Colourimetry
So in the case of finding out the purity of the aspirin I have produced I must look at the chemistry involved.
Salicylic acid is a phenol and all phenols react with a neutral iron III chloride and produce a purple colour
So relating this back to purity, the solid aspirin is made into a solution using a 50:50 ratio of water and ethanol, then neutral iron III chloride is added the concentration of the colour indicate the amount of unreacted salicylic acid present within the sample, therefore the stronger the purple colour the more impure the aspirin is.
To test the purity of the sample I must create a calibration curve using various concentrations of salicylic acid and reacting it with neutral iron III chloride. I then place the solutions in to a colorimeter to give me a numerical value from which I make a calibration curve
I then add my manufactured aspirin to a solution of 50:50 ratio of water ethanol and add neutral iron III chloride and place the aspirin solution into the colorimeter which shall give a numerical value. I than locate the numerical values on the calibration curve and this allows me to find the concentration of salicylic acid in the sample and therefore the purity of my manufactured sample.
Titration
This technique is used to determine the concentration of a solution by reacting it with another solution of known concentration. When the reaction is complete it is signalled by an in indicator which shall change colour.
So for aspirin titration we can use sodium hydroxide, hydrochloric acid, distilled water
First I add a known mass of our manufactured aspirin to a 250 cm3 volumetric flask and then add 25 cm3 of sodium hydroxide and heat to hydrolyse the aspirin and I then add water to the 250 cm3 mark
I then pipette 25 cm3 of the hydrolysed aspirins solution into a 250 cm3 conical flask and add some phenolphthalein indicator. I then titrate 0.1mol dm-3 hydrochloric acid and record the volume of hydrochloric acid needed to turn the pink solution colourless
And I repeat until the concordant results are obtained.
The theory behind this is we can calculate how much hydrochloric acid is needed to neutralise the use sodium hydroxide however the excess amount that has been added will tell us how much salicylic acid was in the solution, which we can work out to tell us the amount of aspirin was present. The more hydrochloric acid need to neutralize the solution the more impure the aspirin is and using the concentration and volume need to hydrolyse the aspirin we can then establish the purity.
Method For An Accurate Titration
Procedural errors can arise if practical technique are not good.
A good technique would include the following:
- The solution in the volumetric flask needs thorough mixing.
- The burette and pipette should be washed out with the solutions being used.
- The conical flask should be thoroughly washed out with distilled water between titrations
-
The end-point of a titration can only be determined accurately if the solution from
the burette is added drop by drop, with swirling, as the end-point is reached
- When an indicator is used in a titration use the same number of drops for each run.
Folowing these method will ensure I do an accurate titration and collect valid results
General safety precaution in the laboratory
ASPIRIN SYNTHESIS
EQUIPMENT LIST
FOR ASPRIN SYNTHESIS
- Access to a fume cupboard
-
100 cm3 conical flask x1
-
10 cm3 measuring cylinder
-
20 cm3 measuring cylinder
-
100 cm3 beaker x2
- Glass rod
- Apparatus for vacuum filtration
- Hirsch funnel
- 2-hydroxybenzoic acid (salicylic acid) (100 g)
-
Ethanoic anhydride (200 cm3)
-
Concentrated sulphuric (VI) acid (20cm3)
-
Glacial ethanoic acid (200 cm3)
- Water bath containing crushed ice
- De-ionised water
- A balance (2 dip.)
- Oven for drying sample
SAFETY INFORMATION
Salicylic acid
Inhalation causes irritation of lungs resulting in breathing difficulties. Contact with skin causes irritation and possible burns, especially if the skin is wet or moist. If absorbed, may cause symptoms similar to those for ingestion. May cause skin rash and eruptions.
Glacial ethanoic acid
It is and must therefore be handled with appropriate care, since it can cause skin burns, permanent eye damage, and irritation to the mucous membranes. These burns or blisters may not appear until hours after exposure. gloves offer no protection, so special resistant gloves, such as those made of should be worn when handling the compound.
CARE: Glacial ethanoic acid should be handled in a fume hood because of the pungent, corrosive vapour.
Ethanoic anhydride
Ethanoic anhydride is an irritant and flammable. Because of its reactivity toward water, alcohol foam or carbon dioxide are preferred for fire suppression. The vapour of acetic anhydride is harmful.
Concentrated Sulphuric acid
Method for Aspirin synthesis
-
Working in a fume cupboar, swirl 8 g of 2-hydroxybenzoic acid (salicylic acid) with 16 cm3 of ethanoic anhydride (CARE Corrosive) in a 100 cm3 conical flask.
-
Add 2cm3 of concentrated sulphuric (VI) acid (CARE Corrosive) and continue agitating the flask for about 10 minutes. Crystals of aspirin will appear and soon the whole solution will form a crystalline ‘mush’.
-
Stir in 16 cm3 of cold glacial ethanoic acid (CARE Corrosive) to dilute the mixture and cool by placing in a water bath containing crushed ice.
