Hi everyone!
This video will discuss the Qualitative Analysis of Group I Cations, the experiment you will
be performing this week.
Before I discuss the lab today, I want to first discuss what qualitative analysis is.
It is an approach we use to identify specific substances, typically ions, that are present
in a mixture.
Here’s a general flowchart or scheme on how to accomplish the identification.
Say you have a sample containing three possible cations, A, B and C. Your goal is to figure
out which of the cations is in your sample.
To be able to identify the cations, you must first separate them from each other.
This is typically done by adding a reagent that will selectively precipitate one of the
ions.
So in our scheme, we add reagent X, which precipitates cation C while leaving cations
A and B still in solution.
In the picture, the cloudiness in the test tube is due to the formation of the solid
as reagent X is added.
Now we need to separate out the precipitated cation from the soluble ones.
This is done through centrifugation, where the sample is spun at high speed to sediment
the solid at the bottom of the test tube.
Once centrifugation is done, we carefully decant the supernatant, which is the soluble
part of the mixture, into a new test tube while leaving the pellet, the solid part of
the mixture in the old test tube.
In our scheme, we now have cation C separated from A and B, so that allows us to perform
a specific confirmation test on cation C. Here we show that cation C forms a yellow
precipitate in its confirmation test.
We must further separate out cations A and B, in this case by adding reagent Y which
will selectively precipitate one of the cations.
Then once we centrifuge to separate the supernatant from the pellet, we can carry out the confirmation
tests for each ion.
The term qualitative analysis is used here because we are not testing for the quantity
of each ion, merely its presence.
Since qualitative analysis requires selective precipitation of specific ions, two main reactions
are typically used to change the state of a specific ion:
The first is precipitation reactions, which mean adding a counter ion that will cause
a specific ion to form a precipitate.
For example, adding sulfate ion to a solution containing barium ion will precipitate the
barium as barium sulfate.
The second is adding a species that can act as ligands to the ion, causing them to form
complex ions which are soluble.
For example, adding thiocyanate to iron ion cause it to form a soluble complex ion, iron
thiocyanate.
As we go through each reaction in a qualitative analysis scheme, you will see that we manipulate
ions to go through one of the states: solid or soluble, by using one of the reactions
here.
In analytical chemistry, a well known scheme used to separate cations is given in this
flowchart, taken from your lab manual.
As you can see, this scheme starts with a large number of cations, which are slowly
separated into various groups of cations, which are then further analyzed.
The first step of the separation is the addition of HCl which will selectively precipitate
3 cations: silver, lead, and mercury, so these cations are called Group I Cations.
Further separation results in groups 2 to 5.
This week you will be working on the cations from group 1, the cations I just mentioned:
silver, lead and mercury.
Importantly, this is a SOLO lab, as is true for the next 3 qualitative analysis labs.
Make sure you write down careful notes from the video so you are prepared to think through
your experimental results when you are in the lab.
The goal in this lab is to work with a known sample containing all three of the group I
cations, called a positive control.
In addition, you will also work with a sample that contains some of the group I cations.
In this second sample, you will have to identify which cation or cations are present.
I have taken the scheme or flowchart from your lab manual and pasted it here to show
you all the steps.
In the next section, I will describe each of the steps in the flowchart, why we do them
and what we expect to get in carryout the steps.
Let’s start with step 1, which is the addition of 6M HCl to your sample.
This is meant to precipitate only the group I cations: silver, lead and mercury and separate
them from all the other ions which will still be soluble.
Centrifugation separates the white precipitate containing the insoluble chloride salts from
the solution which we can assume contains other ions.
Once we are confident that our sample only contains group I cations, we must separate
them from each other before we can confirm their identities.
The first step is by adding water to the precipitate from the previous step and then heating it.
When we do this step, only the lead chloride salt with dissolve while the other two: silver
chloride and mercury (I) chloride will remain insoluble The reason has to do with the Ksp
of these salts.
As you can see, PbCl2 has the largest Ksp of the three salts.
Recall from your Le Chatelier’s Principle lab that the dissolution of these salts are
endothermic reactions.
This means Increasing temperature will result in a right shift, effectively dissolving the
solids.
