✔ 最佳答案
Your question is quite difficult. I try my best to answer it by my knownledge and website information
When we use a known amount of HCl with unknown concentration in conical flask and titrate with known concentration of silver nitrate solution .The reaction happens like this:
Cl− (aq) + Ag+ (aq) → AgCl (s)
White ppt is formed
When all Cl- is reacted with Ag+
An extra amount of Ag+ added to the mixture will react with CrO42-
although in fact it also happens during the above reaction
The solubility of silver chromate is slightly higher than that of silver chloride. So, in a mixture of both ions silver chloride will be formed. Only when no chloride is left, will silver chromate show its color.
Prior to the endpoint the solution has a milky lemon yellow appearance, due to the color of the chromate ion and the precipitate of silver chloride already formed. When approaching the endpoint, additions of silver nitrate lead to steadily slower disappearing red colorations. When the red brownish color remains (with grayish spots of silver chloride in it) the endpoint of titration is reached.
2009-03-25 20:27:28 補充：
you are right
under acidic environment
chromate will change to dichromate making its concentration is too low
we can make use a buffer solution to maintian the pH around 7
as also if it is too alkaline, silver(II) hydroxide will also form
making the titration value not accurate enough
2009-03-25 20:43:36 補充：
you can also know that
2 CrO42− + 2 H3O+ ⇌ Cr2O72− + 3 H2O
is a reversible reaction
if you do not use a buffer
or you afraid that the buffer will affect the concentration of HCl
that means the titration result will still be reliable as using up of CrO42− will shift the position to the left
2009-03-26 22:38:44 補充：
a buffer solution is used to provide a Fixed pH value environment to the solution
that means a buffer with pH7 is provided to the solution
in addition , THAT solution is neutral for the reaction to be carry out
2009-03-26 22:38:48 補充：
as i said before
despite the use of buffer, you can still carry out the experiment in acidic environment
2009-03-27 23:12:53 補充：
it is not related at all
it is another methods that can be use to show the end point of the experiment
Ag+ (aq) + SCN− (aq) → AgSCN (s)
with indictor iron(III)
Fe(OH)(OH2)52+ (aq) + SCN− (aq)→ [Fe(OH2)5(SCN)]2+ + OH−
blood-red [Fe(OH2)5(SCN)]2+ will be formed at the end point
