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Method for determining iron content in white carbon black: o-phenanthroline spectrophotometry. 3.2.1 Principle: Ascorbic acid is used to reduce trivalent iron ions in the sample solution to divalent iron ions. At a pH of 2–9, divalent iron ions can form an orange-red complex with o-phenanthroline; the absorbance of this complex is then measured using a spectrophotometer at its maximum absorption wavelength of 510 nm. 3.2.2 Reagents and solutions 3.2.2.1 Hydrochloric acid (GB 622-77). 3.2.2.2 Hydrochloric acid (GB 622-77): 6N solution. Dilute concentrated hydrochloric acid by half. 3.2.2.3 Ammonia water (GB 631-77): 1+1 solution. Dilute concentrated ammonia solution by half. 3.2.2.4 Sulfuric acid (GB 625-77). 3.2.2.5 Acetic acid (GB 676-78) – sodium acetate monohydrate (GB 693-77) buffer solution: pH≈4.5. 3.2.2.6 Ascorbic acid: 2% solution, valid for 10 days. Weigh 2 g of ascorbic acid and dissolve it in 98 g of water. 3.2.2.7 Neophyrin (GB 1293-77): 0.2% solution. Weigh 0.2 g of o-phenanthroline and add it to 99.8 g of water; heat if necessary to dissolve it. 3.2.2.8 Ammonium ferric sulfate [NH4Fe(SO4)2·12H2O] (GB 1279-77). 3.2.2.9 Iron standard solution: 1 ml contains 0.100 mg of iron. Weighed 0.863 g of ammonium ferric sulfate [NH4Fe(SO4)2·12H2O] into a beaker, added 100 ml of water to dissolve it, then added 10 ml of concentrated sulfuric acid. The mixture was transferred to a 1000 ml volumetric flask and diluted to the mark. 3.2.2.10 Iron standard solution: 1 ml contains 0.010 mg of iron. Dilute 10-fold with an iron standard solution (3.2.2.9). Valid only for the day of use. 3.2.3 Instruments and Equipment: General laboratory instruments and the following instruments. 3.2.3.1 Spectrophotometer: equipped with cuvettes with a thickness of 3 cm. 3.2.4 Preparation of the standard curve 3.2.4.1 Preparation of the standard reference solution: In 7 100 ml volumetric flasks, add 0.00 (reagent blank test), 1.00, 2.00, 4.00, 6.00, 8.00, and 10.00 ml of the iron standard solution (3.2.2.10) respectively (equivalent to 0.00, 0.010, 0.020, 0.040, 0.060, 0.080, and 0.100 mg of iron). 3.2.4.2 Color development: Each volumetric flask is treated in the same manner as specified below: add water to about 40 ml, and adjust the pH to near 2 using hydrochloric acid (3.2.2.2) (check with a pH test strip). Add 2.5 ml of ascorbic acid (3.2.2.6), 10 ml of buffer solution (3.2.2.5), and 5 ml of o-phenanthroline solution (3.2.2.7); dilute to the mark with water and mix well. 3.2.4.3 Determination of absorbance: The absorbance is determined using a spectrophotometer (3.2.3.1) at a wavelength of 510 nm, with water as the control, and in cuvettes with a thickness of 3 cm.