lecture 1 Analytical Chemistry.ppt

Analytical and Inorganic Chemistry
Chapter 1
Introduction in Quantitative Analysis
A- Acid Base Titrations
Prepared by Dr. Hoda Abd El-Shafy Shilkamy
Chemistry Department –Faculty of Science –Sohag University

Introduction
The term titrimetric analysis refers to quantitative chemical analysis carried
out by determining the volume of a solution of accurately known
concentration which is required to react quantitatively with a measured
volume of a solution of a substance to be determined. The solution of
accurately known concentration is called standard solution.
The term volumetric analysis was used for this form of quantitative
determination but it has now been replaced by titrimetric analysis. In
titrimetric analysis the reagent of known concentration is called titrant and the
substance being titrated is termed the titrand.
The standard solution is usually add from a long graduated tube called burette.
The process of adding the standard solution until the reaction is just complete
is termed titration. The point at which this occurs is called equivalence point
or the theoretical (or stoichiometric) end point. The completion of the titration
is detected by some physical change, produced by the standard solution itself
or, more usually, by the addition of an auxiliary reagent, known as an
indicator ;

For use in titrimetric analysis a reaction must have the following conditions:
1- There must be a simple reaction which can be expressed by a chemical
equation; the substance to be determined should react completely with the
reagent in stoichiometric or equivalent properties.
2- The reaction should be relatively fast. (Most ionic reaction satisfy this
condition.) In some cases the addition of a catalyst may be necessary to
increase the speed of a reaction.
3- There must be an alteration in some physical or chemical property of the
solution at the equivalence point.
4- An indicator should be available which, by a change in physical properties
(color or formation of a precipitate), should sharply define the end point of the
reaction.

Definition of some terms
Titration
Titration is the process in which the standard reagent is added
to a solution of an analyte until the reaction between the
analyte and reagent is complete.
Equivalence point and End point
The equivalence point of a titration is a theoretical point that can not be
determined experimentally. Instead, we can only estimate its position by
observing some physical change associated with the condition of equivalence.
This change is called the end point for titration.
Indicators
Indicators are often added to analyte solution in order to give an observable
physical change (end point) at or near the equivalence point. In other wards
indicator is a compound having a physical property (usually color) that
changes abruptly near the equivalence point of a chemical reaction.

Desirable properties of standard solutions
The ideal standard solution for titrmetric method will:
1- be sufficiently stable so that it is only necessary to determine the concentration once,
2- react rapidly with the analyte so that the time required between additions of reagent
is minimized .
3- react more or less completely with the analyte so that satisfactory end points are
realized.
4- Undergo a selective reaction with the analyte that can be described by simple
balanced equation.
Few reagents meet all these ideal perfectly.
Methods for establishing the concentration of standard solutions
Two basic methods are used to establish the concentration of such solutions. The first
is the direct method in which a carefully weighed quantity of primary standard is
dissolved in a suitable solvent and diluted to an exactly known volume in a volumetric
flask.

The second is by standardization the process whereby the concentration of a
reagent is determined by reaction with a known quantity of a second reagent. A
titrant that is standardized against another standard solution is some times
referred as a secondary standard solution. If there is a choice, then solution are
prepared by the direct method. On the other hand , many reagents lack the
properties required for a primary standard and therefore required standardization.
Direct titration and back titration
When a titrant reacts directly with an analyte, the procedure is termed a direct
titration. It is some times necessary to add an excess of standard titrant and then
determine the excess amount by back titration with a second standard titrant. In
other wards back titration is a process in which the excess of standard solution
used to react with an analyte is determined by titration with a second standard
solution. Back – titration are often required when the rate of reaction between the
analyte and reagent is slow or when the standard solution lacks stability. In back
– titration, the equivalence point corresponds to the point when the amount of
initial titrant is chemically equivalent to the amount af analyte plus the amount
of back titrant.

Classification of reaction in titrimetric analysis
The reaction employed in titrmetric analysis fall into four main classes. The
first three of these involve no change in oxidation state as they are dependent
upon the combination of ions. But the fourth class, oxidation-reduction
reactions, involves a change of oxidation state or, expressed another, a transfer
of electron.
1- Neutralization reaction, or acidimetry and alkalimetry.
These include the titration of free bases, or those formed from salts of weak
acids by hydrolysis with a standard acid (acidimetry), and the titration of free
acids, or those formed by the hydrolysis of salts or weak bases, with a standard
base (alkalimertry). The reaction involve the combination of hydrogen and
hydroxide ions to form water. Also under this heading must be included
titrations in non-aqueous solvents, most of which involve organic compounds.

2- Precipitation reaction. These depend upon the combination of ions to form
a simple precipitate as in the titration of silver ion with solution of chloride. No
change in oxidation state occurs.
3- Complex formation reaction. These depend upon the combination of ions,
other than hydrogen or hydroxide ion, to form a soluble slightly dissociated ion
or compound, as in the titration of a solution af a cyanide with silver nitrate.
Ethylendiaminetera-acetic acid, largely as the disodium salt of EDTA, is a very
important reagent for complex formation titration and has become on of the
most important reagents used in titrimetric analysis.
4- Oxidation-reduction reaction. Under this heading are included all reactions
involving change in oxidation number or transfer of electrons among the
reactive substance. The standard solutions are either oxidizing or reducing
agents.

