Page 241 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice
P. 241
232 ACID-BASE DISORDERS
120 The ionization, or dissociation, constant for an acid is an
indication of the strength of that acid. A large value for K a
þ
100 means that [H ] and [A ] are much greater than [HA];
that is, the acid is a strong one and is largely dissociated. A
+
pH = – log 10 [H ] þ
80 small value for K a means that [H ]and [A ] are much
[H + ] (nEq/L) 60 of it is dissociated. Hydrochloric acid (HCl) and sulfuric
smaller than [HA]; that is, the acid is a weak one and little
acid (H 2 SO 4 ) are strong acids and dissociate almost
þ
40 completely in aqueous solutions, whereas NH 4 is a weak
acid (i.e., it is a strong base) and dissociates to a small
extent.
20
Taking the base 10 logarithm of both sides of the
dissociation equilibrium equation yields:
0
7.00 7.20 7.40 7.60 7.80 8.00
þ
pH ½H ½A
Figure 9-1 Exponential relationship between [H ] and logK a ¼ log ½HA
þ
pH. (From Madias NE, Cohen JJ. Acid-base chemistry and buffering. ½A
þ
In: Cohen JJ, Kassirer JP, editors. Acid-base. Boston: Little, Brown, logK a ¼ logð½H Þ þ log
1982: 5.) ½HA
þ
There is an inverse relationship between pH and [H ]: Multiplying by 1 yields:
the greater the [H ], the lower the pH. Furthermore,
þ
þ
pH and [H ] vary not linearly with one another but expo- þ ½H ½A
þ
logK a ¼ logð½H Þ log
þ
nentially as shown in Figure 9-1. The [H ] for a given pH ½HA
within the physiologic range is given in Table 9-1.
Applying the concept of pH to both the hydrogen ion
LAW OF MASS ACTION concentration and dissociation constant, K a :
The law of mass action states that the velocity of a reac- pK ¼ pH log ½A
tion is proportional to the product of the concentrations a ½HA
of the reactants. For the acid just described, there are two ½A
pH ¼ pK þ log
opposing reactions: a
½HA
þ
HA ! H þ A This is the commonly used Henderson-Hasselbalch form
þ
H þ A ! HA of the dissociation equilibrium equation. Occasionally,
the term salt or base is substituted for A and the term
The velocity of the first reaction can be written: acid for HA:
v 1 ¼ k 1 HA½ ½salt
pH ¼ pK þ log
a
½acid
and the velocity of the second reaction:
þ
v 2 ¼ k 2 ½H ½A
CONCEPT OF BUFFERING
At equilibrium, the rates of the two opposing reactions
exactly counterbalance one another and the two velocities A buffer is a compound that can accept or donate protons
are equal: (hydrogen ions) and minimize a change in pH. A buffer
solution consists of a weak acid and its conjugate salt.
þ
k 1 HA½ ¼ k 2 ½H ½A When a strong acid is added to a buffer solution
containing a weaker acid and its salt, the dissociated
Rearranging and substituting a new constant, K a , the ion- protons from the strong acid are donated to the salt of
ization, or dissociation, constant for the acid HA: the weak acid and the change in pH is minimized.
Consider an aqueous solution with equal amounts of
þ
½H ½A Na 2 HPO 4 and NaH 2 PO 4 . The pK a for this buffer pair
k 1 =k 2 ¼ K a ¼
½HA is 6.8:
