Glycerine

Determination of Stability and Quality

1.TITLE

Colour after alkali treatment (Polyol stability test)

2. SCOPE

This APAG standard describes an application for the determination of the colour of glycerine after alkali treatment as a measure of the colour stability in a strongly alkaline medium. It is also known as a ‘Heat stability test with KOH treatment’ and ‘Polyol stability test’. This method is applicable to purified glycerine.

3. DEFINITION

The colour after alkali treatment is defined as the colour of glycerine after treatment with potassium hydroxide and heating for 1 hour at 125 °C under nitrogen. The colour is expressed in Hazen (APHA or Pt-Co) units.

4. PRINCIPLE

An amount of potassium hydroxide pellets is dissolved in a glycerine sample and heated in a flask under nitrogen for 1 hour at 125 °C. The strong base reacts with potential impurities, resulting in yellow coloration. The colour in Hazen (APHA) units is determined immediately after cooling, based on photometric measurements [14.1] (Note 1).

Note 1:
When a photometer is not available, the colour can also be determined by visual comparison to a set of standards according to [14.2].

5. REAGENT

All the reagents with recognised analytical quality:

5.1 NITROGEN with an oxygen content lower than 5 ppm, when nitrogen blanketing is requested (e.g. Air Product 4.8 or equivalent).

5.2 POTASSIUM HYDROXIDE, pellets [CAS-No. 1310-58-3]

6. APPARATUS

Usual laboratory apparatus, in particular:

6.1 HEATING BATH, e.g. a 5 l stainless steel beaker equipped with a mechanical stirrer and filled with a suitable medium (such as liquid paraffin or silicon oil). The heating device must be capable of maintaining a temperature of 125°C with an accuracy of ± 2°C.

6.2 THERMOMETER, conforming to the following specifications:

	Graduation:	1 °C
	Temperature scale:	up to 130 °C

6.3 FLASK, flat bottomed flask with a capacity of 250 ml and a ground neck of 24mm.

It should equipped with a 24 mm ground glass joint with a nitrogen inlet (see Annexe, Figure 1).

6.4 TIMER, capable of measuring a 1 hours time interval.

6.5 COLD water bath

6.6 EQUIPMENT FOR COLOUR MEASUREMENT, suitable to measure colour in the Hazen (APHA) units. It is recommended to use a photometer (e.g. LICO 300 series, Note 2). The pathlength of the sample cells should be selected according to both instrument type and product specifications (e.g. in general, 50 mm for the LICO 300). When not available, the method based on visual comparison to a set of standards should be used [14.2].

Note 2:
Small differences in colour measurements may result from the instrument configuration.

7. SAFETY

The use of this method may involve hazardous chemicals and/or equipment, although the safety aspects have been omitted from this procedure.

Please study and be aware of the Material Safety Data Sheet and correct laboratory performance for the appropriate health and safety precautions that may apply to any of the chemicals and equipment prior to use.

For chemicals the CAS numbers have been included in reagent paragraph.

8. PROCEDURE

8.1 SAMPLING

8.1.1 Take the test sample in accordance with ISO 2096 – 1972 [14.3].

8.2 DETERMINATION

8.2.1 Place the heating bath in a fume cupboard. Set it to 125 °C (Note 3). This temperature should be maintained within ± 2 °C.

8.2.2 Weigh, to the nearest 0.1 g, 95 g of the test sample into a 125 ml flask.

8.2.3 Add 7 ± 1 g of potassium hydroxide pellets and attach the nitrogen inlet tube.

8.2.4 Connect the tube with the nitrogen supply and adjust the flow so that the surface of the test sample is blanketed by nitrogen. This will take about 5 minutes.

8.2.5 Heat and shake carefully until the potassium hydroxide has been completely dissolved (see Note 4).

8.2.6 Place the flask in the heating bath maintained at 125 °C ± 2 °C and set the timer for 60 minutes.

8.2.7 Once the heating time is over, take the flask out of the heating bath. Place it in the cold water bath and let it cool to below 30 °C (Note 5).

8.2.6 Fill a sample cell with the alkali treated glycerine. Read its colour in Hazen (APHA) units (Note 6).

Note 3:
60 mm of the thermometer should be immersed.

Note 4:
The cleanliness of the test tubes is of utmost importance. Washing with detergent and then rinsing with reagent-grade acetone or alcohol. Dichromate solutions should not be used.

Note 5:
Do not interrupt the blanketing with nitrogen during the cooling.

Note 6:
When a photometer is not available, the treated glycerine should be visually compared to a set of standard Hazen solutions as described in [14.2].

9. CALCULATIONS

Not applicable

10. EXPRESSION OF THE RESULTS

Report the results using the Hazen (APHA) units, together with the pathlength of the cell used. Make reference to APAG – GL - 005 and give the conditions used for the test (nitrogen, temperature and heating time).

11. ACCURACY & PRECISION

There is no international standard available.

12. METHOD VALIDATION

There is no data available about both accuracy and precision (Note 7).

Note 7:
The precision, as measured by both repeatability and reproducibility, is generally evaluated by running interlaboratory testing.

13. HISTORIC

This first electronic version of APAG-GL-005 is equivalent to the 1988 version. Minor typographic changes have been made. In addition, to guarantee consistent and reliable colour data, this new version recommends to replace the method based on visual comparison [14.2] by photometric measurements [14.1].

14. REFERENCE

14.1 EN 1557 – 1997 – Colorimetric characterisation of optically clear coloured liquids as X, Y, Z tristimulus values in transmission.

14.2 ISO 2211 – 1973 – Measurement of colour in Hazen units (Pt-Co scale).

14.3 ISO 2096 – 1972 – Glycerols for industrial use – Methods of sampling.

15. ANNEXE

Figure 1: Nitrogen inlet