Buffers

A buffer is a system of molecules and ions that acts to prevent changes in H+ concentration and thus serves to stabilize the pH of a solution. In blood plasma, for example, the pH is stabilized by the following reversible reaction involving the bicarbonate ion (HCO3-) and carbonic acid (H2CO3):

The double arrows indicate that the reaction could go either to the right or to the left; the net direction depends on the concentration of molecules and ions on each side. If an acid (such as lactic acid) should release H+ into the solution, for example, the increased concentration of H+ would drive the equilibrium to the right and the following reaction would be promoted:

Table 2.3

The pH Scale

H+

OH-

Concentration

Concentration

(Molar)*

pH

(Molar)*

Acids

I.o

o

io-14

o.i

l

lo-13

o.oi

2

lo-12

o.ooi

3

lo-11

o.oooi

4

Io-|°

lo-5

S

io-9

lo-6

6

lo-8

Neutral

lo-7

7

lo-7

Bases

lo-8

8

lo-6

lo-9

9

Io-S

io-|0

lo

o.oooi

lo-11

ll

o.ooi

lo-12

l2

o.oi

lo-13

l3

o.i

lo-14

l4

I.o

*Molar concentration is the number of moles of a solute dissolved in one liter. One mole is the atomic or molecular weight of the solute in grams. Since hydrogen has an atomic weight of one, one molar hydrogen is one gram of hydrogen per liter of solution.

*Molar concentration is the number of moles of a solute dissolved in one liter. One mole is the atomic or molecular weight of the solute in grams. Since hydrogen has an atomic weight of one, one molar hydrogen is one gram of hydrogen per liter of solution.

Chemical Composition of the Body

Notice that, in this reaction, H+ is taken out of solution. Thus, the H+ concentration is prevented from rising (and the pH prevented from falling) by the action of bicarbonate buffer.

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