clare, sorry dude - but you're fundamentally misinformed on that. the
difference between de-ionized is practically zero, and for radiator
purposes /is/ zero. if you have experience of "de-ionized" causing
problems, it wasn't de-ionized, it was softened, a whole different
animal and that indeed can be corrosive.
Deionization
The process used for removal of all dissolved salts from water is
referred to as deionization. Deionization requires the flow of water
through two ion exchange materials in order to effect the removal of
all salt content.
Deionization. The terms demineralization and deionization are used
somewhat interchangeably by the industry. While the term
demineralization is generally better understood, deionization is
especially apt.
The passage of water through the first exchange material removes the
calcium and magnesium ions just as in the normal softening process.
Unlike home equipment, deionization units also remove all other
positive metallic ions in the process and replace them with hydrogen
ions instead of sodium ions.
As the metallic ions in the water affix themselves to the exchange
material, the latter releases its hydrogen ions on a chemically
equivalent basis. A sodium ion (Na+) displaces one hydrogen ion (H+)
from the exchanger; a calcium ion (Ca++) displaces two hydrogen ions;
a ferric ion (Fe+++) displaces three hydrogen ions, etc. (Recall that
home softeners also release two sodium ions for every calcium or
magnesium ion they attract.)
This exchange of the hydrogen ions for metallic ions on an equivalent
basis is chemical necessity that permits the exchange material to
maintain a balance of electrical charges.
Now because of the relatively high concentration of hydrogen ions, the
solution is very acid.
At this point the deionization process is just half complete. While
the positive metallic ions have been removed, the water now contains
positive hydrogen ions, and the anions originally in the raw water.
The partially treated water now flows through a second unit, this time
an anion exchange material normally consists of replaceable hydroxyl
anions and fixed irreplaceable cations.
Now the negative ions in solution (the anions) are absorbed into the
anion exchange material. Released in their place are hydroxyl anions.
All that emerges from such a two unit system is ion-free water. It
still contains the positive hydrogen ions released in the initial
exchange plus the negative hydroxyl ions released in the second
exchange.
What has become of these two ions? Through the magic of chemistry they
have combined (positive to negative) to produce water molecules which
are in no way different from the water in which they were produced.
The result of this two-stage ion exchange process is water that is
mineral-free.
Equipment for use in the deionization process may be of several types.
Available are both multiple bed and single bed units. Multiple bed
units have pairs of tanks, one for the cation exchanger, the other for
the anion exchanger. Single bed units incorporate both the cation and
anion exchangers, mixed in a single tank.
Deionized water has a wide range of uses in industry. Chemical
production, pharmaceuticals, electroplating, television tube
production and leather goods processing are among the many diversified
applications for deionized water.
Reverse Osmosis
Reverse osmosis (RO) is the most economical method of removing 90% to
99% of all contaminants. The pore structure of RO membranes is much
tighter than UF membranes. RO membranes are capable of rejecting
practically all particles, bacteria and organics >300 daltons
molecular weight (including pyrogens). In fact, reverse osmosis
technology is used by most leading water bottling plants.
Natural osmosis occurs when solutions with two different
concentrations are separated by a semi-permeable membrane. Osmotic
pressure drives water through the membrane; the water dilutes the more
concentrated solution; and the end result is an equilibrium.
In water purification systems, hydraulic pressure is applied to the
concentrated solution to counteract the osmotic pressure. Pure water
is driven from the concentrated solution and collected downstream of
the membrane.
Because RO membranes are very restrictive, they yield slow flow rates.
Storage tanks are required to produce an adequate volume in a
reasonable amount of time.
RO also involves an ionic exclusion process. Only solvent is allowed
to pass through the semi-permeable RO membrane, while virtually all
ions and dissolved molecules are retained (including salts and
sugars). The semi-permeable membrane rejects salts (ions) by a charge
phenomena action: the greater the charge, the greater the rejection.
Therefore, the membrane rejects nearly all (>99%) strongly ionized
polyvalent ions but only 95% of the weakly ionized monovalent ions
like sodium.
Reverse osmosis is highly effective in removing several impurities
from water such as total dissolved solids (TDS), turbidity, asbestos,
lead and other toxic heavy metals, radium, and many dissolved
organics. The process will also remove chlorinated pesticides and most
heavier-weight VOCs. Reverse osmosis and activated carbon filtration
are complementary processes. Combining them results in the most
effective treatment against the broadest range of water impurities and
contaminants.
RO is the most economical and efficient method for purifying tap water
if the system is properly designed for the feed water conditions and
the intended use of the product water. RO is also the optimum
pretreatment for reagent-grade water polishing systems
_Water Purification Text____________________________________________
According to tis, there is a 5% difference in removal rates of
monovalent ions like sodium, and only a 1% difference in covalent
ions.
_CS____________________________________________________________
Deionized water goes through basically the same process as softened
water - ionic exchange - but substitutes hydrogen ions instead of
sodium ions. The positive and negative hydrogen ions combine to
produce pure water, so they are taken intirely out of the equation.
Deionized water is neutral in pH (7) but very quickly becomes acidic
as it has a strong afinity for CO2 from the air, forming carbonic
acid.
Reverse osmosis water is very similar in this regard but has
somebuffering capacity due to the presence of a low level of
monovalent ions.
Distilled water has a less agressive affinity for CO2, as well as
having a varying buffering capability due to the presence of SOME
monovalent and co-valent ons, so does not acidify quite as quickly.
Both reverse Osmosis and deionized water are considered to be
"agressive" - but in a chemically different way than "hard" or highly
mineralized water.
_CS______________________________________________________________
From the Work Health Organization guides for drinking water quality:
Demineralised water is highly aggressive and if untreated, its
distribution
through pipes and storage tanks would not be possible. The aggressive
water attacks the water distribution piping and leaches metals and
other
materials from the pipes and associated plumbing materials.
_WHO_____________________________________________________________
