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: