EXPERIMENT 1: THE VISIBLE SPECTRA OF SOFT DRINKS.
Objectives
1) To determine the origin of the colour of a soft drink sample from its
absorption spectrum.
2) To determine the wavelength at maximum absorbance for a soft drink
sample.
3) To determine the unknown concentration of a soft drink from the
calibration curve.
Introduction
The word spectroscopy is used to refer to the broad area of science dealing with the
absorption, emission, or scattering of electromagnetic radiation by molecules, ions,
atoms, or nuclei. Spectroscopic techniques are some of the most widely used
analytical methods in the world today. Different regions of the electromagnetic
spectrum such as infrared, visible, ultraviolet, or, X-ray radiation can be used to
interact between light and matter to probe chemical structure.
White light is light that visible and consist of all colour component of rainbow.
When white light hits a coloured solution, electron in the solution will absorb some of
the light, the other will be transmitted through the solution that results in light that can
be seen by our eye. To measure the concentration of dilute solutions, one of the most
accurate and convenient way by using colorimeter or absorption spectrophotometry.
Absorption spectrometry is a widely used technique in analytical chemistry based on
the property of molecules to absorb light at specific wavelengths. The wavelength
chosen is usually that one at which the absorbance, of the species to be analysed, is
at maximum.
In this experiment, Spectronic 20 is been used because it measure the
absorption of lights in the visible spectrum that we perceive as colour, and the
techniques used is called colorimetric. The instrumental components consists of
source, monochromator, and detector. The intensity of the beam from the light source
will be measured by detector. The initial intensity is called as, Iₒ and when the sample
is placed between, the intensity is called as I. The detector compares the two
intensities and reports the results as percent transmittance (%T) or absorbance (A).
These terms are defined to be:
%T = I / Iₒ x 100
(Equation 1)
A = - log T = - log (I / Iₒ) = 2 – log (%T)
(Equation 2)
The amount of light absorbed is depend on how well the substances absorb the light,
the path length of light and the concentration. The equations is
A = ɛbC
(Equation 3)
The symbol ɛ (epsilon) is stand for molar absorptivity, b is the path length of light and
C is for concentration. Molar absorptivity is a characteristics of the absorbing species
that tells how much light is absorbed at a particular wavelength. Since the unit C
typically moles per litre (M) and b is usually in centimetre (cm), the units of ɛ are
commonly expressed as M −1 cm−1 to make A dimensionless. Equation 3 is known as
the Beer-Lambert Kaw or simply Beer’s Law. It is importance because absorbance is
directly proportional w=to concentration of absorbing species.
However, if the concentration is described in units other than moles per litre,
the ɛ is replaced with symbol a or absorptivity, and the Beer’s Law is written as follows:
A = abc
(Equation 4)
Apparatus
The apparatus that been used in the experiment are beaker, pipet, glass rod,
volumetric flask,50 Ml, dropper and retort stand.
Chemicals
The chemicals that been used is a can of soft drink (red colour)
Procedure
A. Preparation of ‘Standard’ Solutions of Soft Drink (Known Concentration)
1. Soft drink was poured into a beaker and the carbonation was removed.
2. 5.00 mL of soft drink was pipetted into a 50.00 mL volumetric flask and been
diluted until calibration marks with distilled water. The volumetric flask been
cover and shake until homogenous. The solution been stored in small
beaker.
3. Step 2 have been repeated using 10 mL, 15 mL, 20 mL, and 25 mL of soft
drink.
B. Operations of the Spectronic 20 and Determination of 𝝀𝒎𝒂𝒙 .
1. Spectronic 20 was turn on and the instrument had been warm up for 15
minutes.
2. Wavelength had been set to 600 nm.
3. The (%T) been adjust to 0% transmittance.
4. Cuvette was obtain. The tube been cleaned and rinse with distilled water,
and ¾ full of blank solution was filled into the tube. The blank solution is
distilled water. The excess solution and finger prints was carefully wipe
away from the outside of the tube by using Kimwipe.
5. 0 absorbance and 100% transmittance was adjust with the cuvette
containing the ‘blank’ in the sample holder. The cuvette was then removed
and been set aside without emptying the distilled water.
6. Another cuvette was been cleaned and rinsed with small amount of the
‘standard’ soft drink. ¾ of the cuvette was been filled with solution, the
cuvette was been wipe with a Kimwipe and been placed in the sample
holder with hash marks aligned. The absorbance was read and recorded.
7. The cuvette then been removed, the top was closed and the wavelength
was setting to 20 nm lower.
8. 0% transmittance was reset if it has changed.
9. The cuvette that contain blank solution was insert and 100% transmittance
was reset. Cuvette was removed.
10. The cuvette that containing the same soft drink that been used in step (6)
was insert.
11. The absorbance was read and recorded in Table 1.2.
12. Step 8 through 11 was repeated until 360 nm. The absorbance reading was
taken at each 20 nm interval.
