chapter 9 applying neutralization titrations introduction to analytical chemistry

CHAPTER 9APPLYING

NEUTRALIZATION TITRATIONS

Introduction toIntroduction toAnalytical ChemistryAnalytical Chemistry

Copyright © 2011 Cengage Learning9-2

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 AcidsSodium CarbonateAcids 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 AcidsTris-(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.



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.



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



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.



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

NitrogenKjeldahl methodSince 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.



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 forDetermining Organic Nitrogen

Dumas methodThe 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 forDetermining 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



SulfurSulfur 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.



Other ElementsTable 9-1 lists other elements that can be determined

by neutralization methods.


Table 9-1


9B-2 Determining Inorganic Substances

Ammonium SaltsAmmonium salts are conveniently determined by

conversion to ammonia.The ammonia is collected and titrated as in the Kjeldahl

method.



Nitrates and NitritesThe method just described for ammonium salts can be

extended to the determination of inorganic nitrate or nitrite.



Carbonate and Carbonate MixturesIn 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



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



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.



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.



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.


THE END

chapter 9 applying neutralization titrations introduction to analytical chemistry

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