I found this to be a great explanation of [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd] and [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] at huge website called The Krib
http://www.thekrib.com/. As I have been trying to figure some of this out for myself, I thought it best to pass it along. By the way, keep those airstones humming at night. I hear its good for the fishies. Also,
let me know what you think of this article too
Betowess
Water Hardness
Contents:
[acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd], carbonate hardness, etc.
by booth-at-hplvec.LVLD.HP.COM (George Booth) (Tue, 6 Apr 1993)
--------------------------------------------------------------------------------
[acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd], carbonate hardness, etc.
by booth-at-hplvec.LVLD.HP.COM (George Booth)
Date: Tue, 6 Apr 1993
This should make it to the [acronym:4fa805c6dd="Frequently asked questions"]FAQ[/acronym:4fa805c6dd] sometime. Does this help?
=========================================
PRACTICAL FRESHWATER HARDNESS
Water hardness is of interest to aquarists for two reasons: to
provide the proper environment for the fish and to help stabilize the
pH in the aquarium. There are two types of water hardness: general
hardness ([acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd]) and carbonate hardness ([acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd]). A third term commonly used
is total hardness which is a combination of [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd] and [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd]. Since it is
important to know both the [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd] and [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd], the use of total hardness can be
misleading and should be avoided.
GENERAL HARDNESS
General hardness is primarily the measure of calcium ([acronym:4fa805c6dd="Calcium"]Ca[/acronym:4fa805c6dd]++) and
magnesium ([acronym:4fa805c6dd="Magnesium"]Mg[/acronym:4fa805c6dd]++) ions in the water. Other ions can contribute to [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd]
but their effects are usually insignificant and the other ions are
difficult to measure. [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd] will not directly affect pH although "hard"
water is generally alkaline due to some interaction of [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd] and [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd].
[acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd] is commonly expressed in parts per million ([acronym:4fa805c6dd="Parts per Million"]ppm[/acronym:4fa805c6dd]) of calcium
carbonate ([acronym:4fa805c6dd="Calcium carbonate"]CaCO3[/acronym:4fa805c6dd]), degrees hardness (dH) or, more properly, the molar
concentration of [acronym:4fa805c6dd="Calcium carbonate"]CaCO3[/acronym:4fa805c6dd]. One German degree hardness (dH) is 10 [acronym:4fa805c6dd="Magnesium"]mg[/acronym:4fa805c6dd] of
calcium oxide (CaO) per liter. In the U.S., hardness is usually
measured in [acronym:4fa805c6dd="Parts per Million"]ppm[/acronym:4fa805c6dd] of [acronym:4fa805c6dd="Calcium carbonate"]CaCO3[/acronym:4fa805c6dd]. A German dH is 17.8 [acronym:4fa805c6dd="Parts per Million"]ppm[/acronym:4fa805c6dd] [acronym:4fa805c6dd="Calcium carbonate"]CaCO3[/acronym:4fa805c6dd]. A molar
concentration of 1 milliequivalent per liter (mEq/l) = 2.8 dH = 50
[acronym:4fa805c6dd="Parts per Million"]ppm[/acronym:4fa805c6dd]. Note that most test kits give the hardness in units of [acronym:4fa805c6dd="Calcium carbonate"]CaCO3[/acronym:4fa805c6dd];
this means the hardness is equivalent to that much [acronym:4fa805c6dd="Calcium carbonate"]CaCO3[/acronym:4fa805c6dd] in water but
does not mean it actually came from [acronym:4fa805c6dd="Calcium carbonate"]CaCO3[/acronym:4fa805c6dd]. Water hardness follows
these guidelines:
0 - 4 dH, 0 - 70 [acronym:4fa805c6dd="Parts per Million"]ppm[/acronym:4fa805c6dd] : very soft
4 - 8 dH, 70 - 140 [acronym:4fa805c6dd="Parts per Million"]ppm[/acronym:4fa805c6dd] : soft
8 - 12 dH, 140 - 210 [acronym:4fa805c6dd="Parts per Million"]ppm[/acronym:4fa805c6dd] : medium hard
12 - 18 dH, 210 - 320 [acronym:4fa805c6dd="Parts per Million"]ppm[/acronym:4fa805c6dd] : fairly hard
18 - 30 dH, 320 - 530 [acronym:4fa805c6dd="Parts per Million"]ppm[/acronym:4fa805c6dd] : hard
higher : liquid rock (Lake Malawi and Los Angeles, [acronym:4fa805c6dd="Calcium"]CA[/acronym:4fa805c6dd])
General hardness is the more important of the two in biological
processes. When a fish or plant is said to prefer "hard" or "soft"
water, this is referring to [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd]. Incorrect [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd] will affect the transfer
of nutrients and waste products through cell membranes and can affect
egg fertility, proper functioning of internal organs such as kidneys
and growth. Within reason, most fish and plants can successfully
adapt to local [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd] conditions, although breeding may be impaired.
