## Reagents

Prod. No. 104003 Formaldehyde solution min. 37 % for analysis

Prod. No. 109099 Iodine solution 0.05 mol I_{2}/l 0.1 N solution

Prod. No. 109147 Sodium thiosulfate solution 0.1 mol/l 0.1 N solution

Prod. No. 109137 Sodium hydroxide solution 1 mol/l 1 N solution

Prod. No. 100716 Sulfuric acid 25 % for analysis

Prod. No. 105445 Zinc iodide-starch solution for analysis

Prod. No. 116754 Water for analysis

## Preparation of a Stock Solution

In a calibrated or conformity-checked 1000-mL volumetric flask make up 2.50 mL of formaldehyde solution min.

37 % to the mark with distilled water. The stock solution prepared according to this procedure has a concentration of approximately 1000 mg/l formaldehyde.

## Precise Assay of the Standard Solution (Stock Solution) for Formaldehyde

Pipette 40.0 mL (full pipette) of the formaldehyde stock solution of approximately 1000 mg/l into a 300-mL ground-glass conical flask, and add 50.0 mL (buret) of iodine solution 0.05 mol/l and 20 mL of sodium hydroxide solution 1 mol/l. Leave to stand for 15 minutes and subsequently add 8 mL of sulfuric acid 25 %. Add 1 mL of zinc iodide-starch solution and subsequently titrate to the end point with sodium thiosulfate solution 0.1 mol/l.

## Calculation of the Exact Content of the Formaldehyde Solution

*C1 = consumption of sodium thiosulfate solution 0.1 mol/l*

C2 = quantity of iodine solution 0.05 mol/l

mg/l Formaldehyde = (C2 - C1) x 37.525

Further investigational concentrations may be prepared from the stock solution exactly determined according to the procedure described above by diluting accordingly.

## Stability

When stored in a cool place (refrigerator), the stock solution remains stable for one week. After this time, the stock solution must be determined anew and the corresponding value must be duly accounted for in the further use. The diluted investigational solutions must be used immediately.