Practical investigations
Read our general notes on Risk Assessment
For my A2 investigation, I am investigating the effects of temperature on Vitamin C content in fruit juices. I started my experiment with the intention of using DCPIP - however, when titrating it with a standardising Vitamin C solution of known concentration, the colour change was incredibly difficult to detect - the solution just gradually turned slightly more orange because the solution was originally orange. In order to check whether this was just due to my DCPIP solution, I added pink food colouring to the Vitamin C solution to see if it did actually turn pink at all. Again, hardly a colour change.
I have been told that iodine is not as a good a method to use, but at least that gives a clear colour change. Is there really a problem with using that method? Or is there a way to make the DCPIP colour change more obvious to detect?
110208
Igloo replies ...
I am puzzled. Presumably you had the DCPIP solution in the burette and the standard vitamin C solution was colourless. At the endpoint the solution turns from colourless to pink. Perhaps you added them the other way round. If so, put the DCPIP solution in the burette next time. If I am barking up the wrong tree, there is another explanation. Perhaps your solid DCPIP is old and unreliable. Freshly purchased DCPIP really does give good and sharp end-points, even with fruit juice solutions which are mildly coloured.
If you’ve been using coloured solutions throughout and you’re still unhappy with the end-point, you could try again, this time decolourising the solution prior to carrying out the titration. To do this you can place a volume of the juice in a beaker, add two or three spatula fulls of activated charcoal, stir for a few minutes, and then filter. A pipette can then be used to draw up known volumes of the filtered decolourised solution, followed by a titration with your DCPIP.
If all else fails you can certainly use iodine (together with starch) since the end point is unmistakable. The only problem is that iodine can oxidise other material in the juice apart from vitamin C, whereas DCPIP is more specific.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
back to Practical investigations
Rate this page or react
Share your views on this page, 0 ratings so far
updated: 12 February 2008
