"pH" represents a terminology used by chemists to indicate the concentration, in a solution, of hydrogen ions (H+). pH is widely measured in stream monitoring because it is relatively easy and inexpensive to do so, but the significance of the measurements are usually not well understood by students or non-chemists. Although pH expresses hydrogen ion concentration, it does so as the logarithm of the reciprocal of H+ activity (pH = -log[H+]). In essence, it is a scale that extends from 0, representing very acidic, to 14, representing very alkaline, with 7 corresponding to neutrality. Put another way, the more hydrogen ions present, the more acidic is the water and the lower its pH (remember reciprocal relationship). pH values from 7 to 14 represent increasing capacity to neutralize acidity, and typically a preponderance of hydroxyl (OH-) ions over hydrogen ions in solution.
pH is more than an esoteric chemistry of solutions: pH is an important property of natural waters influenced by the substances dissolved in the water and influencing chemical reactions and the ability of water to bring other substances into solution. Pure water contains a balance of hydrogen and hydroxyl ions and is thus neutral, having a pH of 7.00. Chemicals put in the atmosphere by industrial processes and dissolved in rain drops, shift the pH to slightly acid in some parts of the world (see Acid Rain). Organic acids from decomposing vegetable matter can make runoff slightly acidic. Thus, mountain stream waters tend to range in pH between 6.0 and 7.5.
Because the pH scale is logarithmic, each unit change represents a ten-fold change in hydrogen or hydroxyl ion concentration. The range of pH suitable for most life in freshwater environments is between about 5 and 9. However, values are seldom encountered outside of the range from 6 to 8 pH units. Values below 6 suggest some input of acidic substances. Values above 8 may mean either mixing with seawater (as in an estuary or muliwai), or high rates of productivity. During the day, photosynthetic activity by phytoplankton or bottom plants will raise DO level as oxygen is produced, and also will raise pH as carbon dioxide (CO2) is taken up. Carbon dioxide forms a weak acid (carbonic) when in solution, and removal of this acid by photosynthesizing organisms can raise pH levels measureably.
Seawater contains several substances which cause the pH to be closer to 8 and buffer the water. A soluition is said to be buffered when it contains chemicals which stabilize hydrogen ion concentration. Within limits, additions of weak acids or weak bases to buffered solutions like seawater do not alter the pH, or alter it only slightly.
Hawaii (US) Fresh water stream -- pH shall not deviate more than 0.5 unit from ambient, nor from the range 5.5 to 8.0 pH units. Elevated wetlands -- pH shall not deviate more than 0.5 unit from ambient, nor from the range 4.5 to 7.0 pH units Estuary -- pH shall not deviate more than 0.5 unit from ambient, nor from the range 7.0 to 8.6 pH units (except Pearl Harbor acceptable range is 6.8 to 8.8) Embayment -- pH shall not deviate more than 0.5 unit from 8.1 (i.e., 7.6 to 8.6) except where depressed by runoff minimum becomes 7.0 Open coastal water -- pH shall not deviate from the range 7.6 to 8.6 pH units except where depressed by runoff minimum becomes 7.0. Republic of Palau Fresh waters -- Class 1 -- pH variation shall not be greater than 0.2 pH units from natural conditions, but not lower than 6.5, nor higher than 8.5 from other than natural causes. Class 2 -- pH variation shall not be greater than 0.5 pH units from natural conditions, but not lower than 6.5, nor higher than 8.5 from other than natural causes.