[MissDee]

Hi,

Today I did a lab experiment dealing with chromatographic separation with food dyes. I had a few questions and I was wondering if anyone is willing to help me better understand the experiment. This is my very first Chemistry class so please be patient with my lack of knowledge, haha.

In the lab I had to take four chromatography papers, label and dot the bottom of the paper with different colored dyes. The papers were then dipped into a n-propyl alcohol/water solvent until the dyes ran up the strip of paper. I then had to calculate the Rf values for each strip.

I didn't have a problem with the calculation for the Rf strips. I believe the formula is Rf1=d1/D. For example: the solvent front distance was 5.35cm; whereas one of the color spot's distance from the middle was 4.25cm. The Rf would then equal .749cm. (Correct me if I'm wrong!)

Anyway, one of the questions is: "What can you say about the solubility of component A and component B in the mobile phase and their attraction to the paper?" Now, my impression is that the solubility can be found by how far the dye traveled up the paper. For example, if I compare the blue dye to the mixed dye (green/yellow), the blue dye travels furthest up the paper - therefore meaning its more soluble. Do I have the wrong impression about this? Does it have to do with how many different colored circles are formed instead?

Another question is: "Are any of the dyes homogeneous? How can you tell?" I'm guessing that the homogeneous dyes are the dyes that formed dots that were similar color to the dyes (?). For example, the blue dye's circles are light blue, blue and purple; whereas the green dye is yellow and pink. The blue dye would be homogeneous since its closer to its original color. Am I totally wrong about this?

"Which dye was the most soluble in the mobile phase? Which dye was the least soluble in the mobile phase?" I think this is similar to the first question asked - about the distance of the dye on the paper, unless I'm totally wrong about that.

"Is chromatography destructive? How can you tell in the experiment?" I would say that anything dealing with organic chemicals, such as the n-propyl alcohol/water solvent used in the experiment, would be dangerous. However, it is just food dyes... maybe its not that dangerous after all? Anyone care to elaborate on this?

I appreciate all the help - I would love for someone to really explain this to me and help me find the answers and understand this thoroughly!

[Yggdrasil]

I didn't have a problem with the calculation for the Rf strips. I believe the formula is Rf1=d1/D. For example: the solvent front distance was 5.35cm; whereas one of the color spot's distance from the middle was 4.25cm. The Rf would then equal .749cm. (Correct me if I'm wrong!)

This is a common mistake, but the R_f is 0.749, not 0.749cm.  R_f values are unitless.  Since they are the ratio of two distances, the cm in the numerator and denominator cancel out.

Anyway, one of the questions is: "What can you say about the solubility of component A and component B in the mobile phase and their attraction to the paper?" Now, my impression is that the solubility can be found by how far the dye traveled up the paper. For example, if I compare the blue dye to the mixed dye (green/yellow), the blue dye travels furthest up the paper - therefore meaning its more soluble. Do I have the wrong impression about this? Does it have to do with how many different colored circles are formed instead?

Correct.  Components that travel farther up the paper are more soluble in the solvent whereas components that do not travel as far have a higher affinity for the paper.

Another question is: "Are any of the dyes homogeneous? How can you tell?" I'm guessing that the homogeneous dyes are the dyes that formed dots that were similar color to the dyes (?). For example, the blue dye's circles are light blue, blue and purple; whereas the green dye is yellow and pink. The blue dye would be homogeneous since its closer to its original color. Am I totally wrong about this?

Homogenous substances are substances that contain only one component as opposed to heterogenous substances which are mixtures of two or more components.  Chromatography is a useful tool in chemistry because it can separate the components in a mixture.  So, the key here is whether your chromatography results show that the dye is a mixture of multiple dyes or a single dye.

"Which dye was the most soluble in the mobile phase? Which dye was the least soluble in the mobile phase?" I think this is similar to the first question asked - about the distance of the dye on the paper, unless I'm totally wrong about that.

Correct.

"Is chromatography destructive? How can you tell in the experiment?" I would say that anything dealing with organic chemicals, such as the n-propyl alcohol/water solvent used in the experiment, would be dangerous. However, it is just food dyes... maybe its not that dangerous after all? Anyone care to elaborate on this?

A destructive analysis is one in which you cannot recover the material after performing the analysis.  For example, some analytical techniques in chemistry require that you vaporize your sample with a laser to study the components.  This type of analysis is, of course, destructive, because you need to destroy your sample in order to analyze it.  Other analytical techniques, such as measuring its density, or measuring its absorbance, are non destructive, because the analytical technique does not permanently damage the sample.

[Borek]

Anyway, one of the questions is: "What can you say about the solubility of component A and component B in the mobile phase and their attraction to the paper?" Now, my impression is that the solubility can be found by how far the dye traveled up the paper. For example, if I compare the blue dye to the mixed dye (green/yellow), the blue dye travels furthest up the paper - therefore meaning its more soluble. Do I have the wrong impression about this? Does it have to do with how many different colored circles are formed instead?

Correct.  Components that travel farther up the paper are more soluble in the solvent whereas components that do not travel as far have a higher affinity for the paper.

Just don't treat this rule too seriously, as this is only approximation. It will be perfectly OK if stated as "between components with identical affinity to the paper components that travel farther up the paper are more soluble in the solvent whereas for components that have identical solubility components that travel farther have a lower affinity for the paper." Drawing conclusions when both solubility and affinity changes can be misleading.

But as a rule of thumb it will work OK.

[MissDee]

Thanks so much for answering some of my questions. However, I'm still having trouble figuring out which dyes were homogeneous. I had four dyes in the experiment: blue, yellow, green, and a mix (blue/yellow). When the dyes were extracted, blue had light blue/blue/purple circles; yellow had pink/yellow circles; green had light blue/blue/yellow circles; and the mixture had light blue/blue/purple/yellow circles. How do I distinguish the homogeneous dyes? Is it the dye whose extracted colors were similar to the original color? For example, would the blue dye be homogeneous since the colors extracted were light blue/blue/purple?

Thanks!

[ARGOS++]

[MissDee]

Hi Argos, thanks for the quick response.

So basically, all of the dyes are homogeneous then?... Since all of the dyes produced different colored circles.

[ARGOS++]

Dear MissDee;

You tell it a little simplified, but that should hold as long as there is no founded suspicion that two different dyes would/could have an "identical" R_f.

Good Luck!
                    ARGOS⁺⁺

[ARGOS++]

                             Copy of the lost replay No. 4

Dear MissDee;

Homogenous Dyes are Dyes you cannot separate by chromatography or by other methods.
But don’t let you misguide from a concentration profile of a spot or a circle, because also the concentration is able to shift a little bit the nuance.

As long as you can clearly distinguish between the circles different shade and/or R_f value it means that each separated circle represents a maybe homogenous Dye.

I hope to have been of help to you.
Good Luck!
                    ARGOS⁺⁺