THE (p)OWER OF (H)YDROGEN, pH
When we talk about pH we are talking about hydrogen ions, the “power of hydrogen,” in aqueous (water) solutions. Hydrogen ion concentration, written [H+], is expressed in moles per liter (mol/L), but the numbers get quickly out of hand, e.g. .00000000000001 moles per liter (10-14 mol/L), an extremely small concentration of hydrogen ions. Reference to “pH” serves as shorthand for these inconvenient numbers.
Mathematically speaking, pH is the negative logarithm of [H+], which means that as pH increases [H+] decreases, and that as pH decreases [H+] increases. More accurately, however, pH is a measure of hydrogen ion activity (aH+), but in dilute solutions, such as in the case of most body fluids (water solutions), [H+] and aH+ are for practical purposes equivalent. Perhaps a simpler way of understanding pH numbers, which range from 1 to 14, is that for each increase there is an additional zero in the measurement, e.g., pH of 1 = 10-1 (0.1), pH of 2 = 10-2 (0.01), … pH of 14 = 10-14 (0.00000000000001), each of which is a successively smaller concentration.
The pH of distilled water is 7.0. Hydrogen ion concentration (activity) is approximately 1 X 10-7. Water also, however, contains an equivalent concentration of hydroxyl ions [OH‾], ions of opposite charge, which offsets [H+], and is thus said to be “neutral” (buffered). The pH of an aqueous (water) solution is changed by adding more hydrogen ions (e.g., lactic acid) and/or other kinds of ions of opposite charge (e.g., bicarbonates). Solutions with a pH below 7.0 are acidic; [H+] is greater than [OH‾]. Solutions with pH levels above 7.0 are alkaline; [OH‾] is greater than [H+].
The range of extracellular fluid pH levels is very restricted. Blood plasma, for example, is a slightly alkaline aqueous (water) solution, with a normal pH range of 7.35 to 7.45. Plasma acidemia is a pH below 7.35 (although this is still alkaline), and plasma alkalemia is a pH above 7.45. Plasma pH levels below 6.9, and above 7.8, are fatal. Levels below 7.35 and above 7.45 can result in physical symptoms, psychological changes, and performance deficits.
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