Titrations: Taking Advantage of Stoichiometric Reactions

Titrimetric Analysis

Titrations: Taking Advantage of

Stoichiometric Reactions

Titrimetric methods include a large and powerful group of quantitative procedures based on measuring the amount of a reagent of known concentration that is consumed by the analyte.

Titrimetry is a term which includes a group of analytical methods based on determining the quantity of a reagent of known concentration that is required to react completely with the analyte.

There are three main types of titrimetry: volumetric titrimetry, gravimetric titrimetry, and coulometrtic titrimetry.

Volumetric titrimetry is used to measure the volume of a solution of known concentration that is needed to react completely with the analyte.

Gravimetric titrimetry is like volumetric titrimetry, but the mass is measured instead of the volume.

Coulometric titrimetry is where the reagent is a constant direct electrical current of known magnitude that consumes the analyte; the time required to complete the electrochemical reaction is measured.

The benefits of these methods are that they are rapid, accurate, convenient, and readily available.

Defining Terms Standard Solution Titration Equivalence Point Back- Titration

Standard Solution: A reagent of a known concentration which is used in the titrimetric analysis.

Titration: This is performed by adding a standard solution from a burette or other liquid- dispensing device to a solution of the analyte until the point at which the reaction is believed to be complete.

Defining terms

Equivalence Point: Occurs in a titration at the point in which the amount of added titrant is chemically equivalent to the amount of analyte in a sample.

Back- Titration: This is a process that is sometimes necessary in which an excess of the standard titrant is added, and the amount of the excess is determined by back titration with a second standard titrant. In this instance the equivalence point corresponds with the amount of initial titrant is chemically equivalent to the amount of analyte plus the amount of back- titrant.

More Defining Terms

End point: The point in titration when a physical change occurs that is associated with the condition of chemical equivalence.

Equivalence Points and End Points

Indicators are used to give an observable physical change (end point) at or near the equivalence point by adding them to the analyte. The difference between the end point and equivalence point should be very small and this difference is referred to as titration error. To determine the titration error: Et= Vep - Veq

Et is the titration error

Vep is the actual volume used to get to the end point

Veq is the theoretical value of reagent required to reach the end point

One can only estimate the equivalence point by observing a physical change associated with the condition of equivalence.

Conductimetric Indication The electrical conductance of a solution

is a measure of its current carrying capacity and is determined by its total ionic strength.

It is a non-specific property. Conductance is defined as the reciprocal

of resistance (Siemans, -1).

Indication Methods

Acid-base titrations especially at trace levels. Relative precision better than 1% at all

levels. Rate of change of conductance as a function

of added titrant used to determine the equivalence point.

High concentrations of other electrolytes can interfere.

Conductimetric Indication

An acid/base indicator is a weak organic acid or a weak organic base whose undissociated form differs in colour from its conjugate base or conjugate acid form.

The behaviour of an acid type indicator is described by the equilibrium;

Theory of Indicator Behaviour


The behaviour of an base type indicator is described by the equilibrium;

In + H2O InH+ + OH-

HIn + H2O In- + H3O+


Primary Standards A primary standard is a highly purified compound that serves as a reference

material in all volumetric and mass titrimetric properties. The accuracy depends on the properties of a compound and the important properties are:

1. High purity

2. Atmospheric stability

3. Absence of hydrate water

4. Readily available at a modest cost

5. Reasonable solution in the titration medium

6. Reasonably large molar mass

Compounds that meet or even approach these criteria are few, and only a few primary standards are available.

Acid-base reactions. Na2CO3, Na2B4O7, KH(C8H4O4), HCl

(cbpt.)

Complex formation reactions. AgNO3, NaCl

Precipitation reactions. AgNO3, KCl

Redox reactions. K2Cr2O7, Na2C2O4, I2

Primary standards for different types of titrations

A substance that can be used for standardisations, and whose concentration of active substance has been determined by comparison to a primary standard.

