Hach-Lange LZV936 - DR 6000 Brewery Analysis software - Handbuch (Seite 30)

English 30
Accessories
• Spectrophotometer, 520 nm
• 10 mm rectangular cuvette OS
• Stop watch
• Water jet pump
Reagents
• 2,6-dichlorophenol-indophenol, 0.005 M (DPI solution, molecular
weight of the sodium salt 290.08):
a. Weigh into a beaker approximately 100 mg DPI, add
approximately 25 mL dist. H
2
O and dissolve by heating (to
approximately 60 °C (140 °F)).
b. Rinse the contents after cooling into a 50 mL volumetric flask
and fill to 50 mL. Filter through a fiberglass filter.
c. Add 10 mL filtrate, 1 g KJ and 2 mL H
2
SO
4
(1+6) to a 150 mL
Erlenmeyer flask and titrate with 0.01 N sodium thiosulfate
until the color changes toward starch paste.
d. Calculate the indicator content:
used mL × 14.5 = mg indicator in 100 mL
e. Dilute the remaining filtrate so that 100 mL solution contains
145 mg indicator.
f. The solution can be stored in brim-full brown bottles at +4 °C
(39 °F) for approximately 1 week.
• Phosphate-citrate buffer, pH =4.35:
• Dissolve 31.60 g di-sodium hydrogen phosphate (Na
2
HPO
4
x 12
H
2
O) and 11.75 g citric acid (C
6
H
8
O
7
x H
2
O) in dist. H
2
O and dilute
to 1 L.
Sample preparation
1. Maintain the beer at 20 °C (68 °F) and remove the carbon dioxide
under vacuum (water jet pump)
2. Pipette the decarbonated beer into a 10 mL reagent flask with
glass stopper. Pour the 0.25 mL 0.005 M DPI solution slowly into
the flask.
3. Seal the reagent flask immediately and invert the flask two times.
Start the stop watch after the first inversion. After the first tip start
the stop watch.
4. Fill a 10 mm rectangular cuvette with the mixture immediately.
Measure the extinction against the blank value (decarbonated beer
without the added reagent) at 520 nm 60 seconds after the addition
of the reagent.
Results specifications
Proportion of the supplied DPI quantity in %, which is reduced by 10 mL
beer in 60 sec.
Accuracy
V
kr
= ± 1 %
Standard values
> 60 very good
50–60 good
45–50 satisfactory
< 45
poor
Literature
MEBAK brew-technical analysis methods
4th edition 2002, Volume II, P. 104 et seq