Hach-Lange DR 5000 brewerie analysis, supplementary software LZV570 - Manual (Page 45)

45
Working procedures
2.12 Iron
Iron in beer may originate from raw materials, filter aids, apparatus,
pipes or cans, or beer foam stabilising agents. Iron has a
disadvantageous effect on colloidal stability, taste and the gushing
tendency of the beer.
Principle
Divalent iron reacts with the disodium salt of
5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine-4,4-disulphonic acids
(Ferrozin) to form a violet-coloured complex with a very high molar
absorbance coefficient. Trivalent iron must be reduced to the
divalent form before the determination is carried out. The colour
intensity is measured with a spectrophotometer.
Measuring range
0–1 mg/l
Accessories
• Analytical balance readable to 0.1 mg
• Pipettes (0.1 ml, 2 ml, 5 ml)
• Spectrophotometer (560 nm)
• 40 mm rectangular cuvette (OS grade)
Reagents
Prepare all solutions with iron-free H
2
O.
• Buffer solution (pH 4.3):
Dissolve 75 g ammonium acetate and 150 g conc. acetic acid
in about 800 ml H
2
O, check the pH and make up to 1 l.
• Ferrozin reagent:
Dissolve 0.257 g Ferrozin or Ferrospectral in 50 ml buffer (the
solution remains stable for 2 weeks).
• Ascorbic acid (2.5%) Prepare freshly each day.
• Hydrochloric acid, conc.
• Iron(III) standard solution for obtaining the calibration curves:
Dissolve 863.4 mg ammonium iron(III) sulphate
[NH
4
Fe(SO
4
)
2
x 12 H
2
O] in H
2
O in a 1 l measuring flask. Add
0.1 ml conc. hydrochloric acid and make up to the mark with
H
2
O. Dilute 50 ml of this solution with H
2
O to 1 l to obtain a
standard solution containing 5 mg/ml Fe 3+ .
Sample preparation
1. Expel carbon dioxide from beer and allow the foam to
completely collapse.
2. Pipette 40 ml beer, 2 ml Ferrozin reagent and 1 ml ascorbic
acid solution into a 50 ml measuring flask.
3. Make up to the mark with H
2
O.