Fatty Acids and Raw Materials
Determination of Stability and Quality
1.TITLE
Colour after heating
2. SCOPE
This APAG standard describes a method for the colour determination after heating as a measure of the heat stability. It is applicable to all distilled fatty acids.
3. DEFINITION
The colour after heating is the colour expressed in Lovibond units (or in the requested unit) after heating the fatty acid sample under air, in accordance with the conditions given in Table 1 below.
Table 1: Selection of temperature and heating time according to the type of fatty acids.
|
Type of fatty acids
|
Temperature (° C)
|
Heating time (hours)
|
|
Fatty acids not containing laurics and IV > 25
|
205
|
1
|
|
Fatty acids not containing laurics and IV = 1 - 25
|
205
|
2
|
|
Fatty acids not containing laurics and IV < 1
|
205
|
6
|
|
Fatty acids containing laurics
|
150
|
2
|
|
Short and medium chain fatty acids (C6 - C14)
|
100
|
6
|
4. PRINCIPLE
A defined quantity of liquid sample is heated under standardised conditions (air, temperature and time). The colour is determined immediately after cooling based on photometric measurements [14.2]. (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 [14.3].
5. REAGENT
None
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, with an accuracy of ± 2°C, temperatures of 100, 150 or 205°C (according to the fatty acid type, as specified in Table 1).
6.2 THERMOMETER, of partial immersion type, conforming to the following specifications:
| Immersion: |
60 mm
|
| Graduation: |
1 °C
|
| Overall length: |
ca 300 mm
|
| Temperature scale: |
up to 250 °C
|
6.3 TEST TUBES, heat proof glass (e.g. boro-silicate) with the following dimensions:
| Length: |
170 mm
|
| Outside diameter: |
25 mm
|
| Inside diameter: |
21 mm
|
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 time intervals of 1, 2 or 6 hours (according to the fatty acid type, as specified in Table 1).
6.6 WATER BATH: In the case of solid samples, an additional water bath is required, which can be controlled at approximately the temperature of the measurements to be made.
6.7 EQUIPMENT FOR COLOUR MEASUREMENT, suitable to measure colour in the Lovibond 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, 11 mm for the LICO 300). When a photometer is not available, the method based on visual comparison to a set of standards should be used [14.3].
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 5555 – 1991 [14.4].
8.1.2 Prepare the sample in accordance with ISO 661 – 1989 [14.5].
If the sample to be tested is not liquid at room temperature, heat it in a water bath. Do not exceed the sample titre by more than 10 °C.
8.2 DETERMINATION
8.2.1 Place the heating bath in a fume cupboard. Set it to the requested temperature or in accordance with the table 1 above (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). Place this tube in the tube holder in the heating bath.
8.2.3 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, in accordance with Table 1.
8.2.5 Once the heating time is over, take the test tube out of the heating bath and cool it to approximately 80 °C for fatty acids.
8.2.6 Preheat a cuvette to approximately 70 °C and fill it with the treated fatty acid. Read its colour according to [14.2] (Notes 5 & 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:
If the colour measured in a given cell is darker than 2 Red or 20 Yellow units, then a smaller cell should be used.
Note 6:
When a photometer is not available, the treated fatty acid sample should be visually compared to a set of standards as described in [14.3].
9. CALCULATIONS
Not applicable
10. EXPRESSION OF THE RESULTS
Report the results using the Lovibond scale (red, yellow and blue units) or in the requested unit (e.g. Gardner or APHA), together with the cell used. Make reference to APAG – FA - 002 and give the conditions used for the test (air, temperature and heating time).
11. ACCURACY & PRECISION
The accuracy and precision are not defined in the A.O.C.S. Td 3a-64.
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-FA-002 is equivalent to the 1987 version. Only 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. REFERENCE
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 BS 684: Section 1.14: 1987 - Determination of colour.
14.4 ISO 5555 – 1991 – Animal and vegetable fats and oils – Sampling.
14.5 ISO 661 – 1989 - Animal and vegetable fats and oils – Preparation of the test sample.
15. ANNEXE
None
