Basic Concepts in Volumetric analysis

Index

S. No Content Page No.

1 INTRODUCTION 2

2 The basic Principles of Volumetric Analysis 4

3 Conditions for Volumetric Analysis 4

4 Precautions of Volumetric Analysis: 5

5 TYPES OF TITRATIONS 6

6 Methods to determine the endpoint 9

7 Primary and secondary standards 10

8 Methods of expressing concentration of solution 15

@ Madhusudhana Reddy Induri Ph.D

INTRODUCTION

Volumetric Analysis was first introduced by Jean Baptiste Andre

Dumas, a French chemist. It is a mode of quantitative analysis which is based on the

determination of the volume of a solution of known concentration (standard)

required to react quantitatively with a solution of the substance to be analysed. It is

also known as titrimetric analysis.

There are many different types of titrations that differ by the titrant used and

substances that can be determined. While every titration is different, they all share

similar characteristic – but before we will start to discuss them let’s define several

important terms.

Titrate: The Substance to be analyzed is called titrate.

Titrant: The reagent of known concentration which is added to the solution of the

substance to be analyzed is called titrant.

Titration Curve: A plot of solution pH versus titrant volume during a titration.

Titration: The process of finding out the volume of the titrant required to react

completely with a known volume of solution (Titrate) under analysis is known as

titration.

Indicator: Auxiliary agents used to determine the end point of titration. A species

added to the analyte to give an observable change at (which is the endpoint) or near

the end point.

End Point: The end point is the point where the system changes when the moles

of the reacting titrant exceed the moles of the substance being titrated. When using

2

@ Madhusudhana Reddy Induri Ph.D

an indicator, the end point occurs when enough titrant has been added to change the

color of the indicator.

Equivalence Point: The equivalence point is the exact point in a titration when

the moles of titrant equal the substance being titrated.

Titration Flask: In volumetric analysis, two solutions are always made to react

in a conical flask known as titration flask.

Standard Solution: It is a solution of definite concentration or of known

strength.

Standardization: It is a process whereby the concentration of a solution is

determined by the known concentration of solution.

Titration Error: It is the smallest difference between equivalence point and end

point. This difference is because an indicator always produces the visual change

either a little before or after the equivalence point.

3

@ Madhusudhana Reddy Induri Ph.D

The basic Principles of Volumetric Analysis:

1. The one solution to be analyzed contains an unknown amount of chemicals.

2. The reagent of known concentration reacts with chemical of unknown amount

in the presence of an indicator to show the end-point. This is the point which

shows the completion of the reaction.

3. The volumes are measured by a titration which completes the reaction

between reagent and solution.

4. The volume and concentration of reagent used in the titration give the amount

of reagent in moles.

5. The amount of unknown chemical in the measured volume of solution is

calculated by using the mole ratio of the equation.

6. The amount of unknown chemical in the original sample is calculated by the

amount of unknown chemical in the measured volume.

Conditions for Volumetric Analysis:

1. Chemical reaction must be simple and should take place quantitatively according

to a definite equation to form known products.

2. The reaction should be instantaneous under the experimental conditions

maintained.

3. There should a marked change in some of the properties of the solution to be

analysed at equivalence point.

4. The reaction should take place essentially to completion under the experimental

conditions maintained.

5. The end point should be well defined either between the reacting substances or

by the use of an indicator.

4

@ Madhusudhana Reddy Induri Ph.D

Precautions of Volumetric Analysis:

Some important points should be remembered in doing volumetric analysis to get the

accurate results.

1. All the equipment’s like burette, beaker, pipette and volumetric flask should

be washed properly with distilled water before taking them in use as the

presence of any other chemical can be the reason for wrong measurement.

2. Index finger should be used for pipetting the solution.

3. The process of filling the pipette should be accurate to avoid excess addition of

solutions.

4. Do not blow through the pipette to expel the last drop of solution from it;

simply touch the inner surface of the titration flask with the nozzle of the

pipette for this purpose.

5. The volumetric flask should be shaking well after adding the indicator and

also the titration flask with addition of each drop of solution from burette.

6. Contamination should be avoided.

7. The indicator should not be used in excess.

8. The flask should be removed as the indicator changes color.

9. Sometimes, an air bubble in the nozzle of the burette can be the reason for

altering the readings, so, it must be removed before taking the initial reading.

