[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Water hardness (long)
Hi Bob,
You are absolutely right, that the nomenclature of water hardness is
totally messed up! This is principally the fault of one publisher of
aquarium literature, who translated German aquarium books into English,
using translators who did not understand the subject and translated
everything literally or, even worse, "made a guess" which made sense to
them, but was technically wrong! These books spread this "knowledge" and
thus created the present confusion. Older American aquarium books,
before the "age of translations", say in the '50s and '60s, although
simple, because not all aspects of water hardness were understood, had
it right nevertheless.
Water hardness - particularly its nomenclature - is a relatively simple
subject to sort out, but must be viewed in context of its development:
Originally, it was used for assessing the water's ability to form lather
during laundering. In "hard water" it was hard to form lather because of
formation of calcium/magnesium salts of fatty acids (the so-called
"bathtub crud"). Because washing clothes is fairly much universal, most
industrial countries had their own "national" water hardness scales,
usually expressed as "degrees". Thus we had American, English, French,
German, etc. degrees of water hardness. The abbreviation "dH" stands for
"deutsche Haerte" (German hardness) and "DH" for "Degrees of Hardness" -
(American hardness). Britain used "degrees Clark" and so on. These
"degrees" were based on different chemical reactions and different
reference volumes, so are not readily interconvertible, but there are
tables to help in that task.
Fairly soon it was evident, that not all "water hardness". is the same,
i.e. it can be formed by different Ca/Mg salts that behave differently.
Thus water hardness was divided into "permanent hardness" and "temporary
hardness", the sum of the two being "total hardness".
The "temporary hardness" component consisted mostly of the BICARBONATES
of calcium and magnesium [Ca(HCO3)2 & Mg(HCO3)2], because the carbonates
[CaCO3 & MgCO3] are very little soluble, their concentration in solution
was low and thus they did not contribute significantly to hardness.
For simplicity's sake, in further discussion, I will refer only to
*calcium*, but it shall be understood, that *magnesium* has a very
similar chemistry and is also an important component of water hardness.
Calcium carbonate [CaCO3] can react reversibly with additional carbon
dioxide [CO2] to form the soluble calcium bicarbonate, which then
contributes to water hardness.
"Temporary hardness" or "carbonate hardness" (in its true sense) can be
removed by boiling the water. Upon heating the soluble bicarbonates
decompose into CO2, [which escapes from solution], water and calcium
carbonate [CaCO3] which precipitates as "boiler stone". This reaction is
also reversible if, upon cooling, a source of additional CO2 is
provided.
The calcium carbonates and bicarbonates in solution form a *buffer* that
resists changes in pH (by the way, it is ALWAYS lower case "p" and
capital "H" = pH). "Buffer" is a concept best handled separately. It
involves the equilibrium between calcium carbonates/bicarbonates.
Suffice it to say here, that the higher the "buffering capacity" of the
water, the better its ability to *resist* changes in pH, particularly
upon addition of acids. In an aquarium "buffering capacity" prevents
large swings in pH and the resultant "pH shock" to the livestock. The
more of these calcium carbonates/bicarbonates are present, the more
alkaline is the water, hence the term *ALKALINITY*.
However, only in certain cases is "alkalinity" equal to "carbonate
hardness", because "alkalinity" can be caused by ANY alkali, while
"carbonate hardness" refers ONLY to the calcium carbonate/ bicarbonate
content. A simple titration can estimate the concentration of calcium
carbonate/bicarbonate - the ALKALINE component of hardness. Thus the
term "carbonate hardness" (KH = from the German "Karbonathaerte") -
which in most German waters actually measured one component of water
hardness, the Ca/Mg carbonates - started to be applied, incorrectly, to
measurements of alkalinity, as if alkalinity was a component of water
hardness. Unfortunately, using the simple analysis kits now available on
the market this difference is NOT distinguishable, but it is REAL!
"Permanent hardness" is comprised of all other salts of calcium and
magnesium, most commonly the sulfates and chlorides. It does not affect
pH to any great extend, nor is it removable by simple means (if one
excludes water softeners as "simple"). It DOES have a physiological
effect on biota. Water high in "permanent" hardness" = "non-carbonate
hardness" is NOT suitable for fish or plants that *require* soft
water.
Total hardness (TH) or "general hardness" (GH - from the German
Gesamthaerte = total hardness!) is simply the sum of permanent and
temporary hardness *or* of carbonate and non-carbonate hardness. In many
natural waters total hardness will be the same whichever set of
parameters one measures. If it is *not* the same, then it indicates that
some other components, besides salts of calcium and magnesium are
present in quantities that affect the measurement.
In practical terms *total hardness* is very easy to measure: To a sample
one adds a buffer, an indicator and titrates (adds drop-by-drop) with
EDTA solution. (EDTA = etylenediaminetetraacetic acid). A color change
from pink to deep blue is your endpoint. Read off the volume of EDTA
solution added and look up/calculate the total Ca/Mg hardness.
As mentioned above, one can determine *alkalinity* by a simple
titration. If all the alkalinity comes from Ca/Mg carbonates/
bicarbonates, then it is a true *carbonate hardness* determination. If
it happens that carbonate hardness is *larger* than total hardness, then
you know that some other component(s) contributed to the alkalinity, but
is being lumped into the "hardness" term. There are methods available to
determine the *true carbonate hardness*.
To summarize:
Carbonate hardness (KH) is NOT the same as alkalinity. It can have the
same value as "total hardness", but cannot be larger than TH, even if
the measurement says so!
Total hardness (TH) is the sum of all hardness causing components
(calcium and magnesium salts) present in the water and *calculated* as
calcium carbonate.
Alkalinity, in the strict sense, is independent of carbonate hardness,
but often parallels it. It is a measure of buffering capacity.
P.S. These days all the scientific, governmental, and many hobby
publications use parts-per-million units to measure hardness and NOT
"degrees", because "degrees" are an imprecise term - which "degree"?.
Alkalinity is commonly measured in milli-equivalents per liter.
I have tried to keep this as simple as possible, did not use any
equations, but not all people can visualize a chemical reaction! Hope
that this somewhat historical overview will not be too confusing. More
confusion on water hardness we don't need!
If you have any questions, ask away.
Best,
George