Pregnant patient wishes to make a folic acid syrup, is this stable and if so, how?

You do not give me a dosage. The following formula can be adjusted.  

Folic acid             100 mg

NaOH   0.1 N         2.5 ml   (à 2.8)  

Concentrated Hydroxybenzoate Solution FN 6   1 g  

freshly boiled and cooled water up to 100 ml  

note: 0.1 N NaOH = 4 G NaOH, aqua AD 1,000 ml

This solution should be compared in a dark brown red bottle and best kept in the refrigerator.  

Shelf life is up to 2   month; some opt for 1 month. pH = 8 is not ideal for the hydroxybenzoates!  

I find   in the Merck Index that bring folic acid in solution is with NaHCO3. The required amount of bicarbonate is easy to calculate. The advantage of this method is the pH obtained, which amounts to only   6.5 to 6.8. More favourable for the preservatives.  

To conclude the case, I must also cite that a suspension is an alternative. For this you can use the Excipiens viscosus sine Saccharo USP, which is described in my booklet Galensich Formularium on page 149. Given the small amount of folic acid, I would nevertheless prepare a 10% trituration with microcrystalline cellulose (Avicel ph 101 or 102) and suspend an aliquot of this trituration.