DeLgAdO;502447; said:
nope
these mineral are insoluable in water.
but what dissolves them?
Lets review our basic water chemistry...
A review what PH is and where the term comes from;
Water is composed of two atoms of hydrogen bound to one atom of oxygen
like this: H-O-H
Water ( the universal solvent) willl dissolve itself and produce two charged ions in solution as follows;
H - O - H <---> H(+) + OH(-)
Note the double headed arrow. This implies that it is an equillibrium reaction that can move in either direction. At any one time hydrogen ( H+) and hydroxide (OH-) ions are combining to form a water molecule at the same rate they are dissolving elsewhere in your fish tank. The final concentration (precentage if you like) of the ions reaches what we refer to as "chemical equilibrium"
The equilibrium for the dissolution of water, Kw, is determined as follows;
Kw (water constant),
' [H+][OH-]
= ---------------- = 1.00 * 10^-14 (1*10 to the power of -14)
' [H-O-H]
where [H+] is the concentration of hydrogen ions (protons)
[OH-] is the concentration of hydroxide ions
Both ions will have a concentration of 1.00 * 10^-7 ( the square root of Kw)
The pH of a solution is defined as the negative log of the hydrogen
ion concentration, so, water at equilibrium contains 1.00 * 10^-7
hydrogen ions, and the negative log of this is equal to 7. This gives
us the basis of the pH chart with pH 7 representing neutrality. Lower
pH numbers represent more hydrogen ions in solution and are more
acidic, higher pH numbers represent more hydroxide ions in solution
and are more basic.
Acids are compounds which give up a hydrogen ion, while bases
accept a hydrogen ion. For instance, hydrochloric acid (an example of a strong
acid) is composed of an atom of hydrogen and one atom of chlorine: H-Cl. When dissolved in water it undergoes the following reaction:
H-Cl <----> H(+) + Cl(-)
A "strong" acid means that it almost entirely exists as the dissociated components, H+ and Cl-.
Calculating the pH of a strong acid solution is relatively straight
forward as you can assume that for each molecule of starting material,
one molecule of hydrogen will be produced. Thus, 0.5 Molar HCl will
produce 0.5 Moles of H+ in solution, which gives:
pH = -log(0.5) = 0.3
More commonly found in biological systems are 'weak' acids which are
not usually found completely dissociated into ions, but exist at
equilibrium with a fair amount of non-ionized starting material. An
example is acetic acid (vinegar): CH3CO2H
O O
|| ||
CH3-C-O-H <----> CH3-C-O- (-) + H(+)
Now, since weak acids do not completely dissociate, we must consider
the amount of starting material in our equation. The equilibrium
constant for the dissociation is defined as Ka (which in this case is
1.8*10^-5 measured by dedicated chemists the world over):
Ka (Acid constant) =
[CH3CO2-][H+]
-------------------- = 1.8*10^-5
[CH3CO2H]
Example: Calculate the pKa (same as pH only for chemicals other than water that are dissolved) of a solution of 0.5M acetic acid Ka =
[CH3CO2-][H+]
---------------- =
[CH3CO2H]
[x][x]
-------- = 1.8*10^-5 =
[0.5M]
[x][x]= (0.5 M )*(1.8*10^-5)
x^2 = (0.5 M )*(1.8*10^-5)
x = 0.003
so pKa = -log(0.003) = 2.5
More complicated examples can be considered by considering how this
reaction actually proceeds in water:
CH3CO2H + H20 <----> H30(+) + CH3CO2(-)
the acetic acid acts as an acid and the water acts as a base on the
left side of the equation. For the reverse reaction (called the
conjugate reaction), the protonated water (H3O+) acts as an acid (the
conjugate acid) and the CH3CO2- acts as a base (the conjugate base).
This is the same if an acid and a base are mixed in solution.
Represented as:
A-H + B <----> A(-) + B-H(+)
where:
A-H = acid
B = base
A(-) = conjugate base
B-H(+) = conjugate acid
a very useful equation for calculating the pH of a solution using the
pKa of the dissolved compounds, is the Henderson-Hasselbach equation:
''''''''''''''''''''''''''''''''[acid]'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
pH = pKa - log ( -------- )
,,,,,,,,,,,,,,,,,,,,,,,,[base],,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
pKa values can be found in a table in one of your old high school chemistry books.
Strong bases obey the same principle as strong acids - they completely
dissociate in solution.
Weak bases obey the same rules as weak acids, they do not dissociate completely.
Think of dissolved compounds as having an acid and a basic component in their equilibrium.
A rich mixture of weak acids and bases in an aquarium will have a strong buffering affect and prevent sudden or swift changes in the pH
Ok enough of the long hair chemistry talk; the point is that a lot of things dissolve in water minerals included.
They form both acid and basic components in solution. How acidic or basic depends on the pH and the amount of other dissolved materials that may absorb these components to form other components etc, etc.
Corals consist primaraly of calcium carbonate but contain magnesium and other elements as well. Coral carbonates will have differing values of pKa depending on where they come from but will act as effective buffering compounds nonetheless.
Limestones also act as excellant buffering agents but tend to dissolve more slowly due to less porosity in their construction. Solubility is also a function of surface area, which is greatly enhanced by porosity.
