Monograph - Metrohm

Practical thermometric titrimetry 31
Fig. 23: Titration plot of the determination
of fluoride with boric acid.
7.5.2. Determination of formaldehyde
Formaldehyde can be determined in electroless copper plating solutions by the addition
of an excess of sodium sulfite solution and titrating the liberated hydroxyl ion with
standard acid.
H 2 C=O + SO 3
2–
+ H 2 O [HO–CH 2 –SO 3– ] + OH –
7.6. Thermometric titration of water
The thermometric titration of water using Karl Fischer (KF) reagent is feasible due to
the large reaction enthalpy involved (approx. –67 kJ/mol). However, other workers have
sought to avoid some of the disadvantages and limitations inherent in the use of the KF
reagent. Sadtler and Wilson (ca. 1980) developed a thermometric titration technique relying
on the endothermic acid-catalyzed reaction of 2,2-dimethoxypropane (DMP) with
water.
H +
CH 3 C(CH 3 O) 2 CH 3 + H 2 O 2CH 3 OH + (CH 3 ) 2 CO
ΔH 0 f (kJ/mol) –459.4 –285.8 2 x –239.2 –239.2
ΔH 0 r = +27.6 kJ/mol
The reagent had seen prior use in the dehydration of histological specimens, and in infrared
and gas chromatographic methods for the determination of water. DMP is of low
toxicity, quite stable, not hygroscopic and only reacts with water in the presence of an
acid. The reaction products (acetone and methanol) are relatively benign. Many applications
have been demonstrated, including the determination of water in concentrated
acids (where the acid itself provides the catalyst for the reaction) (see Figures 24 a and
24 b). Methane sulfonic acid may be used as a catalyst in many applications.