Organic acids cannot be titrated accurately in aqueous solution. What solvent is used?
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Nonaqueous acid-base titration.
Acid-base titration-nonaqueous acid-base titration
Learning objectives:
1. Master the basic principle and operation of non-drop setting.
2. Familiar with non-aqueous alkali quantity method.
3. Understand the non-aqueous acid method.
Non-aqueous titration is a titration analysis method in non-aqueous solution. Non-aqueous acid-base titration is an acid-base titration carried out in non-aqueous solution. Acid-base titration with water as solvent cannot accurately titrate too weak acid-base. Organic acids and bases with low solubility cannot be titrated; It is impossible to titrate multicomponent or mixed acids and bases step by step or separately, and it is also impossible to titrate substances with similar strength. Non-aqueous solvents can increase the solubility of organic matter; Change the acidity and alkalinity of substances; Expand the range of acid-base titration.
1. Classification of non-aqueous solvents:
Proton solvent: a solvent with strong proton donating and accepting ability.
1) acidic solvent organic weak base can significantly enhance its relative alkalinity in acidic solvents, and glacial acetic acid is the most commonly used acidic solvent.
2) Organic weak acid in alkaline solvent can significantly enhance its relative acidity in alkaline solvent, and dimethylformamide is the most commonly used alkaline solvent.
3) Amphoteric solvents are both acidic and alkaline, and methanol is the most commonly used one.
B. aprotic solvent: a solvent with no proton transfer in the solvent molecule.
1) dipolar aprotic solvent
2) inert solvent
C. mixed solvents
2. Properties of solvents
Acidity and alkalinity of (1) solvent
① the acid (HA) is dissolved in the solvent (SH):
Conclusion: The apparent acidity of HA depends on the inherent acidity of HA and the inherent alkalinity of solvent. The stronger the alkalinity of the solvent SH, the more complete the reaction and the stronger the acidity of HA.
For example, HCL is in H.
Acidity of 2O > acidity of HAc (∵H
2
O alkalinity > > HAc)
② Alkali B is dissolved in the solvent SH.
Conclusion: The apparent alkalinity of B depends on the inherent alkalinity of B and the inherent acidity of the solvent. The stronger the acidity of the solvent SH, the more complete the reaction and the stronger the alkalinity of B..
For example: NH
The alkalinity in 3 HAc is greater than that in H.
2
O alkalinity (∵HAc acidity > h
2
o)
(2) Dissociation of solvents
The acidity or alkalinity of the solvent is determined by two pairs of yoke acid-base pairs; In each pair of * * * yoke acid-base pairs, the stronger the acid, the weaker the corresponding * * * yoke base. The size of solvent K _ S has a certain influence on the change of the jump range of acid-base titration. In this paper, by comparing two solvents with different dissociation constants, namely water (p K w= 14.00) and ethanol (p K s = 19. 1), the influence of solvent dissociation constant on the jump range of acid-base titration is illustrated.
The smaller the self-dissociation constant (Ks) of the solvent, the larger the pKs, the larger the titration jump range and the steeper the titration end point. Therefore, weak acids that cannot be titrated directly in water can be titrated in non-aqueous solvents. (3) polarity of solvent
Dielectric constant ε: indicates the energy required for the dissociation of oppositely charged particles in solution.
Ionization: proton transfer occurs between acid and alkali and solvent, and ion pairs are formed under the action of electrostatic attraction.
Dissociation: ion pairs are separated by solvents to form solvated protons or solvent anions.
Conclusion: The polarity ↑, ε↑, f↓, and energy ↓ of the solvent are easier to dissociate and acidic.
For example, the acidity of HAc in water (ε=80.37) is greater than that in ethanol (ε=25).
(4) Homogenization effect and polymerization effect.
Common inorganic acids, such as perchloric acid, hydrochloric acid, sulfuric acid and nitric acid, are all strong acids, and the following acid-base balance exists in water:
Perchloric acid +H2O←→H3O++ClO4-
HCl+H2O←→H3O++Cl-
Sulfuric acid +H2O←→H3O++HSO4-
Nitric acid +H2O←→H3O ++ nitric acid-
In water, inorganic acid is strong acid, while water is alkali. Water accepts the proton of inorganic acid to form another acid hydration proton (H3o+); After releasing protons, inorganic acids are transformed into corresponding yokes (ClO4-, Cl-, HSO4-, NO3-, etc. ). This acid-base reaction proceeds completely to the right. That is to say, no matter how acidic the mineral acids are, after being dissolved in water, their inherent acid strength can no longer be expressed, but all of them are homogenized to the strength level of hydrated protons (H3O+), and as a result, their acid strengths are all equal. The homogenization of this solvent is called homogenization effect or.
