Welcome back everyone to the second video on the Gravimetric Analysis lab!
In the first video, I discussed how you can use just two masses to determine the identity
of the cation in this precipitation reaction.
In this video, I will discuss specific ideas related to the actual experimental steps.
Unlike some other experiments where I provide you with a revised procedure, you can follow
the steps written in your lab procedure for this experiment.
As an overview, the steps are written below.
You will weight a certain amount of your unknown sulfate salt and then dissolve it.
You will add 6M HCl to the solution which will help the crystals of barium sulfate to
form.
Then heat the solution and slowly add barium chloride, which is the second reactant in
the reaction.
This will result in the formation of a white precipitate which is the barium sulfate.
After all the barium chloride has been added, wait 20 minutes for the crystallization process
to complete.
In the meantime, prepare your crucible for the last stage of the experiment.
After 20 minutes, decant and filter the solution to isolate the barium sulfate from the aqueous
reagents.
Then put the filter paper containing the barium sulfate inside the crucible and heat the crucible
in a fume hood.
When all the filter paper disappears, you should weigh the mass of the barium sulfate
product.
In the rest of this video, I will highlight specific steps that you want to be careful
about to ensure that your experiment is a success.
First off, the unknown sulfate salt comes in a small vial that is stored inside a desiccator.
A desiccator is a container meant to protect the samples inside it from moisture.
The sulfate salts easily form hydrates with water from the atmosphere.
When the sulfate salts are hydrated, its molar mass is higher than when it’s dry, which
will result in an error in our calculation.
The 6M HCl is not a reactant, but merely needed to help crystals of barium sulfate to form,
so it acts as a catalyst.
Addition of barium chloride to the hot sulfate solution must be done SLOWLY.
One way to do this is by using a dropper to add the barium chloride as shown in the picture.
Simply get some barium chloride solution and squirt it into the unknown sulfate solution.
The correct way to use a crucible tong is to place the crucible at the wider part of
the tongs.
In certain situations while you’re heating however, it may be more convenient to pinch
the crucible with the tongs.
When you do so, make sure you don’t put too much pressure, otherwise the hot crucible
can easily break.
This experiment requires you to heat the crucible at
different levels of heating, so let me discuss how you can control the type of flame you
see in your bunsen burner.
First off, it’s important to remember that the reaction that goes on when a flame appears
is a combustion reaction between natural gas (or methane) and oxygen from air.
There are two knobs in your burner, the bottom circular knob controls how much natural gas
flows into your burner.
The tall collar can also be turned and controls how much air (which contains oxygen) flows
in to the burner.
To make your flame shorter or taller, adjust the gas or circular knob at the bottom.
To make your flame hotter or cooler, adjust the collar that controls the air.
This picture shows the heat of the flame.
The leftmost flame is the coolest flame.
It is yellow in color and creates a loot of soot, so we often call this dirty flame.
The rightmost blue flame is the hottest flame and burns cleanly.
The shorter cone inside the flame is the hottest part.
To adjust the appearance the flame from the yellow to the blue, turn the burner’s collar.
As you let more air in, the flame will get bluer and hotter.
If you let too much air in though, the air will blow the flame out.
Avoid the yellow flame in your experiment.
When you’re heating your crucible containing the filter paper, start with the blue flame
without the inner cone.
As the filter paper slowly turns to ash, let in more air to the burner until you see the
blue inner cone.
Heat the crucible right at the tip of that cone.
To filter the barium sulfate solution, you have to create a cone filter from an ashless
filter paper.
The way to fold the circular filter paper into a cone shape is shown here.
First you fold the filter paper in half then another half, so the paper is a quarter of
its original size.
Then open one side, so you have a conical shape and put it inside a funnel.
Wet the filter paper with water so it will stick to the funnel.
The correct technique to decant your solution (called the filtrate) is by pouring it into
the filter along a stirring rod.
Another view is shown here so you can better see it.
The stirring rod will make sure the liquid doesn’t splash around which will cause you
to lose your sample.
When you’re heating your filter paper in the crucible in the fume hood, you should
see the following four stages.
When you start, the filter paper is still wet/moist.
You want to heat it using the blue flame without the inner cone.
After heating for a while, the filter paper will turn brown and eventually black.
This is normal.
As you continue heating, the paper will turn to black and gray ash.
At this point, increase the flow of air into your burner so you generate the inner cone
in your burner.
Put the crucible at the tip of the inner cone and let it heat until no more black/grey residue
of the paper is left.
At the end, you should only see a flaky white solid in the crucible.
That is your barium sulfate.
If during your heating, your paper catches fire and you see flame inside your crucible,
quickly cover the crucible with the lid to extinguish the flame.
If at the end of heating, you see some soot or carbon-containing black substance on your
lid, you can remove it by heating the lid upside down using the bunsen burner as shown
in the picture.
Let me wrap up by mentioning the safety procedure for this lab.
You will need to have your full PPE and your gloves to handle the 6M HCl and other reagents.
All reagents and products must be discarded in the hazardous chemical waste container
which is a 1 gallon plastic jar, that looks like the one shown in this picture.
The jar will be located in the waste fume hood.
You will need to check out the crucible and its lid from the stockroom.
At the end, make sure you clean the crucible by brushing all soot and tar away and rinsing
it with water.
Dry both crucible and lid and return them to the stockroom.
That’s the end of the video on Gravimetric Analysis.
Thanks for watching!