Polarimetric Sucrose Content of Molasses with Correction for Invert Sugar Content by Polarimetry and Clarification using Basic Lead Acetate – Accepted

ICUMSA^® Method GS4-1 (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 GS4_/7-1 (2013)), which has been rewritten.

2 Scope and field of Application

2.1 Scope
This method is used to determine sucrose content in beet or cane molasses by polarimetry, taking into account the invert sugar content [1–5]. A diluted molasses sample is clarified using basic lead acetate solution before measuring the polarimetric sucrose content. The polarimetric measurement is repeated after acid hydrolysis of the clarified solution to obtain the invert sugar correction. Historically, this Method is known under the name “Herzfeld-Clerget double polarisation Method”. This method is not suitable for samples that do not produce a valid polarimeter reading for the procedure described.

2.2 Field of Application
This method measures the sucrose content in cane or beet molasses assuming the absence of raffinose, kestoses and other optically active substances [1–5]. ICUMSA^® studies have confirmed such interferences as being usual, so that the result obtained was historically referred to as ‘apparent sucrose’ or ‘apparent sugar’.
This method is intended as a method for assessing the value of molasses, in terms of its polarimetric sucrose content after correction for the presence of invert sugar to satisfy a commercial requirement, such as the basis of a molasses purchasing contract.

3 Normative references

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

• ICUMSA^® Specification and Standard SPS-1 (2026) Polarimetry and the International Sugar Scale – Official [6]
• ICUMSA^® Method GS1-1 (2022) Polarimetric Sucrose Content of Raw Sugar by VIS Polarimetry – Official (Reference)
• ICUMSA^® Specification and Standard SPS-11 (2022) Sampling, Sample Handling and Sample Preparation of Liquid Sugar Products, Syrups and Molasses

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 Cane molasses – is a by-product of either cane raw sugar manufacture or cane sugar refining. It is a dark-coloured, highly viscous liquid from which no further sugar can be crystallised by conventional methods.

4.2 Beet molasses – is a by-product of either beet sugar manufacture or refining. It is a dark-coloured, highly viscous liquid from which no further sugar can be crystallised by conventional methods. Cane and beet molasses differ in both their sucrose content and non-sucrose composition.

4.3 Invert sugar – the equimolar mixture of glucose and fructose. The use of this term is based on acid hydrolysis of sucrose producing equimolar amounts of glucose and fructose. Different quantities of glucose and fructose introduce an error in the original sample, which cannot be corrected by this method.

4.4 Normal sucrose solution – the “normal sucrose solution” [6] is defined as 26.0160 g of pure sucrose weighed in vacuo and dissolved in water at 20.00 °C to a final volume of 100.000 mL. This corresponds to 26.000 g weighed in air under normal conditions (1013 mbar, 20 °C, 50-% relative humidity) and dissolved in water to a final volume of 100.000 mL. The “normal sucrose solution” is expressed as a mass fraction (w_S,pol,ref) and is the reference standard (100 °Z) for polarimetric sucrose content measurement. This reference value is dependent upon the use of a polarimeter tube length of 200 mm for the polarimetric sucrose content determination for the “normal sucrose solution”. For further details of polarimetric sucrose content and sugar solutions, refer to ICUMSA^® SPS-1 (2026).

4.5 Adjusted polarimeter reading of clarified molasses test sample, Z_Mo,dil – is obtained using the prepared test sample (see section 9.2) and a polarimeter that is calibrated in °Z. The polarimeter reading r_Mo,dil (°Z) must be corrected using a correction factor (k_l) for the length (l, in mm) of the polarimeter tube to obtain the adjusted polarimeter reading Z_Mo,dil in °Z.

Z_Mo,dil = r_Mo,dil · k_l

For l = 100, k_l = 8
For l = 200, k_l = 4

4.6 Adjusted polarimeter reading of a clarified molasses test sample after acid hydrolysis, Z_Mo,dil,hyd – is obtained using the prepared test sample (see section 9.3) and a polarimeter that is calibrated in °Z. The polarimeter reading r_Mo,dil,hyd (°Z ) must be corrected using a correction factor (k_l) for the length (l, in mm) of the polarimeter tube to obtain the adjusted polarimeter reading Z_Mo,dil,hyd in °Z.

