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9.4 Chemical monitoring and management: 3. Manufactured products are analysed
| Syllabus reference (October 2002 version) | ||
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3. Manufactured products, including food,
drugs and household chemicals, are analysed
to determine or ensure their chemical composition
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Students learn to: |
Students:
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Extract from Chemistry Stage 6 Syllabus (Amended October
2002). © Board of Studies, NSW.
[Edit: 18Jun10]
Prior learning:Preliminary modules 8.2 (8.2.1, 8.2.3, 8.2.4, 8.2.5) 8.3 (8.3.1, 8.3.2, 8.3.3, 8.3.5), 8.4 (8.4.3, 8.4.4)
HSC modules 9.2 (9.2.3), 9.3 (9.3.1, 9.3.3).
Background: All legally manufactured products under current health and consumer regulations are analysed to ensure quality requirements and health and safety guidelines are adhered to, so as to protect the consumer. In order to comply with this aspect of society, companies conduct a series of accepted tests and report the information accordingly. Analytical chemists conduct the tests, usually following a flow chart concept.
The information below about tests is very closely allied to the information you will need to have worked on in your first-hand investigations in this section of the syllabus.
perform first-hand investigations to carry out a range of tests, including flame tests, to identify the following ions:
- phosphate
- sulfate
- carbonate
- chloride
- barium
- calcium
- lead
- copper
- iron
Background
The tests used to separate ions from a mixture make use of the insolubility of certain cation-anion combinations. These insoluble ion combinations are called precipitates. If the crystals of insoluble salt are colourless, the insoluble salt precipitate appears white due to reflection of white light by the crystal surfaces.
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Perform a first-hand investigation using the procedure provided below. The results can then be used to identify tests for the cations and anions. You may need to modify the procedure depending on the availability of the chemicals or equipment.
Procedure:
- Prepare or select solutions that contain the following cations: barium, calcium, lead, copper, iron(II), iron(III) and silver.
- Prepare or select solutions that contain the following anions: phosphate, sulfate, carbonate, chloride, hydroxide and nitrate.
- Determine the cation-anion combinations that form a precipitate by testing a small quantity of an appropriate cation solution with the same quantity of an appropriate anion solution, as indicated in the table below. (A sample set of results appears in the notes for the next syllabus point.)
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Cations Anions phosphate sulfate carbonate chloride hydroxide nitrate barium ? ? ? ? ? ? calcium ? ? ? ? ? ? lead ? ? ? ? ? ? copper ? ? ? ? ? ? iron(II) ? ? ? ? ? ? iron(III) ? ? ? ? ? ? silver ? ? ? ? ? ? NOTE: Copper, lead and silver compounds should not be disposed of into sink-waste systems. Collect for waste treatment and subsequent recycling or disposal.
- Some cations may form similar colour precipitates with the same anion. Distinguish between these with a flame test by placing a small sample of the precipitates in a colourless Bunsen burner flame (with the air hole open).
- From your table of results, identify tests that can be used to indicate the presence of a particular ion.
- You can confirm your results or identify data that cannot be determined from the first-hand investigation by extracting information from solubility tables in secondary sources, such as a chemistry data book or chemistry textbooks.
gather, process and present information to describe and explain evidence for the need to monitor levels of one of the above ions in substances used in society
The following outlines an appropriate process to study lead ions.
Background
Some foodstuffs are monitored for the presence of particular metals, e.g. lead, to ensure that we are not consuming poisons that can accumulate in our bodies.
When lead was an additive in petrol the concentration of lead in our blood increased when we inhaled air from busy roads. Even after the removal of lead from petrol lead could be found in soil and in dust in houses near busy roads and near industries that emitted lead into the air. The concentration of lead in blood needs to be monitored, especially for populations of children living in polluted areas and workers in lead smelters. Action can then be taken or evidence used to make changes, such as the location of worksites or in work practices.
