Byproducts in Indigo Synthesis? I have been messing around with making Indigo dye while waiting for bacteria to grow. I have been following the Baeyer-Drewson method. Basically, you mix nitrobenzaldehyde with an acetone water mix and slowly add NaOH. The solution turns black and you get a dark sludge. I have been separating the sludge from the liquid by centrifugation. I have noticed that the supernatent has a dark reddish brown color.


I was wondering if anyone knew what the red byproduct was? 
My indigo is black when dried, wikipedia shows nice bright blue chunks. What could be causing this color difference? Maybe the red byproduct? 
Does indigo dissolve in anything without needing to be reduced first? I have had no luck with the organic solvents available in our lab.
After looking further into indigo production, I came across a related structure called indirubin.

Indirubin:

Indigo:


Indirubin is a byproduct of indigo synthesis and has a reddish color. It may also be more soluble than indigo, and indirubin derivatives have some anticancer activity. If you trust this supplier, you can buy indirubin for pretty cheap to compare with your lab's indigo reactions.



I also found a study done on old native american dyes that studied indigo and indirubin in fabric fibers, among other dyes. They provide a UV-VIS spectrum of both compounds, with Ind = indigo and Inr = indirubin. We can see that indigo has an absorption peak in the red region, so blue is reflected. Indirubin has a peak at around 540 nm, which is yellow-green, allowing some blue and red to reflect, but probably more red than blue. This paper also references a 1910 paper** studying indirubin levels in commercially available indigo samples, but I can't find that article. They suggest that higher levels of indirubin are produced when more indoxyl is converted to isatin. If you can find this 1910 article, it may provide some insight into reducing isatin and indirubin levels.



** William Popplewell Bloxam and A. G. Perkin, J. Chem. Soc., Trans., 1910, 97, 1460-1475.