- Filter off the crystals using a Buchner funnel (vacuum
Filtration),
- Wash once with ice-cold water.-to remove any impurities however one must only was it once as you do not want to reduce yield of aspirin.
- Dry your purified sample on filter paper and weigh it.
Analysis of Aspirin
Analysis of aspirin
EQUIPMENT LIST
For
Thin layer chromatography
- Access to a fume cupboard
- UV light source
- Watch glass (3)
- Dropping tubes or melting-point Tubes
- T.L.C. plates (silica-coated, fluorescent ones are ideal)
- Small beaker to hold t.l.c. plate
- cover for the beaker
-
Ethanol (5 cm3)
- Prepared aspirin samples
- Sample of pure aspirin
- Sample of pure 2-hydroxybenzoic acid
- Solvent for chromatography
- Aluminium foil or cling film
SAFETY INFORMATION
Ethanol
Ethanol is extremely flammable and the vapour produced by ethanol can cause respiratory problem, drowsy or dizziness if inhaled. Any spillage must be dealt with as soon as possible. There must be no naked flames around as the liquid and vapours are flammable
Cyclohexane
Cyclohexane is very flammable and may be ignited by contact with a hot surface - a naked flame is not necessary. Wear safety glasses. Before starting work ensure that the working area does not contain any potential sources of ignition.
Make sure that ventilation of the working area is good. If possible use a fume cupboard. Cyclohexane must not be flushed down a sink as it is both an environmental hazard and a serious fire risk. It should be stored for disposal by the technician in charge of the laboratory.
Ethyl Ethanoate
Extremely flammable - Wear safety glasses. Make sure that there is no source of ignition near where you work. The vapour may be ignited by contact with a hot plate or hot water pipe - no naked flame is needed.
Salicylic acid
Inhalation causes irritation of the mucous membrane and upper respiratory tract. Contact with skin causes irritation and possible burns, especially if the skin is wet or moist. If absorbed, may cause symptoms similar to those for ingestion. May cause skin rash and eruptions.
CARE The chromatography solvent is harmful by inhalation (as are the vapours produced by this activity) and highly flammable. Work in a fume cupboard or in a well-ventilated laboratory as directed and avoid inhaling the fumes. Avoid naked flames. The chromatography solvent contains cyclohexane. Return residues containing cyclohexane to a residues bottle. Do not pour them down the sink.
Method for Thin layer chromatography
- Take, a pre-dried thin-layer chromatography plate that will fit into a
small beaker. Try to handle the t.l.c. plate by the edges only.
Draw a fine pencil line about 1 cm from the bottom of the plate. This is the
baseline.
- Take a few crystals of the aspirin that you have prepared and dissolve them
in a minimum volume of ethanol on a watch glass. Try a few drops, then add
more ethanol if required.
- On the baseline place a small spot of your aspirin sample. The spot is best
made by using a very fine dropping pipette or a drawn-out melting-point
tube. Apply a small quantity of the solution at a time; let it dry, and then add
more. Try not to let the diameter of the spot exceed 5 mm.
-
Repeat steps 5 and 6 using pure salicylic acid and then pure aspirin. Ensure
the three spots are well spaced out along the baseline but are not too close
to the side edges of the t.l.c. plate. It is a good idea to label the top of the
chromatography plate (in pencil) so that you know what each spot is
.
- Working in a fume cupboard or a well-ventilated laboratory, pour some of the
chromatography solvent into the beaker to a depth of no more than 5 mm.
- Place the chromatography plate in the beaker, making sure the solvent level
is below the baseline.
- Cover the beaker with cling film (or aluminium foil or a watch glass). Leave
the solvent to rise up the t.l.c. plate. This will take about 15–25 minutes.
- When the solvent has nearly reached the top of the plate, take the
chromatogram out of the beaker. Place the plate in a fume cupboard and
allow the solvent to evaporate.
- A) locate the positions of the substances on the plate by examining it
under UV light. (CARE Do not look directly at the light source.) View the
plate by reflected light.
Analysis of aspirin
EQUIPMENT LIST
For
Titration
- access to a balance
- specimen tubes (1)
- burette
-
100 cm3 conical flask
-
250 cm3 volumetric flask
-
25 cm3 pipette
- White tile
-
95% ethanol (100 cm3)
-
sodium hydroxide solution, 1.0 mol dm–3 (200 cm3)
-
hydrochloric acid 0.1 mol dm–3 (600cm3 )
- phenolphthalein indicator (red)
- deionised water
SAFETY INFORMATION
Sodium hydroxide
Contact with the eyes can cause serious long-term damage .The solid and its solutions are corrosive. Significant heat is released when sodium hydroxide dissolves in water. Always wear safety glasses. Do not allow solid or solution to come into contact with your skin. Small amounts of dilute sodium hydroxide can be flushed down a sink with a large quantity of water
Phenolphthalein Indicator
This chemical may cause irritation if you get it into your eyes or breathe it in. Wear safety glasses. It is also very flammable.
