Description
Ex .5 Spectrophotometric Analysis of a Commercial Aspirin Tablet
A) Standard solution and Beers law calibration curve
1-Fill in the following table :
M1 x V 1 = M2 x V 2
M1= molarity of salicylic acid (SA)
Volume of SA=V1
V2= the total volume of dilute solution =10mL
M2=molarity of dilute solution (u have to find) and write in table below
Solution
SA. 1 (V1=1.0)
SA .2 (V1=3.0)
SA.3 (V1=6.5)
SA .4 (V1=9.0)
ASA of unknown
Absorbance(A)
0.159
0.356
0.589
0.879
0.641
Concentration /M
( find from calibration
curve)
2- Plot a graph (or calibration curve) of Absorbance vs. conc by using the values from the table
B) Analysis of unknown aspirin tablet
1. Mass of aspirin tablet =0.1205g
2. Concentration of ASA unknown from calibration curve =………………….. mol / L
3. Conc of Stock ASA unknown = 25 ml / 1.5 ml ( D.F ) x Conc. of ASA
unknown(step 2) =………………… mol / L
4. Moles of ASA in 50 ml solution = Conc of Stock ASA unknown(step 3) x 50mL=
………….. moles
5.Mass of ASA = moles ASA x Molar Mass ASA (180 g / mol ) = …………………g
6. % of ASA in aspirin = Mass of ASA(step 5) / Mass of aspirin tablet(step 1) x100
=
………………….. %
EXPERIMENT 5
SPECTROPHOTOMETRIC ANALYSIS OF A COMMERCIAL ASPIRIN TABLET
Aim
To find the percentage of active ingredient (ASA) in
commercial tablet using UV/Vis spectrophotometer
Introduction
Aspirin has chemical name Acetyl salicylic acid(ASA).
Aspirin has active ingredient and also some inactive ingredients.
Here we will study about active ingredient in aspirin. It is ASA. The
amount of ASA is different depending on the branded company .
We are doing the reaction of ASA with Fe(III) (coming from FeCl3) .and
will get complex of purple or violet color. The intensity of the color is
directly related to the concentration of aspirin present. The
concentration of this complex formed can be determined
by
spectrophotometric method.
Why we see complex as violet color?
If you observe violet, what color light is being absorbed?
We see violet color.
We see the complement(or opposite) of the light that is absorbed.
Color wheel
absorbed
Part 1 :Making solutions of Iron salicylate and
Beers law calibration curve.
• In part 1 we have to make dilute solutions of volume 10mL by reactions
of SA(salicylic acid) with FeCl3.SA reacts with FeCl3and gives violet or
purple complex .Absorption of each solution is measured and a
calibration curve(Beers plot) is obtained.
General reaction
Iron salicylate
Iron salicylate solutions
Find concentration of each solution
Solution
Concentration /M
M1V1=M2V2
M1=0.000725M
V2=10mL
M2?
SA. 1 (V1=1.0)
0.725 x 10-4
SA .2 (V1=3.5)
2.5×10-4
SA.3 (V1=6.5)
4.71×10-4
SA .4 (V1=9.0)
6.525×10-4
Q 1 From the table which solution has more concentration of complex and why ?
Q2 which solution is having more dark color and why?
PART 2:finding the mass % of ASA in tablet
1.Take tablet mass
2.Dissolve tablet in NaOH and heat on hot plate to make the reaction little fast. NaOH wil hydrolyze
the tablet and wil make salicylate ion.
3.Than this salicylate ion solution is transferred to flask and filled with water. This is concentrated
stock ASA solution .
4.From this solution take 1.5mL in 50mL flask and add FeCl3 to make complex. This is ASA unknown.
A violet complex is formed between ASPIRIN and IRON(III).
Both Complex and ASA are in 1:1 so amount of complex α amount of ASA
Spectroscopy
Spectrophotometer
UV/VIS spectrophotometer
Spectroscopy Is the interaction of
electromagnetic radiation with matter.
Spectrophotometer is an instrument used
to measure light absorbed or transmitted
by sample.
UV/Vis spectrophotometer is a
spectrophotometer which measures
absorbance in visible (400-800nm) and
UV (200-40nm) region.
UV/VIS spectrophotometer
BEERS LAW
This A is at one specific wavelength(λ) of light.
We select that wavelength of light where
absorbance is maximum.
That wavelength is known as λmax
Wavelength: distance between identical part of
wave
Take absorbance of all solutions made in part 1 and last step of part 2 using UV/Vis spectrophotometer
Instrumentation of spectrophotometer
To find absorbance first we wil use solvent (which is FeCl3) in cuvette. This will be blank reading
and absorbance of this must be zero. Than we will take reading of solutions made.
Information from Beers plot /calibration curve
• Using BEERS law we can make Beers plot/calibration curve between
concentration and absorbance.
• We can find a)slope from this curve.
b)concentration of unknown
We can find slope by inserting Beers law equation
into best fit line (y=mx + b)
b=intercept which we set as zero
Q1 :From the plot what is the relationship
between absorbance and concentration?
Solution
Concentration /M Absorbance(A) at
M1V1=M2V2
530nm.
M1=0.000725M
V2=10mL
M2?
SA. 1 (V1=1.0)
0.725 x 10-4
0.356
SA .2 (V1=3.5)
0.487
SA.3 (V1=6.5)
0.589
SA .4 (V1=9.0)
0.879
ASA unknown
( find from
calibration curve)
0.641
Q2 From the table of absorbance and concentration
which solution has the most absorbance and why?
Q 3:At which wavelength the absorbance is maximum
for complex?
Now Beers calibration curve (using Beers law) to find concentration of unknown ASA
How to find C ASA unknown
2 methods to find.
1)Directly from graph .: take value for unknown from table and
mark it on y axis. than draw line from that mark and touch best fit
line and than bring down to x axis.
2)Use best fit line equation. Y=mx
Put the value of y which is absorbance for unknown and find x.
Q4 The blank reading was taken before taking readings of
solutions .What was the solvent used to take blank reading .see
from the video .
Q5 What must be the absorbance of solvent in which we made
solution?
2)Y=mx
(1)
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