LECTURE 2LECTURE 2

Titration methodTitration methodBuffer solutions.

ass. prof. I. R. Bekusass. prof. I. R. Bekus

Titrimetric analysis- is a method of quantitative analysis Titrimetric analysis- is a method of quantitative analysis used to determine unknown concentration of known used to determine unknown concentration of known

substance.substance.

You must know definition of some useful You must know definition of some useful terms:terms:

Titrant Titrant or or Standard solutionStandard solution – a solution of – a solution of accurately known concentration.accurately known concentration.

Titration Titration – the process of determining – the process of determining unknown concentration by adding the small unknown concentration by adding the small increments of standard solution until the increments of standard solution until the reaction is just complete.reaction is just complete.

Titration setup: the titrant drops from the burette into the analyte solution in the flask. An indicator present then changes color permanently at the endpoint.

Overview of Titrimetry:Overview of Titrimetry:Titrimetric methods are classified into four groups based on Titrimetric methods are classified into four groups based on

the type of reaction involved.the type of reaction involved.

These groups areThese groups are 1)1) acid–base titrations, in which an acidic or basic titrantacid–base titrations, in which an acidic or basic titrant

reacts with an analyte that is a base or an acid;reacts with an analyte that is a base or an acid;2)2) complexometric titrations involving a metal–ligand complexometric titrations involving a metal–ligand

complexation reaction; complexation reaction; 3)3) redox titrations, where the titrant is an oxidizing or redox titrations, where the titrant is an oxidizing or

reducing agent; reducing agent; 4)4) precipitation titrations, in which the analyte and titrant precipitation titrations, in which the analyte and titrant

react to form a precipitate.react to form a precipitate...

Titrant - the reagent added to a solution containing

the analyte and whose volume is the signal.

A reagent, called the titrant, of known

concentration (a standard solution) and volume is

used to react with a solution of the analyte, whose

concentration is not known.

equivalence point - the point in a titration where

stoichiometrically equivalent amounts of analyte

and titrant react.

End point - the point in a titration where we stop adding titrant.

Indicator - a colored compound whose change in color signals the end point of a titration.

Titration error - the determinate error in a titration due to the difference between the end point and the equivalence point.

A primary standard is a standard that is accurate enough that it is not calibrated by or subordinate to other standards. A primary standard in chemistry is a reliable, readily quantified substance.

Secondary reagent - a reagent whose purity must be established relative to a primary reagent

A burette (also buret) is a vertical cylindrical piece of laboratory glassware with a volumetric graduation on its full length and a precision tap, or stopcock, on the bottom (or calibrated glass tube).

Even the thickness of the lines printed on the burette matters; the bottom of the meniscus of the liquid should be touching the top of the line you wish to measure from.

Equipment for Measuring Equipment for Measuring VolumeVolume

Analytical chemists use a variety of glassware to Analytical chemists use a variety of glassware to measure volumemeasure volume: : beaker; graduated cylinder;beaker; graduated cylinder; volumetric flask; pipetvolumetric flask; pipettete;; dropping pipetdropping pipettete..

Burette A burette is a device used in

analytical chemistry for the dispensing of variable, measured amounts of a chemical solution. A volumetric burette delivers measured volumes of liquid. Piston burettes are similar to syringes, but with precision bore and plunger. Piston burettes may be manually operated or may be motorized.

(a) (b) (c) (d)

Common types of pipettes and syringes: (a) transfer pipette; (b) measuring pipette; (c) digital pipette; (d) syringe.

chemical dropper

A pipette, pipet, pipettor or chemical dropper is a laboratory tool commonly used in chemistry, biology and medicine to transport a measured volume of liquid, often as a media dispenser.

Laboratory flasks

There are several types of laboratory flasks, all of which have different functions within the laboratory. Flasks, because of their use, can be divided into:

1. Reaction flasks

2. Multiple neck flasks

3. Schlenk flask

4. Distillation flasks

5. Reagent flasks

6. Volumetric flask

Volumetric flask A volumetric flask

(measuring flask or graduated flask) is a piece of laboratory glassware, a type of laboratory flask, calibrated to contain a precise volume at a particular temperature. Volumetric flasks are used for precise dilutions and preparation of standard solutions.

