A pH (potential of Hydrogen) measurement reveals if a solution is acidic or alkaline (also base or basic). If the solution has an equal amount of acidic and alkaline molecules, the pH is considered neutral. Very soft water is commonly acidic, while very hard water is commonly alkaline, though unusual circumstances can result in exceptions.

The pH scale is logarithmic and runs from 0.0 to 14.0 with 7.0 being neutral. Readings less than 7.0 indicate acidic solutions, while higher readings indicate alkaline or base solutions. Some extreme substances can score lower than 0 or greater than 14, but most fall within the scale.

A logarithmic scale means that there is a ten-fold difference between each successive full number on the scale. An acidic solution reading 4.0 represents a ten-fold increase in acidic molecules over a 5.0 solution. The acidic difference between a 4.0 solution and a 6.0 solution is 100 times greater (10x10).

Hydrochloric acid or muriatic acid is an extremely caustic solution that sits at the extreme end of the acidic scale. This chemical is often used to lower the pH of highly alkaline water, such as in the treatment of swimming pools and aquariums. Only small amounts of hydrochloric acid are required, relative to the amount of water being treated.

Also low on the acidic scale at 1.5 – 2.0 is gastric acid, or stomach acids that help us digest food. Colas come in at 2.5, even more acidic than vinegar at about 3.0! It might also be surprising to learn that beer is slightly more acidic than acid rain, and coffee only slightly less acidic.

Pure water has a neutral pH and human saliva hovers close to neutral, while our blood is slightly alkaline. Seawater hits the pH scale between 7.7 and 8.3, and products like hand soap, ammonia and bleach score high on the alkaline scale running from 9.0 – 12.5. Highly alkaline baking soda or sodium bicarbonate is often used to increase the base of acidic water.

Aquarists rely heavily on pH measurements for proper fish keeping. Large bodies of water such as lakes and oceans have very little pH fluctuation, making fish intolerant of pH swings. Decaying plants, left over fish food and even fish waste all have a tendency to lower pH in an aquarium, while certain types of rocks and shells can continuously release trace amounts of calcium, boosting alkalinity. Fish keeping becomes a pH balancing act, as fishes subjected to pH swings are prone to disease and early death.

Due to chemical water treatment and other factors, tap water in many large cities throughout the U.S. tends to be alkaline with a pH close to 8.0. Though drinking tap water with a high (alkaline) pH is not harmful, the declining quality of tap water over the years has resulted in many people opting for faucet or pitcher filters to remove chlorine, chloramines, pesticides and other substances. These filters do not alter the pH of the water. Alternately many people choose to buy purified bottled water or fresh spring water, more likely to have a pH closer to neutral.