## Description

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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