NSWC/WOL/TR 75-3 5 



essentially Nernstian with respect to the ammonia level and 

 approximately follows the equation: 



E = Eo - S log (NH 3 ) 



E = electrode potential in millivolts 



Eo = approximately 180 mv (varies slightly from 

 electrode to electrode) 



S = the Nerst slope, 58 mv/decade at 2 9°C 



The ammonia electrode is almost totally interference free. 

 Anions, cations, and common gases (carbon dioxide, sulfide, cyanide, 

 and sulfur dioxide) do not interfere. Sample color, turbidity, and 

 the presence of suspended solids do not affect the measurement. 

 Used with an expanded scale pH/mv meter, the electrode is capable 

 of precision equal to the best color imetric methods for ammonia. 

 The sensitivity is approximately 10 _ 6 moles/liter (20 ppb) . 



b. Cyanide 



The cyanide specific ion electrode has been used for rapid, 

 direct measurement of cyanide ions in waste waters. The cyanide 

 electrode uses a membrane of silver sulfide and silver iodide. 

 Cyanide ion in the sample reacts with the silver iodide in the 

 membrane to release iodide into the solution. The iodide at the 

 membrane surface is proportional to the level of cyanide and is in 

 turn sensed by the electrode: 



2CN~ + Agl „ Ag(CN)~ + I~ 



As a consequence the electrode responds to cyanide with Nernstian 

 behavior : 



E = Eo + S log(CN~) 



where S is the electrode slope (about 59 mv for a ten-fold increase 

 in the cyanide level at 25°C) . The lowest level of cyanide that can 

 be measured is about 50 ppb. 



20 



Riseman, J. H., "Electrode Techniques for Measuring Cyanide in 



Waste Waters", Am. Lab., 4, 63 (1972). 



46 



