Ebony

Titration

The solubility of aspirin

The more soluble a medicine is, the more quickly it will pass from the digestive system into the bloodstream after being swallowed. You can find out the solubility of aspirin by titrating a saturated solution of it with aqueous sodium hydroxide. Aspirin is a weak acid so the solution will have a pH greater than 7 at the end-point. Phenolphthalein is a suitable indicator to use during this titration.

Equipment

·        Six 100 cm³ conical flasks

·        Filter paper

·        funnel

·        White tile

·        Burette and stand

·        25 cm³ and 10 cm³ pipettes

·        Weighing scales.

·        Phenolphthalein indicator

·         0.020 mol ^dm-3 aqueous sodium hydroxide.

Method

1.      Weigh out 0.5 g of aspirin into a 100 cm3 conical flask, add 50 cm³ of distilled water and swirl for 5 minutes. Filter the solution into a dry 100 cm³ conical flask.

2.       Using a 10 cm³ pipette, transfer four separate 10 cm³ samples into 5 100 cm³ conical flasks.

3.       Add four drops of phenolphthalein to each of the flasks and titrate with 0.020 mol dm–³ aqueous sodium hydroxide until the first permanent pink colouration. Carry out one rough and at least two accurate, titrations and record your results in a table, repeat the titrations until the results are within 0.1cm³ of each titration.

4.       Find the mean volume, V cm³, of aqueous sodium hydroxide needed to react with the aspirin dissolved in 10 cm³ of solution.

Conclusions

·        Using 0.020 mol dm^-3 sodium hydroxide and 10 cm³ samples of dissolved aspirin the titres should be in the range 12–16 cm3 of sodium hydroxide at room temperature

·         The relative molecular mass of aspirin is 180 g mol^-1

·         The solubility varies significantly with temperature and is in the range 0.2–0.4 at room temperature.

The instruments and sensors which were used in this titration were

·         Burette

·         1cm³ glass pipette

·         10cm³ glass pipette

·         A disposable pipette

·         Phenolphthalein indicator

·         100cm³ beaker

·         250cm³ conical flask

When measuring out the volume of the standard solution (aspirin solution) i used a 1cm³ glass pipette as if a 25cm³ glass pipette was used this would be quicker but it would be less accurate and during a titration you need to be very accurate. This solution is placed in the conical flask. Along with 3 or more drops of phenolyphalene indicator solution which should make the solution change colour.

When filling the burette with the solution i made sure the line is below the bottom of the meniscus. The readings from the burette need to be very accurate.  You can top up the amount of the solution by using a disposable pipette until the line is just below the bottom of the meniscus. When adding the solutionto the aspirin solution make sure you watch carefully as the sensor tells you when to record the results by turning a different colour near the end point and suddenly the final colour at the end point. The experiment should be repeated until you get concordant data; the data needs to be within 0.1 cm³ to make it reliable and accurate. When recording the readings on the burette they should be top the nearest 0.05cm³. The mean average is then calculated.

Although using an indicator is not the most accurate sensor for this experiment it is the most effective as it is easy to spot the colour change and the data is obtained quickly.

Many of the instruments and sensors which were used can affect the experiments results due to accuracy as some instruments/ sensors are more accurate than others.

When weighing out volume in a measuring cylinder there is room for error as you have to keep your eye on the meniscus and make sure that the bottom of the meniscus is on the line, if this is not it is because of human error, this is not as accurate but is the quickest method. As if you were to use a 2cm³ pipette you would have the correct amount of volume in the cylinder but this would take much longer to measure out 100cm³ of distilled water than it would to use a 100cm³ measuring cylinder and you may lose count of the amount you have put in. there is also the chance of air bubbles in the pipette which will make your measurements less accurate. There is then less human error if you use a 100cm³ measuring cylinder rather than a 2cm³ pipette. Using the 250cm³ volumetric flask is easier to use in this experiment but it is the same as using a 100cm³ measuring cylinder as all though there is less human error when using one it is more accurate to use a pipette but there is more chance of human error.

When using a burette, a burette only has 1mm gaps and therefore is not as accurate as using a 1mm³ glass pipette, but there is less human error when measuring out the volume in a burette, as If you are measuring out 25cm³ using a 1cm glass pipette there is more room for human error as you could lose count and air bubbles may appear when using a glass pipette, but there is 0.10 gaps therefore it is more accurate but using a 1mm³ glass pipette is very time consuming and it is quicker to use a 25cm³ glass pipette although it is less accurate there is less room for human error. It is also quicker to pure the solution into the burette and adjust with a 2cm³ disposable pipette as long as the line is below the bottom of the meniscus.

When using all of these measuring instruments there is more room for human error when using a lot of something small. I.e. measuring out 100cm³ using a 1cm³ may be more accurate but there is more room for human error. So it is easier and less time consuming to measure out 100cm³ using a 100cm³ measuring cylinder or 25 or 50cm^3.