10.0 mL of freshly prepared formaldehyde stock solution
was transferred into a conical flask, added 25.0 mL of a
standard iodine solution and 10.0 mL of sodium hydroxide
solution. The solution was allowed to stand for 5 minutes.
Then the solution was acidified with 11.0 mL of hydrochloric
acid and titrated for excess iodine by standard sodium thiosulfate
solution. 0.1 mL of starch solution was added when
color of the solution became pale straw. After addition
of starch solution, immediately the color was changed to
deep blue-black. The titration was continued until the color
changes from deep blue-black to colorless. Similarly, the
blank titration was performed. The difference between titration
values of blank and sample was used for calculation of
formaldehyde contents in stock solution