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Higher Physics Topical Investigation Skin Cancer—Prevention and Cure
2
Investigation Brief Suntan creams stop harmful UV radiation
reaching the skin. Manufacturers’ products
are rated with a Sun Protection Factor
(SPF). Suntan creams can have SPF values
from 6 to over 50.
UV radiation monitors normally measure
National 5 Chemistry
Chemical Analysis
Student Guide
National 5 Chemistry
Practical Assignment Chemical Analysis: Student
Page 2
Contents
Page 3 Investigation A1 – “Calcium analysis of water”
Page 6 Investigation A2 – “Calcium analysis of milk”
Page 12 Investigation B – “Iron in tea and cereals”
Page 15 Investigation C – “Chloride in seawater”
National 5 Chemistry
Practical Assignment Chemical Analysis: Student
Page 3
Investigation A1 - “Calcium in water”
Background
Drinking water contains small amounts of salts and minerals dissolved from rocks that the
water has passed through. Across Britain there is considerable variation in the concentration
of different ions present in tap water.
Calcium ions, Ca2+, in drinking water can supplement the calcium in our diet and may be
beneficial to our health. Some popular bottled waters are advertised as being high in
dissolved minerals.
In high concentrations, Ca2+ ions can be a cause of “water hardness”. Hard water is not a
health hazard but can form an unpleasant scum with soap as well as causing washing
machines, irons and heating boilers to break down.
The concentration of calcium ions can be measured by titrating a sample of water using a
chemical known as EDTA.
Ca2+ + Na2C10H14N2O8 Ca C10H14N2O8 + 2Na+
calcium ion EDTA calcium compound sodium ions An indicator called murexide is used which changes from pink to purple when the endpoint is reached. In this experiment, the larger the titre of EDTA, the higher the concentration of calcium ions present in the water sample.
National 5 Chemistry
Practical Assignment Chemical Analysis: Student
Page 4
The Experiment
You will need
0·01 mol l-1 EDTA solution (if your water
sample is very pure, you may need to
use a 0.001 mol l-1 solution)
Murexide indicator
1 mol l-1 sodium hydroxide solution
(NaOH)
Funnel
Clamp and stand 3 cm3 dropper or 5/10 cm3 measuring
cylinder
50 cm3 burette 25 cm3 pipette and safety filler
100 cm3 conical flask
Safety
1 mol l-1 sodium hydroxide is corrosive. Wear goggles.
Method
1. Using the funnel, fill a 50 cm3 burette with 0·01 mol l-1 EDTA solution, making sure
the tip is full and free of air bubbles.
2. Using a pipette, add 25·0 cm3 of your water sample into a 100 cm3 conical flask.
3. Add 2 cm3 of 1 mol l-1 sodium hydroxide to the flask using a dropper or a small
measuring cylinder.
4. Add a spatula tip of murexide indicator powder
5. Remove the funnel from the top of the burette and note the reading on the burette.
6. Titrate the water sample using the 0·01 mol l-1 EDTA solution until the colour
changes from pink to purple and then read the burette to the nearest 0·1 cm3.
7. Repeat the titration until your titres agree to within 0·2 cm3.
National 5 Chemistry
Practical Assignment Chemical Analysis: Student
Page 5
National 5 Chemistry
Practical Assignment Chemical Analysis: Student
Page 6
Investigation A2 - “Calcium in milk”
Introduction
Milk, and other dairy produce are extremely important sources of calcium in the diet. It is very important for:
helping build strong bones and teeth
regulating muscle contractions, including heartbeat
making sure blood clots normally
A lack of calcium could lead to a condition called rickets in children and osteomalacia or osteoporosis in later life.
The concentration of calcium ions can be measured by titrating a sample of milk using a
chemical known as EDTA.
Ca2+ + Na2C10H14N2O8 Ca C10H14N2O8 + 2Na+
calcium ions EDTA calcium compound sodium ions An indicator called murexide is used which changes from pink to purple when the endpoint is reached. In this experiment, the larger the titre of EDTA, the higher the concentration of calcium ions present in the milk sample.
