Spectroquant Pharo 300 - Page 433

Release 06/2014
Spectroquant
®
photometers NOVA 60 and Pharo photometers
Instructions for the Preparation of Standard Solutions
Standard solution of formaldehyde
Preparation of a stock solution:
In a calibrated or conformity-checked 1000-ml volu metric flask make up
2.50 ml of formaldehyde solution min. 37% GR to the mark with distilled
water.
The stock sol u tion prepared according to this procedure has a concen-
tration of approx. 1000 mg/l formal dehyde.
Precise assay of the stock solution:
Pipette 40.0 ml (full pipette) of the formaldehyde stock solution 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%. Sub sequently titrate with sodium thiosulfate solution 0.1 mol/l
until the yellow iodine color has disappear ed, add 1 ml of zinc iodide-
starch solution, and con tinue to titrate until a milky, pure white color
emerge.
Calculation and preparation of a standard
solution:
C1 = consumption of sodium thiosulfate solution 0.1 mol/l (ml)
C2 = quantity of iodine solution 0.05 mol/l (50,0 ml)
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 with distilled water.
Stability:
When stored in a cool place (refrigerator), the stock solution of approx.
1000 mg/l remains stable for one week. After this time, the stock so-
lution must be determined anew. The diluted standard solutions
(in vestigational concentrations) must be used immediately.
Reagents required:
1.04003.1000 Formaldehyde
solution min.
37% GR for
analysis
1.09099.1000 Iodine solution
0.05 mol/l
Titripur
®
1.09147.1000 Sodium thio-
sulfate solution
0.1 mol/l
Titripur
®
1.09137.1000 Sodium hy-
droxide solution
1 mol/l
Titripur
®
1.00716.1000 Sulfuric acid
25% for analysis
EMSURE
®
1.05445.0500 Zinc iodide-
starch solution
GR for analysis
1.16754.9010 Water for
analysis
EMSURE
®