Transcript Chapter 14 Applying Neutralizaton Titrations

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.
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 + 2OHCO32- + 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.
Sulfur
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.