CARBONATE HARDNESS
Carbonate hardness ([acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd]) is the measure of bicarbonate ([acronym:4fa805c6dd="Hydrogen carbonate"]HCO3[/acronym:4fa805c6dd]-) and
carbonate ([acronym:4fa805c6dd="Carbonate"]CO3[/acronym:4fa805c6dd]--) ions in the water. In freshwater aquariums of
neutral pH, bicarbonate ions predominate and in saltwater aquariums,
carbonate ions begin to play a role. Alkalinity is the measure of the
total acid binding capacity (all the anions which can bind with free
H+) but is comprised mostly of carbonate hardness in freshwater
systems. Thus, in practical freshwater usage, the terms carboante
hardness, acid binding, acid buffering capacity and alkalinity are
used interchangeably. In an aquarium, [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] acts as a chemical buffering
agent, helping to stabilize pH. [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] is generaly referred to in degrees
hardness and is expressed in [acronym:4fa805c6dd="Calcium carbonate"]CaCO3[/acronym:4fa805c6dd] equivalents just like [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd].
In simple terms, pH is determined by the negative log of the
concentration of free hydrogen ions (H+) in the water. If you add a
strong acid such as nitric acid to water, it completely dissociates
into hydrogen ions (H+) and its "conjugate base" or "salt", [acronym:4fa805c6dd="Nitrate"]NO3[/acronym:4fa805c6dd]- or
nitrate. The hydrogen ions freed in the reaction then increase the
concentration of hydrogen ions and reduce the pH. Since nitric acid
is the end product of the nitrogen cycle, this explains why aquarium
pH tends to decrease and nitrates tend to increase over time.
When the aquarium has some carbonate buffering in it, the
bicarbonate ions will combine with the excess hydrogen ions to form
carbonic acid (H2CO3) which then slowly breaks down into [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd] and
water. Since the excess hydrogen ions are used in the reaction, the
pH does not change very much. Over time, as the carbonate ions are
used up, the buffering capacity will drop and larger pH changes will
be noted. From this it is clear why aquariums with low [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] seem
unstable - as acid is produced by biological action, the [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] is used
up; when it is gone, the pH is free to drop rapidly as H+ ions are
generated.
ADJUSTING FRESHWATER HARDNESS
If your local water is too hard for the fish and plants you desire,
it can be softened. There are many ways to do this but some are more
suited to aquarium use than others. The best (and most expensive, of
course) is to use a reverse osmosis ([acronym:4fa805c6dd="Reverse osmosis"]RO[/acronym:4fa805c6dd]) deionizer and mix the
resulting water ([acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd]=0) with tap water to get the desired [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd]. Peat
moss can be used to soften and condition the water for use in South
American cichlid tanks, but will add a slight tea color to the water.
Peat filtering may be difficult to control. Peat should be boiled
first to kill any unwanted organisms.
Commerical water softening resin "pillows" can be used on a small
scale, but are not effective for larger amounts of water. Water
softening systems designed for large scale home use (like bath water)
are not suitable since they use an ion exchange principle: usually
sodium ions are substituted for calcium and magnesium ions and excess
sodium is not desired in the aquarium. An even worse practice is to
use a cation exchange resin in the hydrogen ion form and use it to
pull divalent ions out of the water.
If the local [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd] is too low, it can be raised by adding calcium
sulfate and/or magnesium sulfate. This has the drawback of
introducing sulfates (SO4--) into the water, so care should be
exercised. Calcium carbonate can be used, but it will also raise the
[acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] (this is ideal for the lucky few who have naturally soft water).
Various combinations can be used to produce the desired results.
Carbonate hardness can be reduced by boiling the water (inpractical
for all but the smallest aquariums; let it cool before adding to the
tank
or by peat filtering.
Carbonate hardness can be easily increased by adding sodium
bicarbonate. Calcium carbonate will increase both [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] and [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd] in equal
parts.
One teaspoon (about 6 grams) of sodium bicarbonate (NaHCO3) per 50
liters of water will increase [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] by 4 degrees and will not increase
general hardness. Two teaspoons (about 4 grams) of calcium carbonate
([acronym:4fa805c6dd="Calcium carbonate"]CaCO3[/acronym:4fa805c6dd]) per 50 liters of water will increase both [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] and [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd] by 4
degrees. Different proportions of each can be used to get the correct
[acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd]/[acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd] balance dictated by the fish and plants in the tank. Since it
is difficult to accurately measure small quantities of dry chemicals
at home, a test kit should be used to verify the actual [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] and [acronym:4fa805c6dd="General Hardness"]GH[/acronym:4fa805c6dd] that
is achieved.
