Quick answer: Type I water is ultrapure water with a minimum resistivity of 18 megohm-centimeters, used for the most sensitive applications like HPLC, mass spectrometry, and molecular biology. Type II water is highly purified water with a minimum resistivity of 1 megohm-centimeters, used for routine lab work like reagent prep, glassware rinsing, and autoclave feedwater. Type I is the higher purity grade and is typically produced by further polishing Type II water.
When people talk about purified water in a laboratory or industrial setting, they’re rarely referring to a single, uniform product. Water purity exists on a spectrum, and Type I and Type II describe two distinct grades of high-purity water defined by industry standards such as ASTM D1193, ISO 3696, and the Clinical and Laboratory Standards Institute (CLSI). Choosing the wrong grade either wastes money on unnecessary purity or introduces contaminants that compromise sensitive analytical work, manufacturing processes, or lab results.
Why Does Water Purity Matter in the First Place?
Water is considered impure not because it looks dirty, but because it contains dissolved ions, organic compounds, dissolved gases, bacteria, endotoxins, silica, suspended particles, and other contaminants. Most of these are invisible to the naked eye, yet they can interfere with chemical reactions, damage analytical instruments, contaminate products, or skew lab results.
High-purity water systems remove these contaminants through a combination of treatment technologies, including:
- Multimedia filtration
- Activated carbon filtration
- Water softening
- Reverse osmosis
- Deionization
- Ultraviolet oxidation
- Final point-of-use filtration
The final water quality depends on which combination of these technologies is used and how tightly the finished water is controlled.
What Is Type II Water?
Type II water is a highly purified grade of water intended for routine, general-purpose laboratory use rather than ultrapure applications. It’s typically produced using reverse osmosis followed by deionization, and some systems add ultraviolet treatment on top of that.
Typical Type II water specs:
- Resistivity: a minimum of 1.0 megohm-centimeters
- Total organic carbon (TOC):Â below 50 parts per billion
- Production method: reverse osmosis + deionization, sometimes with UV
Because Type II water removes the vast majority of dissolved minerals and contaminants, it’s suitable for most day-to-day lab work where absolute purity isn’t required.
What Is Type II Water Used For?
- Reagent preparation
- Buffer preparation
- Glassware rinsing
- Microbiological media preparation
- Clinical chemistry analyzers
- Autoclave feedwater
- Routine chemical testing
- Feedwater for Type I polishing systems
For many labs, Type II water is the primary purified water source used throughout the facility.
What Is Type I Water?
Type I water, commonly called ultrapure water, is the highest purity grade. It’s typically made by taking already-purified water, usually RO water or Type II water, and polishing it further using mixed-bed deionization, ultraviolet oxidation, and final point-of-use filtration. These extra steps strip out even trace concentrations of dissolved ions, organic compounds, microorganisms, bacteria, endotoxins, silica, and suspended particles.
Typical Type I water specs:
- Resistivity: a minimum of 18.0 megohm-centimeters (near the theoretical maximum for water)
- Conductivity: approximately 0.056 microsiemens per centimeter
- Total organic carbon (TOC):Â below 50 parts per billion
Because Type I water contains almost no dissolved ions, it’s highly reactive. It readily absorbs carbon dioxide and other contaminants from the air, which means it starts losing purity almost immediately once exposed to the atmosphere. That’s why Type I water is usually produced at the point of use rather than stored.
What Is Type I Water Used For?
- High-performance liquid chromatography (HPLC)
- Liquid chromatography-mass spectrometry (LC-MS)
- Inductively coupled plasma mass spectrometry (ICP-MS)
- Inductively coupled plasma optical emission spectroscopy (ICP-OES)
- Molecular biology, PCR, and DNA sequencing
- Cell culture and tissue culture
- Pharmaceutical analytical laboratories
- Semiconductor manufacturing
In many of these applications, contaminants measured in parts per billion, or even parts per trillion, can interfere with results or damage sensitive instruments.
Type I vs. Type II Water: Key Differences at a Glance
| Type I Water | Type II Water | |
|---|---|---|
| Resistivity | > 18 MΩ·cm | > 1 MΩ·cm |
| Total organic carbon | <50 ppb | <50 ppb |
| Production method | RO/Type II water further polished with mixed-bed DI, UV oxidation, and final filtration | Reverse osmosis + deionization, sometimes with UV |
| Storage | Produced at point of use; degrades quickly once exposed to air | Can be stored for routine use |
| Typical applications | HPLC, LC-MS, ICP-MS, molecular biology, PCR, semiconductor manufacturing | Reagent prep, glassware rinsing, autoclaves, routine testing |
| Relative cost | Higher (more equipment, media, maintenance) | Lower |
Do I Need Type I Water for My Whole Lab?
