Water quality classes in the lab

Water is the reagent used in the largest volume in most laboratories and the one least often thought of as a reagent. It is one. Its purity affects blanks, baselines, dissolution, cleaning and every solution made with it, and the standards that classify it treat it with the same seriousness as any other material: a set of measured properties, each with a limit, and a class name that summarizes which limits were met.

The classification systems

Two standards are in common use. ASTM D1193, the Standard Specification for Reagent Water, defines Types I through IV. ISO 3696 defines Grades 1, 2 and 3 for analytical laboratory use. In both, the first class is the most stringent. The systems overlap without being identical, so a procedure should name the standard as well as the class: “Type I” and “Grade 1” describe similar water but are not interchangeable references.

Pharmacopeias define their own grades of water, with requirements set for manufacturing use rather than for analytical blanks. These are a separate system with a separate purpose, and a pharmacopeial water name is not a substitute for a reagent water class, or the reverse.

What gets measured

The properties in the specifications describe what has been removed from the water and how thoroughly:

• Resistivity or conductivity, a measure of dissolved ions. Pure water conducts very poorly, because ions are what carry current. The most stringent classes approach the theoretical limit for pure water, about 18 megohm-centimeters at 25 °C.
• Total organic carbon, a measure of organic contamination that ion measurements cannot see.
• Specific ions such as sodium, chloride and silica, which matter for particular analyses and are listed separately in some classes.
• Microbial counts, specified as an additional requirement for uses that are sensitive to them.
• Particles, usually controlled by a final filter at the point of use.

Each property answers a different question. Water can show a high resistivity and still carry organic contamination, because many organic compounds do not ionize. That is why a single resistivity reading is not a full description of the water.

Why the class only holds at the tap

High-purity water is not stable. The moment it leaves the purification system it begins to pick up what is around it. Carbon dioxide from the air dissolves and forms carbonic acid, lowering the resistivity within minutes. Containers leach ions and organic compounds into it. Water standing in a reservoir or a wash bottle supports microbial growth. A carboy of Type I water filled on Monday is not Type I water on Thursday, whatever the label on the carboy says.

The practical rule follows: the most stringent classes are drawn at the point of use, used promptly, and not stored. Lower classes tolerate storage better, and are often specified for exactly that reason — as feed water, rinse water or water for general solutions.

Matching class to use

The highest class is not the default answer. Trace elemental analysis, sensitive chromatography and the preparation of standards at low concentrations call for the most stringent water, because contaminants at very low levels show up in the result. Glassware rinsing, many buffer preparations and feed water for instruments that do their own polishing are served by lower classes. Using the highest class everywhere costs more in consumables and gives a false sense that water has been dealt with; using a low class for a sensitive method puts its contaminants into the blank.

A method should name the water it needs by standard and class, and give the reason if the reason is not obvious. That turns a habit into a requirement someone can check.

The record a water system should leave

A purification system that produces classified water should show that it did so. That means a resistivity reading at the point of dispense, temperature-compensated and logged, and total organic carbon monitoring where the class requires it. It means records of cartridge and filter changes, of sanitization, and of the calibration of the system’s own sensors. When a result is questioned, the water record for that day is one of the first things a reviewer asks to see. A system with a display and no log can show that the water is good now, and nothing about yesterday.

The same applies to water bought in containers for analytical use. The container should carry a lot number, the class it was made to, and a certificate for that lot. Once opened, it is subject to the same pickup from air and container as any other stored water, and the certificate describes it only as it was when sealed.