[Leica] Re: Deionized Water (more and long)

[William Gower <w.gower@home.com>]

Jim Brick Wrote:

>Deionized water is what all chem/bio/etc. labs use for "pure" water. It is
>at least the equivalent of double distilled water. Many cartridge
>combinations provide triple or better distilled water equivalent.
>So in most laboratory usages (photo lab included) deionized water would be
>better than ordinary distilled water.

>If my chem & photo lab experience has taught me incorrectly, I'm sure Mr.
>Gower will correct me.

Jim...really, "Mr. Gower" is not necessary. I'm only 30 :-)

But, you're right. Generally speaking, deionized water is of a higher
quality than simple steam distilled water that you buy commercially.

The quality of water (deionized or distilled) is measured in units of
conductivity or the inverse, which is resistivity. The less dissolved salts
in the water, the more resistive it is, and therefore, less conductive. It's
as simple as that.

Units of measurement are the ohm-cm3 (resistance) or mho-cm3 (conductivity).
When water gets really pure, the resistance is measured in millions of Ohms,
or meg-ohm or the inverse, micro-mho. Occasionally, microsiemens are used
interchangeably with micro-mho.

So:

Tap water = 0.05 meg-ohm

Distilled water (once) = 0.5 meg-ohm

Distilled water (three times) = 1 meg-ohm

Distilled water (28 times in Quartz) 18.3 meg-ohm

Theoretical quality = 26 meg-ohm

Deionized water is generally on par with 3 times distilled. Is low quality
deionized water. Normally, 2-5 meg-ohm is not uncommon in industrial
processes. I've worked on high purity systems that produce water for
super-critical pressure boilers that power steam turbines for electricity
where 12 meg-ohm was the requirement, and in nuclear power stations where 16
to 18 meg-ohm is the standard. For these systems, the water is purged with
an inert gas so that O2 and CO2 from the air dosen't dissolve into the
water. The action of CO2 can increase the conductivity and drop the pH to
below 4.3.

I use 1 meg-ohm water for rinsing my negs. I get it from work from a
deionizer system that produces water for a low pressure boiler system. This
is a "baby" system that makes around 200 US gallons per minute. So me taking
5 gallons every other week doesn't go missed.

When I worked as an analytical Chemist, 18.3 meg-ohm was the standard for
trace metal analysis. We would routinely measure parts per billion levels of
inorganic material and the water would be lower than the detection limit for
some pretty sophisticated equipment.

18.3 meg-ohm would be over-kill for photographic use, but it would be simple
to build a deionizer to deliver near this quality. (This would be temporary,
as the action of CO2 and O2 would lower the quality, but it would probably
stablilize around 2-4 meg-ohm.)

Water that pure (18 meg-ohm) can be problematic - it can act as an acid. It
can dissolve steel no problem. And quickly, too. You need CPVC or
polypropylene pipes to ensure it doesn't eat through your piping. I saw this
in a power plant in Oklahoma once, it was amazing - a 10" diameter steel
pipe made so thin you could poke your finger through it.

The heart of the deionizer is the exchange resin. This is the material that
removes the dissolved solids in exchange for a hydrogen (H+) or hydroxyl
(OH-) ion. H+ + OH- = H-OH (or H2O). The process is reversible in that the
ions that are trapped on the resin can be removed by washing with dilute
acid or caustic. The time for the ions to accumulate on the resin depends on
the amount that is dissolved in your water. This will vary from person to
person, city to city, etc.

The amount of solids is expressed as total dissolved solids or TDS, in parts
per millions or mg/L of water. The higher the TDS, the higher the
conductivity of your water. Toronto City water (Lake Ontario) has a TDS of
around 250 mg/L solids or around average TDS. 400-600 TDS is getting higher,
and is typical of a carbonate aquifer (think poor Martin). I did a talk in
China where the groundwater being recycled was on the order of 30,000 TDS.
That's pretty brackish water.

As for the home made deionizer, it's going to be pretty simple. PVC or ABS
pipe for the body, polypropylene tubing, resin. Easy as pie. I'll do the
hard stuff - an excel spreadsheet that will calculate the quality of your
water, predict the quality of the deionized water that you can expect, how
long you can go between regenerations and the size of deionizer you need
(based on the flow required).

Now, your homework for this project is to call your local water utility and
get a water analysis. This is a routine request, and should be easy with one
phone call - call your waterworks department and ask for someone in the
engineering department. That's the best place to start.

Give me a week or so to work on the spreadsheet. I'll mail it to everyone
who has contacted me directly and anyone else who asks.

I'm off to the Nations Capital tomorrow evening for a nice long weekend
(that would be Canada's national capital, Ottawa) of winter fun and the
National Gallery of Contemporary Photography - my M2 is all packed up and
ready to go. 

Catch up with you all next week.

Kind regards

William