Graduated cylinder

A graduated cylinder, measuring cylinder or mixing cylinder is a piece of laboratory equipment used to measure the volume of a liquid. Graduated cylinders are generally more accurate and precise than laboratory flasks and beakers

Funnels (laboratory) Laboratory funnels are

funnels that have been made for use in the chemical laboratory. There are many different kinds of funnels that have been adapted for these specialized applications. Filter funnels, thistle funnels (shaped like thistle flowers), and dropping funnels have stopcocks which allow the fluids to be added to a flask slowly. For solids, a powder funnel

BuretteBurette – kind of laboratory glass for exact – kind of laboratory glass for exact measurement of volume of solution used. Burette is measurement of volume of solution used. Burette is graduated and has a burette tap or stopcock at one graduated and has a burette tap or stopcock at one extreme end to control the flow of titrant.extreme end to control the flow of titrant.

Equivalence pointEquivalence point. The point in a titration at which the . The point in a titration at which the amount of titrant added is chemically equivalent to amount of titrant added is chemically equivalent to the amount of substance titration. the amount of substance titration.

End pointEnd point.. The point at which the completion of a The point at which the completion of a reaction is practically observed. When using an reaction is practically observed. When using an indicator, the end point occurs when enough titrant indicator, the end point occurs when enough titrant has been added to change the color of the indicator.has been added to change the color of the indicator.

Three important precautions are needed when working with pipettes and volumetric flasks.

First, the volume delivered by a pipette or contained by a volumetric flask assumes that the glassware is clean.

Second, when filling a pipette or volumetric flask, set the liquid’s level exactly at the calibration mark.

Sird, the liquid’s top surface is curved into a meniscus, the bottom of which should be exactly even with the glassware’s calibration mark.

Burette filling instructionBurette filling instruction Always use a small funnel to fill a buretteAlways use a small funnel to fill a burette To fill a burette, close the stopcock at the bottom. To fill a burette, close the stopcock at the bottom.

You may need to lift up the funnel slightly, to You may need to lift up the funnel slightly, to allow the solution to flow in freelyallow the solution to flow in freely

Fill the burette past the zero markFill the burette past the zero mark Check the tip of the burette for an air bubble. To Check the tip of the burette for an air bubble. To

remove an air bubble you must lift up tip of burette remove an air bubble you must lift up tip of burette and then open stopcock. If an air bubble is present and then open stopcock. If an air bubble is present during a titration, volume reading may be in error!during a titration, volume reading may be in error!

Take the funnel out of the burette Take the funnel out of the burette so that drops of so that drops of solution from the funnel will not fall into the solution from the funnel will not fall into the burette.burette.

When you burette is filled, with no air bubbles, you When you burette is filled, with no air bubbles, you must level of the liquid to exactly the zero mark. Read must level of the liquid to exactly the zero mark. Read the bottom of the meniscus. Be sure your eye is at the the bottom of the meniscus. Be sure your eye is at the

level of meniscus, not above or belowlevel of meniscus, not above or below

After filling burette, a known volume of the unknown After filling burette, a known volume of the unknown concentration solution should be taken with the pipette concentration solution should be taken with the pipette

and placed into the conical flask, along with a small and placed into the conical flask, along with a small amount of the indicator. amount of the indicator.

Slowly release known solution from the burette into Slowly release known solution from the burette into

the conical flask, while swirling the mixture.the conical flask, while swirling the mixture.

The solution should be let out of the burette until the The solution should be let out of the burette until the indicator changes colour and value on the burette should indicator changes colour and value on the burette should

be recorded.be recorded.

Types of TitrationTypes of Titration

Neutralisation (Acid-Base) titrationNeutralisation (Acid-Base) titrationPrecipitation titrationPrecipitation titrationReduction-Oxidation (Redox) Reduction-Oxidation (Redox)

titrationtitrationComplexometric titrationComplexometric titration

Acid-Base TitrationAcid-Base Titration As the second step in this investigation you are now going to As the second step in this investigation you are now going to

compare two solutions (an acid and a base) using a method compare two solutions (an acid and a base) using a method called "titration".called "titration".

In the first procedure you are simply going to add an acid In the first procedure you are simply going to add an acid solution to a basic solution. Each solution will be of a different solution to a basic solution. Each solution will be of a different "strength", or concentration, or amount, and you will simply "strength", or concentration, or amount, and you will simply observe the relative results.observe the relative results.

In the second procedure you are going carry out a number of In the second procedure you are going carry out a number of titrations in which an acid solution is carefully added to a titrations in which an acid solution is carefully added to a basic solution. In each case you have to find the "end point", basic solution. In each case you have to find the "end point", which is the point at which you have added just enough of the which is the point at which you have added just enough of the acid solution to exactly neutralize all the base that was in the acid solution to exactly neutralize all the base that was in the original solution.original solution.

