ASTM D1209 Test Method for Color of Clear Liquids (Platinum-Cobalt Scale)
4. Apparatus
4.1 Spectrophotometer, equipped for liquid samples and for measurements in the visible region.
NOTE 2 - The spectrophotometer used must be clean and in first-class operating condition. The instrument should be calibrated in accordance with the instructions given in the Standards for Checking the Calibration of Spectrophotometers (200 to 1000 nm).
4.2 Spectrophotometer Cells, matched having a 10-mm light path.
4.3 Color Comparison Tubes - Matched 100-mL, tall-form Nessler tubes, provided with ground-on, optically clear, glass caps. Tubes should be selected so that the height of the 100-mL graduation mark is 275 to 295 mm above the bottom of the tube.
4.4 Color Comparator - A color comparator constructed to permit visual comparison of light transmitted through tall-form, 100-mL Nessler tubes in the direction of their longitudinal axes. The comparator should be constructed so that white light is passed through or reflected off a white glass plate and directed with equal intensity through the tubes, and should be shielded so that no light enters the tubes from the side.
5. Reagents
5.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society, where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
5.2 Purity of Water - Unless otherwise indicated, references to water shall be understood to mean reagent water conforming to Type IV of Specification D1193.
5.3 Cobalt Chloride (CoCl2•6H2O).
5.4 Hydrochloric Acid (sp gr 1.19) - Concentrated hydrochloric acid (HCl).
5.5 Potassium Chloroplatinate (K2PtCl6).
6. Platinum-Cobalt Reference Standards
6.1 Platinum-Cobalt Stock Solution - Dissolve 1.245 g of potassium chloroplatinate (K2PtCl6) and 1.00 g of cobalt chloride (CoCl2•6H2O) in water. Carefully add 100 mL of hydrochloric acid (HCl, sp gr 1.19) and dilute to 1 L with water. The absorbance of the 500 platinum-cobalt stock solution in a cell having a 10-mm light path, with reagent water in a matched cell as the reference solution, must fall within the limits given in Table 1.
NOTE 3 - This stock solution is commercially available from reputable chemical suppliers.
6.2 Platinum-Cobalt Standards - From the stock solution, prepare color standards in accordance with Table 2 by diluting the required volumes to 100 mL with water in the Nessler tubes. Cap the tubes and seal the caps with shellac or a waterproof cement. When properly sealed and stored, these standards are stable for at least 1 year and do not degrade markedly for 2 years.
6.2.1 For a more precise measurement of light colors below 15 platinum-cobalt, prepare color standards from the stock solution in accordance with Table 3 by diluting the required volumes to 100 mL with water in the Nessler tubes. Use a semi-microburet for measuring the required amount of stock solution.
7. Procedure
7.1 Introduce 100 mL of specimen into a Nessler tube, passing the specimen through a filter if it has any visible turbidity. Cap the tube, place in the comparator, and compare with the standards.
