Page:NIOSH Manual of Analytical Methods - 2016.pdf/6
FO RM ALD EHY DE: M ETH OD 2016, Issue 2, dated 1 5 Ma rch 2003 - Page 6 of 7 APPE ND IX A -
PREPA RA TIO N AND STANDARDIZATION OF FORMALDEHYDE STOCK SOLUTION (ca. 1 mg/mL)
Preparation. Dilute 2.7 mL 37% aqueous form alin solution to 1 L with distilled, deionized water. This solution is stab le for at least three m onth s wh en s tored at room tem pera ture. Standardization by pH Titration. Place 5.0 m L of freshly prepared 1.13 M sodium sulfite solution in a 50-mL beake r and stir magn etically. Adjust pH to between 8.5 and 10 with base or acid. Record the pH. Add 3.0 to 1 2.0 mL formaldehyde stock solution. The pH should now be greater than 11. Titrate the solution back to its original pH with 0.02 N sulfuric acid (1 mL acid = 0.600 mg formaldehyde; about 17 mL acid needed). If the endpoint pH is overrun, bac k-titrate to the e ndp oint with 0.01 N sodium hydroxide. Calculate the co nce ntration , C s (mg/mL), of the form aldehyde stoc k solution:
W here :
30.0 = 30.0 g/equivalent of formaldehyde N a = normality of sulfuric acid (0.02 N) V a = volume of sulfuric acid (mL) used for titration N b = normality of NaOH (0.01 N) V b = volume of NaO H (mL) used for back-titration V s = volume of formaldehyde stock solution (mL)
Standardization by Colorimetric Titration. Place 50 mL of freshly prepared 0.1 M sodium sulfite and 3 drops of 0.04% thymoph thalein indicator (w/v) in 50:50 ethanol:water into a 250-m L Erlenm eyer flas k. Titrate the contents of the flask to a colorless endpoint with 0.1 N sulfuric acid (usually 1 or 2 drops is sufficient). The indicator is blue at pH values above the endpoint and is colorless at pH values below the en dp oin t. Transfer 3.0 to 12.0 mL of the formaldehyde solution to the same flask and titrate the m ixture with 0.1 N sulfuric acid to a colorless end point. Calculate the concentration, C (mg/mL), of formaldehyde in solution.
W here:
30.0 Na Va Vs
= = = =
g/equivalent of formaldehyde normality of sulfuric acid (0.1 N) volume of sulfuric acid used for titration (mL) volume of formaldehyde stock solution (mL)
NOTE: Sulfuric acid (0.1 N) is substituted for 0.1 N hydrochloric acid, which is specified in OSHA Method 52, in order to prevent possible formation of bis(chloromethyl)ether, a potent carcinogen , by reac tion of form aldehyde with hydrochloric a cid [12 ]. This colorimetric titration was adapted from OSHA Method 52 [5], which was based on the procedure of W alke r [15].
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition