8.5 Colorimetry & UV-VIS Spectroscopy

Question 1

Beer-Lambert law

Answer 1

A = Ɛlc

A = absorbance
epsilon = molar absorption coefficient
l = sample cell length
c = molar conc

Question 2

simplified Beer-Lambert law

Answer 2

A = kc

absorbance is directly proportional to molar concentration of solution

Question 3

Beer-Lambert law log form

Answer 3

A = log(Io / I)

Io = original light intensity
I = exiting light intensity

Question 4

absorbed colour and observed colour are…

Answer 4

complementary colours

Question 5

range of visible light

Answer 5

100 - 800mm

Question 6

range of UV light

Answer 6

100 - 400mm

Question 7

colorimeter elements

Answer 7

1. light source
2. colour filter
3. sample solution
4. detector
5. recorder

Question 8

UV-VIS spectrophotometer elements

Answer 8

1. light source
2. monochromator
3. sample solution
4. detector
5. recorder

Question 9

example: absorbance of iron Fe2+

Answer 9

oxidise to Fe3+ since Fe2+ doesn’t absorb colours well

react with SCN-

Fe3+ + SCN- –> FeSCN2+ (aq)

bright red colour, all equimolar

Question 10

complementary colour mnemonic

Answer 10

ROY GBV

Question 11

2 uses of colorimetry and UV-VIS spec

Answer 11

1) chromium levels in workplace: pump samples air and solids are collected on filter paper, converted to yellow CrO4 (2-) ions for analysis

2) phosphate levels in waterways, harmful to environment, causes eutrophication (excessive algae growth).
ammonium molybdate and tin(II) chloride aka SnCl2 –> molybdenum blue for analysis