Hopefully the below will be of interest, it has been
derived from various sources. In summary, my way of thinking about
this is distilled is less pure than demineralised which is less pure
than deionised. Where manufacturers specify a particular water grade
it is because they do not want any residual minerals or organics in
the water so I would always follow their recommendations.
Distilled, demineralised and deionised water and measuring of the
It is quite difficult to find clear definitions and standards for
distilled, demineralised and deionised water. Probably the easiest
way to familiarise in the topic of producing (ultra) pure water is
to start with the oldest and best-know method: distilling.
Distilled water is water that has been boiled in an apparatus called
a “still” and then recondensed in a cooling unit (“condenser”) to
return the water to the liquid state. Distilling is used to purify
water. Dissolved contaminants like salts are left behind in the
boiling pot as the water vapour rises away. It might not work if the
contaminants are volatile so that they also boil and recondense,
such as having some dissolved alcohol.
Very elegant stills can selectively condense (liquefy) water from
other volatile substances, but most distillation processes allow
carry-over of at least some volatile substances, and a very little
of the non-volatile material that was carried into the water vapour
stream as bubbles burst at the surface of the boiling water.
Maximum purity (this is discussed towards the end of this post) from
such stills is usually 1.0 MWcm; and since there is no protection
from carbon dioxide (CO2) dissolving into the distillate the pH is
generally 4.5-5.0. Additionally, you have to be careful not to
re-contaminate the water after distilling it.
Demineralisation - this is generally any process used to remove
minerals from water, however, commonly the term is restricted to ion
Deionisation - this is a process which utilises special-manufactured
ion-exchange resins to remove ionised salts from water. This can
theoretically remove 100 % of the salts dissolved in water.
Deionisation typically does not remove organics, virus or bacteria
except through “accidental” trapping in the resin and specially made
strong base anion resins which will remove gram-negative bacteria.
Deionisation entails removal of electrically charged (ionised)
dissolved substances by binding them to positively or negatively
charged sites on a resin as the water passes through a column packed
with this resin. This process is called ion exchange can be used in
different ways to produce deionised water of various qualities.
Deionised water can also be produced with reverse osmosis plants.
Reverse osmosis is capable of rejecting bacteria, salts, sugars,
proteins, particles, dyes, and other constituents that have a
molecular weight of greater than 150-250 Daltons.
Ultra purer water - this is highly-treated water of high resistivity
and no organics; usually used in the semiconductor and
Measuring Water Purity
Water purity may be measured in various ways. You can attempt to
determine the weight of all of the dissolved material (“solute”);
this is most easily done for dissolved solids, as opposed to
dissolved liquids or gases. In addition to actually weighing the
impurities, one can estimate their level by the degree to which they
increase the boiling point or lower the freezing point of water. The
refractive index (a measure of how transparent materials bend light
waves) is also affected by solutes in water. Alternately, water
purity can be quickly estimated on the basis of electrical
conductivity or resistance - very pure water conducts electricity
poorly, so its resistance is high.
Quality Ultra Pure Water Pure Water Purified Water
Typical Resistivity 10-18 MOhm. cm 1-10 MOhm. cm 1-0.02 MOhm. cm
Conductivity 0.1-0.0555 microS/cm 1.0-0.1 microS/cm 1.0-50 microS/cm
Pure water by definition is slightly acidic and distilled water will
test out around pH 5.8. The reason is that distilled water dissolves
carbon dioxide from the air. It dissolves carbon dioxide until it is
in dynamic equilibrium with the atmosphere. That means that the
amount being dissolved balances the amount coming out of solution.
The total amount in the water is determined by the concentration in
the atmosphere. The dissolved carbon dioxide reacts with the water
and finally forms carbonic acid.
2 H2O + CO2 --> H2O + H2CO3 (carbonic acid)
Newly produced distilled water has a pH value of approximately 7,
but is affected by the presence of carbon dioxide it will reach a
slightly acidic pH-value within a couple of hours.
Additional, it is important to mention that the pH of ultra-pure
water is difficult to measure. Not only does high-purity water
rapidly pick up contaminants - such as carbon dioxide (CO2) - that
affect its pH, but it also has a low conductivity that can affect
the accuracy of pH meters. For instance, absorption of just a few
ppm of CO2 can cause the pH of ultra pure water to drop to 4.5,
although the water is still of essentially high quality.
F. N. Kemmer; The Nalco water handbook; 2. Edition; 1988
Degremont; Water treatment handbook; sixth edition; 1991
Osmonics Pure Water Handbook; 2. Edition; 1997