Colour in Solution of Sugars with more than 10 and less than 16,000 ICUMSA^® Units and pH Adjustment with MOPS Buffer– Official (Reference)

ICUMSA Method^® GS9-8 (2026)

1 Warnings

Warning and Safety Precautions
Persons using this ICUMSA^® Method should be familiar with normal laboratory practice. This standard does not address all the safety issues, if any, associated with its use. It is the responsibility of the user to establish appropriate safety and health practices and to ensure compliance with any national regulatory conditions.

Disclaimer
The mention of specific companies or of certain manufacturers’ products does not imply that they are endorsed or recommended by the International Commission for Uniform Methods of Sugar Analysis (ICUMSA^®) in preference to others of a similar nature that are not mentioned.

General advice
ICUMSA^® Methods are updated from time to time following scientific and technical development. Please check on https://www.icumsa.org, whether the Method in hand is the current version.
This method edition cancels and replaces the previous (ICUMSA^® Method GS9-8 (2011)), which has been rewritten and revised.

2 Scope and field of application

2.1 Scope
This method is used for the determination of colour in solution for plantation white, raw, white and specialty sugars [1, 2].

2.2 Field of application
The method can be applied to plantation white sugars, raw sugars, intermediate products of sugar manufacture and very low colour sugars provided that a pH-adjusted filtered test solution can be prepared by the procedure specified in the method. The method is suitable for all sugars that have colour in solution from 10 IU to 16 000 IU [3].

3 Normative references

There are no normative references in this document.

4 Terms and definitions

ICUMSA^® maintains a terminological database (under development) for use in standardisation at https://www.icumsa.org.
For the purposes of this document, the following terms and definitions drawn from the database apply:

4.1 Transmittance of a solution. If P_λ⁰ represents the radiant energy incident upon the first surface of the solution and P_λ represents the radiant energy leaving the second surface of the solution. Then:

\displaystyle T(l)=\frac{{{{P}_{\lambda }}}}{{P_{\lambda }^{0}}}     internal transmittance of the solution with pathlength l

4.2 Transmittance ratio. Let T_soln represent the transmittance of a cell containing the solution and let T_solv represent the transmittance of the same or duplicate cell containing the pure solvent. Then:

\displaystyle {{R}_{\text{T}}}=\frac{{{{T}_{{\text{soln}}}}}}{{{{T}_{{\text{solv}}}}}}     transmittance ratio of the solution

4.3 Absorbance

A(λ)_S = –lg R_T(λ)_S     absorbance of the solution

4.4 Absorption coefficient. Let  l represent the length (cm) of the absorbing path between the boundary layers of the solution and let β represent the mass concentration (g/mL) of the sugar solution. Then:

\displaystyle \varepsilon {{(\lambda )}_{\text{S}}}=\frac{{A{{{(\lambda )}}_{\text{S}}}}}{{l\cdot \beta }}    absorption coefficient of the solution

4.5 Reagent blank. A mixture of any solvent(s) and/or reagent(s) that would be presented to the detector for analysis of a sample. A reagent blank is analysed to determine if it contributes to the measurement signal and is often used with techniques such as spectrophotometry to zero the instrument before measuring test samples. For this method the reagent blank is 0.02 mol/L MOPS buffer (6.5) and after filtering (9.1), the filtered solution is used as a blank/zeroing of the instrument as part of the procedure to measure the colour in solution of a test sample (8.2).

4.6 ICUMSA^® Colour. The value of the absorption coefficient multiplied by 1000 for the absorbance at 420 nm of the solution at pH 7.0 is reported as ICUMSA^® Colour (C_IU). The resulting values are designated as ICUMSA^® Units (IU).

5 Principle

Sugar is dissolved in water and MOPS (3-(N-morpholino) propane sulphonic acid) buffer pH 7.0 (6.4) is added; the solution is then filtered through a membrane filter to remove turbidity [4, 5]. The absorbance of the filtered solution is measured at a wavelength of 420 nm and the ICUMSA^® colour in solution is calculated.
The solution concentration and cell length are chosen to give an absorbance in the preferred range of 0.1 to 0.8.

