Abstract：The determination of chromium oxides and metallic chromium in coating surface of chrome plate is very important. However, the total content of chromium oxides (in chromium, similarly hereinafter) and metallic chromium was usually determined in previous studies. On the basis of determination method of chromium oxides in chrome plate surface in appendix of A657/A657M-03 standard, the surface of chrome plate before and after immersion in sodium hydroxide solution was analyzed and compared by glow discharge spectrometry. It proved that the chromium oxides in coating surface of chrome plate could be dissolved by NaOH solution, while the metallic chromium could not be dissolved. The dissolution conditions of chromium oxides and the determination conditions by spectrophotometry were optimized. The results indicated that the chromium oxides in coating surface of chrome plate could be dissolved completely in 300 g/L NaOH solution at 90 ℃ for 20 min. Then, H2SO4(1+1) was added into sample solution for neutralization at pH 7. After addition of 5.0 mL of mixed sulfuric acid-phosphoric acid solution (3+3+4), 2.0 mL of AgNO3 solution and 5.0 mL of (NH4)2S2O8 solution, the solution was heated to boiling and kept for 10 min to decompose the oxides. After cooling to room temperature, 3.0 mL of diphenylcarbazide solution was added. Then, the sample solution was diluted to 100 mL. The content of chromium was determined at 542 nm with 2 cm cuvette. It was found that the content of chromium showed good linearity in range of 0.24-100 μg. The linear equation was A=0.002 1+0.013 mCr(μg) with correlation coefficient of 0.999 7. The detection limit of method was 0.24 μg, and the low limit of determination was 1.2 μg. The measured amount of chromium oxides in coating surface of chrome plate was divided by sample area to finally obtained the content of chromium oxides in unit area of coating surface of chrome plate. The proposed method was applied to the determination of chromium oxides in coating surface of two types of chrome plate. The results were 12.8 mg/m2 and 11.1 mg/m2, respectively. The relative standard deviation (RSD) were 3.1%(n=5) and 3.6%(n=5), respectively. The recovery test was also conducted, and the recoveries of chromium oxides were between 92% and 101%.
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