2.
The composition of the buffer solution
The buffer solution HAc-NaAc is composed of weak acids and their salts
.
In addition to HAc-NaAc, other weak acids and their salts can also form buffer solutions, such as HCN-NaCN buffer solutions
.
The weak acid and weak acid in the buffer solution are called buffer pairs
.
The formula for calculating the pH of a buffer solution composed of weak acid and weak acid salt is
Similarly, weak bases and their salts can also form buffer solutions, such as NH 3 -NH 4 Cl solution
.
Add a small amount of strong acid to the system, the NH 3 molecules in the system will react with H + to generate NH 4 + ; and add a small amount of strong base to the system, OH - will react with NH 4 + to generate NH 3
.
Since the ratio of NH 4 + and NH 3 concentrations did not change much, the pH of the system remained basically unchanged
.
Equilibrium constant
Therefore
Using the concentration of the buffer pair, the pOH of the buffer solution composed of a weak base and a weak base salt can be calculated, and then the pH can be calculated
.
In addition to weak acids and weak bases and their corresponding salts can form buffer solutions, acid salts and their secondary salts can also form buffer solutions, such as NaH 2 PO 4 -Na 2 HPO 4 , NaHCO 3 -Na 2 CO 3 and so on
.
The pH of the system can be calculated by the following formula
At the same time, the acid salt solution itself is also a buffer solution, such as NaHCO 3 , Na 2 HPO 4 , NaH 2 PO 4, etc.
They can interact with strong acids or alkalis to keep the pH of the system basically unchanged
.
Borax is dissolved in water to produce H 3 BO 3 and [B(OH) 4 ]- in the amount of other substances , and its solution is also a good buffer solution
.
3.
Buffer range
by
It can be seen that the concentration ratio of a weak acid or a weak base to its salt determines the change in pH of the buffer solution
.
The greater the concentration of the buffer pair, the smaller the change in the concentration ratio of the weak acid or base and its salt, and the smaller the pH change of the buffer solution, that is, the greater the buffer capacity of the solution
.
The buffer capacity is large, it can resist the influence of more acid and alkali and the influence of water dilution
.
When the concentration of the two substances in the buffer pair is the same, the ability to resist strong acid and strong alkali is more balanced, and the buffer solution has the best buffer effect
.
It is generally believed that the concentration ratio of the buffer pair should be controlled within 10
.
Thus, the buffer range of the solution is
Weak acid-weak acid salt pH=pKa Θ ±1
Weak base-weak base salt pOH=pKb Θ ±1
According to the buffer range of various buffer solutions and the pH of the solution to be prepared, the corresponding weak acid or weak base can be selected and the concentration ratio should be adjusted appropriately to prepare the required buffer solution
.
[Example 7-6] Existing formic acid (HCOOH, Ka Θ = 1.
8×10 -4 ) and acetic acid (CH 3 COOH, Ka Θ = 1.
8×10 -5 )
.
(1) To prepare a pH=3.
00 buffer solution, which buffer is the best?
(2) What is the concentration ratio of the buffer pair?
Solution (1) pKa Θ values of two acids
According to the buffer range of the buffer solution (pH=pKa Θ ±1), HCOOH-HCOONa should be used to prepare a buffer solution with pH=3.
00
.
(2) Derived
