Spectophotometry

Question 1

What is spectrophotometry?

Answer 1

it is a method to measure how much a chemical substance absorbs light by measuring the intensity of light as a beam of light passes through a sample solution

Question 2

How can one select a narrow range of light?

Answer 2

using filters (photometers)
using prisms/gratings (spectrometers)

Question 3

The basic principle of spectrophotometry

Answer 3

each compound transmits or absorbs light over a certain range of wavelength

Question 4

Basic structure of spectrophotometers

Answer 4

light source –> lens –> monochromator(prism) –> wavelength selector –> solution sample in cuvette –> detector –> digital display/ meter

Question 5

The 2 laws of spectrophotometry

Answer 5

Beer’s Law

Lambert’s Law

Question 6

Beer’s Law

Answer 6

Absorbance is directly to concentration over a certain concentration range or inversely proportional to the logarithm of transmitted light

Question 7

Lambert’s Law

Answer 7

Absorbance is directly proportional to the length of light path through the sample

Question 8

Beer-Lambert Law

Answer 8

Absorbance of a solution is directly proportional to the concentration of the absorbing species in the solution and the path length

Question 9

Mathematical expression of Beer-Lambert Law

Answer 9

A ∝ cl
A= εcl
ε = absorptivity coefficient

Question 10

What unit is absorbance measured in?

Answer 10

has no unit

Question 11

What is the absorptivity co-efficient?

Answer 11

the absorption obtained for 1M solution of a pure compound with a light path of 1cm under standard conditions of solvent, temperature and wavelength

Question 12

Absorptivity co-efficient is ………….. specific

Answer 12

wavelength

Question 13

Derivations of the Beer-Lambert law

Answer 13

1. When materials are exposed to radiation, part of the incident radiation is absorbed, part is transmitted and part is scattered.
2. The fraction of incident light absorbed depends on the thickness of the absorbing medium
3. The intensity of the light absorbed is related to the concentration of the absorbing substance

Question 14

What 2 quantities relate the change in intensity of light before and after interaction with a matter

Answer 14

Transmittance

Absorbance

Question 15

What is transmittance?

Answer 15

the fraction of light that reaches the detector after passing through a sample
T= It/Io

Question 16

Formula for absorbance w.r.t transmittance

Answer 16

A= -log(T)
A= -log(It/Io)
A= log(Io/It)