimes termed as volumetric analysis as

measurements of volume play a vital role.• Here reagent is used as a standard solution, and they are

called titrant.• Titrant Volume is defined as the volume of a titrant that is

reacted.

• Complexometric Titration is in the detection ofmixtures of different metal ions present in thesolution.

• When each drop of titrant is added, the reactionreaches an equilibrium state swiftly.

• There would be no chance for any interferingsituations.

• The equivalent point can be identified veryaccurately using a complexometric titration.

• EDTA is used as a titrant and it is pretty much wellestablished.

• Indicators• Calmagite and Eriochrome BlackT (EBT) are such indicators that

change from blue to pink when they complex with calcium or magnesium. The endpoint of a complexometric EDTA titration using either Calmagite or EBT as the indicator is detected as the colourchanges from pink to blue.

• The endpoint detection in complexometric titration can be done by two methods.

• 1. Visual Method• One of the most common methods for determination of endpoint

owing to its simplicity, least cost and accuracy. Following are some of the visual methods used for determining the end point of the complexometric titrations.

• Metallochromic or PM indicators• pH indicators• Redox indicators

EDTA Complexometric Titration

• EDTA called as ethylenediaminetetraacetic acid is a complexometricindicator consisting of 2 amino groups and four carboxyl groupscalled as Lewis bases.

• Edta is a hexadentate ligand because of its competence to denotesix pair of lonely electrons due to the formation of covalent bonds.

• Even the presence of small metal ions would lead to a distinctchange in the color. This leads to the formation of a weak complex.

• Complexing agents are less soluble in water, and most of them arefree acids.

• They are used in volumetric Solutions. Before using them, they areconverted into sodium salts that are feasible in water.

• Since they are characterised with less solubility in water, they areused for titration.

• Sometimes simple titration methods are used to determine thesimple metal ions present in water. But to determine the exactnumber of metal ions present complexometric titration is used, andit is conducted using EDTA.

Iodometry and Iodimetry

• http://www.federica.unina.it/agraria/analytical-chemistry/iodometry/

• Acid Base titrations from book