You are at The Krib ->Plants ->CO2 [E-mail]

Hardness (incl. History)


  1. Water hardness (long)
    by George Slusarczuk <yurko/> (Sun, 14 Jun 1998)
  2. More water hardness (long & boring)
    by George Slusarczuk <yurko/> (Wed, 17 Jun 1998)

Water hardness (long)

by George Slusarczuk <yurko/>
Date: Sun, 14 Jun 1998

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

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

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.



More water hardness (long & boring)

by George Slusarczuk <yurko/>
Date: Wed, 17 Jun 1998

Hi Roger,

>From your last post on the subject of water hardness (APD V3 #329) I see
that we agree on several things. For instance we agree that the
commercial "carbonate hardness" kits actually measure alkalinity; that
the use of "carbonate hardness" to describe alkalinity should be
avoided; that "hardness" always refers to concentrations of various
calcium and magnesium species and not other parameters.

We seem to hold different views on whether "temporary hardness" is the
same as "carbonate hardness" (in the true sense of the word) and whether
"temporary hardness" is of any use for aquarists. 

You searched the literature without finding a confirmation of the
identity of temporary hardness = carbonate hardness.I am really not
surprised, because you probably searched only the relatively recent
handbooks. One has to go to books from before the war (WW II, that is)
to find it. In the mainstream technical/scientific literature these
terms went out of use just after the war, when analytical chemistry
started turning instrumental. There was no need for them anymore! This
is exactly why I presented a *historical* overview. In my *opinion* a
historical perspective offers a clearer view of nomenclatural and
technical origins.

I don't think that these days there are many commercial analytical
laboratories using such manual analyses. Most, if not all, do it by
means of ICP or AA and ion chromatography. It is only the hobbyists that
still do it by hand, but even here each year there appear more and more
electronic analytical gadgets designed for the hobbyist. They are
covenient to use and are slowly displacing the chemical kits.

Unfortunately, currently I do not have a convenient access to a good
university library to look things up in order to give you an
authoritative "chapter and verse" that temporary hardness = carbonate
hardness. If I remember correctly (and that's an IF), in the old "Bible"
- - Analysis of Waters and Wastewaters, published by AWWA with other
interested Societies - and here one must find an old copy, with, I
think, a red cover (not blue or the present green) is just such a
description. There they also describe the "soap foam" method of
determining water hardness, relly ancient stuff.

While water hardness was "mainstream science", aquarists did not worry
(or know) about it. "The Aquarium Book" by E.G. Boulenger (THE
Boulenger!), published in 1925, does not even mention water hardness! I
think that water chemistry for aquarists became fashionable only after
WW II. The "Handbook of Tropical Aquarium Fishes" by H.R. Axelrod and
L.P Schultz, published by McGraw-Hill in 1955, has about 1/2 of a page
on hardness, recommending the soap-suds method, even providing an
address where to buy the soap.

You say:
> Certainly if we were to accept Mr. Slusarczuk's definition of 	> carbonate hardness=temporary hardness, then it would not be the > same as alkalinity. My tap water, for instance has 2 degrees of > temporary hardness, but 7 degrees of alkalinity.  If I mix a 	> solution of sodium bicarbonate and test it, I find no temporary > hardness, but plenty of alkalinity.

I am not certain HOW you measured temporary hardness (sensu stricto),
but I agree with you, that it is not the same as alkalinity - that's
EXACTLY my point!. As to the semantic identity of carbonate hardness =
temporary hardness, let's look at it from another angle:

There should be (is?) general agreement, that water hardness can be
divided into PERMANENT and TEMPORARY hardness. There should also be no
argument, that boiling the water will precipitate Ca/Mg carbonates i.e.
the *temporary* hardness. It does not take much of a logical leap, that
if temporary hardness is comprised of the Ca/Mg CARBONATES, it IS
"carbonate hardness"! In the "good old days" alkali and alkaline earth
carbonates could most conveniently be quantitated by titration with an
acid - an identical procedure to ALKALINITY determination. It is not
difficult to perceive how the two terms were synonymized in the popular
aquarium literature and in the minds of aquarists. That, however, does
not make "carbonate hardness" & "alkalinity" identical. 

For lack of a better source in my personal library, I quote from
"Aquarium Water Chemistry" by Dr. R. Geisler and published by TFH
(1963), page 6: - "By hardness we mean the content of dissolved calcium
and magnesium in the water. Depending how much calcium and magnesium is
tied in with carbon dioxide or other acids, we speak of carbonate
hardness and residual hardness, both of which combine to give us total

A more recent book "Aquarium Plants" by Barry James, published by
Salamander Books Ltd. (1986) on p. 14 states: "Since the carbonates
produced can be easily broken down by boiling, this type of hardness is
called temporary hardness or carbonate hardness".
> As aquarists we really have no particular interest in temporary 			> hardness, but we do have a substantial interest in alkalinity.  			> Temporary hardness just gives us a breakdown of total hardness 				> and for our purposes I think we only need to know about total 				> hardness. (snip)

Because temporary=carbonate hardness CONTRIBUTES to *alkalinity*, but
alkalinity, aside from the Ca/Mg carbonate contribution, has NO formal
contributory relationship to hardness, it would seem to me that some
attention should be paid to it, if one minds alkalinity at all.

> Temporary hardness has no inherent relation to buffer capacity.  			> An alkalinity test is an excellent measure of bicarbonate in 				> most unpolluted natural waters.

I agree with the second statement, but disagree with the first: In most
"natural" waters temporary=carbonate hardness IS the *buffering
component* of Total Hardness! It's the famous carbonate/bicarbonate
equilibrium equation that describes the buffering capacity of natural
waters. That does NOT mean that other species do not contribute to
alkalinity, because they do!

> My view on these definitions:  simplify, simplify, simplify
> "carbonate hardness", when used in the hobby refers to 						> alkalinity. Otherwise it appears to be a meaningless term and 				> should be avoided. If there is an accepted definition that 					> equates carbonate hardness with temporary hardness then this 				> only emphasizes our need to avoid using the term.

I could not agree more for the need of simplification. As the popular
saying goes "I agree one thousand percent"! Then why do some people
insist on using "carbonate hardness" when they mean "alkalinity"?
Because of "tradition"? "Simplicity?" 

I do not think that any member of this list has NOT heard, or does not
understand, the term "alkalinity" (certainly not after THIS discussion)
so, why not decide to use "alkalinity" instead of "carbonate hardness",
never mind whether they are identical or not!

I realize, that for most members of this list this discussion on the
finer points of water hardness is not that interesting. In that spirit I
have decided not to post any more "wisdom" on this topic on the list.
Anybody who has further questions/views/ arguments is invited to contact
me directly at <>

I thank the many who posted or e-mailed their support.


George "S"

(The "S" is so that there can't be any confusion with another George on
this list. He is totally blameless for my views!)

Up to CO2 <- Plants <- The Krib
This page was last updated 29 October 1998