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difference between MQ water and distilled water - stupid question (Apr/27/2006 )

Can anyone tell me the difference?



Milli Q

"Water that has been distilled is fed through a special ion exchange cartridge, which increases its purity. The purity of the water is assessed by measuring the resistance of the water to conducting electricity. As the resistance of the water to conductivity increases so does its purity."

From Wikipedia, the free encyclopedia

"Milli-Q water is water that has been purified using an ion exchange cartridge. The purity of the water is monitored by measuring the conductivity. The higher resistance, the fewer ions in the water. A value greater than 18.2 MΩ cm-1 (@ 25 °C) is desirable. As a final step, the water is normally dispensed through a 0.22 μm membrane filter."

So essentially Milli-Q water is just distilled water that has gone through more filters.


distilled water is made by continuous (as opposed to fractional) distillation of a water source. many times it is distilled a second and even a third time. the problem with it is that since it is continuous distillation, some contaminants will come across with the water.

milli-q water is made by passing the source water through mixed bed ion exchange and organics (activated charcoal) cartridges. the source does not have to be distilled or deionized (reverse osmosis) but the cartridge life will be shorter if it is not. finally the water is passed through a filter which should pull out any intact organisms. sometimes there is a uv irradiator attached, as well.

milli-q is a brand name, there are other brands that perform as well as the milli-q system.


ASTM Standard Specification for Reagent Water
Conductivity @ 25°C: Milli-Q (Type I) 0.056, distilled (Type II) 1.0 microhms/cm
Resistivity @ 25°C: Milli-Q 18, distilled 1.0 megohms/cm