Determination of
formaldehyde concentration by Iodine titration
This protocol is based on the following reactions:
CH2O + I2
→ CH2O2
+ 2 I- + I2 (excess)
I2(excess) + 2 S2O32-
→ 2 I- + S4O62-
By titrating excess I2 with a known quantity of S, the original number of moles of formaldehyde (CH2O) can be back calculated. The only solutions that need to be measured very precisely are the diluted formaldehyde solution and the 0.1 M sodium thiosulfate solution.
Solutions:
Prepare the following reagents in advance:
Iodine solution (0.05 M or equivalently 0.1 N): weigh 0.6305 g of I2 and 1.5 g of KI (KI is necessary to dissolve the I2. If the I2 does not dissolve completely, add more KI). Dilute to 50 mL with H2O.
Starch indicator solution: Suspend 1 g of starch into 200 mL H20 (supermarket corn starch will work just fine). Bring to boil for 5 minutes (not all starch will dissolve). Add 6 mL concentrated HCl and dilute to 500 mL with H2O. Let sediment, pour off supernatant.
Sodium thiosulfate (0.1 M; use a volumetric flask fo accuracy): weigh 2.4 g Na2S2O3·5 H2O and dissolve to 100 mL with H2O.
Sodium hydroxide (1 M): weigh 2 g of NaOH pellets, dilute to 50 mL with H2O.
Sulfuric acid (25%): add 12.5 ml H2SO4 to ~ 30 mL H2O, dilute to 50 mL with H2O.
Diluted formaldehyde solution: 0.625 mL of original commercial stock (~37% w/v) diluted to 250 mL with H2O (use a volumetric flask).
Now
you are ready to do the experiment
To two 20 mL beakers with a stir bar add in the following order:
4 mL of diluted formaldehyde solution in one beaker, 4 mL of H2O in the other beaker as the
control.
5 mL of Iodine solution into both.
2 mL 1 M NaOH into both. (Solution will turn light yellow from a dark brown; WAIT
15 MINUTES).
Add sulfuric acid into both
beakers until solution is dark brown again (~2-3 mL).
Add starch indicator solution to both beakers, 1 mL.
With a burete, pipette or a glass syringe titrate with the thiosulfate solution until solution is perfectly clear and free of any particulates (~2-5 mL).
Results
C1 = Volume of thiosulfate added (in mL) to control sample.
C2 = Volume of thiosulfate added (in mL) to Formaldehyde sample.
Original Manufacturer's formaldehyde stock solution:
(% w/v)= 15.015*(C1-C2).
Method contributed by Rick B. Cooley. Graduate Student at the Arp laboratory.
