USP Purified Water (PW)

Purified Water is described in the USP 23 monograph as follows:
“Purified Water is water obtained by distillation, ion-exchange treatment, reverse osmosis, or other suitable process. It is pre-pared from water complying with the regulations of the U.S. Environmental Protection Agency (EPA) with respect to drinking water. It contains no added substances.”
Microbial Quality
Regarding the bacteriological purity of PW, the monograph (legally enforceable section) states only that PW must comply with the EPA regulations for drinking water. The EPA regulations only specify limits for coliform bacteria. In the informational section of the USP 23, which deals with action guidelines for the microbial control of ingredient water, it says:
“A total microbial (aerobic) count that may be used for source drinking water is 500 colony-forming units (cfu) per mL. A gen-eral guideline for Purified Water may be 100 cfu/mL.”
These numbers for cfu/mL are only advisory guidelines that represent recommended alert/action limits, not reject levels. The informational section also suggests that the microbial action limits for PW should be based on the intended use of the water and the nature of the product being made. It recognizes that microbial limits for PW require being defined on a case-by-case basis.
USP 23 Supplement 5, effective since November 1996, specifies the method for total bacteria counts. It states “Heterotrophic Plate Count of a 1-mL sample, using Plate Count Agar at an incu-bation temperature of 30 to 35°C for a 48-hour period (minimum).” There is some controversy (Collentro 1996) because this method will underestimate “starved” bacteria in high-purity water.
Chemical Quality
Effective November 15, 1996, the former inorganic chemistry tests (for calcium, sulfate, chloride, ammonia, and carbon dioxide) were replaced with a three-stage conductivity test. The conductiv-ity limit is pH-dependent but, for example, at pH 7.0, conductivity should be less than 5.8 microSiemen/cm (μS/cm). The former test for oxidizable substances was replaced with a Total Organic Carbon (TOC) limit of 0.05 mg/L. TOC is an indirect measure of organic molecules pre-sent in water measured as carbon. The new tests allow continuous in-line monitoring of water using instrumentation rather than lab work.