phenolphalein colour change

The time needed for colour change of phenolphthalein is very short. Therefore, what happens as described in the question is NOT due to time taken for colour change of phenolphthalein. Actually, this is due to that the alkali added is not uniformly distributed in the not well-shaken solution but concentrates at a certain region.
酚酞顏色改變需時很短。因此，題目所述的情況並非由於酚酞需要時間轉色。事實上，這是由於在未搖勻的溶液中，所加的鹼並非平均分佈於溶液中，而是集中於某區域之內。

Before addition of the alkali from the burette, the acid in the conical flask is uniformly distributed. When a small amount of alkali is added without shaken the conical flask, the alkali added would concentrate in a certain region due 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 becomes uniform. The alkali originally excess in the region is completely neutralized and the resulting solution is acidic. This makes the whole solution becomes back to colourless.
由滴定管加入鹼前，錐形瓶中的酸是平均分佈的。當在不搖晃錐形瓶下加入小量的鹼，由於溶液沒有搖勻，所加入的鹼會集中於某一區域之內。雖然所加入鹼的體積遠少於終點所需，但所加入的鹼足以把該區域中所有的酸中和，而在該區域中過量的酸會令該區域內的酚酞轉紅。可是，當溶液搖勻後，溶液變成均勻，原本在該區域中過量的鹼被中和，所得溶液呈酸性，令全溶液變回無色。

2010-05-29 16:21:50 補充：
When the end-point is near, the amount of excess acid in the conical flask is small. 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.