Analytical Chemistry
Lecture No. 2
Date :2 /12/ 2012
Dr. Mohammed Hamed
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Analytical Chemistry
Definition: Analytical chemistry is the chemistry discipline concerned with
the chemical composition of materials. Analytical chemistry also is concerned
with developing the tools used to examine chemical compositions.
Analytical Chemistry deals with methods for determining the chemical
composition of samples of matter. A qualitative method yields information
about the identity of atomic or molecular species or the functional groups in
the sample; a quantitative method, in contrast, provides numerical information
as to the relative amount of one or more of these components.
Analytical methods are often classified as being either classical or
instrumental.
classical methods :In the early years of chemistry, most analyses were carried out by separating
components of interest in a sample by precipitation, extraction, or distillation.
For quantitative analyses, the separated components were then treated with
reagents that yielded products that could be recognized by their colors, boiling
points or melting points, their solubility in a series of solvents, their odors,
their optical activities, or their refractive indexes.
Qualitative analysis consists of methods for establishing the qualitative
chemical composition of a substance—that is, the identification of atoms,
ions, and molecules that enter into the composition of the substance being
analyzed. The most important characteristics of all methods of qualitative
analysis are specificity and sensitivity. Specificity characterizes the ability to
detect the presence of an unknown element in the presence of other
elements—for example, iron in the presence of nickel, manganese, chromium,
vanadium, or silicon. Sensitivity is defined as the smallest quantity of an
element that can be detected by a given method. Sensitivity of modern
methods is expressed in magnitudes of the order of 1 mg (one millionth part
of a gram).
Quantitative analysis consists of methods for determining the
quantitative composition of materials—that is, the quantitative amounts of the
chemical elements or of certain compounds in the analyzed substance. Along
with specificity and sensitivity, the most important characteristic of every
method of quantitative analysis is accuracy.
quantitative analyses, the amount of analyte was determined by
gravimetric or by titrimetric measurement.
Gravimetric Methods – the mass of the analyte or some compound produced
from the analyte was determined.
Titrimetric Methods – the volume or mass of a standard reagent required to
react completely with the analyte was measured.
Instrumental Methods:Measurements of physical properties of analytes, such as conductivity,
electrode potential, light absorption, or emission, mass to charge ratio, and
fluorescence, began to be used for quantitative analysis of a variety of
inorganic, organic, and biochemical analyte.
Highly efficient
chromatographic and electrophoretic techniques began to replace distillation,
extraction, and precipitation for the separation of components of complex
mixtures prior to their qualitative or quantitative determination. These newer
methods for separating and determining chemical species are known
collectively as instrumental methods of analysis.
Instrumentation can be divided into two categories: detection and quantitation.
Measurement of physical properties of analytes - such as conductivity,
electrode potential, light absorption or emission, mass-to-charge ratio, and
fluorescence-began to be employed for quantitative analysis of inorganic,
organic, and biochemical analytes.
Efficient chromatographic separation techniques are used for the separation of
components of complex mixtures.
Solution Concentration
Normality:- The normality ‘N’ of a solution is, given by the number of
equivalents of solute per litre of solution
No.of equivalents of Solute
N= --------------------------------Volume of solution in litre
W/ EW
OR
N=--------------V
Where,
N = Normality
W = Weight of solute
EW = Equivalent weight of solute
V = Volume of solution in litre
Normality varies according to the reaction; as the equivalent weight of a
substance may vary according to the reaction, which the solute participates.
The equivalent weight is generally calculated after determining the specific
changes in chemical reaction of the solute.
Percent Concentration:- Concentration is many-a-times expressed in
terms of percent (parts per hundered). Percent composition of a solution
can be expressed as :
mass of solute
1. Per cent w/w = ---------------------× 100
mass of solution
volume of solute
2. Per cent v/v = --------------------------× 100
volume of solution
mass of solute
3. Per cent w/v =----------------------------- × 100
volume of solution
Percent w/w is frequently employed to express the concentration of
commercial aqueous reagents, per cent v/v is used to specify the
concentration of a solution prepared by dilution a pure liquid with another
liquid and percent w/v is employed to indicate the composition of dilute
queous solutions of solid reagents.
Molality :- Number of moles of solute dissolved in 1 kg (1000 g) of
solvent
moles of solute
Molality (or Molal Concentration) = -------------------------kg of solvent
In units of molal = m
One of the most commonly used concentration units among chemists is the
Molarity concentration term. Molarity is defined as the moles of solute
divided by the volume of the solution in liter units. An alternative
definition is the number of millimoles of solute divided by the volume of
the solution in milliliters. The symbol that we use is the capital letter M.
For example, if we have a 3 M HCl solution, then we have 3 moles of HCl
in every liter of that solution. Alternatively, we can say that we have 3
millimoles of HCl in every milliliter of solution. A shortened term for this
concentration term is the word "molar".
Application of the Molarity Term: Example
Problem: A sample of NaNO3 weighing 8.5 grams is placed in a 500 ml
volumetric flask and distilled water was added to the the mark on the neck
of the flask. Calculate the Molarity of the resulting solution.
Solution:
1. Convert the given grams of solute to moles of solute by dividing by the
molecular weight of NaNO3:
2. 1 mole NaNO3 = Molecular mass of NaNO3 expressed in grams = 23 +
14 + 3(16) = 85grams
8.5 grams NaNO3 x 1 mole NaNO3 / 85 grams NaNO3 = 0.1 mole NaNO3
3. Convert given ml of solution to liters by dividing by 1000:
1 liter = 1000 ml
500 ml x1 liter / 1000 ml = 0.500 liters
4. Apply the definition for Molarity:
Molarity = moles NaNO3 / volume of the solution in liters
M = 0.1 mole / .500 liters = 0.200 Molar NaNO3
Since molarity involves a basis of solution volume, it is apparent that the
molarity of a solution will change as volume changes which is associated
with changes in temperature. Formal Concentration: Some substances do
not exist in molecular form, whether in solid or solution form, they remain
in ionic form in solid state as well as in solution. In such cases instead of
molecular weight, formula weight is used in preparation of solution and its
concentration is expressed in terms of formality, for example Concentration
of NaCl and FeCl3 can be defined very simply as number of formula
weight of a solute per litre of solution. It is represented by ‘F’.
Weight of solute in g / Formula weight
F = ----------------------------------------------------------Volume of solution in litre
Formal solutions generally show changes in formality where volume
changes associated with temperature.