Determination of Stability and Quality
Colour after heating
This APAG standard describes a method for the colour determination of glycerol after heating as a measure of the heat stability. It is applicable to refined glycerine.
The colour after heating is the colour expressed in Hazen (APHA or Pt-Co) units after heating the glycerine sample under nitrogen for 2 hours at 220°C.
A defined quantity of glycerine sample is heated under standardised conditions (nitrogen, temperature and time). The colour in Hazen (APHA) units is determined immediately after cooling, based on photometric measurements [14.2] (Note 1).
When a photometer is not available, the colour can also be determined by visual comparison to a set of standards according to [14.3].
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).
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 220°C with an accuracy of ± 2°C.
6.2 THERMOMETER, of partial immersion type, conforming to the following specifications:
||ca 300 mm
||up to 250 °C
6.3 TEST TUBES, heat proof glass (e.g. boro-silicate) with the following dimensions:
These tubes should have a 24 mm ground glass joint equipped with a nitrogen inlet (see Annexe, Figure 1). They should be marked with a line at 125 mm from the bottom, to indicate the sample volume to be used.
6.4 TEST TUBE HOLDER, suitable for 6 tubes and allowing the heating medium to circulate properly.
6.5 TIMER, capable of measuring a 2 hours time interval.
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 [14.3].
Small differences in colour measurements may result from the instrument configuration.
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.
8.1.1 Take the test sample in accordance with ISO 2096 1972 [14.4].
8.2.1 Place the heating bath in a fume cupboard. Set it to 220 °C (Note 3). This temperature should be maintained within ± 2 °C.
8.2.2 Fill a clean and dry test tube with the liquid sample to the level mark (Note 4).
8.2.3 Attach the nitrogen inlet, connect the tube with the nitrogen supply and adjust the flow of the nitrogen so that the surface of the test sample is blanketed by nitrogen. It takes about 5 minutes.
8.2.4 Place this tube in the tube holder in the heating bath.
8.2.5 Adjust the tube holder so that the medium in the bath is on level with the content of the tube. The bath temperature should not drop more than 5 °C below the minimum bath temperature, and the recovery time to reach the bath temperature should not exceed 5 minutes.
8.2.4 Set the timer for 120 minutes.
8.2.5 Once the heating time is over, take the test tube out of the heating bath. Place it in a beaker filled with cold water and let it cool to below 30 °C (Note 5).
8.2.6 Fill a sample cell with the treated glycerine. Read its colour in Hazen (APHA) units (Note 6).
60 mm of the thermometer should be immersed.
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: D
o not interrupt the blanketing with nitrogen during the cooling.
When a photometer is not available, the treated glycerine should be visually compared to a set of standard Hazen solutions as described in [14.3].
10. EXPRESSION OF THE RESULTS
Report the results using the Hazen (APHA) units, together with the cell used. Make reference to APAG GL - 003 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).
The precision, as measured by both repeatability and reproducibility, is generally evaluated by running interlaboratory testing.
This first electronic version of APAG-GL-003 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 [14.3] by photometric measurements [14.2].
14.1 A.O.C.S. Official method Td 1a-64 (Re-approved in 1997), replaces L 15a-58 - Colour after heating.
14.2 EN 1557 1997 Colorimetric characterisation of optically clear coloured liquids as X, Y, Z tristimulus values in transmission.
14.3 ISO 2211 1973 Measurement of colour in Hazen units (Pt-Co scale).
14.4 ISO 2096 1972 Glycerols for industrial use Methods of sampling.
Figure 1: Nitrogen inlet