70% isopropyl alcohol is the better choice for wiping the stopper of a bacteriostatic water vial. The intuition that a stronger solution must work better is wrong for alcohols: the 30% water in a 70% solution is what makes it effective.

Why 70% outperforms 99%

• Contact time. Alcohol kills microorganisms while it is in contact with them. 99% isopropyl alcohol evaporates from a stopper in a few seconds, before it has had time to act. The water in a 70% solution slows evaporation and keeps the surface wet for the 20 to 30 seconds that give a useful kill.
• Penetration. Alcohol works by denaturing proteins and disrupting membranes. Very high concentrations denature the outer proteins of a cell so fast that they form a protective layer, and the alcohol does not reach the interior. The water in 70% carries the alcohol through the cell wall so the whole cell is affected.
• Spores. Neither concentration is reliable against bacterial spores; a stopper wipe is a reduction step, not a sterilization step. This is one reason clean technique at every entry still matters and why the preservative in the vial is there at all.

Technique

1. Use a fresh 70% isopropyl alcohol swab or a lint-free wipe wetted with 70% isopropyl alcohol; do not reuse a swab between vials.
2. Wipe the exposed center of the stopper with one firm pass. Scrubbing back and forth redistributes whatever was on the stopper.
3. Let the stopper air-dry completely, about 30 seconds. Drying is when most of the antimicrobial action happens, and it prevents alcohol being carried into the vial. Do not fan or blow on it.
4. Enter the stopper with a fresh sterile transfer device only once it is dry.

What 99% isopropyl alcohol is for

Anhydrous isopropyl alcohol has its place: cleaning electronics and optics where water would cause damage, and applications where a residue-free, fast-evaporating solvent is wanted. Disinfecting a rubber stopper is not one of them.

Stopper materials and alcohol

Bromobutyl and chlorobutyl stoppers tolerate isopropyl alcohol well; a brief wipe does not affect them. Repeated soaking is a different matter and is not part of normal practice. See vial stoppers explained.

Related

How to open a bacteriostatic water vial · How many times can a vial be punctured? · Common mistakes

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