A box of food coloring, available from a grocery store, contains one small bottle each of the color — Chromatography Chemistry Question
Problem Context
A box of food coloring, available from a grocery store, contains one small bottle each of the color red, yellow, blue, and green. You have been given two vials, one containing some green food coloring and the other a mixture of the blue and the yellow.
Devise and carry out a procedure to determine:
1) Which solvent-paper combination gives the best separation of the dyes present in the mixture of the blue and yellow food coloring, and
2) Which dye(s) is (are) present in both the mixture of the blue and yellow food coloring and in the green food coloring.
Give a brief description of your experimental plan.
Model Answer
An average response indicated that the experiment relied on paper chromatography. It gave a plan for using all six combinations (2 types of paper x 3 eluents) on the mixture of blue and yellow food coloring, then using the most effective combination to analyze the green food coloring. (Using all six combinations on each of the two types of food coloring was also reasonable.) An excellent response gave more details about the mechanics of paper chromatography, for example by drawing a sketch of the proposed setup.
Record your data and observations.
Model Answer
An average response clearly linked sets of observations with particular conditions (eluent, paper, food coloring sample), usually in the form of a table or tables. The degree of separation was described in qualitative terms, and included the color of the spots ("blue faster than yellow"). A difference in the order of elution, with blue dye eluting faster than yellow on white paper but yellow faster than blue was observed. An excellent response described the observations both qualitatively and quantitatively (by Rf or by a quantitatively accurate sketch of the chromatogram). The presence of a red spot in the blue/yellow mixture was noted.
Which solvent-paper combination gives the best separation?
Model Answer
The white paper/salt water combination worked best.
Which dye(s) is (are) present in both a mixture of blue and yellow food coloring and in the green food coloring?
Model Answer
An average response noted that blue and yellow dyes were present in both samples. An excellent response made the comparison with respect to elution rates (i.e., Rf values) as well as colors. A poor response alluded to the presence of a green dye (no green dye was present in any of the samples, but overlap between spots might produce the appearance of a green smear).
Provide the order in which the colors are separated with the brown paper-salt water combination. Compare that order with the order of the white paper-salt water combination. Describe a possible explanation for the difference.
Model Answer
An average response noted the inversion of order mentioned in (2) and attributed to different affinities of the dyes for the surfaces of the brown vs. white paper. An excellent response was concrete in connecting the change in affinity to the difference in elution times, e.g., that the greater affinity of the brown paper for the blue dye made the blue dye run more slowly on the brown paper. It might also have mentioned possible reasons for the change in affinity, e.g. different hydrophilicity of the paper surfaces. A poor response attributed differences to factors unrelated to paper chromatographic separation (e.g., the molecular weights of the dyes).