Antimony-based pH electrodes

Simultaneous and continuous measurements of extracellular pH, potassium K+, and lactate in an ischemic heart were carried out to study lactic acid production, intracellular acidification, and cellular K+ loss and their quantitative relationships [6, 7], The pH sensor was fabricated on a flexible kapton substrate and the pH sensitive iridium oxide layer was electrodeposited on a planar platinum electrode. Antimony-based pH electrodes have also been used for the measurement of myocardial pH in addition to their application in esophageal acid reflux detection. 

Two methods are used to measure pH electrometric and chemical indicator (1 7). The most common is electrometric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instmment measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal—metal oxides, eg, antimony—antimony oxide (see Antimony AND ANTIMONY ALLOYS Antimony COMPOUNDS), have also found use as pH sensors (8), especially for industrial appHcations where superior mechanical stabiUty is needed (see Sensors). However, because of the presence of the metallic element, these electrodes suffer from interferences by oxidation—reduction systems in the test solution. 

Metal oxide film coated metal electrodes can provide reliable potentiometric pH response. The miniaturized version of them gained application in potentiometric SECM. For example, a microdisc antimony pH electrode was used by Toth et al. [47] for investigating measuring function of silver iodide based ion selective cyanide electrode. Iridium oxide based microelectrode was applied by Wipf and coworkers [48] for pH imaging. 

For adverse conditions (in terms of either temperature or vibration) the antimony electrode has proved useful, particularly for industrial processes with extreme environmental problems. The electrode is not particularly reliable for precise measurements, but its simple form (consisting of antimony metal embedded in an insulating material) allows pH measurements under such adverse conditions. The principle of the electrode is based on a half-reaction whereby the metal and its metal oxide are both insoluble and the electrode s response is dependent on hydronium ion activity ... 

Acetylcholineesterase A stock solution of 0.52mg/mL of the pesticide trichlorophen in lOmM phosphate buffer of pH 7.5 was diluted with buffer to various concentrations. The obtained solutions were then analyzed using an ACh biosensor based on the inhibition effect of trichlorophen on the function AChE which promotes the hydrolysis of the natural neurotransmitter, acetylcholine. The sensor was fabricated by immobilizing AChE onto the surface of an antimony disc electrode, which was then used in conjunction with a double junction Ag/AgCl (0.1 M-KC1) reference electrode with a 0.1 M lithium acetate salt bridge. 

While the majority of enzyme electrodes fabricated have been rather large devices, there have been some recent reports concerning the development of miniaturized and even microsensors. For example, MeyerhoflF (M5) prepared an essentially disposable urea sensor (tip diameter 3 mm) by immobilizing urease at the surface of a new type of polymer-membrane electrode-based ammonia sensor (see Fig. 4). Alexander and Joseph (Al) have also prepared a new miniature urea sensor by immobilizing urease at the surface of pH-sensitive antimony wire. Similarly, lannello and Ycynych (II) immobilized urease on a pH-sensitive iridium dioxide electrode. In these latter investigations, ammonia liberated from the enzyme-catalyzed reaction alters the pH in the thin film of enzyme adjacent to the pH-sensitive wire. 

The pH of the gel can be determined by inserting a microelectrode at points along the gel, or a small section can be cut out and dissolved, and the pH measured with a regular set of electrodes. A common set of microelectrodes includes antimony wire for the indicator electrode and a micro calomel electrode for the reference. If you use this pair, you must correct the pH meter reading based upon the half cell reaction. 

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