Secondary Standards

Standard SolutionsStandard solutions play a key role in

titrimetric methods.Desirable Properties of Standard Solutions:1. Sufficiently stable2. React rapidly with analyte3. React completely with analyte4. Endure a selective reaction with analyte

Types of Titrations Volumetric

Acid-Base/Neutralisation Redox

Gravimetric Precipitation Complexometric Coulometric

Neutralisation Titrations

The neutralisation reactions between acids and bases used in chemical analysis.

These reactions involve the combination of hydrogen and hydroxide ions to form water.

Neutralisation Titrations For any actual titration the correct end

point will be characterised by a definite value of the hydrogen ion concentration.

This value will depend upon the nature of the acid and the base, the concentration of the solution and the nature of the indicator.


A large number of substances called neutralisation indicators change colour according to the hydrogen ion concentration of the solution.

The end point can also be determined electrochemically by either potentiometric or conductimetric methods.

Example of volumetric titration set up

http://wine1.sb.fsu.edu/chm1045/notes/Aqueous/Stoich/Aqua02.htm


Titrations Titration Curve

1.) Graph showing how the concentration of one of the reactants varies as titrant is added.

Understand the chemistry that occurs during titration Learn how experimental control can be exerted to influence the

quality of an analytical titration- No end point at wrong pH- Concentration of analyte and titrant and size of Ksp influence end point- Help choose indicator for acid/base and oxidation/reduction titrations

Sharpness determined by titration condition

Monitor pH, voltage, current, color, absorbance, ppt.

Titrations Precipitation Titration Curve

4.) Three distinct regions in titration curve Before, at and after the equivalence point.

before

after

at

Titration curves

Weak Acid Titration Curve

2

4

6

8

10

12

0 10 20 30 40 50

Buret Volume (mL)

pH

equivalence point

mid point

A Typical Titration Curve


2

4

6

8

10

12

0 10 20 30 40 50

Buret Volume (mL)

pH

equivalence point

Find the Equivalence Point (Geometric method)

1) using a ruler, draw lines that follow the flat, more horizontal part of the curve

2) draw a line that follows the flat, more vertical part of the curve


2

4

6

8

10

12

0 10 20 30 40 50

Buret Volume (mL)

pH

equivalence point


3) using a ruler, measure the distance between the top intersection and the bottom intersection

4) the geometric center of this line segment is the equivalence point


2

4

6

8

10

12

0 10 20 30 40 50

Buret Volume (mL)

pH

equivalence point


5) draw a vertical line from the equivalence point to the x-axis

6) where the line crosses the x-axis is the volume at the equivalence point(28.7 mL in this case)

Theory of Indicator Behaviour Acid-base indicators take

advantage of the rapid change in pH of the solution being titrated as the equivalence point is reached.

The choice of an indicator is determined by the pH of the solution at the equivalence point.


Figure 19.4 Colors and approximate pH range of some common acid-base indicators

Strong Acid-Strong base Strong Acid-Strong base TitrationTitrationpH starts low and increases gradually as acid pH starts low and increases gradually as acid is neutralized by the added baseis neutralized by the added baseClose to equivalence point pH rises steeplyClose to equivalence point pH rises steeplyBeyond this, pH increases slowly with Beyond this, pH increases slowly with addition of more baseaddition of more baseEquivalence pointEquivalence point

– the mol OHthe mol OH1- 1- = mol H= mol H1+ 1+

–The pH = 7The pH = 7

Figure 19.6 Curve for a strong acid-strong base titration

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Weak Acid-Strong Base Weak Acid-Strong Base TitrationTitrationInitial pH of weak acid starts higher than pH of a Initial pH of weak acid starts higher than pH of a strong acid of the same molarity and then increases as strong acid of the same molarity and then increases as the acid is neutralized by added basethe acid is neutralized by added baseGradually rising curve is called buffer region as the Gradually rising curve is called buffer region as the weak acid HA reacts with the strong base to produce weak acid HA reacts with the strong base to produce its conjugate base Aits conjugate base A1- 1-

At the mid-point of buffer region half of original At the mid-point of buffer region half of original HA has reacted with the base to produce AHA has reacted with the base to produce A1- 1-

