【讨论】sodium monofluorophosphate procedure (USP)

问题:
1 Add about 50 ml of water, mix to effect solution, dilute with water to volume, and mix.
加大概50ml水,混合均匀后,用水定容,混匀?

2 transfer 20.0 ml of this solution to the reaction flask of a suitable fluoride steam distilling apparatus containing 10 lime-glass beads measuring 5 mm in diameter and 70 ml of dilute sulfuric acid(1 in 2). distill, by passing steam through the solution in the reaction flask and applying heat to the flask, collecting the distillate in a 250 ml volumetric flask.
移取20.0ml该溶液到可用于氟蒸馏的反应烧瓶中,烧瓶内装有10粒钙玻璃珠和70ml(1:2)的硫酸.蒸馏,passing steam through the solution in the reaction flask and heat to the flask.馏分收集在250ml容量瓶中.

3 change receivers when the flask is filled to volume with distillate, and continue distilling into a 400-ml beaker until an additional 150ml to 200ml of distillate tailing has been collected. mix the solution in the 250ml volumetric flask,
当容量瓶收集到馏分后改用400ml的烧杯做承接器,至收集到150-200ml馏分,混合250ml容量瓶中的馏分.
为什么要分两个容器来接馏分呢?
加入的量是20ml 母液和70ml硫酸,怎么会有150-200ml的馏分收集呢?
是否我哪里理解错了

4 transfer 50.0ml to a 150 ml beaker. to the 150ml beaker add 50ml of water and 3 drop of sodium alizarinsulfonate TS, and mix. adjust the acidity of the solution by the careful addition, successively, of sodium hydroxide solution( 1 in 50) and dilute hydrochloric acid( 1 in 160), until the pink color has just been discharged. add 1 ml of monochloroacetate buffer, of titrant with 0.025M thorium mitrate ot a permanent pink color, and record the volume, in ml, of titrant consumed as Va, add 3 drop of sodium alizarinsulfonate TS to the distillation tail in the 400ml beaker, proceed as directed previously with the adjustment of the acidity, the addition of monochloroacetate buffer, and the titration with 0.025 M thorium nitrate, and record the volume, in ml, of titrant consumed as Vb, calculate the percentage of Na2PFO3 in the sodium monofluorophosphate taken by the formula:
[(2)(143.95)(5Va+Vb)(M)/(W) ](143.95/18.9984)(F)
M是Th(NO3)4的摩尔浓度,W是Na2PFO4的重量
为什么要分开两次滴定呢?
公式中为什么要乘2, Va为什么又要乘5?谢谢

[quote]原文由 christe 发表:3 change receivers when the flask is filled to volume with distillate, and continue distilling into a 400-ml beaker until an additional 150ml to 200ml of distillate tailing has been collected. mix the solution in the 250ml volumetric flask,
当容量瓶收集到馏分后改用400ml的烧杯做承接器,至收集到150-200ml馏分,混合250ml容量瓶中的馏分.
为什么要分两个容器来接馏分呢?
加入的量是20ml 母液和70ml硫酸,怎么会有150-200ml的馏分收集呢?
是否我哪里理解错了


250 ml 容量瓶收集至刻度后（满）后换成烧杯
一个可能性是因为没有更大的容量瓶（大于400），而预知馏分在400左右。如果直接用烧杯，容易溅出（或者其他原因），因此要先用250ml容量瓶。

因为导入是蒸汽，会不会蒸汽冷却后也进入到了馏分呢？

上文提到by passing steam through the solution in the reaction flask，所以我觉得一部分馏分是来自于蒸汽中的水。

dilute hydrochloric acid是稀盐酸，我在翻译法国资料时遇到过，挺常见的用法。

分两次滴定和公式中的5Va+Vb都是因为，馏分是在两个容器中的。装在容量瓶中的第一组馏分体积是250ml，而滴定时只用了50ml，所以应该乘以5。装在烧杯中的第二组馏分全部用于滴定，所以不需要乘以5。总体积就是5Va+Vb。

为什么要分两次蒸馏：可能是因为最初的馏分中含量很高，（所以滴定时只取五分之一进行滴定，也因此需要用容量瓶定容。），而最后的馏分中含量已经很低了（所以全部用于滴定）。

楼主不是学化学的吧？

如果是这样的话,那是怎么样的一个装置图呢?,想象不出来

三口烧瓶？你用“三口烧瓶”为关键词在google上搜索一下图片就知道了。不过我化学学得不好，也没做过这个试验，不知道具体的装置是怎样的。