The properties of the acid solution are standardized, and fully The properties of the acid solution are standardized, and fully known. So, by finding the exact amount of acid that known. So, by finding the exact amount of acid that neutralizes a known solution of base, it is possible to carry out neutralizes a known solution of base, it is possible to carry out a calculation and find out the molecular weight of the base.a calculation and find out the molecular weight of the base.

These titrations are based on the neutralization reaction These titrations are based on the neutralization reaction that occurs between an acid and a base, when mixed that occurs between an acid and a base, when mixed

in solution. in solution. A neutralization reaction in A neutralization reaction in aqueous solution is a reaction of an acid and a aqueous solution is a reaction of an acid and a

hydroxide base to produce a salt and waterhydroxide base to produce a salt and water

An acid-base titration is the determination of An acid-base titration is the determination of the concentration of an acid or base by the concentration of an acid or base by

exactly neutralizing the acid/base with an exactly neutralizing the acid/base with an acid or base of known concentration. This acid or base of known concentration. This

allows for quantitative analysis of the allows for quantitative analysis of the concentration of a unknown acid or base concentration of a unknown acid or base

solution.solution.An acid-base titration in which a base is An acid-base titration in which a base is

titrated with a standard solution of an acid is titrated with a standard solution of an acid is called called AcidimetricAcidimetric

An acid-base titration in which an acid is An acid-base titration in which an acid is titrated with a standard solution of an alkali titrated with a standard solution of an alkali

(a base) is called (a base) is called AlkalimetricAlkalimetric

Precipitation TitrationPrecipitation TitrationPrecipitation Titration it is a volumetric titration method where the reaction between Precipitation Titration it is a volumetric titration method where the reaction between

the titrant and sample solution yield precipitate (low solubility, usually ionic the titrant and sample solution yield precipitate (low solubility, usually ionic compounds)compounds)

The most important precipitating reagent is The most important precipitating reagent is silver nitrate.silver nitrate.Titrimetric methods based upon silver nitrate are sometimes termed Titrimetric methods based upon silver nitrate are sometimes termed argentometric argentometric

methods.methods.Argentometry, where the titrant is a standard AgNOArgentometry, where the titrant is a standard AgNO33 solution is the most common solution is the most common

precipitation titrimetric method, because precipitation titrimetric method, because silver precipitates are usually highly insolublesilver precipitates are usually highly insoluble many species form steichiometric precipitates with Ag+ (e.g. Cl-, Br-, I-, F-, CN-, many species form steichiometric precipitates with Ag+ (e.g. Cl-, Br-, I-, F-, CN-,

SCN-, CrO42-, PO43- etc.)SCN-, CrO42-, PO43- etc.) these precipitates are formed quicklythese precipitates are formed quicklyTitrant is a standardized AgNOTitrant is a standardized AgNO33 solution. The titrant needs to be stored in a dark solution. The titrant needs to be stored in a dark

(brown) container.(brown) container.Argentometry is most often used for determination of chloride ions, but it can be used Argentometry is most often used for determination of chloride ions, but it can be used

for other halides (bromide, iodine).for other halides (bromide, iodine).There are 3 techniques of end point determination:There are 3 techniques of end point determination: method of Mohr (indicator: potassium chromate)method of Mohr (indicator: potassium chromate) method of Volgard (indicator: ferric salt)method of Volgard (indicator: ferric salt) method of Fajans (indicator: fluorescein)method of Fajans (indicator: fluorescein)The most often used Mohr methodThe most often used Mohr method

Mohr methodMohr method

Mohr titration is used for determination Mohr titration is used for determination of halide in a solution.of halide in a solution.

Potassium chromate can serve as an Potassium chromate can serve as an indicator for the determination of indicator for the determination of chloride, and bromide ions by chloride, and bromide ions by reacting with silver ion to form a reacting with silver ion to form a brick-red silver chromate (Agbrick-red silver chromate (Ag22CrOCrO44) ) precipitate in the equivalence-point precipitate in the equivalence-point region.region.

