108
Exercise 8
situations does inorganic carbon become a
limiting factor to photosynthesis of planktonic
algae.
observed pH and the capacity for buffering of a
given body of water. The lethal effects of most
acids appear when pH < 5.5 and of most alkalis
near pH 9.5, although the tolerances of many
organisms are considerably more restricted
within these pH extremes. Thus, the buffering
capacity of natural waters to resist changes in pH
can be of great importance to the maintenance of
life.
Natural waters exhibit wide variations in this
relative acidity and alkalinity, not only in actual
pH values, but also in the total amount of
dissolved material producing the acidity or alkalinity. The concentrations of these compounds
and the ratios of one to another determine the
pH
Ions in solution are capable of conducting an electrical current. Even pure water will
conduct a current to a slight degree because a small number of water molecules
dissociate into ions:
H 2 0 ~ H+ +OHIn this case an equal number of hydrogen and hydroxyl ions is formed. Thus the water
is neither acidic nor alkaline, but is neutral.
By careful measurements it has been found that, when pure water at 25°C ionizes,
0.0000001 g of H + is liberated per liter. Since 1 mole of protons weighs 1 g, the
concentration of protons in pure water is also 1 x 10 -7 moijl which is conveniently
described on a negative logarithmic scale simply as 7. The pH scale is a series of
numbers ranging from 0 to 14 that denote various degrees of acidity or alkalinity.
Values below 7 and approaching 0 indicate increasing acidity, while values from 7 to 14
indicate increasing alkalinity. Mathematically,
1
pH = -logJ..{+ ~oncentration and
1
-log - - = 7
10- 7
the pH of pure water with a H + concentration of 10 -7 at 25 u C. By Le Chatelier's
Principle:
(H + concentration) x (OH - concentration)
" - - - - - - - - - - - - - - - - - - - - = a constant
H 2 0 concentration
Because the concentration of H 2 0 is very large relative to the equilibrium
concentration of H+ and OH-, it may be taken as a constant, and the equation
becomes [H +] [OH -] = a constant with a value of 10 -14. According to this equation,
if the H+ concentration were to increase, the OH - concentration would decrease
proportionally, and vice versa. With an equal concentration of each ion, the solution is
neutral and its pH = 7. IfOH - ions (a base) were added to such a solution, there would
be a decrease in the H + ion concentration, the solution would become more alkaline,
and the pH value would increase. Similarly, ifH + ions (an acid) were added to a neutral
solution, the OH- concentration would decrease, the solution would become more
acidic, and the pH value would decrease.
The farther the pH value is from the neutral point of 7, the greater is the
concentration of either H + or OH - ions. Since these are logarithmic values, each
integer represents a H + concentration ten times that of the next higher number. A pH
of 2 represents 0.01 mol of H + ions per liter, ten times the H + concentration at pH 3
Exercise 8
situations does inorganic carbon become a
limiting factor to photosynthesis of planktonic
algae.
observed pH and the capacity for buffering of a
given body of water. The lethal effects of most
acids appear when pH < 5.5 and of most alkalis
near pH 9.5, although the tolerances of many
organisms are considerably more restricted
within these pH extremes. Thus, the buffering
capacity of natural waters to resist changes in pH
can be of great importance to the maintenance of
life.
Natural waters exhibit wide variations in this
relative acidity and alkalinity, not only in actual
pH values, but also in the total amount of
dissolved material producing the acidity or alkalinity. The concentrations of these compounds
and the ratios of one to another determine the
pH
Ions in solution are capable of conducting an electrical current. Even pure water will
conduct a current to a slight degree because a small number of water molecules
dissociate into ions:
H 2 0 ~ H+ +OHIn this case an equal number of hydrogen and hydroxyl ions is formed. Thus the water
is neither acidic nor alkaline, but is neutral.
By careful measurements it has been found that, when pure water at 25°C ionizes,
0.0000001 g of H + is liberated per liter. Since 1 mole of protons weighs 1 g, the
concentration of protons in pure water is also 1 x 10 -7 moijl which is conveniently
described on a negative logarithmic scale simply as 7. The pH scale is a series of
numbers ranging from 0 to 14 that denote various degrees of acidity or alkalinity.
Values below 7 and approaching 0 indicate increasing acidity, while values from 7 to 14
indicate increasing alkalinity. Mathematically,
1
pH = -logJ..{+ ~oncentration and
1
-log - - = 7
10- 7
the pH of pure water with a H + concentration of 10 -7 at 25 u C. By Le Chatelier's
Principle:
(H + concentration) x (OH - concentration)
" - - - - - - - - - - - - - - - - - - - - = a constant
H 2 0 concentration
Because the concentration of H 2 0 is very large relative to the equilibrium
concentration of H+ and OH-, it may be taken as a constant, and the equation
becomes [H +] [OH -] = a constant with a value of 10 -14. According to this equation,
if the H+ concentration were to increase, the OH - concentration would decrease
proportionally, and vice versa. With an equal concentration of each ion, the solution is
neutral and its pH = 7. IfOH - ions (a base) were added to such a solution, there would
be a decrease in the H + ion concentration, the solution would become more alkaline,
and the pH value would increase. Similarly, ifH + ions (an acid) were added to a neutral
solution, the OH- concentration would decrease, the solution would become more
acidic, and the pH value would decrease.
The farther the pH value is from the neutral point of 7, the greater is the
concentration of either H + or OH - ions. Since these are logarithmic values, each
integer represents a H + concentration ten times that of the next higher number. A pH
of 2 represents 0.01 mol of H + ions per liter, ten times the H + concentration at pH 3