column about pH ended with a short introduction of alkalinity.
This month I will finish our discussion about the difference
between pH and alkalinity. Alkalinity is a measure of
the acid-neutralizing capacity of a water. Acid-neutralizing
capacity means the ability to accept acid without a
subsequent drop in pH. Alkalinity is basically a measure
of how much antacid is dissolved in the water. The more
acid that can be added to a water before the pH starts
to drop, the higher the alkalinity. Normally, adding
acid to a solution would cause a drop in the pH since
this increases the number of hydrogen ions. But if a
hydrogen ion can be neutralized by a base then it will
not contribute to the pH so the pH will not change.
A water that can accept acid without a subsequent drop
in pH is said to be buffered. Therefore, alkalinity
is the amount of buffering in a water.
Alkalinity is not the measurement of just one substance
or ion. It is the combined property of many anions
(negative charged ions). To a great extent, the alkalinity
of most waters is determined by the amount of bicarbonate
and carbonate present in the water. But other substances
such as hydroxide, phosphates, silicates and borates
also contribute to the alkalinity.
Much of the confusion between alkalinity and pH may
stem from the fact that pH is used to measure alkalinity.
As the accompanying figure shows, alkalinity is determined
by 'titration'. During titration, acid is added to
the water sample. At first, the acid is neutralized
by the bases present and the pH remains stable. With
further acid additions the bases are consumed and
the pH will drop more quickly. Finally, each additional
drop of acid will cause a large decline in the pH
as there are no more bases available to neutralize
the acid. In the alkalinity titration procedure an
acid of known concentration is slowly added to the
water sample until a specific pH is reached. The designated
pH is called the endpoint pH and the most common value
is 4.5 (but in cases of low alkalinity, less than
20 mg/L CaCO3, a pH of 4.9 is used). When comparing
the alkalinity from one sample to the next it is important
to make sure the pH endpoint is the same. The pH can
be determined with a pH probe and meter (see Method
1 of diagram) or by adding the acid with a dropper
to the water sample in which a pH color indicator
is added (Method 2). Most test kits available to hobbyists
use a chemical called bromcresol green as the pH indicator
because it turns colors (from blue to green) very
sharply at a pH of 4.5. With either method the amount
of acid (milliliters or drops) added to the sample
is then multiplied by a conversion factor to arrive
at an alkalinity value, commonly in units of mg/L
calcium carbonate (CaCO3). (The test kit instructions
will indicate the appropriate conversion factor.)
While calcium carbonate is the most common way to
express alkalinity the aquarist must realize that,
as stated above, there may be many more substances
contributing to the alkalinity then just carbonate.
Further, calcium has nothing to do with alkalinity.
In fact, the water may not even have calcium in it
but the units are still reported as calcium carbonate.
(To eliminate this confusion marine biologists and
limnologists express alkalinity in terms of milliequivalents
per liter.) The use of calcium carbonate is just a
convenient way to standardize the measurement and
its reporting so data from many different types of
water can be compared. It is analogous to a group
of tourists travelling to many different countries
and upon their return wanting to compare how much
money they have. The money is in many different currencies
so they mentally convert it to U.S. dollars. None
of the tourists actually have U.S. dollars but for
comparison purposes they determine the equivalent
amount in common currency to see who has the greater
Alkalinity is important for several reasons. If you
decide to rear Discus and want to decrease the pH
of your breeding tanks you may consider adding acid
to your normal source of aquarium water. The higher
the alkalinity of the source water, the more acid
you will have to add to reach the desired pH as the
alkalinity will neutralize the first acid additions.
Alkalinity is also the reason why the pH seems to
'bounce' after adding a pH decreasing chemical. Immediately
after adding acid the pH will decrease but after a
time and some stirring of the water the pH will rise
towards the initial value (this is the bounce). This
is due to the bases neutralizing the acid you have
added. You will have to repeat the acid additions
until the bases are consumed, at which time, the pH
will drop quickly and not bounce back.
Alkalinity also helps neutralize the tendency of aquarium
water to become acidic. The nitrogen cycle in an aquarium,
a natural and necessary process, adds hydrogen ions
to the water. The hydrogen ions will slowly cause
the pH to drop which can be harmful to the inhabitants
and even to the nitrifying bacteria. Alkalinity neutralizes
the hydrogen ion produced by the nitrogen cycle slowing
the downward pH trend.
Alkalinity can be replenished by adding buffers to
the water. The most common buffer is sodium bicarbonate
(baking soda). For marine aquariums, calcium carbonate
is widely used to add both calcium and alkalinity
to the sea water. Care must be taken when adding some
buffers as they can cause the pH to increase very
quickly, shocking the fish. The best way to replace
alkalinity is by regular water changes.
Another term for alkalinity is carbonate hardness.
The use of this term should be curtailed as it just
adds to the confusion between hardness and alkalinity
which are not at all related.
Acidity is the opposite of alkalinity. Acidity is
a measure of the ability of a water to accept bases
without the pH increasing. Acidity is rarely important
to aquarists as none of the fish and invertebrates
we keep live in the low pH of water normally associated
with high acidity.
Test kits for both pH and alkalinity are widely available
and easy to use. I recommend you test the water once
a week and note the values in your aquarium logbook.
When the values start to drop, a partial water change
is needed. Make these procedures part of your regular
maintenance and your fish will live longer, healthier
©1995, Timothy A. Hovanec
Originally published in
Aquarium Fish Magazine, Feb. 1995