2.
铬元素总浓度的测定:准确移取\(\rm{25.00 mL}\)含\(\rm{Cr}\)\(\rm{{\,\!}_{2}}\)\(\rm{O}\)\(\rm{\rlap{_{7}}{^{2-}}}\)和\(\rm{Cr}\)\(\rm{{\,\!}^{3+}}\)的酸性废水,向其中加入足量的\(\rm{(NH}\)\(\rm{{\,\!}_{4}}\)\(\rm{)}\)\(\rm{{\,\!}_{2}}\)\(\rm{S}\)\(\rm{{\,\!}_{2}}\)\(\rm{O}\)\(\rm{{\,\!}_{8}}\)溶液将\(\rm{Cr}\)\(\rm{{\,\!}^{3+}}\)氧化成\(\rm{Cr}\)\(\rm{{\,\!}_{2}}\)\(\rm{O}\)\(\rm{\rlap{_{7}}{^{2-}}}\),煮沸除去过量的\(\rm{(NH}\)\(\rm{{\,\!}_{4}}\)\(\rm{)}\)\(\rm{{\,\!}_{2}}\)\(\rm{S}\)\(\rm{{\,\!}_{2}}\)\(\rm{O}\)\(\rm{{\,\!}_{8}}\);向上述溶液中加入过量的\(\rm{KI}\)溶液,充分反应后,以淀粉为指示剂,向其中滴加\(\rm{0.015 mol·L}\)\(\rm{{\,\!}^{-1}}\)的\(\rm{Na}\)\(\rm{{\,\!}_{2}}\)\(\rm{S}\)\(\rm{{\,\!}_{2}}\)\(\rm{O}\)\(\rm{{\,\!}_{3}}\)标准溶液,终点时消耗\(\rm{Na}\)\(\rm{{\,\!}_{2}}\)\(\rm{S}\)\(\rm{{\,\!}_{2}}\)\(\rm{O}\)\(\rm{{\,\!}_{3}}\)溶液\(\rm{20.00 mL}\)。计算废水中铬元素总浓度\(\rm{(}\)单位:\(\rm{mg·L}\)\(\rm{{\,\!}^{-1}}\),写出计算过程\(\rm{)}\)。
已知沉淀过程中发生的反应如下:
\(\rm{①2Cr}\)\(\rm{{\,\!}^{3+}}\)\(\rm{+3S}\)\(\rm{{\,\!}_{2}}\)\(\rm{O}\)\(\rm{\rlap{_{8}}{^{2-}}}\)\(\rm{+7H}\)\(\rm{{\,\!}_{2}}\)\(\rm{O═Cr}\)\(\rm{{\,\!}_{2}}\)\(\rm{O}\)\(\rm{\rlap{_{7}}{^{2-}}}\)\(\rm{+6SO}\)\(\rm{\rlap{_{4}}{^{2-}}}\)\(\rm{+14H}\)\(\rm{{\,\!}^{+}}\)
\(\rm{②Cr}\)\(\rm{{\,\!}_{2}}\)\(\rm{O}\)\(\rm{\rlap{_{7}}{^{2-}}}\)\(\rm{+6I}\)\(\rm{{\,\!}^{-}}\)\(\rm{+14H}\)\(\rm{{\,\!}^{+}}\)\(\rm{=2Cr}\)\(\rm{{\,\!}^{3+}}\)\(\rm{+3I}\)\(\rm{{\,\!}_{2}}\)\(\rm{+7H}\)\(\rm{{\,\!}_{2}}\)\(\rm{O}\)
\(\rm{③I}\)\(\rm{{\,\!}_{2}}\)\(\rm{+2S}\)\(\rm{{\,\!}_{2}}\)\(\rm{O}\)\(\rm{\rlap{_{3}}{^{2-}}}\)\(\rm{=2I}\)\(\rm{{\,\!}^{-}}\)\(\rm{+S}\)\(\rm{{\,\!}_{4}}\)\(\rm{O}\)\(\rm{\rlap{_{6}}{^{2-}}}\)