Buffers our friends

"Why do we have to learn about buffers? Is it really necessary to understand the acid dissociation constant?"

Certainly, you have heard those questions from your students.

Thus, before you derive the equations start with a simple experiment and explain about our blood system. Without a buffer system in our body even drinking a small amount of sweet drinks (Coca-Cola...) would be devastating for our health. But we are save (http://ayurveda.md/education?id=124).

Observational learning is very important, and everyday examples will spur the interest of the students.

In our laboratory we perform an experiment with Bromothymol blue, a chemical indicator used to detect weak acids and bases. It displays a color change from the protonated form (HBB) to the non-

protonated form (BB-).

Consequently, at low pH (acidic), the solution will be yellow, while at higher pH the solution will be blue.

The students follow the pipetting scheme

and determine qualitatively the color change.

Afterwards, the spectrophotometer is used to quantitatively determine the acid dissociation constant.

Some interesting web-pages:

"Experiment 1: The pH of solution A is 7.0 i.e. it’s neutral. When we add 10 mL of 0.2M HCl to it, the pH decreases to 1.5. On the other hand, when we add 10 mL of 0.2M NaOH to solution A the pH shoots up to 12.5. In both these cases we see a drastic change in pH due to either the increase or decrease of proton concentration. Remember, pH of a solution is dependent on the concentration of hydronium ions.

Experiment 2: Let’s see what happens when we repeat the same experiment with solution B, whose pH is also maintained at 7.0. When we add 10 mL of 0.2M HCl to it, the pH decreases by only 0.2 units to 6.8. Next, when we add 10 mL of NaOH to solution B, the pH just slightly rises to 7.2 from 7.0. In both these cases we do not see a drastic change in pH, as we observed with solution A.

So, our observation from this experiment is that solution A underwent drastic changes in pH upon addition of a strong acid or a strong base, while solution B resisted a change in pH.

Can you guess what the difference might be between solutions A and B?

Solution A is pure ‘water’, while solution B is a ‘buffer’."

The pH of blood is maintained at ~ 7.4 by the carbonic acid – bicarbonate ion buffering system.