Mohr titration has to be performed at a neutral or weak Mohr titration has to be performed at a neutral or weak basic solution of pH 7-9 (or 6-10), because silver basic solution of pH 7-9 (or 6-10), because silver

hydroxide forms at high pH, while the chromate forms hydroxide forms at high pH, while the chromate forms H2CrO4 at low pH, reducing the concentration of H2CrO4 at low pH, reducing the concentration of chromate ions and delaying the formation of the chromate ions and delaying the formation of the

precipitate.precipitate.If Ag+ solution is add to a Cl- solution containing of small If Ag+ solution is add to a Cl- solution containing of small

quantity of CrO4-, then AgCl will firstly precipitated, quantity of CrO4-, then AgCl will firstly precipitated, while Ag2CrO4 has not yet, and concentration Ag+ while Ag2CrO4 has not yet, and concentration Ag+

increases progressively until solubility product of the ions increases progressively until solubility product of the ions reach the value of Ksp Ag2CrO4 (2,0·10-12) to form reach the value of Ksp Ag2CrO4 (2,0·10-12) to form

brick-red precipitate.brick-red precipitate.Before titration small amount of sodium or potassium Before titration small amount of sodium or potassium chromate is added to the solution, making it’s slightly chromate is added to the solution, making it’s slightly

yellow colour. During titration, as long as chlorides are yellow colour. During titration, as long as chlorides are present, concentration of Ag+ is too low for silver present, concentration of Ag+ is too low for silver

chromate formation. Near equivalence point concentration chromate formation. Near equivalence point concentration of silver cations rapidly grows, allowing precipitation of of silver cations rapidly grows, allowing precipitation of

brick-red silver chromate which signals end point.brick-red silver chromate which signals end point.

Reduction-Oxidation (Redox) TitrationReduction-Oxidation (Redox) TitrationA A redox titrationredox titration is based on an oxidation-reduction reaction between is based on an oxidation-reduction reaction between

analyte and titrant.analyte and titrant.In this experiment you will use a standard solution of potassium In this experiment you will use a standard solution of potassium

permanganate (KMnOpermanganate (KMnO44) to determine the of iron (as Fe2+) in an ) to determine the of iron (as Fe2+) in an unknown solution. unknown solution.

Permanganate ion reduces to a manganese (II) ion in the acidic Permanganate ion reduces to a manganese (II) ion in the acidic solution. This reaction requires 5 electrons and 8 hydrogen ions: solution. This reaction requires 5 electrons and 8 hydrogen ions:

MnOMnO44-+ 8H+ + 5 e- = Mn2+ + 4H-+ 8H+ + 5 e- = Mn2+ + 4H22OO Only one electron is necessary to reduce Fe (III) to Fe (II) Only one electron is necessary to reduce Fe (III) to Fe (II) Fe3+ + e- = Fe2+Fe3+ + e- = Fe2+ Therefore, 1 mole of MnO4-(the oxidizing agent) reacts with 5 moles Therefore, 1 mole of MnO4-(the oxidizing agent) reacts with 5 moles

of Fe2+ (the reducing agent) to form 5 moles of Fe3+ and 1 mole of of Fe2+ (the reducing agent) to form 5 moles of Fe3+ and 1 mole of Mn2+. Thus, in net ionic form: Mn2+. Thus, in net ionic form:

MnOMnO44- + 5Fe2+ + 8H+ = 5Fe3+ + Mn2+ + 4H- + 5Fe2+ + 8H+ = 5Fe3+ + Mn2+ + 4H22OO

Reactions in which electrons are transferred from one Reactions in which electrons are transferred from one species to another are known as redox reactions, or species to another are known as redox reactions, or 

oxidation--reduction reactions. reactions.

2 Na + Cl2 Na + Cl22 2 NaCl 2 NaCl

A redox reaction is made up of two reactions:A redox reaction is made up of two reactions:

reduction -- gain of electron(s)reduction -- gain of electron(s)

oxidation -- loss of electron(s)oxidation -- loss of electron(s)

Writing Redox EquationsWriting Redox Equations

In a redox reaction, the number of electrons In a redox reaction, the number of electrons lost by the species being lost by the species being oxidized must  must balance the number of electrons gained by balance the number of electrons gained by the species being the species being reduced..

