Re: [Killietalk] What's in my water

["Edd Kray" <edd at catfishnthecrawlers_com>]

Putting together all this chemistry talk (good) with my question on RO
water:

I don't like the idea of adding any of my tap water back in because it's the
Ca/Mg I want to make "go away" and it will put it back in along with the
very strong buffering that keeps my water above 8.

I don't like using some commercial product when no-one can tell me what's in
it and what I'm adding back.

Why not just plain Salt (NaCl), It'll give me a know chemistry, add back
some ionic strength and conductivity, counteract osmotic stress and it's
cheap (lots cheaper that a product like Equillibrium)

See any problems with that?

-----Original Message-----
From: killietalk-bounces+edd=catfishnthecrawlers_com at aka.org
[mailto:killietalk-bounces+edd=catfishnthecrawlers_com at aka.org] On Behalf Of
Koran, David HQ02
Sent: Friday, February 03, 2006 11:16 AM
To: killietalk at aka_org
Subject: [Killietalk] What's in my water

As Charlie has stated you are measuring apples and trying to state the
result
as oranges.  When you measure conductivity this is a sum of a concentration
of charges so to speak.  You sum the molal conductivities of the components
of your solution.  Molality is a concentration term based on weight per
weight, i.e., the number or fraction of atomic/molecular weights of an ion
in
a kilogram of water.  Molal conductivity is also a function of the charge of
the ion.  To simplify matters you can approximate an average value for ionic
conductivities and then use this artificial factor to relate the electrical
measurement to weight of dissolved solids in your solution.

To find the weight of dissolved solids you must eliminate the solution
(water) or "dry the sample".

Osmotic pressure is also the sum of osmotic pressures of the solution
components.  In this case the component osmotic pressure is a linear
function
of the molality of the component.  The net directional flow of water across
a
permeable membrane (i.e., cell wall) is determined the net difference of
component osmotic pressures.

Hardness is a subjective measurement of the ability of soap in a water
sample
to form a lather or bubbles.  You can measure the concentration of
multivalent cations in a solution by a "complexometric titration" and
multiple by a factor to relate to hardness.  Hardness test kits add a simple
alkaline buffer to your test solution and an indicator complexing agent
which
quickly binds to your multivalent ions in the test solution (that ions with
a
charge of +2 or more, namely calcium and magnesium but this will also hold
true for iron and other metal cations which are probably in not much of a
concentration).  When you add your 3rd solution you are actually titrating
your metal ions with a more effective complexing agent than the indicator.
This titrating agent is solution of a known concentration of EDTA (you see
this in a lot of products! And is the chemical abbreviation for
ethylenediaminetetraacetic acid which has 6 binding sites and effectively
blankets each metal ion).  You add titrating solution until the EDTA
scavenges every multivalent ion in solution and then out-competes the
indicator for the metal ions it previously tied up.  At your end point or
read out point you see a color change and that is when you have an excess of
freed indicator (acid) over the metal ion-indicator complex ion.  You chose
your indicator such that the color change (in solution) between the two
forms
is dramatic.  By rigging the concentration of solution you drop into your
test solution to some concentration which is a multiple of what maybe
doubling the volume (by addition of test solution) you can count drops and
get an approximation of what the initial concentration of your multivalent
ions were.  Remember, here again you have ions of different atomic weight
but
for simplicity sake you assume all multivalent ions you titrate/complex are
calcium.  For the test kits you buy, calmagite is usually the indicator and
in the free ionic acid form it is produces a blue solution (actually the
conjugate base is a blue ion or absorbs light in all but the blue region of
the spectrum) whereas when it is complexed with calcium ion it imparts a red
color to water.

Notice that in all of this discussion nothing was said about measuring
monovalent ions or the ions of your akali metals; sodium, potassium and
lithium.  It should also be noted that in measuring conductivity, calcium
and
magnesium ion will produce twice the conductivity of the akali metel ions.
As hardness goes, it relates to the amount of multivalent ions present.  You
form soaps and lubricants by breaking down polyglycols with a strong base
(lye or sodium hydroxide) and form a salt of a fatty acid.  These soaps are
much more soluble than the parent fatty acid but when paired with a
multivalent ion like calcium or magnesium they readily form a precipitate
and
drop from solution.  You, therefore, don't simulate hard water by simply
adding salt, i.e., sodium chloride.

I know, ack, chemistry lessons on a Friday afternoon...you can now look for
your beer.

Dave K 

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