Sciencemadness Discussion Board

Quantitative analysis of sorbic acid

goldberg - 7-8-2020 at 09:30

I'm looking for ways to determine concentration of sorbic acid salts in aqueous solution that
also contains caproic acid.
Sorbic acid contains two double bonds that can be oxidized. I do not have access to gas nor chromatography so i need to exclude this method.

Potassium permanganate can oxidize double bonds, but in basic solution(needed to keep sorbic acid in solution as a salt) it will yield green manganate ions so determination of endpoint of titration would be hard.
Unfortunately i do not have barium salts that could be used to precipitate insoluble barium manganate and getting clear solution. How about EDTA or other chelating agent to decolorise this solution?

Or maybe spectrophotometric determination in visible light after addition of constant amount of basic potassium permangnate?

Or maybe some electrochemical method like cyclic voltamperometry?

I'm intrested also in general case of determination of alkenes in aqueous solution.
I do not have ICl so i can not use iodine number method.

Any ideas?

Pumukli - 7-8-2020 at 10:09

Bromatometry? Known ammount of Br2 (in excess), bromine consumed by the addition to the double bonds, excess Br2 determined by an appropriate method.

unionised - 7-8-2020 at 14:44

Sorbic acid is fairly soluble- about 2 g / litre if I remember rightly.
That's about 0.01M
And you should be able to titrate that.

Failing that, you can do titrations in mixtures of acetone/ water.
If you are really stuck, you can do the titration in a 2 phase system.
You need to do it slowly, with lots of shaking.

I guess you can't do UV spectroscopy which is a pity.

Boffis - 8-8-2020 at 00:21

The method of chlorination I desribed in the sorbic acid thread was developed from a method of analyzing sorbc acid in food using sodium chlorite and hydrochloric acid and then titration of the excess oxidizing capacity. Check it out. You could probably do something similar with NaBr/NaBrO3 and titrate the excess bromine with iodide + thiosulphate or sodium sulphite at a push.

There are a lot of methods as this is a commercially important analyte in food and soft drinks so the methods tend to be tolerant to impurities.

[Edited on 8-8-2020 by Boffis]

goldberg - 8-8-2020 at 00:32

Thanks. I'm thinking which method is better: permanganometry or bromatometry?
Solution of potassium permanganate is not very stable as analytical reagent, the question is how long i can store it?
Preparing each time fresh solution and titration with sodium oxalate would be quite labour intensive.

OTOH titration of iodine is a little tricky because iodine escapes from flask as a gas introducing error.
Permanganate method seems to be simpler for me but i can be wrong.

outer_limits - 8-8-2020 at 02:22

My analytical textbook says that the concentration should be checked every 3-4 months.
This paper shows that the rate of degradation could be even so small as 1% in 4 years (