什么叫返滴定法返滴定法的适用范围是什么
参考答案：返滴定法是在一定条件下，向试液中加入已知过量的EDTA标准溶液，然后用另一种金属离子的标准溶液滴定过量的EDTA，由两种标准溶液的浓度和用量，即可求得被测物质的含量。
这种方法主要适用于下列情况：被测离子与EDTA的反应速度太慢，在反应要求的pH值条件下，被测离子发生水解等现象干扰测定；用直接法滴定时，无适宜的指示剂，或被测定离子对指示剂有封闭作用。
| 完形填空：先通读下面的短文，掌握其大意,然后在每小题的A、B、C、D四个选项中选出最佳选项。 | ||||
| Deng Yuanyuan is the hero of 20 children at the Sunshine Home in Shanghai. It is not an easy 1 to teach a class of kids with special needs. But for the 32-year-old girl, it is not difficult at all. Deng became a voluntary English teacher six months 2 . People usually think these kids are special and treat them 3 , Deng said. " But in my eyes , they are just lovely kids like every child." She remembered that the first time she 4 into the classroom, some kids 5 , some were mad at the stranger, and some were just talking to 6 . Deng then embraced(拥抱) every child. She taught only one English word " 7 " the whole day. When Deng came into the classroom 8 the following day, she was 9 that all the children were greeting her with hellos. She became popu  lar among the children.Gradually(逐步地), Deng 10 her friends to the class and introduced the Sunshine Home to more people. " Some of them even volunteered to teach with me," she said with a smile. | ||||
|