二苯卡巴肼光度法测定镀铬板表面铬氧化物含量

测定镀铬板表面镀层中铬氧化物及金属铬意义重大,但以往测定的大都是镀铬板表面铬氧化物量(以铬计,下同)和金属铬量的总和。实验在A657/A657M-03标准附录中测定镀铬板表面铬氧化物方法的基础上,通过采用辉光光谱法对氢氧化钠溶液浸泡镀铬板前后的表面进行对比分析,证实了氢氧化钠溶液仅对镀铬板表面的铬氧化物有溶解作用,而不能溶解金属铬,同时优化了铬氧化物的溶解条件和光度法测定条件。实验表明:将镀铬板浸泡到300 g/L 90 ℃的NaOH溶液中20 min可将镀铬板表面铬氧化物溶解完全;在样品溶液中加入H₂SO₄(1+1)中和至pH为7,再加入5.0 mL硫磷混酸(3+3+4)、2.0 mL AgNO₃溶液以及5.0 mL(NH₄)₂S₂O₈溶液,加热至沸腾并保持约10 min以破坏过氧化物,冷却至室温后加入3.0 mL二苯卡巴肼溶液并稀释至100 mL,在542 nm处于2 cm比色皿中测定,Cr质量在0.24～100 μg之间有较好的线性关系,其线性方程为A=0.002 1+0.013 m_Cr(μg),相关系数为0.999 7。方法检出限为0.24 μg,测定下限为1.2 μg。将最终所测得镀铬板表面铬氧化物的量除以镀铬板试样面积,最终可计算出单位面积内镀铬板表面铬氧化物的含量。采用实验方法对两类镀铬板表面铬氧化物进行测定,所得平均含量分别为12.8 mg/m²和11.1 mg/m²,其相对标准偏差分别为3.1%(n=5)和3.6%(n=5)。对镀铬板试样进行加标回收,测得铬氧化物的加标回收率在92%～101%之间。

Determination of chromium oxides in chrome plate surface by diphenyicarbazide spectrophotometry

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 m_Cr(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%.