Dilute hydrocloric acid
Contact with the eyes or skin can cause serious permanent damage
Concentrated solutions of this acid are extremely corrosive; very dilute solutions are mildly corrosive. Toxic by inhalation - the concentrated solution releases dangerous quantities of hydrogen chloride vapour. Always wear safety glasses. Do not allow the acid or a solution of it to come into contact with your skin. Wear gloves if handling the concentrated acid.
Method for Titration
Analysis of aspirin
EQUIPMENT LIST
For
Colourimetry
- A Colorimeter
-
volumetric flasks (100 cm3 ) x 7
- 100 cm3 beaker x2
- 2-hydroxybenzoic acid (salicylic acid) (20 g)
- De ionised water
- A balance (2 dip.)
- Weighing boat
-
50cm3 pipette
-
10cm3 pipette
-
Ethanol (95%) (600cm3)
SAFETY INFORMATION
Salicylic acid
Inhalation causes irritation of the mucous membrane and upper respiratory tract. Contact with skin causes irritation and possible burns, especially if the skin is wet or moist. If absorbed, may cause symptoms similar to those for ingestion. May cause skin rash and eruptions.
Ethanol
Ethanol is extremely flammable and the vapour produced by ethanol can cause respiratory problem, drowsy or dizziness if inhaled. Any spillage must be dealt with as soon as possible. There must be no naked flames around as the liquid and vapours are flammable
Method for colourmetric analysis
Calibration curve
It is necessary to prepare a calibration curve before using the aspirin with the colorimeter.
To create a calibration curve, a range of salicylic acid concentrations must be made. The reaction between salicylic acid and neutral iron III chloride produces a purple colour, so I will need to use a filter that is the opposite of this colour for absorption. This is green so I use the filter 530 nm.
I will create solutions of salicylic acid of known concentrations. I will create this solution using ethanol, water, salicylic acid and neutral iron III chloride (to create a purple complex)
Step one create the stock solution (Solution A)
I will need to create a stock solution, which will be 0.1 mol dm-3 concentration of salicylic acid. Using this stock solution I will sequentially dilute it to create different known concentrations of salicylic acid
-
I will weigh 1.38 g of salicylic acid and add it to a 100 cm3 volumetric flask I then add a solution of ethanol and water (50:50) to the mark on the volumetric flask. Invert the falsk several times to fully dissolve the powder.
The concentration of salicylic acid in the volumetric flask is 0.1mol dm-3
The calculations behind this:
Mr of salicylic acid = 138.12
Step 2 the varying the concentration of the stock solution
I will then need to create various solutions of different concentrations using the stock solution
-
0.01mol dm-3 concentration of salicylic acid (Solution B )
-
I will extract 10 cm3 of the stock solution and pour it into a clean 100 cm3 volumetric flask
-
I will then add a solution of ethanol and water (50:50) to the mark on the volumetric flask
This newly made solutions has a concentration of salicylic acid of 0.01mol dm-3
-
0.001mol dm-3 concentration of salicylic acid (Solution C )
-
I will extract 10 cm3 of solution B and pour it into a clean 100 cm3 volumetric flask
-
I will then add a solution of ethanol and water (50:50) to the mark on the volumetric flask
This newly made solutions has a concentration of salicylic acid of 0.001mol dm-3
-
0.0001mol dm-3 concentration of salicylic acid (Solution D )
-
I will extract 10 cm3 of solution C and pour it into a clean 100 cm3 volumetric flask
-
I will then add a solution of ethanol and water (50:50) to the mark on the volumetric flask
This newly made solutions has a concentration of salicylic acid of 0.0001mol dm-3
-
0.00001mol dm-3 concentration of salicylic acid (Solution E )
-
I will extract 10 cm3 of solution D and pour it into a clean 100 cm3 volumetric flask
-
I will then add a solution of ethanol and water (50:50) to the mark on the volumetric flask
This newly made solutions has a concentration of salicylic acid of 0.00001mol dm-3
-
0.05mol dm-3 concentration of salicylic acid (Solution F )
-
I will extract 50 cm3 of the stock solution and pour it into a clean 100 cm3 volumetric flask
-
I will then add a solution of ethanol and water (50:50) to the mark on the volumetric flask
This newly made solutions has a concentration of salicylic acid of 0.05mol dm-3
-
0.005mol dm-3 concentration of salicylic acid (Solution G )
-
I will extract 10 cm3 of solution F and pour it into a clean 100 cm3 volumetric flask
-
I will then add a solution of ethanol and water (50:50) to the mark on the volumetric flask
This newly made solutions has a concentration of salicylic acid of 0.005mol dm-3
-
0.0005mol dm-3 concentration of salicylic acid (Solution H )
-
I will extract 10 cm3 of solution G and pour it into a clean 100 cm3 volumetric flask
-
I will then add a solution of ethanol and water (50:50) to the mark on the volumetric flask
This newly made solutions has a concentration of salicylic acid of 0.0005mol dm-3
-
0.00005mol dm-3 concentration of salicylic acid (Solution I )
-
I will extract 10 cm3 of solution ‘I’ and pour it into a clean 100 cm3 volumetric flask
-
I will then add a solution of ethanol and water (50:50) to the mark on the volumetric flask
This newly made solutions has a concentration of salicylic acid of 0.00005mol dm-3
Colourimeter readings
- I will chose the appropriate filter for my absorbance 530 nm = Green
- I will reset the colourimeter with a cuvette filled with water
- I will then fill a cuvette with the sample I want to study
- I will then place my cuvette into the colourimeter and note down my readings
- I will then repeat step 2 and 4. Twice more to get accurate results
- I will do this process for all the different concentration of salicylic acid and create a calibration curve
- I will reset the colourimeter with a cuvette filled with water
- I will then fill a cuvette with the aspirin sample solution that has been reacted with iron III chloride (forming a pale purple solution)
- I will then place my cuvette into the colourimeter and note down my readings
BELOW IS A DIAGRAM EXPLAINNG HOW THE DIFFERENT CONCENTRATIONS ARE MADE
Follow up investigation
Does temperature affect the purity and yield of aspirin?
