Ariel Grove
February 2013
Introduction:
A UV-Vis is a spectrophotometer instrument that is very common to analyzing compounds in ultraviolet
and visible regions of the electromagnetic spectrum. In comparison to Infrared which looks at vibrations,
UV-Vis looks more at the electronic transitions. This instrument allows for the determination of specific
wavelengths and maximum absorbance’s of particular compounds. This instrument does have its limits
in detection, in which we can switch over to the fluorescence instrument, which can detect trace
concentrations to the point of 1 ppt. This instrument uses two separate wavelengths along the magnetic
spectrum to determine the quantitative measurements of an analyte. It sends light into the sample,
which excites the electrons. Then, the instrument reads the radiation released when the electrons
return back down to their ground state.
Purpose:
The purpose of this lab was to run scans on salicylic acid (in hexanes) samples, to obtain the
wavelengths and maximum absorbance values of the compounds to determine the levels of salicylic acid
in aged aspirin via the UV-Vis instrument. The recorded samples would then be used to make a standard
calibration curve. Also, a calibration curve would be made from the recorded reading of quinine and
sulfuric acid solutions, from the fluorescence instrument to determine the level of quinine in the tonic
water sample.
Procedure:
UV-Vis
-
Prepare dilutions of salicylic acid in hexanes (10 ppm, 20ppm,30ppm,40ppm and 95ppm)
Prepare an aspirin sample by crushing a standard aspirin tablet (around 325mg) and
dissolving in a 100mL flask filled with hexanes
Follow the standard SOP for the UV-Vis provided to complete the scans
Fluorescence
-Prepare dilutions of quinine in 0.1M sulfuric acid (0.1ppm, 1ppm, 2ppm, 3ppm)
- prepare an unknown solution, 5mL of tonic water in 95mL of 0.1M sulfuric acid
Follow the standard SOP for the Fluorescence instrument provided to complete the scans
Data/Calculations:
Salicylic acid in hexane dilutions:
From 1000 ppm stock:
(1000ppm)(V1)=(10ppm)(50 mL)
V1= .5 mL of 1000 ppm stock
(1000ppm)(V1)=(20ppm)(50 mL)
V1= 1 mL of 1000 ppm stock
(1000ppm)(V1)=(30ppm)(50 mL)
V1= 1.5 mL of 1000 ppm stock
(1000ppm)(V1)=(40ppm)(50 mL)
V1= 2 mL of 1000 ppm stock
(1000ppm)(V1)=(95ppm)(50 mL)
V1= 4.75 mL of 1000 ppm stock
Quinine in .1 M sulfuric acid dilutions:
From 100 ppm stock:
(100 ppm)(V1)=(.1ppm)(100mL)
V1=.1 mL of 100ppm stock
(100 ppm)(V1)=(1ppm)(50mL)
V1=.5 mL of 100ppm stock
(100 ppm)(V1)=(5ppm)(50mL)
V1=2.5 mL of 100ppm stock
(100 ppm)(V1)=(10ppm)(50mL)
V1=5 mL of 100ppm stock
Tonic water concentration
205.0787=194.3x+22.84
x=.938 ppm
Aspirin Tablet Concentration
1.2627=.01921x-.07102
x=62.03 ppm
Results:
UV-Vis
Concentration
(ppm)
10
20
30
40
95
Aspirin Sample
Trial 1
Trial 2
Trial 3
Average
.046
.083
.392
1.264
1.602
1.263
.042
.082
.388
1.271
1.607
1.265
.042
.082
.387
1.276
1.611
1.260
.0433
.0823
.3890
1.2703
1.6067
1.2627
Calibration curve
Concentration vs. Absorbance
r2=.7942
y=.01921x-.07102
Absorbance (nm)
2.0
1.5
1.0
0.5
0.0
0
20
40
60
80
100
Concentration (ppm)
Fluorescence
Concentration
(ppm)
.1
1
2
3
Unknown
Trial 1
Trial 2
Trial 3
Average
14.824
235.556
457.540
570.982
207.295
14.150
233.519
457.023
568.223
204.469
14.037
234.732
457.648
570.982
203.472
14.3370
234.6023
457.4037
570.0623
205.0787
(Optimal wavelength: emission 451, excitation 316)
Calibration Curve
Concentration vs. Intensity
Peak Intensity (nm)
800
r2=.9755
y=194.3x+22.84
600
400
200
0
0
1
2
3
4
Concentration (ppm)
Conclusions:
When both graphs were observed, the Fluorescence graph turned out much more accurate and had a
much higher r squared value, for precision was higher for that particular experiment. During the UV-Vis
experiment, it seemed as my partner and I kept running into various issues. Wild results were being
observed and likewise, the calculated ppm concentration value for the aged aspirin wouldn’t be a very
value to use, hence seeing the inaccurate values collected in the calibration. On the other hand, the
tonic water calculated ppm would be a reasonable conclusion for the values seemed close to predictions
and came out with nice values in the calibration curve. Again as mentioned before, with the UV-Vis
portion of this lab, dilutions had to keep being made, which most likely was due to human error in
measuring the correct volumes and doing the correct calculations. This lab would definitely need to be
run again, with precise calculations and precision being a key part to get more reliable values. Mostly
focusing on the UV-Vis portion rather than the fluorescence.