Assay of aspirin by using Back titration pdf.pdf

Assay of aspirin by using Back titration

1. MEDICINAL
CHEMISTRY PRACTICE
4
th
STAGE

Assay of Aspirin by
using Back Titration
Lab.4
1

2. Objective:
Determination of amount of aspirin in tablets
(determination is done by alkaline hydrolysis and back
titration)

2

3. Theory:
Assay: is an analytical procedure in laboratory medicine for
qualitatively assessing or quantitatively measuring the
presence or amount or the functional activity of a target entity
(the analyte), which can be a drug or biochemical substance.

The main methods used in quantitative determination of ASA
either in its pharmaceutical preparations or powdered pure
form are:
1- Spectrophotometric methods
2- Chromatographic methods: such as: GC, TLC, and HPLC.

3

4. Back titration
4
3- Titration (volumetric) methods: are very simple and
accurate methods; therefore, these method is the most
widely used for the quantitative determination of ASA
such as:
Back titration method: is one of volumetric methods
which includes the addition of an excess of standard
volumetric sol. to a weight amount of a sample and then
determination for the excess unreacted (no required or
utilized by the sample), then the amount of volumetric
sol. used by the substance is determined.

5. When Is a Back Titration Used?
5
A back titration is used when the molar concentration of an
excess reactant is known, but the need exists to determine the
strength or concentration of an analyte.
Back titration is typically applied in acid-base titrations:
When the acid or (more commonly) base is an insoluble salt
(e.g., calcium carbonate)
When direct titration endpoint would be hard to discern (e.g.,
weak acid and weak base titration)
When the reaction occurs very slowly

6. What is Aspirin ?
6
Chemically:
It contain one benzene ring with carboxyl group as well as
acetyl group. That’s why it is acidic in nature.
Its chemical name is 2-Acetoxybenzoic acid. Also called as
Acetylsalicylic acid.
 A part of a drug family known as NSAIDs or non-steroidal
anti-inflammatory drugs.

7. Physical properties of Aspirin
7
Physical State: crystalline
 Appearance: white
Odor: odorless
Nature: Aromatic and acidic
 pH: 3.5
Melting Point: 136°C
Solubility: in organic solvents.
Specific Gravity/Density:1.350
 Molecular Formula:C9H8O4
 Molecular Weight:180.16

8. How Aspirin works
8
Aspirin works by reducing the production of prostaglandins.
Prostaglandins are pain-triggering hormones. It produces pains
like headaches etc. Aspirin, sticks to COX-2 (Cyclooxygenase 2
enzyme)to suppress pain.
Aspirin blocks production of TxA2 by acetylating a serine
residue near the active site of platelet cyclooxygenase-1
(COX-1).
The action of aspirin on platelet COX-1 is permanent, lasting
for the life of the platelet (7-10 days).

9. How Aspirin works
9

10. Uses of Aspirin
10
 Aspirin is used to treat or prevent heart attacks, strokes,and
chest pain (angina).
 It is also used as pain killer.
 Aspirin should not be given to a child who has a fever, especially
if the child also has flu symptoms or chickenpox. Aspirin can
cause a serious and sometimes fatal condition called Reyes
syndrome in children.
Avoid drinking alcohol while you are taking this medication.
Alcohol may increase your risk of stomach bleeding.

11. Chemical principle


11
Aspirin is a weak acid that also undergoes slow hydrolysis; i.e.,
each aspirin molecule reacts with two hydroxide ions.
Aspirin Sodium salicylate

12. Procedure:
12
1.Grind 5 aspirin tablets into a fine powder by using a mortar
and pestle.
2.Place the powdered aspirin in a conical flask and add 25 ml
of 0.5 N sodium hydroxide (NaOH).
3.Boil gently for 10 minutes.
4.Titrate the excess alkali with 0.5 N hydrochloric acid using
phenol red as indicator( the end point is pink colorless)
5.Repeat the same procedure without aspirin.

13. Calculation for concentration of Aspirin
13
Each ml of 0.5 N sodium hydroxide is equivalent to
0.04504 g of aspirin.
V2 of blank – V1 of aspirin = V3 of NaOH
(V2 of blank − V1 of aspirin) * 0.04504= amount of
aspirin in 5 tablets (g).

14. Calculation of Chemical factor
14
 Estimate each ml of 0.5 N sodium hydroxide is equivalent
to 0.04504 g of aspirin!
1 mole of aspirin = 2 moles of NaOH
180 gm of aspirin = 2 Equivalent no. of NaOH
180 gm/2 of aspirin = 1000 ml of 1N NaOH
90/2 gm of aspirin = 1000 ml of N/2 NaOH
0.045 gm of aspirin = 1ml of N/2 NaOH

15. Questions:
15
How to prepare 0.5 N of HCl in 500 ml of water ?
How to prepare 0.5 N of NaOH in 250 ml of
water?
Note: M.wt of HCl = 36.46 gm/mole
M.wt of NaOH = 40 gm/mole