Question

A pharmacist is asked to formulate the simple syrup containing 55% (w/v) of sucrose. Answer the following questions regarding the compounding of this formulation:

a) Can this formulation be considered self-preserving? explain your answer

b) How much alcohol USP is required to add to this formulation if the prescription calls for 750 mL of the syrup. Show calculations.

Answer 1

Ans. #A. No

The minimum sugar content in a neutral syrup (i.e. there is no other preservative or ingredient other than sugar) required for self-preservation is 85.0 % (w/v).

Since the given sugar content is less than 85 % (w/v), the 55% (w/v) formulation is most likely to exhibit microbial contamination and deterioration upon exposure to air.

#B. Mass of sucrose (simple sugar) in 750 mL syrup = 55 % (w/v) of 750 mL

                                                            = (55 g / 100 mL) x 750 mL

                                                            = 412.5 g

# Now,

            Volume preserved by sugar, V1 = 412.5 g x (0.53 g/ mL) = 218.625 mL

            Volume occupied by sugar, V2 = 412.5 g x (0.647 g/ mL) = 266.8875 mL

            Free water equivalence = Total Volume – (V1 + V2)

                                                = 750.0 mL – (218.625 mL + 266.8875 mL)

                                                = 264.4875 mL

# The volume of water V1 and V2 are NOT available to microbes, so can’t get contaminated and remains self-preserved. However, the remaining “free-water equivalence” is susceptible to microbial contamination.

The alcohol content required to preserve free water = 18.0% (v/v)

So,

            Required amount of absolute alcohol = 18.0 % (v/v) of free water equivalence

                                    = (18.0 mL absolute alcohol / 100.0 mL water) x 264.4875 mL water

                                    = 47.60775 mL

Therefore, required volume of absolute alcohol = 47.61 mL

Ref: https://pharmlabs.unc.edu/labs/solutions/preserving.htm