Unreacted [HA] = [AUnreacted [HA] = [A1- 1- ] so Ratio [A] so Ratio [A1- 1- ] / [HA] = 1 ] / [HA] = 1

Figure 19.7

Curve for a weak acid-strong base

titration

Titration of 40.00mL of 0.1000M HPr with 0.1000M NaOH

[HPr] = [Pr-]

pH = 8.80 at equivalence point

pKa of HPr = 4.89

methyl red


Weak Acid-Strong Base Weak Acid-Strong Base TitrationTitrationUsing the Henderson-Hasselbalch equationUsing the Henderson-Hasselbalch equationpH = pKpH = pKa a + log ([A+ log ([A1- 1- ] / [HA] )] / [HA] )pH = pKpH = pKa a + log (1) + log (1) pH = pKpH = pKa a Experimental procedure used for estimating KExperimental procedure used for estimating Ka a of of unknown acidunknown acidClose to equivalence point pH rises steeplyClose to equivalence point pH rises steeplyAt the equivalence point pH is greater than 7 due to At the equivalence point pH is greater than 7 due to presence of the weak base Apresence of the weak base A1- 1-

Figure 19.8

Curve for a weak base-strong acid

titration

Titration of 40.00mL of 0.1000M NH3 with 0.1000M HCl

pH = 5.27 at equivalence point

pKa of NH4+

= 9.25


Weak Base-Strong Acid Weak Base-Strong Acid TitrationTitrationInitial pH of weak base starts higher than 7 but Initial pH of weak base starts higher than 7 but

lower than pH of a strong base of the same lower than pH of a strong base of the same molarity and then decreases as the weak base is molarity and then decreases as the weak base is neutralized by added acidneutralized by added acidGradually dropping curve is called buffer region Gradually dropping curve is called buffer region as the weak base B reacts with the strong acid to as the weak base B reacts with the strong acid to produce its conjugate acid BHproduce its conjugate acid BH1+ 1+

At the mid-point of buffer region half of original At the mid-point of buffer region half of original B has reacted with strong acid to produce BHB has reacted with strong acid to produce BH1+ 1+

Unreacted [B] = [BHUnreacted [B] = [BH1+ 1+ ] so Ratio [B] / [BH] so Ratio [B] / [BH1+ 1+ ] = 1 ] = 1

Neutralisation Titrations- Polyfunctional acids

e.g. Phosphoric acid

Yield multiple end points in a titration.Yield multiple end points in a titration.

pKa = 7.19

pKa = 1.85

Figure 19.9

Curve for the titration of a weak polyprotic acid.

Titration of 40.00mL of 0.1000M H2SO3 with 0.1000M

NaOH


Redox Titrations

Titration of a reducing agent by an oxidizing agent or titration of an oxidizing agent by a reducing agent.

Potassium Permanganate Potassium Dichromate

Cerium IV Iodine

Redox Titrations Potassium Permanganate

MnO4- + 8H+ + 5e- Mn2+ + 4H2O

StandardisationStandardisation Sodium oxalate or arsenic (III) Sodium oxalate or arsenic (III)

oxideoxide

Many AnalysesMany Analyses

Redox Titrations

Persulphates: Add an excess of iron (II) S2O8

2- + 2Fe2+ + 2H+ 2Fe3+ + 2HSO4-

The excess iron (II) is determined by back titration against standardised permangenate.

MnO4- + 8H+ + 5Fe2+ Mn2+ + 5Fe2+ + 4H2O

Redox Titrations

Hydrogen Peroxide: 2MnO4

- + 5H2O2 + 6H+ 2Mn2+ + 5O2 + 8H2O

Nitrites: 2MnO4

- + 5NO2- + 6H+ 2Mn2+ + 5NO3

- + 3H2O

Complexometric Titrations

Titrations between cations and complex forming reagents.

The most useful of these complexing agents are organic compounds with several electron donor groups that can form multiple covalent bonds with metal ions.


Most metal ions react with electron-pair donors to form coordination compounds or complex ions.