Not usually. This is one of the most common misconceptions in water purification. Producing ultrapure water throughout an entire facility would technically work, but it’s often unnecessary and considerably more expensive. Ultrapure systems require additional treatment equipment, more frequent replacement of polishing media, higher maintenance, and greater operating expense. Using Type I water for routine glassware rinsing or autoclave feedwater usually provides little practical benefit.
How Do Labs Typically Combine Type I and Type II Systems?
Most labs use a staged purification approach:
- Type II water is produced in larger volumes for routine applications, glassware rinsing, buffer and reagent prep, autoclaves, and general lab work.
- A portion of that Type II water is then sent through a polishing system to produce Type I water for analytical instruments and other sensitive applications.
This approach delivers the required water quality for sensitive work while reducing equipment costs, resin consumption, maintenance, and overall operating expense.
Is Resistivity Enough to Measure Water Purity?
Not on its own. Resistivity measures dissolved ionic contamination, but it doesn’t directly measure organic compounds, bacteria, endotoxins, suspended particles, or pyrogens. Water can have excellent resistivity and still contain contaminants that interfere with a specific application. That’s why high-purity water systems typically monitor several parameters together: resistivity, conductivity, total organic carbon, bacterial counts, endotoxins, silica, sodium, and particle counts.
How Is Water Purity Specified Outside the Lab?
Outside laboratory settings, “Type I” and “Type II” come up less often. Industrial facilities tend to specify water quality using measurable performance criteria instead, such as conductivity, resistivity, silica, sodium, boron, dissolved oxygen, or total organic carbon. For example:
- Semiconductor manufacturing may require ultrapure water that exceeds traditional Type I lab specs.
- Pharmaceutical facilities typically specify USP Purified Water or Water for Injection.
- Power plants focus on conductivity, silica, sodium, and dissolved oxygen.
How Do I Choose the Right Water Quality for My Process?
When designing a water treatment system, the question isn’t whether the system can produce Type I water. The more useful question is: what water quality does the process actually require? Producing water purer than necessary drives up capital costs, operating expenses, resin usage, maintenance, and energy consumption without improving process performance. A well-designed system delivers the required purity level while minimizing lifecycle cost and maximizing reliability.
Type I and Type II water aren’t competing standards; they’re complementary grades built for different jobs. Type II is the workhorse behind most routine lab operations and doubles as feedwater for Type I polishing systems. Type I represents the ceiling of purity, reserved for the analytical techniques and manufacturing processes where even trace contamination changes the outcome.
The goal was never to produce the purest water possible. It’s to produce the right water quality for the application, at the lowest practical lifecycle cost.
Frequently Asked Questions
What resistivity does Type I water have? A minimum of 18 megohm-centimeters, which corresponds to a conductivity of about 0.056 microsiemens per centimeter.
What resistivity does Type II water have? A minimum of 1 megohm-centimeters.
Is Type I water the same as distilled water? No. Distillation alone doesn’t remove all dissolved ions, organics, or microorganisms to Type I levels. Type I water requires additional polishing steps like mixed-bed deionization, UV oxidation, and final filtration.
Can Type II water be used for HPLC or mass spectrometry? Generally not recommended. These techniques are sensitive enough that trace contaminants in Type II water can interfere with results. Type I water is the standard for HPLC, LC-MS, and ICP-MS.
Which standards define Type I and Type II water? ASTM D1193, ISO 3696, and the Clinical and Laboratory Standards Institute (CLSI) each define grades of laboratory water, with some variation in exact specifications between standards.
Why can’t Type I water be stored? Because it contains almost no dissolved ions, Type I water readily absorbs carbon dioxide and other contaminants from the air. It starts losing purity almost immediately after exposure, so it’s typically generated at the point of use rather than stored in tanks.
Whether your process calls for Type I, Type II, or something in between, Puretec designs and services water treatment systems engineered to deliver exactly the purity level your application requires, consistently and reliably. Talk to a water treatment specialist for a system assessment.
