Subject: Testing Vitamin C in Orange Juices using Acid/Base Titration

I want to test the concentration of vitamin c (ascorbic acid) 
using an acid/base titration using 0.1 mol/L NaOH and bromothymol 
blue indicator.  However I'm aware that using this titration, 
there may be a problem because of the presence of citric acid.  
The NaOH will react with the citric acid (twice) before reacting 
with ascorbic acid.  How do I separate the citric acid from 
orange juice?  Are there common indicators (that can be found in 
a high school lab) that can indicate when the first two citric 
acid has been neutralized?  

THANX

Re: Testing Vitamin C in Orange Juices using Acid/Base Titration