National 5 Chemistry
Practical Assignment Chemical Analysis: Student
Page 7
The Experiment
You will need
0·1 mol l-1 EDTA solution Murexide indicator
1 mol l-1 sodium hydroxide solution
(NaOH)
Funnel
Clamp and stand 3 cm3 dropper or 5/10 cm3 measuring
cylinder
50 cm3 burette 10 cm3 pipette and safety filler
100 cm3 conical flask 100 cm3 measuring cylinder
Distilled water White tile
Safety
1 mol l-1 sodium hydroxide is corrosive. Wear goggles.
Method
1. Using a funnel, fill the burette with 0·1 mol l-1 EDTA solution, making sure the tip is
full and free of air bubbles.
2. Using a pipette, add 10·0 cm3 of milk to the 100 cm3 conical flask.
3. Using the measuring cylinder, add 40 cm3 of distilled water to the flask.
4. Add 5 cm3 of 1 mol l-1 sodium hydroxide using a 3 cm3 Pasteur pipette or a small
measuring cylinder.
5. Add a spatula tip of murexide indicator powder.
6. Remove the funnel from the top of the burette and note the reading on the burette.
7. Titrate with the 0·1 mol l-1 EDTA until the colour changes from pink to purple*. Read
the burette to the nearest 0·1 cm3.
8. Repeat the titration until the titres agree to within 0·2 cm3.
National 5 Chemistry
Practical Assignment Chemical Analysis: Student
Page 8
* The colour change can sometimes be difficult to see. It is easiest to do a test run first so you can have a ‘target’ colour in front of you to compare.
National 5 Chemistry
Practical Assignment Chemical Analysis: Student
Page 9
Investigation B – “Analysis of Iron in foods”
Background
Iron is an essential nutrient in our diets. It is needed for many things but especially
because it forms the heart of the haemoglobin protein that allows our blood to carry
oxygen around the body.
Many foods contain iron and breakfast cereals have iron added to them to increase their
nutritional value – sometimes as fine iron filings. Tea leaves are also a good source of
iron.
The quantity of iron present can be measured by titration.
Nitric acid is added to a sample of food, releasing iron(III) ions, Fe3+.
When potassium iodide is added iodine, I2, is formed.
2Fe3+
+ 2Iˉ 2Fe2+
+ I2
This iodine is titrated with a solution of thiosulfate, S2O32- .
I2 + 2S2O32-
2Iˉ + S4O62-
Starch solution is added as an indicator and changes from blue/back to colourless when the endpoint is reached.
The overall reaction in this experiment can be shown as:
3+ 2- 2+ 2-
2 3 4 62Fe 2S O 2Fe S O
iron ions thiosulfate
The larger the titre of thiosulfate, the greater the mass of iron present in the food sample.
National 5 Chemistry
Practical Assignment Chemical Analysis: Student
Page 10
The experiment
You will need
Preparing the solution
Sample of food or tea 2 mol l-1 nitric acid solution
Access to a balance (2dp) crucible
Bunsen burner, tripod and pipe-clay
triangle
100 cm3 beaker
25 or 100 cm3 measuring cylinder 50 cm3 volumetric flask
Funnel and filter paper
The titration
20 cm3 pipette and safety filler 100 cm3 flask
funnel 0·01 mol l-1 sodium thiosulfate solution
1% starch solution burette and stand
Dropper (for adding starch) white tile
Safety
2 mol l-1 nitric acid is corrosive. Wear goggles.
Method
Preparing the solution
1. Accurately weigh about 2.0g of a dry food sample into a crucible and roast it in a
fume cupboard for several minutes until all the food has turned to ash and no more
smoke is coming off.
2. Allow the ash to cool and wash it into a beaker using 2 mol l-1 nitric acid.
[CORROSIVE]
3. Add a further 20 cm3 of 2 mol l-1 nitric acid [CORROSIVE] is added and boil the
mixture for 5 minutes.
4. Let the mixture cool again and then filter it using the filter paper and funnel it.
5. The filtrate is then placed in a 50 cm3 standard flask and made up to the mark using
distilled water.
