OBJECTIVES 1. To identify the source of the colour of a soft drink sample based on its absorption spectrum. 2. To investigate the wavelength of the soft drink sample at its maximum absorbance. 3. To investigate the concentration of the unknown soft drink based on the calibration curve.
INTRODUCTION Spectroscopy
is
the
study
of
the
relationship
and
interaction
between
electromagnetic radiation and matter. Spectroscopic techniques is one of the technique that is mainly used nowadays. Infrared, visible light, UV and X-ray are some of the example of electromagnetic radiation and can be used to interact with matter. The interaction between light and matter to probe chemical structure are one of chemical analysis that are important in instrumental method. White light is light that is 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 the 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
calorimetry
or
absorption
spectrophotometry.
Absorption spectrophotometry is a widely used technique in analytical chemistry based on the property of molecules to absorb light at specific wavelengths. Based on the techniques, the amount of light energy a solution absorbs from a beam of light on a particular wavelength will be measure. The colour of the aqueous solution results from the absorption of certain wavelengths of white light falling on the solution. The colour depends on the structure of the solute as water is transparent. The wavelength of maximum absorption is designated as lambda max and is characteristic of the material absorbing the light. The DR 2800 will be used in this experiment instead of spectonic 20. The Hach Lange DR2800 is a visible light spectrophotometer with pre-programmed Hach Lange test methods, and the ability
for users to input their own test methods, allowing flexibility(www.edie.net). With the visible absorption measurement, qualitative analysis to determine the wavelength of maximum absorbance from the absorption spectrum as well as absorptivity value of a specific molecule and quantitative analysis to determine the concentration of unknown solution from standard calibration curve can be done.
APPARATUS
Beaker, Pipet, glass rod, Volumetric flask (25 mL), DR 2800 Hach, cuvette, kimwipe.
MATERIALS
A can of carbonated drinks, distilled water.
Procedure A. Preparation of “standard “ solutions of soft drink of (known concentration). 1. Soft drink was pour into a beaker and strirred to remove the carbonation. 2. 2.5 mL of soft drinks then pipet and pour into 25 mL volumetric flask and was diluted up to mark by using distilled water. The volumetric then cover and shaken and homogeneous solution was form. The solution the poured into small beaker. 3. Step 2 was repeated by replacing the 2.5 mL soft drinks with 5 mL, 7.5 mL, 10 mL and 12.5 mL of soft drink. B. Operation of the DR 2800 hach and determination of λmax. 1. The DR 2800 was turned on and warmed up for about 15 minutes. 2. The wavelength was set to 600 nm. 3. The transmittance was adjusted to 0%. 4. Cuvette was obtained, cleaned and rinsed by using distilled water and then filled until the mark or roughly about ¾ full with blank solution. Kimwipe was then used to wipe away any impurities on its surface.
5. 0 absorbance and 100% transmittance was adjusted with the cuvette containing the blank in the sample holder. The cuvette the removed and set aside without even emptying the distilled water. 6. Another cuvette were cleaned and rinsed with small amount of the standard soft drink that the absorbance was going to be measured. Then, the cuvette were filled with ¾ full with the solution and cleaned by using kimwipe. It then palced on the sample holder. The absorbance were read and recorded. 7. The cuvette were removed, the top closed and the wavelength were change to a setting which is 20 nm lower. 8. The 0% transmittance was reset. 9. The cuvette containing distilled water were inserted and was reset to 100?T. The cuvette the removed, 10. The cuvette containing same soft drink solution in step 6 was inserted. 11. The absorbance were read and recorded in table 1.2. 12. Step 8 until 11 was repeated until 360 nm.. The absorbance were taken at each 20 nm intervals. 13. By using the graph paper, the absorption spectrum of the soft drink were plotted and λmax was determined. C. Preparation of Unknown soft drink sample The unknown soft drink sample was prepared by the lab assistant by using small amount of soft drink that we used. D. Quantitative analysis of the Soft drink solution. 1. The DR 2800 hach was setted to the λmax obtained from part B which is 520 nm. 2. 0 and 100%T was set. 3. The absorbance of each 5 standard soft drink solution was measured and recorded. 4. The absorbance of the unknown soft drink solution was also measured and recorded. E. Cleaning Up The waste was poured down the drain and all the apparatus used were cleaned.
