“Ultrapure” and “sterile and preserved” describe different kinds of quality. Type I water is the purest water a laboratory produces, measured by how little it contains. Bacteriostatic water is a packaged product, measured by what it guarantees: sterility, endotoxin control and a working preservative. Neither is a substitute for the other.
What Type I means
Under ASTM D1193, Type I reagent water has a resistivity above 18 MΩ·cm at 25 °C (conductivity below 0.056 µS/cm), total organic carbon below 50 µg/L, and sodium, chloride and silica each in the low microgram-per-litre range. It is produced at the point of use by a polishing system fed with pretreated water and is used immediately, because it absorbs carbon dioxide and leaches ions from any container within minutes. It is not sterile unless the system includes a final sterilizing filter or ultraviolet unit, and it carries no endotoxin claim unless that is specifically monitored.
Side by side
| Property | Ultrapure Type I water | Bacteriostatic water |
|---|---|---|
| Conductivity | below 0.056 µS/cm | a few µS/cm, from the packaged WFI-grade water |
| Total organic carbon | below 50 µg/L | on the order of 7 g/L, all of it benzyl alcohol |
| Sterility | not claimed by default | guaranteed in the sealed vial |
| Endotoxin | not claimed by default | not more than 0.5 EU/mL |
| Preservative | none | 0.9% benzyl alcohol |
| Shelf life | minutes; drawn fresh | years sealed; 28 days opened |
| Container | none; dispensed from a system | 3 mL, 10 mL, 30 mL vials |
| Typical use | trace analysis, chromatography, reagent preparation | preserved multi-use diluent |
Why the TOC line matters
By the Type I standard, bacteriostatic water is spectacularly impure: its organic carbon content is more than a hundred thousand times the Type I limit, because the preservative is an organic compound added on purpose. That is not a defect; it is the design. But it means bacteriostatic water is the wrong choice for any measurement in which organic background or ultraviolet absorbance matters. See physical properties of bacteriostatic water and vs. HPLC-grade water.
Why Type I water is not a diluent replacement
Freshly drawn Type I water is not sterile, is not preserved, has no endotoxin claim and cannot be stored. Anything that specifies a sterile, preserved, multi-use diluent cannot be prepared from it without adding back everything the specification asks for. See why laboratories buy bacteriostatic water sealed.
Related
USP water grades explained · Bacteriostatic water vs. deionized water · Alternatives
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