In a balanced redox reaction equation:In a balanced redox reaction equation:* the number of atoms of each element * the number of atoms of each element

must be balancedmust be balanced* the total charge on the ions on the left * the total charge on the ions on the left

hand side of the equation will equal the hand side of the equation will equal the total charge on the ions on the right hand total charge on the ions on the right hand side of the equationside of the equation

In the redox titrations, we need a In the redox titrations, we need a chemical species that can change chemical species that can change

colour in the potential range colour in the potential range corresponding to the sharp change at corresponding to the sharp change at the end point. A chemical substance, the end point. A chemical substance,

which changes colour when the which changes colour when the potential of the solution reaches a potential of the solution reaches a

definite value, is termed as an definite value, is termed as an oxidation-reduction or redox indicatoroxidation-reduction or redox indicator..

InoxInox ++ nne e → → InredInredcolour A colour A  colour Bcolour B

PermanganatometryPermanganatometry

Potassium permanganate is a very strong oxidizing agent and is employed in the estimation of reducing agents like ferrous salts, oxalic acid, arsenious oxide, etc. The permanganate ion, MnO4-, gets reduced to Mn2+ ion in acidic medium and to MnO2 in neutral and alkaline media. Titrations involving potassium permanganate are usually carried out in acidic medium. Since MnO4– is intense purple while Mn2+ is colour less, the reaction mixture at equivalence point is colour less and even a single drop of the permanganate would impart sufficient pink colour to the solution acting as self indicator. The reducing agent in the titration to be discussed is oxalic acid here. The composition of it is H2C2O4·2H2O. In spite of being a dehydrate it is a good primary standard as its composition is unchanged during storage or weighing. This redox reaction can be split apart in two parts- one showing the oxidation and the other reduction

This titration is carried out in warm conditions This titration is carried out in warm conditions (temperature about 60 C). The reaction at room (temperature about 60 C). The reaction at room temperature is slow because of the equilibrium temperature is slow because of the equilibrium nature of this reaction. COnature of this reaction. CO22 is highly soluble in is highly soluble in

water and thus heating removes all dissolved carbon water and thus heating removes all dissolved carbon dioxide out of the solution. dioxide out of the solution.

While noting the burette readings, it should be taken While noting the burette readings, it should be taken into account that the solution is so intensely into account that the solution is so intensely

coloured that the lower meniscus of the solution coloured that the lower meniscus of the solution may not be clear. Thus for permanganate titrations may not be clear. Thus for permanganate titrations

the upper meniscus in the burette is noted.the upper meniscus in the burette is noted.

Complexometric titrationComplexometric titrationErio - T indicator or Eriochrome Erio - T indicator or Eriochrome Black-T indicator is used in Black-T indicator is used in this titration.this titration.

EDTA is a versatile chelating EDTA is a versatile chelating agent. A chelating agent is a agent. A chelating agent is a substance whose molecules substance whose molecules can form several bonds to a can form several bonds to a single metal ion. Chelating single metal ion. Chelating agents are multi-dentate agents are multi-dentate ligands. A ligand is a ligands. A ligand is a substance that binds with a substance that binds with a metal ion to form a complex metal ion to form a complex ion. Multidentate ligands are ion. Multidentate ligands are many clawed, holding onto many clawed, holding onto the metal ion to form a very the metal ion to form a very stable complex. EDTA can stable complex. EDTA can form four or six bonds with a form four or six bonds with a metal ion.metal ion.

The picture on the left shows the color of the indicator The picture on the left shows the color of the indicator

before titration.before titration.

This color change from wine red to violet to This color change from wine red to violet to blue is due to the compact nature of the blue is due to the compact nature of the

complex. The statement "the compact nature complex. The statement "the compact nature of the complex" means when the indicator is of the complex" means when the indicator is added to the hard water, the indicator Erio-T added to the hard water, the indicator Erio-T forms a complex with the Ca+2 ions that is forms a complex with the Ca+2 ions that is

pink in color. As EDTA is added to the pink in color. As EDTA is added to the solution, the EDTA forms a complex with solution, the EDTA forms a complex with

the Ca+2 leaving the indicator Erio-T the Ca+2 leaving the indicator Erio-T uncomplexed, which is blue in color.uncomplexed, which is blue in color.

pH scalepH scale ACIDACID NEUTRAL NEUTRAL BASE (ALKALINE)BASE (ALKALINE)

0-----------------------------0-----------------------------7-7--------------------------------14-------------------------------14

Techniques of titrimetric analysis.

Washing up and drying ware Preparation of standard solutions Sample preparation Titration:

- Measurement of volumes

- An indicator choice Calculations

Acid - Base indicatorsAcid - Base indicators

Acid - Base indicators (also known as pH indicators) are Acid - Base indicators (also known as pH indicators) are substances which change colour with pH. They are substances which change colour with pH. They are usually weak acids or bases, which when dissolved in usually weak acids or bases, which when dissolved in water dissociate slightly and form ions.water dissociate slightly and form ions.