Leveling effect The solvent with homogeneous function is called homogeneous solvent. Water is the homogenizing solvent of the above inorganic acids.
But water can't level hydrochloric acid and acetic acid, because the alkalinity of water is too weak for acetic acid, and the proton transfer reaction is very incomplete.
HAc+H2O←→H3O++Ac-
There are a lot of acetic acid molecules in the solution, but few hydrated protons. Because hydrochloric acid and acetic acid have different reactions in solvent water, they show different acid strengths. This effect of distinguishing the strength of acid (base) is called polymerization effect. Solvents with polymerization are called identification solvents. For hydrochloric acid and acetic acid, water is a good solvent to distinguish.
(3) Selection of non-aqueous solution conditions-selection of solvent
1 should have good solubility for samples and titration products.
The purity should be high, and water should be removed.
3 should be able to enhance the pH value of the acid-base to be measured.
4 viscosity should be small and volatility should be small.
Non-aqueous acid-base titration-application
A, alkali titration (Ch. P is called non-aqueous alkali method)
(1) Scope of application: Non-aqueous titration is mainly used to determine the contents of organic bases and their hydrohalides, phosphates, sulfates or organic acid salts, as well as alkali metal salts of organic acids, and also used to determine the contents of some organic weak acids. Non-aqueous titration is mainly used to determine the content of raw materials and drugs.
(2) Method
1. titration system: generally, weakly basic compounds can enhance their alkalinity in acidic solvents, that is, they can be titrated with strong acid. Most of the above drugs can be titrated in glacial acetic acid or acetic anhydride, and sometimes in some inert solvents (benzene, carbon tetrachloride, chloroform, etc.). ) or solvents (nitromethane, dioxane, acetone, etc.) that can affect solute dissociation and significantly affect pH value. ).
Titration solution is mostly glacial acetic acid solution of perchloric acid.
2. Methods of indicating the end point: potentiometric titration and indicator method. Generally speaking, the end point is indicated by potentiometric titration. If an indicator is used, the color change of the end point should be based on the color change of the indicator at the end point of potentiometric titration. Indication is a method often used in practical operation.
When using glacial acetic acid as solvent and acid titration solution to titrate alkali, the most commonly used indicators are crystal violet, α -naphthol benzyl alcohol (0.2% glacial acetic acid solution, its alkalinity is yellow and its acidity is green) and quinaldine.
Red (0. 1% methanol solution, red in alkalinity and colorless in acidity)
Crystal violet indicator (0.5% glacial acetic acid solution): its basic color is purple and its acid color is yellow. It is complicated to change color under different acidity. The color change from alkaline area to acidic area is: purple → blue → blue-green → green → yellow-green → yellow. When titrating different alkalis, the final color changes are different:
(1) When titrating strong alkali, the end point should be blue or blue-green;
(2) When titrating weak alkali, blue-green or green should be the end point. In the acid-base titration of non-aqueous solution, potentiometric titration is the basic method to determine the end point besides indicating the end point with indicator. Because many substances are titrated by non-aqueous titration, no suitable indicator has been found at present. When selecting indicators and determining the end color of indicators, potentiometric titration is needed as a control.
(3) In the Pharmacopoeia of China (20 15 edition), there are many examples of using perchloric acid titration solution to determine the content of weakly basic drugs. For example:
1. Organic weak base
K
b
Organic weak base > 10- 10, glacial acetic acid as solvent, directly titrated with perchloric acid titrant.
K
b
For extremely weak alkali < 10- 12, the mixed solution of glacial acetic acid and acetic anhydride in a certain proportion should be selected as the solvent and titrated directly with perchloric acid titration solution.
2. Alkali metal salts of organic acids
Because the acidity of organic acid is weak, its yoke (organic acid radical) is strongly alkaline in glacial acetic acid, so it can be titrated with glacial acetic acid solution of perchloric acid.
3. Hydrohalide salts of organic bases
Alkaloids are insoluble in water and unstable, and often exist in the form of hydrohalides. Because of hydrohalides,
It is acidic in glacial acetic acid solution, which makes the reaction incomplete and requires the addition of Hg(Ac).
