They can be combined without any chemical reaction, but the result is not bacteriostatic water any more. Sterile water contains no preservative, so mixing it with bacteriostatic water dilutes the benzyl alcohol below the 0.9% at which its effectiveness was tested. Equal volumes give 0.45%; one part bacteriostatic water in ten gives 0.09%. Below the tested concentration the preservative cannot be relied on, and the mixture has to be treated as unpreserved: used once and discarded.

Two mistakes the question usually hides

• Topping up a preserved vial. Adding sterile water to a partly used vial of bacteriostatic water to make it last longer does the opposite: it lowers the preservative concentration in the whole vial and introduces a new opportunity for contamination.
• Making sterile water “preserved”. Adding some bacteriostatic water to a container of sterile water does not preserve it. Even a large proportion of bacteriostatic water leaves the mixture below 0.9%, and a small proportion contributes almost nothing.

When a mixture is fine

A mixture is acceptable when the preservative is not being relied on: for example, when a solution is being prepared with a specified total benzyl alcohol content for an assay blank, or when the mixture will be used immediately and discarded. In those cases the proportions are recorded and the mixture is labeled with its actual benzyl alcohol concentration and a same-day discard time. See what 0.9% means in other units and labeling and record-keeping for prepared solutions.

What to do instead

If the goal is more preserved diluent, open a new vial of bacteriostatic water. If the goal is a preservative-free diluent, use sterile water on its own. If a vial is regularly left unfinished at 28 days, move to a smaller vial size rather than stretching a larger one. See preserved or preservative-free: a decision guide and how labs choose a vial size.

Related

Bacteriostatic water vs. sterile water · Shelf life after opening

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