The acid and its conjugate base have different colours. The acid and its conjugate base have different colours. At low pH values the concentration of H3O+ is high At low pH values the concentration of H3O+ is high and so the equilibrium position lies to the left. The and so the equilibrium position lies to the left. The equilibrium solution has the colour A. At high pH equilibrium solution has the colour A. At high pH values, the concentration of H3O+ is low - the values, the concentration of H3O+ is low - the equilibrium position thus lies to the right and the equilibrium position thus lies to the right and the equilibrium solution has colour B.equilibrium solution has colour B.

Methods to determine the end point visual indicators: Colour change: In some reactions, the solution changes colour without

any added indicator. This is often seen in redox titrations, for instance, when the different oxidation states of the product and reactant produce different colours.

Precipitation: If the reaction forms a solid, then a precipitate will form during the titration. A classic example is the reaction between Ag+ and Cl- to form the very insoluble salt AgCl. This usually makes it difficult to determine the endpoint precisely. As a result, precipitation titrations often have to be done as "back" titrations (see below).

Physical and chemical methods with the subsequent analysis of curves of the titration showing changes which occur in the course of titration (change of physical and chemical parametres standard solutions)

Types of titrimetric determinations.

Titration can be: direct titration back-titration (on residue) substitute-titration (displacement titration) revertive titration

direct titration – titrant add to an analyte solution and react with determined substrance

Requirements to reactions in direct titration reaction involving the titrant and analyte must be of

known stoichiometry, quantitatively the titration reaction must occur rapidly a suitable method must be available for determining the

end point with an acceptable level of accuracy Reactions should proceed by room temperature Titration should not be accompanied by collateral

reactions which deform the results of the analysis Reactions should be specific a suitable indicator is available

А + Т = product

Back titration. A titration in which a reagent is added to a solution containing the analyte, and the excess reagent remaining after its reaction with the analyte is determined by a titration.

This titration is used, when: the titration reaction is too slow, a suitable indicator is not available, there is no useful direct titration reaction the standard solution lacks of stability (fugitive)

А + Тexcess = product1 + Тresidue

Тresidue + Тpadding = product2

displacement titration. A titration in which the analyte displaces a species, usually from a complex, and the amount of the displaced species is determined by a titration.

This titration is used, when: the analytes are unstable substance It is impossible to indicate the equivalent (end) point in direct reaction Analyte doesn’t react with titrant reaction involving the titrant and analyte mustn’t be of known

stoichiometry, quantitatively

Most commonly used indicators in acid-base titration are:Most commonly used indicators in acid-base titration are:

Calculate concentration of primary standard

VM

mC

M

VmEm

NC

V/mT

2

112 V

VCC N

N

А) a measured volume of another primary standard solution

Establishment of secondary standard Establishment of secondary standard concentrationconcentration

where:CCN2N2 and VV22 are concentration and volume

of secondary standard solutionCCN1N1 and VV11 are concentration and volume

of primary standard solution

B) a weighed quantity of a primary standard

VE

mC

mN

1000

where: CN and V are concentration

and volume of secondary standard solution

m and Em are mass and equivalent weight of primary standard

N1V1=N2V2N1V1=N2V2 From the total volume of known solution needed to react the end point, the From the total volume of known solution needed to react the end point, the

concentration of the unknown solution can be calculatedconcentration of the unknown solution can be calculated.. N1 – normality of solution with known concentration N1 – normality of solution with known concentration V1 – volume of solution with known concentration V1 – volume of solution with known concentration N2 – normality of solution with unknown concentration N2 – normality of solution with unknown concentration V2 – volume of solution with unknown concentrationV2 – volume of solution with unknown concentration Example:Example:Problem.Problem. 30 ml of 0.10N NaOH neutralised 25.0 ml of hydrochloric acid. Determine the 30 ml of 0.10N NaOH neutralised 25.0 ml of hydrochloric acid. Determine the

concentration of the acid.concentration of the acid.Solution. Solution. N1-N1- normality of normality of NaOH = 0,1 mol-equiv/lNaOH = 0,1 mol-equiv/lV1 - V1 - volume of volume of NaOH = 30 mlNaOH = 30 mlV2 - V2 - volume of HCl = 25 mlvolume of HCl = 25 mlN2 - normality of HCl - ?N2 - normality of HCl - ?

lequivmolV

VNN /12,0

25

301,0

2

112

BUFFER SOLUTIONS

Buffers are solutions which can resist changes in pH by addition of acid or alkali.

