The antimony electrode

Antimony electrodes, formed by simply casting the material in air with the formation of surface oxide, behave reversibly with respect to hydrogen ions in solution according to 

While of little practical use as a laboratory tool, requiring calibration for any type of solution used, it has proved useful on an industrial scale for monitoring pH in plants where its robustness is a particular asset. 

The glass electrode is the most widely used indicator electrode for pH determinations used in the laboratory. It operates on the principle that the potential difference between the surface of a glass membrane and a solution is a 

When used in practice it must be coupled with a reference electrode also dipping into the working solution, e.g. 

Since the potential of the silver-silver chloride electrode is constant and the potential difference between the inner surface of the glass membrane and the hydrochloric acid solution is constant, the only potential difference which can vary is that between the outer surface of the membrane and the working solution. The overall potential of the system is thus a function of the pH of the working solution only. 

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The antimony electrode used by Rosenfeldt et al. [156] did respond proportionally to the myocardial pH changes in dogs produced by infusion of sodium bicarbonate or inhalation of carbon dioxide. However, pH measured by the antimony electrodes was consistently about 0.26 units higher than that which was measured with the Paratrend optical... 

The corrosion of antimony electrodes was also measured using ICP-MS (inductively coupled plasma mass spectrometry) for dissolved antimony in vivo [156], After the electrodes were inserted in the plasma, the antimony concentration showed a linear rise with time at a rate approximately of 94 j,g/L/h (r2 = 0.997). Although the projected antimony concentration is lower than the safe limit, accumulation of dissolved antimony and localized toxic effects in tissue may prevent the antimony electrode from long-term implantable applications. 

In addition to glass electrodes [159], the pH sensors incorporated in the catheter for esophageal pH monitoring were reported to be mostly antimony electrodes such as the Slimline from Medtronic Inc. [95, 158, 164], In a recent study, Pandolfino el al. [33] compared the accuracy of the Slimline antimony pH monitoring system to that of a conventional glass electrode catheter pH system during ambulatory conditions in 18 patients. They reported that the antimony electrodes had acceptable performance... 

For those redox couples that involve a metal ion plus the metal, the logical electrode system is the metal itself. In other words, if the measured quantity is to be cupric ion [copper(II)], a practical indicator electrode is a piece of copper metal. All second-class electrodes involve an active metal in combination with an insoluble compound or salt. Thus, the silver/silver chloride electrode actually is a silver/silver ion electrode system that incorporates the means to control the silver ion concentration through the chloride ion concentration [Eq. (2.14)]. A related form of this is the antimony electrode, which involves antimony and its oxide (an adherent film on the surface of the antimony-metal electrode) such that the activity of antimony ion is controlled by... 

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 ... 

The antimony electrode is an example of electrode of the type metal-oxide. It consists of a rod of metallic antimony, coated with a natural film of antimony trioxide and immersed into a solution with a definite hydroxyl ion concentration. Its function can be explained as follows antimony emits at first ions into the solution... 

According to this equation the antimony electrode is reversible with respect to hydroxyl ions. The same can be said about the hydrogen ions too, because their concentration is indirectly proportionate to that of ions OH. Therefore, when substituting the activity of the hydrogen ion for that of the hydroxyl ion ... 

The antimony electrode is used for pH determination when an accurate measurement is not strictly required its advantage in comparison with other types of electrodes is a simple design and low sensitivity to various poisons. 

IV. The Antimony Electrode.—The so-called antimony electrode is really an electrode consisting of antimony and its trioxide, the reaction being... 

This approach is, of course, only possible with solid-state ISE and only with those rapidly electrochemically regenerated. Unless removed by purging, oxygen is always present in the solution and alleviates the need for addition of a redox mediator that could alter the rest potential of the antimony electrode and interfere with the pH. Moreover, the normalization of the approach curve with the limiting current in the bulk removes the need for the knowledge of the bulk concentration and diffusion coefficient of oxygen. 

Antimony Electrode. The antimony electrode is perhaps the best representative of a whole class of metal/metal-oxide redox electrodes that respond to pH. The potential is probably developed as a result of an oxidation-reduction reaction involving antimony and a skin of antimony(III) oxide which forms on the surface of the metal ... 

In actual practice, the antimony electrode does not give highly accurate results. The previous history, preparation, and surface characteristics of each antimony billet used affect the response. Each electrode must be carefully calibrated over the pH range to be used and factors such as dissolved oxygen and the composition of the buffer solution affect response. Nevertheless, the ruggedness, simplicity, very low resistance, and low cost of the antimony electrode have made it useful, for example, in continuous industrial-process monitoring when high precision and accuracy is not required. 

The quinhydrone electrode is of simpler construction than the antimony electrode. A piece of 24-gauge platinum wire about 1/2 in. long is introduced into the capillary, which is then sealed and beaded to the wire leaving a short tip exposed externally. The platinum wire should be cleaned in aqua regia briefly and washed in distilled water before using. After the bead has cooled, self-wetting solder is introduced into the lumen and the assembly is heated until the solder melts. After the solder has bonded to the platinum, but while it is still molten, a piece of clean copper wire is inserted into it to form the electrode connection when the solder cools. 

The consequence of these three calibration curves is shown in Figure 2 where the pH of phosphate solutions measured with a glass electrode is compared with the apparent pH of the same solutions measured with antimony electrodes previously calibrated with 67mM phosphate buffer standards. If, for example, one has a solution containing 2.6 mM phosphate with a true pH of 7.40, the reading with calibrated antimony electrode, will be an apparent pH of 6.80. It would therefore appear that to get true pH value with the antimony electrode, the buffers used in the calibration process should have the same phosphate concentration as the unknown solution. 

The data showed that a linear relationship does not exist between the pH determined with the antimony electrode and that with... 

III, Does the Antimony Electrode Behave Simply as a Metal-Metal Oxide Electrode in Air J. Chem, Soc. London. 752-755 (1948). 

Comment (Puschett) We have now had a fair amount of experience with an ultramicro technique for measuring pH both with antimony and glass microelectrodes. We noted that there was very little change in the pH measured with the antimony electrode as time elapsed, once equilibration had occurred, when we used electrodes that were 3-4 weeks old as compared to using those made the same day. We presumed this was due to the fact that the electrode had, in fact, become somewhat oxidized in the interim. 

The use of antimony microelectrodes which are metal/metal oxide electrodes, is shown in Fig. la. They consist of antimony-filled glass capillaries, which are drawn out in order to obtain a fine bevelled tip, which is pH-sensitive and can be introduced into the tubular lumen. These electrodes have also been used to measure titratable acidity and ammonia in an in vitro" system, according to Solomon et al (16) and Karlmark (6). In this paper we will restrict ourselves to the discussion of some aspects of tubular acidification which can be studied due to the rapid response of the antimony electrode system to pH changes, permitting the kinetic study of the tubular acidification mechanisms. 

The potential of the antimony electrode is given by an equation of the form... 

The pH electrode is another suitable transducer for the construction of a urea biosensor. The classical glass electrode is sensitive to H ions and urease is attached in a gel of either polyacrylamide [107] or methacrylamide-aoylamide cqxdymer [108]. me metallic electrodes are also sensitive to H ions (for example, the antimony electrode) and can also be used in conjunction with a urease membrane [109]. The enzyme pH electrode detects a very weak variation in proton concentration arising from an enzymatic reaction, and the signal amplitude is determined by the buffering capacity of the solution. Both the nature of the buffer solution and the working pH value of the biosensor can reduce its practical use. A differential measurement at different pH values can be used to correct for any variation. 

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