Herapathite Explained

Herapathite, or iodoquinine sulfate, is a chemical compound whose crystals are dichroic and thus can be used for polarizing light.

It was discovered in 1852^[1] by William Bird Herapath, a Bristol surgeon and chemist. One of his pupils found that adding iodine to the urine of a dog that had been fed quinine produced unusual green crystals. Herapath noticed while studying the crystals under a microscope that they appeared to polarize light.^[2]

In the 1930s, invented a process to grow single herapathite crystals large enough to be sandwiched between two sheets of glass to create a polarizing filter; these were sold under brand name "Bernotar" by Carl Zeiss. Herapathite can be formed by precipitation by dissolving quinine sulfate in acetic acid and adding iodine tincture.^[3]

Herapathite's dichroic properties came to the attention of Sir David Brewster, and were later used by Edwin H. Land in 1929 to construct the first type of Polaroid sheet polarizer. He did this by embedding herapathite crystals in a polymer instead of growing a single large crystal.

Structurally, herapathite consists of quinine (in a cationic doubly-protonated ammonium form), sulfate counterions, and triiodide units, all as a hydrate. They combine as 4C₂₀H₂₆N₂O₂•3SO₄•2I₃•6H₂O, or sometimes other ratios and higher polyiodides.^[4]

Further reading

• Bernauer, F. (1935). "Neue Wege zur Herstellung von Polarisatoren". Forschritte der Mineralogie, Kristallographie und Petrographie Neunzehnter Band.
• Land . E.H. . 1951 . Some aspects on the development of sheet polarizers . Journal of the Optical Society of America . 41 . 12. 957–963 . 10.1364/josa.41.000957.
• Marks . A. M. . 1969 . Electrooptical Characteristics of Dipole Suspensions. Applied Optics . 8 . 7. 1397–1412 . 10.1364/AO.8.001397 . 20072446 . 1969ApOpt...8.1397M.

Notes and References

1. W. B. Herapath. XXVI. On the optical properties of a newly-discovered salt of quinine, which crystalline substance possesses the power of polarizing a ray of light, like tourmaline, and at certain angles of rotation of depolarizing it, like selenite. 1852. Phil. Mag.. 3. 17. 161–173. Taylor & Francis. London. 10.1080/14786445208646983.
2. Kahr . Bart . Freudenthal . John . Phillips . Shane . Kaminsky . Werner . 2009 . Herapathite . Science . 324 . 5933. 1407 . 10.1126/science.1173605. 19520951 . 2009Sci...324.1407K . 220089099 .
3. Book: Gabba, Luigi . 1884 . Trattato Elementare di Chimica Inorganica ed Organica . Elementary Treatise on Inorganic and Organic Chemistry . it . Francesco Vallardi . 516 .
4. C. D. . West . Crystallography of herapathite . American Mineralogist . 22 . 5 . 1937 . 731–735 .