Lyophilized (freeze-dried) research materials are supplied as a dry cake or powder that has to be brought back into solution before use. Bacteriostatic water is one of several diluents used for this. It is the right choice only when the material’s documentation specifies it, or when a preserved solution that will be drawn from more than once is wanted and the preservative is compatible with the material. This guide describes the general laboratory procedure. It does not cover any particular material: the data sheet supplied with the material governs the diluent, the concentration and the storage of the resulting solution.
1. Confirm that bacteriostatic water is the specified diluent
Check the material’s data sheet or certificate of analysis for the recommended diluent. Benzyl alcohol is not compatible with everything: it interferes with some assays, absorbs ultraviolet light near 257 nm, and is not used for cell-culture work. If the data sheet specifies plain sterile water, a buffer or an organic solvent, that is what is used. See when the preservative interferes and bacteriostatic water and cell culture.
2. Calculate the volume
Volume of diluent (mL) equals the amount of material (mg) divided by the target concentration (mg/mL). For example, 5 mg of material at a target concentration of 1 mg/mL needs 5 mL of diluent. A vial of bacteriostatic water holds slightly more than its labeled volume (see fill volume and overfill), so choose a vial size that covers the calculated volume without leaving a large opened remainder that will pass its 28-day in-use limit unused. The vial-size recommender does this arithmetic.
3. Technique
- Bring both the material and the water to room temperature. Inspect the water: clear, colorless, within its expiration date and, if previously opened, within 28 days of opening. See what color should bacteriostatic water be?
- Wipe both stoppers with 70% isopropanol and allow them to dry.
- Withdraw the calculated volume with a calibrated sterile transfer device. Add the water slowly down the inside wall of the material’s vial rather than directly onto the cake: a jet of liquid can foam or scatter a delicate cake.
- Swirl or roll the vial gently until the material has dissolved. Do not shake unless the data sheet says shaking is acceptable; foaming can damage some materials and makes the solution impossible to inspect.
- Inspect the solution. It should be clear and free of undissolved material unless the data sheet says otherwise.
4. Label and record
Label the solution with the material name, concentration, diluent (“bacteriostatic water, 0.9% benzyl alcohol” and its lot number), date prepared, discard date and the preparer’s initials, and enter the preparation in the log. See labeling and record-keeping for prepared solutions.
5. Store as the data sheet directs
The 28-day in-use convention applies to the opened vial of bacteriostatic water. The stability of the prepared solution is set by the material’s data sheet, which may specify a shorter period, refrigeration or protection from light. Bacteriostatic water itself is not frozen, and freezing a prepared solution is done only if the data sheet permits it. See can bacteriostatic water be frozen?
Related
Preparing a dilution series · Laboratory uses of bacteriostatic water · How many mL are in a vial?
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.