How many grams of anhydrous dextrose should be used in preparing 1 liter of a 1/2% isotonic ephedrine sulfate nasal spray?

Question

Provide step-by-step rationale and context/indications about why a certain approach to this problem is the correct one.

Answer 1

0) Isotonic refers to osmotic strength; this is a colligative property, so #discrete particles (ions or neutral) per L is what this refers to.

1) Determine #particles/L for "isotonic", from standard consideration that 0.9%w/v NaCl solution is isotonic (you may be able to find more precise datum!), bearing in mind that NaCl has 2 particles per formula unit!

2) Determine molarity of 1/2% ephedrine sulfate. You may need to check pKa's etc. Then determine #particles/L just from ephedrine sulfate.

3) From the total #particles/L for isotonic in (1), subtract the #particles/L for the stated conc of ephedrine sulfate.

4) The difference must be made up by dextrose; use the MW of anhydrous dextrose to calculate how much you need.