2012-04-04

Qualitative Analysis of Metal Cations - Week 2

This week in lab you're going to be analyzing an unknown mixture of the metal cations you studied last week.  To do this, you need to look over the tests you did last week and find a way to sequentially use some or all of those tests to separate the cations.  This is NOT a "run all the tests and figure it out later" experiment, you have to have a plan.
To develop your plan, assume you are starting with a mixture of all 5 cations, and you want to separate them into 5 different containers.  The key to developing a good flow chart is the ability to separate solids from each other when multiple things precipitate.  For example, if the first step of your flow chart is "add chromate", you will precipitate all 5 cations as their chromate salts.  You have no way to separate these solids from each other, so this would be a VERY bad first step.  Look over some of the multi-step tests you did last week.  If adding some reagent causes 2 or 3 of the cations to form precipitate, and you have a way to separate those precipitates from each other (with another step in the multi-step test you performed last week), then that might be a good place to start.
As an example, what if you had a mixture of NaCl, NaNO3, sand, and sawdust.  How could you separate them?  If you added water, the NaCl and NaNO3 would dissolve, the sand would sink, and the sawdust would float.  That accomplishes some of the separation, but what about the dissolved salts?  If there was a reagent you could add to make chloride ions form a precipitate, like maybe Pb2+(aq), you would be able to separate the chloride from the nitrate.
{OK, picky people in the crowd, that doesn't exactly separate "NaCl" from "NaNO3", but it illustrates the point!}
Given the tests you performed, there are a few different flow charts that will work to separate the 5 cations you're working with, so if you have something that's a little different from someone else, that's OK.  It would be great if you compared your flowchart to someone else's and had a discussion about the similarities and differences, it might lead you BOTH to make better flowcharts/plans.

No comments:

Post a Comment