Why is indigo insoluble in water but indigo carmine is soluble?

Answer 1

Indigo pigment derived from the indigo plant. It must be carefully extracted and it requires optimum condition during treatment (before which it's invisible). Indigo carmine is a bluish colored derivative of the cochineal insect (carmine is normally red to burgundy). To the best of my knowledge, they are two completely different substances with different properties.

Carmine is also insoluble in water and oil; though it dissolves in mild alkali solutions.

Carmine comes mainly from bugs in Peru and the Canary Islands. Inigo comes from a number of regions, including France. Indigo production is mainly an artesian affair while carmine is used in food and beverages, cosmetics, and microbial stains. One company, Carminic Industries, is unique in that it markets carmine products derived from microbes. They might be based in either California or Oregon.

In short, they are different substances.

I'm not quite sure if that was what you were looking for, but that's what I know. Hope it helped.

Answer 2

Indigo is insoluble in water due to its structure, which lacks polar groups facilitating water interaction. Indigo carmine, on the other hand, contains sulfonate groups that increase its solubility in water by forming ion-dipole interactions with water molecules.