Titration Curves
To find the end point we monitor some property of the titration reaction that has a
well defined value at the equivalence point. For example, the equivalence point for
a titration of HCl with NaOH occurs at a pH of 7.0. We can find the end point,
therefore, by monitoring the pH with a pH electrode or by adding an indicator that
changes color at a pH of 7.0.

Suppose that the only available indicator changes color at a pH of 6.8. Is this end
point close enough to the equivalence point that the titration error may be safely
ignored? To answer this question we need to know how the pH changes during the
titration.
titration curve provides us with a visual picture of how a property, such as
pH, changes as we add titrant. We can measure this titration curve
experimentally by suspending a pH electrode in the solution containing the
analyte, monitoring the pH as titrant is added. We can also calculate the
expected titration curve by considering the reactions responsible for the
change in pH. However we arrive at the titration curve, we may use it to
evaluate an indicator's likely titration error.
Applications
• Provide standard pharmacopoeial methods for the assay of unformulated drugs
and excipients and some formulated drugs, e.g. those that lack a strong
chromophore.
• Used for standardisations of raw materials and intermediates used in drug
synthesis in industry. Suppliers of raw materials may provide these materials at a
specified purity which has been assayed titrimetrically to a pharmacopoeial
standard.
• Certain specialist titrations, such as the Karl Fischer titration used to estimate
water content, are widely used in the pharmaceutical industry.

1- Acid–base titration
Neutralization titrations are widely used to determine the amounts of acids and
bases and to monitor the progress of reactions that produce or consume hydrogen
ions. In addition, we investigate titration curves that are plots of pH vs. volume of
titrant, and present several examples of pH calculations.
Acid–base titrations depend on the neutralization between an acid and a base when
mixed in solution. In addition to the sample, an appropriate pH indicator is added
to the titration chamber, reflecting the pH range of the equivalence point. The
acid–base indicator indicates the endpoint of the titration by changing color. The
endpoint and the equivalence point are not exactly the same because the
equivalence point is determined by the stoichiometry of the reaction while the
endpoint is just the color change from the indicator.

A Solutions and indicators for acid/base titrations Neutralization titrations depend on a
chemical reaction of the analyte with a standard reagent. There are several different types
of acid/base titrations.
1- The titration of a strong acid, such as hydrochloric or sulfuric acid, with a
strong base, such as sodium hydroxide.
2- The titration of a weak acid, such as acetic or lactic acid, with a strong
base.
3- The titration of a weak base, such as sodium cyanide or sodium salicylate,
with a strong acid.
A titration is a procedure for carrying out a chemical reaction between
two solutions by the controlled addition from a buret of one solution (the
titrant) to the other, allowing measurements to be made throughout the
reaction. For a reaction between an acid and a base, a titration is useful for
measuring the pH at various points throughout the reaction.
A titration curve is a graph of the pH as a function of the amount of titrant
(acid or base) added.

1- Strong Acid-Strong Base Titrations
Here is an example of a titration curve, produced when a strong base is added
to a strong acid. This curve shows how pH varies as 0.100 M NaOH is added
to 50.0 mL of 0.100 M HCl.
The equivalence point of the titration is
the point at which exactly enough titrant
has been added to react with all of the
substance being titrated with no titrant left
over. In other words, at the equivalence
point, the number of moles of titrant added
so far corresponds exactly to the number of
moles of substance being titrated according
to the reaction stoichiometry. (In an acid-
base titration, there is a 1:1 acid:base
stoichiometry, so the equivalence point is
the point where the moles of titrant added
equals the moles of substance initially in
the solution being titrated.)

Titrations Involving a Weak Acid or Weak Base
Titration curve of a weak acid being titrated by a strong base:
Here,
Here, 0.100 M NaOH is being added to 50.0
mL of 0.100 M acetic acid.
There are three major differences between
this curve (in blue) and the one
we saw before (in black):
1- The weak-acid solution has a higher
initial pH.
2- The pH rises more rapidly at the start, but
less rapidly near the equivalence point.
3- The pH at the equivalence point does not
equal 7.00.

POINT OF EMPHASIS :
The equivalence point for a weak acid-strong
base titration has a pH > 7.00.
For a strong acid-weak base or weak acid-strong base titration, the pH will
change rapidly at the very beginning and then have a gradual slope until
near the equivalence point. The gradual slope results from a buffer
solution being produced by the addition of the strong acid or base, which
resists rapid change in pH until the added acid or base exceeds the buffer's
capacity and the rapid pH change occurs near the equivalence point.
Titration curve of a weak base being titrated by a strong acid:
.
The weak-acid solution has a lower
initial pH.
2- The pH drops more rapidly at
the start, but less rapidly near the
equivalence point.
3- The pH at the equivalence point
does not equal 7.00.