However, since both silver and mercury (I) chlorides have such low Ksp, even at high
temperatures, their solubilities are still very low, meaning that only lead chloride
will dissolve when we add the hot water.
Note also that by adding water, we are increasing volume, causing the reaction to shift to the
right and increasing solubility.
Once you are fairly sure that all of the lead chloride has dissolved, centrifuge to separate
it from the still solid mercury (I) and silver chlorides.
The next step is to confirm that the lead ion is actually present.
To do this we add chromate ions, which form a yellow precipitate of lead chromate when
lead is present.
The next step is to separate the remaining two ions: silver and mercury (I).
Here we will add NH3.
For silver, NH3 is a ligand, so its addition results in the formation of a complex ion,
Ag(NH3)2+, which is soluble.
At the same time, NH3 undergoes a disproportionation reaction; a type of oxidation reduction reaction
where one substance, in our case mercury (I) is both oxidized and reduced into two new
compounds.
In our situation, the mercury (I) ion is reduced into liquid mercury as well as oxidized into
mercury (II) ion in the form of a compound called mercuric amidochloride.
As shown in this picture, these two compounds appear as a grayish solid mixture because
liquid mercury is silvery and mercuric amidochloride is white.
This appearance confirms that mercury (I) is present in your original sample.
Once you see that the solid mercury mixtures are formed, centrifuge the test tube and decant
to separate the soluble silver ion from the solid mercury mixtures.
The last step is to verify that silver ion is present by adding 6M HNO3.
The reason is shown in the following set of reactions.
First we know that silver currently exists as the complex ion Ag(NH3)2+.
At the same time, we know that chloride ions are still present in the solution, since we
have not taken any step to specifically remove it.
We know that chloride and silver can form the white precipitate, silver chloride, but
only if the silver is present as free silver ion.
To convert the silver from its complex ion form to the free form, we add nitric acid.
The proton from the strong acid will react with ammonia, a weak base in a product-favored
reaction.
This reduces the concentration of NH3, thus forcing the complex ion reaction to shift
left, releasing free silver.
The free silver ion can now form a white precipitate with chloride.
So that’s the conceptual background of the group I cation qualitative analysis.
Let’s now discuss some of the experimental steps.
First, you will get an unknown sample from your instructor.
You will also make a positive control or known sample, which contains all 3 ions.
The purpose of the positive control is to familiarize yourself with the experimental
steps, to see the actual results you’re supposed to get, to check the quality of the
reagents and make sure the equipment is functioning properly.
Running a positive control experiment is typical in all research laboratories.
To make it easy for you to spot differences, run the positive control and unknown side
by side.
Some guidelines for using the centrifuge: first, use medium size test tubes.
These fit the best in the centrifuge.
Check that the test tubes are not cracked on the bottom or the side before you put them
in the centrifuge.
In addition, make sure each test tube is balanced with another right opposite it in the centrifuge.
Unbalanced or slightly cracked test tubes will break at the high speed spin of the centrifuge.
Spin about 1 minute to ensure that your solid is completely sedimented to the bottom of
the test tube.
You should decant gently so as not to disturb the pellet at the bottom of the tube.
If some of the pellet gets carried to the tube that is only supposed to have solution,
you now a contaminating ion which will complicate your observations and may lead you to the
wrong conclusion.
The most common problem in this analysis is the incomplete separation of lead from the
other two ions.
This occurs because the lead ion is not completely dissolved.
To ensure complete dissolution, the water bath used to heat your ion mixture must be
boiling.
You have to use your stirring rod to mix so that all the lead is dissolved.
Lastly, centrifuge the hot test tube.
If you wait till it cools down, your lead will reprecipitate.
A washing step is included in your procedure.
This is done to remove ions that are not group I. Washing is done as follows: add 1 ml of
water into a solid, mix the solid and water with your stirring rod, centrifuge and discard
the supernatant.
Repeat as many times as needed.
Lastly, I want to discuss the safety and waste disposal instructions.
All the ions in this group are known to be biologically toxic.
That means you don’t want to spill any of it on you or others, as the ions can be absorbed
through your skin.
Make sure you have your gloves at all times during the experiment.
In addition, at the end, all solutions should be discarded in the chemical waste container.