Type 1 water: Purity that doesn’t last

Why immediate use matters and why storage may put compliance at risk 

Written by Elise Mosley from Avidity Science 

Laboratories depend on the reliability of their results, which in turn depend on water quality being consistent and compliant with internal Standard Operating Procedures (SOPs) and external regulatory standards.

However, what is less well understood is that Type 1 water does not stay Type 1 for long once it has been produced and dispensed.

How Quickly Type 1 Water Degrades

By definition, Type 1 ultrapure water has a resistivity of 18.2 MΩ·cm at 25°C. Yet the moment it leaves the purification system and is exposed to air, degradation begins:

• Within minutes to 1 hour: Absorption of carbon dioxide from the air causes resistivity to drop sharply, often to 10–12 MΩ·cm
• Within a few hours: Resistivity can fall further to less than10 MΩ·cm
• Over longer storage: Microbial regrowth and leaching from containers drive quality further down, outside Type 1 specification

Even in sealed containers, water quality declines over hours to days, since CO₂ and trace contaminants diffuse through plastics where there is no recirculation to maintain purity.

What happens below 18.2 MΩ·cm?

It is important to note that 18.2 MΩ·cm is the threshold for Type 1 (ultrapure) water. When resistivity drops below this level, the water no longer meets Type 1 specifications. Depending on its resistivity, total organic carbon (TOC), and microbial content, the water may fall into Type 2 range (1–18 MΩ·cm) or even lower laboratory water grades.

In practical terms, this means that water, which was Type 1 when produced, may already be out of specification for SOPs within minutes of being stored in an open container. 

Why point-of-use systems are the answer

To ensure that true Type 1 water is always available when and where it is needed, laboratories increasingly turn to dedicated Type 1 water purification systems. These systems:

• Continuously monitor quality parameters (resistivity, TOC), giving documented proof of compliance
• Minimise the risk of failed experiments, wasted reagents, and repeat testing
• Support audit readiness and regulatory assurance

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