# Water grades and purification trains

Why autoionization caps resistivity at 18.2 MΩ·cm, what conductivity, TOC and bacterial limits each grade actually measures, and why water decays from the moment it leaves the purifier.

The 18.2 MΩ·cm ceiling is not a performance figure but the conductivity of water itself; every grade system is a distance-from-that-ceiling measurement, and storage quietly undoes what the purifier achieved.

Source: https://en.bioecon.ru/docs/bioproduction-equipment/cleanroom-facilities/water-systems/
Updated: 2026-09-08



The cleanest water ever measured sits at 18.2 MΩ·cm resistivity — 0.0555 µS/cm — and nothing can push past that, because at that point the current is carried by water itself. Pure water autoionizes into H₃O⁺ and OH⁻ at a fixed equilibrium, and those ions, not contaminants, set the floor. Every laboratory water grade is therefore a measurement of distance from this ceiling: the farther resistivity falls below 18.2 MΩ·cm, the more dissolved ions the water carries.

## What the grades actually measure

Grade systems bundle three different impurity axes under one label. Ionic contamination is measured by conductivity; organic contamination by total organic carbon; biological contamination by bacterial count. ISO 3696 grade 1 asks for 10 MΩ·cm, ASTM D1193 Type I for the full 18.2 MΩ·cm with TOC below 50 ppb, and the pharmacopoeias add enforcement: USP Purified Water must stay below 1.3 µS/cm at 25 °C with an action limit around 100 CFU/mL. A system can meet one axis while failing another — freshly deionized water teeming with bacteria is still electrically clean, which is why grades specify all three axes and why conductivity alone is a poor proxy for quality.

## The physics of each separation stage

Each stage in a purification train removes a different class of impurity by a different mechanism. Distillation exploits the volatility gap between water and dissolved solids; it is energetically expensive because the latent heat of vaporization — about 2.26 MJ per kilogram, roughly 0.63 kWh — must be paid for every litre, twice in a bidistiller. Reverse osmosis is a pressure-driven size-and-charge sieve: polyamide membranes reject 95–99% of dissolved ions while passing water. Electrodeionization combines ion-exchange resin with a DC field that continuously regenerates the resin, polishing RO permeate into the 10–18 MΩ·cm band without the acid and caustic cycles that batch deionization needs. The final polish uses 185 nm UV light to oxidize organics into CO₂ that the subsequent cartridge captures, and ultrafiltration or a 0.22 µm membrane to hold back pyrogens, nucleases and particles.

## Why storage is the real enemy

Ultrapure water begins to decay the instant it leaves the purifier. Carbon dioxide diffuses in from the air and forms carbonic acid, pulling resistivity down measurably within minutes; the container leaches ions, organics and particulates back into what is, electrostatically and thermodynamically, one of the most aggressive solvents known. The claim that 18.2 MΩ·cm water attacks glass and plastics is real but easy to overstate: the effect is measurable in leachate analysis yet modest at bench timescales, and vendors who market "aggressive water" hazard claims are selling concern rather than physics. What follows is engineering, not chemistry: keep the water moving in sanitary loops, minimize dead legs, monitor conductivity and TOC continuously, and put the final filter at the point of use — because the quality that matters is the quality in the flask, not the quality in the tank.

## The honest summary

A purification train does not create purity; it manages a gradient, and every hour of storage, every metre of piping and every open beaker spends some of it back.