CONTENTS
CONTENTS
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PAGE
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Abstract
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2
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Introduction
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3
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Literature
review
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4
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Objectives
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6
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Methodology
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7
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Results
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8
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Discussion
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9
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Conclusion
and recommendation
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11
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Reference
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12
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Appendix
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Abstract
This experiment wasabout concentration
dependent absorbance values, determined using a UV-vis spectrometer which gives
the value of absorbance of a solution based on the amount of light absorbed by
the solution. The primary objective of this experiment is to determine the
absorbance of copper sulphate solutions of different concentrations. Besides, the
absorbance of a sodium chloride sample solution was also determined by using
the same method.
In this experiment, copper sulphate
solutions of concentrations 28g/L, 14g/L, 7g/L, 3.5g/L, and 1.75g/L were
prepared in volumetric flasks from solid copper sulphate. The five solutions
were labeled c1 to c5 following the sequence. A solution
of NaCl was also prepared and labeled as X. These solutions, and distilled
water were placed into the UV-vis spectrometer to determine their absorbance. Before
the measurement was taken, the UV-vis spectrometer was adjusted so that the
wave length of the emitted light was at a wavelength of 800nm. The absorbance
value of distilled water was recorded as reference.
While measuring the absorbance of the
solutions, the quartz cells containing the solutions must be clear from
fingerprints because the fingerprints might affect the amount of UV light that
reached the detector in UV-vis spectrometer. Therefore, tissue paper is used to
wipe off the fingerprints before inserting the quartz cells into the UV-vis
spectrometer.
The experiment basically showed that
absorbance increases with the concentration of a solution, obeying
Beer-Lambert’s Law.
Introduction
A UV-vis spectrophotometer is a
research instrument used to gather information about a chemical sample by
determining the absorbtion or transmission of UV-vis light by the sample. It can
also be used to measure the concentration of the absorbing materials based on
the calibration curves produced.
A UV-vis spectrophotometer exposes a
chemical solution to the ultraviolet and visible region of the electromagnetic
spectrum when the chemical solution is placed in the UV-vis beam. Depending on
the type of chemical, a certain amount of light gets absorbed by the chemical
which causes electrons to be promoted from one energy level to another. The
amount of light which is not being absorbed will pass through the chemical to
the detector. The amount of light that reaches the detector is then recorded as
a spectrum. A spectrum is a graphical representation of the amount of light
absorbed or transmitted by matter as a function of the wavelength. Since the
samples are prepared in known concentrations, the graphed results make a
calibration curve from which the unknown concentration can be determined by its
absorbance.
A UV-visible spectrophotometer
measures absorbance or transmittance from the UV range from which the human eye
is not sensitive to the visible wavelength range to which the human eye is
sensitive to.
Literature review
When light passes through a
substance, light of certain wavelength is being absorbed by the substance,
while the rest of the light will pass through the substance, or being reflected
by the substance. For example, when light passes through a solution of copper
sulphate, the copper ions in the solution absorbed the visible lights from the
red end of the spectrum. The blue light reflects into our eyes and this is why
the copper sulphate solution appears to be blue to our eyes.
However, substances do not only
absorb lights from the visible region of the wavelength. They also absorb
invisible light, for example UV rays, dependent on the type of substance.
Since different substances absorb
light of different wavelength, this can be used to determine the type of
substance in a sample.
A UV-visible spectrometer can be used
to measure the absorbance of solutions. light beams of wavelength in the
visible region, and the UV region are passed through the solutions, where light
of certain wavelengths are absorbed, and the rest will reach the detector. The detector converts the incoming light into a current. The
higher the current, the greater the intensity of the light.
For each
wavelength of light passing through the spectrometer, the intensity of the
light passing through the reference cell is measured. This is usually referred
to as Io, where I is the intensity. The intensity of the light
passing through the sample cell is also measured for that wavelength, given the
symbol I.
The relationship
between absorbance,A and the two intensities is given by:
From the equation, it is shown that A is a
value without unit.
The derived equation, A=ebc
Where
A is absorbance
e is the molar absorbivity
b is the path length of the sample
c is the concentration of the compound in solution
shows that concentration of a solution is
directly proportional to its absorbance value.
A UV-vis spectrometer is generallly
used in analytical chemistry, especially in the quantitativeanalysis of transition
metal ions, highly conjugated organic compounds, and biological macromolecules.
Determination is usually carried out in solutions.
Objectives
1.
To determine absorbance of
solutions at different concentrations.
The absorbance values of copper
sulphate solutions of different concentrations is to be determined using a
UV-vis spectrometer.
2.
To determine the
concentration of given samples.
Methodology
Reagents and equipment
Solid
CuSO4, distilled water, solid NaCl, 5 100ml volumetric flasks,
measuring cylinder, dropper, 200ml beaker, glass rod, 10-mm path length quartz
cells, UV-vis spectrometer.
Method
1. A CuSO4 stock solution of 2.8g is prepared in 100ml
distilled water.
2. Four sample solutions is
prepared in the 100ml volumetric flasks by diluting the stock solution,
followed by each previously prepared solution, using sample concentrations of
14g/L, 7g/L, 3.5g/L and 1.75g/L.
3. A small amount of NaCl
(,0.1mg) is added in a 100 ml volumetric flask and the flask is filled with the
stock solution. This sample is labeled as X.
4. The absorbance of
distilled water is measured and used as a reference.
5. The absorbance values of
the stock solution and of each of the other solutions are measured at 800nm.
sets 1. slamat mencuba kawan2..