Z_Mo,dil,hyd = r_Mo,dil,hyd · k_l

4.7 Polarimetric sucrose content of a molasses sample after correction for invert sugar, w_S,pol,Mo, – from the adjusted polarimeter readings in °Z of both the diluted molasses sample before hydrolysis (4.5) and the same sample after acid hydrolysis (4.6) the polarimetric sucrose content after correction for invert sugar can be calculated:

\displaystyle {{w}_{{\text{S}\text{,pol}\text{,Mo}}}}\text{=}\frac{{100\left( {{{Z}_{{\text{Mo}\text{,dil}}}}-{{Z}_{{\text{Mo}\text{,dil}\text{,hyd}}}}} \right)}}{{132.66-0.5\left( {t-20} \right)}}

where w_S,pol,Mo, is reported in g/100 g and t is the temperature in °C at which the measurements were carried out. The use of g/100 g is for convenience in reporting to satisfy a commercial requirement for assessment of polarimetric sucrose content of molasses and assumes that the °Z values determined by this Method are equivalent to g/100 g.

5 Principle

A diluted solution of beet (4.1) or cane molasses (4.2) is mixed with a clarification agent, filtered and analysed by polarimetry. An aliquot of the filtered solution is then hydrolysed using acid. The original sample contains glucose and fructose, the presence of which modify the polarimetric sucrose content calculated from the optical rotation. After hydrolysing sucrose with acid into glucose and fructose (invert sugar), the optical rotation of the hydrolysed sample is determined. From the two polarimeter readings, the polarimetric sucrose content of the original molasses sample, corrected for deviations due to glucose and fructose, can be calculated.
Other optically active non-sucrose compounds, present in the sample, are not considered. Their presence may influence the result but the influence is not considered to be significant [1].

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.

Warning and Safety Precautions
Users of this method are advised to consult their national health and safety legislation before handling basic lead acetate.

6.1 Basic lead acetate – Pb₃(OH)₄(CH₃CO₂)₂, analytical reagent grade (CAS#1335-32-6)

6.2 Basic lead acetate solution – into a suitably-sized vessel (7.1), weigh (7.4) 560 g of basic lead acetate (6.1) and add approximately 1000 mL of distilled water. Using a hotplate or suitable head source (7.3), boil the solution for about 30 minutes and allow to settle. Decant the supernatant liquid and dilute to 1.24 specific gravity or 24.4 g PbO/100 mL total lead content, using recently boiled distilled water. Check the total lead content either by measuring the specific gravity accurately or by determining total lead titrimetrically as described in Method GS1-1 (2022). The total lead specification calls for a specific gravity of (1.24 ± 0.01) or a total lead content of (24.4 ± 1.0) g PbO/100 mL. The basic lead content must be between 9.5 and 10.5 g PbO/100 mL [2]. If the quantity of basic lead lies above this range, adjust the basic lead content by adding glacial acetic acid. After adjustment, redetermine both the total lead and basic lead contents.
Keep the prepared basic lead solution in a vessel fitted with an airlock mechanism to avoid any contact with carbon dioxide in the air. Flush the vessel with nitrogen gas before closing.

6.3 Hydrochloric acid, concentrated – HCl, analytical reagent, ρ₂₀ ≈ 1.18 g/mL (CAS#7647-01-0).

6.4 Hydrochloric acid solution, approx. 2.5 mol/L – Prepare by adding 215 mL of concentrated hydrochloric acid (6.3) to a 1000 mL volumetric flask (7.1) containing approximately 200 mL de-ionised water. Mix the solution gently, then make up to a volume.

6.5 Potassium oxalate monohydrate – (K₂C₂O₄) ∙ H₂O, reagent grade, (CAS#6487-48-5).

6.6 Di-sodium oxalate – Na₂C₂O₄, reagent grade, (CAS#62-76-0).

6.7 Filter aid – inert, silica-based/diatomaceous earth powder with high porosity.

6.8 Zinc powder – reagent grade, coarse (CAS#7440-66-6).

7 Apparatus

7.1 Glassware – suitable for the preparation of samples (section 9) and reagents. A selection of volumetric flasks and stoppers (100, 250, 500, 1000 mL), measuring cylinder (50 mL recommended), beakers/dishes (suitable for use in weighing of samples and preparation of reagents), pipettes (50 mL recommended).

7.2 Precision balance – readable to 1 mg.

7.3 Hot plate or heat source – suitable for use in preparation of basic lead acetate solution (6.2). The heat provide must be sufficient to boil the reagent or water.

7.4 Orbital mixer/shaker – suitable device for use in mixing of samples (section 9).

7.5 Filter paper – qualitative, hardened, ashless, with high wet-strength, high chemical resistance and a large nominal particle retention for fast analytical filtration. The size must be suitable for funnels (7.6) used in filtration (section 9) (18 cm diameter is recommended).