I will carry out the exactly same procedure of aspirin synthesis (Pg.17) as I did when the solutions were made at room temp 20°c
I will be make aspirin again at 3 different temperatures:
The additional apparatus I will need are:
- a water bath –with temperature control
- thermometer x2
I will then analyse the aspirin produced from the different temperatures and see using my various analytical method which temperature produces the best yield and the purest sample of aspirin.
Results
Aspirin synthesis
Results
Below is a table which shows the yield of aspirin powder produced at different temperatures
Thin layer chromatography
Results
I did thin layer chromatography on all of the samples of aspirin I produced
First I did thin layer chromatography on pure aspirin and salicylic acid
Rf values use
To get the Rf value you this equation
Rf value of salicylic acid = = 0.64
Rf value of aspirin = = 0.46
Below are the Rf values of our manufactures aspirin
- Rf value of aspirin manufactured at 10°c =
= 0.45 = could be aspirin
= 0.6.1 = could be salicylic acid
- Rf value of aspirin manufactured at 20°c (room temp) =
= 0.44
- Rf value of aspirin manufactured at 40°c =
=0.46
- Rf value of aspirin manufactured at 60 °c =
=0.46
Colourimetry results
Calibration curve of different concentrations of salicylic acid using a 520 cm-1 nm filter
Below are the results of my colourmetric analysis
Back Titration
Aspirin made at 10’c
Average titre = 47.75 cm3
Aspirin made at 20’c
Average titre = 26.45 cm3
Aspirin made at 40’c
Average titre = 20.95cm3
Aspirin made at 60’c
Average titre = 13.60cm3
Pure Aspirin
Average titre: 3.10cm3
Analysis
Percent yield
After I had made my aspirin I filtered it using vacuum filtration method. I then placed the wet product on a watch glass and then placed it an oven and allowed it to dry. I then weighed the dry product taking into account the weight of the watch glass by calibrating the weighing scale. I did this for all four of my aspirin batches.
Below is a table which shows the yield of aspirin obtained at different temperatures.
I can now work out the percent yield of each batch of aspirin and obtain a value from which I can I find the method that has the best yield:
In my method I used 8g of salicylic acid.
So I will work out how many moles of salicylic acid I used as my reactant using the equation below
Moles =
Mr of salicylic acid =138.12
= 0.057921 moles of salicylic acid
One mole of salicylic acid will produce one mole of aspirin
So in my method I have used 0.057921 moles of salicylic acid so theoretically I have produced 0.057921 moles of aspirin.
So using the equation below me can work out the theoretical mass of aspirin
Mass (g) = Moles x Mr
Mr of aspirin= 180.15
0.057921 x 180.15
= 10.43 g
So theoretically the mass of aspirin I should obtain is 10.43g
I can work out percent yield using the equation below
Percent yield =
I can now used this equation to work out the percent yield of all my manufactured aspirin
So according to my results the method which produced the highest yield was the aspirin made at 40°c and the method which produced the poorest yield was the aspirin made at 60°c
Colourimetry
I made a calibration curve by creating several concentrations of salicylic acid. I then added iron III chloride which reacted with the salicylic acid to produce a purple solution. I then measured the intensity of the purple colour using the colorimeter. I repeated each reading 3 times to get an average. I used a green filter at wavelength of 520nm cm-1
These are my results
*N/A = colour too dark for colorimeter to analyse
I then made up solutions of my manufactured aspirin. I did this by accurately weighing out 1 gram of my manufactured aspirin from each batch and placing them in four different, 100cm3 volumetric flasks and then diluting them with a 50/50 mixture of water and ethanol to the 100cm3marker. I then carefully added iron III chloride drop by drop until there was no colour change to the solution
I then placed them into the colourimeter and took readings using the same method as when I created my calibration curve.
