NSWC/WOL/TR 75-3 5 



their use is also quite similar to that for the glass pH electrode. 

 They are used with a reference electrode, such as saturated calomel 

 or silver-silver chloride, and a high impedance potential measuring 

 device, such as a pH meter with a millivolt scale. A calibration 

 plot of voltage versus the logarithm of concentration of the test 

 ion is made, generally for at least a 10-fold range of concentra- 

 tion. The voltage for the test solution is measured, and the test 

 ion concentration is then calculated from the calibration plot. For 

 on-the-spot, plant or field measurements, a portable, direct-reading 

 specific ion meter is used (Orion Series 400 specific ion meters) . 



The major advantages of specific ion electrodes for use in 

 water analyses are their speed and ability to be used for in-situ 

 monitoring. Minimal sample and reagent preparation prior to analysis 

 are required. The method also offers a wide concentration range, 

 and a precision and accuracy comparable to other accepted methods. 

 The system is relatively inexpensive. A portable, direct-reading 

 specific ion meter costs $450 and prices of individual specific ion 

 electrodes range from $150 to $300. The cost for analysis is 

 approximately $10 per sample. 



a. Ammonia 



The use of an ammonia specific ion electrode in the 

 determination of ammonia in surface waters, sewage samples, and 

 saline waters has been recently developed. 19 The electrode is an 

 Orion Model 95-10 gas sensing electrode, incorporating an internal 

 reference electrode and a diffusion-type membrane. 



Dissolved ammonia from the sample diffuses through a highly 

 gas-permeable membrane until a reversible equilibrium is established 

 between the ammonia level of the sample and the internal filling 

 solution. Hydroxide ions are formed in the internal filling solution 

 by the reaction of ammonia with water, 



NH 3 + H 2 « NH4 + +OH- 



The hydroxide level of the internal filling solution is measured by 

 the internal sensing element and is directly proportional to the 

 level of ammonia in the sample. The electrode potential is 



19 Thomas, R. F. and Booth, R. , "Selective Electrode Measurement of 

 Ammonia in Water and Wastes", Environ. Sci. Technol., 1_, 523 (1973) 



45 



