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Buffered Solutions - Concept

Teacher/Instructor Jacqueline Spivey
Jacqueline Spivey

Ph.D.,U.C.Santa Cruz
Teaching at a top-ranked high school in SF

She teaches general and chemistry at a top-ranked high school in San Francisco. Prior to that, she lead and published a number of research studies and lectured at SF State University.

Buffered solutions are solutions that are buffered by the presence of weak acid and a conjugate base. Buffered solutions resist change in pH when acids or bases is added to it by reacting with any added hydrogen or hydroxide ions so that they do not accumulate. Therefore, buffered solutions are used to keep pH at a constant value.

So in this segment let's talk about buffered solutions, so this is riding on the heels of acid base and acid base equilibrium so one thing I want to say in the beginning is to recall that pH is equal to minus the log of the concentration of protons of H+ and so when you increase the concentration of protons your pH goes down and vice versa.

Okay so what then is a buffered solution? A buffered solution is a solution that resists change in its pH even when you add a strong acid or a strong base so remember that strong acids disassociates completely in solution giving you an increase in proton concentration and strong bases disassociates in solutions to give you a strong increase in hydroxide ions okay so usually if a solution is not buffered you dump in a bunch of protons then your pH is going to go down making it more acidic, if you dump in a bunch of hydroxide ions your pH should go up making it less acidic so how do buffered solutions manage to keep the pH in the range that you want? So basically what happens is that they react with any added protons or hydroxide ions so that these ions do not appreciably accumulate in solution so how do we do that? Well when you're making a buffered solution, you want to make it with a weak acid and its conjugate base so for this first example the weak acid will be represented by HA and indicating that this is the proton will be be be lost and the conjugate base will be A- when you loose this proton in solution.

Okay so then if you have a buffered solution and you add a proton an H+ it will react with the base with the conjugate base and so then you have the proton plus the conjugate base to give you back your weak acid. Okay so likewise if you added a strong base, they will give you hydroxide ions then it will react with the weak acid HA so you have your hydroxide ions interacting with your weak acid to form water and your conjugate base so this allows your solution to be what we call buffered. So lets do a little more in depth example of that so lets consider a mixture of acidic acid which is HC2H3O2 and sodium acetate which is the sodium in front C2H3O2, remember sodium acetate the conjugate base when its in solution will break that into its ionic species so sodium +1 acetate -1 okay so acidic acid here is our weak acid for our buffered solution and our conjugate base would then be sodium acetate. At some point you're probably thinking to yourself where do the sodium go? And if you recall from your understanding of just general Chemistry reactions sodium is a spectator ion so its just kind of around and its important just re-bouncing the charge on that acetate ion.

Okay so assuming we have this mixture of this weak acid acidic acid and this conjugate base sodium acetate, what happens if we add a strong acid say HCl? Okay so a strong acid again HCl is going to completely disassociates in solution to give you protons H+ and chlorine ions Cl- okay so normally you dump into solution you're going to increase your concentration of protons and thereby decreasing your pH making it more acidic so but this buffered solution that we have has this acetate ion right with the minus charge and once we dump in our protons plus then what are we going to make? We're just going to make more of our weak acid, acidic acid so therefore basically this acetate ion comes in kind of captures the protons that we put in so there's no appreciable build-up of the concentration of protons and therefore we don't have an appreciable change in our pH. It doesn't become more acidic in this case so let's do the other side of that what happens if we add a strong base in the form of sodium hydroxide to this mixture of acidic acid and sodium acetate? So again a strong base much like a strong acid is going to disassociate completely in solution so sodium hydroxide would disassociate to give us a sodium ion with a plus charge and a hydroxide ion with a minus charge and so normally if it wasn't a buffered solution we dump this in to make our solution more basic or to basically increase the pH right because an increase in hydroxide ions makes the pH go up which makes it more basic.

In a buffered solution though again remember here we have our acidic acid so HC2H3O2 and then we add in our hydroxide ions and these guys right here this base basically the hydroxide ion is a better base or stronger base than the acetate and so it is going to remove this proton from the acidic acid to make water and then we have acetate ion leftover so again we still have the basic makeup of our initial solution right here we've got some water and then we've got acetate ion lying around okay so again basically when we dumped in the strong base the hydroxide ions, remove the proton from the acidic acid to make the acetate conjugate base and again our solution has not taken any appreciable changes in its pH so that's pretty much the basics of how you do acid base.