13. Plot the absorption spectrum of soft drink and determine the 𝜆𝑚𝑎𝑥 using
graph paper.
C. Preparation of ‘Unknown’ Soft Drink Sample.
1. Some of the soft drink was poured into a beaker and been stir to remove
carbonation.
2. The soft drinks, without measuring the volume had been poured into 50 mL
of volumetric flask and been diluted with distilled water until calibration mark.
The flask been homogenize by closed with stopper and the shake. The
unknown solution should not be darker than the most concentrated solution.
3. The ‘unknown’ sample is been put into cuvette ¾ full.
D. Quantitative Analysis of the Soft Drink Solution.
1. The spectronic 20 was set to the maximum wavelength ( 𝜆𝑚𝑎𝑥 ) obtained
from Part B.
2. 0 absorbance and 100% T was set as given in the procedure above.
3. The absorbance of each of 5 ‘standard’ soft drink was measured and
recorded,
4. The absorbance of ‘unknown’ soft drink solution was measured and
recorded.
E. Cleaning Up.
1. The soft drink water can be poured down to the drain.
2. The cuvette been cleaned and dried.
DATA
Table of The wavelength and Absorbance of Soft Drink.

Wavelength (nm)
Absorbance
600
0.001
580
0.039
560
0.117
540
0.158
520
0.181
500
0.163
480
0.120
460
0.073
440
0.043
420
0.034
400
0.026
380
0.027
360
-
Maximum wavelength: 520 nm.
Table of Soft Drink Concentration (Volume %) and Absorbance.
Solutions
Concentration (Volume %)
Absorbance
1
10
0.179
2
20
0.359
3
30
0.565
4
40
0.738
5
50
0.899
Unknown sample
18
0.335
CALCULATIONS
Sample calculation for the concentration of solution 2:
10 𝑚𝑙
50 𝑚𝑙
x 100 = 20%.
The calculation for others concentration solution are the same except for unknown.
Concentration of unknown : 18%
The concentration of unknown had been obtained from the Beer’s Law plot graph.
RESULTS
Absorbance versus Concentration (volume%)
1
0,9
0,8
ABSORBANCE
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
0
10
20
30
CONCENTRATION %
Beer’s Law plot.
40
50
60
Questions
1. Based on Beer’s Law, when the concentration of an analyte increases, how
the following be affected (increase, decreasing, no change)?
i.
Absorbance.
= The absorbance will be increased.
ii.
Transmittance.
= The transmittance will decrease.
2. Why is it important to first obtain the absorption spectrum of the soft drink
before making a calibration curve?
= To determine the wavelength of the maximum absorption.
3. What is the purpose of the ‘blank’ solution?
= To trace sources of artificially introduced contamination.
4. What colour (s) is/ are being absorbed in your soft drink?
= The colour that been absorbed in soft drink is blue green.
Discussion
In this experiment, operator carried out an experiment to determine the absorbance
soft drink. Soft drink that been used in the experiment is red in colour. The soft drink
is been diluted into certain concentration which in volume percent which is 10, 20, 30,
40 and 50. Before we analysis the absorbance of different concentration solutions of
soft drink, operator need to know the wavelength at maximum absorbance. Hence, to
identify the wavelength at maximum absorbance, one of the standard is been analysed
at different wavelength in Spectronic 20, the device been used in the experiment.
Based on the data, the reading of absorbance were relatively increasing from
360nm with 0 absorbance until it reach the peak which is at 520 nm with 0.181. After
520 nm, the absorbance reading relatively decreasing. Based on the table on the
introduction of the lab manual, the absorbed colour is at blue-green range. Thus, the
origin of the soft drink that been obtained is blue-green in colour.
After that, we can carried out experiment of the relationship of different
concentration and absorbance. Based on the results, the absorbance is increasing
proportionally with the concentration. Based on the Beer’s Law plot, we can conclude
that as the concentration increase, the absorbance increase follow the Beer’s Law
theory. The concentration of the unknown can be known by drawing line on the
absorbance reading which is at 0.335 and thus based on the plotted graph the %
concentration is 18.
During the experiment, few precautions were taken to minimize the error. The
cuvette must been wipe with KimWipe thoroughly before putting in the spectronic 20.
Next, the cuvette that been used for the experiment must been clear without any
scratch to get better results. When pipette the soft drink, make sure to rinse with the
same soft drink to remove impurities that might presents in the glassware. Then, when
diluting the soft drink, make sure the distilled water not more or less than calibration
mark.
Conclusion
To conclude the experiment, the source of colour of soft drink that been obtained is
blue-green. The wavelength at maximum absorbance is 520 nm. The concentration %
for the unknown obtained from the Beer’s Law Plot is 18.
References
1) Lecturer’s note.
2) Docslide.us