INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS 575
There is an on-line instrument for laboratory and field
use to carry out ASV.^83 In the cell a glassy carbon electrode
is automatically coated with a thin film of mercury. (When
the replating of the electrode is necessary it is carried out
automatically.) A pump and a set of five solenoid stream-
switching valves direct sample and reagents to the cell. By
means of a microprocessor and keyboard all steps and ana-
lytical parameters can be facilitated. The detection limits are
1 ppb for bismuth, cadmium, copper, lead, thallium and zinc
that plate on mercury and 50 ppb for gold and silver that do
not plate on mercury and mercury itself.
A.c. voltammetry employs a variety of electrodes with
similar operations and response described in the polarogra-
phy section above.
Figures 26A and B give information relating to the detec-
tion limits in terms of the molarity of the analyte and the reso-
lution between peaks or waves expressed as volts. Therefore,
comparisons can be made among the techniques illustrated.(2) Amperometry
(a) Dissolved oxygen electrode
Dissolved oxygen, DO, in water is determined by DO
probes using galvanic or electrolytic type cells. These
DO probes have their electrodes protected by an oxygen
permeable membrane and are referred to as membrane elec-
trode systems. These systems contain a cathode working
electrode and a Ag/AgCl reference electrode and an electro-
lyte of 1M KCl.
In an electrolytic probe the working electrode may be a
platinum (Clark electrode^84 ), gold, carbon, or silver micro-
electrode (see Figure 28). Gold seems to give the best service.
A membrane electrode system using a gold electrode, an Ag/
AgCl reference electrode with a KCl gel electrolyte, and a
polarizing voltage of 0.80 is commercially available.^85 The
cell reactions are as follows:
Gold cathodeO 2 H 2 O 4e → 4OH^ ^ (36)silver anode (Cl^ ^ ,a 1)4Ag 4Cl^ ^ → 4AgCl 4e. (37)The design, based on the Clark electrode, is commonly used
by a number of manufacturers. 85,86,87
A DO probe operating on the galvanic principle is one
developed by Mancy et al.^88 In this cell a spontaneous elec-
trochemical reactions occurs as follows:
Pb anode2Pb 60H^ ^ → 2PbOOH^ ^ 2H 2 O 4e (38)Ag cathodeO 2 2H 2 O 4e → 4OH^ ^. (39)The electrolyte, 1 M KOH, is used since hydroxide, OH^ ^ ,
ion is formed during the reduction of the oxygen. For higher
sensitivities a saturated potassium bicarbonate electrolytesolution is employed because of the insolubility of the lead
salt formed. A 1 mil, polyethylene membrane separates the
cell from the analyte sample. Several galvanic electrode sys-
tems are available. 89,90,91
The DO probe output current, from both types of cells,
is sensitive to temperature changes. To compensate for this
effect, thermisters are placed in series with the load circuit^92
or in a bridge circuit.^93
(c) Potentiometric, amperometric and conductometric
titrations
Electrochemical means are used in potentiometric,
amperometric and conductometric titrations to determine
the endpoint of a titration. These titrations are so named to
indicate the mechanism of endpoint determination. The elec-
trodes used in the potentiometric and amperometric methods
are sensitive to analyte and/or titrant redox species; whereas
in conductometric titrations total ion content is detected.
In amperometric titrations the magnitude of current flow is
determined as a function of the volume of titrant. The elec-
trode potentials are measured as a function of the titrant
volume in potentiometric titrations. Conductance measure-
ments are used in conjunction with titrant volumes in this
third method.(1) Potentiometric titrations^94
The electrodes discussed in the previous section on
potentiometric instruments may be used in potentiomericTHERMISTORSCAPTEFLON MEMBRANE GOLD CATHODEELECTROLYTESILVER ANODEFILL PORT
SCREWPVC BODYCONNECTORFIGURE 28 Clark DO membrane probe. (Courtesy of Rosemount
Analytical, Inc., La Habra, CA.)C009_005_r03.indd 575C009_005_r03.indd 575 11/23/2005 11:12:27 AM11/23/2005 11:12:27