Science Buddies: "Ask an Expert"

Hello
I need help doing some pre-lab calculations for the "Investigate the Vitamin C Level in Bell Peppers During Various Stages of Ripeness" experiment.
So for my experiment I was going to use DCPIP as a titrant (this is the only titrant our school is allowing us to use). We have been told that we can use a concentration of 0.1% w/v or below.
We are supposed to do a back calculation which is based on the known amount of analyte (e.g. vit C) in red bell peppers (127.7mg/100g), determine how much titrant you will need per trial - if this titrant volume is very high, that means you need to dilute your food sample since your food item is very rich in vit C - this also means you need to complete a sample calculation for the dilution of your food item such that it allows you to use a more reasonable quantity of titrant per trial.

However I have been having difficulty in figuring out how to do this. My teacher said that the titrant should not exceed more than 10mL per trial.
Could you please help to outline steps that I can take to attain the correct calculations.

Hello anna00030,

I'm assuming that you're working on the project described here:

https://www.sciencebuddies.org/science- ... ss#summary

And from your post it sounds like you've worked out some of your experimental procedure. You might want to review the titration tutorial for some additional information:

https://www.sciencebuddies.org/science- ... -titrating

The calculations you asked about can be worked out in the following steps.

What is the molar concentration of DCPIP that you will use?
You mentioned 0.1% wt/vol. This equates to 1 g of DCPIP per liter of water. Use the molecular mass of DCPIP to convert to moles per liter.

How many moles of DCIP are in the 10 mL addition you want in your titration?
Use the molar concentration from above to calculate the number of moles in 10 mL of the DCPIP solution.

How many moles of vitamin C (ascorbic acid) should you titrate?
One mole of DCPIP reacts with one mole of vitamin C. So the solution you titrate should contain the same number of vitamin C moles as the number of DCPIP moles calculated in the previous step.

What mass of vitamin C is being titrated?
Use molecular mass of vitamin C and the number of moles from the previous steps to compute the mass of vitamin C that should be titrated.

What mass of vegetable material is needed?
Use the bell pepper vitamin C content (127/7 mg/100 g) and vitamin C mass to be titrated to calculate the mass of bell pepper needed for your titration solution.

I hope this helps. Please post again if you have more questions.

A. Norman