6.3 Percent (%) Solutions
The % solutions can be expressed as below:
(i) Weight/volume (%, w/v): It is the weight of a solute in grams dissolved in
100 ml of solution, i.e., 4 g pellet of NaOH is dissolved in 100 mlfinal volume
of distilled water.
(ii) Volume/volume (%, v/v): The volume of a solute in ml dissolved in 100 ml of
solution, i.e., 20% ethanol (v/v) means that 20 ml of ethanol is mixed with 80 ml
of distilled water to makefinal volume 100 ml.
6.4 Buffer Solutions
A buffer is defined as the solution that resists changes in the pH upon addition of
small amount of acid or base. A buffer is anaqueous solutionof a weak acidand its
conjugate baseor weak base and its conjugate salt. The role of buffers in maintaining
the pH of a solution can be explained with the aid of the Henderson-Hasselbalch
equation.
pH¼pKaþlog½conjugate base
½conjugate acideThe conjugate acid and conjugate base act together to resist large changes in pH
by partially reacting with H+or OHions added to the buffer solution. Let us
consider the example of acetate buffer that contains acetic acid and sodium acetate.
Acetic acid ionizes weekly, and sodium acetate ionizes to a large extent in solution.
When H+ions (i.e., HCl) are added to this buffer solution, the acetate ions present in
buffer bind with H+to form acetic acid, and the H+ions are taken out of circulation.
The acetic acid is ionized very weakly, and hence, pH change due to addition of HCl
is resisted by buffer. Similarly, if OH ions (i.e., NaOH) are added to buffer solution,
the H+ions present in buffer combined with OHions, and OHions are taken out
of circulation, and pH change is again resisted by buffer. The amount of change in
pH of buffer depends on the strength of the buffer and the ratio of conjugate base/
conjugate acid. The efficiency of a buffer to resist changes in pH on addition of acid
or base is called as the“buffer capacity.”Buffer capacity can be defined as the
number of moles of H+or OHthat must be added to 1 litre of the buffer in order to
change the pH by one unit. The buffering capacity of the buffer solution is maximum
when pH¼pKa, i.e., when the concentrations of conjugate acid and conjugate base
are equal. In general, buffers should not be used at a pH greater or lower than 1 pH
unit from their pKa. The most commonly used buffers in the laboratory are acetate
buffer, phosphate buffer, citrate buffer, and Tris buffer.
24 6 Preparation of General Laboratory Solutions and Buffers