12.2 Flashcards
12.2 Investigating the copper content of brass screws• To research and plan a short investigation
Brass is an alloy containing copper. You will plan how to determine the copper content in a sample of brass.
One method of finding the concentration of Cu2+ ions in a solution is titration using sodium thiosulfate, Na2S2O3(aq).
You will use research skills to find the necessary information, and then carry out your planned procedure.
Chemicals
Your teacher will demonstrate the dissolving of brass screws, and provide you with a sample of the resulting solution.
You will also be provided with
• a standard solution of sodium thiosulfate, Na2S2O3, 0.100 mol dm–3
• a standard solution of potassium iodide, KI(aq), 0.500 mol dm–3
• a starch solution.
Analysing your data
- Use your results to calculate the percentage by mass of copper in the provided brass screws.
- Compare your calculated percentage mass of copper with the percentage mass in different types of brass.
Consider your method once you have finished the procedure.
• Are you happy with the results you have found?
• Is the mass of copper you found close to the value you expected?
• If not, can you explain where errors may have occurred, and suggest how to improve the method?
• Compare the results from other methods in your class and evaluate the pros and cons of the various methods.
No hazard classification
HNO3
DANGER
Causes severe skin burns and eye damage.
Na2CO3(aq)
Currently no hazard classification at this concentration.
CH3COOH(aq)
Currently no hazard classification at this concentration
Eq
uipmentL
will be selecting appropriate apparatus for the experiment. It is likely that they will require: • safety spectacles • conical flasks (250 cm3) • volumetric flask (250 cm3) • burette, stand and clamp • white tile • pipette (25.0 cm3) and filler • dropping pipette • wash bottle containing distilled or de-ionised water (about 300 cm3 will be required) • glass beakers (250 cm3 and 100 cm3) • spatula
If learners will be dissolving the brass screws themselves, they will require
• additional glassware and apparatus
• access to a balance
• gloves, goggles and access to a fume cupboard for the reaction with concentrated nitric acid.
Health & Safety
- The Student sheet assumes that learners will be provided with pre-prepared samples of dissolved brass (or mock ‘dissolved brass’ as described in the Chemicals section). Learners could dissolve the brass screws themselves, but due to the use of concentrated nitric acid and release of NO2, this should be done only if the teacher is confident that the risks can be adequately controlled.
- Learners should wear eye protection throughout the experiment. Goggles or a face shield should be used when handling concentrated nitric acid.
- The dissolving of the screws should be carried out in a fume cupboard and chemical-resistant gloves should be worn when handling concentrated nitric acid.
- Learners are asked to conduct a risk assessment as part of their research and planning. Ensure that learners do not carry out the procedure before you are satisfied that they are fully aware of all risks and appropriate precautions.
The procedure for dissolving the brass screws is as follows:
- 2.5 g of brass will dissolve in 10 cm3 concentrated nitric acid – use as little acid as possible to dissolve all the brass. This should be carried out in a fume cupboard as toxic fumes of nitrogen dioxide, NO2, are produced.
- Transfer the resulting solution to a 250 cm3 graduated flask, and add 0.5 mol dm–3 sodium carbonate drop by drop until a precipitate of copper carbonate starts to appear. This neutralises the solution.
- Add 1 mol dm–3 ethanoic acid drop by drop until the precipitate just dissolves.
- Make the solution up to 250 cm3 so that repeated titrations can be carried out.
- It may not be possible practically for learners to carry out this process themselves. This process can be demonstrated and learners provided with pre-prepared samples, or samples of a ‘dissolved brass’ solution prepared as a 0.25 mol dm–3 solution of CuSO4•5H2O
note
Note that some references for the dissolving of brass recommend additional steps to remove excess nitric acid or any iron(III) aqueous ions. For the purposes of this investigation the above method should be sufficient.
Teachers should make suitable off-line resources available, which may include safety data sheets to inform the risk assessment, and textbooks containing information about I2/S2O32− titrations. Learners should also use online resources to research, and be encouraged to consider which sources may be considered reliable.
Some examples of online sources giving experimental details for the titration method are:
http: //www.titrations.info/iodometric-titration-copper
http: //www.csudh.edu/oliver/che230/labmanual/copbrass.htm
http: //rod.beavon.org.uk/brass.htm
note note
How to detect the end-point of the titration
The iodine solution produced from reacting the brass solution with copper iodide is brown. As the titration reaches the end-point, the brown colour fades. Once the colour is quite pale (but not yet colourless), a little starch is added. The iodine is adsorbed to the starch, forming a deep blue product. Further titration removes this remaining iodine, causing the blue colour to face. The end-point is reached when the blue colour has just gone; the mixture will be a creamy white.
The starch should only be added near the end-point, not at the beginning of titration. If starch is added when the iodine concentration is high, the resulting disappearance of the blue colour will be gradual, rather than giving a clear end-point.
How to convert copper in brass into a neutral solution containing Cu2+ ions See above. Some learners may find more elaborate methods in their research. Note that learners may also find sources referring to colorimetric methods for determination of the concentration of copper(II) ions in the brass solution. If you have the time and resources, you may like to offer this as an alternative method. If different groups in the class follow different methods they can be compared in terms of ease, accuracy and efficiency. Naturally, following a different method will affect which apparatus and techniques learners can be assessed on in this activity
