The pH scale measures the concentration of H+ and OH. The pH of a solution is a negative
logarithm of hydrogen ion concentration. The pH scale puts a number to it ranging
from 0 to 14.
The pH scale was developed because the concentration of a solution can vary by so
much over time. A pH scale is the easiest way to express this solution pH change.
The pH scale ranges from 0 to 14. At the 0 end the concentration is increasingly
acidic. Moving up to 2 is lemon juice and stomach juices. Around 3 are vinegar, beer
and cola. At 4 is tomato juice. At 5 is black coffee and rainwater. Urine is 6. Pure
water and human blood are 7. Most biological fluids are between pH 6 and pH 8.
Between 8 and 9 is seawater. Milk of magnesia is 10. Household ammonia is 11, household
bleach is 12. Between 13 and 14 is oven cleaner. Products at the two extremes (less
than pH 1 or greater than pH 13) are extremely oppressive and corrosive. Examples
include sulfuric and hydrochloric acid on the acid end, and caustic soda on the alkaline
end.
The internal pH of most living cells is close to 7. When there is even a slight change
in pH, it can be extremely harmful because the chemical processes of the cell are
sensitive to the concentration of hydrogen and hydroxide ions. Biological fluids resist
change to their pH because of the presence of buffers. Buffers in human blood keep
blood pH close to 7.4. A person can not survive if the pH of their blood drops to
7 or rises to 7.8.
Acid adds hydrogen ions to a solution and it removes hydroxide ions because of the
tendency of H+ to combine with OH- to form water. The base has the opposite effect
with an increasing OH- concentration and reducing H+ concentration by forming water.
Each pH unit represents a tenfold difference in the H+ and OH- concentration. This
makes the pH scale compact. For example, a solution of pH 2 is not twice as acidic
as a solution of pH 4, but 100 times more acidic. When the pH of a solution changes
a little, the concentrations of H+ and OH- change a lot.
