bsc chemistry 2.1 practicals

1. Name : MunasheC Mazodze
Program : Bachelor of Science HonorsDegree in Chemistry
Reg no : C14123361T
Course : laboratory Chemistry (III)
Experiment : 5
Level : 2:1
Title : Flame Atomic Absorption Spectrophometry
Date : 28 April2014
Aims : to determineamountof aspirin in a pharmaceutical
: manipulatethe techniques behind use of the floroumeter
: Detection of possible chemical interferences and sensitivity of
the

2. Floroumeter
Introduction
Fluorescence is the emission of light by a molecule which has absorbed radiant
energy Skoog (2007). When an atom or molecule absorbs energy, eletrons are
promoted from their ground state to different vibrational energy levels of an
excited state. The excited species can quickly give up its excess energy and relax to
its ground state through non radiative relaxation and radiative relaxation. There are
2 types of nonradiative relaxation, vibrational relaxation and internal conversion.
Electrons in the excited state will drop to the lowest vibrational energy level via
vibration relaxation. Vibrational relaxation takes place during collisions between
excited molecule and molecule of the solvent. Nonradiative relaxation between the
lowest vibrational level of another electronic state can also occur. This type of
radiation is called internal conversion. The mechanisms by which this type of
relaxation occurs are not yet fully understood. according to Skoog the net effect of
nonradiative is a slight increase in temperature of the medium. Molecules can relax
from lowest vibrational energy of the excited state to any vibrational energy levels
of the ground state through fluorescence, in which excess energy is given up as
emissions of light at longer wavelength than absorbed. The other radiative
relaxation process phosphorescence is not discussed here. When absorption A ≤
0.05 the relationship between fluorescence intensity and sample concentration can
be expressed as
F ≈ 2.3 K’€bc P0
Where K’ is a constant that depends on the quantum efficiency of the fluorescence,
€bc is the absorbance A, P0 is the power of the beam incidence on the solution, F is
the fluorescence intensity. At a constant P0
F ≈ Kc
Where K is the new constant equal to 2.3 K’€b P0
A plot of the fluorescence intensity vs. concentration is used as a calibration curve
for quantitative analysis and ideally should be linear Skoog (2007) if A is smaller
than 0.05. Extremely dilute samples can be analyzed because of high intensity of
the method.

3. Reagents andapparatus
Flour meter
Cuvettes
60ml separation funnel
Volumetric flasks, 50ml, 100ml
Beaker, 300-400ml, 40
Pipettes, 1 2, 5, 10ml
Graduated cylinder, 10ml
Sodium Hydrogen carbonates
NaOH
Salicylic acid
Chloroform
Aspirin tablet
Experimental procedure
The solutions were prepared as follows:
1% Na2CO3 – was made by dissolving 2,5g of Na2CO3 in 250ml of distilled water
0.5M NaOH –was made by dissolving 2g pellets of NaOH and dilute up to the
mark with water in 100ml volumetric flask
100ppm salicylic acid –was made by dissolving 0.01g of salicylic acid in 100ml
volumetric flask and diluting up to the mark with water
Salicylic acid standard solutions 1ppm, 2ppm, 5ppm, 10ppm, -were made by
transferring 1, 2, 5, 10ml respectively from 100ppm standard stock solution to a

4. 100mL volumetric flask separately and diluting all standards to the mark with
distilled water which was adjusted to pH 11 with NaOH.
Aspirin tablet sample solutions- 3 aspirin tablets were weighed and 0.1g of each
was dissolved in 20ml chloroform and 3 extractions were done, twice using 1%
Na2CO3 and once with water using 60mL extraction funnel then the aqueous phase
was collected into 50mL volumetric flasks and 20mL of 0.5M NaOH was added
into the collected aliquots and then diluted up to the mark with water. 0.4mL of
each solution was transferred into 100mL volumetric flasks and diluted up to the
mark with a 1% Na2CO3.
10ppm solution was used to find the maximum wavelength for excitation and
emission by scanning the excitation wavelength from 250 to 350nm while holding
the emission wavelength at 400nm, then the excitation wavelength was set at
maximum and the emission wavelength was scanned from 350 to 500nm. The
emission wavelength was set at the maximum and a blank was scanned for zero
emission from 350nm. After setting the excitation wavelength at maximum the
emission of the standard solution and aspirin tablets were scanned and the
fluorescence intensity at maximum wavelength was recorded.
Results and calculations
Table 1:fluorescence ofsalicylic acid
type flourescence concentation
1 25.917 1
2 29.201 2
3 45.168 5
4 64.078 10
Sample1 37.123 3.580
Sample2 33.499 2.739
Sample3 40.500 4.363

5. Discussion
The large R2 value (0.9941) shows that the data used to generate the calibration
curve is very linear, and is therefore well described by Beer’s law Robinson (2008).
The calibration curve was used to determine the concentration (and mass) of
acetylsalicylic acid in 3 tablets, diluted to 3 different concentrations (Table 1). The
experimental design includes method errors that affect the average mass of aspirin
determined to be in the tablets. First, only 3 tablets were used to for the trials. This
small sample size increases the possibility of indeterminate errors affecting the
results Bauer (2007) and Skoog (2007). To reduce this error, additional tablets
would need to be tested. Second, both the hydrolysis reaction of acetylsalicylic
acid with sodium hydroxide and the reaction of salicylic acid with iron sodium
carbonate are assumed to proceed to completion. This assumption is reasonable
because the NaOH and Na2CO3 solutions were present in excess compared to the
molar amount of acetylsalicylic acid. If the reactions did not proceed to completion,
however, the amount of acetylsalicylic acid present in the tablet would be
underestimated by this technique in agreement with Peters (2006) and Reynolds
(2004). The direction of error caused by incomplete reactions is consistent with the
result of a lower measured value of acetylsalicylic acid compared to the value on
the bottle. This experiment determined the amount of acetylsalicylic acid in an
aspirin tablet, which was hypothesized to be the same as the amount on aspirin
package paper. A difference of 2.46% between the amount measured and the
amount on the bottle supports this hypothesis. A variety of factors support the
suitability of this experiment to measure the amount of acetylsalicylic acid in a
tablet: the reaction to create the colored chromophore proceeded to completion (or
y = 4.3123x + 21.686
R² = 0.9941
0
10
20
30
40
50
60
70
0 2 4 6 8 10 12
flouresence(nm)
concentation (ppm)
Calibration curve for Salicylic acid determination

6. near completion), and the calibration curve generated with standard solutions of
the chromophore was fit to a linear equation (Beer’s law), which allows for a
precise determination of the amount of acetylsalicylic acid in the unknown.
REFERENCES
D.A Skoog, Principles of instrumental Analysis, 4th edition, chapter 10,
2007
Robinson and Robinson, Contemporary instrumental analyses, Prentice hall
373391,2008
R.G Reynolds Atomic Absorption Spectroscopy A Prentice Guide Barnes
and Noble New York 1972
Bauer, Christian and O’ Riley Atomic Absorption Spectroscopy chapter 10
2004
D.G Peters, J.M.Hayes and G.M Hieftje, Chemical Separations and
Measurements, Saunders, Philadelphia 1974 Chapter 29 2006