The technical expression is that pH is the negative logarithm of the
hydrogen ion concentration. What does this mumbo jumbo mean?
Chemically speaking, pure water isn't just
water molecules; a very, very small, but nevertheless important
proportion of any group of water molecules dissociates (falls apart) into the
component ions.
H2O <-> H+ + OH-
This is where the water (H2O) is in equilibrium
(balance) with its two ions, hydrogen (H+) and hydroxyl (OH-).
These ions have an electrical charge, which is quite small, but for
their size, very important.
In pure water, the concentrations of these ions are both 10-7,
chemists write this as [H+] = 10-7. This means one in ten million. The square brackets are used to denote that we're referring to the concentration.
Should you see H
The logarithm of the number 100, which is 10 x 10, or 102 is
simply "2". The logarithm of 1000 (103) is "3".
Multiplying, 100 x 1000 =100,000 or 105 , its log is, of
course, "5". Multiply the numbers, but add the powers, or logarithms.
Now taking ten to the power of minus something. 10-1 is 1/10
or 0.1; 10-2 = 0.01, and its log is "-2". Now the log of [H+],
10-7, is "-7", which is much easier to write than putting in
all the noughts every time.
Multiplying the concentrations of these two pure water ions
[H+] x [OH-] =10-14 (Each one is 10-7).
In pure water, the hydrogen ion concentration is 10-7 , so
the pH, which is the negative of the log for this concentration (-7),
is +7, or simply 7.
So we can say that the pH of pure water is 7! Water with this pH value we call neutral
In any water- based solution, whether lake, river, seawater or lemonade, the product of these two concentrations is always 10-14 . Water molecules will dissociate or reassociate to make this value.
Water is a very good solvent, many things dissolve in it very easily. Pure water is something that doesn't exist for long. Distilled water is very quick to take in gases from the air and dissolve its container, given the chance. So it won't stay pure for long.
Chemicals from nature, have dissolved in water to form the salt
water in the world's oceans.
Rivers running into the sea dissolve some of the rocks they flow over
and through, and may also take up other substances, such as humic acids
from peat.
Naturally acidic water has a higher concentration of hydrogen ions than
pure water.
When we say that water has a pH of 5, we actually are
saying [H+] =10-5 (so [OH-] =10-9).
Of more interest to marine enthusiasts, if the water is alkaline, this
means that minerals in the water have set free more hydroxyl ions, or
made an inbalance which has lowered the hydrogen ions.
So if pH = 8,
then [H+] =10-8. What should be clear is that a
change of only "1" in pH value is actually expressing a ten- fold
change in the concentration of hydrogen ions! A change from pH 7 to pH
9 means a one hundred- fold change.
pH is a measure of the actual state of the water. Increases in CO2
in the night will cause a lowering of pH, which reverts to its daytime
value after the lights have been on an hour or so.
To maintain a stable pH value, as in the ocean, we need to consider the
effect of buffering, which is the stabilising of the pH to a constant
value using substances which absorb the potential changes. But first we
need to think about
Aquarists usually measure pH in one of two ways, electrically with a pH
electrode and meter, or with a colour test kit.
Simply stated, a pH electrode measures the number of hydrogen ions
moving in the liquid, this very small signal is amplified and can be
read out on a meter. This system is very accurate and reliable,
but needs to be regularly calibrated using one (or better 2, if your
meter allows it) standard solutions with a fixed pH value. pH electrode
behaviour changes slowly but steadily over its lifetime, until it finally can no longer be calibrated and needs to be replaced. If a pH electrode
is being used continuously, it should be kept clear of algal and
bacterial growth, i.e. in the dark and wiped off and recalibrated
weekly. This is even more important if used as a pH controller, where
its output is used to switch some equipment.
Colour tests involving an indicator are basically a mixture of dyes, each of which changes its composition, and therefore colour, at particular pH values, often by direct reaction with the hydrogen ions or by loss of a hydrogen ion. Indicators do not all change colour at pH 7, some need less encouragement, some more. So by mixing the dyes, a steady set of clear colour changes can be produced for a particular range. Other pH ranges, other dyes!
Buffering is the attempt to stabilise a particular pH value
The level of salt(s) of around 35 grams per litre in the world's oceans is remarkably stable. Some regions may be slightly higher (Red Sea) or lower (Arctic) but still stable. We need to be able to keep the water in our aquariums stable, and so need to be able to measure the values in some way for producing synthetic seawater or to adjust for any changes following evaporation by topping up with RO water.
Obviously we can't take a litre of seawater and evaporate the water and measure the salt left over, so an indirect method must be used. In aquariums there are 3 common methods, two manual and the other electronic
also called a Hydrometer. The names are, for our purposes, interchangeable. This is the simplest and cheapest piece of equipment and you will find it indispensible. It is simply a device which floats in the water. The higher it floats, the denser the water. Reading off the scale where the water surface lies gives you a direct reading. This floating relates also to temperature, items for aquarium use are calibrated for use at 25 Celsius. Use of those for wine will not give a correct result. Not just because they are for a different temperature, but also because the density range of interest is not the same as the narrow range required by aquarists.
KH is a term used for what may be called the ability to bind with acids and buffer the seawater,
.Calcium is normally fairly undesirable in freshwater, causing 'Hardness', so called because it is hard to make a lather with soap. Scum in a bath is caused by the reaction of soap with dissolved calcium ions.
In seawater it is absolutely essential for much of life, in our aquariums it is used in particular by stony corals to build their 'skeletons'. So we need to maintain the steady level found in natural seawater of about 400-420 mg/L.
Calcium is not found in isolation, the reactions involved by which corals take up calcium, and not lose it again are complicated, involving a very similar metal ion, magnesium, and numerous other substances, including the organics and phosphate fairly common in our aquariums. For more depth, the article on chemistry-and-the-aquarium-calcium in the online publication reefs.com is certainly interesting reading
What one might glean from the article is that we need to keep our systems as near to clean sea water as possible, and to keep our calcium levels up. The original method in the Berlin System involved adding Limewater dropwise from a glass of RO water containing slaked lime (Calcium Hydroxide).