How much Sodium Benzoate should I add to preserve 200ml of Orange juice?
From NCBI (National Center for Biotechnology Information):
Sodium benzoate (0.4 g) was added to 400 mL orange juice.
So 200 ml of orange juice, being half the amount of 400 ml, may need 0.2 g of sodium benzoate, which is half of 0.4 g.
"Concentration as a food preservative is limited by the FDA in the U.S. to 0.1% by weight"
So, m maximum of 0.1% per weight of the weight of your 200ml of orange juice.
Let's say 200ml of juice is at least 200g (more or less equivalent to water), you'd need to add at most 0.2g of Sodium Benzoate.