Science Makes Sense-Week 25: Lead and Tin

The recently revealed shocking story of lead-poisoned water for the residents of Flint, Michigan for almost a year (Refs.1,2) brought back memories of lead paint poisoning of children living in poor, older inner city neighborhoods.(Ref.3) The tragedy is that this old story of lead paint has resurfaced in many African-American communities in Chicago once again.(Ref.4) Both situations could have been avoided since there is ample evidence to point out that even low level continuous ingestion of lead causes damage in hearing, learning abilities and coordination.(Ref.5)
Why has lead been used from the time of the Greek-Roman civilizations in spite of knowledge regarding its toxicity?(See Nuggets) This is because lead has a high corrosion resistance, it is soft, easy to work with and has a low melting point.(Ref.6) Furthermore, it has been durable for many uses, especially till recent times for lead-based paints and lead pipes.(Ref.5)
The word ‘plumbing’ derived from ‘Plumbum’, gave rise to the chemical symbol Pb for lead since lead was used a lot in the field of plumbing.(Ref.3)
This shiny blue-white soft metal,lead, when exposed to the air containing oxygen and carbon dioxide, becomes coated with a dull grey layer of basic carbonate that adheres to the surface and protects it from further change. A similar kind of protection takes place in the presence of sulfuric acid: it forms a layer of lead sulfate. Because of this property, lead is often used to sheathe cables, to carry tanks of sulfuric acid and to protect roofs from the atmosphere.(Ref.6)
In recent times, it is mainly used in lead-acid storage batteries.
The electron configuration of Pb is 6s2 6p2 which means there are 2 electrons each in the 6s and 6p level respectively. Lead exhibits the oxidation state +2, when it loses the ‘p’ electrons and is generally basic. However, the oxidation state +4 also exists and in it, lead is more acidic. This property where a metal exhibits both acidic and basic characteristics like another metal, aluminium, Al, is called amphoterism. Tin also exhibits this behavior in the +4 oxidation state.(Ref.6)
Lead exists as many oxides, carbonate and chromate and shows brilliant colors that were useful for paint pigments till the 20th century when the toxic nature of ingested lead was documented. The reason it was used for so long is because no one imagined that lead paints would be ingested till there was evidence to show that children were licking lead paint off frames in old homes.
The other area where lead was used extensively was in motor fuel as an anti-knock agent called tetraethyl lead. The addition of lead to the fuel reduced engine knock and so was used for years. Here the lead replaced the carbon in methane and each of the hydrogen atoms was replaced by an ethyl group, fooling the motor fuels into letting it dissolve. In the cylinder, the heat would knock the ethyl radicals off and form a cloud of PbO, lead oxide. The ethyl radicals stopped any explosions and let the fuel charge not detonate and burn smoothly. This permitted the use of less expensive straight-chain hydrocarbons as fuel versus the more expensive branched and aromatic hydrocarbons of higher octane rating.(Higher octane rating improves engine efficiency.) The lead in the air was found to be especially harmful for younger children and so leaded fuel was finally discontinued. It is important to note that most adults working with lead are not unduly affected by it, unless ingested continuously. Its widespread use in paints and in fuels made it dangerous, since children and growing young adults are particularly vulnerable to it; yet lead is not as toxic as mercury.(Ref.6)
Tin is another metal similar to lead in some of its properties, and in the same Group in the Periodic Table. Known with the chemical symbol Sn, today we mainly think it is used with other metals in alloys as well as coating other metals.
Known from the Bronze Age where tin and copper formed the very useful alloy bronze, tin like lead has an ancient history. Just like lead it has a low melting point, but its boiling point is very high. This large range where it is a solid helps in forming alloys without loss in vaporization. (Ref.7)
Tin also exists in two oxidation states having the same outermost electronic configuration like lead. This leads to its amphoteric behavior just like lead. Tin forms chlorides and stannous chloride is a reducing agent. Metallic tin is safe and is not toxic when ingested like lead. (Ref.7)
Organo-tin compounds were used as a biocide, but because of their toxicity, these compounds have been phased out. (Ref.8)
Tin and lead have much in common, both known from ancient times, both are amphoteric, low melting point solids. Lead has been less expensive and even though many of its uses have been phased out, it is still useful in certain specific areas. Tin though is coming back to be used more often than before. (See Nuggets) These two metals cannot be dismissed that easily!
Activities for Middle School Teachers:
What are the other metals in this group besides tin and lead? What are their properties and uses? How are they similar and different in their properties and uses?
Lead is toxic when ingested; what about mercury? Compare and contrast how both lead and mercury are being used in different ways today compared to earlier because of their toxicity.
Nuggets of Information:
The Roman architect, Vitruvius Pollio during the first century BC recognized the dangers of using lead.(Ref.3)
In the mid 19th century lead poisoning was called’painter’s colic’.(Ref.3)
The slightly sweet taste of lead made it a good additive in Roman wine and there is a belief that this use of lead could have been one of the causes for the fall of the Roman Empire.(Ref.3)
The relative higher density of lead, compared to copper and iron, has made it suitable in the manufacture of bullets and sinkers.(Ref.6)
Even though many European countries banned the use of interior lead paints in 1909, the US waited till 1971 to do so.(Ref.3)
Tin was also called white lead, being a soft white metal.(Ref.7)
Tin exists in three allotropic forms:Below 18 degrees C, it exists as alpha tin or grey tin with a diamond structure (like carbon, silicon and germanium)and is a semiconductor. From 18 to 161 degrees C, the stable form is beta-tin or white tin with a slightly higher density and a tetragonal crystal structure. Above 161 degrees C to the melting point is gamma-tin with about the same density but rhombic crystal form. When beta-tin is bent it emits a sound called “tin cry’ from the shearing movement between the crystal grains.(Ref.7)
Apart from being used most often in the alloy, bronze, tin is also used in several other alloy forms like pewter(tin and lead), a superconducting wire alloy of tin and niobium,and Babbitt metal which is a combination of tin,copper and antimony. The superconducting wires are used in the manufacture of very powerful magnets and Babbitt metal is used for bearings. Tin plating which is a coating of tin over steel is still used for canned goods and to prevent corrosion (Refs.8,9)
The biggest use of tin is for soldering especially electrical circuits. Molten glass used in windows is floated on tin to get a flat surface; stannous flouride is used in toothpaste and organo-tin products are needed to stabilize PVC plastics.
(Ref.9)
Indonesia and China are the largest producers of tin.(Ref9)

Chicago Tonight, April 2016, had a story about the presence of lead in service lines that could leach and enter homes and become a crisis like the Flint, Michigan water situation.(Ref.10)

References:

1.http://www.theguardian.com/us-news/2016/feb/17/flint-water-crisis-michigan-contaminated-water-bills-food-water-watch-study
2.http://www.democracynow.org/2016/2/17/thirsty_for_democracy_the_poisoning_of_an
3.http://www.toxipedia.org/display/toxipedia/History+of+Lead+Use
4.http://www.chicagotribune.com/news/watchdog/ct-lead-poisoning-chicago-met-20150501-story.html
5.http://www.dhr.virginia.gov/pdf_files/LeadPaint1.PDF
6.http://mysite.du.edu/~jcalvert/phys/lead.htm
7.http://mysite.du.edu/~jcalvert/phys/tin.htm
8.http://humantouchofchemistry.com/history-of-tin.htm
9.http://wanttoknowit.com/uses-of-tin/
10.http://chicagotonight.wttw.com/2016/02/18/chicago-s-lead-pipes-what-you-need-know

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