Calibration curve of different concentrations of salicylic acid using a 520 cm-1 nm filter
Below are the results of my colourmetric analysis
Now have the concentrations of unreacted salicylic acid in my manufactured aspirin
I can now work out the purity of my aspirin
Purity of aspirin made at 10°c
To do this I first need to work out the moles of salicylic acid in my 100 cm3 solution
I will use this equation:
Moles = concentration (mol dm-3) x volume (dm-3)
0.0062 mol dm-3 x 0.1 dm3
= 0.00062 moles of salicylic acid
Now I have the moles I can now work out the mass of salicylic acid in my 1 gram of aspirin
I use this equation
Mass (g) = Moles x Mr
0.00062 x 138.12
= 0.085634g
So now I have the mass of salicylic acid in my 1 gram of aspirin I can now find out the percentage of salicylic acid in 1 gram of manufactured aspirin
=8.6%
So the aspirin I manufactured at 10°c was 91.4% pure aspirin
Below is table that show the percent purity of my manufactured aspirin made at different temperatures using the same calculation procedure as above
Thin layer chromatography (T.L.C)
Thin layer chromatography is a qualitative test it basically tells you which substances are present in your sample as it are based on the principle on affinity, different molecules will have different affinity to the chromatography solvent so they will travel different distances
We can identify substances by their Rf value which is calculated using this equation
= Rf value
First I did used T.L.C to find the Rf values of pure aspirin and pure Salicylic Acid. The Rf values I get from these two substances will be the standard Rf values. I will then compare these standard Rf values with the Rf values I get with my manufactured aspirin
RF value of salicylic acid = = 0.64
RF value of aspirin = = 0.46
Below is a table showing my RF values for my manufactures aspirin
The results of thin layer chromatography confirm that I have made aspirin in all 4 temperature conditions. It also tells me the presents of other substances. The result for the T.L.C of the aspirin made at 10°c indicate that I have made aspirin however there is also salicylic acid present in my sample. This leads me to believe that the aspirin made at 10°c will be the least purist aspirin however this is not a test of purity, only quality.
Back titration results
A back titration will allow me to find how pure my aspirin is.
I add a known amount of sodium hydroxide (25cm3) to a known mass of aspirin (2g) and heat them gently. This will then hydrolyse the aspirin. I then titrate 25cm3 of this solution with hydrochloric acid. I can then work out how much sodium hydroxide will react with the hydrochloric acid. I can then work out how much sodium hydroxide reacted with aspirin. I then implement my knowledge that one mole of aspirin will react with 2 moles of sodium hydroxide I can then work out the moles of aspirin and then I can work out the mass of pure aspirin.
For me to do repeats of my back titrations I had to make up a standard solution of my aspirin so I added 2 g of aspirin with 25cm3 of 1 mol dm-3 in a 250cm3 volumetric flasks and gently heated over a Bunsen burner. When it cooled I then added purified water to the 250cm3 mark. I did this for all four batches of my manufactured aspirin and for the pure aspirin.
Pure aspirin
I am now going to work out how the purity of the pure aspirin which was provided by the school
My average titre was 3.10 cm3 of 0.1 mol hydrochloric acid to a 25 cm3 solution of sodium hydroxide and aspirin
First I will work out how many moles of hydrochloric acid I used
Moles = concentration (mol dm-3) x volume (dm-3)
0.1 mol dm-3 x 0.0031dm
= 0.00031moles of HCl
I will now work out how many moles of NaOH I used
1 mol dm-3 x 0.025 dm
=0.025 moles of NaOH
However I diluted this 10 times when I made up my standard solution
So the actual molarity is 0.0025 moles
I will now subtract the amount of moles of HCl that reacted with the NaOH
0.0025 moles – 0.00031 moles
= 0.00219 moles of excess NaOH
2 moles of NaOH reacts with one mole of aspirin
So 0.001095 moles of aspirin reacted with 0.00219 moles of NaOH
However I diluted the aspirin 10 times when I made up my standard solution
So the true molarity is 0.01095
Now I have the moles of the aspirin I can now work out the mass of pure aspirin acid in my manufactured aspirin
Mass (g) = Moles x Mr Mr of aspirin = 180.15
0.01095 X 180.15 = 1.97g
I can now work out the purity of my aspirin by working out the mass of pure aspirin against the starting mass of aspirin
=98.5 % pure aspirin
Purity of aspirin made at 60°c
I am now going to work out how the purity of the aspirin I made at 60°c
My average titre was 13.6 cm3 of 0.1 mol hydrochloric acid to a 25 cm3 solution of sodium hydroxide and aspirin
First I will work out how many moles of hydrochloric acid I used
Moles = concentration (mol dm-3) x volume (dm-3)
0.1 mol dm-3 x 0.0136dm
= 0.00136 moles of HCl
I will now work out how many moles of NaOH I used
1 mol dm-3 x 0.025 dm
=0.025 moles of NaOH
However I diluted this 10 times when I made up my standard solution
So the actual molarity is 0.0025 moles
I will now subtract the amount of moles of HCl that reacted with the NaOH
0.0025 moles – 0.00136 moles
= 0.00114 moles of excess NaOH
2 moles of NaOH reacts with one mole of aspirin
So 0.00057 moles of aspirin reacted with 0.00114 moles of NaOH
However I diluted the aspirin 10 times when I made up my standard solution
So the true molarity is 0.0057
Now I have the moles of the aspirin I can now work out the mass of pure aspirin acid in my manufactured aspirin
Mass (g) = Moles x Mr Mr of aspirin = 180.15
0. 0.0057 X 180.15 = 1.026g
I can now work out the purity of my aspirin by working out the mass of pure aspirin against the starting mass of aspirin
=51.3 % pure aspirin
Using the same calculation method above me then calculated the purity of the rest of my manufactured aspirin.
