Not as a labeled requirement, but keeping vials in their carton is good practice. Bacteriostatic water is supplied in clear glass vials, and most labels specify only controlled room temperature and protection from freezing. Water itself is unaffected by light. The preservative is the reason to be careful: benzyl alcohol oxidizes slowly to benzaldehyde and benzoic acid, and light and dissolved oxygen both accelerate that reaction.
Sealed vials
In a sealed vial the oxygen available is limited to the small headspace, so the reaction is slow and the manufacturer’s stability studies, done in the clear vial, already account for ordinary indoor light. There is no need for amber glass or foil. Direct sunlight on a windowsill is a different matter, mainly because of the temperature it produces rather than the light itself. See controlled room temperature explained.
Opened vials
Once a vial has been entered, each withdrawal admits air, so oxygen is no longer limiting and light becomes a larger factor in how fast benzaldehyde forms over the 28-day in-use period. A vial that will be entered repeatedly is best kept in its carton or a closed drawer between uses. This matters most where the solution will be used for ultraviolet measurements, because benzaldehyde absorbs UV far more strongly than benzyl alcohol. See benzyl alcohol oxidation and bacteriostatic water as a UV-Vis blank.
What to do
- Store cartons closed, on a shelf away from windows and heat sources, at 20–25 °C.
- Return an opened vial to its carton between withdrawals.
- Follow the label if it does state a light requirement; the label always governs.
Related
How to store bacteriostatic water · Does bacteriostatic water need to be refrigerated?
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