Lead and lead salts

Lead and lead salts. On your List of Recommended Chemicals for Science in Australian Schools, you say that lead nitrate can be used in Years 7–12. They are classed as 'high risk' substances with uncertain or unpredictable risk levels in Education QLD’s guidelines. I don’t, therefore, understand why lead and lead salts are included on your list?

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Publication Date: 23 October 2015
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Answer by ginny.r.ward on question Lead and lead salts

Expert Answer

Thank you for highlighting the hazards associated with the use of lead and lead salts.

Lead poses a significant risk of harm to humans, and especially to children, as it accumulates in the body and can have long-term effects.[1,2]   The use of lead and its salts in schools are banned in some Australian jurisdictions, and approved or restricted in others.

On the List of Recommended Chemicals, we have included only three lead-containing substances: lead metal, lead nitrate (in solution for students) and lead (II) oxide.  These are included because we consider that they allow for some worthwhile demonstrations and activities and that their associated hazards can be reduced to an acceptable level with safe practices.

We have included lead metal on the List of Recommended Chemicals, so that it may be used to demonstrate the difference in its malleability compared with that of other metals.   Elemental lead also has applications in physics activities, which employ its high density compared with that of other metals, also lead shot is used in determinations of specific heat.[3]

Lead is not readily absorbed through the skin,[2] however, there is the risk of lead ingestion from eating or touching the mouth after handling lead.   To minimise exposure, pieces of lead metal can be wrapped in plastic clingwrap or provided in a zip-lock plastic bag prior to handling.  Students should be instructed to wash their hands well after handling lead.

Lead nitrate in solution is included on the List of Recommended Chemicals in order that the reaction of lead nitrate and potassium iodide to give a yellow precipitate of lead iodide may be conducted.  This reaction gives a dramatic colour change and is effective on a microscopic scale; using one drop each of the reactants on a white tile, spotting tile or flat acrylic sheet.  Making a precipitate in a test tube is not recommended because of the difficulty in cleaning up the only very slightly soluble lead iodide.  A teacher demonstration of this reaction is also a good alternative.

Lead nitrate in solution may also feature in senior chemistry activities such as the demonstration of displacement reactions and qualitative inorganic analysis, although the use of lead salts is not strictly necessary in any of these activities.  The amount of heavy metal waste produced from displacement reactions can be minimised by using spotting tiles.[4]   Lead salts are not necessarily required as unknowns in inorganic analysis activities.  Lead salts are also not necessary as reagents in these activities; barium chloride can be used to indicate the presence of sulfate ions and silver nitrate can be used to indicate the presence of iodide ions.

Lead monoxide, PbO, is included for the purpose of demonstrating its facile reduction to lead metal when heated in the presence of carbon.[5]  This reaction is best conducted in a fume cupboard and as a teacher demonstration.  Students could then perform the less hazardous extraction of iron from iron oxide on a match head.[6,7]

As lead is not biodegradable and persists in the environment, waste lead and lead compounds should not be disposed of in the general waste and should be stored for collection by a licenced waste disposal contractor.

It is recommended that any reactions using lead or its compounds be conducted on a conservative scale and that other materials be substituted for lead where possible (e.g. using lead-free solder in electronics).   Science ASSIST will be developing further guidance regarding the use of lead and its compounds in the coming months.

References

[1] Flora, G., Gupta, D., & Tiwari, A. (2012). Toxicity of lead: A review with recent updates. Interdisciplinary Toxicology, 5(2), 47–58. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3485653/

[2] Abadin H, Ashizawa A, Stevens YW, et al., Toxicological Profile for Lead. Atlanta (GA): Agency for Toxic Substances and Disease Registry (US); 2007 Aug. https://www.ncbi.nlm.nih.gov/books/NBK158766/

[3] Nuffield Foundation, Institute of Physics, The specific thermal capacity of lead, 2015, http://practicalphysics.org/specific-thermal-capacity-lead.html

[4] Royal Society of Chemistry, Learn Chemistry: TeacherExpt:Displacement reactions between metals and their salts, 2013, http://www.rsc.org/learn-chemistry/wiki/index.php%3Ftitle%3DTeacherExpt:...

[5] Royal Society of Chemistry, Learn Chemistry: Extracting metals with charcoal, 2015, http://www.rsc.org/learn-chemistry/resource/res00000417/extracting-metal...

[6] Royal Society of Chemistry, Classic Chemistry Experiments: The Reduction of Iron oxide by Carbon, 2015, http://www.rsc.org/learn-chemistry/resource/res00000419/the-reduction-of-iron-oxide-by-carbon?cmpid=CMP00000489

[7] Bianchi, Zoe, Extracting Metal on a Match Head, 2013, http://asd54.blogspot.com.au/2013/04/extracting-metal-on-match-head.html

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