Nonaqueous titration

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• Nonaqueous titration

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• Nonaqueous titration is the titration of
  substances dissolved in nonaqueous solvents.
• It is the most common titrimetric procedure used
  in pharmacopoeial assays and serves a double
  purpose
• it is suitable for the titration of very weak
  acids and very weak bases, and it provides a
  solvent in which organic compounds are soluble.

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Theory

• The theory is that water behaves as both a weak
  acid and a weak base thus, in an aqueous
  environment, it can compete effectively with very
  weak acids and bases with regard to proton
  donation and acceptance, as shown below
• H2O H ? H3O
• Competes with RNH2 H ? RNH3
• or
• H2O B ? OH- BH
• Competes with ROH B ? RO- BH

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The effect of this is that the inflection in the
titration curves for very weak acids and very
weak bases is small, because they approach the pH
limits in water of 14 or 0 respectively , thus
making endpoint detection relatively more
difficult.
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A general rule is that bases with pKa lt 7 or
acids with pKa gt 7 cannot be determined
accurately in aqueous solution.
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Substances which are either too weakly basic or
too weakly acidic to give sharp endpoints in
aqueous solution can often be titrated in
nonaqueous solvents. The reactions which occur
during many nonaqueous titrations can be
explained by means of the concepts of the
Brønsted-Lowry theory. According to this theory
an acid is a proton donor, i.e. a substance which
tends to dissociate to yield a proton, and a base
is proton acceptor, i.e. a substance which tends
to combine with a proton. When an acid HB
dissociates it yields a proton together with the
conjugate base B of the acid
HB ? H B-
acid ? proton base
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HB ? H B-
acid ? proton base
Alternatively, the base B will combine with a
proton to yield the conjugate acid HB of the base
B, for every base has its conjugate acid and,
every acid has its conjugate base.
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It follows from these definitions that an acid
may be either an electrically neutral
molecule, e.g. HCl, or a positively charged
cation, e.g. C6H5NH3, or a negatively charged
anion, e.g. HSO4-. A base may be either an
electricially neutral molecule, e.g. C6H5NH2, or
an anion, e.g. Cl-.
Substances which are potentially acidic can
function as acids only in the presence of a base
to which they can donate a proton. Conversely
basic properties do not become apparent unless an
acid also is present.
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Nonaqueous solvents used

• Aprotic solvents are neutral, chemically inert
  substances such as benzene and chloroform.
• They have a low dielectric constant, do not react
  with either acids or bases and therefore do not
  favor ionization.

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The fact that picric acid gives a colorless
solution in benezene which becomes yellow on
adding aniline shows that picric acid is not
dissociated in benzene solution and also that in
the presence of the base aniline it functions as
an acid, the development of yellow color being
due to formation of the picrate ion.
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Protophilic solvents are basic in character and
react with acids to form solvated protons. HB
Sol. ? Sol.H B- Acid Basic solvent ?
Solvated proton Conjugate base of acid
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weakly basic solvent has less tendency than a
strongly basic one to accept a proton. Similarly
a weak acid has less tendency to donate protons
than a strong acid. As a result a strong acid
such as perchloric acid exhibits more strongly
acidic properties than a weak acid such as acetic
acid when dissolved in a weakly basic solvent. On
the other hand, all acids tend to become
indistinguishable in strength when dissolved in
strongly basic solvents owing to the greater
affinity of strong bases for protons. This is
called the leveling effect. Strong bases are
leveling solvents for acids, weak bases are
differentiating solvents for acids.
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Protogenic solvents are acidic substances, e.g.
sulfuric acid. They exert a leveling effect on
bases.
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Amphiprotic solvents have both protophilic and
protogenic properties. Examples are water,
acetic acid and the alcohols. They are
dissociated to a slight extent. The dissociation
of acetic acid, which is frequently used as a
solvent for titration of basic substances, is
shown in the equation below CH3COOH ? H
CH3COO- Here the acetic acid is functioning as
an acid. If a very strong acid such as perchloric
acid is dissolved in acetic acid, the latter can
function as a base and combine with protons
donated by the perchloric acid to form protonated
acetic acid, an onium ion HClO4 ? H ClO4-
CH3COOH H ? CH3COOH2 (onium ion)
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Since the CH3COOH2 ion readily donates its
proton to a base, a solution of perchloric acid
in glacial acetic acid functions as a strongly
acidic solution.
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When a weak base, such as pyridine, is dissolved
in acetic acid, the acetic acid exerts its
levelling effect and enhances the basic
properties of the pyridine. It is possible,
therefore, to titrate a solution of a weak base
in acetic acid with perchloric acid in acetic
acid, and obtain a sharp endpoint when attempts
to carry out the titration in aqueous solution
are unsuccessful.
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HClO4 CH3COOH ? CH3COOH2 ClO4- C5H5N
CH3COOH ? C5H5NH CH3COO- CH3COOH2 CH3COO-
? 2CH3COOH Adding HClO4 C5H5N ? C5H5NH
ClO4-
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Visual indicators