Buffers are mainly of two types: (а) mixtures of weak acids with their salt

with а strong base (b) mixtures of weak bases with their salt

with а strong acid. А few examples are given below:Н2СО3 / NаНСО3 (Bicarbonate

buffer;carbonic acid and sodium bicarbonate)

СН3СООН / СН3СОО Na (Acetate

buffer; acetic acid and sodium acetate)Na2HPO4/ NaH2PO4 (Phosphate buffer)

Acid–Base Concept1) The Arrhenius theoryACID - a substance that provides H+ ions in water

BASE - a substance that provides OH- ions in water

2) The Brønsted-Lowry Theory

All Brønsted–Lowry bases have one or more lone pairs of electrons:

3) The Lewis Acids and Base theoryLEWIS ACID An electron-pair acceptor

LEWIS BASE An electron-pair donor

Acids

Lemon juice contains citric acid, and vinegar contains ethanoic acid.

Some strong acids are hydrochloric acid, sulphuric acid and nitric acid.

Some weak acids are ethanoic acid, citric acid and carbonic acid.

There are many acids present in our everyday lives.

Alkalis

When the oxides of some metals dissolve in water they make an alkali solution.

Alkalis react with acids and neutralise them.

Many everyday substances are alkalis. They feel soapy. They are corrosive.

Alkalis

Alkalis are present in many cleaning substances in use in our homes.

Kitchen cleaners are alkaline because they contain ammonia or sodium hydroxide, which attack grease.

Calcium hydroxide and sodium hydroxide are strong alkalis. The most recognisable and common weak alkali is ammonia.

An acid (НА) reacts with а base (Н2О) to form

the conjugate base of the acid (А-) and the conjugate acid of the base (H3O

+).

Accordingly, the acetate ion (СН3СОО- ) is the

conjugate base of acetic acid (СН3СООН), and the

ammonium ion (NH4+ ) is the conjugate acid of

ammonia (NН3). The acid-base reaction is frequntly

abbreviated НА = Н+ + А- with the participation of H2O

implied.

The relationship between the pH of а solution and the concentrations of an acid and its conjugate base is easily derived.

[НА] [Н+]= K ---------- [А-]Taking the negative log of each term [А-]рН = - log К + log --------- [А-] [А-]рН = pК + log --------- [А-]This relationship known as the Henderson-Hasselbalch equation.

Factors Affecting pH of а Buffer

The pH of а buffer solution is determined by two factors:

1. The value of pK: The lower the value of pK, the lower is the pH of the solution.

2. The ratio of salt to acid concentrations: Actual concentrations of salt and acid in а buffer solution may be varied widely, with по change in рН, so long as the ratio of the concentrations remains the same.

Buffer Capacity On the other hand, the buffer capacity is

determined by the actual concentrations of salt and acid present, as well as by their ratio. Buffering capacity is the number of grams of strong acid or alkali which is necessary for а change in pH of one unit of one litre of buffer solution.

The buffering capacity of а buffer is, definеd аs the ability of the buffer to resist changes in pH when an acid or base is added.

Buffers Act When hydrochloric acid is added to the acetate buffer, the salt reacts

with the acid forming the weak acid, acetic acid and its salt. Similarly when а base is added, the acid reacts with it forming salt and water. Thus, changes in the pH are minimised.

СН3СООН + NaOH = СН3COONa + Н2О СН3СООNа + HCI = СН3СООН + NaCI The buffer capacity is determined by the absolute concentration of

the salt and acid. But the рН of the buffer is dependent on the relative proportion of the salt and acid (see the Henderson - Hasselbalch's equation). When the ratio between salt and acid is 10:1, the pH will be one unit higher than the pKa. When the ratio between salt and acid is 1:10, the pH will be one unit lower than the pKa.

Mechanisms for Regulation of pH

1.   Buffers of body fluids, 2.   Respiratory system, 3. Renal excretion. These mechanisms are

interrelated. Acidic solutions have a high H+

concentration. Base solutions have a low H+ concentration. The pH scale is used to indicate the acidity or alkalinity of a solution. Pure water with an equal number of hydrogen and hydroxide ions has a pH of 7.

Thank you for attention