Chemical quantities (AS)
Read our general notes on Risk Assessment
Last week we did an acid-base titration practical. We had to determine the concentration of HCl that reacted with sodium carbonate. Why is sodium carbonate suitable as a primary standard, whereas sodium hydroxide is not?
The important things about a primary standard are that it should be a stable solid, reasonably soluble in water, and capable of being weighed out accurately. Sodium hydroxide suffers from two problems: it is deliquescent (absorbs water from the air); it also absorbs carbon dioxide from the air to give a surface layer of sodium carbonate and sodium hydrogencarbonate.
Anhydrous sodium carbonate is not deliquescent, but it does absorb carbon dioxide and water from the air to give sodium hydrogencarbonate. When heated, this sodium hydrogencarbonate decomposes back to sodium carbonate again, but the sodium carbonate does not decompose on heating. So, for use as a primary standard, sodium carbonate is first heated and then cooled out of contact with the air. It is then weighed in a container with a lid before being dissolved in water and made up in the usual way.
Ulex
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
back to Chemical quantities (AS)
Rate this page or react
Share your views on this page, 8 ratings so far
, rated at 
updated: 21 August 2003
