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Alkalinity - The Buffering Capacity of a Water

by Timothy A. Hovanec

Last month's 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 total.

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 lives.

©1995, Timothy A. Hovanec

Originally published in Aquarium Fish Magazine, Feb. 1995

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