Because the USP monograph sets a 30 mL maximum container size for bacteriostatic water. The monograph requires that it be packaged in single-use or multi-use containers not larger than 30 mL. The largest vial any manufacturer can label as USP bacteriostatic water is therefore 30 mL, and the sizes in common supply are 3 mL, 10 mL and 30 mL.
Why the limit exists
The size cap is tied to the preservative. USP treats products that contain an antimicrobial preservative as small-volume products: the preservative is there so a container can be entered repeatedly, and keeping the container small bounds the quantity of preservative in any one container and keeps the in-use period of a multi-use container practical. Plain sterile water, which contains no preservative, is available in much larger containers precisely because it is meant to be used once and discarded.
What it means for laboratories that need volume
Since 30 mL is the largest vial, larger requirements are met by pack count rather than vial size: BAC Water Supply packs contain 4, 8, 12, 24 or 36 vials. A laboratory that uses about 300 mL a month, for example, would order ten to twelve 30 mL vials, or more, smaller vials if each preparation only needs a few millilitres. The 28-day in-use convention for an opened vial also means a 30 mL vial is only economical when it will be used up within that window. See pack sizes explained, how to estimate monthly usage and buying in bulk.
Related
How many mL are in a vial? · 30 mL vials explained · 3 mL vs. 10 mL vs. 30 mL
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