What Is Ultrapure Water? A Simple Guide to Purity, Grades and Uses
What is ultrapure water? It’s water that has been cleaned until almost nothing but water is left. Minerals, salts, tiny bits of dirt, germs, and even dissolved gases are cut down to a few parts per billion or less. Its purity benchmark is 18.2 MΩ·cm at 25 °C, the highest reading pure water can reach.
People often call it UPW for short. It’s used anywhere a tiny trace of dirt would ruin the result, like making computer chips, medicines, and lab tests.
What is ultrapure water, and what does 18.2 MΩ·cm mean?
We check water purity with electricity. Dissolved salts carry electric current, a bit like cars on a road. Fewer salts means fewer cars, so current struggles to flow.
That struggle is called resistivity, measured in megohm centimeters (MΩ·cm). Its opposite is conductivity, in microsiemens per centimeter (µS/cm). So 18.2 MΩ·cm equals 0.055 µS/cm.
Even perfect water isn’t a perfect insulator. A tiny share of water molecules always split into H⁺ and OH⁻ ions, and those carry a little current. That sets a hard ceiling of about 18.2 MΩ·cm at 25 °C. No water can read higher at that temperature.
Resistivity only sees ions, though. It can’t spot dust, bacteria, or some organic chemicals, so standards also limit total organic carbon (TOC), particles, and germs.
Ultrapure water vs deionized, distilled, and RO water
Each method removes different impurities, and UPW combines several of them.
| Water type | How it’s made | Removes well | Can leave behind |
| Reverse osmosis (RO) | Pushed through a very tight filter membrane | Most salts, larger organics, particles | Some ions and dissolved gases like CO₂ |
| Distilled | Boiled, then the steam is cooled back to water | Minerals and salts | Some organics that boil over, plus CO₂ from air |
| Deionized (DI) | Passed through ion exchange resin beads | Charged salts and minerals | Uncharged organics, bacteria, particles |
| Ultrapure (UPW) | RO, ion exchange, UV light, degassing, and fine filters in a closed loop | All major impurities, down to ppb or less | Very little, as long as it stays sealed and moving |
So all ultrapure water is deionized, but not all deionized water is ultrapure.
Ultrapure water standards and grades
The answer to what is ultrapure water changes a little by industry, because each one uses its own rulebook.
For labs, the ASTM D1193 standard sorts reagent water into four types:
| ASTM D1193 | Resistivity (MΩ·cm) | TOC (ppb) | Sodium (ppb) | Silica (ppb) |
| Type I | above 18 | below 50 | below 1 | below 3 |
| Type II | above 1 | below 50 | below 5 | below 3 |
| Type III | above 4 | below 200 | below 10 | below 500 |
| Type IV | above 0.2 | not set | below 50 | not set |
Type III has a stricter resistivity limit than Type II, but Type II is tighter on TOC and silica.
Other fields set their own bar:
- Computer chip makers follow ASTM D5127 and SEMI F63, the strictest rules of all. Top grades need TOC below 1 ppb.
- Many lab equipment makers call 18.2 MΩ·cm water with TOC below 10 ppb “Type 1” water.
- Drug makers follow USP rules. Their limit is conductivity of 1.3 µS/cm and TOC of 500 ppb, plus strict germ limits.
What ultrapure water must be free of
| Impurity | Everyday example | Why it matters |
| Particles | Dust, pipe flakes | Can block tiny circuits on a chip |
| Bacteria | Slimy film in pipes | Leaves residue behind |
| Positive ions (cations) | Sodium, calcium | Spoil chips and lab tests |
| Negative ions (anions) | Chloride, silica | Cause deposits and corrosion |
| Organic carbon | Leftovers from plants or plastic | Messes with chemistry |
| Dissolved gases | Oxygen, carbon dioxide | Oxygen rusts surfaces, CO₂ lowers purity |
How ultrapure water is made
Plants stack several steps, and each one catches what the last one missed:
- Pretreatment filters, softens, and uses carbon to protect the equipment that comes next.
- Reverse osmosis, often done twice, removes most salts and organics.
- Degassing pulls out oxygen and carbon dioxide.
- Electrodeionization (EDI) or ion exchange removes most leftover ions.
- UV light breaks down organics and controls germs.
- A mixed bed polisher brings the water up to about 18.2 MΩ·cm.
- Ultrafiltration catches the last tiny particles.
- A loop keeps the water moving, so germs can’t settle in.
The final polish: mixed bed resin
We make the resin for this step. A mixed bed holds two kinds of resin beads blended together. One grabs positive ions, and the other grabs negative ions.
As water flows through, it meets both bead types again and again. It acts like many cleaning stages in a row, which is how it reaches such high purity.
Our YHJ 18MH gel mixed bed resin is made for product water above 18 MΩ·cm. It works at up to 60 °C.
The negative ion beads, our strong base anion resin, have a tough job. Silica is weakly charged and slips through more easily as the beads wear out. So watch silica, not just resistivity, as a bed nears the end of its life.
Where ultrapure water is used
| Industry | What UPW does |
| Computer chips | Rinses wafers between steps. One 200 mm wafer can use about 5,600 liters. |
| Medicines | Used as an ingredient and for cleaning |
| Laboratories | Used in sensitive tests like HPLC, mass spectrometry, and PCR |
| Power plants | Feeds boilers and turbines, where deposits cause damage |
| Solar panels and screens | Cleans glass and cells |
Advanced chip plants can use several million gallons of UPW a day. You can see more of these uses on our electronics and water treatment applications page.
Why ultrapure water doesn’t stay pure
Ultrapure water pulls in anything it touches, and air is the biggest problem. Carbon dioxide dissolves in, forms a weak acid, and adds ions.
ResinTech measured how fast this happens when UPW sits open to air:
| Time open to air | Resistivity |
| Fresh | 18 MΩ·cm |
| 1 hour | about 16 MΩ·cm |
| 6 hours | about 10 MΩ·cm |
| 24 hours | about 3 MΩ·cm |
| 48 hours | about 1 MΩ·cm |
That’s why UPW is made right where it’s used and kept flowing in closed loops. A few rules help:
- Dispense it right before you need it.
- Don’t store it in open beakers.
- Measure resistivity inline. A carried sample picks up CO₂ and reads falsely low.
Can you drink ultrapure water?
Some websites say drinking it is deadly, but the sources we checked don’t support that for a single glass. The documented concern is long term: a 1980 WHO review found that regularly drinking demineralized water increased electrolyte loss.
So don’t drink it. It has no useful minerals, and lab systems aren’t built to food safe standards.
FAQs
Is Type 1 water the same as ultrapure water?
In labs, usually yes. Type 1 means the top lab grade. Chip makers use even stricter grades.
What is EDI?
EDI stands for electrodeionization. It uses resin beads plus electricity to pull out ions, so it doesn’t need acid and caustic regeneration like regular resin beds.
Why does temperature matter for the 18.2 reading?
Resistivity changes with temperature. That’s why the 18.2 figure is always given at 25 °C, and good meters correct for temperature.
Is more purity always better?
No. Pick the grade your job needs. A stricter grade costs more and may give you no real benefit.
Can Haitron help me choose resin for an ultrapure water system?
Yes. Just send us your feed water analysis and your target resistivity. Our team will match a mixed bed resin to your system.
Key Takeaway
So, what is ultrapure water? It’s water cleaned to about 18.2 MΩ·cm, with almost no salts, organics, particles, germs, or gases left. It powers chip making, medicine, and lab work, and it stays pure only while it’s sealed and moving.