FRESHWATER CHEMISTRY DETAILS
In more detail, the pH of a buffered solution can be expressed by the
Henderson-Hasselbach equation:
base
pH = pK + log ----
acid
where pK is one or more "equilibrium dissociation constants" of the
weak acid. In the bicarbonate and carbonate buffering cases, this is:
[acronym:4fa805c6dd="Hydrogen carbonate"]HCO3[/acronym:4fa805c6dd]- [acronym:4fa805c6dd="Carbonate"]CO3[/acronym:4fa805c6dd]--
pH = 6.37 + log ----- and pH = 10.25 + log -----
H2CO3 [acronym:4fa805c6dd="Hydrogen carbonate"]HCO3[/acronym:4fa805c6dd]-
Note that the pK values are affected rather appreciably by temperature
and chlorinity. If you plot the pH versus the ratio of base to acid,
you will get a logarithmic graph something like this:
100% base ,--------- ,----------
/ /
H2CO3 | [acronym:4fa805c6dd="Hydrogen carbonate"]HCO3[/acronym:4fa805c6dd]- | [acronym:4fa805c6dd="Carbonate"]CO3[/acronym:4fa805c6dd]--
50% mix + [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd] |<- pH=6.37 |<- pH=10.25
| |
/ /
100% acid --------` --------`
pH ...5...6...7...8...9...10..11..12...
Bicarbonate buffering is effective over ratios from 1:100 up to
100:1. This gives an effective pH range of 4.37 to 8.37, which, not
coincidently, defines the pH range of most aquatic life.
If you add bicarbonate ions (for example, by adding sodium
bicarbonate or calcium carbonate), the base to acid ratio will
increase and the pH will increase. From the graph, the rate of
increase will be determined by the pH you started with: at pH = 6.37,
you will need a lot of bicarbonate ions to change the pH; at pH=7.5,
you will need a lot less. (Note: the chemical equilibria of the
various components of the carbonate system ([acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd], H2CO3, [acronym:4fa805c6dd="Hydrogen carbonate"]HCO3[/acronym:4fa805c6dd]- and
[acronym:4fa805c6dd="Carbonate"]CO3[/acronym:4fa805c6dd]--) are very complex and are beyond the capability of the author to
fully describe).
The rise in pH that occurs when [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] is added will be balanced to a
degree by the dissolved [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd] in the water. Fortunately, [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd] is also a
result of the nitrification process and fish and plant respiration so
it is readily available. The [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd] will form small amounts of carbonic
acid and bicarbonate which will tend to reduce the pH. This mechanism
gives us a way to regulate pH in the aquarium.
If the pH of an aquarium is determined PRIMARILY by the carbonate
buffering system, then the relation of pH and [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] and dissolved [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd] is
fixed. You can change either [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] or [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd] to set the pH. An automatic [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd]
injection system will measure pH and inject [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd] to lower it if it
exceeds a set point. In this case [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] is fixed. As the [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd] is used by
plants and diffuses into the atmosphere, the pH will rise. The
controller cycles the [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd] on and off to keep the pH around a fixed
value.
The following chart shows dissolved [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd] levels in [acronym:4fa805c6dd="Parts per Million"]ppm[/acronym:4fa805c6dd] for a range of
[acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd] and pH values:
degrees [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd]
2 3 4 5 6
+------------------------
6.6 | 15 23 30 38 46
6.7 | 12 18 24 30 36
6.8 | 9 14 19 24 28
pH 6.9 | 7 12 15 19 23
7.0 | 6 9 12 15 18
7.1 | 5 7 9 12 14
7.2 | 4 6 8 9 11
7.3 | 3 4 6 7 9
7.4 | 2.4 3 5 6 7
7.5 | 1.9 2.5 3.5 5 5
7.6 | 1.5 2 2.5 3 4
7.7 | 1 1.5 2 2.5 3
7.8 | 0.9 1.1 1.5 2 2
7.9 | 0.6 0.9 1 1.2 1.6
8.0 | 0.5 0.7 0.9 1 1.2
Note that typical dissolved [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd] levels in a moderately stocked
aquarium will be in the range of 2-3 [acronym:4fa805c6dd="Parts per Million"]ppm[/acronym:4fa805c6dd]. From the chart, it is clear
that almost any carbonate hardness will produce a pH in the mid 7
range unless extra [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd] is added. For a typical planted aquarium,
pH=6.9, [acronym:4fa805c6dd="Carbonate Hardness"]KH[/acronym:4fa805c6dd]=4 and [acronym:4fa805c6dd="Carbon dioxide"]CO2[/acronym:4fa805c6dd]=15 [acronym:4fa805c6dd="Parts per Million"]ppm[/acronym:4fa805c6dd] is just about ideal.
[/b]
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