The results of my titration indicate that the purest aspirin was the aspirin made at 60°c.
Anomalous results
In my results box above there are no results recorded for the aspirin made at 10°c and 20°c the reason for this is because when I did my calculations it gave a negative value, the reason for this could be due to the high level of impurities within the sample which have affected the pH of the solution giving such strange results.
These results could also be caused by human error. The concentrations for the Sodium Hydroxide and Hydrochloric acid had to be adjusted for the titration as I needed 0.1 mol dm-3 of HCl and 1 mol dm-3 of NaOH. However I was supplied with 1 mol dm-3 of HCl and 2 mol dm-3 of NaOH I diluted the Hydrochloric acid with deionised water and created a large batch of which was sufficient enough for 20 titrations. I then did my back titrations for the ‘pure aspirin’ and the aspirin I made at 60°c and 40°c. I then had to create another batch of dilute HCl (0.1 mol dm-3) to do my titrations for the aspirin made at 10°c and 20°c. Here I think there may have been contamination as I was dealing with several colourless liquids at the same time. This contamination could have changed the pH, the concentration or even added chemicals to the Hydrochloric acid, thus giving such strange results.
Conclusion and Evaluation
Conclusion
My aim of this investigation was to find out whether temperature effects purity and yield of aspirin ... the answer is yes
In my investigation I found that the purest aspirin was the aspirin that was synthesised at 60°c the back titration and Colourimetry result proved that it was the purest out of the four batches. However the purity of the aspirin differed depending on analytical technique.But, I am certain that the aspirin made at 60°c was the purest of the four.
The Maxwell–Boltzmann distribution describes the probability of molecules interacting with each other( by short collisions) is calculated by the particle's speed, the temperature of the system and the mass of the particle, so by increasing the temperature the molecules will have more kinetic energy so they will collide more and react. It also mean there will be a greater proportion of molecules that will have the require activation energy (Ea) to react changing the shape of the Maxwell–Boltzmann distribution curve to the right
So relating it back to context, at higher temperature there will be more salicylic acid reacting with the ethanoic anhydride forming more aspirin, this also means there will be less unreacted salicylic acid present in the end product therefore making the product more pure. So at low temperature the molecules do not have enough kinetic energy to react so there is less salicylic acid reacting with the ethanoic anhydride so there is more unreacted salicylic acid in the end product, making it less pure.
In my investigation I found that the aspirin that was synthesized at 40°c produced the highest yield the aspirin that was synthesised at 60°c may have produced the most purest aspirin it however produced the poorest yield this could be due to the fact at high temperature there are several by product formed . So when I filtered and washed my aspirin the by-products dissolved in the washings and was filtered off therefore producing the poorest yield.
In the pharmaceutical industry yield is not as important as purity. In modern day aspirin manufacture the salicylic acid is refluxed with ethanoic anhydride at 90°c for several hours. It is then cooled, filtered and re-crystallised.
Evaluation
On the whole I am pleased with the results of my investigation. I carefully followed my plan reading the instructions several times to avoid misconceptions. I used only the material I needed and I used only the amount I need to avoid waste of chemical. In terms of collecting data I was through making repeats where possible and making measurements with accuracy e.g. putting a sheet of paper behind the burette to improve visibility of the scale and being level with the meniscus to avoid any parallax error and increasing the accuracy of my results.
I believe I carried out the experiment in a safe manner wearing the appropriate clothing and glove and eye protection. I washed my hands every time I entered or left the lab. I cleaned my work stations frequently and positioned my equipment effectively to avoid any accidents my myself or others around me i.e. placing all flasks and glass ware in the middle of the table to avoid them being knocked over. When aspirin comes in contact with water it hydrolyses back in to its products producing salicylic acid and ethanoic acid again so to avoid this I kept my aspirin in air tight glass specimen
Tubes to prevent any contact with the moisture in the air, by doing this I tried to prevent any further hydrolysis of my aspirin which would pose a risk of ruining my results.
