chemistry

Purple of Cassius

Purple of Cassius is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Purple of Cassius rather than just read about it. In short: Purple of Cassius is a purple pigment formed by the reaction of gold salts with tin(II) chloride. It has been used to impart glass with a red coloration (see cranberry glass), as well as to determine the presence of gold as a chemical test.

Key takeaways

Purple of Cassius belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Purple of Cassius to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Purple of Cassius from memory before moving on to harder problems.

Reference excerpt

Purple of Cassius is a purple pigment formed by the reaction of gold salts with tin(II) chloride. It has been used to impart glass with a red coloration (see cranberry glass), as well as to determine the presence of gold as a chemical test. Generally, the preparation of this material involves gold being dissolved in aqua regia, then reacted with a solution of tin(II) chloride. The tin(II) chloride reduces the chloroauric acid from the dissolution of gold in aqua regia to a colloid of elemental gold supported on tin dioxide to give a purple precipitate or coloration. When used as a test, the intensity of the color correlates with the concentration of gold present. This test was first observed and refined by a German physician and alchemist, Andreas Cassius (1600–1676) of Hamburg, in 1665. Berzelius later made a detailed study of the purple of Cassius. The colour also attracted attention from Michael Faraday. Richard Adolf Zsigmondy, who earned the 1926 Nobel Prize for chemistry, says that "Several of the red gold divisions prepared with formaldehyde as well as those reduced with phosphorus appeared perfectly clear in ordinary daylight (like good red wine). They did not settle out their gold, and I was therefore able to call them rightly chemical solutions. In Thomas Graham’s dialysis, however, they behaved like colloidal suspensions: the gold particles did not pass through the parchment membrane. This showed my gold divisions their proper place, namely, that they belonged to the colloidal suspensions." Recently, a new way for production of the pigment SiO2:AuNPs by using heating of sol-gel composites has been proposed. The physicochemical procedure, used for preparation of the pigment, allows also the production of a week purple color, arising from cristobalite doped with AuNPs. The optical properties of the composites are explained by the theory of Gustav Mie.

References

Partially translated from the German Wikipedia article, Goldpurpur.

L. B. Hunt (1976). "The True Story of Purple of Cassius" (PDF). Gold Bulletin. 9 (4): 134–139. doi:10.1007/bf03215423. S2CID 138485909.

Worked examples

Example 1 — a first encounter with Purple of Cassius

Start with the simplest possible case. Write down what Purple of Cassius claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Purple of Cassius before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Purple of Cassius ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Purple of Cassius

In research: Purple of Cassius appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Purple of Cassius in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Purple of Cassius is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chemical tests, Glass dyes, Gold, so understanding it makes those chapters shorter.
In everyday life: Look for Purple of Cassius outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Purple of Cassius in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Purple of Cassius means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Purple of Cassius out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Purple of Cassius in simple terms?

Why does Purple of Cassius matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Purple of Cassius?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Purple of Cassius.