In this video, you're going to learn how to titrate.
Before watching this video, make sure that you've seen the meniscus and burette videos.
Volumetric titrations are used in chemistry to determine the concentration
of an analyte solution.
To do this, you use a solution of a known concentration,
and react it with a known volume of your analyte.
You do this until you reach the end point.
This is when all of the analyte is assumed to have reacted.
The end point of the reaction can be indicated using
several different methods
such as color change by indicator,
or change in pH by a pH meter.
Because it's the simplest to demonstrate, and most common,
I'll be showing you color change by an indicator.
I'll be using phenylethylene.
The first thing to do is to make sure that you have everything set up
and ready to go.
You should have your burette already cleaned
three times with the ionized water, and three times with your solution of a known concentration,
and set up properly in the retort stand.
You'll now want to lower it to eye level
so that you can add your solution.
Make sure your stopcock is closed
and then with a cleaned funnel,
add your solution to the burette.
The solution held in the burette is referred to as the titrant.
Make sure that you add enough solution so that
you won't run out midway through the titration.
With the burette back in its proper position,
place the waste beaker under the tip, and allow a small amount of solution to drain.
This prevents the formation of bubbles.
Once you've done this, remove the waste beaker
and place a blank white paper
underneath the burette tip.
This will be used as a blank background to see the color change more easily.
Next, you will want to transfer an accurate volume of your analyte into your receiving vessel.
The best way to do this is using a clean pipette.
This will allow you to transfer a very precise volume.
which you will need to know
in order to do your calculations at the end of the titration.
Use the pipetting skills shown to you in the pipetting video.
Then transfer your solution into your receiving vessel.
In most titrations, Erlenmeyer flasks are used.
This is because they prevent solution from splashing back out of the flask.
You'll also want to make sure that you are
using a flask of an adequate volume.
You don't want it to be too small that solution will splash out
while you are swirling it.
You now need to add some of your indicator.
As I said earlier, I am using phenolphthalein.
This means that the solution will
turn pink when the end point is reached.
You will only need a few drops.
You'll want to give your solution a quick swirl to make sure it is mixed
and then place it under the burette.
Make sure that there is enough room
to swirl the flask without it bashing into the tip of the burette
but you still want the tip to be
below the rim so that you don't lose any of your solution.
Initially, you can add large amount of titrant,
but as you get closer to the end point
you'll want to slow down.
It will be difficult to judge the first time around
but you'll be able to do it better with practice.
A good indicator that you're nearing your end point
is when there is initial color change
but it fades quickly with swirling.
As you get even closer to your end point,
it will take longer and longer for the color to fade.
When you get to this point, you'll have to reduce your aliquot to drop wise
or even half drop wise.
Making half drop additions takes
a lot of practice. What you need to do is first let a small amount of titrant out of
the burette, but only enough so that the drop hangs on the tip
and doesn't drop down into your solution.
Then, touch the drop to the side of your receiving flask
and wash the droplet down
with your deionized water.
Make sure as you get even closer to your end point that
to rinse down the sides of your flask with deionized water.
This helps to get all of the unreacted titrant into the bottom of the flask.
Doing this won't affect your results
but it will dilute the solution, so make sure not to add
too much, or it will make the color change harder to see.
When the end point is reached, the color is persistent
but faint. It may be difficult to
notice, but that's what using the white paper helps you to see.
The end point is marked when the color
remains visible for at least 20-30 seconds.
Different indicators will give you different results and different transitions.
some will be very sharp, and some will be very gradual.
They will use a variety of different colors,
so make sure that you know what you're looking for
before you start.
After you have reached your end point,
make sure to let the burette rest for a few seconds
and then record the final volume.
Using the initial and final volume,
in your burette, you'll get the volume of transferred solution into the reaction.
Using this, and the reactin stoichiometry,
you'll be able to determine the concentration of your analyte solution.
When performing a titration, it's
common to use at least three different aliquots of your analyte.
This helps you to get reproducability in your results.
It's also common for students to overshoot the end point.
This helps you to get more practice in.
When you've finished all of your titrations
make sure to clean your burette
as was shown to you in the burette video, and store it upside down for later use.
Volumetric titrations are very daunting to new students to chemistry
but with a lot of practice, you'll eventually get the hang of it.