I have a demonstration of pH buffers, and I'm going to compare a buffer to pure water,
and I'm going to start off with four solutions that are at pH 7—they're neutral solutions.
The water is mixed with a universal indicator, and a universal indicator is a mixture of indicators
that has a color of violet at basic pH; neutral, it's green; and it's orange
at acidic pHs, and the range is about from 11 to 7 to pH 3 for those color changes. I've got
hydrochloric acid, 1 molar with a dropper, and sodium hydroxide, one molar with a dropper,
so that we can add just a tiny amount of acid or base to see how it changes the pH, and we're going
to observe that pH change by observing the color change. I'm going to start off with the water...
...and i'm going to add a drop each of acid on the left—on my left—and base on my right...
So you can see where they dropped, there's a color change, but i'm going to mix them up by swirling,
and you can see that that one drop of acid, one drop of base has changed the pH of those water
solutions significantly, from seven to around three, from seven to around 11. Now we're going
to take a look at the buffer, and the buffer is a mixture of acetic acid and ammonia, it has a
buffer pH of around seven, and it will buffer against changes in acidity or basicity, so...
I'm going to do the same addition of a single drop of acid and base to compare...
You can see that locally you get a little bit of a change, but once I swirl it up
it's like I didn't change anything at all,
those solutions are both neutral. So now, I'm going to add one two three four five drops
and one two three four five drops, and let's see what happens...
So even adding a significant amount of acid and base
only changes the pH a little! Now, to my eye, this side is starting to look a little bit yellow, and
this side is starting to look a little bit bluish, from you know, so they're...they're diverging
away from the green color that they started, but still it's taken a whole lot of addition to change
those colors. Now buffers aren't magic, if I add more acid or base than the buffering capacity,
then I'm going to be able to get these solutions to change color—change
pH. Once I exceed the buffering capacity of these mixtures,
then as you can see on the acid side, I pushed right past it; on the base side, not so much.
So what we can look at though, is these are still buffered, right? If I add a couple of drops
of base to the water, and a couple of drops of acid to the base, and mix them up...
...then they're still...
...really sensitive to the addition.... The hydrochloric acid and the sodium hydroxide
form salts, so it's an it's these are strong acids and a strong acid and a strong base,
so they form a neutral conjugate, they form a neutral solution, so I can just push these guys
back and forth without a problem. However if I add acid and base to the buffered solution...
...you can see that...
...they're heading back towards neutral, but the pH doesn't change as much as with the pure water.
So this is a demonstration of of a buffer, and how they resist changes in pH, and of
water pure water, very sensitive to changing pH on addition of a strong acid or a strong base.
Thanks for coming to lecture hall B in the Science Center, COVID era! aaahhhhh