Clark cells

With sodium thiosulphate, (ti) 395 with Clark cell, (amp) 639 Oxygen, dissolved effect in polarography, 603, 618... 

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.

Calculate the free energy change (heat change) of the cell reaction (AH) in calories for two battery systems (a) A lead-acid cell with an open-circuit voltage of 2.01 V at 15 °C and a temperature coefficient of resistance (dE/dT) of 0.0037 V/K. (b) A Zn-Hg cell (Clark cell) with an open-circuit potential of 1.4324 V at 15 °C and a temperature coefficient of 0.00019 V/K. (Bhardwaj)... 

The method is usually adopted when the transferring component is 02, since it is possible to monitor the dissolved oxygen concentration with a polarographic electrode(Clark cell). The method requires some precautions. The probe has an intrinsic time constant tp (the time needed to attain 63% of the reading after a stepwise concentration change) so that its output is not directly related to the actual concentration, especially when the concentration in the liquid phase changes rapidly. Indeed the measurement may be... 

Clark cell — Electrochemical -> standard cell showing a particularly stable and reproducible cell voltage. A zinc and a mercury electrode (half-cell) are combined according to... 

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. 

Oxygen sensors — Device for measurement of oxygen concentration or activity. See - Clark cell, - lambda-probe. 

Standard cell — An electrochemical cell composed of two - half-cells containing electrodes built according to standard (normal) conditions. Frequently the term is also used for electrochemical cells showing a well-defined, reproducible, and stable cell voltage suitable for calibration purposes (- Weston cell, - Clark cell). 

R. Gerber, J. D. Ryan, and D. S. Clark, Cell-based screen of HMG-CoA reductase inhibitors and expression regulators using LC-MS, Anal. Biochem. 329 (2004), 28-34. 

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 ... 

Clark cell. Standard cell for measuring electrical potential with mercury and zinc amalgam electrodes in zinc sulfate solution. 

Use Chemical (admixed with sulfuric acid as a catalyst, in oxidation of naphthalene to phthalic acid), batteries (Clark cell, Weston cell). 

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—... 

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

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.). 

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