7.6 Funnel and filtrate receiver – suitable for the preparation of samples (section 9).

7.7 Thermometer – suitable for temperature measurement of sample solutions and water bath, in the range of (0 to 100) °C with an accuracy of ±0.5 °C.

7.8 Polarimeter or saccharimeter – measurement wavelength: e. g. 589 nm (Na-D-line; VIS, preferably equipped with temperature compensation, calibrated in °Z at 20.0 °C (see ICUMSA^® SPS-1 (2026)).

7.9 Polarimeter tubes – Flow-through or side-filling polarimeter tubes of length specific to the polarimeter used (200 mm or 100 mm in length) (see ICUMSA^® SPS-1 (2026)).

7.10 Heated water bath – controlled at (69.0 ± 0.5) °C.

7.11 Timer – suitable for monitoring the hydrolysis procedure (9.3).

7.12 Cooled water bath – controlled at (20.0 ± 0.5) °C or cool running water to achieve the required temperature (9.3).

8 Sampling, sample handling and sample preparation

For sampling of molasses (4.1 and 4.2), the guidelines in ICUMSA^® SPS-11 (2022), Sampling, Sample Handling and Sample Preparation of Liquid Sugar Products, Syrups and Molasses should be followed. Ensure each sample is thoroughly mixed before sub-sampling for analysis.

9 Procedure

9.1 Preparation of samples. Into a beaker or dish (7.1), weigh (7.2) 65.000 ± 0.002 g sample and transfer without loss to a 500 mL volumetric flask (7.1) with the aid of approximately 100 mL of de-ionised water, then make up the volume to 150 mL and mix the solution by swirling the flask (7.4 or manually). Add 40 mL of basic lead acetate solution (6.2) and mix by swirling. Once the solution is thoroughly mixed, make the solution up to volume with de-ionised water and mix gently by inverting the flask.
Filter the solution using filter paper (7.5) fitted to a funnel and receiver flask (7.6). Reject the first 10 mL of filtrate and collect at least 100 mL of filtrate. Add sufficient dry potassium oxalate (6.5) or sodium oxalate (6.6) to the filtrate and mix to completely precipitate the excess lead. Filter the mixture using a fresh filter paper (7.5) rejecting the first 10 mL of filtrate. If the filtrate remains cloudy it may be necessary to use a small quantity of filter aid (6.7) to aid filtration. The clear filtrate obtained is used to prepare the unhydrolyzed test sample (9.2) and the hydrolysed test sample (9.3).

9.2 Preparation and measurement of the unhydrolyzed diluted molasses test sample. Pipette (7.1) 50 mL of the filtrate (prepared in 9.1) into a 100 mL volumetric flask (7.1). Make up to volume at 20.0 ± 0.5 °C (7.7). Add the unhydrolysed test sample (prepared using the solution prepared in 9.1) to a 200 mm polarimeter tube (7.9). A 100 mm tube may be used if the polarimeter fails to produce a valid reading using a 200 mm tube. Record (7.8) the polarimeter reading (r_Mo,dil ) in °Z. Record the temperature (t) at which the reading is taken.

9.3 Preparation and measurement of the hydrolysed diluted molasses test sample. Pipette (7.1) a further 50 mL of the filtrate (prepared in 9.1.) into a 100 mL flask (7.1) and add 23 mL of hydrochloric acid solution (6.4) and 7 mL of de-ionised water. Insert a thermometer (7.7) into the flask and place the flask into the water bath at 69 °C (7.10). Swirl the flask in the water bath for 5 minutes (7.11) timed from the moment the contents reach 65.0 ± 0.5 °C.
Remove the flask from the water bath and cool rapidly (7.12) to (20.0 ± 0.5) °C. Remove the thermometer from the flask, rinsing it carefully into the flask with de-ionised water. Make the volume up to 100 mL at 20 °C.
Add the hydrolysed test sample (prepared using the solution from 9.1) to the 200 mm polarimeter tube (7.9). Record (7.8) the polarimeter reading (r_Mo,dil,hyd) of the hydrolysed test sample. Solutions which fail to produce a valid reading may be decolourised by the addition of 0.5–1.0 g of zinc powder (6.8). A 100 mm tube may be used if the polarimeter fails to produce a valid reading using a 200 mm tube. Record the temperature (t) at which the reading is taken.

10 Calculation and expression of results

10.1 Calculation. The polarimetric sucrose content of a molasses sample after correction for invert sugar content (w_S,pol,Mo) is determined using the adjusted polarimeter readings of the diluted molasses test sample before (4.5) Z_Mo,dil and after (4.6) Z_Mo,dil,hyd hydrolysis (respectively), using the calculations shown in 10.1.1, 10.1.2 and 10.1.3.

