PH value and buffer solution--the ion product of the first water section and the PH value of the solution
The chemical reactions carried out in the solution, especially the chemical reactions in the organism, often need to be carried out under certain pH conditions. All human body fluids have a certain PH value, and it is not easy to change, so it can ensure the normal physiological activities of the human body. The reason why human body fluid has a certain pH value is that it is a buffer solution itself, which has the ability to resist a small amount of strong acids or bases from outside, so as to stabilize the PH value of the solution. There are three purposes for studying this chapter: one is to grasp the PH value and its relationship with the acidity and alkalinity of the solution, and the theory of acid-base indicator; the second is to master the knowledge and methods required to prepare the buffer solution; The basic equilibrium theory provides the necessary basic knowledge.
The first ionic product of water and the PH value of the solution
1. Ion product of water
Water is an amphoteric substance that can both release and accept protons. To a certain extent, water also dissociates weakly, and protons are transferred from one water molecule to another, forming H3O + and OH-.
When equilibrium is reached, the dissociation constant Ki of the available water is obtained
Or [H2O +] [OH-] = K1 [H2O] 2
Since the dissociation degree of water is extremely small, the [HO] value can be regarded as a constant, so that K1 [H2O] 2 is equal to another new constant Kw
[H3O +] [OH-] = Kw
Kw is called the ion product constant of water, or simply the ion product of water. The above formula indicates that at a certain temperature, the product of the hydrogen ion concentration and the hydroxide ion concentration in water is a constant (Table 3-1). At 25 ° C, it was experimentally determined that [H3O +] and [OH-] were 1.0 × 10-? 7mol·L-1 in pure water. The hydrated ion H3O + is usually abbreviated as H +, so that at normal temperature:
Table 3-1 Ion product of water at different humidity
Temperature / ℃
KW
I temperature / ℃
KW
0
1.2 × 10-15
50
5.5 × 10-14
10
3.0 × 10-15
60
9.6 × 10-14
20
6.8 × 10-15
70
1.6 × 10-18
25
1.0 × 10-14
80
2.5 × 10-13
30
1.5 × 10-14
90
3.8 × 10-13
40
2.9 × 10-14
100
5.5 × 10-13
K = 1.0 * 10-7 * 1.0 * 10-7 = 1.0 * 1014
[H +] [OH-] = 1.0 * 10-14 (3-2)
Because water absorbs a lot of heat when it dissociates, so the temperature rises and the dissociation degree and KW of water also increase accordingly.
The ion product principle of water is not only applicable to pure water, but also to all dilute aqueous solutions. In any dilute aqueous solution, no matter how [H +] and [OH-] change, their product is always equal to KW.
2. PH value of the solution
PH value and buffer solution-the ionic product of the first water section and the PH value of the solution in pure water or neutral solution at 25 ℃
When acid is added to the water, the [H +] in the solution will increase. When the new equilibrium is reached, the [H +] of the solution is 1.0 × 10-2mol·L-1, because [H +] [OH-] = 1.0 × 10-14, then
It can be seen that in acidic solutions, [H +]> 1.0 × 10-7 mol·L-1, and [OH-] <1.0 × 10-7 mol·L-1.
If the alkali is added to pure water, the [OH-] in the solution will increase. When the new equilibrium is reached, the [OH-] of the solution is 1.0 × 10-2mol·L-1, and the [H + ] = 1.0 × 10-12 mol·L-1. It can be seen that in alkaline solution, [OH-]> 1.0 × 10-7mol·L-1, and [H +] <1.0 × 10-7 mol·L-1. From the above three situations:
In pure water or neutral solution [H +] = 1.0 × 10-7 mol·L-1 = [OH-]
In acidic solution [H +]> 1.0 × 10-7 mol·L-1> [OH-]
In alkaline solution [H +] <1.0 × 10-7 mol·L-1 <[OH-]
Of course, [H +] or [OH-] can be used to indicate neutral, acidic or alkaline in the solution, but in practical applications, [H +] is often used. However, the [H +] of many important solutions in biology and medicine is often a very small value, and it has a negative index. It is not convenient to use [H +] to indicate the acidity and alkalinity of the solution. For example, human blood [H +] is 0.0000000398 mol·L-1, which is 3.98 × 10-8 mol·L-1. It is not easy to see whether the blood is acidic or alkaline. Sorensen first proposed to express the acidity and alkalinity of the aqueous solution with PH value.
The pH of the solution is the negative logarithm of the hydrogen ion concentration.
Its mathematical expression is: pH = -lg [H +]
That is, [H +] = 10-pH (3-3) strictly speaking, when considering the activity:
Pα + = lgαH + (3-4)
It must be noted that when the PH value differs by one unit, the [H +] difference is 10 times; when the PH value differs by two units, the [H +] difference is 100 times; and so on.
To express the acidity and alkalinity of dilute aqueous solution with PH value, there is "
In pure water or neutral solution, [H +] = 1.0 × 10-7 mol·L-1 PH = 7
In an acidic solution, [H +]> 1.0 × 10-7 mol·L-1 PH <7, the lower the pH, the stronger the acidity.
In alkaline solution, [H +] <1.0 × 10-7 mol·L-1 PH> 7, the greater the PH, the stronger the alkalinity.
Similar to PH, [OH-] and KW can also be expressed by their negative logarithms, ie
pOH = -lg [OH-] (3-5)
pKw = -lgKw (3-6)
Since at 25 ℃, [H +] [OH-] = KW = 1.0 × 10-14
Take the negative logarithms on both sides of the equation, then
-lg [H +]-lg [OH-] =-lgKw = -lg1.0 * 10-14
and so
pH + pOH = pKw = 14 (3-7)
The relationship between [H +], [OH-], PH, POH value and the acidity and alkalinity of the solution in the aqueous solution is shown in Table 3-2.
Relationship between value and acidity and alkalinity of solution
[H +] 100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14 PH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Acid
Alkaline
Acid gradually strengthened Alkaline gradually strengthened POH 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 [OH-] 10-14 10-13 10-12 10-11 10-10 10-9 10-8 10 -7 10-6 10-5 10-4 10-3 10-2 10-1 100
In practical applications, the PH value is generally limited to the range of 0-14. When [H +] or [OH-] is greater than (that is, 100), the PH value is no longer used, and [H +] or [OH-] is still used to indicate the acidity and alkalinity of the solution.
It must be noted that the PH value indicates the acidity or effective acidity of the solution rather than the acid concentration. Acidity or effective acidity refers to the concentration of H + in the solution, strictly speaking, the activity of H +, and refers to the concentration of the dissociated acid. The acid concentration is also called total acidity or analytical concentration. It refers to the amount of acid substance contained in 1 liter of solution, including the total concentration of dissociated and undissociated two parts of the acid, and its size is determined by titration analysis. Acidity or effective acidity is measured with PH test paper or PH meter. Latent acidity refers to the concentration of the undissociated part, that is, the difference between the total acidity and the effective acidity. For example, the concentration of 0.01mol·L-1HCL and 0.01mol·L-1HOAc are the same, but the effective acidity is different. The total acidity of 0.01 mol·L-1HCL solution is 0.01 mol·L-1, and its effective acidity [H +] is also the same value. At 25 ℃, the HOAc solution with total acidity of 0.01 mol·L-1, its effective acidity [H +] Only 4.2 × 10-4mol·L-1.
Example 1 Obtain the pH values ​​of 0.1mol.L-1 HCl solution and 0.01mol.L-1 HOAc solution respectively, and it is known that their [H +] are 0.01mol.L-1 and 4.2 * 10-4mol.L-1, respectively .
Solution: pH of HCl solution = -lg0.01 = -lg10-2 = 2.0
HOAc solution pH = -lg (4.2 * 10-4)
= [0.62 + (-4)]
= 3.38
Example 2 Given the pH of a solution = 4.60, calculate the hydrogen ion concentration of the solution.
Solution: -lg [H +] = pH = 4.60
lg [H +] = 4.60 = -5 + 0.40 = 5.40
Check the inverse of 0.4 is 2.512, so
[H +] = 2.512 * 10-5mol.L-1
3. Application of PH value in medicine
PH is commonly used in medicine to indicate the acidity and alkalinity of body fluids (see Table 3-3). The PH value is of great significance in medicine. For example, the pH value of normal human plasma is fairly constant, maintained between 7.35 and 7.45. If the pH value of the blood is greater than 7.5, it will show obvious alkalosis clinically. When the PH value of blood is less than 7.3, it shows obvious acidosis.
There are many methods to determine the PH value in the solution, and the PH test paper is commonly used in clinic to determine the PH value of the patient's urine. For a more accurate determination of the PH value, use a PH meter.
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