93 Elemental analysis
Elemental analysisis the process for determining the partial or complete chemical formula
for a substance. Most commonly, it involves the complete combustion in air or oxygen of
the substance and then quantifying the amount of elemental oxides produced. In the case of
organic compounds, the carbon is converted to carbon dioxide and the hydrogen to water.
From these, the percent carbon and percent hydrogen in the substance can be found and
compared with a proposed chemical formula for the substance at hand.
Element test: Put a small amount of the solid into a small piece of Na metal then roll it
around the solid, followed by introduction into a fusion tube. The tube is heated with a
gentle flame at a slow rate (in order not to evaporate N 2 present in solid) then strong heating
till the bottom of the tube become red hot. The tube is then put in a beaker containing
a minimal amount of water then heated, cooled, filtered and the filtrate divided into three
parts.
- Testfornitrogen: The filtrate and ferrous sulphate are boiled and cooled and dilute
sulphuric acid is added. If green or blue color occurs the solid contain nitrogen.
The chemistry behind what happened:
Na+C+N --> NaCN
FeSO 4 +NaCN gives Fe[CN] 2
Fe[CN] 2 +4NaCN give the complex Na 4 [Fe(CN) 6 ]
ferrous oxidizes to ferric by the acid so 3Na 4 [Fe(CN) 6 ]+4Fe3+--> Fe 4 [Fe(CN) 6 ] 3
- Testforsulphur: The filtrate is exposed to dilute acetic acid and lead acetate, yielding
a brown or black precipitate.
The chemistry behind what happened:
Na+S --> Na 2 S
Na 2 S +Pb(CH 3 CO 2 H) 2 yields lead sulphide, a black precipitate.
or: The filtrate and sodium nitro prusside yield a violet color
Na 2 S+Na 2 [Fe(CN) 5 NO] --> Na 4 [Fe(CN) 5 NOS] = violet color
- Testforchlorine: