Ultramarines are bright blue pigments based on a zeolite (socialite) that hosts the colour centres [S2] and [S31 . Ultramarines include lapis lazuli and have been in use for over 5500 years. The UV-VIS spectrum of fS2]“ exhibits a broad band centred at 370 nm, while |S3] ' absorbs at 595 nm.
(a) Draw Lewis structures for [S2]~ and [S3]–. What shape is the [S3] ion?
(b) Why can EPR spectroscopy be used to study these ions? Suggest a reason why both ions behave isotropically even at low temperatures.
(c) Explain why an ultramarine containing only [S3]– colour centres appears violet-blue in colour.
(d) Why does [S2]– contribute to the colour of ultramarine pigments even though λmax is 370 nm,i e. in the UV region?
(e) In synthetic ultramarines, the ratio of [S3]–:[S2]– can be altered to produce pigments ranging from violet-blue through blues to green. Account for this.
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