10.1.1 Calculation of the adjusted polarimeter reading of a clarified and diluted molasses test sample.

Z_Mo,dil = r_Mo,dil · k_l

10.1.2 Calculation of the adjusted polarimeter reading of a clarified and diluted molasses test sample after acid hydrolysis.

Z_Mo,dil,hyd = r_Mo,dil,hyd · k_l

10.1.3 Calculation of polarimetric sucrose content of a molasses test sample after correction for invert sugar content.

\displaystyle {{w}_{{\text{S}\text{,pol}\text{,Mo}}}}\text{=}\frac{{100\left( {{{Z}_{{\text{Mo}\text{,dil}}}}-{{Z}_{{\text{Mo}\text{,dil}\text{,hyd}}}}} \right)}}{{132.66-0.5\left( {t-20} \right)}}

10.2 Expression of results. Express the result in g/100 g to two decimal places.

10.3 Test report. The test report shall include information 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. Data from SUPS performance of Method GS4_/7-1 (now GS4-1) was presented at the 28^th Session of ICUMSA^® in 2012 [7]. An estimation of repeatability and reproducibility from the SUPS data was included in the 2013 version of the Method. A collaborative study was performed and reported at the 34^th Session of ICUMSA^® in 2025 [8].

11.2 Repeatability and Reproducibility. Estimation of repeatability results based on reported SUPS data [7] is that the difference between 2 results obtained in a single laboratory should not exceed 1.5 g/100 g at the 20 g/100 g level and 2.5 g/100 g at the 40 g/100 g level.
For molasses samples with polarimetric sucrose content between 30.1 and 49.5 g/100 g, the absolute difference between two results, obtained under repeatability conditions, (r), should not be greater than 2.8 g/100 g [8]. For molasses samples with polarimetric sucrose content between 30.1 and 49.5 g/100 g, the absolute difference between two results, obtained under reproducibility conditions, (R), should not be greater than 5.2 g/100 g [8].

12 Bibliography

1. Saillard E. (1936): Referee’s Report on Subject 6, The Testing of Molasses, Proc. ICUMSA^® 9^th Session, London, 18–22
2. Roche M. (1970): Referee’s Report on Subject 7, Sucrose in Factory and Refinery Products excluding Beet, Cane and Crystalline Sugars (Physical Methods), Proc. ICUMSA^® 15^th Session, London, 52–60
3. Stachenko S. (1974): Referee’s Report on Subjects 7 and 8, Sucrose in Factory and Refinery Products excluding Beet, Cane and Crystalline Sugars, Proc. ICUMSA^® 16^th Session, Ankara, 85–101
4. Dutton J.V. (1978): Referee’s Report on Subjects 7 and 8 Sucrose in Factory and Refinery Products excluding Beet, Cane and Crystalline Sugars, Proc. ICUMSA^®17^th Session, Montreal, 103
5. Shelton C.J. (1994): Referee’s Report on General Subject 4, Molasses, Proc. ICUMSA^® 21^st Session, Havana, 78–87
6. ICUMSA® (2026): ICUMSA^® Methods Book: Specification and Standard SPS-1, Verlag Dr. Albert Bartens KG, Berlin, 1–7
7. Eggleston G. (2012): Referee’s Report on General Subject 4, Molasses, Proc. ICUMSA^® 28^th Session, Cambridge, 118
8. van Berchum S. (2025): Referee’s Report on General Subject 4, Molasses, Proc. ICUMSA^® 34^th Session, Delhi, in press

13 Historical

The double-polarisation (Herzfeld-Clerget) method for determination of sucrose content was first published in the 1970 Methods Book. It was rewritten as GS4_/7-1 1 (now GS4-1) in 1994 and adopted with Accepted status. The Method was revised in 2013 when proficiency testing scheme precision data was added. Work on improvement and validation was continued, resulting in the collaborative testing data presented at the 34^th Session in 2025. The Method was then rewritten as GS4-1 to align it to the new ICUMSA^® Method format and include the validation and preparation data determined from collaborative study performed.

Annex 1 – Collaborative Study data

The following Table were reported in the 34^th ICUMSA^® Proceedings [8] but they have been reproduced here, as it is relevant to the performance of the method. The sample material used to determine the results in the table was molasses (cane and beet).

Table A1: Performance data for the polarimetric sucrose content of molasses determined using basic lead acetate as a clarification agent and Method GS4-1 (2025) (outliers removed)