The determination of the total inorganic nitrogen content is of great analytical importance in pollution and process control, and in food, clinical and agricultural analyses. Provided that the total nitrogen is present as inorganic nitrate, nitrite or as ammoniacal nitrogen, the content may be determined by conventional procedures, which have been greatly simplified by the advent of ammonia gas-sensing electrodes. As the gas electrode measurement is rapid, the time required for analysis is substantially reduced compared to the conventional methods involving distillation. If organic nitrogen-confining compounds are present, the conventional methods are not applicable. In order to preserve all the advantages of ammonia gas-sensing electrodes, the reduction procedure used must be capable of reducing inorganic nitrogen-containing compounds selectively to ammonia.
Although the use of ammonia gas sensors is very attractive, practical difficulties -such as poor mechanical properties of the membrane and possible clogging of the membrane pores -often limits the utility of these highly selective sensors. These shortcomings and limitations are eliminated in the recently introduced air-gap electrode which has been used successfully in various determinations, such as of ammonium ions in waste water [l] and in serum [ 2 J ; of urea IS] (via. enzymatic degradation) and of hy~ogenc~bonate [ 2 J in full blood, plasma and serum; of suIphur dioxide in wine [ 41; and of the total inorganic and the total organic carbon contents in waters [ 5 1. The air-gap electrode is based on the same principle as other gas sensors, except that it has no gas-permeable membrane. The membrane is replaced by an air gap which separates the surface of the indicator electrode from the sample solution, the entire system being contained in a gas-tight measuring chamber. The electrode is covered by a very thin film of electrolyte, the pH of which is being measured. The greatest advantage of this design is that the electrode never comes into direct physical contact with the sample solution, which can therefore contain components that normally affect the function of all other gas electrodes. Simplicity of design, ease of renewal of electrolyte, and fast response are further advantages gained by replacing the gas-permeable membrane with the air gap.