Method 8.4 - Refined Sugar: ICUMSA 420 Colour: 1. Rationale
Method 8.4 - Refined Sugar: ICUMSA 420 Colour: 1. Rationale
Method 8.4 - Refined Sugar: ICUMSA 420 Colour: 1. Rationale
1. Rationale
This method is applicable to all refined and white sugars and is used to determine the
ICUMSA colour of refined sugar in solution.
2. Principle
The sugar is dissolved in distilled water and the solution filtered through a membrane
filter (the type of which is specified by ICUMSA) to remove turbidity. The pH of the
solution is only adjusted to 7.00 ± 0.02 using basic or acidic solutions if the colour as
determined at a pH of 7.00 is more than 50 IU. The Brix and absorbance of the filtered
solution is measured at a wavelength of 420 nm and the ICUMSA colour of the solution is
calculated.
3. Definitions
If I1 represents the radiant energy incident upon the first surface of the solution, and I 2
represents the radiant energy leaving the second surface of the solution. Then:
I2
T = = transmittance of the solution
I1
and 100 × T = percentage transmittance
3.2 Transmittancy
L e t Tsoln represent the transmittance of a cell containing the solution and let Tsolv
represent the transmittance of the same cell containing the pure solvent. Then:
Tsoln
Ts = = transmittancy of the solution
Tsolv
3.3 Absorbancy (extinction) m easured in absorbance units (AU)
Let b represent the length (mm) of the absorbing path between the boundary layers of the
solution and let c represent the concentration (g/cm 3 ) of the sugar solution. Then:
As
Ai = = absorbancy index of the solution.
bc
The value of the absorbancy index multiplied by 10 000 is reported as the ICUMSA
Colour of the solution and the resulting value is expressed in ICUMSA Units (IU). Since
the wavelength at which the determination of colour in solution is used is set at 420 nm
the value is designated as being the ICUMSA 420 Colour.
4. Apparatus
4.1 Spectrophotometer capable of light transmission measurements at a wavelength
of 420 nm with the narrowest practical bandwidth, e.g. ± 10 nm
4.13 Pipettes: 2 × 10 cm 3
5. Reagents
Hydrochloric acid (HCl, 32%) is a corrosive acid and contact with the skin, eyes and
through inhalation must be avoided. Work in a fume cupboard while wearing gloves and
safety glasses.
Pipette 10 cm 3 of the 1 M hydrochloric acid solution into a 200 cm 3 volumetric flask and
make to the mark with distilled water.
Sodium hydroxide (NaOH) is a corrosive base and contact with the skin and eyes should be
avoided. Wear gloves and safety glasses during use.
Weigh 40.0 g sodium hydroxide pellets and dissolve in some distilled water. This
dissolution is exothermic and the solution will therefore heat. Allow the solution to cool
and dilute to 1 000 cm 3 in a volumetric flask.
Pipette 10 cm 3 of the 1 M sodium hydroxide solution into a 200 cm 3 volumetric flask and
make to the mark with distilled water.
6. Procedure
Following the manufacturer’s directions, calibrate the pH meter using the 4.00 and 7.00
pH buffer solutions (compensated for a temperature different from 20°C) while stirring at
a constant rate. Calibrations should be done at the beginning of each day or shift using
fresh buffer solutions only. The buffer solutions should be at room temperature.
Mix the sample of sugar thoroughly. Weigh 30.00 ± 0.02 g of the sugar into a 250 cm 3
beaker. Add 70 cm 3 of distilled water and dissolve using a magnetic stirrer and follower.
Filter the solution through a 0.45 µm cellulose nitrate membrane under vacuum into a
clean, dry Buchner flask.
If the pH of the solution is to be adjusted to 7.00 ± 0.02 transfer the solution to a 100
cm 3 beaker. Stir the solution on the magnetic stirrer and adjust the pH of the solution to
7.00 ± 0.02 using either hydrochloric acid (0.05 M) to bring the pH down or sodium
hydroxide (0.05 M) to bring the pH up. Allow ample time for the pH reading to stabilize (1
minute).
7. Calculations
Use the Brix reading at 20.0°C to obtain the concentration of total solids in g/cm 3 using
the formula indicated below.
where a = 0.9971843
b = 3.85738×10-3
c = 1.254916×10-5
d = 8.125659×10-8
e = 5.611455×10-10
A s × 10 000
ICUMSA 420 Colour =
bc
8. Example
Obtain the concentration of total solids in g/cm 3 from the Brix reading at 20.0°C using
the equation in 7.
0.091 AU × 10 000
ICUMSA 420 colour =
50 mm × 0.352 g/cm 3
= 51.7 IU
Report as 52 IU
9. Precision
10. References
ICUMSA (2003). White sugar solution colour at pH 7.0. ICUMSA Methods Book, GS2/3-9.
ICUMSA (2003). White sugar solution colour. ICUMSA Methods Book, GS2/3-10.
Mellet P, Lionnet GRE, Kimmeling ZJ and Bennett PJ (1982). Standards for the analytical
precision of sugar and molasses analyses. Proc S Afr Sug Technol Ass, 56: 55-57.
SASTA (1985). Laboratory Manual for South African Sugar Factories. 3rd Edition: 337.
SMRI (2004). Determination of the colour and turbidity of white sugar in solution. SMRI
Test Methods, TM025.