- Gather information by looking in
encyclopaedia, as well as searching the Internet using key
words, such as monitoring, lead and
levels. If you have access to CD–ROMs, such
as Encarta or Encyclopaedia Britannica,
search in them under lead.
- Process your information, making sure
you assess its reliability by comparing information from
various sources.
- Present information in the form of an
explanation, describing and explaining evidence
for the need to monitor levels of lead in substances used
in society, such as food and fuel.
- An explanation has the following structure:
| Section and features | An explanation |
|---|---|
| A title or heading | Example:
Why lead should be monitored in our environment |
| A phenomenon identification: a general statement informing the reader what is being explained. | |
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An explanation sequence: a number of
paragraphs in logical sequence. It is often useful to
begin by describing the phenomenon or issues related
to the explanation.
The text should be characterised by the use of:
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One place to start could be:
Lead Poisoning 
Parenting and Child Health, Children, Youth and Women's Health, South Australia
deduce the
ions present in a sample from the results of tests
Ion identification flow charts
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There are several flow charts that can be used to assist in the identification of ions present in an aqueous sample. Two such flow charts follow. Assume that only one cation and one anion will be in your sample. Once those ions are identified there is no need to continue. In real-life water testing situations, this assumption cannot be made.
Cation identification
Anion identification
- You can use your own first-hand data to draw up your own flow charts to show how to identify, in a suitable sequence, the anions: phosphate, sulfate, carbonate and chloride, and the cations: barium, calcium, lead, copper, iron(II) and iron(III).
identify data, plan, select equipment and perform first-hand investigations to measure the sulfate content of lawn fertiliser and explain the chemistry involved.
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For each of the procedures described below:
- Determine the type of data that needs to be collected and explain the quantitative analysis that will be required for this data to be useful.
- Plan your investigation by predicting possible issues that may arise during the investigation and identify strategies to address these issues if necessary. The preparation of flow charts for the procedures for each of the investigations would help here. There are several flow charts possible for some of these analyses.
- Perform the investigation to analyse the concentration of the substance required. Modify the procedure to suit your equipment and materials and analyse the effect of any adjustment made. Check the label of the product to compare your analysis results with that stated for the product. More than likely, your values will be slightly different.
Some chemical analysis techniques
Sulfate content of lawn fertiliser
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Sulfate in ammonium sulfate fertiliser can be determined by precipitation as barium sulfate.
An excess of soluble barium salt, such as barium nitrate, is added to a solution containing a known weight of ammonium sulfate.
The particles of barium sulfate are too small to be trapped by ordinary filter paper. A sintered glass filter is needed to trap the BaSO4. The white residue of BaSO4 is washed to remove other salt ions, the sintered glass filter dried until a constant weight. Knowing the original weight of the filter, the mass of BaSO4 can be calculated.
Sulfate content is
analyse information to evaluate the reliability of the results of the above investigation and to propose solutions to problems encountered in the procedure
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A significant problem in separating solid barium sulfate from solution is the very small size of the crystals precipitated.
- Compare your results for this experiment with others to evaluate the reliability of your results. Remember that repeating your experiment does not guarentee reliable results.
- Search out information on ways of increasing crystal size and ways of effectively separating the solid barium sulfate from the solution.
gather, process and present information to interpret secondary data from AAS measurements and evaluate the effectiveness of this in pollution control.
- AAS allows the detection of very small concentrations of metals from samples of air, water or food. This activity depends on your ability to manipulate data and dilution factors. The absorbance values obtained using solutions of known concentration enable you to draw a calibration graph. Use the specific absorbance data provided to read off the corresponding concentration for the sample. The following information relates to the monitoring of arsenic and its analysis will allow you to evaluate the use of AAS.
A case study in the monitoring of arsenic
Arsenic-rich ground water is a serious threat to 20 million people in Bangladesh. Solar oxidation and removal of arsenic (SORAS) is a simple method that uses irradiation of water with sunlight in PET plastic, or other UV transparent bottles, to reduce arsenic levels in drinking water.
