phenolphalein colour change

发布网友发布时间：2023-06-30 16:18

共1个回答

热心网友时间：2023-10-09 01:42

The time needed for colour change of phenolphthalein is very short. Therefore
what happens as described in the question is NOT e to time taken for colour change of phenolphthalein. Actually
this is e to that the alkali added is not uniformly distributed in the not well-shaken solution but concentrates at a certain region. 酚酞颜色改变需时很短。因此，题目所述的情况并非由于酚酞需要时间转色。事实上，这是由于在未摇匀的溶液中，所加的碱并非平均分布于溶液中，而是集中于某区域之内。 Before addition of the alkali from the burette
the acid in the conical flask is uniformly distributed. When a *** all amount of alkali is added without shaken the conical flask
the alkali added would concentrate in a certain region e to the solution is not well shaken. Although the volume of alkali added is far less than that needed in end point
the added alkali neutralizes all the acid in such a region
and the excess alkali in such region turns the colourless of phenolphthalein in such region to red. However
when the solution is mixed well
the solution bees uniform. The alkali originally excess in the region is pletely neutralized and the resulting solution is acidic. This makes the whole solution bees back to colourless. 由滴定管加入碱前，锥形瓶中的酸是平均分布的。当在不摇晃锥形瓶下加入小量的碱，由于溶液没有摇匀，所加入的碱会集中于某一区域之内。虽然所加入碱的体积远少于终点所需，但所加入的碱足以把该区域中所有的酸中和，而在该区域中过量的酸会令该区域内的酚酞转红。可是，当溶液摇匀后，溶液变成均匀，原本在该区域中过量的碱被中和，所得溶液呈酸性，令全溶液变回无色。 2010-05-29 16:21:50 补充： When the end-point is near
the amount of excess acid in the conical flask is *** all. Therefore
the addition of alkali would cause regional excess of alkali much easier than that at the beginning of the titration. To prevent the this
the conical flask must be shaken for a longer period of time.
参考: fooks