Ciencias
Chromatography and the Rf Value
Chromatography separates soluble substances by how far each travels up the paper. Rf is spot distance over solvent distance, and always lands between 0 and 1.
Chromatography
Chromatography separates the substances in a mixture by how far each one travels through a stationary material while a moving solvent carries it along.
- También llamado
- Paper chromatography, Thin-layer chromatography, or TLC
- Dónde lo encuentra el alumnado
- Grade 7 science, in the practical on separating inks or food colourings, and again in Grade 10 chemistry as a test of whether a substance is pure.
La respuesta corta
Chromatography separates the substances in a mixture by how far each one travels through a piece of paper as a solvent soaks along it. Each substance is described by an Rf value: the distance the spot moved divided by the distance the solvent front moved. Rf always comes out between 0 and 1.
Un ejemplo
Rf = distance moved by the spot ÷ distance moved by the solvent = 6.0 ÷ 8.0 = 0.75
Both distances are measured from the same pencil baseline. The solvent front is where the wet paper stops, at 8.0 cm here. The spot is measured to its centre, not its lower or upper edge, at 6.0 cm. Divide and the answer is 0.75, with no units, because centimetres have cancelled. Write the working out as a fraction and it is very hard to divide the wrong way round.
Two phases and a race
Chromatography works because every substance in the mixture is pulled two ways. The paper is the stationary phase and holds substances back; the solvent is the mobile phase and carries them forward. A substance strongly attracted to the solvent and weakly held by the paper travels a long way. One held tightly by the paper barely leaves the line.
That is why the spots end up spread out along the strip rather than moving together, and why a mixture of four dyes gives four spots. It also means the order is not about size or weight — it is about the balance of those two attractions in the particular solvent you chose.
What the Rf value is for
Distances in centimetres are useless on their own, because a longer run of solvent moves every spot further. Dividing one by the other cancels that out and leaves a number that describes the substance rather than the experiment. Since a spot can never overtake the solvent carrying it, the answer is always between 0 and 1.
Rf values only match when the conditions match: same solvent, same type of paper, similar temperature. That is why an unknown is not identified by looking its Rf up in a book but by running a known reference sample alongside it on the same sheet. Two different substances can share an Rf by coincidence, and the way to expose that is to run the plate again in a second solvent.
Pencil, and the level of the solvent
The baseline is drawn in pencil because pencil is graphite, which does not dissolve and therefore does not move. A line drawn in ink would separate into its own spots and travel up the paper along with the samples, ruining the measurement it was supposed to anchor.
The solvent in the tank must start below the baseline. If the line is under the surface, the spots simply dissolve away into the solvent instead of being carried up the paper, and nothing separates. One more rule follows from the same reasoning: a pure substance produces a single spot in every solvent tried, which is exactly how chromatography is used to check purity.
- The range every Rf value falls in, because a spot cannot overtake the solvent carrying it
- 0 to 1The range every Rf value falls in, because a spot cannot overtake the solvent carrying it
Preguntas frecuentes
How do you calculate an Rf value?
Measure from the baseline to the centre of the spot, then from the same baseline to the solvent front, and divide the first by the second. The result has no units and lies between 0 and 1. If you get a number above 1, the two measurements have been divided the wrong way round.
Why must the start line be drawn in pencil?
Because graphite is insoluble in the solvent, so the line stays exactly where it was drawn. Pen ink is a mixture of soluble dyes, and it would separate and rise up the paper like any other sample, both contaminating the result and moving the line every measurement is taken from.
What does it mean if a sample gives only one spot?
It suggests the sample is a pure substance, since a mixture would separate into several spots. The evidence is stronger if it holds in more than one solvent — two substances can happen to travel the same distance in one solvent but rarely do so in a second with different properties.
Why does the solvent have to stay below the pencil line?
Because the spots need to be carried up by solvent soaking through the paper, not sitting in a pool of it. If the baseline is submerged, the samples dissolve straight into the tank and wash away. Nothing travels up the paper, and there is no separation and nothing to measure.
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