2
Make it a reality
HgX
2. At this time, alkaloids exist in the form of acetate, so HClO can be used.
four
Titration with glacial acetic acid solution.
Second, the acid titration (Ch. P is called non-aqueous acid method)
The commonly used titration solution for acid titration is sodium methoxide solution.
When titrating mixed acids, methyl isobutyl ketone is usually used as a distinguishing solvent, and mixed solvents such as methanol-benzene or methanol-acetone are often used.
B. When titrating weak acid, use alcohol as solvent.
C. When titrating weak acid and extremely weak acid, alkaline solvent ethylenediamine or dipole solvent dimethylformamide is often used as the solvent.
Tan Tao of Chongqing Medical College.
￥
five
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Nonaqueous acid-base titration.
Acid-base titration-nonaqueous acid-base titration
Learning objectives:
1. Master the basic principle and operation of non-drop setting.
2. Familiar with non-aqueous alkali quantity method.
3. Understand the non-aqueous acid method.
Non-aqueous titration is a titration analysis method in non-aqueous solution. Non-aqueous acid-base titration is an acid-base titration carried out in non-aqueous solution. Acid-base titration with water as solvent cannot accurately titrate too weak acid-base. Organic acids and bases with low solubility cannot be titrated; It is impossible to titrate multicomponent or mixed acids and bases step by step or separately, and it is also impossible to titrate substances with similar strength. Non-aqueous solvents can increase the solubility of organic matter; Change the acidity and alkalinity of substances; Expand the range of acid-base titration.
Page 1
1. Classification of non-aqueous solvents:
Proton solvent: a solvent with strong proton donating and accepting ability.
1) acidic solvent organic weak base can significantly enhance its relative alkalinity in acidic solvents, and glacial acetic acid is the most commonly used acidic solvent.
2) Organic weak acids in alkaline solvents can significantly enhance their relative acidity in alkaline solvents, and dimethylformamide is the most commonly used alkaline solvent.
3) Amphoteric solvents are both acidic and alkaline, and methanol is the most commonly used one.
B. aprotic solvent: a solvent with no proton transfer in the solvent molecule.
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Enthusiastic netizens cannot accurately titrate organic acids in aqueous solution. What solvent is used?
***5 answers
System default value
answer the question
Ask a question
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More than 225 users adopted TA's answer.
Nonaqueous acid-base titration.
Acid-base titration-nonaqueous acid-base titration
Learning objectives:
1. Master the basic principle and operation of non-drop setting.
2. Familiar with non-aqueous alkali quantity method.
3. Understand the non-aqueous acid method.
Non-aqueous titration is a titration analysis method in non-aqueous solution. Non-aqueous acid-base titration is an acid-base titration carried out in non-aqueous solution. Acid-base titration with water as solvent cannot accurately titrate too weak acid-base. Organic acids and bases with low solubility cannot be titrated; It is impossible to titrate multicomponent or mixed acids and bases step by step or separately, and it is also impossible to titrate multicomponent or mixed acids and bases with similar strength. Non-aqueous solvents can increase the solubility of organic matter; Change the acidity and alkalinity of substances; Expand the range of acid-base titration.
1. Classification of non-aqueous solvents:
Proton solvent: a solvent with strong proton donating and accepting ability.
1) acidic solvent organic weak base can significantly enhance its relative alkalinity in acidic solvents, and glacial acetic acid is the most commonly used acidic solvent.
2) Organic weak acid in alkaline solvent can significantly enhance its relative acidity in alkaline solvent, and dimethylformamide is the most commonly used alkaline solvent.
3) Amphoteric solvents are both acidic and alkaline, and methanol is the most commonly used one.
B. aprotic solvent: a solvent with no proton transfer in the solvent molecule.
1) dipolar aprotic solvent
2) inert solvent
C. mixed solvents
2. Properties of solvents
Acidity and alkalinity of (1) solvent
① the acid (HA) is dissolved in the solvent (SH):
Conclusion: The apparent acidity of HA depends on the inherent acidity of HA and the inherent alkalinity of solvent. The stronger the alkalinity of the solvent SH, the more complete the reaction and the stronger the acidity of HA.
For example, HCL is in H.
Acidity of 2O > acidity of HAc (∵H
2
O alkalinity > > HAc)
② Alkali B is dissolved in the solvent SH.
