Separtion by paper chromatography

[moonlight1]

Hi does anyone know why it is imporatnt not to permit the solvent front to reach the top of the chromatogram?

 

[ShawnD]

When the solvent reaches the top, it totally screws up your ratios. Before reaching the top, you can say the components move X distance for every Y distance the solvent moves. Now that the solvent can't move anymore, that ratio doesn't apply, so you can no longer calculate that ratio. You would have to start over again.

 

[moonlight1]

Then why is it impotant to keep the indictor spots very small? is it beacuse separation is efficient?

 

[ShawnD]

If you use too much sample for the spots, you get huge streaks, and the different parts can overlap. Suppose 3 dots are supposed to come out, one is blue, one is yellow, and one is green. If you use too much, you'll get big yellow and blue streaks that overlap to make a greenish area. Is the desired green component even there, or is the green area just overlap of yellow and blue? How can you tell?

 

[moonlight1]

Whats the point to use acid-base indicators as the subanctes instead of more commom substances such as sugar or salt?

is it casue its colorless in soultion so we canot track their movement