Groundwater in Bangladesh contains Fe2+ ions and Fe3+ ions. Fe3+ forms an insoluble hydroxide precipitate. Arsenic with an oxidation state of three, As(III), is only weakly adsorbed but arsenic with an oxidation state of five, As(V), is strongly adsorbed to the surface of iron(III) hydroxide particles as they precipitate out of solution.
The SORAS method involves adding about 6 drops of lemon juice to a litre of water in a 1.5 L PET bottle. The bottle is shaken vigorously for 30 seconds, then placed horizontally in sunlight for 4 to 5 hours. The UV energy, oxygen and water in the bottle produce oxidising conditions:
At the end of the day, the bottle is stood vertically. The As5+ is adsorbed onto the surface of the brown Fe(OH)3 as it precipitates overnight. The next morning, the liquid is decanted off or filtered through fine cloth leaving the last 100 mL, containing iron(III) hydroxide and arsenic(V), to be discarded. The citric acid from the lemon juice enhances the photochemical oxidation of the arsenic(III) and leads to much faster formation and settling out of precipitate.
Here are data that can be used to produce an AAS calibration graph for the arsenic levels in this study.
Here are some AAS arsenic absorbance measurements for an investigation into the SORAS method:
- Draw a calibration curve of absorbance vs total arsenic concentration. Use the curve to gather data to determine the arsenic concentration before and after SORAS treatment.
- People generally require about two litres of water a day and the recommended daily intake of arsenic by an adult is set at 150 micrograms (150 µg). Process the information extracted from the data by assessing the importance of the data and information gathered in relation to the acceptable levels of arsenic.
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Present your findings. By referring to
the precision of AAS and to the quantities of arsenic in
drinking water before and after treatment, evaluate the
effectiveness of:
- the SORAS method in reducing arsenic levels in drinking water to acceptable levels
- the use of AAS in monitoring and controlling pollution in this situation.
Atomic-absorption spectroscopy Virginia
Tech Chemistry Department, USA
describe the use of atomic absorption spectroscopy (AAS), in detecting concentrations of metal ions in solutions and assess its impact on scientific understanding of the effects of trace elements
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Each element has its own characteristic absorption spectrum that is related to its electron energy levels.
Atomic absorption spectroscopy (AAS) detects minute concentrations of an element in a sample of solution.
The flame containing the vapourised sample absorbs light at the particular wavelengths characteristic of the element in the flame and re-emits it in all directions. A detector records the intensity of light emerging from the flame. The intensity of light detected drops sharply at the wavelengths of light absorbed by the elements in the flame, thus producing an absorption spectrum. The relative intensity and pattern of changes of intensity within each of the bands in the absorption spectrum indicate the concentration of the element in the test sample.
- The study of the concentration of pollutants in our
environment has been greatly enhanced and is more accurate
and reliable since the development of AAS by the CSIRO
scientist, Alan Walsh, in the 1950s. As Alan Walsh stated
"the AAS method is a quick, easy, accurate and highly
sensitive means of determining the concentrations of over
65 elements". It is used in a range of areas, such as
medicine, agriculture, mineral exploration, metallurgy,
food analysis, biochemistry and environmental monitoring.
It has been described as the most significant advance in
chemical analysis of the 20th Century.
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Trace elements are elements needed in very small amounts
by living things. AAS enabled the measurement of the
concentrations of many metals in the bodies of plants and
animals and in their surrounding environments. This has
proved to be enlightening in many practical situations.
Two such situations include the following:
- In coastal south-western Australia, animal health could not be maintained on seemingly good pastureland. AAS showed cobalt deficiencies in the soil and the pasture.
- Arid parts of Victoria could not support legume crops until molybdenum deficiencies were detected by AAS and rectified.
Alan Walsh and AAS Australian Academy of
Science.
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