formaldehyde - Sciencemadness Dot Org

QUANTITATIVE ANALYSIS 259
The amount of sodium hydroxide consumed as determined by the titration
of the unreached alkali and the formaldehyde content is calculated from
this figure.
Procedure: Fifty cc of normal sodium hydroxide solution and approximately 25
cc of neutral 6-7 per cent hydrogen peroxide (prepared by adding 5 cc of pure commercial
100-volume peroxide to 20 cc water J are placed in a 500-ce Erlenmeyer flask.
To this mixture is added a carefully measured sample of the formaldehyde to be
analyzed. The flask is then covered with a porcelain crucible cover, ajritsted thoroughly,
and after one minute placed in a wattM- bath kept at apnroximaieh- SO'C
for five minutes. After standing without heating for another five minutes. :Lr mixture
is cooled to room temperature and titrated with normal sulfuric or hydrochloric
acid using 6 drops bromothymol blue solution* as indicator. The end point is taken
n-hen the color of the solution changes from blue to green. A blank titration is made
vritk SO cc of the normal alkali and 25 cc of the dilute peroxide. The formaldehyde
content of the sample is calculated by means of the following equation:
_ (Blank titer — Sample titer) X Normality of acid X 3.002
% Formaldehyde =
Weight of Sample
The above procedure has been found satisfactory for industrial use*
However, Biichi 10 , who employs a 50-cc volume of 3 per cent peroxide for
the oxidation, prefers a 15-niinute heating period at approximately 60°C7
since Mach and Herrmann 30 have found that impurities sometimes slow up
the oxidation reaction. At room temperature a minimum reaction time of
Ik hours is required.
Phenolphthalein, thymol blue, and broniothymol blue are satisfactory
indicators. Litmus and methyl red are less satisfactory because of their
slow and uncertain color transformations, diethyl red is reported to give
low results.
With reference to the effect of impurities on the alkaline peroxide method7
Biichi 10 found that neither methyl nor ethyl alcohol interferes with the
analysis. Homer 19 , who prefers 2;4-dinitrophenol as indicator, states that
commercial formaldehyde of abnormally high methanol content requires
the application of a correction factor, viz., 0.09 per cent for every 10 per cent
methanol present. Acetone, when present at concentrations of 7 per cent
or below, has a negligible effect 10 . Acetaldehyde gives high results when
present in concentrations greater than 0.5 per cent. According to Kiihl- 5 ,
hydrolysis products of proteins cause incorrect results, since hydrogen
peroxide reacts with amino acids.
The hydrogen evolved in the alkaline peroxide oxidation of formaldehyde
has been used as a basis for a gas volumetric method of formaldehyde determination
by Frankforter and West 13 .
(3) Iodimetric Method. Particularly suitable for the determination of
small quantities of pure formaldehyde is the iodimetric procedure devised
* This indicator solution is prepared hy dissolving 0.5 g dibromothymol sulfophthalein
in 150 ee methanol, diluting with water to a volume of 300 cc and neutralizing
to a distinct green.