*** Do your own risk assessment – who am I? Your mother / head technician?? ***
This practical is adapted from an example calculation in Advanced Level Practical Work for Chemistry (2004) by Andrew Hunt. It is an example of a back titration so quite a fun calculation and has plenty of sources of error to quantify and discuss. I’d recommend setting aside 90 mins for this experiment – the titration is relatively quick but making the standard solutions takes time.
BTEC Level 3 Scenario: You are working for Defra (Department for Environment, Food and Rural Affairs) and have been asked to investigate the health and welfare of battery hens. Your supervisor has suggested that healthier hens have a higher percentage of calcium carbonate in the shells of the eggs they lay. Your task is to calculate the percentage calcium carbonate in a battery hen’s eggshell and a free-range hen’s eggshell through back titration. You will then analyse the errors in the experimental error and conclude whether there is a real difference between the calcium carbonate content of eggshells from hens kept in different conditions. You will submit your findings in a lab report written in the style of a scientific journal article, including abstract and discussion. [Teaching Note: can be used as part of P5M5D3 in Unit 22 or P1M1D1 in Unit 19.]
Preparing the Standard solution
- Crack a free-range egg and dispose of the contents. Rinse inside with distilled water and remove membrane / air sac. Allow to dry. Weigh on a 2d.p. balance.
- Grind egg to a fine powder using a pestle and mortar.
- Measure 40 cm3 of 1.20 mol dm-3 hydrochloric acid into a 250 cm3 beaker. Add the ground eggshell to the beaker and stir until no more bubbles of gas form.
- Take a filter funnel and paper, place in neck of a 250cm3 volumetric flask. Decant the solution into the flask. [NB The acid is in excess. We want the unreacted acid to go into the flask – try to prevent the shell from getting into the filter funnel as it will block it.]*
- Wash the remaining shell with approx. 50cm3 of distilled water and decant this solution into the volumetric flask through the filter funnel.
- Top up the volumetric flask to the meniscus with distilled water. Invert 10 times to mix.** Label as Excess Acid from Free Range Shell, with your name and the date.
- Repeat steps 1-6 for a battery-farmed egg and label as Excess Acid from Battery Shell
The Back Titration
- Fill a burette with 1.0 mol dm-3 sodium hydroxide solution.
- Pipette 25 cm3 of Excess Acid from Free Range Shell into a 100 cm3 conical flask. Add 3-5 drops of methyl orange indicator.
- Perform a rough titration.
- Perform additional titrations until 2 concordant results have been obtained.
- Repeat steps 1-4 for Excess Acid from Battery Shell.
* I have found that this stage takes a long time and so is suitable to be left across a break. Ensure the filter paper isn’t block – if it is, replace it with a fresh funnel/filter paper. This of course will lose some acid soaked into the original filter paper. Vacuum filtration is an option but has its own risks of contamination.
** Bubbles of gas often form in the volumetric flask. I have removed these using a Pasteur pipette; I have taken the meniscus as being the line of the solution, ignoring the level of these bubbles.
Students will calculate the number of moles of acid in the original 40cm3. Using the balanced equation and the average titre, they will calculate the number of moles of hydrochloric acid neutralised in the titration. The difference will be the number of moles of acid that reacted with the egg.
From the balanced equation, students will calculate the number of moles of calcium carbonate that reacted. They will then convert this into grams and work out the percentage of calcium carbonate found in the eggshell.
They will repeat this process for the other eggshell.
By real difference (mentioned in the scenario) I mean that after calculating the percentage error in measurement, the student should calculate the range in which the true percentage calcium carbonate for each egg is found. If these ranges do not overlap, there is a real difference.