NON-AQUEOUS TITRATIONS

3 NON-AQUEOUS TITRATIONS
3.1 Revision of aqueous titration curves
You may remember from Instrumental Tests 3 that the size of the endpoint break in an acid-base
titration was affected by the strength of the acid and base involved, as shown in Figure 3.1.
Eventually, by around an acid strength of pKa 10, the endpoint break vanishes entirely.

Does this mean an acid with a pKa of 10 doesn’t react with NaOH? No, it still undergoes the
traditional acid-base to salt plus water reaction, but there is obviously a problem.
CLASS EXERCISE 3.1
What is the conjugate base of an acid?
What can you say about the strength of the conjugate base of a very weak acid?
3. Non-aqueous Titrations
AIT 3.2
The conjugate bases of very weak acids such as this one are strong enough to react with water.
Because there is so much water present – it is the solvent after all – the reaction is quite significant
(remember Le Chatelier’s Principle). So, the reaction between weak acid and base gets reversed, and
therefore the pH change around endpoint is masked because the acid reactant keeps getting reformed,
as shown in Equation 3.1.
Weak acid + NaOH Conjugate base + water Eqn 3.1
This reduces the portion of neutralised acid to less than 99.9%, the accepted limiting value for
quantitative applications. The other way to consider this, and it is useful in the context of non-aqueous
solvents, is that the solvent water becomes a significantly stronger acid, relative to the strength of
the acid, HA.
CLASS EXERCISE 3.2
What would happen when you titrated a very weak base with HCl in water?
3.2 Titrations in non-aqueous solvents
There is no reason why acid-base titrations have to be carried out in aqueous solution. In fact, this
introduces problems when dealing with organic acids and bases for a variety of reasons, including:
• lack of solubility – this can be overcome by back titration, but this is less accurate
• the loss of 99.9+% reaction completion – as explained above, water reacts with the conjugate
acid or base which is the product of the reaction, and causes the reaction to reverse
In the case of the weak acid, water is too strong an acid, so a less acidic solvent is required. For the
titration of a very weak base, water would again be a problem, this time because it is too strong a base.
So what this means is that water as the solvent is a problem, which must be removed, so we use a
different solvent for our sample and titrant.
Organic liquids can be used as titration solvents, and fit into four basic categories:
• neutral – such as hexane, trichloromethane and toluene,
• amphiprotic – such as methanol and ethanol
• basic – such as 1-butanamine and 4-methyl-2-pentanone
• acidic – such as ethanoic acid
The solvent is chosen so that it will not react with the product of the titration reaction.
CLASS EXERCISE 3.3
Complete the following table.
Analyte Reaction Product Solvent Class
Acid
Base
3. Non-aqueous Titrations
AIT 3.3
The most obvious solvent would be a neutral one for there is no chance of it reacting with anything.
However, there is an option that works even better. What is equally unlikely to react with a base?
Another base! Why is this better? It increases the size of the endpoint break relative to that for a
neutral solvent, making the endpoint easier to detect.
So the best solvent to choose is one of the same nature (acid or base) as the reaction
product, and therefore, the opposite nature to the analyte. Thus, for a weak acid, a basic solvent,
and for a weak base, an acidic solvent.
CLASS EXERCISE 3.4
Choose a suitable solvent for the titration of a very weak base.
If you think about the answer to Exercise 3.4 a little, it starts to look a bit strange (or maybe lot
strange). After all, what will you be titrating the base with? An acid, of course. But you have just
dissolved the sample and analyte in a bucketful of acid!! Surely this can’t work.
CLASS EXERCISE 3.5
(a) What would you expect to happen when the weak base analyte in Exercise 3.4 is
dissolved in the solvent you have just chosen?
(b) What is the product of the reaction?
(c) If there was 0.001 mole of basic analyte in the sample, how many moles of basic
compound are now in the solution?
(d) How does the strength of the “new base” compare to that of the analyte?
Practical aspects
Table 3.1 lists the typical titrants and primary standard used for non-aqueous acid-base titrations. You
might be confused about potassium hydrogen phthalate (KHP) being a primary standard for acidic
titrants, when it is used to standardise NaOH in water. KHP is a weak base due to the COO- group. It
must be said that it has some solubility problems in non-aqueous solvents, being ionic.
3. Non-aqueous Titrations
AIT 3.4
TABLE 3.1 Common titrants and primary standards used in non-aqueous titrations
Analyte Titrant Primary Standard
Basic perchloric acid KHP
Acidic tetraalkylammonium hydroxides
(R4N+OH-, R often butyl) and
sodium alkoxides (Na+RO-, R
frequently ethyl)
benzoic acid
The best titrant for the purpose is, as always, the strongest acid or base. Our “normally strong” acids,
including hydrochloric, nitric and perchloric acid, do not completely dissociate in other solvents. In
non-aqueous solvents, perchloric acid is in fact stronger than hydrochloric acid. Thus, the former is
used in pure ethanoic acid. In practice, HClO4 is supplied as a 72% aqueous solution. When mixed
with ethanoic acid, the water present would cause problems in the titration. It is removed by addition
of ethanoic anhydride, which reacts with water to yield ethanoic acid.
Endpoint detection
pH has no meaning in non-aqueous solution, and thus a pH meter operating in this mode is useless.
However, the glass (pH) electrode directly measures electric potential in the solution, on the meter’s
mV setting. The electrode responds to the change in concentration, producing the usual titration
curve, from which the endpoint may be obtained by the first derivative method.
As you should be aware, glass electrodes are routinely stored in aqueous solution when not
being used. Therefore, they cannot be used immediately for non-aqueous titrations. They must be
dewatered by immersion for 30 minutes in a solvent such as anhydrous methanol. However, this also
limits the time that electrode will produce a response, and usually after about two hours, it stops
working. It then must be soaked in dilute aqueous HCl to restore it to health.
Indicators can be used, but which one is suitable depends not purely on the nature of the
reactants, but also the solvent, and trial and error is required. It generally isn’t worthwhile bothering
with them.
Applications
Apart from the obvious organic acids and bases that can be titrated, other more unusual species, such
as nitrate and chloride ions, can also be determined, since these are the conjugate bases of “weak”
acids in non-aqueous media.
Non-aqueous titrations are important in the pharmaceutical industry where many significant
species, for example sulfa drugs (weak acids) and alkaloids (weak bases) cannot be analysed by
normal titration methods.
What You Need To Be Able To Do
• explain the problems associated with acid-base titrations in water
• explain why a non-aqueous solvent solves these problems
• choose an appropriate solvent and titrant, given a particular analyte
• outline the method of endpoint detection