Introduction to Analytical Chemistry CHAPTER 9 APPLYING NEUTRALIZATION TITRATIONS

9A Reagents For Neutralization Titrations Strong acids and strong bases cause the most pronounced change in pH at the equivalence point.

9A-1 Preparing Standard Acid Solutions Hydrochloric acid is widely used for titration of bases. Perchloric acid and sulfuric acid are also stable and are useful for nitric acid are seldom encountered because of their oxidizing properties.

9A-2 Standardizing Acids Sodium Carbonate Acids are frequently standardized against weighed quantities of sodium carbonate. As shown in Figure 8-4, two end points are observed in the titration of sodium carbonate. The second end point is always used for standardization. An even sharper end point can be achieved by boiling the solution briefly to eliminate the reaction products,

9A-2 Standardizing Acids Other Primary Standards for Acids Tris-(hydroxymethyl)aminomethane, (HOCH₂)₃CNH₂ , known also as TRIS or THAM, is available in primary-standard It possesses the advantage of a substantially greater mass per mole of protons consumed (121.1) than sodium carbonate

Figure 9-1 Figure 9-1 Titration of 25.00 mL of 0.1000 M Na2CO3 with 0.1000 M HCl. After about 49 mL of HCl have been added, the solution is boiled, causing the increase in pH shown. The change in pH on further addition of HCl is much larger.

9A-3 Preparing Standard Base Solutions Sodium hydroxide is the most common base for preparing standard solutions, although potassium hydroxide and barium hydroxide are also encountered. None of these is obtainable in primary-standard purity.

9A-3 Preparing Standard Base Solutions The Effect of Carbon Dioxide upon Standard Base Solutions If the end point of a titration occurs in acidic solution 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.

9A-3 Preparing Standard Base Solutions The Effect of Carbon Dioxide upon Standard Base Solution Most titrations that make use of a standard base have basic end points In these basic solutions, The effective concentration of the base is thus diminished by absorption of carbon dioxide, and a systematic error (called a carbonate error) results.

Example 9-2

Example 9-2 The effective concentration cNaOH of NaOH for acetic acid is

9A-3 Preparing Standard Base Solutions The solid reagents are always contaminated by significant amounts of carbonate ion. The presence of this contaminant does not cause a carbonate error provided the same indicator is used for both standardization and analysis.

9A-3 Preparing Standard Base Solutions Distilled water, should be boiled briefly to eliminate the gas. The water is then allowed to cool because hot alkali solutions rapidly absorb carbon dioxide. The concentration of a sodium hydroxide solution will decrease slowly (0.1 to 0.3% per week) if the base is stored in glass bottles.

9A-4 Standardizing Solutions of Bases Potassium Hydrogen Phthalate, KHC₈H₄O₄ nonhygroscopic crystalline high molar mass (204.2 g /mol)

9A-4 Standardizing Solutions of Bases Other Primary Standards for Bases Benzoic acid is obtainable in primary-standard purity Potassium hydrogen iodate, KH(IO₃)₂

9B-1 Elemental Analysis Nitrogen Kjeldahl method Since most proteins contain approximately the same percentage of nitrogen, multiplication of this percentage by a suitable factor (6.25 for meats, 6.38 for dairy products, and 5.70 for cereals) gives the percentage of protein in a sample.

9B-1 Elemental Analysis 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.

Feature 9-2 Other Methods for Determining Organic Nitrogen Dumas method The sample is mixed with powdered copper(II) oxide and ignited in a combustion tube to give carbon dioxide, water, nitrogen, and small amounts of nitrogen oxides. A stream of carbon dioxide carries these products through a packing of hot copper, which reduces any oxides of nitrogen to elemental nitrogen. The mixture then is passed into a gas buret filled with concentrated potassium hydroxide. The only component not absorbed by the base is nitrogen, and its volume is measured directly.

Feature 9-2 Other Methods for Determining Organic Nitrogen The newest method for determining organic nitrogen involves combusting the sample at 1100°C for a few minutes to convert the nitrogen to nitric oxide, NO. Ozone is then introduced into the gaseous mixture, which oxidizes the nitric oxide to nitrogen dioxide. This reaction gives off visible radiation (chemiluminescence), the intensity of which is measured.

