The distance travelled relative to the solvent is called the R f value. For each compound it can be worked out using the formula:. For example, if one component of a mixture travelled 9. In the example we looked at with the various pens, it wasn't necessary to measure R f values because you are making a direct comparison just by looking at the chromatogram.
You are making the assumption that if you have two spots in the final chromatogram which are the same colour and have travelled the same distance up the paper, they are most likely the same compound. It isn't necessarily true of course - you could have two similarly coloured compounds with very similar R f values. We'll look at how you can get around that problem further down the page. In some cases, it may be possible to make the spots visible by reacting them with something which produces a coloured product.
A good example of this is in chromatograms produced from amino acid mixtures. Suppose you had a mixture of amino acids and wanted to find out which particular amino acids the mixture contained. For simplicity we'll assume that you know the mixture can only possibly contain five of the common amino acids. A small drop of a solution of the mixture is placed on the base line of the paper, and similar small spots of the known amino acids are placed alongside it.
The paper is then stood in a suitable solvent and left to develop as before. In the diagram, the mixture is M, and the known amino acids are labelled 1 to 5. The position of the solvent front is marked in pencil and the chromatogram is allowed to dry and is then sprayed with a solution of ninhydrin.
Ninhydrin reacts with amino acids to give coloured compounds, mainly brown or purple. The left-hand diagram shows the paper after the solvent front has almost reached the top. The spots are still invisible. The second diagram shows what it might look like after spraying with ninhydrin.
There is no need to measure the R f values because you can easily compare the spots in the mixture with those of the known amino acids - both from their positions and their colours.
And what if the mixture contained amino acids other than the ones we have used for comparison? There would be spots in the mixture which didn't match those from the known amino acids. You would have to re-run the experiment using other amino acids for comparison.
Two way paper chromatography gets around the problem of separating out substances which have very similar R f values. I'm going to go back to talking about coloured compounds because it is much easier to see what is happening. You can perfectly well do this with colourless compounds - but you have to use quite a lot of imagination in the explanation of what is going on!
This time a chromatogram is made starting from a single spot of mixture placed towards one end of the base line. It is stood in a solvent as before and left until the solvent front gets close to the top of the paper. In the diagram, the position of the solvent front is marked in pencil before the paper dries out.
This is labelled as SF1 - the solvent front for the first solvent. I have done this lab a few times in the past with high school kids. Looking forward to teaching another course where I can use it again.
The technique does have a wide range of applications. Glad you enjoyed the hub. Good luck in your studies! I had a chemistry lab last semester on both paper and TLC chromatography. The paper lab done was using different eluents to see which was the most effective we used a Sharpie dot on the paper. The TLC lab was a row of different analgesic drugs and then a mystery drug made up of one or more of the known drugs.
We had to find out the mystery drug based on how the results compared to the known drugs. It was a pretty awesome lab Marine Biology. Electrical Engineering. Computer Science. Medical Science. Writing Tutorials. Performing Arts. Visual Arts. Student Life. Vocational Training. Standardized Tests. Online Learning. Social Sciences. Legal Studies. Paper chromatography offers many advantages like low-cost, unattended, hassle-free operation and simplicity.
What actually happens in paper chromatography? As we all know, ink is a solution containing a number of different molecules. Different characteristics such as solubility and size are present in these molecules. When pulled along the piece of paper toweling, each molecule travels at a different speed because of their different characteristics. Compared to the heaviest particles, the lightest particles move more quickly and at a greater distance.
The pigments that make up an ink sample are thus separated out. Over the years, paper chromatography has evolved and has found widespread applications in molecule separation of different polarities. An effective technique used for separating colored pigments from a mixture. How does it work? A few drops of the colored pigments mixture are placed on the filter paper and then it is slowly submerged into a jar of solvent. Depending on their polarity, it dissolves the molecules present in the mixture, as the solvent rises up the paper.
The separated components will now be separated through the same or different solvent used. Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. Types of Paper Chromatography Paper Chromatography has been classified into 5 types. These are Read also: Minerva mills vs Union of India A case that evolved basic structure doctrine.
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