Analysis of Aspirin Tablets
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To discover the percentage of acetylsalicylic acid in a sample of
In order to do this, the amount of moles that react with the sodium
hydroxide must be known. This is achieved by using the method of back
titration. In this, the amount of moles in the sodium hydroxide
solution after it has been reacted with the aspirin is found using
titration, and then compared with the amount of moles it had without
the aspirin being added. The difference in moles is the number of
moles of sodium hydroxide that reacted with the aspirin, and therefore
the number of moles of acetylsalicylic acid. A simple sum can then be
done to figure out the mass of aspirin, which when compared to the
mass of the tablets, reveals the percentage of acetylsalicylic acid in
Aspirin is an analgesic (pain relieving) and an antipyretic drug (a
drug that lowers body temperature). The main constituent of aspirin is
2 - ethanoythydroxybenzoic acid, also known as acetylsalicyclic acid
(shown below right). It was originally made from just salicylic acid
(which is found in the bark of a willow tree) when used by the Ancient
Greeks to counter fever and pain, but its bitterness and tendency to
irritate the stomach caused problems. These were resolved by the
German chemist Felix Hoffman, who made the acetyl derivative of
salicylic acid in the 1890's.
When aspirin is taken, it travels unaffected through the acidic
conditions of the stomach until it reaches the intestines, where it is
hydrolysed into acetate and salicylate ions by the alkaline juices
found there. Here, the salicylates lower body temperature in feverish
patients, and they also relieve certain types of pain such as
headaches and rheumatism.
Aspirin is currently the first choice drug for fevers and inflammation
due to arthritis, acting at the site of the tissue damage rather than
the pain centers of the brain.
Despite its usefulness, there is a
danger to taking too many aspirin. Over time aspirin can cause
gastronomical bleeding that leads to iron deficiency, and also gastric
ulcers may occur. They should not be given to children as it increases
the risk of contracting Reye's syndrome, a serious and often fatal
disease of the brain and some abdominal organs.
· About 1.5g of aspirin tablets - usually 4 or 5 tablets
· 1.0 mol dm¯³ Sodium Hydroxide
· 0.1 mol dm¯³ Hydrochloric Acid
· Distilled Water
· Methyl Orange indicator
· Conical Flask
· Burette Stand Acetylsalicylic acid
· Safety Filler
· Bunsen burner
· Bunsen Mat
· 250cm³ Standard Flask
· Aspirin tablets - These contain the acetylsalicylic acid, which will
react with the sodium hydroxide
· Sodium Hydroxide - This will hydrolyse the aspirin into salicylic
and acetate ions
· Hydrochloric Acid - This will neutralise the sodium hydroxide
· Distilled Water - This is used to make up the correct solutions and
to wash equipment. Normal water will not do because of the impurities
· Methyl Orange indicator - The colour of this indicates when the
sodium hydroxide has been neutralised by the hydrochloric acid.
· Conical Flask - This is used to react the aspirin tablets with the
sodium hydroxide. It is more appropriate to use as the shape of it
makes it less likely that any should spill out.
· Burette - This is used to add the hydrochloric acid to the sodium
hydroxide. It makes the results as accurate as possible, as it allows
the hydrochloric acid to be added drop by drop when the sodium
hydroxide is close to neutralising.
· Burette Stand - This holds the burette steady in place.
· Funnel - This allows hydrochloric acid to be poured into the burette
so none is spilt.
· Pipette - This is used to accurately draw out the correct amount of
a substance from a solution.
· Safety Filler - This is used to draw the substance up the pipette as
safely as possible.
· Bunsen burner - This is used to gently heat up the sodium hydroxide
and aspirin solution, increasing the rate at which they react.
· Bunsen Mat - This is used to protect the table
· Wooden splint - Used to light the Bunsen safely
· Matches/Lighter - used to light the wooden splint
· Tripod - Used as a stand to hold the solution in the Bunsen flame
· Gauze - Acts as a fireproof table surface for the flask to sit on
· Tongs - To safely pick up the hot conical flask
· White Tile - So the colour of the solution is easier to see
·250cm³ Standard flask - This is used to mix up solutions accurately.
