What is an acid?
We’ve all seen cartoons with bright green liquids that ooze and can eat through anything,
but what about in the real world?
You’ll have heard of many examples of acids that you might find around your home.
Sulphuric acid is found in strong cleaning products like drain cleaners, citric acid
is found in all citrus fruits and you put acetic acid on your chips.
Just because something is an acid, doesn’t necessarily make it dangerous, but some acids
can be incredibly harmful.
So how do you determine the potency of an acid?
We can look at the concentration of the acid in the solution, and that will tell us a lot.
A 1 molar solution of hydrochloric acid is fairly corrosive, but a 6 M solution would
cause a lot of damage to me if I spilled it.
Not all acids are equal though: a 1M solution of citric acid is not as corrosive as a 1M
solution of hydrochloric acid.
That’s because hydrochloric acid is far more ready to dissociate into hydrogen ions
and chloride ions.
In the case of strong acids, all the molecules in the solution will dissociate.
This makes it easy to calculate the number of hydrogen ions in a solution.
A 1M solution of hydrochloric acid has 1 mole of hydrogen ions per litre, and one mole of
chloride ions per litre too.
The concentration of hydrogen ions is quite a difficult thing to imagine, so to make things
simpler, there is an easy to follow scale of acidity, which you know as the pH scale.
pH means potential of hydrogen and can be simply calculated from an equation.
The pH is equal to −log10([H+]), so a 1M solution of hydrochloric acid, with [H+] = 1
M, gives a pH of 1.
A 1 mM solution of hydrochloric acid has a pH of 3.
In a bottle of pure water, out of every 10 million molecules, one of the H2O molecules
will have dissociated into H+ and HO−.
−log10([1x10−7]) = 7.
The pH of pure water is 7.
On the other side of the pH scale, bases are molecules that can accept a hydrogen ion.
For example, sodium hydroxide can take a hydrogen ion and form H2O, which is very stable, and
a sodium ion.
In basic solutions, hydrogen ions are so readily taken up by base molecules that the concentration
of hydrogen ions is really low.
There is very little potential for hydrogen: in our equation, because of the – sign at
the front of it, a low concentration of hydrogen ions means a high pH.
If we take an acid, which wants to lose H+ ions and mix it with a base, which wants to
take up H+ ions, we will make a neutral solution – the two balance each other out.
To get the pH to exactly 7, the amount of acid molecules must match the amount of base
molecules.
The best way to achieve this is with a titration.
We can use a burette to add a very precisely known volume of one solution to another.
If the concentration of the acid and the base was equal, then the volume we would need to
add would also be equal.
If we don’t know the concentration of one of the solutions, we can make up a solution
with a well-known concentration using a volumetric flask, and titrate our standard solution into
the unknown solution until we get a perfect neutral green colour on the pH scale.
To make a standard solution, we take a volumetric flask.
It is shaped this way, with a large bulb at the bottom and a thin stem, to allow you to
more accurately add the right amount of liquid.
This flask is a 500 mL volumetric flask.
The ceramic line on the neck of the flask represents 500 mL.
Since the neck of the flask is so narrow, a small amount of volume will make a large
change to the height of the meniscus in the flask.
I’m going to add a weighed amount of sodium hydroxide to the volumetric flask, and then
use deionised water to fill the flask.
I’m going to slow right down once the liquid is near the mark, so that I can accurately
add water to get the meniscus to the perfect level.
The bottom of the meniscus should rest along the mark.
Now we have a standard solution of sodium hydroxide.
We know the concentration is 0.05 M. I’m going to titrate this against an unknown concentration
solution of hydrochloric acid.
I’ll fill the burette carefully, you must lower it so that when you pour the liquid
the top of the burette is below your eye level.
This reduces the danger of spilling the solution.
Once the burette is filled, I’ll note down the volume measured in the burette.
Now I’ll open the tap and allow the solution to pour gently into the acid.
Once the colour begins to change, we are close to neutral so I’ll slow right down and allow
the solution to mix thoroughly before adding more.
Now, drop by drop, the base is neutralising the acid.
Once the solution is green, we can note down the final burette volume, and subtract that
from the initial volume.
The volume we have added, or titre, is ____ mL.
Our solution concentration was 0.05 mol L−1 and we have used y mL, or x L, so the amount
of base we have added is 0.05x mol.
The volume of acid we used was 10 mL, so therefore, the number of moles of acid in 10 mL was 0.05x.
To change this to a M concentration, we need to scale up from 10 mL to 1 L, which means
multiplying 0.05x by 1/0.01 to give our concentration as 5x M.
We’ve seen how to make a volumetric solution and accurately calculate the concentration
of an unknown solution – this is crucial to analytical chemistry, which can be used
in a forensic examination.
As well as that, we’ve looked at acids and bases, and understanding those is important
to many chemical reactions which involve making and breaking bonds with hydrogen atoms.