Example 9-3 A 0.7121-g sample of a wheat flour was analyzed by the Kjeldahl method. The ammonia formed by addition of concentrated base after digestion with H₂SO₄ was distilled into 25.00 mL of 0.04977 M HCl. The excess HCl was then backtitrated with 3.97 mL of 0.04012 M NaOH. Calculate the percent protein in the flour.

Example 9-3

Example 9-3

9B-1 Elemental Analysis Sulfur Sulfur in organic and biological materials is conveniently determined by burning the sample in a stream of oxygen. The sulfuric acid is then titrated with standard base.

9B-1 Elemental Analysis Other Elements Table 9-1 lists other elements that can be determined by neutralization methods.

Table 9-1

9B-2 Determining Inorganic Substances Ammonium Salts Ammonium salts are conveniently determined by conversion to ammonia. The ammonia is collected and titrated as in the Kjeldahl method.

9B-2 Determining Inorganic Substances Nitrates and Nitrites The method just described for ammonium salts can be extended to the determination of inorganic nitrate or nitrite.

9B-2 Determining Inorganic Substances Carbonate and Carbonate Mixtures In a solution containing sodium carbonate, sodium hydrogen carbonate, and sodium hydroxide. No more than two of these three constituents can exist in appreciable amount in any solution. The analysis of such mixtures requires two titrations, one with an alkaline-range indicator, the other with an acid-range indicator. The composition of the solution can then be deduced from the relative volumes of acid needed to titrate equal volumes of the sample.

Table 9-2

Figure 9-3 Figure 9-3 Titration curves and indicator transition ranges for the analysis of mixtures containing hydroxide, carbonate, and hydrogen carbonate ions.

Figure 9-3 Figure 9-3 Titration curves and indicator transition ranges for the analysis of mixtures containing hydroxide, carbonate, and hydrogen carbonate ions.

Figure 9-3 Figure 9-3 Titration curves and indicator transition ranges for the analysis of mixtures containing hydroxide, carbonate, and hydrogen carbonate ions.

Example 9-4 A solution contains NaHCO₃, Na₂CO₃ , and NaOH, either alone or in permissible combination. Titration of a 50.0-mL portion to a phenolphthalein end point requires 22.1 mL of 0.100 M HCl. A second 50.0-mL aliquot requires 48.4 mL of the HCl when titrated to a bromocresol green end point. Deduce the composition, and calculate the molar solute concentrations of the original solution.

Example 9-4 Because less than half of this amount is involved in the first titration, the solution must contain some NaHCO₃ in addition to Na₂CO₃. When the phenolphthalein end point is reached, the originally present is converted to .

Example 9-4 The titration from the phenolphthalein to the bromocresol green end point involves

9B-2 Determining Inorganic Substances The method described in Example 9-4 is not entirely satisfactory because thepH change corresponding to the hydrogen carbonate equivalence point is not sufficient to give a sharp color change. Relative errors of 1% or more must be expected as a consequence. The accuracy of methods can be greatly improved by taking advantage of the limited solubility of barium carbonate in neutral and basic solutions.

9B-2 Determining Inorganic Substances In the Winkler method analysis of carbonate/hydroxide mixtures, both components are titrated with a standard acid to the end point with an acid-range indicator. An unmeasured excess of neutral barium chloride is then added to a second aliquot of the sample solution to precipitate the carbonate ion; the hydroxide ion is titrated to a phenolphthalein end point.

9B-2 Determining Inorganic Substances Carbonate and hydrogen carbonate ions can be accurately determined in mixtures by first titrating both ions with standard acid to an end point with an acid-range indicator. carbonate in a second aliquot is converted to carbonate by the addition of a known excess of standard base. After a large excess of barium chloride has been introduced, the excess base is titrated with standard acid to a phenolphthalein end point.

9B-3 Determining Organic and Biochemical Substances The analysis of esters involves heating the sample in the presence of an excess of standard base. The resulting hydrolysis, which is called saponification, consumes one mole of the base per mole of ester. After saponification is judged to be complete, the excess base is titrated with a standard acid solution.

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