10. The burette should not be leaked during titration.

11. Keep eyes in the level of the liquid surface during the time of taking the

burette reading or measuring flask and pipette etc.

12. Lower meniscus and upper meniscus are always read in case of color less and

colored solutions respectively.

5

@ Madhusudhana Reddy Induri Ph.D

TYPES OF TITRATIONS

Based on the nature of solvents and chemical reaction:

The titrimetry can be categorized into four types which are based on the type of

reaction involved in the process.

TITRATION

Aqueous titrations Non-Aqueous Titrations

1. Acid-Base 1. Acid-Base

2. Redox 2. Redox

3. Complexometric a. Iodometry

4. Precipitation b. Iodimetry

Chemicals to be Indicators to

Types Reagents to be used
analyzed be used

Acid / Base acid or base alkali or acid pH indicator

compound containing
ion that form

Precipitation the other ion needed to conductivity
insoluble salt

form the insoluble salt

natural color
oxidizing or reducing reducing or

Redox change or
agent oxidizing agent

redox indicator

metal ion that form metal ion
Complexometric complexing agent

complexes indicator

6

@ Madhusudhana Reddy Induri Ph.D

Acid-base Titrations: The strength of an acid can be determined using a standard

solution of base is called as acidimetry. In the same way, the strength of a base can be

found with the help of standard solution of an acid is known as alkalimetry. The acid-

base titration is based on the reaction that neutralization between a base or an acidic

and analyte

Redox Titrations: The redox titration is also known as an oxidation-reduction

reaction. In this type of titration, the chemical reaction takes place with a transfer of

electrons in the reacting ions of aqueous solutions.

Precipitation Titrations: The titration is based on the insoluble precipitate

formation when the two reacting substance are brought into contact are called as

precipitation titration.

Complexometric Titrations: This type of titration is similar to precipitation

titration in that a solid precipitates out of the sample when a reagent is added. The

difference is that in complexometric titration, the solid is formed more quickly and

more completely than in precipitation titration, which reduces errors in

measurement. Ethylenediaminetetraacetic acid, an acidic powder better known as

EDTA, is commonly used in this type of titration because it readily bonds with

metals. This type of titration can be used to measure the ingredients within soaps

and detergents.

7

@ Madhusudhana Reddy Induri Ph.D

Based on method of titration:

They are of three types of titration based on the method used in process of titration.

TITRATION

Direct In-direct Back

1. Direct Titration: As the name indicates, it is basic titration. In this method,

the solution of the substance to be determined quantitatively is directly

titrated with a suitable titrant by using an appropriate indicator or a suitable

instrument to locate the end point.

Example: The titration of strong acids such as HNO3, H2SO4, HCl, etc. with

strong alkalis such as NaOH, KOH.

2. In-direct titration: Theoretically it is converting a substance into an acid

and analyzing with a base (also vice-verse). This is a methods extrapolated to

use titration for non-readily reactive substances. A substance can be weakly

acidic and so it does not permit for precise analysis by direct titration. So first

that substance is chemically altered to be more reactive in acidic or basic form

and then analyzed by adding a titrant.

3. Back Titration: This titration is followed when the direct titration is not

possible. For example, the reaction between determined substances and

titrant can be too slow, or there can be a problem with end point

determination.

8

@ Madhusudhana Reddy Induri Ph.D

In this method, the substance to be analyzed is dissolved in a known sufficient

volume of a standard solution of acid or alkali. The excess of acid or alkali

remaining in the solution is back titrated by using suitable indicator. A blank

determination (without substance) is done from which the difference is the

volume required for the substance is found out.

Back titrations are used when:

1. One of the reactants is volatile, for example ammonia.

2. An acid or a base is an insoluble salt, for example calcium carbonate.

3. A particular reaction is too slow.

Direct titration of weak acid/base: the end point is very difficult to observe.

METHODS TO DETERMINE THE ENDPOINT

1. pH indicator: This is a substance which shows the chemical change by

changing the color.

2. A potentiometer: This is used to measure the electrode potential of the

solution. These are used for redox titration. They show the end point with

changing potential of the working electrode.

3. pH meter: It is a ion-selective electrode. In pH meter the potential of

electrode depends on the amount of H+ ion present in the solution. The pH of

the solution can be measured in the whole titration. This gives more correct

result than indicators.

