Ink chromatography
Black ink is rarely one dye. It is usually a blend of several coloured dyes that together absorb nearly all light. When water creeps through the paper, each dye travels at its own pace, depending on how strongly it clings to the paper fibres, and the black comes apart into bands of the colours it was hiding. Type a word and drop water on it, or run the classic school experiment on a paper strip.
The four dyes
| Dye | Rf (this solvent) | Measured |
|---|
Rf is how far a dye travels divided by how far the solvent front travels. Every ink here is a different mix of these four dyes, so each ink comes apart into a different set of bands. The paper strip measures Rf from the bands as they form.
Why black comes apart
- Ink is a mixture. A yellow dye absorbs blue light, a magenta absorbs green, a blue absorbs red, and together they leave almost nothing to reflect. That is why marker black often looks slightly purple or green when it is thin.
- Two phases. As the solvent moves through the paper, each dye molecule keeps switching between being carried along in the liquid and sticking to the cellulose fibres. A dye that spends most of its time dissolved rides near the front; one that clings lags behind. That fraction is its Rf.
- Change the solvent, change the order. Water carries the polar dyes; alcohol prefers the less polar ones. With rubbing alcohol selected, the bands here come out in a different order. Chemists choose a solvent to pull apart exactly the substances they care about.
A short history
In the 1850s Friedlieb Ferdinand Runge let drops of dye solutions spread on blotting paper and published the patterns as "pictures that paint themselves". In 1903–1906 the Russian botanist Mikhail Tsvet separated leaf pigments by washing them through a column of chalk. The bands of green and yellow gave the method its name: chromatography, "colour writing". In 1944 Consden, Gordon and Martin introduced paper chromatography to separate amino acids, and Archer Martin and Richard Synge shared the 1952 Nobel Prize in Chemistry for partition chromatography. Its descendants (gas chromatography, HPLC, the columns in a drug-testing lab) are among the most used tools in chemistry, and forensic examiners still compare inks this way.
How the page works, and what is made up
The page uses the watercolour model from the watercolour text page: water flows over the paper and wicks along its fibres by invasion percolation. Four dyes are tracked instead of one, each carried with the wicking water at its own fraction of the water's pace, its Rf. Colour comes from Beer–Lambert absorption, multiplying each dye's absorbance on white filter paper.
The dyes, their colours and their Rf values are illustrative, not measurements of real commercial inks. Real Rf values depend on the paper, the solvent mix and the temperature. The measured column shows that the simulation reproduces the Rf it was given, which is a check on the model, not on chemistry.