Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Elemental analysis

Elemental analysis is 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 N2 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.

1. Test for nitrogen: 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
FeSO4+NaCN gives Fe[CN]2
Fe[CN]2+4NaCN give the complex Na4[Fe(CN)6]
ferrous oxidizes to ferric by the acid so 3Na4[Fe(CN)6]+4Fe3+ --> Fe4[Fe(CN)6]3

2. Test for sulphur: The filtrate is exposed to dilute acetic acid and lead acetate, yielding a brown or black precipitate.

The chemistry behind what happened:
Na+S --> Na2S
Na2S +Pb(CH3CO2H)2 yields lead sulphide, a black precipitate.

or: The filtrate and sodium nitro prusside yield a violet color

Na2S+Na2[Fe(CN)5NO] --> Na4[Fe(CN)5NOS] = violet color

3. Test for chlorine:

a) In the absence of N or S: The filtrate is exposed to dilute nitric acid and silver nitrate. Formation of a white precipitate suggests the presence of chlorine.

b) In the presence of N and/or S :

The filtrate is exposed to dilute sulphoric acid then boiled to 1/3 initial volume and cooled. Formation of a white precipitate after the addition of dilute nitric acid and silver nitrate suggests the presence of chlorine.

Equations:

NaCN+AgNO3 --> AgCN white ppt
Na2S+AgNO3 gives Ag2S black ppt
In the presence of N or S these two precipitates may interfere with the white colour of the result of the chlorine test. Therefore dilute sulphoric acid is added because in the presence of N or S: Na2S+ dilute H2SO4 gives H2S gas NaCN+ dilulte H2SO4 gives HCN gas

There is no interference with the white colour expected from the chlorine test in solid.

Last modified on 5 June 2010, at 01:54