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Hi everyone, in this video I will discuss the pH and Buffer lab, which you'll be doing next.
This lab has five components in it so please pay careful attention to all the videos because
each of the videos will describe one or two parts of the lab
that you'll be doing and all the different experiments that you'll be doing are
connected to the concept of
pH measurement
as well as
Buffers
which will discuss in the next chapter.
So the five parts of this experiment are
first one is: you're going to measure pH
using acid-base indicators.
the second part is also about pH measurement, but in this case you're going to use a pH
meter to do it
The third and fourth parts are both about calculating Ka,
For the third part you'd be calculating Ka using an ICE table,
for the fourth part you're going to be calculating Ka using the Buffer method,
and lastly for the fifth part, you're going to make a buffer,
at a specific pH that I would give to you.
So let's start with part 1 which is acid-base indicator.
Just for a general piece of information acid-base indicators
are compounds that change colors at a specific pH.
In this lab, you'll be given a series of acid-base indicators which have specific
ranges of pH.
By observing
the colors of specific acid-base indicators on specific solutions, you'll have an idea of what the pH of the solution should be.
Well here's an example of some acid-base indicators before we actually talk about
what you'll be using in the lab.
For example, the acid base indicators that you might be the most familiar
with is phenolphtalein,
which of course is shown in this picture right here.
where when it's acidic, it has no color and when it's basic it has a pink color.
And this will be
the color of phenolphtalein when it's in the range between the acidic and basic
condition
and that's has a particular value of pH.
In the case of phenolphtalein, it's in the range of pH 6-8,
between pH 6-8.
Methyl orange is another
pH indicator that you'll encounter.
And as you can see here, there are three
different colors ranging from acidic
color of methyl orange which is
more on the reddish side and a yellow color when it's on the basic side,
and then in-between you have this orange color.
So for methyl orange itself, the pH range is from 3.1-4.4.
So between that particular pH (range)
that's where the color is going to be the in-between color.
Lastly of course you're
familiar with litmus,
which also has a range
for pH.
And as you know in acidic media,
litmus will be red
and in
basic media, it will be blue.
You can have the in-between color which is purple
when you're just about neutral. So as a matter of fact for litmus, the
pH range is about neutral, so this is from
about (pH) 5-7.
Now let's talk about some
how to use this to narrow down the pH of the solution that you might have.
So let's take a look at this picture which lists
several pH indicators
some you might see in the lab itself, some you might not.
The value of
pH is given
here from 0 to 14.0
So here is neutral which is 7.0
So 1, 2, 3, 4, 5, 6
and then you could keep going to 14.0.
Now let's say you have a solution X which you don't know
the pH of
and you
put in a couple of drops of
methyl orange
which is the first indicator
and you see an orange color.
An orange color for methyl orange is neither red nor yellow, but it's somewhat
in the middle
so we can safely say that maybe it's between this pH range, which is
between 3.0 - 5.0.
When you take
another sample of solution X and you
put
a couple of drops of methyl red, which is our second indicator you see a color
of red,
which means that it has to be
around this area.
So then what you need to do to narrow down the range is just to
basically shade the areas so they overlap with each other.
So these areas overlap with each other which means that
the pH of this solution must be somewhere
around 3-5 in this case.
Now, can you narrow it down further?
You can add other indicators, maybe not the ones that are listed here but we have
other indicators that might help you narrow it down to between 3-4.
But just using these two
results we can narrow it down to about 3-5.
Now in the next experiment
you will be measuring pH with a pH meter, so
what I'll do is I'll discuss that in the next video.