pH indicator問題~~希望有人識得可以答我......

because the indicator molecule can exist as acidic and alkaline form. Both form have different colour.

for example,
HIn <====> In- + H+
HIn is the acidic form of methly orange, which is red
In- is the alkaline form of methly orange, which is yellow

pH = pK + log([In-]/[HIn])
as pK is a constant, when thier is any change in pH, the ratio of HIn to In- will adjust.
as a result, colour of the solution will change its colour.

2007-01-30 22:41:30 補充：
***** of the above paragraph is [p K], which is a constantthe system cannot display [p K] =P

First, you must know that indicator is a "weak acid". It is a "dye".
It can be eaily protonated吸H+, giving one distinct colour.
when it is deprotonated 放H+, by reacting with base, another colour will be given.
Secondly, only a few drops of indicator is used in order to prevent the disturbance 打擾
of the pH value of the solution to be tested.

The basic principle of indicator
Hln ------> H+ + ln-
ka = [H+][ln-]/[Hln]
ka/[H+] = [ln-]/[Hln]

The colour change of the indicator is due to the change in the ratio of [ln-] to [Hln]

因為是ka固定(i.e. 10),當[H+] is 5,[ln-]/[Hln] becomes 2

當[H+] is 2,在鹼性溶液時, [H+] decreases, [ln-]/[Hln] becomes 5. Change of ratio of [ln-] to [Hln], gives colour change.

當[H+] is 10,在酸性溶液時, [H+] increases, [ln-]/[Hln] becomes 1. Change of ratio of [ln-] to [Hln], gives colour change.


如果你覺得用比例說明為何會變色都有少少深,你可以這樣看以下說明
這式是雙向的Hln ------> H+ + ln- (用雙向箭頭表示,但print不到)
當[H+]增加(在酸性溶液時),化學平衡向左移,即多一些Hln 形成
當[H+] 被移走時,(在鹼溶液時)化學平衡向右移,即多一些ln- 形成
因為Hln 和ln-都有自己的顏色,所以當在酸性溶液時色,pH indicator會變色.
如果你不懂化學平衡,問一問化學老師

希望你明白