Page:NIOSH Manual of Analytical Methods - 3500.pdf/5

 FORMALDEHYDE: METHOD 3500, Issue 2, dated 15 August 1994 - Page 5 of 5

METHOD WRITTEN BY: Eugene R. Kennedy, Ph.D., NIOSH/DPSE.

APPENDIX: PREPARATION AND STANDARDIZATION OF FORMALDEHYDE STOCK SOLUTION (ca. 6.6 mg/mL) Dilute 2.7 mL 37% formalin solution to 1 L with distilled, deionized water. This solution is stable for at least three months. Standardize by placing 5.0 mL of freshly prepared 1.13 M sodium sulfite solution in a 50-mL beaker and stir magnetically. Adjust pH to between 8.5 and 10 with base or acid. Record the pH. Add 10.0 mL formaldehyde solution. The pH should now be about 11. Titrate the solution back to its original pH with 0.02 N sulfuric acid (1 mL acid = 0.600 mg HCHO; about 17 mL acid needed). If the endpoint pH is overrun, back-titrate to the endpoint with 0.01 N sulfuric hydroxide. Calculate the concentration, C s (mg/mL), of the formaldehyde stock solution:

where: 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 NaOH (mL) for back-titration V s = volume of formaldehyde stock solution (10.0 mL).

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94