Proton theoryIt is expounded that the molecule or ion that can give protons is an acid, and the molecule or ion that can receive protons is a base. The acid gives protons and converts them into corresponding bases, and the base receives protons and converts them into corresponding acids. This pair of acids and bases that transform each other due to the gain or loss of protons are called conjugate acid base pairs.[1]
Chinese name
Conjugated acid base pair
Foreign name
conjugate acid-base pairs
Concept
A pair of acids and bases that transform each other due to the gain or loss of protons
According to Bronsted and Laurie'sAcid base proton theoryIt is believed that acid is a substance that can give protons and base is a substance that can receive protons.An acid loses its proton and becomes the conjugated base of the acid. After obtaining the proton, it becomes the conjugated acid of the base. The acid base that has this relationship is called the conjugated acid base pair.According to the proton theory of acids and bases, acids or bases can be neutral molecules, cations or anions.A substance with both acid and alkali properties is called an amphoteric substance. Similarly, a solvent with both acid and alkali properties is called an amphoteric solvent.The loss or gain of protons in an acid or base is always carried out in a certain solvent, and the transfer reaction of protons between amphoteric solvent molecules can occur itself, that is, proton self transfer.[2]
Conjugated acid base pair
Announce
edit
Conjugated acid base pair is a pair of acids and bases linked by proton gain loss relationship.According to the acid base proton theory, acids and bases always exist in correspondence. Acids give protons to become their conjugated bases, and bases get protons to become their corresponding conjugated acids. This relationship is called conjugation relationship.Such as acetic acid (HAc) and acetate (Ac), ammonia (NHthree）And ammonium ion (NHfour）Are conjugated acid base pairs.In analytical chemistry, this conjugated system is often used as an acid-base buffer system, that is, to keep the analytical solution at a certain pH value and not be affected by the addition of a small amount of acid or base from the outside and dilution of the solution.[3]
In buffer solution
Announce
edit
The biggest difference from the ionization theory is that the proton theory has only the concept of acid and base, but not the concept of salt.Therefore, in thebuffer solutionThe buffer pair is a pair of conjugate acids and bases.In fact, using the concept of conjugated acid base pairs, buffer solutions can be divided into two categories: weak acids and their conjugated bases, and weak bases and their conjugated acids.In addition, at high concentrations ofStrong acid and strong baseIn solution, due to
or
The concentration of is already very high, and adding a small amount of acid or base will not have a great impact on the acidity of the solution. In this case, strong acid and strong base are also buffer solutions, which are mainly buffer solutions at high acidity (pH<2) and high alkalinity (pH>12).[1]
Application of Conjugated Acid Base Pair in Buffer Principle
Taking HB-B-Buffer solution system as an example, HB and
The initial concentration of HB is very high, and there are conjugated acid HB and its conjugated base in the system.According to the principle of acid-base reaction, it is not difficult to know that if a small amount of strong acid is added to this solution, the conjugate base in the solution is acceptable
, generate HB to resist
Impact on pH.If a small amount of strong base is added to this solution, the conjugated acid HB in the solution is acceptable
, generating
To resist
Impact on pH.Because of the existence of conjugated acid-base pairs, it can resist a small amount of added acid-base, thus keeping the pH value of buffer solution basically unchanged.[1]
Application of Conjugated Acid Base Pair in the Calculation Formula of pH Value of Buffer Solution
Conjugated acid base pair[4]
From the proton theory, the buffer solution composed of conjugated acids and bases is the most common and typical. Applying the concept of conjugated acid base pairs, we can get the formula:
Then we can know from the calculation that:
The formula can also be expressed as:
Where pKa is the acid constant of conjugated acid.The calculation formula can be used to calculate the proton concentration (pH) of the buffer solution composed of conjugated acids and their conjugated bases.[1]
Application of Conjugated Acid Base Pairs in the Expression of Buffer Range
Theoretical research shows that buffer solution exceeds
(weak acid and its conjugate base, or weak acid and its corresponding salt) or
(Weak bases and their conjugated acids, or weak bases and their corresponding salts) no longer have buffer capacity.Applying the concept of conjugated acid base pairs, the buffer range can only be
or
Is shown in Table 2.[1]
Buffer solvent example
Application of Conjugated Acid Base Pair in Buffer Pair Selection
When a buffer solution with a certain pH value needs to be prepared in actual work, the following steps can be followed:
According to ionization theory, select a buffer pair to make pK of weak acid (or weak base)a(or pKb）Equal to or close to the required pH (or pOH), such as pKaOr (pKb）If it is not completely equal to the required pH (or pOH) value, the required concentration ratio of weak acid (or weak base) and salt can be calculated according to the required pH (or pOH) value using the buffer equation.According to proton theory, only "salt" is replaced by "conjugated base" (or "conjugated acid").[1]