4. Conductance: The conductivity of solutions is also changed in titration and

it depends on the ions present in the solution, mobility of ions and ions

concentration.

5. Color change: In the redox reactions, the color of solution changes without

use of indicator. This is due to different oxidation states of the product.

9

@ Madhusudhana Reddy Induri Ph.D

PRIMARY AND SECONDARY STANDARDS

Standards: The solution whose concentration is exactly known is called standard

solution. Standards are materials containing a precisely known concentration of a

substance for use in quantitative analysis. A standard provides a reference that can

be used to determine unknown concentrations or to calibrate analytical instruments.

There are two types of standard solutions used in volumetric analysis. They are:

1. Primary Standard and 2. Secondary Standard

Primary Standard

From the name itself it is obvious that this is a standard which comes first. That’s

why the name is primary.

A primary standard is a chemical or reagent which has certain properties such as

a) It is extremely pure,

b) Highly stable

c) It is anhydrous

d) It is less hygroscopic

e) Has very high molecular weight

f) Can be weighed easily

g) Should be ready to use and available

h) Should be preferably non toxic

i) Should not be expensive

Let us understand each point one by one:

A primary standard material should be extremely pure which means that it

should be a chemical of high grade of purity, preferably 99.98%. In a chemistry lab

you will come across chemicals of different grade of purity. If you check the label you

10

@ Madhusudhana Reddy Induri Ph.D

will notice a number with percentage termed as purity. So, when a chemical has

purity of 99.98% or more it is a suitable material to be considered for primary

standard. Usually those chemicals that exceed the requirement of

American Chemical Society (ACS) are extremely pure and can be used for

making primary standard. It is an analytical reagent of extreme purity which is

specially manufactured for the purpose of being used as primary standard.

It should be highly stable which means it usually does not react easily when

kept in its pure form. Or in other words it should have very low reactivity. This is

important because if a reagent reacts easily with atmospheric oxygen or water or

changes its property over time then it is unreliable. We can never use such unstable

and unreliable chemicals as standard.

It should be anhydrous which means that it does not contain any water

molecule in its molecular structure. For example, in a chemistry laboratory you will

come across same chemical with different number of water molecules attached with

it. Let us see the example of Epsom salt. The chemical name is magnesium sulphate.

So we write the formula is MgSO4. But the chemical Epsom salt which is found in

grocery or drug store is a chemical with formula MgSO4.7H2O. Therefore if you want

to prepare a primary standard of magnesium sulphate you should purchase an

anhydrous MgSO4 preferably an analytical reagent grade chemical and with purity

greater than 99.98%. Remember such reagents are usually available with common

vendors. Just being anhydrous is not sufficient. The chemical preferably should be

less hygroscopic that is on opening the container it should not absorb water

molecules from atmosphere. Why water should not enter into chemical? This will be

clearer in the following point where it is explained how presence of water molecule

11

@ Madhusudhana Reddy Induri Ph.D

can affect the simple calculation of standard concentration making the entire

standardization procedure unreliable.

Has very high molecular weight compared to its other similar forms. If we take

the same example of Epsom salt you can understand this statement. Let us take 1

gram of MgSO4 fit for making a primary standard and we call it salt A. Now take 1

gram of MgSO4.7H2O fit for common uses and name it salt B. Now you compare the

actual weight of magnesium sulphate to make a standard solution for both chemicals.

The molecular weight of MgSO4 is 108 but for MgSO4.7H2O it is 234.

In case of one molecule of salt A the weight of actual MgSO4 will be 108 atomic

mass unit. But In case of one molecule of salt B the weight of actual MgSO4 will be

108 out of its total weight of 234 atomic mass unit.

108 gram salt A (MgSO4) will give 108 gram of MgSO4

So 1 gram MgSO4 salt will give = 108/108 = 1 gram of MgSO4

But 234 gram salt B (MgSO4.7H2O) will give 108 gram of MgSO4

So 1 gram MgSO4.7H2O salt will give = 108/234 = 0.461 gram of MgSO4

Therefore if you by mistake make a standard out of salt B, you will actually be taking

0.461 gram of MgSO4 and calculating it as 1 gram. So with this faulty standard

estimation of MgSO4 in other unknown solution will give less result than the actual

concentration Therefore it is important that primary standards must be anhydrous

and of high molecular weight.

