Hi everyone!
Welcome to the video on calculating the cost effectiveness of antacid using titration.
There are two goals to this experiment, one is to determine the amount of acid that can
be neutralized by a gram of antacid.
The second goal is to compare two antacids and determine which one is more cost effective.
We can determine both of these goals by using acid-base reaction stoichiometry.
Let’s start by discussing antacids.
As you know, these are compounds you take when you have excess acid in your stomach.
As a result, these compounds contain weak bases as their active ingredients such as
aluminum hydroxide, calcium carbonate, and magnesium hydroxide.
These bases neutralize excess acids in the stomach.
For example, if the antacid contains calcium carbonate, the reaction with H+ or proton,
which is what an acid produces, can be shown here.
As you see, the final product is water and CO2 gas.
It is important to distinguish the two terms used here.
Antacid refers to the tablet that we buy and active ingredient refers to the actual compounds
in the antacid that can neutralize acids.
The active ingredient is a small percentage of the antacid tablet.
The rest of the tablets is composed of materials such as binders that allow the tablet to be
easily digested.
Our first goal is to determine how many moles of H+ or acid, can be neutralized by one gram
of an antacid tablet.
We can do this by carrying a procedure called a back titration.
What this process requires is that we add excess amount of acid, in this case HCl, to
a given mass of antacid.
Because the HCl is in excess, some will be left over after all the antacid has been neutralized.
The remaining HCl is then neutralized by addition of NaOH, in this case using the technique
of titration.
By using stoichiometry, we can calculate the number of moles of HCl that reacts with the
antacid.
We can say that there is a certain quantity of active ingredients that neutralizes the
HCl.
The unit of this quantity is equivalent, which is a unit that defines the quantity of a substance
based on the substance’s ability to react with another substance.
In this case, one equivalent of the active ingredient is defined as the quantity of active
ingredient that can neutralize exactly 1 mol of HCl.
You may wonder why we don’t just use mol for the unit of the active ingredient.
The reason is because we don’t know which compounds make up the active ingredient.
As a result, it is impossible for us to stoichiometrically calculate the exact number of moles.
So once we finish our experiment, what we can find out is how many equivalents of active
ingredient we have in a gram of antacid tablet.
After that, we want to know how much it costs for us to buy a certain equivalent of active
ingredient.
We will use the cost information of the antacid to calculate that.
Let’s work this out in an example.
Here a student reacts 0.326 g with excess HCl.
The remaining HCl is neutralized by NaOH.
We’re given the cost information and asked to calculate the cost per equivalent of the
antacid.
So we first have to calculate the number of equivalent of active ingredient.
This can be calculated by taking the difference between the total HCl used and the HCl that
is neutralized by NaOH.
So let’s do the calculation.
To get the total HCl, we multiply the concentration of HCl by its volume which gives us 3.346
x 10-3 HCl.
We can then calculate the number of moles of HCl that is neutralized by NaOH.
Since HCl and NaOH react in a 1:1 stoichiometry, we can calculate the number of moles of NaOH
used first, which is just its molarity times its volume, and then we multiply with the
stoichiometric relationship which gives us 1.765 x 10-3 mol HCl.
Subtracting one from the other and then multiplying by the relationship between equivalent and
mol gives us 1.581 x 10-3 equivalent of the active ingredient.
Now remember that we want to know how much active ingredient we have in a gram of antacid
tablet.
To do that, we will take the number of equivalents we just calculated and divide it by the mass
of antacid we used in the experiment, 0.326 g, which in the end gives us 4.85x 10-3 equivalent
of active ingredient per gram of antacid tablet.
The last part is to find out how much we pay for an equivalent of the active ingredient.
In this case, we’re told that we pay $5.99 for 100 tablets which each weighs 0.650 g.
Multiplying those two numbers and the number of equivalent per gram we got earlier, results
in a cost of $19 per equivalent of active ingredient.
So this calculation is the one you will be performing in your experiment today.
Here I am giving you the cost information for the two brands of antacids you will use.
Your goal is to compare the cost effectiveness of these two brands by calculating the $/eq
for each brand and seeing which one is cheaper.