· Goggles - Used to protect the eyes throughout the experiment
· Laboratory coat - Used to protect the skin and clothing
· Sodium Hydroxide is extremely caustic, and may cause severe burns if
it comes into contact with the skin. It is particularly dangerous if
it gets in the eyes, even in the most dilute solutions, so labcoats
and eye protection is essential whilst handling. In order to dispose
of, it should first be neutralised using an acid
· Hydrochloric acid is hazardous, so care should be taken not to
inhale or swallow, as it can cause severe burns and even kill. It is
extremely corrosive to skin and may produce burns and ulcerations. If
it gets into the eyes, it may cause permanent irritation. Continual
exposure may cause laryngitis, bronchitis, pulmonary edema and even
Four aspirin tablets (1.36 grams) were dropped into a clean conical
flask. A pipette was placed in the 1.0 mol dm¯³ sodium hydroxide
solution, and a safety filler was fixed onto the end. By winding it
upwards, the pressure inside the filler came so low, the solution
became sucked up inside the pipette. When the solution was sucked up
past the 25cm³ mark, the safety filler was removed, and replaced by a
thumb. By carefully lifting the thumb on and off the top of the
pipette, the level of the solution dropped down. When this dropped so
that the base of the discus was equal the 25cm³ mark, the solution was
taken away and released into the conical flask on top of the aspirin
The Bunsen burner was then set up. It was placed onto a Bunsen mat,
and the tubing was secured to a gas tap. A tripod was placed over
this, and on this a gauze mat was placed. The gas was then turned on,
and the Bunsen was lit using a burning splint. The valve was then
opened, giving the hotter blue flame, which is more suitable for
heating. The conical flask was placed on top, and was occasionally
picked up with tongs and shaken around, to make sure it never quite
boiled. After about 10 minutes, when the acetylsalicylic acid was
fully hydrolysed, the conical flask was left to allow the solution to
cool. This was important because if the hot solution was poured into a
cold flask, it could cause the glass to expand and break.
After it cooled, the solution was poured into a standard flask. This
was poured carefully, as all the moles from the conical flask needed
to be transferred to the standard flask. It was okay to use distilled
water to wash into the very bottom of the conical flask to get all the
moles out, as the next step anyway was to dilute it to the 250cm³
mark, so the base of the discuss was on the line.
After the dilution, 25cm³ of the now 0.1mol dm¯³ solution was taken up
in a pipette (once again using a safety filler) into a conical flask.
A couple of drops of methyl orange indicator were then added, turning
it a slightly yellow colour. The flask was placed on a white tile
beneath the burette on the stand. The tap of the burette was closed,
and a funnel placed on top. Holding the funnel carefully, 0.1 mol dm¯³
Hydrochloric acid was poured into the burette. When the burette was
filled, the funnel was removed carefully, shaking of the last drop
into the burette. The level of the discus in the burette was then
marked down as a starting point in a table as shown below. The tap was
then opened, and the hydrochloric acid mixed with the sodium
hydroxide, slowly neutralizing it. The solution had to be picked up
and swilled around for more accuracy. The nearer the solution got to
being completely neutralised, the paler the indicator colour became.
As it neared the end, the acid would be added drop by drop, until the
indicator turned the slightest little bit pink. The discus level was
then marked down as the end point of the experiment. The amount used
to neutralise the solution could then be used to find out how many
moles reacted with the aspirin. This experiment was repeated 4 times
to get the most accurate results possible, and they were all recorded
in the table below.
Average = 17.5 + 17.7 + 17.6 + 17.7 = 17.625
17.625cm³ of Hydrochloric Acid is needed to neutralise the Sodium
Hydroxide after it has reacted with the aspirin tablets.
Reliability of results
Sodium + Hydrochloric º Sodium + Water
Hydroxide Acid Chloride
NaOH + HCl º NaCl + H2O
(1/1000) x 25 = 0.025
There are 0.025 moles in 25cm³ of 1.0 mol dm¯³ NaOH. Therefore, 0.025
moles of HCl are needed to react with it.
(1/1000) x 17.625 = 0.017625
The titration showed that 0.017625 moles of HCl reacted with the NaOH.
This means that only 0.017625 moles are left after the NaOH has
reacted with the aspirin tablets.
0.025000 - 0.017625 = 0.007375
0.007375 moles of NaOH reacted with the aspirin
Acetylsalicylic + Hydroxide ºAcetate + Salicylic + Water
Acid ions ions ions
CH3COOCH4COOH + 20H¯ºCH3COO¯ + HOC6H4COO¯+ H20
There is a 1:2 ratio of acetylsalicylic acid to hydroxide ions
(0.007375/2) = 0.0036875
Therefore, 0.0036875 moles of acetylsalicylic acid reacted with the
hydroxide ions. The relative molecular mass of the acetylsalicylic
acid is 180
0.0036875 x 180 = 0.66375
0.66375 grams reacted, so the mass of each tablet that reacts is
(0.66375/4) = 0.1659375 grams
Each Tablet has a mass of 0.34 grams, so the percentage of
acetylsalicylic acid in each tablet can now be found out
(0.1659375/0.34) x 100 = 48.805147
Therefore, in one aspirin tablet with a mass of 0.34 grams, there is
approximately 48.8% of acetylsalicylic acid.