XXII
Release 07/03
Standard solution of formaldehyde
Preparation of a stock solution:
In a calibrated or conformity-checked 1-l volu-
metric flask make up 2.50 ml of formaldehyde
solution min. 37 % GR to the mark with DI 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 % GR. 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 pre-
pared 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 further dilu-
tion solutions (investigational concentrations) must
be used immediately.
▼
Reagents required:
1.04003.1000
Formaldehyde solution min.
37 % GR
1.09099.1000
Iodine solution 0.05 mol
I2/l 0.1 N solution
1.09147.1000
Sodium thiosulfate solution
0.1 mol/l 0.1 N solution
1.09137.1000
Sodium hydroxide solution
1 mol/l 1 N solution
1.00716.1000
Sulfuric acid 25 % GR
1.05445.0500
Zinc iodide-starch solution GR
1.16754.9010
Water GR
Instructions for the Preparation of Standard Solutions
Standard solution of chlorine dioxide
acc. to DIN EN ISO 7393
Preparation of a KlO
3
stock solution:
In a 1000-ml volumetric flask dissolve 1.005 g
of KlO
3,
art.1.02404, in 250 ml of distilled water.
Subsequently make up to the mark with distilled
water.
▼
Preparation of a KlO
3
/Kl standard solution:
Transfer 10.5 ml of the KlO
3
stock solution to a
1000-ml volumetric flask, add approx. 1g of Kl,
art. 1.05043, and make up to the mark with
distilled water.
1 ml of this solution is equivalent to 0.020 mg
of chlorine dioxide.
▼
Preparation of the chlorine dioxide
standard solution:
Pipette 10 ml KlO
3
/Kl standard solution into a
100-ml volumetric flask, add 2.0 ml of H
2
SO
4
0.5 mol/l, art. 1.09072, leave to stand for 1 min,
and then add NaOH 2 mol/l, art. 1.09136,
dropwise (approx. 1 ml) until the solution just
loses its colour. Subsequently make up the
solution to the mark with distilled water.
The concentration of the solution is 2.00 mg/l
chlorine dioxide.
▼
Stability:
The KlO
3
stock solution remains stable for 4
weeks when stored in a cool place. The KlO
3
/Kl
standard solution can be used for 5 hours when
stored in a cool place. The dilute chlorine dioxide
standard solution remains stable for a maximum
of 5 minutes.
▼
Reagents required:
1.02404.0100
Potassium iodate, volum. standard
1.05043.0250
Potassium iodide GR
1.09072.1000
Sulfuric acid 0.5 mol/l
1.09136.1000
Sodium hydroxide solution 2 mol/l
