The technical definition of pH is that it is a measure of the activity of the hydrogen ion (H+) and is reported as the reciprocal of the logarithm of the hydrogen ion activity. Therefore, a water with a pH of 7 has 10-7 moles per liter of hydrogen ions; whereas, a pH of 6 has 10-6 moles per liter. The pH scale ranges from 0 to 14.
In general, a water with a pH < 7 is considered acidic and with a pH > 7 is considered basic. The normal range for pH in surface water systems is 6.5 to 8.5 and for groundwater systems 6 to 8.5. Alkalinity is a measure of the capacity of the water to resist a change in pH that would tend to make the water more acidic. The measurement of alkalinity and pH is needed to determine the corrosivity of the water.
The pH of pure water (H20) is 7 at 25 °C, but when exposed to the carbon dioxide in the atmosphere this equilibrium results in a pH of approximately 5.2 because CO2 in the air dissolves in the water and forms carbonic acid. Because of the association of pH with atmospheric gasses and temperature, it is strongly recommended that the water be tested for pH as soon as possible after the water sample is taken. The pH of the water is not a measure of the strength of the acidic or basic solution and alone does not provide a full picture of the characteristics or limitations of the water supply.
In general, a water with a low pH (< 6.5) could be acidic, soft, and corrosive. Therefore, the water could leach metal ions such as iron, manganese, copper, lead, and zinc from the aquifer, plumbing fixtures, and piping. Consequently, a water with a low pH could contain elevated levels of toxic metals, cause premature damage to metal piping, and have associated aesthetic problems such as a metallic or sour taste, staining of laundry, and the characteristic "blue-green" copper staining of sinks and drains. The primary way to treat the problem of low pH water and corrosive water is with the use of a neutralizer. The neutralizer feeds a basic solution into the water to prevent the water from reacting with the house plumbing or contributing to electrolytic corrosion; a typical neutralizing chemical is soda ash (Na₂CO₃). Neutralizing with soda ash increases the sodium content of the water.
A water with a pH > 8.5 could indicate that the water is hard or scale-forming. Hard water does not pose a health risk, but can cause aesthetic problems. These problems include:
Formation of a "scale" or precipitate on piping and fixtures causing water pressures and interior diameter of piping to decrease (‘atherosclerosis’ of the pipes);
Causes an alkali taste to the water and can make coffee taste bitter;
Formation of a scale or deposit on dishes, utensils, and laundry basins;
Difficulty in getting soaps and detergents to foam and formation of insoluble precipitates on clothing, etc.; and
Decreases efficiency of electric water heaters; the formation of scale on the heating elements inhibits the transfer of heat to the water.
Typically these problems are encountered when the Hardness exceeds 100 to 200 milligram (mg) CaCO3/liter (L), which is equivalent to 12 grains per gallon. Water can be softened through the use of ion-exchange or the addition of a lime-soda ash mixture, but both processes increase the sodium content of the water.
Note: "The ideal pH level of alkaline ionized water for long-term human consumption is between 8.5 and 9.5. (Source: Bawell Water Ionizers)".
Note that humans are much more tolerant of acidic beverages than liquids which are basic; probably everything you drink is at least slightly acidic.
Synergy is the process whereby two or more substances combine and produce effects greater than their sum. For example, 2 + 2 = 4 (mathematically). But synergistically, 2 + 2 = much more than 4! Synergy is a mathematical impossibility but it is a chemical reality. Here's how it works.
When acid water (water with low pH values) come into contact with certain chemicals and metals, this often makes them more poisonous than normal. As an example: fish that usually can withstand pH values as low as 4.8 will die at pH 5.5 if the water they're swimming in contains as little as 0.9 mg/L of iron. Mix an acid water with small amounts of aluminum, lead, or mercury, and you have a similar problem - one that far exceeds the usual dangers of these substances separately. Heavy metals can accumulate on the gills of fish or cause deformities in young fish, reducing their chance of survival.
Changes in the pH value of water are important to many organisms. Most organisms have adapted to life in water of a specific pH and may die if it changes even slightly. This is especially true of aquatic macroinvertebrates and fish eggs and fry.
The pH is a critical factor determining the health of a waterway. The factors that control it are obviously complicated. As with many environmental concerns, we need to be aware of the implications of any impacts we have upon the environment.