Spectroscopy

Question 1

Electro-analytical methods

Answer 1

A class of techniques in analytical chemistry, which deals with study of an analyte (test solution) by using suitable instrumental techniques.

Question 2

Spectrophotometry and Spectroscopy

Answer 2

A method to measure intensity of light
absorbed by a chemical substance when a beam of light passes
through a sample solution.

The branch of science that deal with the study of
interaction of matter with electromagnetic radiation of particular
wavelength and vice-versa

Question 3

Lambert’s Law

Answer 3

When a monochromatic radiation is passed through a light absorbing species (solution), the decrease in the intensity of radiation with thickness of the a light absorbing species (solution)
is directly proportional to the intensity of incident light

Question 4

Beer’s Law

Answer 4

The decrease in the intensity of incident radiation due to absorption is directly proportional
to the concentration of absorbing species.

Question 5

Absorbance and Molar Extinction coefficient

Answer 5

Absorbance :
No Unit, It is the logarithm of the ratio of the incident light (I0) to the transmitted light (I).

Molar Extinction coefficient :
It is defined as the absorbance of that solution which has concentration of the absorbing species 1 mol.dm-3 and the path
length is 1 cm

A = ε . C . l

Where, ε = molar absorptivity, dm3mol-1cm-1

C = concentration, mol.dm-3

L = path length

A = 2- log10 %T
A = -log T = log (1/T)

Question 6

Limitations of Lamberts and Beers

Answer 6

Linearity of the Beer-Lambert Law is limited by chemical and instrumental factors
Not valid for concentration greater than 10^-2
Molar extinction coefficient depends on the R.I. of the solution and changes with it.
Temperature fluctuations and stray light affect absorbance measurement.

Question 7

Single beam spectrophotometer

Answer 7

Light source :
Deuterium and hydrogen lamps (190-400 nm) (For UV light)
Tungsten filament lamp (300-750 nm) (For visible light)

Sample containers :
Cuvettes (Quartz)

Mono - chromators :
Prisms can be used as mono - chromator
Diffraction gratings

Detectors :
Photomultiplier tube
Photodiode
Photodiode array

Working :
Monochromator
Through a quartz cuvette containing sample solution
Transmitted radiation falls on photoelectric cell which gives a electrical signal as absorbance reading.

Question 8

Double beam Spectrophotometer

Answer 8

Light source :
Deuterium and hydrogen lamps (190-400 nm) (For UV light)
Tungsten filament lamp (300-750 nm) (For visible light)

Sample containers
Cuvettes (Quartz)

Beam splitters
Mirrors

Mono-chromators
Prisms can be used as mono-chromator
Diffraction gratings

Detectors
Photomultiplier tube
Photodiode
Photodiode array

Working :
Variation in intensity of light is compensated by splitting it.
One beam through blank soln and one through sample soln
And as in single beam the beam falls on photoelectric cell which converts radiation to electrical signal as absorbance.

Question 9

Applications of UV Visible Spectroscopy

Answer 9

Detector in HPLC
To study absorbance of organic compounds
Magnitude of molar absorptivity
Identification of inorganic and organic species

Question 10

Conductometric titration

Answer 10

Constant measuring of conductance of a mixture while reactant is being added, sudden change is the equivalence point.

When one electrolyte is added to another the conductance will alter if ionic reaction takes place, if not then conductance will simply increase.

Question 11

Procedure for Conductometric titration

Answer 11

Definite volume of solution is pipetted out.
Dip type conductivity cell dipped in it, distilled water added if not completely dipped.
Cell connected to conductometer and conductance is measured.
Then titrant added to burette, small proportions added to solution and stirred.
7-8 reading beyond equivalence point.

Question 12

Strong Acid with Strong Base

Answer 12

Before NaOH conductance is high due to hydrogen ions.
Then conductance drops due to addition of NaOH which causes OH and H ions to form undissociated water.
Once H ions are extinguished the Solution only contains NaCl, then addition of NaOH increases conductance due to presence of OH.