It can be weighed easily because it is so pure that its weight is in fact a true

representative of number of moles present in its actual weight. One of the uses of

primary standard is to standardize a volumetric solution. That means they are used

12

@ Madhusudhana Reddy Induri Ph.D

for standardization of titration of solutions. It can be used for titration of acids as

well as bases.

Some of the Examples for primary standards:

1. Acid-base titrations

a. Sodium Carbonate (Anhydrous)

b. Potassium Hydrogen Phthalate

c. Oxalic acid

2. Redox titrations

a. Potassium Dichromate

b. Potassium Bromate

c. Potassium Iodate

d. Oxalic acid

e. Arsenic Trioxide

3. Precipitation titrations

a. Silver Nitrate

4. Complexometric titrations

a. Pure Metallic Zinc

b. Zinc Chloride

Secondary Standard

A secondary standard is a substance which may be used for standardizations,

and whose content of the active substance has been found by comparison against a

primary standard. A secondary standard is used by standard laboratories such as

companies involved in preparation of reagents, kits or laboratories responsible for

producing quality control material for other labs. Secondary standard is used for the

13

@ Madhusudhana Reddy Induri Ph.D

purpose of calibration of control material in smaller lab for analysis of unknown

concentration of a substance. So basically, secondary standard serves the purpose of

external quality control for smaller labs. This makes it essential that the secondary

standard must first be standardized against the primary standard.

A secondary standard is a chemical or reagent which has certain properties such as

a) It has less purity than primary standard

b) Less stable and more reactive than primary standard

c) But its solution remains stable for a long time

d) Titrated against primary standard

Usually a chemical fit for being a standard chemical yet does not meet the

requirements of a primary standard.

The best example is anhydrous sodium hydroxide. It is extremely hygroscopic.

As soon as the bottle is opened sodium hydroxide starts absorbing moisture from

atmosphere and within no time it becomes moist. So, sodium hydroxide cannot be

used as a primary standard for the reason that it absorbs water and carbon dioxide

from the atmosphere and the composition of its solution is subject to wide variations

at different periods.

Another example is potassium permanganate (KMnO4) very often as

secondary standard. It is a good oxidizing agent or in other words it is reactive so less

stable. More often due to its reactivity, its own oxidized product manganese oxide

(MnO2) contaminates the content. That’s why it is unsuitable for being a primary

standard. But it can be used very well as a secondary standard.

Similarly Sodium thiosulphate absorbs CO2 from the atmosphere and gets

decomposed. A deposit of sulphur settles at the bottom.

Next question is why is secondary standard called still a standard? This is so

because secondary standard is used as a calibrator by smaller laboratories involved

in actual analysis of unknown samples.

14

@ Madhusudhana Reddy Induri Ph.D

METHODS OF EXPRESSING CONCENTRATION OF SOLUTION

In chemistry, a solution is a homogeneous mixture composed of only one phase. It’s

either solid, liquid or a gas. The solubility of a solute is the amount of solute that

will dissolve in a given amount of solvent at particular temperature to produce a

saturated solution.

Solute: A substance dissolved in another substance, usually the component of a

solution present in the lesser amount.

Solvent: A substance in which another substance is dissolved, forming a solution.

The concentration of a solution is a macroscopic property, represents the amount of

solute dissolved in a unit amount of solvent or of solution, and can be expressed in a

variety of ways (qualitatively and quantitatively).

1. Qualitative Expressions of Concentration

2. Semi-Quantitative Expressions of Concentration

3. Quantitative Expressions of Concentration

Qualitative Expressions of Concentration

A solution can be qualitatively described as:

Dilute: A solution that contains a small proportion of solute relative to solvent

Concentrated: A solution that contains a large proportion of solute relative to

solvent

Semi-Quantitative Expressions of Concentration

A solution can be semi-quantitatively described as:

Unsaturated: A solution in which more solute will dissolve

Saturated: A solution in which no more solute will dissolve

15

@ Madhusudhana Reddy Induri Ph.D

For example, at 0oC, we can dissolve a maximum of 35.7 g of solid NaCl in 100 mL of

water (a saturated solution). Any additional solid NaCl that we add to the saturated

solution simply falls to the bottom of the container and does not dissolve.

