Titrimetry Methods
Acid-Base Titrations
Aqueous Acid-Base Titrations
General
Proton transfer reactions in aqueous solutions are quite fast. Aqueous acid-base titrations are thus suitable for the analysis of any Bronsted acid or base. Practically, the pKa or pKb of the analyte should be less than about 10 (i.e., pKa or pKb < style="font-weight: bold;">Nonaqueous Acid-Base Titrations
General
Sometimes acid-base titrations are performed using a solvent other than water. There are several reasons why nonaqueous acid-base titrations may be used instead of aqueous titrations:
1. The sample is insoluble in water.
2. Sample and/or titrant reacts with water in undesirable ways.
3. For the analysis of very weak acids or bases. As mentioned previously, an aqueous acidimetric
titration is limited to bases with pKb less than about 10. Otherwise, the reaction between titrant (i.e., H3O+) and analyte will be incomplete. One solution to this problem for weak bases would be to use a stronger titrant – an impossibility in aqueous solutions. However, using glacial acetic acid as the solvent would solve that problem, since the strongest possible acid is H2OAc+ (a strong acid indeed). Most strong acids do not completely dissociate in acetic acid. Thus, perchloric acid in acetic acid is a much stronger titrant than the same acid in water. Similar considerations apply to alkalimetric titrations.
4. Selectivity is sometimes enhanced in nonaqueous solutions (analysis of analytes with similar dissociation constants). In aqueous solutions, a difference of 2 pK units is necessary to observe distinct endpoints. However, careful choice of solvent can sometimes allow the observation of distinct endpoints that cannot be measured in aqueous solution.
Common Titrants for Nonaqueous Acid-Base Titrations
Alkalimetry in Nonaqueous Solutions
• HCl in isopropanol
• HClO4 in glacial acetic acid
• these titrants may be standardized by tris, just as in aqueous titrations
Acidimetry in Nonaqueous Solutions
• KOH in ethanol, methanol, or isopropanol
• sodium methylate, NaOCH3, in methanol or chlorobenzene
• nonaqueous alkalimetric titrants may usually be standardized by benzoic acid, which is soluble in most of the solvents commonly used Instrumental Endpoint Detection in Acid-Base Titrations
Potentiometric endpoint detection using a pH meter is the universal instrumental method used for acid-base titrations, both in aqueous and nonaqueous solvents. Special care of the pH electrode is necessary for nonaqueous titrations – in particular, the electrode must not be allowed to become dehydrated.
Example Applications of Acid-Base Titrations
KJeldahl Analysis of Organic Nitrogen
The Kjeldahl procedure is a method for the analysis of organic nitrogen in the –3 oxidation state. The sample is digested with sulfuric acid to convert the organic nitrogen to ammonium, NH4+. The digested sample is then basified and ammonia is then distilled into acid. The ammonia may be distilled into excess standard HCl; the amount of HCl remaining after the distillation is determined by alkalimetric titration. Alternately, ammonia may be distilled into excess boric acid, H3BO3; the dihydrogen borate, H2BO3–, formed by reaction with ammonia is determined by acidimetric titration. The following figure shows apparatus that can be used for Kjeldahl analysis. Kjeldahl analysis is often used in the analysis of surface water and wastewater. The total Kjeldahl nitrogen (TKN) content of a water sample is a measure of the total concentration of nitrogen in the –3 oxidation state in the sample: ammonia/ammonium plus organic nitrogen. Kjeldahl analysis is also widely used to determine the protein content of food samples.
Buffering of Natural Waters The ability of an aqueous solution to resist changes in pH upon the addition of acid or base is termed the buffering capability of the solution. The ability of a natural water body to resist a decrease in pH is very important due to the ubiquitous presence of acid rain. The alkalinity of a water body is defined as the number of moles of H+ needed to bring a 1L sample to pH = 4.5. The higher the acid neutralizing capacity (ANC) of the water, the more acid must be added to the 1L sample to bring the pH to 4.5. Acidimetric titration to pH=4.5 (rather than to an endpoint) is thus widely used to characterize the ability of a water body to resist acidification. If potentiometric detection is not used, bromcresol green (perhaps mixed with methyl red) is used as a chemical indicator; the color change signifies the end of the titration.
Literature :
Arthur I. Vogel "A Text-Book of Quantitative Inorganic Analysis", 2nd edition.
HA Laitinen, WE Harris, "Chemical Analysis, 2 nd edition.