• The following indicators are in common use

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Potentiometric titration

• The end point of most titrations is detected by
  the use of visual indicator but the method can be
  inaccurate in very dilute or colored solutions.
  However under the same conditions, a
  potentiometric method for the detection of the
  equivalence point can yield accurate results
  without difficulty. The electrical apparatus
  required consists of a potentiometer or pH meter
  with a suitable indicator and reference
  electrode. The other apparatus consists of a
  burette, beaker and stirrer.

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??????? ??? ? ?????? ????? ??????? ?? ??????????
??? ??? ????

• ???) ???? ??? ????? ??? ???? ???
• ??? ???? B ?? ?? ????? ??? ????? ??? ? ?? ?????
  ???? ?????? ????? ??? ?????? ????
• B H3O ? BH H2O
• K BH/B H3O Kb/Kw
• B HCOOH2 ? BH HCOOH
• K BH/BHCOOH2 Kb/Ks
• ????? Kb ? Ks ?????
• ???? ?? ???? ???? ????? ???? ??? ????? ????

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?) ????? ???? ????? ?? ???? ?????? ?? ????? ????
?? ??? ?????? ?? ??????? ??????? ?? ???? ?????
????? ????? ???? ? ???????? ????? ?? ??? ????
????? ?????? ????? ????? ????? ?? ?????? ????????
? ??? ????? ?? ????? ??????? ????? ?? ???? ??
????. ?? ????? ?????? ????? ?? ?? ????? ?? ?????
?????? ??? ?????? ?? ??? ?????? ?? ????? ???? ??
??? ??? ?????. C6H5NH2 HOAc ? C6H5 NH3
OAc- Kb ?? Kb ?????? ???. C6H5NH2 H2O ?
C6H5 NH3 OH-
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???????? ????? ????? ?? ???? ? ??????? ????
????? ????? ?? ??? ?????? ????? ? ???????? ??
????? ??????? ???? ??? ???? ?? ????. ?? ??? ????
??? ???? ????? ?? ???? ?? ??? ????? ?? ???. ?? ?
??????? ????????? ????? ?????? ???????? ?????
??? ?? ??????? ?????? ???? ?????.
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?) ????? ???? ?? ??????? ?? ????? ???? ??
??? ???? ?? ??????? ?? ???? ???? ????? ????? ??
???? ??????? ???? ?? ??? ????? ???. ?? ?? ???? ??
???? ?? ??????? ???? ????? ?? ??????? ?? ??? ??
??? ???? ?? ?? ??? ?? ??? ???? ???? ?? ????? ???
???. HA SH ? SH2 A- Or B SH ? BH
S- ???? ????? ???? ????? ?? ?? ???? 10-5 ??? ??
?????? ?? ?????? 10-10 ???. ?????? ?? ????? ?????
???? ??? ??? ????? ?????? ???? ?? ???? ?? ???
????? ??? ????? ???? ?? ??????? ???? ???
???? BH SH ? SH2 B A- SH ? HA S-
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?????? ??????? ?????? ???? ????????? ??? ???? ???

• ???? ??? ?????? ????? ??? ???? ????.
• ???? ?? ?? ????? ?? ????? ?? ??????? ?????? ??
  ????????? ?? ??? ????? ???? ?? ????? ?? ????
  ?????? ? ?? ????????? ?? ???? ???? ?? ???? ??
  ????? ??????? ?????? ?????? ?????.
• ???? ?? ??????? ?? ?? ?? ?????? ???? ???? ???.

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???? ???? NH4 ????? ????? ?? ?? ???? ?? ???? ??
????????? NH4 ?0.2F ?? ???) NaOH ?0.2F ?? ?????
??? ? ?) ?? C2H5ONa 0.2F ?? ?????? ?? ??
?????? ????. ???? ??? ??? ?????? ????? ???? ?? ?
?????? ?? ????? ????? ?? 1x1o-14 ? 8x10-20 ? ????
????? ????? ???? NH4 ?? ?? ?????? ????? ?? 6 x
10-10 ? ?? ????? ????? ?? 1x10-10 ???.
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?? ???? ???