If I were to do this experiment differently I would:
- I would have large quantities of my reactants so I may do repeats when synthesising aspirin as I only made one batch of aspirin for each different temperature doing this would give me more reliable results
- I would have solutions at the concentration I need before the performing experiment this would save me time during the investigation and also decrease the chance of contamination and miscalculation when creating new concentrations of solutions ( important factor during titration )
- For the Colourimetry test I would have the appropriate filter of 530nm instead of the 520 nm filter that I had used in my experiment (530nm filter was not available).
- During the thin layer chromatography (T.L.C) I would not use capillary tubes as they tend to break, I would use a larger t.l.c plate and used a fine drop pipette to accurately drop the solution onto the t.l.c plate spot thus, saving me time as I would not have to continually reapply the solution wait for it to dry and then reapply the solution once more (which is what I had to do using the capillary tubes).
Validity and reliability
I believe the results I obtained are valid, as in the purest aspirin was the aspirin made at 60°c. And if someone else did this experiment using the same method they would reach the same conclusion. The chemical ideas and theories behind this experiment support this as higher temperatures would produce purer aspirin.
I took many precautions throughout my experiment to ensure my results were valid and reliable;
- I cleaned my equipment thoroughly with purified water before using my equipment
- I labelled the glassware containing my solutions as many of the solutions I was using were colourless.
- I did repeats where possible to ensure my results were reliable e.g. performing many titrations to collect concordant results
- When I created my aspirin I monitored the temperature constantly to maintain my desired temperature
- When I used pipettes and burettes; I made sure I was eye level with the meniscus, use a white sheet of paper behind (to improve visibility) and I made sure that the bottom meniscus was on the line before transferring solutions and taking readings
- I inverted the volumetric flask several times to fully homogenise the solutions
- I always calibrated equipment before using it e.g. calibrating the colourimeter with water before inserting my test sample
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In terms of equipment I have tried (where possible) to minimise my error by using equipment with low error. e.g. I would use a 10cm3 pipette to transfer 10cm3 of solution instead of a 10cm3 volumetric cylinder as the 10cm3 pipette has lower equipment error
I believe doing these various protocols has ensured my results are reliable and valid.
Evaluation of Methods
Aspirin synthesis:
The method of aspirin synthesis I chose in my opinion is the most straight forward method and best suited the aim of my hypothesis as I had full control of the temperature of the system. The other method I had seen was to use a microwave however I would not have control over the temperature and therefore it did not suit my investigation. The bad side of my chosen method was the pungent fumes produced ethanoic anhydride, despite the fact that the ethanoic anhydride was in the fume cupboard the whole time it still made the whole room smell like bad vinegar.
Colourimetry
I chose Colourimetry because it would allow me to utilize the fact that phenols have with iron III chloride (producing a purple complex). This would then allow me to get quantitative results from which I could find the purity of my aspirin based on the concentration off the salicylic acid molecule. The downside of this method is it took a long time preparing the samples and creating the calibration curve. In addition the phenol- iron III chloride reaction would only work well if iron III chloride was freshly made. In terms of analysing purity this method only analyses the amount of salicylic acid in the Aspin sample it does not take into account any other impurity which could be unwanted by-products or even the presence of Ethanoic acid
Thin layer chromatography
T.L.C. allowed me to identify which substances were present in my sample this allowed me to confirm that I had in fact made aspirin. However the down side of using thin layer chromatography is it is very tedious when “spotting” the sample on the pencil line of the T.L.C. plate and involves you having to constantly re apply you sample.
Back titration
I chose to do a back titration rather than a direct titration because In the direct titration of aspirin samples with sodium hydroxide there is always the possibility that the amount of hydrolysis of the aspirin that occurs as a side reaction during the titration, this may be sufficient to affect the accuracy of the final reading. For this reason it is usually suggested that direct titration should be carried out quickly and that the mixture is cooled in ice, so that any hydrolysis is minimized.
Back-titration avoids these complications as the aspirin is completely hydrolysed before it is titrated. On the other hand if you do not put in enough alkali to fully hydrolyse the aspirin the result you obtain may not be valid. Determining the end point in my personal opinion it is easier to see the disappearance of color than the appearance of colour.
Evaluation of Equipment
I believe my choice of equipment was appropriate to my investigation, I did not request for anything I did not need thanks to good planning skills, I was able to gather the equipment I needed before I needed them, saving me time as I did not have to spend a long time looking for equipment. When I measured out volume of solutions I took into account the amount I needed. If I needed 10cm3 of a solution I used a 10cm3 burette instead of a 10 cm3 measuring cylinder which has a larger equipment error or if I needed 15 cm3 of a solution I used a 20 cm3 measuring cylinder instead of a 25 cm3 or 50 cm3 measuring cylinder as the scale it bigger and clearer. When I prepared my standard solutions I always tried to make it up to 100cm3 for two main reasons; first, it made calculations easer to do compared to preparing solutions in a 250cm3 volumetric flask to and second, it minimise chemical waste.
When I did my Colourimetry I would have liked to have a 530nm filter instead of a 520 nm filter as the 530nm is best suited for this type of test.
The balance I used was at 2 decimal places, which was sufficient to accommodate my needs however a 3dp balance would increase the accuracy of my measurements.