Conclusion: The apparent alkalinity of B depends on the inherent alkalinity of B and the inherent acidity of the solvent. The stronger the acidity of the solvent SH, the more complete the reaction and the stronger the alkalinity of B..
For example: NH
The alkalinity in 3 HAc is greater than that in H.
2
O alkalinity (∵HAc acidity > h
2
o)
(2) Dissociation of solvents
The acidity or alkalinity of the solvent is determined by two pairs of yoke acid-base pairs; In each pair of * * * yoke acid-base pairs, the stronger the acid, the weaker the corresponding * * * yoke base. The size of solvent K _ S has a certain influence on the change of the jump range of acid-base titration. In this paper, by comparing two solvents with different dissociation constants, namely water (p K w= 14.00) and ethanol (p K s = 19. 1), the influence of solvent dissociation constant on the jump range of acid-base titration is illustrated.
The smaller the self-dissociation constant (Ks) of the solvent, the larger the pKs, the larger the titration jump range and the steeper the titration end point. Therefore, weak acids that cannot be titrated directly in water can be titrated in non-aqueous solvents. (3) polarity of solvent
Dielectric constant ε: indicates the energy required for the dissociation of oppositely charged particles in solution.
Ionization: proton transfer occurs between acid and alkali and solvent, and ion pairs are formed under the action of electrostatic attraction.
Dissociation: ion pairs are separated by solvents to form solvated protons or solvent anions.
Conclusion: The polarity ↑, ε↑, f↓, and energy ↓ of the solvent are easier to dissociate and acidic.
For example, the acidity of HAc in water (ε=80.37) is greater than that in ethanol (ε=25).
(4) Homogenization effect and polymerization effect.
Common inorganic acids, such as perchloric acid, hydrochloric acid, sulfuric acid and nitric acid, are all strong acids, and the following acid-base balance exists in water:
Perchloric acid +H2O←→H3O++ClO4-
HCl+H2O←→H3O++Cl-
Sulfuric acid +H2O←→H3O++HSO4-
Nitric acid +H2O←→H3O ++ nitric acid-
In water, inorganic acid is strong acid, while water is alkali. Water accepts the proton of inorganic acid to form another acid hydration proton (H3o+); After releasing protons, inorganic acids are transformed into corresponding yokes (ClO4-, Cl-, HSO4-, NO3-, etc. ). This acid-base reaction proceeds completely to the right. That is to say, no matter how acidic the mineral acids are, after being dissolved in water, their inherent acid strength can no longer be expressed, but all of them are homogenized to the strength level of hydrated protons (H3O+), and as a result, their acid strengths are all equal. The homogenization of this solvent is called homogenization effect or.
Leveling effect The solvent with homogeneous function is called homogeneous solvent. Water is the homogenizing solvent of the above inorganic acids.
But water can't level hydrochloric acid and acetic acid, because the alkalinity of water is too weak for acetic acid, and the proton transfer reaction is very incomplete.
HAc+H2O←→H3O++Ac-
There are a lot of acetic acid molecules in the solution, but few hydrated protons. Because hydrochloric acid and acetic acid have different reactions in solvent water, they show different acid strengths. This effect of distinguishing the strength of acid (base) is called polymerization effect. Solvents with polymerization are called identification solvents. For hydrochloric acid and acetic acid, water is a good solvent to distinguish.
(3) Selection of non-aqueous solution conditions-selection of solvent
1 should have good solubility for samples and titration products.
The purity should be high, and water should be removed.
3 should be able to enhance the pH value of the acid-base to be measured.
4 viscosity should be small and volatility should be small.
Non-aqueous acid-base titration-application
A, alkali titration (Ch. P is called non-aqueous alkali method)
(1) Scope of application: Non-aqueous titration is mainly used to determine the contents of organic bases and their hydrohalides, phosphates, sulfates or organic acid salts, as well as alkali metal salts of organic acids, and also used to determine the contents of some organic weak acids. Non-aqueous titration is mainly used to determine the content of raw materials and drugs.
(2) Method
1. titration system: generally, weakly basic compounds can enhance their alkalinity in acidic solvents, that is, they can be titrated with strong acid. Most of the above drugs can be titrated in glacial acetic acid or acetic anhydride, and sometimes in some inert solvents (benzene, carbon tetrachloride, chloroform, etc.). ) or solvents (nitromethane, dioxane, acetone, etc.) that can affect solute dissociation and significantly affect pH value. ).
Titration solution is mostly glacial acetic acid solution of perchloric acid.