Acid-Base Titrations
Aqueous Acid-Base Titrations
General
Proton transfer reactions in aqueous solutions are quite fast. Aqueous acid-base titrations are thus suitable for the analysis of any Bronsted acid or base. Practically, the pKa or pKb of the analyte should be less than about 10 (i.e., pKa or pKb < style="font-weight: bold;">Nonaqueous Acid-Base Titrations
General
Sometimes acid-base titrations are performed using a solvent other than water. There are several reasons why nonaqueous acid-base titrations may be used instead of aqueous titrations:
1. The sample is insoluble in water.
2. Sample and/or titrant reacts with water in undesirable ways.
3. For the analysis of very weak acids or bases. As mentioned previously, an aqueous acidimetric
titration is limited to bases with pKb less than about 10. Otherwise, the reaction between titrant (i.e., H3O+) and analyte will be incomplete. One solution to this problem for weak bases would be to use a stronger titrant – an impossibility in aqueous solutions. However, using glacial acetic acid as the solvent would solve that problem, since the strongest possible acid is H2OAc+ (a strong acid indeed). Most strong acids do not completely dissociate in acetic acid. Thus, perchloric acid in acetic acid is a much stronger titrant than the same acid in water. Similar considerations apply to alkalimetric titrations.
4. Selectivity is sometimes enhanced in nonaqueous solutions (analysis of analytes with similar dissociation constants). In aqueous solutions, a difference of 2 pK units is necessary to observe distinct endpoints. However, careful choice of solvent can sometimes allow the observation of distinct endpoints that cannot be measured in aqueous solution.
Common Titrants for Nonaqueous Acid-Base Titrations
Alkalimetry in Nonaqueous Solutions
• HCl in isopropanol
• HClO4 in glacial acetic acid
• these titrants may be standardized by tris, just as in aqueous titrations
Acidimetry in Nonaqueous Solutions
• KOH in ethanol, methanol, or isopropanol
• sodium methylate, NaOCH3, in methanol or chlorobenzene
• nonaqueous alkalimetric titrants may usually be standardized by benzoic acid, which is soluble in most of the solvents commonly used Instrumental Endpoint Detection in Acid-Base Titrations
Potentiometric endpoint detection using a pH meter is the universal instrumental method used for acid-base titrations, both in aqueous and nonaqueous solvents. Special care of the pH electrode is necessary for nonaqueous titrations – in particular, the electrode must not be allowed to become dehydrated.
Example Applications of Acid-Base Titrations
KJeldahl Analysis of Organic Nitrogen
The Kjeldahl procedure is a method for the analysis of organic nitrogen in the –3 oxidation state. The sample is digested with sulfuric acid to convert the organic nitrogen to ammonium, NH4+. The digested sample is then basified and ammonia is then distilled into acid. The ammonia may be distilled into excess standard HCl; the amount of HCl remaining after the distillation is determined by alkalimetric titration. Alternately, ammonia may be distilled into excess boric acid, H3BO3; the dihydrogen borate, H2BO3–, formed by reaction with ammonia is determined by acidimetric titration. The following figure shows apparatus that can be used for Kjeldahl analysis. Kjeldahl analysis is often used in the analysis of surface water and wastewater. The total Kjeldahl nitrogen (TKN) content of a water sample is a measure of the total concentration of nitrogen in the –3 oxidation state in the sample: ammonia/ammonium plus organic nitrogen. Kjeldahl analysis is also widely used to determine the protein content of food samples.
Buffering of Natural Waters The ability of an aqueous solution to resist changes in pH upon the addition of acid or base is termed the buffering capability of the solution. The ability of a natural water body to resist a decrease in pH is very important due to the ubiquitous presence of acid rain. The alkalinity of a water body is defined as the number of moles of H+ needed to bring a 1L sample to pH = 4.5. The higher the acid neutralizing capacity (ANC) of the water, the more acid must be added to the 1L sample to bring the pH to 4.5. Acidimetric titration to pH=4.5 (rather than to an endpoint) is thus widely used to characterize the ability of a water body to resist acidification. If potentiometric detection is not used, bromcresol green (perhaps mixed with methyl red) is used as a chemical indicator; the color change signifies the end of the titration.
Literature :
Arthur I. Vogel "A Text-Book of Quantitative Inorganic Analysis", 2nd edition.
HA Laitinen, WE Harris, "Chemical Analysis, 2 nd edition.
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