6 Reagents and materials

General advice
During the analysis, unless otherwise stated, use reagents only of analytical grade and only de-ionised or distilled water.

6.1 Sodium hydroxide – NaOH, ≥98%, anhydrous, analytical reagent grade (CAS#1310-73-2).

6.2 Sodium hydroxide solution, approx. 1 mol/L – Weigh (7.1) 40 g of sodium hydroxide pellets (6.1) into a 500 mL beaker (7.2), carefully add about 200 mL of de-ionised water and stir (7.3) to dissolve. Let the solution cool, then transfer to a 1000 mL volumetric flask (7.2) and make to the mark. It is also possible to purchase prepared 1.0 mol/L sodium hydroxide solution from laboratory chemical suppliers, either as the ready-to-use solution or as a certified concentrated solution (requiring volumetric dilution to achieve the required concentration).

6.3 3-(N-morpholino) propane sulphonic acid (MOPS) – C₇H₁₅NO₄S, ≥99%, analytical reagent grade (CAS#1132-61-2).

6.4 MOPS buffer solution, 0.2 mol/L – Weigh (7.1) (41.8 ± 0.1) g of MOPS (6.3) into a 1000 mL beaker (7.2) and dissolve with stirring (7.3) using approximately 800 mL of deionised water. Clean and dry the electrode of the pH meter (7.4) and immerse it in the MOPS solution. Adjust the solution pH to (7.00 ± 0.01) by adding approx. 1 mol/L sodium hydroxide solution (6.2). Stir the solution continuously with a magnetic stirrer (7.3) while the pH is being adjusted. (About 80 mL of sodium hydroxide solution may be required.) Remove the pH electrode. Transfer the solution to a 1000 mL volumetric flask (7.2) and make up to volume with deionised water. Store this solution in a refrigerator and discard any remaining solution after two weeks (risk of microbial growth if stored for longer).

6.5 MOPS buffer solution, 0.02 mol/L – Using a pipette (7.2), add (10 ± 0.1) mL MOPS buffer solution (6.4) to a 100 mL volumetric flask (7.2) then make up to volume with deionised water. This solution is the reagent blank (4.5).

7 Apparatus

7.1 Laboratory balance – readable to 0.01 g.

7.2 Glassware – Volumetric flasks of 1 L, 100 mL and possibly lower (see 8.3) capacity. Beakers of 1 L, 500 mL and (10 to 25) mL capacity. A 10 mL capacity bulb pipette (or equivalent). Conical flasks suitable for preparation of the sample and reference solutions and vacuum filtration (see 9.1).

7.3 Mixing equipment – magnetic stirrer and bar, orbital mixer or similar, for mechanical mixing of the sample solution. The use of a magnetic stirrer is recommended for stirring when performing pH adjustment.

7.4 pH meter – to measure to 0.01 pH.

7.5 Membrane filter – Use commercially available cellulose nitrate filter material, pore size 0.45 μm, diameter 50 mm.

7.6 Membrane filter holder – preferably fitted with stainless steel support.

7.7 Vacuum pump or vacuum supply – suitable for applying low-pressure vacuum for filtration.

7.8 Ultrasonic bath – for de-aeration of the filtered sugar solution.

7.9 Instrument – Spectrophotometer or colorimeter capable of light transmission measurements at a wavelength of 420 nm with the narrowest practical bandwidth. The instrument should be fitted with a grating, prism or interference filter monochromator.

7.10 Associated optical cells – length (1.000 ± 0.005) cm, (2.00 ± 0.01) cm, (5.00 ± 0.02) cm and (10.00 ± 0.02) cm. A cell length of at least 10 cm or more (accuracy commensurate with that stated for 10.00 cm cells) is to be preferred for white sugars.

8 Sampling, sample handling and sample preparation

8.1 Sampling procedure
For sampling of granular sugar, the guidelines in SPS-10 (2022), Sampling, Sample Handling and Sample Preparation of Free-Flowing Sugars, should be followed [6].