RESULTS AND DATA
Wavelength (nm)
Absorbance
600
0.166
580
0.270
560
0.507
540
0.662
520
0.747
500
0.691
480
0.572
460
0.391
440
0.307
420
0.285
400
0.275
380
0.247
360
0.275
Table 1.2: The wavelength and absorbance of soft drink. Λmax = 520 nm
Solutions
Concentration ( Volume
Absorbance
%) 1
2.5
0.189
2
5
0.751
3
7.5
1.247
4
10
1.424
5
12.5
1.838
Unknown sample
?
0.153
Table 1.3 : Table of soft drink concentration (volume %) and absorbance.
CALCULATIONS: Sample calculation for the concentration of solution 2:
Concentration of unknown:
QUESTION: 1) Based on Beer’s Law, when the concentration of any analyte increases, how will the following be affected (increase, decrease, no change)?
a) Absorbance
As A=abc thus, it will also increase
b) Transmittance
As A= - log T, thus it will decrease 2) Why is it important to first obtain the absorption spectrum of the soft drink before making calibration curve? The importance of obtaining the absorption spectrum before making the calibration curve is generally we will select the wavelength of maximum absorbance for a given sample and use it in our absorbance measurements. 3) What is the purpose of using blank solution?
A blank solution is a solution containing little to no analyte of interest, it is usually used to calibrate instruments in this case the DR 2800.
4) What colour is being absorb in your soft drink? Based on the lambda max, the colour absorb by our soft drink is blue-green.
DISCUSSION For this experiment, initially for our group we supposly to be determining the absorbance for blue soft drinks. However, the solution gave an unrealistic results that turn out to be ununiformly increase and decrease drastically for the reading of the data. Then we changed the soft drink with another canned carbonated drinks. Based on the results, the reading of the absorbance were relatively increasing from 360 nm with 0.275 until it reach the peak which is at 520 nm with 0.747. After 520 nm, the absorbance reading relatively decreasing. Based on the data obtained, the lambda max or the wavelength of the maximum absorbance turn out to be 520 nm. Based on the table on the introduction of the lab manual, the absorbed colour is at blue green range. Thus, based on the graph, the origin of the soft drink that we gain turn out to be blue and green. For the reading of the soft drink concentration versus absorbance, the reading of the absorbance
keep
increase
propotionally
woth
the
concentration.
As
the
concentration of the soft drink increase, so does the absorbance. As the plotted graph, the concentration os directly propotional to the absorbance. For the concentration of solution 2, to get the formula of A=£bc is used and to get the c or the concentration the £ is gain by using the gradient of the graph that turn out to be 0.1452. Based on the absorbance gain in the data sheet, at 520 nm which is the maximum absorbance, the absorbance turn out to be 0.747. By substituting all data inside the formula, then we gain the concentration of solution 2 to be 2.0853 M. Meanwhile for the concentration of the unknown solution can be detrermine by drawing line on the absorbance reading which is at 0.153 and thus based on the plotted graph the concentration of it turn out to be 0.8 M. During the experiment, a few precaution were taken to minimized any error that might present on collecting the data. First, before using the pipet we rinse it by using some of the soft drink to remove any other impurities that might present on the glassware. when pipet the carbonated drinks, the eye were ensure to be parallel with the reading needed. Then, when diluting the soft drink, the distilled water were ensure to be not more or less than the calibration mark of the volumetric flask. Then, before pouring the solution into a small beaker, the beaker were rinsed first with a
small amount of the solution. When using the cuvette, the surface of it were wiped using kimwipe each time before inserting it on the DR 2800. CONCLUSION The source of the colour of the souft drink is from blue and green spectrum. The wavelength of soft drink at maximum absorbance is at 520 nm and the concentarion of th unknown soft drink turn out to be 0.8 M. REFERENCES 1. Chemistry 111 LAB: intro to spectrophotomeetrey, june 2005..
MARA UNIVERSITY OF TECHNOLOGY FACULTY OF APPLIED SCIENCE DIPLOMA IN SCIENCE CHM 260 EXPERIMENT 1 THE VISIBLE SPECTRA OF SOFT DRINKS
NAME : AHMAD ISMI ZULFADLI BIN ISHAK STUDENT ID : 2010852022 LAB GROUP 1 DATE OF SUBMISSION : 31ST DECEMBER 2012 LAB PARTNER NAME: A.ZUL RAFIQ SAPILIH ALEXANDER TAGANDAP ATIQAH ARIMI AUDRINA SHIRLEY NORBERT ABDUL HADI BIN MOHD YUSSOF LECTURER’S NAME: MISS NOOR EZAWANIEE HJ MOULTON