National 5 Chemistry
Practical Assignment Chemical Analysis: Student
Page 11
The titration
1. Using a funnel, fill the burette with 0·01 mol l-1 sodium thiosulfate solution, making
sure the tip is full and free of air bubbles.
2. Using a pipette and safety filler, add 20·0 cm3 of the food extract to a conical flask.
3. Add 1·0 g of potassium iodide. The solution should now go brown.
4. Remove the funnel from the top of the burette and note the reading on the burette.
5. Titrate the solution in the conical flask using the 0·01 mol l-1 sodium thiosulfate in the
burette.
6. When the yellow colour has almost gone, add 1 cm3 of starch solution to produce a
dark blue/black solution.
7. Continue titrating until the solution goes clear and colourless (and remains clear
and colourless for at least 1 minute). Read the burette to the nearest 0·1 cm3.
8. Repeat the titration until the titres agree to within 0·2 cm3.
Colour change of iodine/starch indicator
Initial colour iodine fading starch added final end-point
National 5 Chemistry
Practical Assignment Chemical Analysis: Student
Page 12
Investigation C – “Chloride in sea water”
Background
For many of us, our earliest memory of the sea is getting a mouthful of seawater and
finding out that it tastes horribly salty.
The concentration of salt in sea water can vary from place to place. Near the mouth of a
river, where fresh water enters the sea the concentration of salt will be less. In very hot
areas, where a lot of water is lost from the sea by evaporation, the salt concentration will
be higher.
One of the most common ions found in seawater is the chloride ion, Clˉ. The concentration
of chloride ions can be measured by titrating a sample of sea water using silver(I) nitrate
solution.
AgNO3(aq) + Clˉ(aq) AgCl(s) + NO3ˉ(aq)
Potassium chromate is added to indicate the endpoint of the titration as it will form a red-brown precipitate when the endpoint is reached. In this experiment, the larger the titre of silver(I) nitrate, the higher the concentration of chloride ions present in the water sample.
National 5 Chemistry
Practical Assignment Chemical Analysis: Student
Page 13
The Experiment
Equipment Needed
Preparing dilute samples of seawater
20 cm3 pipette and safety filler 100 cm3 volumetric flask
Titration
diluted sea water sample 250 cm3 conical flasks
10 cm3 and 100 cm3 measuring cylinders 0·1 mol l-1 silver nitrate
1 mol l-1 potassium chromate indicator burette and stand
white tile funnel
Preparing dilute samples of sea water
* If the water contains traces of solid matter such as sand or seaweed, it must be
filtered before use.
Dilute seawater by pipetting a 20 cm3 sample into a 100 cm3 volumetric flask and making it up to the mark with distilled water.
Safety
0·1 mol l-1 silver nitrate is and irritant.1 mol l-1 potassium chromate is also an irritant and
can cause health hazards with long-term exposure. Wear gloves and goggles. Both
chemicals are harmful to the environment and should not be poured down the sink.
Titration
1. Using a funnel, fill the burette with 0·1 mol l-1 silver(I) nitrate solution, making sure
the tip is full and free of air bubbles.
2. Pipette a 10·0 cm3 sample of diluted seawater into a conical flask and add about
50 cm3 distilled water and 1 cm3 of potassium chromate indicator.
3. Remove the funnel from the top of the burette and note the reading on the burette.
4. Titrate the solution in the flask using the 0·1 mol l-1 silver nitrate solution from the
burette. Although the silver chloride that forms is a white precipitate, the chromate
indicator initially gives the cloudy solution a faint lemon-yellow colour. Before the
addition of any silver nitrate the chromate indicator gives the clear solution a lemon-
yellow colour.
National 5 Chemistry
Practical Assignment Chemical Analysis: Student
Page 14
5. The endpoint of the titration is identified as the first appearance of a red-brown
colour of silver chromate. Read the burette to the nearest 0·1 cm3.
6. Repeat the titration until the titres agree to within 0·2 cm3.
Colour change of potassium chromate indicator
Initial colour silver chloride near end-point silver chromate precipitate end-point