Friday, 3 February 2012

Chemistry - Soda

The properties, uses and manufacture of the various forms of soda (sodium carbonate, sodium hydrogen carbonate, sodium hydroxide) are relevant to KS3 and KS4 topics involving alkalis, the chemical industry, pollution, and the alkali metals and their compounds.

If you ask a cook what “soda” is they will probably point you to baking soda (sodium hydrogen carbonate) which is used to make cakes light and fluffy. It gives off carbon dioxide gas when heated or reacted with acids such as lemon juice or tartaric acid.  A quick search of the internet will provide lots more uses for baking soda. Baking soda can be bought from supermarkets. You are less likely today to find packets of washing soda (sodium carbonate).  This was used to soften water for washing clothes and prevent soap forming scum.  Caustic soda (sodium hydroxide) is present in many oven cleaners – it is a strong alkali that breaks down grease. 

The usefulness of soda has been known for centuries.  An 1805 document suggested many uses including using a teaspoon of soda to about a litre of water to wash containers used for milk or cream.  The main use of soda was as the alkali used to turn fats and oils in to soap.  For over two thousand years the source of this alkali was the ashes of plants. 

In the Middle East or Arabia, a plant that grew in the salt marshes beside estuaries was used.  The ashes were called “al-qili”.  You can see how we got the term “alkali”.   In the eighteenth century Britain and France competed for supplies of wood ash from Canada.  When France lost control of Canada it was short of alkali for its soap industry.  A prize was awarded for a new process.  The winner was Nicolas Leblanc but in the confusion of the French Revolution he did not receive the prize and had his factories confiscated.   An Irishman, James Muspratt, brought Leblanc’s process to the UK and in the 1820s lots of alkali factories sprang up on Merseyside, Teesside and Glasgow. 

Merseyside was a very good site as it was a seaport, close to the Lancashire textile mills and had sources of salt, coal and limestone nearby.  These three substances, with sulfuric acid, were the raw materials for the Leblanc process.  Muspratt had trouble persuading soapmakers that his soda was as good as the alkali made from ashes but the business was soon doing well. The process was extremely wasteful however.  Huge chimneys were built to disperse the vast quantities of hydrogen chloride gas given off and heaps of waste called “galligu” (calcium sulfide) piled up around the factories and the workers’ homes.  The result was dreadful damage to the environment – no trees could survive within two miles of a factory – and to the health of the workers and their families. 

The Alkali Acts of the 1860s improved matters but a new process helped.  A Belgian, Ernest Solvay invented the process named after him and it was licensed to John Brunner and Ludwig Mond to produce soda in England.  They built their factory in Cheshire and were soon producing soda more cheaply and cleanly than the Leblanc factories.  The Solvay process uses salt and limestone as raw materials together with ammonia gas which is recovered and re-used.  Soda is still produced on the same site and is now part of Tata Chemicals.

In the 1890s an electrolysis method was developed to manufacture caustic soda (sodium hydroxide). The Castner-Kellner process passes an electric current through sodium chloride solution using a mercury cathode.  A factory was built at Runcorn on Merseyside and still runs today.  Mercury cathode cells are being replaced by membrane cells to stop the escape of small amounts of poisonous mercury.

1 Compare the properties of baking soda, washing soda and caustic soda.  Test a solution of each with an indicator and with pH paper.  Find out the effect of adding an acid (lemon juice or hydrochloric acid) to each of the solutions. 
(Use 0.5 solutions of each, and wear goggles as the solutions, particularly caustic soda are irritants)

2 Make some alkali by burning plant material (wooden splints will do), mixing the ashes with water and filtering off the solution.  Test the solution with indicator and acid (as above).
(Care with burning significant quantities of plant material)

3 Test some of the uses of baking soda listed on websites.  Does it work?  Is it more or less effective than other products that are for sale?
(Wear goggles.  Baking soda is relatively harmless but can irritate eyes.)

4 Investigate the Leblanc and Solvay processes. 
• What are the raw materials? 
• What reactions takes place? 
• What waste materials were formed? 
• Why was the Solvay process more economical than the Leblanc process?

5 What are the other products of the electrolysis of sodium chloride solution?  Why was the process economical despite the cost of electricity?  Why are membrane cells replacing mercury cathode (Castner-Kellner) cells?

6 What other uses are there for sodium carbonate (soda) and sodium hydroxide (caustic soda)?

7 Why was the manufacture of soda important to the industrial revolution in the UK?

8 Find out about the damage caused by Leblanc soda works and what was done about it in the 1860s.

9 Prepare a display on the life and work of the people involved in this story – Nicolas Leblanc, James Muspratt, Ernest Solvay, John Brunner, Ludwig Mond, Hamilton Castner, Karl Kellner.

Peter Ellis

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