The Clark cell

Since the cathode is held at a potential corresponding to the diffusion-controlled reduction of oxygen, the concentration of oxygen at x = 0 is c = 0. Let C2 be the oxygen concentration in the electrolyte at x = dg, C2 be the corresponding value at x = dg in the membrane, and C3 the concentration at X = dg -L dm inside the membrane. Accordingly, when 

Practically speaking, this has the undesirable consequence that the sensor response is highly sensitive to the membrane thickness (4n) and membrane properties (Pm)- Calibration of the sensor is thus essential. 

The Clark Cell.—This galvanic combination measures the affinity of the reaction 

In ordinary circumstances the system concerned is not a condensed one, but there are also processes going on in solution. If, however, we cool down to — 7°, ice appears as a solid phase, and then the above reaction occurs between pure substances simply. The heat of this reaction is well known and, in addition, all the reactants have been measured to a sufficiently low temperature (that of boiling hydrogen). The heat change is first calculated at T = 234, at which temperature the mercury is solid and the heat therefore changes by 2 x 555 cals, (heat of fusion of mercury). From this point the U-curve can be drawn down to the absolute zero, and the A-curve calculated with the aid of the Heat Theorem. This calculation, for which we have to thank Pollitzer (46), gives for T = 266 an E.M.F. of 1456 volts, which is identical with the observed value. Consequently the potential, which is of course also a measure of the 

to be sure, accidental that this concordance is so dose, for in this case, on account of the large number of atoms taking part in the reaction, the influence of the spedfic heats is relatively large, and the error of experiment may therefore easily cause an error of o-oi volts. 

The accompanying curves (Fig. 19) give a good idea of the conditions. From low temperatures up to the melting-point of mercury there is hardly any appreciable difference between A and U at this point, for example, 

Owing to the melting of mercury U now falls a little bdow A, and A must therefore bend a little upwards. At T = 266, since the heat of dilution can be neglected, U increases suddenly by the very considerable heat of fusion 

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Numerous commercial oxygen analysers are available, based on the principle of the Hersch cell, but all being protected from the analyte medium, either gaseous or liquid, by means of a membrane (usually Teflon). This membrane detector is known as the Clark cell (see later under membranes as measurement aids, p. 352) for gas analysis we may mention the Beckman Models 715 (also for liquids), 741, 743 (for flue gas), 755 and 778119. 

A standard cell produces a precise voltage and, before the advent of reliable voltmeters, was needed to calibrate medical and laboratory equipment. It is generally agreed that the first standard cell was the Clark cell (see p. 299), but the most popular was the Weston saturated cadmium cell, patented in 1893. 

The Clark cell was patented by Latimer Clark in the 1880s, and was the first standard cell. 

Weston s cell was much less temperature sensitive than the previous standard, the Clark cell. We recall how the value of AG changes with temperature according to Equation (4.38). In a similar way, the value of AG(Ceii) for a cell relates to the entropy change A ceii) such that the change of emf with temperature follows... 

Figure 7.5 Graph of cell emf against temperature for the Clark cell Hg HgS04, ZnS04(sat d) Zn. We call the gradient of this graph the temperature voltage coefficient ...

Figure 3.2 Determination of the temperature eoeffieient of voltage, from which AS e, may be calculated. Data relate to the Clark cell, i.e. Zn ZnS04(samnited). HgS04(s, Hg.

Normal element (cell) - Galvanic element of exceptional reproducibility of the cell voltage. Since 1908 the -> Weston normal element is used as the international standard because it has a higher stability than the - Clark cell. 

Very rapid and efficient stirring of the water in the calorimeter was necessary and a correction for the heat produced by this (in some cases 10 per cent of the whole energy supplied) was necessary. Temperatures were read on a mercury thermometer compared with a standard mercury thermometer and a platinum thermometer. Griffiths, who took the e.m.f. of the Clark cell as 1 4342 volt, found, in 15° g.cal. and nitrogen thermometer temperatures, 4=572 60 at 40 15° and 578 70 at 30 00°, and by combining his results with those of Dieterici at 0° and of Regnault between 63° and 100° he concluded that ... 

Let us consider the case of the Clark cell This cell at ordinary room temperature contains the zinc salt m aqueous solution We have already seen that it is desirable to work with a cell, each phase present being a single pure substance To realise this in the Clark cell we must work at the cryohydnc temperature, namely - 90 C, at which ice separates out The reaction is indicated above The following heat data are available (at 170 C ) —... 

These values, Nernst points out, aie in several instances not very accurate, and fuither they do not all obtain for the same temperatuie They must suffice, however The Nernst Theorem thus gives for the case of the Clark cell—... 

No analogous investigation of the Weston cell has yet been made. There are certain difficulties in the way because, on account of transformation of the saltCdS04j HaO, the cryohydric point cannot be reached directly. The appreciably smaller temperature coefficient of this cell is partially explained, of course, by the much smaller amount of water of crystallization (S mols compared with 7 in the Clark cell) but additional reasons are the great solubility of cadmium sulphate at low temperatures, and the fact that the cadmium salt, in contrast with the zinc salt, dissolves with evolution of heat. 

The agreement must be regarded as extremely satisfactory, particularly as the specific heats of the two cadmium salts were only measured down to the temperature of liquid air. A more detailed discussion appears to be desirable of the question how the small temperature coefficient of the Weston cell compared with that of the Clark cell is to be explained, and how, in a more accurate calculation, account is to be taken of the heat of dilution at the cryohydric point. The cryhohydric point for the Weston cell was found to be T = 256° (cf. also my remarks (p. 118), and Landolt and BGmstein, Tabellen, p. 464, IV Aufl.). 

Clark cell A type of cell formerly used as a standard source of e.m.f. It consists of a mercury cathode coated with mercury sulfate, and a zinc anode. The electrolyte is zinc sulfate solution. The e.m.f. produced is 1.4345 volts at 15°C. The Clark cell has been superseded as a standard by the Weston (Trademark) cadmium cell. The cell is named for the English engineer Josiah Latimer Clark (1822-98). 

Clark cell A type of voltaic cell consisting of an anode made of zinc amalgam and a cathode of mercury both immersed in a saturated solution of zinc sulphate. The Clark cell was formerly used as a standard of e.m.f. thee.m.tat IS C is 1.4345 volts. It is named after the British scientist Hosiah Clark (d. 1898). 

In this paper Clark described the battery composed of mercury-mercurous sulfate, zinc and saturated zinc sulfate (known as the Clark cell). 

The standards of electromotive force available up to the present time did not come up to the ideal which Mr. Weston had set before him, even the best of these, the Clark cell, being subject to a variation of 0.077 per degree centigrade. 

The Clark cell is a standard cell, in which mercury forms the positive pole and a 10% zinc amalgam the negative pole. The electrolyte is a... 

The Clark cell was at one time widely used as a standard cell, but it has now been rejected, in favour of the Weston cell (q.v.), owing to its high temperature coefficient of e.m.f. 

Another cell, sometimes used as a standard of electromotive force, is the Clark cell. 

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