2. Methods of indicating the end point: potentiometric titration and indicator method. Generally speaking, the end point is indicated by potentiometric titration. If an indicator is used, the color change of the end point should be based on the color change of the indicator at the end point of potentiometric titration. Indication is a method often used in practical operation.
When using glacial acetic acid as solvent and acid titration solution to titrate alkali, the most commonly used indicators are crystal violet, α -naphthol benzyl alcohol (0.2% glacial acetic acid solution, its alkalinity is yellow and its acidity is green) and quinaldine.
Red (0. 1% methanol solution, red in alkalinity and colorless in acidity)
Crystal violet indicator (0.5% glacial acetic acid solution): its basic color is purple and its acid color is yellow. It is complicated to change color under different acidity. The color change from alkaline area to acidic area is: purple → blue → blue-green → green → yellow-green → yellow. When titrating different alkalis, the final color changes are different:
(1) When titrating strong alkali, the end point should be blue or blue-green;
(2) When titrating weak alkali, blue-green or green should be the end point. In the acid-base titration of non-aqueous solution, potentiometric titration is the basic method to determine the end point besides indicating the end point with indicator. Because many substances are titrated by non-aqueous titration, no suitable indicator has been found at present. When selecting indicators and determining the end color of indicators, potentiometric titration is needed as a control.
(3) In the Pharmacopoeia of China (20 15 edition), there are many examples of using perchloric acid titration solution to determine the content of weakly basic drugs. For example:
1. Organic weak base
K
b
Organic weak base > 10- 10, glacial acetic acid as solvent, directly titrated with perchloric acid titrant.
K
b
For extremely weak alkali < 10- 12, the mixed solution of glacial acetic acid and acetic anhydride in a certain proportion should be selected as the solvent and titrated directly with perchloric acid titration solution.
2. Alkali metal salts of organic acids
Because the acidity of organic acid is weak, its yoke (organic acid radical) is strongly alkaline in glacial acetic acid, so it can be titrated with glacial acetic acid solution of perchloric acid.
3. Hydrohalide salts of organic bases
Alkaloids are insoluble in water and unstable, and often exist in the form of hydrohalides. Because of hydrohalides,
It is acidic in glacial acetic acid solution, which makes the reaction incomplete and requires the addition of Hg(Ac).
2
Make it a reality
HgX
2. At this time, alkaloids exist in the form of acetate, so HClO can be used.
four
Titration with glacial acetic acid solution.
Second, the acid titration (Ch. P is called non-aqueous acid method)
The commonly used titration solution for acid titration is sodium methoxide solution.
When titrating mixed acids, methyl isobutyl ketone is usually used as a distinguishing solvent, and mixed solvents such as methanol-benzene or methanol-acetone are often used.
B. When titrating weak acid, use alcohol as solvent.
C. When titrating weak acid and extremely weak acid, alkaline solvent ethylenediamine or dipole solvent dimethylformamide is often used as the solvent.
Tan Tao of Chongqing Medical College.
￥
five
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Nonaqueous acid-base titration.
Acid-base titration-nonaqueous acid-base titration
Learning objectives:
1. Master the basic principle and operation of non-drop setting.
2. Familiar with non-aqueous alkali quantity method.
3. Understand the non-aqueous acid method.
Non-aqueous titration is a titration analysis method in non-aqueous solution. Non-aqueous acid-base titration is an acid-base titration carried out in non-aqueous solution. Acid-base titration with water as solvent cannot accurately titrate too weak acid-base. Organic acids and bases with low solubility cannot be titrated; It is impossible to titrate multicomponent or mixed acids and bases step by step or separately, and it is also impossible to titrate multicomponent or mixed acids and bases with similar strength. Non-aqueous solvents can increase the solubility of organic matter; Change the acidity and alkalinity of substances; Expand the range of acid-base titration.
Page 1
1. Classification of non-aqueous solvents:
Proton solvent: a solvent with strong proton donating and accepting ability.
1) acidic solvent organic weak base can significantly enhance its relative alkalinity in acidic solvents, and glacial acetic acid is the most commonly used acidic solvent.
2) Organic weak acids in alkaline solvents can significantly enhance their relative acidity in alkaline solvents, and dimethylformamide is the most commonly used alkaline solvent.
3) Amphoteric solvents are both acidic and alkaline, and methanol is the most commonly used one.
B. aprotic solvent: a solvent with no proton transfer in the solvent molecule.
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