8.2 Preparation of the test sample
Mix the sample of sugar thoroughly.

8.3 Sample preparation
Weigh the quantity of sugar shown in Table 1 and wash with water into a 100 mL volumetric flask (7.2), dissolve/mix by swirling at room temperature, manually or mechanically (7.6). It is vital that the mass of sugar is weighed precisely, as even small changes in the mass can affect the concentration. Once the sugar has dissolved, add (10 ± 0.1) mL MOPS buffer solution (6.2) and make up to the mark with distilled water. The mass of sample used to prepare the sample solution should produce a colour in solution value in the appropriate range for the cell length in use, e.g. for very low colour in solution sugars, a 50 g mass should be used.

Table 1 Aliquots of sugar for colour measurement [7]

If the sample of sugar is not free-flowing or it is difficult to weigh to the precision shown in Table 1, it is necessary to calculate the mass concentration (β) for the actual mass weighed as follows:

β = m_SA/V

9 Procedure

9.1 Filtration of the reagent blank. Filter the reagent blank solution (6.5) through a membrane filter (7.5) mounted in a membrane filter holder (7.6) under vacuum (7.7), into a clean dry conical flask (7.2). Discard the first portion of the filtrate if cloudy.

9.2 Filtration of the sample solution. Filter the sample solution (8.3) following the same procedure as described in 9.1. Discard the first portion of the filtrate if cloudy. If de-aeration of the solution is required, place the flask in an ultrasonic bath (7.7) for a short time (not more than 3 min).

9.3 Colour measurement. Set up the spectrophotometer or colorimeter (7.9) according to the manufacturer’s instructions and adjust the wavelength to 420 nm. Rinse the optical cells (7.10) using filtered reagent blank (9.1) for one cell and filtered sample solution (9.2) for the other cell. Determine the absorbance (A(420)_SOL) of the filtered sample solution (9.2), using the cell (7.10) containing the filtered reagent blank solution (9.1) as the reference standard for zero absorbance. Record the absorbance of the sample test solution at 420 nm to 0.001 units.

10 Calculation and expression of results

10.1 Calculation. The absorption coefficient of the test solution (4.4) is calculated using the mass concentration of sample solids in solution (β) in g/mL (from Table 1) for the test solution.

\displaystyle {{\varepsilon }_{\text{S}}}=\frac{{A{{{(420)}}_{{\text{SOL}}}}}}{{l\cdot \beta }}

From the definition given in 4.6 the ICUMSA^® colour C_IU is:

\displaystyle {{C}_{{\text{IU}}}}={{\varepsilon }_{\text{s}}}\cdot 1000
\displaystyle {{C}_{{\text{IU}}}}=\frac{{A{{{(420)}}_{{\text{SOL}}}}\cdot 1000}}{{l\cdot \beta }}\cdot \text{IU}

10.2 Expression of results. For results above 250 IU, express results to the nearest 10 IU; for results below 250 IU, express results to the nearest 1 IU.

10.3 Test report. This clause specifies which information is to be included in the test report. The clause shall require information to be given on at least the following aspects of the test:

• the sample
• the ICUMSA^® Method used (including its year of publication)
• the method used (if the standard includes several)
• the result(s), including a reference to the “Calculation” clause
• any deviations from the procedure
• any unusual features observed
• the date of the test

11 Precision

11.1 Interlaboratory test. A collaborative study was performed and reported at the 34^th (2025) Session of ICUMSA^® for the current Method [3]. Collaborative studies have been performed for previous versions of the Method, where the filtration procedure was different to that used in the current version; these studies were reported at the 22^nd (1998) [8] and 24th (2004) Sessions of ICUMSA^® [9].

11.2 Repeatability and reproducibility. Repeatability and Reproducibility results for sugars of between 10 IU and 16 000 IU [8] are given in Table 2.

