According to the definition of modern inorganic chemistry, all substances that can give protons are acids, and all substances that can combine with protons are bases. This relationship between acids and bases is called acid base conjugate relationship, and the corresponding acid base is called conjugate acid base.
According to the proton theory proposed by Br ö nsted and Lowry in 1923, any substance (molecule or ion) that gives a proton is an acid;Any substance that accepts protons is a base.In short, acid is the donor of proton, while base is the acceptor of proton.The relationship between acid and base is shown as follows:
At the same time, it can be seen that acid and base are unified in the relationship with protons: acid becomes base after releasing protons, and base becomes acid after receiving protons.In order to express the relationship between them, the relationship between acid and base is often called conjugated acid and base.The base formed after an acid emits protons is called the conjugate base of the acid;An acid formed after a base receives protons. It is calledConjugated acid。We call the corresponding acid and base that differ by one protonConjugated acid base。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.[1]
give an example
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PressAcid-base proton theoryAcids and bases are not isolated from each other, but unified in the relationship with protons, which can be expressed as:
Acid=base+proton
That is,Acids give protons and become bases, and bases receive protons and become acids.A pair of acids and bases that meet the above relationship are calledConjugated acid base（conjugate acid-base pair）。
E.g. HCN-CN-Constitutes aConjugated acid base pair。HCN is CN-Conjugate acid, in turn, CN-It is the conjugate base of HCN.
Another example: HClH++ HCl
HCOH+ HCO-
NHH++ NH
HPOH+ HPO-
[Al(HO)]H2++ [Al(HO)OH]+
It can be seen from the above pairs of conjugated acids and bases that protonic acids can be molecules, such as HCl, or cations, such as NHfour+, oranion, such as HPOfour2-；Proton bases can be molecules, such as NH, or cations, such as [Al (HO) OH]+, or anion, such as HPO-Etc.[1]
Acid base strength
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According to the proton theory of acids and bases, the substance that easily emits protons is a strong acid, but it is not easy to form a base after it emits protons, so it is a weak base because of its weak binding ability with protons.In other words, the stronger the acid, the weaker its conjugated base;On the contrary, the stronger the base, the weaker its conjugated acid.
According to the proton theory of acids and bases, the strength of acids and bases in solution is not only related to the nature of acids and bases, but also to the nature of solvents.What we can measure is the relative strength of acid and base in a certain solvent.If the same acid or base is dissolved in different solvents, their relative strength will be different.For example, HAc is weak acid in water, butliquid ammoniaIt shows strong acid, because the ability of liquid ammonia to capture protons (i.e. alkalinity) is much stronger than that of water.This phenomenon further explains the relativity of acid-base strength.[1]
Significance and limitations
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The acid-base proton theory expands the meaning of acid-base and the scope of acid-base reaction, gets rid of the limitation that acid-base must occur in water, and solves the problem of nonaqueous solutionOr acid-base reaction between gases, and dissociate, neutralizehydrolysisThe general situation of such reactions is one kind of reaction, namelyProton transferAcid base reaction of formula.However, the proton theory is limited to the release and acceptance of protons, so it must contain hydrogen and cannot explain the reaction of a class of compounds without hydrogen.It contains all alkaline substances and is still limited to hydrogen.[2]
application
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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 H+Or OH-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).[2]
Application of Conjugated Acids and Bases in Buffer Principle
HB-B-Buffer solution system, HB and B-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 conjugated base in the solution can accept H+, generate HB to resist H+Impact on pH.If a small amount of strong base is added to this solution, the conjugated acid HB in the solution can accept hydroxyl and generate B, thus resisting the influence of OH 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.[2]
Application of Conjugated Acids and Bases 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 pKa (or pKb) of weak acid (or weak base) equal to or close to the required pH (or pOH). If pKa or (pKb) is not completely equal to the required pH (or pOH) value, calculate the required concentration ratio of weak acid (or weak base) to salt according to the required pH (or pOH) value and using the buffer equation.According to proton theory, only "salt" is replaced by "conjugated base" (or "conjugated acid").[2]