POINT OF EMPHASIS: The equivalence point for a weak base-strong
acid
titration has a pH < 7.00.
Titrations of Polyprotic Acids
An example of a polyprotic acid is H2CO3 which neutralizes in two steps:
H2CO3 (aq) + OH- (aq) → H2O (l) + HCO3 - (aq)
HCO3 - (aq) + OH- (aq) H2O (l) + CO3 2- (aq)

Methods of Expressing Concentration of solutions
There are many systems of expressing the contraction of solutions.
Weight of solute present in a given volume of solution
Moles of solute present in one litre of solution, called molarity
Grame quivalents of solute present in one litre of solution, called normality
Percentage of solute by weight in a solution of known specific gravity.
Weight of Solute per Unit Volume of Solution
In preparation of solution using this system, a known weight of the solute is dissolved
and diluted to known volume.
If accuracy is required, the solute is weighed and transferred to a volumetric flask, and
diluted to a known volume.
The concentration is usually expressed as grams of solute per mil lilitre of solution.
Any multiple or fractional part of such solution will contain a known weight of solute.

Molarity
a. Amole: The molecular weight expressed in grams of any substance is called as
mole or gram molecule of the substance. For example, sodium chloride has
molecular weight 58.5g of sodium chloride means one mole of it.
b. Mille mole: The mole is very large unit, hence a smaller unit which is one
thousand thofit,known as mille mole is used. Thus 1 mole= 1000 mille moles.
c. Molar Solution: A molar solution is defined as the solution containing one mole
of the solute in a liter of its solution. For example 40 g of NaOH is one liter of
solution is called as 1M NaOH solution
Weight of solute in grams
Number of Moles = --------------------------------
Molecular weight of the solute

Molarity of Solution: It is defined as the number of moles of the solute present
in one liter of its solution. It can also be defined as number of mille moles of a
solute present in one mille liter of its solution.
Number of moles of the solute
Molarity= --------------------------------------
Volume of the solution in liter
Number of moles = Molarity Liter
Number of mille moles of the solute
Molarity= ------------------------------------------
Volume of the solution in milliliter
Number of mill moles= Molarity Milliliter's

3. Normality
The concentration of solution can also be expressed in terms of normality.
Equivalent Weight :
The equivalent weight of a substance ( element or compound)as:
“The number of parts by weight of it, that will combine with or displace directly or
in directly1.008partsbyweightofhydrogen,8partsbyweightofoxygen, 35.5 parts by
weight chlorine or the equivalent parts by weight of another element”.
Milli equivalent Weight : It is obtained by dividing the equivalent weight by
1000.
Gram Equivalent Weight : It is the equivalent weight expressed in grams
Equivalent weight o f a substance=1 equivalent of a substance=Equivalent
weight of substance in grams
Milligram Equivalent Weight: It is obtained by dividing the gram equivalent
weight by 1000.

Normality of Solution: Normality is a system of expressing concentration based
on number of equivalents of solute present in one liter of solution or the number
of mille equivalents of solute present in one mille liter of a solution.
Number of equivalents of the solute
Normality= -------------------------------------------
Volume of titration in liter
Number of mille equivalents of the solute
= -------------------------------------------------
Volume of the solution in milliliter
Number of gram equivalents = Normality x Liters

Problems
Q4: Two solutions of a substance (non-electrolyte) are mixed in the
following manner. 480 mL of 1.5 M first solution + 520 mL of 1.2 M
second solution. What is the molarity of the final mixture?
a) 1.20 M
b) 1.50 M
c) 1.344 M
d) 2.70 M
Q5: With increase in temperature, which of these changes?
a) molality
b) fraction of solute present in water
c) mole fraction
d) weight fraction of solute

Q6: The density of a solution prepared by dissolving 120 g of urea
(mol.mass = 60 u) in 1000 g of water is 1.15 g/mL. The molarity
of this solution is:
a) 2.05 M
b) 1.02 M
c) 0.50 M
d) 1.78 M

Q7: The density (in g.mL–1) of a 3.60 M
sulphuric acid solution that is 29% H2SO4
(Molar mass = 98 g.mol–1) by mass will be:
a) 1.45
b) 1.64
c) 1.88
d) 1.22

Q8: What is the difference between Molarity and Molality?
Answer:
The difference between a solution and a solvent is a fundamental
distinction between molality and molarity.
The ratio of the moles of a solute to the total litres of a solution is known as
molarity. Both the solute and the solvent are present in the solution.
Molality, on the contrary, is the ratio of a solute’s moles to a solvent’s
kilograms.

Molarity (M) Molality (m)
Measure of Concentration Concentration
Definition
The moles of a solute per
litre of a solution.
The moles of a solute per
kilogram of a solvent.
Units M m
Equation
M = moles solute/litres
solution
m = moles solute/kg
solvent
Ratio of moles to Volume (in litres) Mass (in kilogra

lecture 1 Analytical Chemistry.ppt

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