2019 Year 12 IB Extended Essays

B ACKGROUND R ESEARCH L IESEGANG RINGS DISCOVERY

Chemical, physical, and biological pattern formation, such as Agate stones and Zebra stripes has been an interest within the scientific community especially in the 20 th century. Yet even more intriguing have been reactions, know as periodic precipitation reactions, in which chemical patterning is seen (Kemsley, 2011). One of the first mentions of such chemical patterning was done by German chemist Frederic Ferdinand Runge in 1855. Runge used a simple filter-paper wet by an electrolyte solution to create such patterning or as he called it “self-grown pictures” (Bussemas, 1994). Yet Runge’s work on such reactions came too early as precipitate patterns fell off the scientific radar until 1896 were Raphael Eduard Liesegang, a German photographer and chemist, reintroduced the fascination for such reactions. Liesegang was working with gelatine layers and potassium-dichromate when he accidently dropped a crystal of silver-nitrate into a specimen of the gelatine only to find a uniquely ring- shaped precipitate to have formed. Liesegang further investigated this phenomenon and as his studies on such precipate patters were the first, the phenomenon came to be called the Liesegang phenomenon or the Liesegang rings (Büki, 2007). Liesegang rings are a series of usually concentric bands of precipitate which can be formed in gels but can be seen in other mediums within nature (Encyclopaedia Britannica, 1998). For a precipitation reaction to occur cations and anions combine within a aqueous solution to form a solid called a precipitate. The reason the solid becomes visible is because it is insoluble (Schaffer & Herman, 2019). The formation of such precipitate can vary from reaction to reaction. Factors like temperature and concentration can affect formation. This is important to note in consideration with Liesegang ring formations as they are an example of a precipitate reaction. C HEMICAL O SCILLATION L IESEGANG RINGS

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