,TYPICAL SLOPE ~ I MEGOHM 

 Co 



4 0.6 0.8 



ELECTRODE BIAS -VOLTS 



Fig. 2. The concentration polarization response 

 of a simple solion diode. 



AD 



n FN 



(1) 



where A is effective area of cathode electrode, 

 W is iodine concentration or normality, JL is 

 effective thickness of diffusion layer, n is 2, 

 number of electrons involved in reaction and 

 F is Faraday constant, 96,500 coulombs/g-mole. 



The diffusion coefficient, D, is given by 



KT 

 a 



(2) 



Fig. 3- Solion flow detector. 



the shape illustrated makes greatest use of the 

 constant current feature. 



Another unusual feature of the solion is the 

 "double" source impedance characteristic. 

 Although the "signal" output appears to originate 

 from a 1 megohm source, the cell appears as a 

 100 ohm source to any 60 cps "pickup" that might 

 appear in the external electrical circuit. 



"Reverse" voltage characteristics of the diode 

 are very similar to those shown in the curve of 

 Fig. 2. The anode and cathode leads are now 

 interchanged with the cathode becoming an elec- 

 trode of 10 times the previous area. The polari- 

 zation curve will again have a similar shape 

 except that the limiting current will be approxi- 

 mately 10 times the "forward" limiting current, 

 due strictly to the increased area of the cathode 

 electrode. The "front to back" ratio is there- 

 fore primarily determined by the ratio of the 

 electrode areas. Values in excess of 100:1 have 

 been obtained . 



where T is absolute temperature in degrees Kelvin, 

 a is viscosity of the solution and K is a con- 

 stant for the particular system. Therefore, for 

 a solion diode the limiting current is primarily 

 determined by the cathode area (a cathodic con- 

 trolled reaction), the iodine concentration and 

 the absolute temperature. 



The electro rhemical reactions occurring at 

 the electrodes consist of reducing iodine at the 

 cathode and oxidizing iodide back to iodine at 

 the anode. The process is illustrated by the 

 following: 



At the cathode: Ip + 2e -» 21" 



At the anode: 21 -» I 2 + 2e 



(3) 



CO 



The electrode material itself has not entered 

 into the reaction and the completely reversible 

 process can continue indefinitely. The shape of 

 the "knee" of the curve can be adjusted by 

 varying the potassium iodide concentration but 



THE EFFECTS OF HYDROACOUSTIC FLOW 



The diode has been discussed to illustrate 

 some of the basic principles of the electro- 

 chemical system utilized in the solion. The 

 assumption was made that hydraulic flow was 

 absent. By proper design, the solion can be 

 utilized as a hydraulic flow or pressure detector. 



Consider the design of a solion, similar to 

 that indicated in Fig. 3- The rigid plastic 

 body of the diode has been replaced with a cell 

 of different design. The cathode element is now 

 mounted in a solid web at the center of the 

 plastic housing so that any fluid flow between 

 chambers is possible only through the cathode 

 electrodes. The electrolyte solution is con- 

 strained by compliant diaphragms which permit 

 limited volume flow between chambers . 



Since the cathode structure tends to obstruct 

 any flow of fluid between chambers it is con- 

 sidered as an acoustic resistance, R, measured 



l6k 



