First, I'd like to thank Alejo for posting this page. It is very helpful. Then, I' d like to refer you to the comments page in compound chem ( dated May 18,2017 from the author of the page),for this procedure. One of the comments was that the indicators tendeded to "go off" after a while. The author responded by saying he extended the time by refrigerating the indicator solution. Another comment mentioned freezing the solutions into Ice cubes to extend life. The first comment( about the solutions going off after a short period of time) unfortunately matches my experience. In my case, the time was a few days. In industry, pure indicator solutions usually are dissolved in alcohol. I don't know if adding some alcohol to these solutions would extend their life or not.

Hi @Ag8n! Thanks for your reply! I had this cabbage solution in fridge almost a week and I didn't see an appreciable color change. I don't know if alcohol could be a good preservant, I'll would try it.

I did this! We tested vinegar, baking soda, lemon juice, pickle juice, and carbonated water. A lot of acids easily available in the kitchen. Then we mixed it all together at the end 😃

For a couple other reads, look up "The Effect of pH on Color Behaviour of Brassica oleracea Anthocyanin" in Journal of Applied Sciencies, volume 11(13):2406-2410,2411. They use a colorimeter based on human vision, so the coordinate system may not be familiar. But it is valid. Another quick and simple prep method is scienceprojectlab.com/red-cabbage-indicator.html. I was hoping these could be used as titration indicators. That usually means storing in the solution phase for at least a month. The liquid form does not appear to be stable. Although the"Applied Science" paper did have a dried form ( probably purchased).

Interesting paper on this same subject. " Application of Butterfly Pea extract as an indicator of acid - base titrations " by Nyi Mekar Saptarini et. al., Journal of Chemical and Pharmaceutical Research 2015 7(2):275-280. It also used water extractions and gives very good information on using these flowers for titrations. No expiration date is given for the solution(not a good sign).

Some natural indicators can have shelf lives of up to 90 days in solution. See "Hippeastrum hybridum anthrocyanin indicators of acid - base titrations" by Robert Byamukama et. al., Int. J. Biol. Chem. Sci. 10(6): 2716-2727 Dec 2016

Check this reference: "Evaluation of red cabbage dye as a potential natural color for pharmaceutical use" by Neela Chigurupati, et. al. Industrial Journal of Pharmaceutics, 241(2002)293-299.

You need to go to section 3.4 "Stability of red cabbage solutions in water". To quote "the dye was most stable at room temperature and pH3(percentage of degradation was only 1-5% over a period of 10 days). However, it was least stable at 50C and pH 8(percentage of degradation was 79-98% over a period of 10days)." Section 3 .5 had some interesting info on using it as an indicator, as well. So, at the very least, the cabbage indicator needs to be stored in acidic media.

Clarification. Acidic media is pH 3.

The acid (from the pH 3 solution ) for the cabbage indicator could mess up some titrations. A blank might need run(and, depending on the titration, the blank could be substantial). A better approach might be freezing the indicator. Then adding the frozen indicator to the flask and letting it melt before titrating. Freezing was mentioned in one of the first comments on compound chem's page. But I've never tried it.

The research shows that natural titration indicators, with good shelf life, do exist ( see Hippeastrum, above). However, Red cabbage does not appear to be one of these indicators.

The data shows that natural titration indicators with long shelf life (90 days) do exist ( see the Hippeastrum note above). However, Red cabbage does not appear to fit in this category.