Ammonia containing sensors

Another classification is based on the analytes that occur in gas or liquid phases. The CL gas sensors are applied to the analysis of compounds in gases or vapors, such as 02, 03, NO, N02, chloride, ammonia-containing compounds... 

American scientists prepared the organo-silica sol-gel membranes60 and demonstrated in a single layer format for pH measurement and multiple-layer format for both C02 and NH3. The sensors used a hydroxypyrenetrisulfonic acid (HPTS) as the indicator immobilizes in a base-catalyzed sol-gel containing poly(dimethyl)siloxane, aminopropyl-triethoxysilane (APTES) and tetraethylorthosilicate (TEOS). This indicator gel was over coated with a hydrophobic sol-gel to reduce cross reactivity to pH when either carbon dioxide or ammonia were examined. 

Among potentiometric enzyme sensors, the urea enzyme electrode is the oldest (and the most important). The original version consisted of an enzyme layer immobilized in a polyacrylamide hydrophilic gel and fixed in a nylon netting attached to a Beckman 39137 glass electrode, sensitive to the alkali metal and NHj ions [19, 2A Because of the poor selectivity of this glass electrode, later versions contained a nonactin electrode [20,22] (cf. p. 187) and especially an ammonia gas probe [25] (cf. p. 72). This type of urea electrode is suitable for the determination of urea in blood and serum, at concentrations from 5 to 0.05 mM. Figure 8.2 shows the dependence of the electrode response... 

SCR is a process by which NO gases in diesel exhausts can be reduced to levels that will meet future legislation. SCR is based on the reduction of NO in the catalytic converter by the injection of ammonia or urea into the exhaust gases before they enter the catalytic converter, where NO and NHj react to form and H O. The ammonia injection process may be controlled by measuring either the ammonia or nitric oxide slip after the catalytic converter. Such an ammonia sensor should be able to tolerate contaminants such as particles in the exhaust gases and should show very low cross sensitivity to NO and HC. Typical diesel exhaust contains 3-9% CO 50-250 ppm CO, 6-12% O, 200-1,000 ppm NO, and 130-260 ppm HC. Furthermore, the response to NH3 should have a time constant in the order of 1 second. 

Konig et al. [80-84] demonstrated that microbial sensors are suitable for the summary quantification of nitrifiable compounds (see also Sect. 3.3.1) as well as for the detection of nitrification inhibiting effects. Such biosensors, which contain a mixed population of the nitrifying bacteria Nitrosomonas sp. and Nitrobacter sp., exhibit a specific supplementary metabolic capacity. This enables the amperometric determination of ammonia according the following scheme of nitrification ... 

Several combined electrodes have been developed to measure dissolved gases such as carbon dioxide (CO2), ammonia (NH3) and sulphur dioxide (S02). These sensor-like electrodes contain an internal solution that is isolated from the external solution by an impermeable membrane. The membrane forbids the passage of water or ions but is transparent to gas molecules dissolved in solution (Fig. 18.4). 

The steady-state current depended on the concentration of ammonia. A linear relationship was observed between the current decrease and the ammonia concentration below 42 mg l-1 (current decrease 4.7 iA). The minimum concentration for the determination of ammonia was 0.1 mg 1" (signal to noise, 20 reproducibility, 5 %). The current decrease was reproducible within 4 % of relative error when a sample solution containing 21 mg 1 of ammonium hydroxide was employed. The standard deviation was 0.7 mg 1 in 20 exper-ments. The response time of the sensor was within 4 min. 

The gas-sensing electrodes also are used for the potentiometric measurement of biologically important species. An enzyme is immobilized at or near the gas probe. The gas sensor measures the amount of characteristic gas produced by the reaction of the analyzed substance with the enzyme. For example, an enzyme electrode for urea [NH2C(0)NH2] determination is constructed by the immobilization of urease onto the surface of an ammonia-selective electrode. When the electrode is inserted into a solution that contains urea, the enzyme catalyzes its conversion to ammonia ... 

Gas-sensing electrodes are examples of multiple membrane sensors these contain a gas-permeable membrane separating the test solution from an internal thin electrolyte film in which an ion-selective electrode is immersed. For example, for the ammonia sensor, the pH of the recipient layer is determined by the Henderson-Hasselbach equation [Eq. (18)], derived from the chemical equilibrium between solvated ammonia and ammonium ions ... 

The catalytic detection of ammonium ions has not been extensively investigated in contrast with the large variety of potentiometric and amperometric chemical sensors and optical sensors described in the literature [236], Similarly, the detection of ammonia in air has merited diflierent approaches in the field of chemical sensors. Screen-printed electrodes modified with Meldola s Blue and covered with a polycarbonate membrane constitute the basis of the catalytic detection of NHj. The measurement is based on the electrocatalytic reduction of NADH upon addition of glutamate dehydrogenase to a stirred solution containing NADH, 2-oxoglutarate and ammonium ions. The rate of current decrease (nA s ), measured at 50 mV, correlates to the concentration of ammonium ions in the sample. Recoveries of ammonium ions in spiked pond and tap waters at the level of 0.1 ppm are close to 100%, which demonstrates the feasibility of this assay for the detection of ammonium ions in waters [237],... 

Extensive research and development of microbial sensors has been carried out by Suzuki et al. (89-94) and Rechnitz et al. (95-97) (see Table III). Microbial sensors consisting of membrane-bound whole cells and an oxygen electrode were constructed for the determination of substrates such as assimilable sugars, acetic acid, alcohols and ammonia, and for the estimation of biochemical oxygen demand (BOD) (98-104). Glutamic acid was determined with a microbial sensor which consists of membrane-bound whole cells containing glutamate decarboxylase and a carbon dioxide gas electrode. These microbial sensors have been applied and evaluated for on-line measurements in fermentation processes (105,106). 

In potentiometric enzyme electrodes lyases producing carbon dioxide or ammonia are used as terminal enzymes of sequences. In fact, the term enzyme sequence electrode was introduced on the occasion of the design of a potentiometric D-gluconate sensor containing gluconate kinase (EC 2.7.1.12) and 6-phosphogluconate dehydrogenase (EC 1.1.1.44) (Jensen and Rechnitz, 1979). The authors found that for such a sensor to function the optimal pH values of the enzymes and the transducer should be close to each other. Furthermore, cofactors, if necessary, must not react with one another nor with constituents of the sample. It was concluded that the rate of substance conversion in multiple steps cannot exceed that of the terminal enzyme reaction. A linear concentration dependence is obtained when an excess of all enzymes of the sequence is provided, i.e. complete conversion occurs of all substrates within the enzyme membrane. Different permeabilities of the different substrates results in different sensitivities. This is particularly important with combinations of disaccharidases and oxidases, where the substrate is cleaved to two monosaccharides of approximately the same molecular size. The above... 

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