The donor species, or LIGAND, must have at least one pair of unshared electrons available.


The number of bonds a cation forms with an electron donor is called the COORDINATION NUMBER.

Typical values are 2, 4 and 6. The species formed as a result of

coordination can be electrically positive, neutral or negative.


A chelate is produced when a metal ion coordinates to two or more donor groups within a single ligand.

For example the copper complex of glycine.


Cu2+ + 2 H C

NH2

H

CO

OH

O

NH2

C

CH2

O

CuO

NH2

C

CH2

O

+ 2H+


A ligand with a single donor group is called unidentate.

Glycine is bidentate. Tri, tetra, penta and hexadentate

chelating agents are also known.


Multidentate ligands have two advantages over unidentate ligands.

They react more completely with cations to provide a sharper endpoint.

The reaction is a single step process.


Tertiary amines that also contain carboxylic acid groups form remarkably stable chelates with many metal ions.

Ethylenediaminetetraacetic Acid EDTA

CH2 CH2 NNHOOCCH2 CH2COOH

CH2COOHHOOCCH2

Complexometric Titrations EDTA can complex a large number of

metal ions. Approximately 40 cations can be

determined by direct titration. EDTA is usually used as the disodium

salt, Na2H2EDTA

H2EDTA2- + M2+ [M(EDTA)]2- + 2H+


Because EDTA complexes most cations, the reagent might appear at first glance to be totally lacking in selectivity.

However, great control can be acheived by pH regulation and the selection of suitable indicators.


Indicators are generally complexing agents which undergo a colour change when bonded to a metal ion.

H2EDTA2- + [M(Ind)] [M(EDTA)]2- + Ind2- + 2H+


Typical indicators are: Murexide Solochrome black Calmagite Bromopyrogallol red Xylenol orange


Typical applications: Determination of cations Hardness of water

Precipitation Titrations Titrations between analytes and

reagents resulting in the formation of a precipitate.

The most useful of these precipitating reagents is silver nitrate.

Titrimetric methods based upon the use of silver nitrate are sometimes called Argentometric titrations.

Precipitation Titrations

Used for the determination of many anions including: halides divalent anions certain fatty acids


Precipitation titrations are based on the SOLUBILITY PRODUCT of the salt, KSP.

The smaller KSP, the less soluble the silver salt and the easier it is to determine the endpoint


Endpoint determination is by coloured indicators (usually back titrations) or turbidity methods.

The most accurate is the VOLHARD METHOD.


VOLHARD METHOD A back titration of thiocyanate

ions against the excess silver ions using an iron (II) salt as the indicator.


Blood RedBlood Red

AgAg++ + SCN + SCN- - AgSCN AgSCN

FeFe3+3+ + SCN + SCN- - FeSCNFeSCN2+2+

Titrations Spectrophotometric Titrations

1.) Use Absorbance of Light to Follow Progress of Titration Example:

- Titrate a protein with Fe3+ where product (complex) has red color- Product has an absorbance maximum at 465 nm- Absorbance is proportional to the concentration of iron bound to protein

Analyte(colorless)

(red)titrant(colorless)

As Fe3+ binds protein

solution turns red

Titrations Spectrophotometric Titrations

1.) Use Absorbance of Light to Follow Progress of Titration Example:

- As more Fe3+ is added, red color and absorbance increases, - When the protein is saturated with iron, no further color can form- End point – intersection of two lines (titrant has some absorbance at 465nm)

As Fe3+ continues to bind proteinred color and absorbance increases.

When all the protein is bound to Fe3+,no further increase in absorbance.

References Skoog, D., West, D., Holler, F.J., &

Crouch, S. (2000). Analytical Chemistry: An Introduction. 7th ed. Thomson Learning, Inc: United States of America.


www.psigate.ac.uk/newsite/ reference/plambeck/chem1/p01173.htm

http://www2.hmc.edu/~karukstis/chem21f2001/tutorials/tutorialStoichiFrame.html

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Titrations: Taking Advantage of Stoichiometric Reactions