Cane Sugar Refining - Purolite

ICUMSA Method for Color Measurement
39
Cane Sugar Refining with Ion Exchange Resins
Spectrophotometer. For routine measurements it is not necessary to use a
spectrophotometer; a photometer with a filter with a narrow band width (±10nm)
is suitable. The design of the instrument should be such as to eliminate as far as
possible the inclusion of forward-scattered light in the measurement of the transmitted
light. This is achieved by restricting the size of the receiving aperture so that it only
accommodates the restricted beam.
Cells. For measurements of white sugar, a cell length of 10 cm is recommended. A second
or reference cell may be used, provided that a test with distilled water has shown that
the two cells are within 0-2% of being identical (with the instrument reading 100%
transmittance on one cell, the other should give a reading between 99.8 and 100.2%).
Also, membrane filters, 50 mm diameter, pore size 0.45 µm (mercury intrusion
method), and membrane filter holders are required.
Reagent. Kieselguhr, analytical grade.
Procedure. Sample Preparation. The sugar to be tested is dissolved in unheated distilled
water. The following concentrations are used:
White sugars 50g/100g
Darker-colored sugars As high as practicable, consistent with reasonable filtration
rates and cell depths.
Liquor, syrups, and juices Diluted to 50% solids or original density, unless dilution is
required to obtain reasonable filtration rates or cell depths.
The solution is filtered under vacuum; white sugar solution and light-colored liquors
are filtered through a membrane filter, pore size 0.45µm.
Slower-filtering solutions are filtered with Kieselguhr (1% on solids) through filter
paper. The first portion of the filtrate is discarded if cloudy. The pH of darker-colored
solutions is adjusted to 7.0±0.2 with dilute hydrochloric acid is removed under vacuum
or in an ultrasonic bath, care being taken to minimize evaporation. The density of
solution is checked after de-aerating.
Distilled water filtered through a membrane filter is used as a reference standard.
Color Measurement. The measuring cell is rinsed three times with the sugar solution
and then filled. The absorbency of the solution is determined at 420nm using filtered
distilled water as the reference standard for zero color. The cell length is chosen so that
the instrument reading will be between 0.2 and 0.8 absorbancy, except for solutions
of white sugar, where the cell length should be as long as possible.
Result. The molar absorption coefficient A S of the solution is calculated as follows:
a S
-Log T S
bc
A S
bc
T S = transmittance
A S = absorbance
b = cell length (cm)
c = concentration of total solids (mols/liter) determined
refractometrically and calculated from density.
a S = molar absorption coefficient (liter/mol cm)