Ion-selective electrodes ammonium

Directions are provided for constructing and characterizing an ammonium ion-selective electrode. The electrode is then modified to respond to urea by adding a few milligrams of urease and covering with a section of dialysis membrane. Directions for determining urea in serum also are provided. 

Fig. 10. Response of an ammonium ion-selective electrode which was made from nonactin dissolved in a PVC membrane mounted in a Phillips electrode body (ISE-561, Glasblaserie Mitller, Zurich) referenced against an SCE to a spHt-stream EIA system, (a) EIA response to injected standards containing both glutamine and ammonium chloride lines A—E contain both glutamine and ammonium chloride E—G contain only glutamine, (b) Strip-chart...

The sensor is an ammonium ion-selective electrode surrounded by a gel impregnated with the enzyme mease (Figme 6-11) (22). The generated ammonium ions are detected after 30-60 s to reach a steady-state potential. Alternately, the changes in the proton concentration can be probed with glass pH or other pH-sensitive electrodes. As expected for potentiometric probes, the potential is a linear function of the logarithm of the urea concentration in the sample solution. 

FIGURE 6-11 Urea electrode, based on the immobilization of urease onto an ammonium ion-selective electrode. 

Fig. 7.6. The dependence of the potential of an ammonium ion-selective electrode to anunonium ions alone (o) and in the presence of 0.15 M Na, 0.005 M K, which is the appropriate ionic composition of blood serum ( ). (After Scholer and Simon [186].)...

Figure 4.15 — (A) Tubular flow-through electrode 1 Perspex body 2 conducting epoxy cylinder 3 mobile carrier PVC membrane 4 electric cable 5 channel (1.2 mm ID) 6 holders 7 screws 8 0-rings. (B) Schematic diagram of a system for on-line monitoring of ammonia ISE tubular flow-through ammonium ion-selective electrode R reference electrode W waste. (Reproduced from [137] with permission of the Royal Society of Chemistry).

Ammonium nitrate dissociates in aqueous solution to NHj and NO3 ions. The former may be measured by ammonium ion-selective electrode and the latter by nitrate ion-selective electrode. The solid may be heated carefully at low temperature (around 90°C) and the evolved ammonia and nitric acid vapors are absorbed in water and measured by selective ion electrodes, respectively. 

Elemental composition Ca 50.03%, C 14.99%, N 34.98. A measured amount of the compound is hydrolyzed with water. The product CaCOs is filtered, dried and determined by gravimetry. Calcium carbonate or the parent calcium cyanamide may be digested with nitric acid, diluted appropriately, and analyzed for Ca by AA or ICP spectroscopy. The hydrolysis product in solution, ammonia, may be measured by ammonium ion selective electrode, or by colorimetry followed by Nesslerization. 

Elemental compostion Ce 25.56%, H 1.47%, N 20.44%, 0 52.53%. The aqueous solution of the compound may be analyzed for Ce by AA or ICP spectrophotometry. Also, the solution may be measured for NH4 ion by ammonium ion-selective electrode and the NO3 ion by nitrate ion-specific electrode, ion chromatography or cadmium-reduction colorimetry. For all these measurements, the solution may require sufficient dilutions. For quantitation, its solution may be standardized by titration with a reducing agent such as sodium oxalate in the presence of iron and ferroin indicator. 

Fig. 6.27 Schematic of potentiometric enzyme electrode for urea, based on ammonium ion-selective electrode...

The electrode is an ammonium ion-selective electrode surrounded by a gel impregnated with the enzyme urease [Fig. 6.13 (34)]. The generated ammo-... 

The sensor mentioned under (iii) uses an ammonium ion selective electrode carrying a PVC nitrobenzene-dibenzo-18-crown-6 gel. The enzyme was immobilized by crosslinking with glutaraldehyde on a Teflon membrane. After addition of urea to the measuring solution the ammonia produced in the enzymatic reaction reaches the internal solution layer (pH 9.7) of the sensor, where it is partially converted to ammonium ion which is transferred into the nitrobenzene phase. As a result, at 370 mV a current flow was observed which was related to urea concentration. The peak current was proportional to the concentration of urea between 1 pmol/1 and 2 mmol/1. The response time of the sensor was about 1 min. 

Petersson (1988b) developed an efficient urea analyzer for undiluted blood samples by using a urease-covered ammonium ion selective electrode in an FIA system. Forty samples per hour could be determined with a useful measuring range up to 40 mmol/l and a serial CV of 1%. The sensor was stable for 25 days. The correlation coefficient with a routinely used method was 0.99. Variations in the hematocrit level had only a small effect on the measurement. 

One of the primary applications of entrapment immobilization has been to prepare enzyme-electrode-based biosensors [27], and one of the first functional enzyme electrodes utilized urease entrapped in an acrylamide film to detect urea using an ammonium ion selective electrode [28]. Highly hydrophobic bilayer lipid membranes and liposomes have also been used to entrap highly labile biomolecules (see chapter 9). Such films and layers are, however, inherently unstable themselves and are useful primarily as research tools. 

To create a potentiometric creatinine sensor, creatinine iminohydrolase, which catalyzes the production of ammonia from creatinine, can be immobilized on the surface of an ammonium ion selective electrode. There is no interference from creatine but some from the ammonium ions in blood and urine. 

The first urea electrode was prepared in 1969 by immobilizing urease in a polyacrylamide matrix on nylon or Dacron nets. These nets were then placed onto a glass cation-selective electrode. To improve the selectivity, an ammonium-ion selective electrode with... 

Immimodiagnostics and enzyme biosensors are two of the leading technologies that have a greatest impact on the food industry. The use of these two systems has reduced the time for detection of pathogens such as Salmonella to 24 hr and has provided detection of biological compounds such as cholesterol or chymotrypsin [15]. Biosensors analyses Beta lactams in milk and presence of urea in milk that lead to production of synthetic milk, the biocomponent part of the urea biosensor is an immobilized urease yielding bacterial cell biomass isolated from soil and is coupled to the ammonium ions selective electrodes of a potentiometric transducer. The membrane potential of all types of potentiometric cell based probes is related to the activity of electrochemically-detected product [16]. 

Ammonium ion-selective electrodes with permeable membrane and gas-sensing polymer membrane electrodes (see subsection 2.1.5) have been applied for the determination of ammonia [344] and are applicable to any species yielding ammonia. The extensive use of membrane electrodes selective to nitrogen-containing compounds on the basis of corresponding ammonium salts incorporated into a PVC matrix was reviewed by Hansen et al. [345]. 

Palleschi G., Mascini M., Martinez-Fabregas E. and Alegret S. (1988) Urea determination in human sera with an ammonium ion selective electrode made with solid inner electric contact and immobilised urease. Anal. Letters, 21,1115-1129. 

Katsu TK, Nishimura N (2000) Organic ammonium ion-selective electrodes using acyclic neutral carriers developed for inorganic-selective electrodes. Anal Sci 16 523-526... 

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