Question

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

Answer 1

Answer:

I am trying to make indigo aluminum lake for food use, and the dissolution of indigo just get me stuck. As declared in papers and in wikipedia , the indigo does not dissolve in water or have quite quite low solubility in water (990 µg/L according to wikipedia ). That is why indigo carmine has been used as a substitute. However, my indigo dyes from both sigma (purity > 95% )and a local reagent company in China (purity >97%) get pronounced solubility. 0.1g of the two indigo dyes were respectively added into 100g DI water, and after stirring without heating for 30 min, very deep dark blue solution appeared. I left the solution on bench for two hours, and only a small amount of particles precipitated on the bottom of the beaker. 10 ml of the supernate was added into 100 ml DI water, and clear blue solution appeared without sediment. It looks the indigo in the supernate was dissolved. The diluted indigo solution (10 ml into 100 ml) was detected using a uv-visible spectrometer in wavelength rang from 200 to 800 nm. The adsorption spectra of the indigo (from sigma)solution with two further diluted solutions were as shown in the attached figure. The indigo from local reagent company had quite similar spectra like the one from sigma (not shown here). What bring me more confusion is that the absorption peak in the visible wavelength rang located at ~695 nm. According to the color wheel theory, the complementary color for the ~695 nm is green, but what i observed by eyes is blue. All the questions are really puzzled, but i feel there must be something behind this. Is there anyone could help me ?

Answer 2

Answer:

Steeping the indigo leaves in warm water breaks down the waxy coating on the leaves releasing indican into the water. Adding soda ash raises the pH making the solution alkaline and transforming indican into molecules of indoxyl. ... This pigment is then insoluble in water and it settles on the bottom of the container.