on the Carritt-Kanwisher data, which is design- 

 ed for fishery biologists . Many biologists with 

 small allotments cannot afford the expense of 

 this instrument. Therefore, we developed 

 circuitry and construction details for a home- 

 made instrument which measures both oxygen 

 and temperature and has manual temperature 

 compensation. 



The construction and operational details 

 are given below, but for a more comprehensive 

 discussion of polarographic electrodes of this 

 and other types, one should consult the refer- 

 ences cited in this paper. Only a minimum 

 amount of theory is necessary to construct and 

 operate the device described here . 



THEORY 



The polarographer is usually interested 

 in the complete polarographic wave which is 

 formed when an electrode pair is operated at 

 various voltages differing from each other by 

 about 0.1 volt. However, in the case of oxygen 

 determination in content or tension, the fishery 

 biologist is interested only in the amount of 

 diffusion current. A steady diffusion current 

 is established within a definite voltage range for 

 various chemicals and this range is easily 

 established; in fact, the data are already in the 

 literature for most common elements and com- 

 pounds. For oxygen, in 0.1 N KCl electrolyte 

 with bare platinum vs. a silver-silver chloride 

 electrode, the diffusion current plateau occurs 

 between about -0.3 and -0.75 volts. With the 

 plastic covered Carritt-Kanwisher electrode 

 (in 0.5 N KOH electrolyte) the steady diffusion 

 current occurs between -0.75 and -1.15 volts 

 (fig. I). Carritt and Kanwisher (1959) gave 

 similar data and demonstrated (f i g. 2, their 

 paper) that the current-voltage curve shifted to 

 the rig^t (i.e. more negative) with increasing 

 temperatures. Their data and ours indicate 

 that the best operating voltage for water temper- 

 ature from 4.5° C. - 30° C, wi± 0.5 N KOH 

 as the electrolyte, is between -0.9 and -1.2 

 volts . Indeed, in actual practice any voltage 

 within this range gives satisfactory results. 

 The difference in diffusion current voltages in 

 KCl and KOH probably depends on the pH of the 

 electrolyte, changing the voltage at which de- 

 composition currents of the electrolyte will occur. 



The operational voltage on the cell in 

 our instrument is established by the combin- 

 ation of resistors R-3 and R-4 (fig. 3, 

 Schematic) which acts as a fixed voltage divider, 

 since the complete polarographic wave is not 

 wanted and a variable voltage is unnecessary. 

 If a battery of 1.5 v. is selected and an operating 

 voltage of -1.0 volt is desired, then: 1.0:0.5:: 

 R4 : R3 : therefore, R4 and R3 can have ohmic 

 values that have the approximate ratio of 1: 0.5. 



Figiire 3: — Schematic diagram of the 

 oxygen-temperature circuit. 



In actual practice we use 1800 ohms for 

 R3 and 3600 ohms for R4. One can install a 1 .5 

 volt battery and operate at -1 .0 v.; a 1.4 mer- 

 cury battery and operate at -0.0 volts. Mercury 

 batteries hold their voltages very well under load 

 and continuous operation, and should be employed 

 if possible. These batteries will maintain the 

 calibration point for many hours or days, which 

 eliminates the need of a variable voltage divider 

 and attendant voltmeter circuit to check the op- 

 erating voltage . 