Table 2

12 Bibliography

1. Parkin G. (2004): Referee’s Report on General Subject 2, White Sugar, Proc. ICUMSA^® Interim 24^th Session, Atlanta, Verlag Dr. Albert Bartens KG, Berlin, 92
2. Parkin G. (2010): Summary Chapter, Proc. ICUMSA^® 27^th Session, Berlin, Verlag Dr. Albert Bartens KG, Berlin, 22
3. Keskar V. (2025): Referee’s Report on General Subject 9, Plantation White Sugar, Proc. ICUMSA^® 34^th Session, Delhi, Verlag Dr. Albert Bartens KG, Berlin, in press
4. Nguyen H.H., Rowe G.S., McCunnie K.A., Player M.R. (1996): Rapid analytical methods for colour and ash in raw sugar, Proc. 18^th Conf. Austral. Soc. Sugar Cane Technol., 341–349; reprinted in Sugar Journal (May 1997), 25–28
5. Nguyen H.H., Rowe G.S., McCunnie K.A., Player M.R. (1997): Rapid analytical methods for colour and ash in raw sugar, Proc. 19^th Conf. Austral. Soc. Sugar Cane Technol., 387–395
6. ICUMSA^® Methods Book (2022): Specification and Standard SPS-10, Verlag Dr. Albert Bartens KG, Berlin, 470–475
7. Keskar V. (2018): Referee’s Report on General Subject 9, Plantation White Sugar, Proc ICUMSA^® 31^st Session, Skukuza, Verlag Dr. Albert Bartens KG, Berlin, 89
8. Urquhart R.M. (1998): Referee’s Report on General Subject 1, Raw Sugar, Proc. ICUMSA^® 22^nd Session, Berlin, Verlag Dr. Albert Bartens KG, Berlin, 100
9. Parkin G. (2004): Referee’s Report on General Subject 2, White Sugar, Proc. ICUMSA^® Interim 24^th Session, Atlanta, Verlag Dr. Albert Bartens KG, Berlin, 90–92

13 Historical

Method GS1-8 (first published in the 2000 Methods Book Supplement) was renumbered as GS9_/1/2-8 in 2004 when white sugar and plantation white sugar were added to the scope. Further work on the Method was undertaken, including revision of precision data and the addition of speciality sugars to the scope, with the revised Method being published as GS9_/1/2/3-8 Official (Reference) in 2011. Further revisions of the Method were suggested in the 2016 and 2018 GS9 Reports, focussing on the preparation of solutions for different ranges of colour in solution and cell path lengths. The method was re-written in 2023 as part of the work to harmonise colour in solution methods and update the method to the ICUMSA^® method format, but further validation work was required to assess the effects of change in filtration procedure. A collaborative study of the GS9-8 (2011) and the proposed changes to the Method was performed and reported at the 34th Session of ICUMSA^® in 2025.

Annex 1 – Collaborative Study data

Table A1 data was reported in the ICUMSA^® Proceedings 34^th Session [8] but has been reproduced here, as it is relevant to the performance of the method. The original values as reported have been rounded to the nearest whole number (Colour in solution values (C_IU) are expressed as whole numbers only).

Table A1: Statistical data for collaborative test on colour in solution colour by Method GS9-8 (2026) (outliers removed)

Table A1: continued

Annex 2 – Collaborative Study data

Table A2 data was reported in the ICUMSA^® Proceedings 22^nd Session [7] but has been reproduced here, as it is relevant to the performance of the method. The original values as reported have been rounded to the nearest whole number (Colour in solution values (C_IU) are expressed as whole numbers only).

Table A2: Statistical data for collaborative test on sugar solution colour by the MOPS buffer method (1998) (outliers removed)

Annex 3 – Collaborative Study data

Table A3 data was reported in the ICUMSA^® Proceedings 24^th Session [8] but has been reproduced here, as it is relevant to the performance of the method. The original values as reported have been rounded to the nearest whole number (Colour in solution values (CIU) are expressed as whole numbers only).

Table A3: Statistical data for collaborative test on sugar solution colour by ICUMSA^® method GS1-8 (2004) (outliers removed)

Table A3: continued