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1.4 More on alkalinity and simplified calculations (Water for coffee)




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This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.

1.4 More on alkalinity and simplified calculations (Water for coffee)

There are two items that need to be noted prior to doing
calculations from published or test data.
Most tests measure hardness and alkalinity. They do not
measure total dissolved solids (this requires a conductivity test).
But, since calcium, magnesium, and carbonates form the preponderance
of dissolved solids in most natural drinking water, and since total
dissolved solids plays only a small role in the calculations, the
higher of the hardness or alkalinity measures (in CaCO3 equivalent
units) can serve as the total dissolved solids measure with only a
minimal loss of accuracy (The LI will come out about 0.1 to 0.2 too
high for water with lots of other minerals).
Second, acidity in natural water comes from dissolved CO2
(fizz), which can vary quickly. A recent improvement on the Langelier
index, called the Puckorius index, replaces the water's current pH
with its equilibrium pH (pHeq), which derives from the alkalinity
value. When water is exposed to air or heated, this works better as a
long term pH estimate than any current pH reading. The formula is:

pHeq = 1.465*log(A) + 4.54

The following table shows the alkalinity value that is equivalent to
the medium pH levels in drinking water:

EQUILIBRIUM WATER pH VERSUS BICARBONATE ALKALINITY IN mg/L CaCO3
----------------------------------------------------------
      pHeq   6.  .2  .4  .6  .8  7.  .2  .4   .6   .8   8.
Alkalinity   10  14  19  26  35  48  65  90  123  168  230
----------------------------------------------------------

By using these facts, one gets the following simplified, state
of the art, Langelier index formula (LI*) that requires only
temperature (T), hardness (H), and alkalinity (A):

LI* = 13.12*log(T +273) +log(H) +2.465*log(A) -log(max(A,H))/10 -39.61

Note that alkalinity has about 2 1/2 times the effect on the
LI as hardness, since changes to it effects both the water pH and the
calcium carbonate saturation. Regretably, most water information is
misleading on this, since only hardness values are discussed.

 

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