Testing for halogenoalkanes
There is no need to make this reaction go to completion. The silver nitrate test is sensitive enough to detect fairly small concentrations of halide ions.
The mixture is acidified by adding dilute nitric acid. This prevents unreacted hydroxide ions reacting with the silver ions. Then silver nitrate solution is added.
Various precipitates may be formed from the reaction between the silver and halide ions:
Silver nitrate solution can be used to find out which halogen is present in a suspected halogenoalkane. The most effective way is to do a substitution reaction which turns the halogen into a halide ion, and then to test for that ion with silver nitrate solution.
Doing the reaction
The halogenoalkane is warmed with some sodium hydroxide solution in a mixture of ethanol and water. Everything will dissolve in this mixture and so you can get a good reaction.
The halogen atom is displaced as a halide ion:
| ion present |
observation |
| Cl- |
white precipitate |
| Br- |
very pale cream precipitate |
| I- |
very pale yellow precipitate |
It is actually quite difficult to distinguish between these colours, especially if there isn't much precipitate. You can sort out which precipitate you have by adding ammonia solution.
| original precipitate |
observation |
| AgCl |
precipitate dissolves to give a colourless solution |
| AgBr |
precipitate is almost unchanged using dilute ammonia solution, but dissolves in concentrated ammonia solution to give a colourless solution |
| AgI |
precipitate is insoluble in ammonia solution of any concentration |
آزمایش تالن یا نیترات نقره آمونیاکی
محلول تالن یک کمپلکس نقره و آمونیاک است که از واکنش یون نقره و محلول آمونیاک (هیدروکسید آمونیم) در مجاورت هیدروکسید سدیم به دست می آید.
در این آزمایش یون نقره در مجاورت قند احیا کننده احیا میگردد و به صورت فلز نقره رسوب میکند. رسوب حاصله به جدار لوله چسبیده، به صورت آئینه در می آید. به همین سبب به آن آزمایش آئینه نقره نیز میگویند.
Confirming the precipitates
عضو شوید
عضویت سریع
آمار مطالب
آمار بازدید