Quantitative Expressions of Concentration

There are a number of ways to express the relative amounts of solute and solvent in a

solution. Which one we choose to use often depends on convenience. For example, it

is sometimes easier to measure the volume of a solution rather than the mass of the

solution.

Note that some expressions for concentration are temperature-dependent (i.e., the

concentration of the solution changes as the temperature changes), whereas others

are not. This is an important consideration for experiments in which the temperature

does not remain constant.

1. Percentage: It refers to the amount of the solute per 100 parts of the solution.

It can also be called as parts per hundred (pph). It can be expressed by any of

following four methods,

Mass Percentage [% w/w]: When the concentration is expressed as the percent

of one component in the solution by mass it is called mass percentage. It is defined as

the amount of solute in grams present in 100 grams of the solution.

Question: What is the mass percentage of urea present in the solution, when 10 g of

urea is dissolved in 90 g of water?

16

@ Madhusudhana Reddy Induri Ph.D

Volume Percentage [% v/v]: It is defined as the volume of solute in mL present

in 100 mL solution. Sometimes we express the concentration as a percent of one

component in the solution by volume, it is then called as volume percentage and is

given as:

Question: 25 mL of alcohol is added to a certain amount of water to make a total of

200 mL solution. What is the volume percent of alcohol now?

Mass by Volume Percentage: It is defined as the mass of solute present in 100

mL of solution.

For example, a 10% mass by volume solution means that 10 gm solute is present in

100 mL of solution.

2. Parts per million (ppm) and parts per billion (ppb): When a

solute is present in trace quantities, it is convenient to express the concentration in

parts per million and parts per billion. It is the number of parts of solute per

million or per billion parts of the solution. It is independent of the temperature.

3. Strength: The strength of solution is defined as the amount of solute in grams

present in one litre (or) of the solution. It is expressed in g/litre.

4. Molarity (M): It is defined as the number of moles of solute

per liter of solution.

Molarity = no. of moles of solute/volume of solution liters

Note that molarity is spelled with an “r” and is represented by a capital M.

17

@ Madhusudhana Reddy Induri Ph.D

Question: Calculate the molarity of a solution prepared by dissolving 15.5 g solid

KBr (Mol. Wt: 119 g/mol) in enough water to make 1.25 L solution?

5. Molality (m): It is defined as the number of moles of solute

per kilogram of solvent.

molality = no. of moles of solute/mass of solvent in kg

Question: Determine the molality of 3000 g of solution containing 37.3 g of KCl

(Mol. Wt: 74.55 g/mol)?

Although their spellings are similar, molarity and molality cannot be

interchanged. Molarity is a measurement of the moles in the total volume of the

solution, whereas molality is a measurement of the moles in relationship to the

mass of the solvent.

When water is the solvent and the concentration of the solution is low, these

differences can be negligible (d = 1.00 g/mL). However, when the density of the

solvent is significantly different than 1 or the concentration of the solution is high,

these changes become much more evident.

6. Normality (N): It is defined as the number of gram equivalents (equivalent

weight in grams) of a solute present per liter of the solution. Normality changes with

temperature since it involves volume. Gram equivalent weight is the measure of the

reactive capacity of a molecule.

Solutions in term of normality generally expressed as,

18

@ Madhusudhana Reddy Induri Ph.D

Number of gram equivalent of solute = Mass of solute in gram/ equivalent

weight of solute

Equivalent weight of solute (E) = Molar mass of solute/ Valence factor

Valence factor for base = acidity of base

Valence factor for acid = basicity of acid

Valence factor for element = Valency

Question: What is the normality of a solution of 23.9 g of sodium hydroxide in 1.25

L of solution)?

7. Mole fraction: The mole fraction of one component in a solution is defined as

the number of moles of that component divided by the total number of moles of all

components in the solution.

It is denoted by the symbol X. Let us suppose that a solution contains two

components A and B and suppose that nA moles of A and nB moles of B are present in

the solution then,

Mole Fraction of A (XA) =

Mole Fraction of B (XB) =

And, XA+XB = 1

Question 1: How much water would be present in 100 mL of 20% aqueous solution

of sugar by volume?

Question 2: What would be concentration of the solution formed by adding two

moles of solute in 1 kg water?

Question 3: What will be mole fraction of solute of a binary solution if that of its

solvent is 0.2?

19