wash in hydrochloric acid. A potential is applied between the two electrodes to m a i n t a i n the p l a t i n u m at t h e cor rect potential for t h e chlorine reduc tion. O t h e r chlorine m e a s u r e m e n t cells t h a t have been successful include the gold/copper cell which, acting galvanically, requires no externally a p plied potential. T h e gold working elec t r o d e will dissolve very slightly in t h e chlorine-containing samples, a n d this helps to keep it clean. Membrane-clad cells are similar in principle to other voltammetric cells except t h a t t h e working electrode is separated from the sample by a gaspermeable hydrophobic m e m b r a n e . T h i s restricts t h e i r application t o those dissolved gases t h a t m a y be measured voltammetrically, in prac tice, oxygen a n d chlorine. T h e great advantage of these sensors, which has m a d e t h e m the preferred type of sen sor for these gases in most process control applications, despite t h e slight increase in response time caused by t h e m e m b r a n e , is t h a t t h e working electrode is protected from fouling a n d poisoning. T h e m e m b r a n e also improves t h e selectivity of the mea s u r e m e n t by preventing ionic redox species in the samples [e.g., Cr(VI)] from interfering. For particularly dirty samples t h e sensor is incorporated in
a flow-chamber giving him sample ve locity across t h e m e m b r a n e . T h i s re duces t h e rate a t which material col lects on the sensor. T w o typical constructions of m e m brane-clad voltammetric cells are shown in Figure 4. T y p e Β is a twoelectrode (Ag/Pb) galvanic cell as orig inally reported by M a c k e r e t h (9). T h i s design involves a particularly large cathode t h a t makes t h e cell both sen sitive a n d robust. I t is widely used in t h e water industry. For boiler feed water analysis, a version of this cell has been developed (10) in which its sensitivity is greatly increased by use of a m u c h more permeable m e m b r a n e . T h e cell is built into a monitor flow system to ensure a high sample veloci ty across t h e m e m b r a n e . T h e monitor m a y be used to measure oxygen con centrations in t h e range 0-20 μg/L·. T h e other t y p e of cell depicted (Type C) is a three-electrode cell (11). T h i s cell has t h e advantage of an easi ly replaceable F E P m e m b r a n e a n d m a y be used for either oxygen or chlo ride analyses in b o t h aqueous a n d gas eous process streams. It m a y be m a d e selective for chlorine in t h e presence of oxygen by lowering t h e cathode p o tential to approximately 0 V, a poten tial a t which t h e oxygen is not re duced. Applications t h a t have been
High Speed Spectroscopy
described include t h e monitoring of dissolved oxygen in t h e brine/lime slurry wastes from t h e a m m o n i a soda process (11). T h e sensitivity of t h e chlorine cell in aqueous samples is low unless t h e sample is strongly acidified to convert all chlorine species to dis solved chlorine gas. Acknowledgment T h e a u t h o r t h a n k s Electronic In s t r u m e n t s Limited for permission t o use Figures 1, 2, 3, a n d 4B. References (1) G. Mattock, "pH Measurement and Ti tration", Heywood, London, England, 1961. (2) G. Mattock, Trans. Soc. Instrum. TechnoL, 16,173 (1964). (3) P. L. Bailey, "Analysis with Ion-Selec tive Electrodes", Heyden, London, En gland, 1976. (4) P. Van den Winkel, J. Mertens, and D. L. Massart, Anal. Chem., 46, 1765 (1974). (5) E. L. Eckfeldt and W. E. Proctor, ibid., 47, 2307 (1975). (6) D. Warmoth and K. Porter, Kent Tech. Rev., 19, 17 (1977). (7) J. F. Haller, U.S. Patent 2,651,612 (8th Sept. 1953); I. R. Weingarten, U.S. Pat ent 3,694,338 (26th Sept. 1972). (8) R. J. Baker, Ind. Water Eng., 6, 20 (1969). (9) F.J.H. Mackereth, J. Sci. Instrum., 41, 38 (1968). (10) M. Riley and P, L. Bailey, Kent Tech. Rev., 11,7(1974). (11) P. O. Kane and J. M. Young, J. Electroanal. Chem., 75, 255 (1977). (12) J. E. Harwood, Water Res., 3, 273 (1969).
1024 Spectral Channels Simultaneously From The Princeton Applied Research OMA If your single channel technique is slowing you down, maybe it's time you joined the switch to the multi channel approach.
Write or call for our OMA brochures: Princeton Applied Research Corp., P.O. Box 2565, Princeton, NJ 08540 609/452-2111
OMA's (Optical Multichannel Anal yzers) feature the following: • Up to 1024 parallel channels • Spectral coverage from UV to thermal IR • Vidicon or Self-scanned Diode Array Detectors • Time resolved spectro scopy to 40 nanosecond resolution • Microcomputer data handling and much more.
Peter Bailey is the factory general manager at the factory of Electronic Instruments Limited in Richmond, where their potentiometric sensors PRINCETON APPLIED RESEARCH and pH meters are made. His re search interests include the develop EG&G COMPANY AN ment of gas-sensing membrane probes 421 and ion-selective electrodes.
Circle # 1 6 3 for A d d i t i o n a l I n f o r m a t i o n O n l y .
706 A · ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