Question 13

Weak Acid with Strong Base

Answer 13

Initial conductance low due to feeble ionization of acetic acid.
On adding base it decreases further due to replacement of H+ by Na+ and due to suppression of acetic acid due to common ion acetate.
Graph is curved near equivalence point due to hydrolysis of salt CH3COONa.
Conductance increases after point due to OH ions from NaOH.

Question 14

Strong Acid Weak Base

Answer 14

Initially high, then decreases due to replacement of H+, after equivalence point the graph becomes almost horizontal as excess aqueous ammonia is not ionized in the presence of ammonium sulphate.

Question 15

Weak Acid and Weak Base

Answer 15

Initial curve is similar to weak acid strong base up till the equivalence point. After point almost horizontal as weak base is feebly ionized and therefore not much conducting.

Question 16

Mixture of Strong and Weak Acid vs Strong Base or Weak Base

Answer 16

Two breakpoints, first corresponds to neutralization of strong acid, second one for neutralization of weak acid which is only neutralized after strong acid is completely extinguished. If a strong base is added the conductance increases after second point, if weak base then it remains almost same.

Question 17

Advantages and Limitations of Conductometric Titrations

Answer 17

Advantages :
Colored solutions can be titrated
Works well for dilute solutions too.
Observations far away from the equivalence point are important.
Extremely weak acid or base can be titrated.

Limitations :
In dilute solutions, obtuse curves are obtained. With obtuse curves it is difficult to locate the equivalence point accurately.

Accuracy is limited due to technique.

Question 18

pH Metery

Answer 18

pH is defined as the negative base 10 logarithm of the hydrogen ion concentration.

pH = - log10[H+]

Measured using two electrodes, usually combined, when in a solution a small galvanic cell is formed.

E = E ind – E ref
E = Eo – (RT/nF) ln[conc.]

A reference electrode is an electrode which has a stable and
well-known electrode potential

Question 19

Saturated Calomel Electrode.

Answer 19

Small tube at the bottom consisting mercury.
Covered with a paste of HgCl2
Aqueous phase is saturated solution of potassium chloride in water, this is contact with Hg+Hg2Cl2
The electrode is normally linked via a porous frit(salt bridge) to the solution in which the other electrode is immersed.

Advantages:

It is simple to construct
Results of cell potential
measurements are
reproducible
Stable over a long period

Question 20

Ion Selective Electrode

Answer 20

Made of a doped glass membrane that is sensitive to a specific ion.

Study diagram well and understand construction.

Working :
Outer surface in contact with sample being studied and inside surface contacts the filling solution.

Question 21

Measurement of pH

Answer 21

System consists of a pH probe, reference probe,
temperature sensor, pH meter and the sample to be measured.

When the pH probe
is in contact with a solution a potential forms between the pH probe and the reference probe. The meter measures the potential and converts it, using the calibration curve parameters, into a pH value.

A two point standardization method is used to standardize the pH meter. First adjusted to 4pH using a known 4pH solution, then to 9.2. It is rinsed well using distilled water and cleaned between the two measurements.
Start the measurement more than 5 minutes after switching on.

Precautions :
When analysis is complete, put pH meter in stand-by mode. Remove electrode from solution and rinse thoroughly with water. Blot dry and put back in yellow pH storage buffer. Place parafilm over the hole and around the bottle to minimize evaporation.

Question 22

IR Spectroscopy

Answer 22

Near IR 15000 cm-1 to 3000 cm-1
Mid IR 4000 cm-1 to 400 cm-1
Far IR 200 cm-1 to 10 cm-1
Most used 4000 cm-1 to 670 cm-1

IR is mainly thermal energy, it causes great deal of vibration in covalent bonds.

Question 23

Modes of Vibration

Answer 23

Its a periodic motion of the atoms of a molecule relative to each other such that the center of gravity stays the same.

Non linear molecules with N atoms has (3N-6) Modes of vibration

Linear molecule has (3N-5) Modes of vibration.

Stretching : in which the distance between the two atoms increases or decreases but the atoms remain in the same bond axis.

Bending: in which the position of the atom changes relative to the bond axis. Covalent bonds can vibrate in several modes, including
stretching, rocking, and scissoring.