Let’s now go through the experimental steps.
On the right is a picture of the various components of this experiment.
You will start with one of the antacids.
Get a tablet and grind it with the mortar and pestle shown here.
Weigh a certain amount of the antacid as described in your lab procedure.
Then dissolve the antacid in HCl, which comes in one the reagent bottles shown here.
Heat the solution, which helps to remove the CO2 formed in the reaction between the antacid
and HCl.
To the hot solution, add bromophenol blue.
This comes in a small vial and is a solution needed to help us determine when to stop our
titration.
Carry out a titration to the end point of bromophenol blue, i.e. until the bromophenol
blue changes its color.
Records the volume of HCl and NaOH used in the titration as well as their respective
molarities.
Repeat the procedure with a second brand of antacid.
Ok, let’s now discuss some important details in the experiment.
Both the HCl and NaOH will be delivered using burets.
As usual, when you use burets you must clean them before and after use.
The procedure to clean a buret before use is shown here.
Basically you will have to rinse the buret with water, the way to hold the buret to rinse
is shown in this picture.
After you’re sure the buret is clean, which is when no water droplet remains inside the
surface, then rinse the buret with the solution you want to use, so either HCl or NaOH.
When you’re done with the buret, rinse again with water before you return the buret to
the stockroom.
A quick reminder also to flush out the air bubble from the buret before use or your volume
reading will be wrong.
As I mentioned earlier, after you add HCl to your antacid, you will boil the mixture.
This is done to remove CO2 that is produced from the reaction between the antacid and
HCl.
If the CO2 is not removed, it will increase the acidity of the solution and will cause
us to titrate with too much NaOH.
We want to titrate to the equivalence point, which is the point when the number of moles
of NaOH is equal to number of moles of remaining HCl.
Since both HCl and NaOH are colorless, we use a third substance, called an indicator
to tell us when to stop titrating.
The indicator changes color when the titration is near the equivalence point.
The point when the indicator changes color is the called its end point.
The indicator we will use in this experiment is bromophenol blue which has the following
colors.
When the solution is acidic, it is yellow and when the solution is basic, bromophenol
blue is blue/purple.
When it transitions from one color to another, it has a greenish color.
So our goal is to titrate until our indicator is greenish or light purple.
How do we know that we are at the end point?
As you’re adding the NaOH into your solution, you will see a color change as soon as the
drop of NaOH hits the HCl, as shown in the picture.
But that color will immediately return back to its original color upon mixing.
So this tells you that you’re still far away from the end point.
As you’re adding more NaOH, you will find that the blue color will take longer mixing
to disappear.
This is a sign that the end point is near.
At this point, you want to add your NaOH drop by drop.
When the green/blue color appears after addition of 1-2 drops of NaOH and stays for 20 seconds
or more, then you know that you have reached the end point.
This picture here shows the correct way to carry out a titration.
One hand should be used to control the buret’s stopcock while the other hand is used to swirl
the Erlenmeyer flask as the NaOH is being added.
Note that each student should do one titration since in total there are two titrations to
do.
Because the antacid tablet contains materials beyond the active ingredient, you will see
that some parts of the antacids won’t dissolve even in the hot HCl solution.
This is normal, but you do want to make sure that as much of the tablet is dissolved as
possible.
Note that if you have an intense blue color in your solution, you have gone way past the
endpoint.
This doesn’t mean that you have to repeat the whole experiment again.
What you have to do is reverse the titration by adding enough HCl to bring the color of
the solution back to yellow.
After that happens, you can then slowly titrate back with the NaOH to get the color to the
endpoint of green/light purple.
If you do carry out the reverse titration, make sure you record all the additional volumes
of HCl and NaOH that you use.
Ok let’s briefly discuss the safety and waste disposal.
As usual, you want to have your complete PPE of goggles, lab coat and closed-toe shoes.
You should combine all unused HCl and NaOH in one beaker and then discard this waste
in the sink because they are non hazardous.
Note that you will need to check out two burets from the stockroom.
Rinse and dry them before returning to the stockroom.
This concludes the video on this experiment.
Thanks for watching and see you in lab!