Dr. Maha Al-Sedik

Objectives:  Electromagnetic Radiation  White light  Beer’ s law  Spectrophotometer  Components of spectrophotometer  Types of spectrophotometer

 Light is the most important example of electromagnetic radiation.  Light moves in waves.  different types of light have different wavelengths.  Wave length: distance between 2 peaks and is expressed by nanometers. Electromagnetic Radiation

 Do you think (a) or (b) has longer wave length and lowest amplitude?

 The human eye responds to radiant energy between 380 and 750.  Sun light is mixture of radiant energy of different wavelengths that eye recognize it as white.

White light

COLORWAVELENGTH (λ in nm) Ultraviolet< 380 Violet380 – 435 Blue436 – 480 Greenish-blue481 – 490 Bluish-green491 – 500 Green501 – 560 Yellowish-green561 – 580 Yellow581 – 595 Orange596 – 650 Red651 – 780 Near Infrared> 780 Visible Light

Each object when exposed to light, It absorbs some of lights and transmit some others.

Q- When do we see solution with green light? A- if it transmits light maximally between ( green wave length) and absorbs light at other wave length.

All colors absorbed except red color

 Beer’s Law talks about the relationship between color and concentration.  Beer’s Law states that the concentration of substance in solution is directly proportional to the light absorbed by the solution or inversely proportional to light transmitted within limits.

A α b x c A : absorbance. α : directly proportional. b : light path in cm. c : concentration of substance. © : constant A =© x b x c

 If we suppose that I have two solution one with known concentration we will name it standard (s). The second with unknown concentration and we will name it unknown (u). As = © x bs x cs Au = © x bu x cu ©= As /bs x cs ©= Au /bu x cu As /bs x cs=Au /bu x cu

B ….light path in cm is constant As / cs = Au / cu cu = Au x cs / As

Beer’ s law is followed only if the following conditions are met :  The light must be monochromatic.  The solute concentration is within given limits.  A chemical reaction does not occur between the molecule of interest and another solute.

Spectrophotometer

 Photometry: measurement of intensity of light.  Spectrophotometry: measurement of intensity of light at selected wave length.

Principle:  The solutions of many compounds have characteristic colors.  The intensity of a color is directly proportional to the concentration of the compound.  Light is absorbed by dissolved substances.  The concentration of the substance is directly proportional to the amount of light absorbed.

Q- Is spectrophotometer quantitative or qualitative technique ? A- Both

Components of spectrophotometer: Light source Monochromator Filter Cuvet Photo detector Signal processor Record

Components of spectrophotometer:

1-Light source:  Tungsten Filament Lamps: the most common source of visible radiation.  Deuterium Lamps: also may be used.

2-Monochromator:  Used as a filter: the monochromator will select a narrow portion of the spectrum of a given source.  Monochromator may be filters or prism and grating.

3-Cuvet:  It may be Glass or plastic.  It may be round, square or rectangular.  It must be clean and clear.  The most popular is 1 cm light path.  Cleaned in solution of concentrated HCL : water : ethanol = 1 :3 :4

3- Photodetector:  Photodetector transforms light to electrical impulses.  When light strikes the photodetector, electrons are exited in the semiconducting material.  The electricity is directly proportional to the intensity of light.

4- SIGNAL PROCESSOR:  Digital signals from the blank are subtracted from standered and uknown 5- RECORD:  Gives the net results on the monitor.

General rules and precautions in spectrophotometer:  A blank should be used for each set of determinations.  Cuvet should be clean, clear and dry.  For each size of cuvet there is a specified minimum volume.  The solution must be free of air.  Cuvet must be put with the same side facing light every time.  Cuvet must be covered before each reading.

Types of spectrophotometer:  Single beam spectrophotometer.  Double beam spectrophotometer.