Glass electrode, and

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. 

Electrode for e pH meter. The pH of a solution can be determined with the aid of a "glass electrode." The voltage between the glass electrode and the reference electrode is directly related to pH. The leads from the electrodes are connected to a pH meter. 

So-called combination electrodes may be purchased in which the glass electrode and the saturated calomel reference electrode are combined into a single unit, thus giving a more robust piece of equipment, and the convenience of having to insert and support a single probe in the test solution instead of the two separate components. 

The urease is incorporated into a polyacrylamide gel which is allowed to set on the bulb of the glass electrode and may be held in position by nylon gauze. Preferably, the urease can be chemically immobilised on to bovine serum albumin or even on to nylon. When the electrode is inserted into a solution containing urea, ammonium ions are produced, diffuse through the gel and cause a response by the ammonium ion probe ... 

Ref 11). The pH of a satd soln in w was measured with a glass electrode and found to be 0.6 (Ref 25, p 2) CA Registry No 517-25-9. The Hercules Powder Co of Wil-mington, Del has been granted a trademark for Nitroform , a synthetic w-insoluble compd for controlled release of N for agricultural use (Ref 40). This should not be confused with TNMe... 

An operational approach to the determination of the acidity of solutions in deuterium oxide (heavy water) was suggested by Glasoe and Long. This quantity, pD, is determined in a cell consisting of an aqueous (H20) glass electrode and a saturated aqueous calomel reference electrode on the basis of the equation... 

The search for models of biological membranes led to the formation of a separate branch of electrochemistry, i.e. membrane electrochemistry. The most important results obtained in this field include the theory and application of ion-exchanger membranes and the discovery of ion-selective electrodes (including glass electrodes) and bilayer lipid membranes. 

It has been emphasized repeatedly that the individual activity coefficients cannot be measured experimentally. However, these values are required for a number of purposes, e.g. for calibration of ion-selective electrodes. Thus, a conventional scale of ionic activities must be defined on the basis of suitably selected standards. In addition, this definition must be consistent with the definition of the conventional activity scale for the oxonium ion, i.e. the definition of the practical pH scale. Similarly, the individual scales for the various ions must be mutually consistent, i.e. they must satisfy the relationship between the experimentally measurable mean activity of the electrolyte and the defined activities of the cation and anion in view of Eq. (1.1.11). Thus, by using galvanic cells without transport, e.g. a sodium-ion-selective glass electrode and a Cl -selective electrode in a NaCl solution, a series of (NaCl) is obtained from which the individual ion activity aNa+ is determined on the basis of the Bates-Guggenheim convention for acr (page 37). Table 6.1 lists three such standard solutions, where pNa = -logflNa+, etc. 

Jagner [28] has also described a semi-automatic titration for high-precision determination of chlorine in seawater, where it has been used for the potentiometric determination of total halides (silver electrode) and alkalinity (glass electrode), and for the photometric titration of total alkaline-earth metals. Several titrations can be effected simultaneously. 

Oxide ratio, 18 815 Oxides, 16 598 acidic, 22 190-191 bond strengths and coordination numbers of, 22 570t diorganotin, 24 819 glass electrodes and, 14 28 gold, 22 707 iron, 14 541-542 lead, 14 786-788 manganese, 15 581-592 nickel, 27 106-108 niobium, 27 151 plutonium, 29 688-689 in perovskite-type electronic ceramics, 14 102... 

H+] is measured potentiometrically with a glass electrode. Briefly, the method involves the use of a glass electrode and a double-junction calomel reference electrode in the titration cell ... 

Two types of cell have been described. In Fig. 1 a cell with a rotating disk electrode is shown. Connections to a pH-stat and to the interface are indicated simultaneously with recording CMT measurements, the metal and reference electrode and a counter electrode (not shown in Fig. 1) can be connected to a potentiostat, so that electrochemical measurements can be recorded intermittently. The volume of solution in the cell is ca. 400 ml. What matters for safe and reliable conditions of measurement is that the disk electrode rotating at a speed of no less than 1000 rpm ensures efficient stirring, so that the effect of alkali formed at the corroding metal (or sometimes at the counter electrode located ca. 1 cm below and parallel to the metal electrode) is immediately sensed effectively by the glass electrode and also the effect of acid supplied from the autoburet is quickly detected. 

Figure 3.9 Schematic diagrams of pH electrodes, showing (a) a glass electrode, and (b) a pH combination electrode (note that the glass and the bulb are the same in both cases).

Accurate and precise measures of pH can be made with a pH meter. Typical pH meters usually contain a glass electrode and reference electrode arranged similar to an electrochemical cell. We discuss electrochemical cells in Chapter 14. For now, though, consider a pH meter as essentially a modified voltmeter in which the voltage measured is directly proportional to the hydrogen ion concentration of the solution. Simple pH meters are capable of measuring pH within 0.1 pH units, while more sophisticated instruments are precise to within 0.001 pH units. 

Ion solvation has been studied extensively by potentiometry [28, 31]. Among the potentiometric indicator electrodes used as sensors for ion solvation are metal and metal amalgam electrodes for the relevant metal ions, pH glass electrodes and pH-ISFETs for H+ (see Fig. 6.8), univalent cation-sensitive glass electrodes for alkali metal ions, a CuS solid-membrane electrode for Cu2+, an LaF3-based fluoride electrode for l , and some other ISEs. So far, method (2) has been employed most often. The advantage of potentiometry is that the number and the variety of target ions increase by the use of ISEs. 

Using a pH Merer, equipped with a glass electrode and a saturated calomel electrode,... 

We have recently devised a rapid and convenient method for determination of the ionization constant for water in mixed aqueous organic solvents (11-16). The method utilizes glass electrodes and gives results in satisfactory agreement with earlier work. 

Experimental measurements were made by immersing a pair of the electrodes in a 15-ml portion of purely aqueous solution A, B, C, or D and allowing the potentials to stabilize. When the potential became stable, a portion of the nonaqueous cosolvent was added to the solution in the cell and the potential was again recorded. This procedure was continued until 50 ml of the cosolvent had been added. The temperature of the cells was kept constant to within 0.05°C of the reported temperatures throughout the experiments. The potential measurement-cosolvent addition experiments were performed at least twice with each combination of glass electrodes and silver-silver chloride electrodes on at least two independently prepared solutions. 

When the concentration of H+ is different on either side of the membrane, a potential difference is generated, which is related to the activity of H+ ions in solution, i.e. pH. The latter is determined using an electronic millivoltmeter, the pH meter, which monitors the potential difference between the glass electrode and an internal reference electrode of Ag/AgCl (currently preferred to the mercurous chloride (Hg) electrode for environmental purposes). After calibration, the instrument will directly yield the pH of a solution. 

Table 9-2 lists pA-., values for common buffers that are widely used in biochemistry. The measurement of pH with glass electrodes, and the buffers used by the U.S. National Institute of Standards and Technology to define the pH scale, are described in Chapter 15. 

Figure 11-10 Titration ot a mixture of acids with tetrabutylammonium hydroxide in methyl isobutyl ketone solvent shows that the order of acid strength is HCI04 > HCI > 2-hydroxybenzoic acid > acetic acid > hydroxybenzene. Measurements were made with a glass electrode and a platinum reference electrode. The ordinate is proportional to pH. with increasing pH as the potential becomes more positive. [D. B. Brass and G. E. A. Wyld. Methyl Isobutyt Ketone as a Wide-Range Solvent for Titration of Acid Mixtures and Nitrogen Bases," Anal Chem. 1957, 29.232.]...

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