Hi Elaine,

You have not mentioned the solvent that is used for sonication and also the concentration. Due to a high concentration in the water medium, it turns into dark brown. Check with UV-Vis spectroscopy, apart from graphene oxide peak, any other peak present or not.

Hello, Elaine.

Actually, the 'pristine graphite oxide' reacts with water to gain more conjugated structure, thus darkens the color of the solution of GO. 

Please see the following article for more details.

Pristine Graphite Oxide

http://pubs.acs.org/doi/abs/10.1021/ja211531y

maybe dehydration has occurred. Did you coagulate GO by adding ether? Did you use concentrated HCl during work-up?  

Thank you all for your feedback. I Only used water, no ether and I used only conc H2SO4 during work up. 5% HCL only for washing. However, during the previous months I have worked on GO, I noticed that ambient temperature has an effect on my GO quality, hence, I have adjusted my procedure whenever it is winter time. No matter how much effort we put in controlling humidity and temperature in our work environment, I have noticed some variances the color of the GO although the XRD, FTIR, SEM and most other analyses confirm that the structure and the quality is almost the same. I guess, nothing really to worry about on color, but I am glad I have raised this so that those who would work on GO will also get an idea when dealing with it.