Question

These are post lab questions regarding paper chromatography. Some background. The solvent used was a mix of HCl and acetone. We spotted the paper with 4 ions including Co(II), Cu(II), Fe(III), and Ni(II). After the solvent nearly reached the top, the paper was removed and measurements were made. Paper was dried and sprayed with ammonia. Changes observed and the paper was dried again and sprayed with sodium sulfide. Pleas help!

a)You used a pencil to mark the chromatography paper since the carbon in the pencil lead does not dissolve in the solute. Why does carbon not dissolve? What type of pen(ink) will also not produce a chromatogram when watert is used as the solvent? Explain using principles based on the polarity of particles.

b)You are to minimize hand contact with the paper since amino acids, oils, etc. on your skin will form a smudge on the cellulose paper. Why will using corroded tweezers also cause a smudge when sodium sulfide is sprayed on the paper?

c) What type of complexes appear when the spots are sprayed with Ammonia and why do new colors appear?

d) Same question as 'c' but being sprayed with sodium sulfide

Answer 1

a) Elemental carbon as in pencil is non-polar in nature. Going with the principle of like-dissolvesi-like, a non-polar substance such as carbon would not dissolve in acetone (acidfied). Infact elemental carbon in insoluble in all the solvent. A non-polar organic ink gel, would not produce a chromatogram whne water is used as a solvent. Water is highly polar (dielectric constant 80) and thus would not be a suitable for dissolving non-polar components of ink.

b) Corroded tweezers have a layer of iron oxide on it. This when in acidic medium would react with the S2- to form iron sulfide and devlop on the chromatogram along with the other mixture components. Thus interfering with the chromatographic separation. It is thus adviced not to use corroded tweezers during chromatograhy.

c) Complexes formed after NH3 spray,

Co2+ + 6NH3 <==> [Co(NH3)6]^2+ (blue-green)

Cu2+ + 4NH3 <==> [Cu(NH3)4]^2+ (dark blue)

Fe3+ + 3NH3 + 3H2O <==> Fe(OH)3 (reddish brown) + 3NH4+

Ni2+ + 6NH3 <===> [Ni(NH3)6]^2+ (light blue)

d) When sprayed with Na2S

Co2+ + S2- <==> CoS (black)

Cu2+ + S2- <==> CuS (black)

2Fe3+ + 3S2- <==> Fe2S3 (black)

Ni2+ + S2- <===> NiS (black)