formaldehyde - Sciencemadness Dot Org

STATE OF DISSOLVED FORMALDEHYDE 35
centrations (3 per cent or less) are those of a monomolecular reaction.
Lesser degrees of dilution give rate figures which are influenced by reverse
reactions of a higher order. The temperature, coefficient of the reaction
rate is calculated as 2,7 per 10°C. This means that the rate is almost
tripled for a temperature rise of ten degrees. The hydrogen and hydroxyl
ion concentrations of the solution have a considerable influence on this
rate, an effect which is at a minimum in the pH range 2.6 to 4.3 and increases
rapidly "with higher or lower pH values. The hydroxyl ion was
found to be 10 7 times stronger in its effect than the hydrogen ion. Mean
rate measurements for solution having a pH of 4.22 were 0.0126 at 20°C
and 0.0218 at 25.5°C. Neutral salts such as potassium chloride have no
noticeable influence on the rate. However, methanol and similar substances
which lower the ion product of water, lower the rate, as will be
seen below;
% Added Rate Constant far Dilution
Methanol from 36.5 to 2$1%
0 0.0126
2 0.0106
4 0,0056
At low temperatures, the depolymerization which follows dilution of
formaldehyde is a very slow reaction. A solution in the pH range 2.6 to
4.3 requires more than 50 hours to attain equilibrium after dilution from
36.5 to 3 per cent at 0°C.
Wadano points out that these kinetic studies cast considerable light on
the mechanism of the depolymerization reaction, showing that it possesses
a marked similarity to the hydrolysis of acids and amides, and to the
mutarotation of sugar. These reactions also have a pH range of minimum
rate and possess activation heats of approximately the same magnitude.
The heat of activation for the depolymerization of dissolved polymeric
formaldehyde hydrates was calculated by Wadano from rate measurements
as 17.4 kcal per gram mol of formaldehyde (CH20). The activation heat
for ester hydrolysis is 16.4-18.5 kcal7 amide hydrolysis 16.4-26.0 kcal and
mutarotation 17.3 to 19.3 kcal. The figures for glucoside splitting are
much higher, indicating a fundamental difference. It is highly probable
that formaldehyde hydrates are amphoteric in the pH range for minimum
reaction rates, but form anions and cations under more acidic or alkaline
conditions. Polymerization and depolymerization reactions probably
involve these dissociated molecules and ions.
Wadano concludes that the following equilibria occur in dilute formaldehyde
and are predominant in the pH ranges indicated:
pH Range Equilibrium
Below 2.6 H2C(OH)3 ^ H3C + —OH + OH~
2.6 to 4.5 HnC(OE)2 ^ HaC + —0" + OH" + H+
Above 4.5 H2C(OH)£ ^± H£C(OH)0- + H +