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I asked about the colour of sunset yellow in water in the previous post (Changes of colour of sunset yellow in water).

And here, I would like to raise another question based on sunset yellow.

Sunset yellow powder comes in several different colours, including, red, orange and yellow. And as mentioned, mine is red. I am intrigued by the reason behind the colour variations.

I come up with two possibilities (and feel free to point out my mistakes):

  1. Sunset yellow is in crystalline form. And the colour variation is caused by the amount of water of crystallisation in the crystals. This idea is attributed to the fact that anhydrous copper(II) sulphate and hydrated copper(II) sulphate show different colours. And I wonder if this phenomenon can be applied to sunset yellow.

  2. The manufacturers treat the sunset yellow powder with acidic chemicals, since sunset yellow, as a pH indicator, shows red-orange colour in acidic solution.

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