0.9% benzyl alcohol means 0.9 grams of benzyl alcohol in every 100 mL of solution (a weight-per-volume percentage), which is 9 mg per mL. The USP monograph requires the label to state the preservative and its proportion, and 0.9% w/v is the concentration used across commercial bacteriostatic water.
The same concentration in other units
| Unit | Value | Working |
|---|---|---|
| Percent w/v | 0.9% | 0.9 g per 100 mL |
| mg/mL | 9 mg/mL | 0.9 g ÷ 100 mL |
| g/L | 9 g/L | 9 mg/mL × 1000 |
| ppm (mg/L) | about 9,000 ppm | 9 g/L expressed as mg/L; strictly ppm by mass is 9 g per 999 g of solution, also about 9,000 |
| Percent w/w | about 0.9% | the solution density is 0.999 g/mL, so w/v and w/w differ only in the third decimal |
| Molarity | 0.083 mol/L (83 mmol/L) | 9 g/L ÷ 108.14 g/mol |
| Molality | about 0.083 mol/kg | same as molarity to two significant figures in a dilute aqueous solution |
Amount of preservative per vial
| Labeled volume | Benzyl alcohol |
|---|---|
| 3 mL | 27 mg |
| 10 mL | 90 mg |
| 30 mL | 270 mg |
Vials contain slightly more than the labeled volume, so the actual amount is a few percent higher. See fill volume and overfill.
What dilution does to the concentration
Whenever bacteriostatic water is mixed with a preservative-free liquid, the benzyl alcohol concentration falls in proportion. Equal volumes of bacteriostatic water and sterile water give 0.45%; one part in ten gives 0.09%. Below its tested concentration the preservative can no longer be relied on, which is why a diluted mixture is treated as unpreserved. See can you mix bacteriostatic water with sterile water? and preparing a dilution series.
Why 0.9% and not more or less
0.9% is high enough to pass antimicrobial effectiveness testing with a margin and far below the roughly 4% solubility limit of benzyl alcohol in water, so the preservative stays dissolved at any labeled storage temperature. See antimicrobial effectiveness testing (USP <51>) and benzyl alcohol properties.
Related
Bacteriostatic water ingredients · How the preservative content is verified
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.