Electrode glass-calomel

An operational definition has been repeatedly suggested (5, 9) and that sponsored by IUPAC is based chiefly on the work of Bates (I, 2). The pH is defined in terms of a cell—e.g., calomel electrode] solution glass or hydrogen electrode. 

With the mixture maintained at a temperature of 37° 0.1°, add 0.1 A sodium hydroxide, from a microburet inserted through an opening in the cover, to adjust potentiometrically the pH to 9.20, using a calomel-glass electrode system Add... 

Potentiometric measurements are simple the redox potential is measured compared to a reference electrode. For pH measurements, commercially available electrodes, comprising the working (glass electrode) and reference electrodes, can be used. For redox potential measurements, the working electrode is usually an inert (e.g., platinum) electrode, and the reference electrode can be a hydrogen electrode, calomel, or other electrodes. Ion-selective electrodes are also based on potential measurements. 

Most pH determinations are made by electrometric methods, the pH of the unkown solution (X) being calculated from that of a known standard (S) and the emf ( x and s) of a cell composed of a hydrogen ion-responsive electrode (for example, a glass electrode or a hydrogen gas electrode) coupled with a reference electrode (calomel, silver-silver chloride). This cell is filled successively with the standard solution S and with the unknown solution X. A liquid junction potential j exists where these solutions make contact with the concentrated KCl solution of the reference electrode. From the Nernst equation for the cell reactions and assuming an ideal hydrogen ion response ... 

This conclusion was reinforced in another way. The observed emf for the glass electrode-calomel cell (with liquid junction) in eight solutions with pH 2-9 differed by a constant amount from that for the glass-AgCl Ag combination (without liquid junction) in the same cell. The range was from 9.0 mV in the acetate buffer to 9.6 mV in the tris buffer the mean was 9.43 mV, and the standard deviation was 0.19 mV. 

The direct reading type of instrument, although possibly less accurate than potentiometric is also used exclusively in modem soil laboratories. The e.m.f of the glass electrode-calomel electrode cell is applied across a resistance, and the resulting current after amplification is passed through an ammeter causing deflection of the pointer across a scale marked in pH rmits. These instruments are available to operate on mains A.C. current. In most pH meters temperature control knob is provided to adjust at temperature of the test solution. 

Fig. 7-38. Potentials in a glass electrode-calomel electrode system commonly used for pH measurement.

Figure 3. Brown s potentiometer for measurement of hydrogen ion concentration using glass electrode. B and B - calomel electrodes A - glass electrode D - high voltage battery C - potentiometer GR - ground Q - quadrant electrometer. (Reproduced with permission from Ref.17 Copyright 1925 American Institute of Physics.)...

Fig. 2. (a) Calomel reference electrode (b) glass electrode (c) solid-state electrode. 

The mode of action of the glass electrode is very complex and, of all the theories put forward, no single one can account for all the observed properties. It is very likely, however, that an important stage involves the absorption of hydrogen ions into the lattice of the glass membrane. The potential of the glass electrode/calomel cell may be expressed as... 

The evolution of nitrogen aids in removing dissolved air. A salt bridge (4 mm tube) attached to the saturated calomel electrode is filled with 3 per cent agar gel saturated with potassium chloride and its tip is placed within 1 mm of the mercury cathode when the mercury is not being stirred this ensures that the tip trails in the mercury surface when the latter is stirred. It is essential that the mercury-solution interface (not merely the solution) be vigorously stirred, and for this purpose the propeller blades of the glass stirrer are partially immersed in the mercury. 

One form of calomel electrode is shown in Fig. 15.1(a). It consists of a stoppered glass vessel provided with a bent side tube fitted with a three-way tap which carries a short upper and a long lower tube the latter is drawn out to a constriction at the bottom end. A short platinum wire is fused into the bottom of the vessel so that it protrudes into the interior, and a narrow glass tube sealed to the bottom of the vessel is bent round parallel to the vessel. A little mercury placed in the bottom of this tube provides electrical connection with the interior of the vessel through the sealed-in platinum wire. Mercury and mercury compounds must be handled with care (see Section 16.8). 

To measure the hydrogen ion concentration of a solution the glass electrode must be combined with a reference electrode, for which purpose the saturated calomel electrode is most commonly used, thus giving the cell ... 

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 H-type cell devised by Lingane and Laitinen and shown in Fig. 16.9 will be found satisfactory for many purposes a particular feature is the built-in reference electrode. Usually a saturated calomel electrode is employed, but if the presence of chloride ion is harmful a mercury(I) sulphate electrode (Hg/Hg2 S04 in potassium sulphate solution potential ca + 0.40 volts vs S.C.E.) may be used. It is usually designed to contain 10-50 mL of the sample solution in the left-hand compartment, but it can be constructed to accommodate a smaller volume down to 1 -2 mL. To avoid polarisation of the reference electrode the latter should be made of tubing at least 20 mm in diameter, but the dimensions of the solution compartment can be varied over wide limits. The compartments are separated by a cross-member filled with a 4 per cent agar-saturated potassium chloride gel, which is held in position by a medium-porosity sintered Pyrex glass disc (diameter at least 10 mm) placed as near the solution compartment as possible in order to facilitate de-aeration of the test solution. By clamping the cell so that the cross-member is vertical, the molten... 

Ref 4 contains the following requirements and criteria for silica (1) finely divided similar to Cab-O-Sil , Grade M-5, made by the Cabot Corp of Boston, Mass, (2) surface area as ml of NaOH titrant used to achieve a pH of 9.0 175 to 225m2/g, (3) density as the wt of a known vol of silica 2.31bs/cu ft max, (4) moisture as loss in wt 1.5% max, (5) pH as the measurement made using a Beckman Model G pH Meter with glass vs calomel electrodes ... 

Figure 17. PMC behavior in the accumulation region, (a) PMC potential curve and photocurrent-potential curve (dashed line) for silicon (dotted with Pt particles) in contact with propylene carbonate electrolyte containing ferrocene.21 (b) PMC potential curve and photocurrent-potential curve (dashed line) for a sputtered ZnO layer [resistivity 1,5 x 103 ft cm, on conducting glass (ITO)] in contact with an alkaline electrolyte (NaOH, pH = 12), measured against a saturated calomel electrode.22...

A glass electrode, a thin-walled glass bulb containing an electrolyte, is much easier to use than a hydrogen electrode and has a potential that varies linearly with the pH of the solution outside the glass bulb (Fig. 12.11). Often there is a calomel electrode built into the probe that makes contact with the test solution through a miniature salt bridge. A pH meter therefore usually has only one probe, which forms a complete electrochemical cell once it is dipped into a solution. The meter is calibrated with a buffer of known pH, and the measured cell emf is then automatically converted into the pH of the solution, which is displayed. 

FIGURE 12.11 A glass electrode in a protective plastic sleeve (left) is used to measure pH. It is used in conjunction with a calomel electrode (right) in pH meters such as this one. 

Monolayers of l-tert-bntyl-l,9-dihydrofullerene-60 on hydrophobized ITO glass exhibited three well-defined rednction waves at -0.55 V, -0.94 V, and -1.37 V (vs. satn-rated calomel electrode, SCE), with the first two stable to cycling [283]. Improved transfer ratios near nnity were reported. The peak splitting for the first two waves was 65-70 mV, mnch less than reported for the pnre C60-modified electrodes. The rednction and oxidation peak cnrrents were equal however, the peak currents were observed to be proportional to the sqnare root of the scan rate instead of being linear with the scan rate as normally expected for snrface-confined redox species. 

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