Centro Escolar University (2022-2023)
Determination of the concentration of an Unknown Solution
using a colorimeter or UV-VIS Spectrometer ( BEER’s LAW)
INTRODUCTION:
Colorimeter
- The colorimeter is designed to analyze
monochromatic ("one-color") light that passes
through a solution.
- The computer interfaced colorimeter monitors the
light received by the photocell as either an
absorbance or a percent transmittance value.
- The measure of the amount of light that passes
through a solution is called "Transmittance".
Transmittance is a ration of the intensity of the
transmitted light to the intensity of the original light,
and is usually expressed in percentage.
- Absorbance is related to transmittance. The light
absorbed by a solution depends on the:
1. absorbing capability of the solution,
2. the distance traveled by the light through the
solution, and
3. the concentration of the solution.
- The relationship of absorbance to transmittance is A
= 2 - log % T
Spectrometry
❤
- A method of analysis that deals with the
measurement of spectra
- A “spectrum” is a system of color bands, it is a
continuum of color formed when a beam of light is
dispersed
- Types of Spectrum
- Color Spectrum
- Electromagnetic Spectrum
- Significance:
- Determination of the identity of a certain
compound
- Determination of the quantity of the active
ingredient
Apparatus :
-Spectrophotometer/ & Spectrometer - is a
photometer (a device for measuring light intensity)
that can measure intensity as a function of the color
and wavelength of light.
- An instrument for measuring the TRANSMITTANCE
or ABSORBANCE of a sample
- It uses a radiant energy DISPERSING DEVICE ( prism
or grating)
- The associated electronics permit the
measurement of wavelength and radiant
power.
- Parts:
-Radiation source - for ultraviolet
measurement, it is usually a hydrogen or
deuterium lamp and a tungsten for visible
measurements.
-Monochromator - to select a narrow band of
wavelength to pass through the sample cell,
an important property of a monochromator is
its bandpass or bandwidth.
-Sample cell or cuvette - this contains the
sample to be analyzed, should have parallel
sides that are perpendicular to the radiation
source.
Cells made of plastic
- Usually acrylic, and are lowest in costs
- Not resistant to all solvents
- Absorbs strongly below 300 NM
Cells made of Glass
- Slightly more expensive than plastic.
- More durable and last for years with proper care
- Absorbs strongly below 320 NM
Cells made of fused Quartz
- Reasonably transparent down to 320 NM
- Best Cells in Spectrometric Analysis.
TYPES OF CELLS
1. Open-Topped rectangular cell
2. Apertured cell
3. Micro cell
4. Flow through Cell
- UV - silica or quartz
- VIS - glass or plastic
-Detector - changes the radiation transmitted
from the UV-VIS spectrophotometer into a
current or voltage for the readout device to
use.
- Types: phototube, photomultiplier tubes
(wavelength region 150-1000 nm).
-Readout devices - use the current or voltage
produced by the detector and amplify it. The
readout device then converts the current or
voltage into a form that the analyst can read
as a print out.
-Readings: absorbance, transmittance,
concentration
Types of Spectrophotometer
1. Single-beam spectrophotometer - Measures the
relative light intensity of the beam before and after a
test sample is inserted.
2. Double beam spectrophotometer - Compares the light
intensity between two light paths, one path
containing a reference sample and the other the test
sample.
3. Split Beam spectrophotometer
4. Complex Diode Array spectrophotometer.
UV-VIS SPECTROPHOTOMETER
Spectrophotometry:
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