Chapter 16 Application of Neutralization Titrations Neutralization titrations are widely used to determine the concentration of analytes that are themselves acids or bases or that are convertible to such species by suitable treatment. Water is the usual solvent for neutralization titrations because it is readily available, inexpensive, and nontoxic. Its low coefficient of expansion with temperature is an added virtue. Some analytes, however, are not titratable in aqueous media because their solubilities are too low. Nonaqueous solvents such as methyl and ethyl alcohol, glacial acetic acid, and methyl isobutyl ketone often make it possible to titrate such analytes in a solvent other than water.

Preparing Standard Acid Solutions Hydrochloric acid is widely used for titration of bases. Dilute solutions of HCl are stable indefinitely and do not cause troublesome precipitation reactions with most cations. Solutions of perchloric acid and sulfuric acid are also stable and are useful for titrations where chloride ion interferes by forming precipitates. Standard solutions of nitric acid are seldom encountered because of their oxidizing properties. Standard acid solutions are ordinarily prepared by diluting an approximate volume of the concentrated reagent and subsequently standardizing the diluted solution against a primary-standard base.

(HOCH2)3CNH2 + H3O+ (HOCH2)3CNH3+ + H2O Standardizing Acids Acids are frequently standardized against weighed quantities of sodium carbonate. Tris-(hydroxymethyl)aminomethane, (HOCH2)3CNH2, known also as TRIS or THAM, is available in primary-standard purity from commercial sources. It possesses the advantage of a substantially greater mass per mole of protons consumed (121.1) than sodium carbonate (53.0). (HOCH2)3CNH2 + H3O+ (HOCH2)3CNH3+ + H2O Sodium tetraborate decahydrate (Na2B4O7.10H2O) and mercury (II) oxide have also been recommended as primary standards. The reaction of an acid with the tetraborate is B4O7 2- + 2H3O+ + 3H2O 4H3BO3

Preparing Standard Base Solutions Sodium hydroxide is the most common base for preparing standard solution, although potassium hydroxide and barium hydroxide are also encountered. None of these is obtainable in primary-standard purity, so standardization is required after preparation. The Effect of Carbon Dioxide In solution as well as in the solid state, the hydroxides of sodium, potassium, and barium react rapidly with atmospheric carbon dioxide to produce the corresponding carbonate: CO2 + 2OH- CO32- + H2O

…continued… Although production of each carbonate ion uses up two hydroxide ions, the uptake of carbon dioxide by a solution of base does not necessarily alter its combining capacity for hydronium ions. If the end point of a titration occurs in acidic solution and thus requires an acid-range indicator, each carbonate ion produced from sodium or potassium hydroxide will have reacted with two hydronium ions of the acid CO3 2- + 2H3O+ H2CO3 + 2H2O

…continued… Because the amount of hydronium ion consumed by this reaction is identical to the amount of hydroxide lost during formation of the carbonate ion, no error is incurred. Unfortunately, most titrations that make use of a standard base have basic end points and require an indicator with a basic transition range. In these basic solution, each carbonate ion has reacted with only one hydronium ion when the color change of the indicator is observed: CO32- + H3O+ HCO3- + H2O The effective concentration of the base is thus diminished by absorption of carbon dioxide and a systematic error (called a carbonate error) results.

Standardizing Solutions of Bases Several excellent primary standards are available for the standardization of bases. Potassium hydrogen phthalate (KHC8H4O4) is an ideal primary standard. It is a nonhygroscopic crystalline solid with a high molar mass (204.2 g/mol). For most purposes, the commercial analytical-grade salt can be used without further purification. Benzoic acid is obtainable in primary-standard purity and can be used for the standardization of bases. Potassium hydrogen iodate, KH(IO3)2, is an excellent primary standard with a high molecular mass per mole of protons.

APPLICATIONS OF NEUTRALIZATION TITRATIONS Neutralization titrations are used to determine the innumerable inorganic, organic, and biological species that posses inherent acidic or basic properties. Equally important, however, are the many applications that involve conversion of an analyte to an acid or base by suitable chemical treatment, followed by titration with a standard strong base or acid.

Elemental Analysis Several important elements that occur in organic and biological systems are conveniently determined by methods that involve an acid/base titration as the final step. Generally, the elements susceptible to this type of analysis are nonmetallic and include carbon, nitrogen, chlorine, bromine, and fluorine. Pretreatment converts the element to an inorganic acid or base that is then titrated.

Nitrogen Nitrogen occurs in a wide variety of substances of interest in research, industry, and agriculture. Examples include amino acids, proteins, synthetic drugs, fertilizers, explosives, soils, potable water supplies, and dyes. Thus, analytical methods for the determination of nitrogen, particularly in organic substance, are of singular importance. The most common method for determining organic nitrogen is the Kjeldahl mehod, which is based on a neutralization titration. In the Kjeldahl method, the sample is decomposed in hot, concentrated sulfuric acid to convert the bound nitrogen to ammonium ion. The resulting solution is then cooled, diluted, and made basic. The liberated ammonia is distilled, collected in an acidic solution, and determined by a neutralization titration.

The sulfuric acid is then titrated with standard base. Sulfur in organic and biological materials is conveniently determined by burning the sample in a stream of oxygen. The sulfur dioxide (as well as the sulfur trioxide) formed during the oxidation is collected by distillation into a dilute solution of hydrogen peroxide. S(s) + O2(g)  SO2(g) SO2(g) + H2O2  H2SO4 The sulfuric acid is then titrated with standard base.

Determining Inorganic Substances Numerous inorganic species can be determined by titration with strong acids or bases. Ammonium salts Ammonium salts are conveniently determined by conversion to ammonia with strong base followed by distillation. The ammonia is collected and titrated as in the Kjeldahl method. Nitrates and Nitrites The method just described for ammonium salts can be extended to the determination of inorganic nitrate or nitrite. These ion are first reduced to ammonium ion by Devarda’s alloy(50% Cu, 45% Al, 5% Zn). Granules of the alloy are introduced into a strongly alkaline solution of the sample in a Kjeldahl flask. The ammonia is distilled after reaction is complete.