But overall I am please to say that the equipment I used allowed me to carry out my investigation with minimal hassle and stress and to a very high level of accuracy and precision
Sources of error
Aspirin synthesis
- The temperature of the water bath may not have been the same as the temperature inside of the test tube. However I did place the reactants (salicylic acid and ethanoic anhydride) in the water bath for a few minutes and used a thermometer to check the temperature prior to mixing them.
- There is also equipment error e.g. the balance may not have been calibrated properly or on the wrong setting which could have influenced the mass of salicylic acid used.
Yield
- I may not have fully dried the samples properly which could influence the results as I am not just measuring the aspirin but also any moisture within the aspirin sample.
- The balance may not have been calibrated properly or on the wrong setting which could have influenced the yield of Aspirin
Colourimetry
- I may not have put in enough iron III chloride in my standard solution at the start which could have affected the solutions of low concentrations as there may not have been enough iron II chloride in the solution to react with the salicylic acid to and change in colour
- There is a chance of contamination as the preparation involved diluting several times from several different flasks however I did try to label my burettes to avoid this.
- The colorimeter could have been faulty or calibrated wrongly therefore giving me invalid results
Titration
- I may have contaminated my solutions as I was diluting several different, colourless solution to make up the concentrations I needed
- Judgement of end point may be wrong
- I May not have put in enough NaOH at the beginning to fully hydrolyse the aspirin
- The hydrochloric acid and sodium hydroxide may not have been pure to begin as several other students were using it as me therefore giving be invalid result
Equipment error
Aspirin synthesis
The equipment I used during aspirin synthesis
Level uncertainly with the mass of salicylic acid
Level Uncertainty with the volume of Ethanoic anhydride
Level of uncertainty of the volume of sulphuric acid
Total level of uncertainty is 28%
Colourimetry – standard solutions
The equipment I used for Colourimetry:
Level Uncertainty with the mass of salicylic acid
Level of uncertainty of a 10 cm3 pipette
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I used the graduated pipette 7 times so that brings my total percentage error using a 10 cm3 pipette to 7%
Level of uncertainty of a 50 cm3 pipette
Level of uncertainty of a 100cm3 volumetric flask
0.08%
I used nine, 100 cm3 volumetric flasks so my total error using 100 cm3 volumetric flasks is 0.72%
Total level of uncertainty of creating my standard solution = 8.4%
Titration
Equipment used
Level Uncertainty with the mass of aspirin
Level of uncertainty with the volume of sodium hydroxide using a 25 cm3 pipette
Level of uncertainty of a 100cm3 volumetric flask
-
Level of uncertainty with using a 50cm3 burette
10°c aspirin titre
20°c aspirin titre
40°c aspirin titre
References
HISTORY OF ASPIRIN
Reference 1
Edward stone - Wikipedia accessed on 06/01/2010
Reference 2
Time line of aspirin – chemical heritage accesses on 09/01/2010
Reference 3
Structural diagram of aspirin accessed on 013/01/2010
Reference 4
Kolbe synthesis accessed on 019/01/2010
And from A2 chemical storylines, third edition, published by Heinemann of Pearson Education Ltd 2009. © University of York –page 8
HEALTH AND SAFETY
Reference 5
Safety information on chemicals –index of all chemicals accessed on 19/01/2010
E.g. iodine →
Reference 6
Laboratory safety signs accessed on 19/01/2010
Reference 7
Chemistry Laboratory Safety Rules accessed on 19/01/2010
http://chemistry.about.com/od/healthsafety/a/aa080104a.htm
Methods of synthesis
Reference 7
Method 1. Adapted from “Experiments in organic chemistry” Edition: 3 by Louis Frederick Fieser – published in 1955 - Page 205
Reference 8
Method 2 adapted from Salter’s Advanced Chemistry-‘what’s in a medicine’ practical booklet, Pearson Education Ltd 2009. © University of York- page 11
Analytical methods Reference 9
Method for thin layer chromatography - adapted from Salter’s Advanced Chemistry-‘what’s in a medicine’ practical booklet, Pearson Education Ltd 2009. © University of York- page 13
Reference 10
Method for titration - adapted from Dr McCord’s chemistry website (accessed on the 3/1/2010) -
Reference11
Background information on Colourimetry -adapted from Salter’s Advanced Chemistry-‘Steel story’ practical booklet, Pearson Education Ltd 2009. © University of York- page 73
Reference 12
Chemistry Review- Project Page from January 1997 issue of Chemistry Review, Volume 6, Number 3, Pages 18 and 19. Chemistry Review published by Philip Allan Updates
Reference 13
Reading a burette accurately
“CHEMISTRY 002.130: Laboratory Manual produced by the Department of Chemistry of The University of Manitoba (2003-2004 Edition, pages 34 and 35).
Reference 14
Maxwell Boltzmann distribution
Chemical ideas, Third edition published by Pearson Education Ltd 2009. © University of York- page 212-215