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Hi. I'm doing an investigation on the vitamin C and total acid content of various orange juices. I am using DCPIP to test vitamin C, and NaOH for the overall acid content. Are these chemicals the best to use for these tests? If not, what can I use?
Would you also be able to tell me the best ways to get a standard for these two tests? I've read some of the other questions on the site and am still not so clear as how to do so. I've been having trouble with the DCPIP. I have used 20 ml of orange juice (diluted to 1:25) and I only add 1 drop of DCPIP and the solution goes pink. Does this class as the end-point? Of should I aim for the blue colour? Are you aware of any common percentages/ concentrations for these two tests?
020608
Igloo writes ...
Although there are other types of titration available for vitamin C analysis, your choice of DCPIP is fine. It is a discriminating enough reagent to give reliable results. As for the total acid content, sodium hydroxide is also a good choice.
In both cases you need to standardise your two reagents. With DCPIP, you can use a solution of pure vitamin C made up to a known concentration (200 mg dm-3 is a common choice since fruit juice samples are often of a comparable concentration). Carry out a titration with this first (with the DCPIP solution in a burette), and then you can carry out subsequent titrations with your orange juice samples. Comparison of your titres (by mathematical proportion) will enable you to deduce the concentrations of vitamin C in the fruit juices. As for the sodium hydroxide, you need to use a reliable primary standard such as potassium hydrogenphthalate.
Pink is certainly the endpoint, not blue. As well as being a redox indicator DCPIP also acts as an acid-base indicator and is coloured pink in acidic solution (e.g. a solution of orange juice).
If your endpoint is obtained after only a drop has been added, then your DCPIP solution must be too concentrated. Have you successfully standardised it against a standard solution of vitamin C?
As for typical values, I usually quote a value in the region of 100 - 400 mg per 100 cm3 of juice, and this range is a very wide one because so much depends on whether the juice is freshly prepared by liquidising a fresh orange, is a juice concentrate, or is simply a commercial sample of “orange drink”.
I thought you might be interested in having a look at the following website which classifies various fruits according to their vitamin C content. It also gives typical values expressed in mg of vitamin C per 100g of pulped fruit
http://www.naturalhub.com/natural_food_guide_fruit_vitamin_c.htm
Good luck with the rest of your experimental work.
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updated: 08 June 2008
