Page 375 - Instrumentation Reference Book 3E

358 Chemical analysis: electrochemical techniques

solutions of a weak base and one of its salts, such
as ammonium hydroxide mixed with ammonium
chloride (as explained above in Section 17.4.6),
undergo relatively little change of pH on the
further addition of acid or alkali and the pH is
almost unaltered on dilution. Such solutions are
called buffer solutions; they find many applica-
tions in quantitative chemical analysis. For ex-
ample, many precipitations are made in certain
ranges of pH values, and buffer solutions of dif-
ferent values are used for standardizing pH meas-
uring equipment.
Buffer solutions with known pH values over a
wide range can be prepared by varying the pro-
portions of the constituents in a buffer solution;
the value of the pH is given by

The weak acids commonly used in buffer solu-
tions include phosphoric, boric, acetic, phthalic, Solution Solution
succinic, and citric acids with the acid partially
containing i( containing 1 Metal 2
neutralized by alkali or the salt of the acid used Metal 1 ions of metal 1 ions of metal 2
directly. Their preparation requires the use of
pure reagents and careful measurement and
weighing, but it is more important to achieve cor-
rect proportions of acid to salt than correct con-
centration. An error of 10 percent in the volume of
water present may be ignored in work correct to
0.02 pH units.
National Bureau of Standards (USA) standard
buffer solutions have good characteristics and for
pH 4. pH 7, and pH 9.2 are available comnier-
cially, as preweighed tablets, sachets of powder or
in solution form. Those unobtainable commer-
cially are simple to prepare provided analytical
grade reagents are used, dissolved in water with a
specific conductance not exceeding 2 pSlcm.

17.5 Electrode potentials

17.5.1 General theory
When a metallic electrode is placed in a solution,
a redistribution of electrical charges tends to take
place. Positive ions of the metal enter the solution
leaving the electrode negatively charged, and the
solution will acquire a positive charge. If the
solution already contains ions of the metal, there
is a tendency for ions to be deposited on the
electrode, giving it a positive charge. The elec-
trode eventually reaches an equilibrium potential
with respect to the solution, the magnitude and
sign of the potential depending upon the concen-
tration of metallic ions in the solution and the Zinc/ Co@er
nature of the metal. Zinc has such a strong ten- sulpha fe sulpha fe
dency to form ions that the metal forms ions in all Figure 17.13 Daniell cell.