Calomel half cell

Experimentally, the aqueous iron(II) is titrated with cerium(IV) in aqueous solution in a burette. The arrangement is shown in Figure 4.6, the platinum indicator electrode changes its potential (with reference to a calomel half-cell as standard) as the solution is titrated. Figure 4.7 shows the graph of the cell e.m.f. against added cerium(IV). At the equivalence point the amount of the added Ce (aq) is equal to the original amount of Fe (aq) hence the amounts of Ce (aq) and Fe (aq) are also equal. Under these conditions the potential of the electrode in the mixture is ( - - f)/2 this, the equivalence point, occurs at the point indicated. 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

The apparatus is assembled and electrical connections are duly completed with dropping mercury electrode (C) as cathode and saturated calomel half-cell as anode,... 

Steady-state potential, negative to saturated calomel half-cell. 

VOLTS SATURATED CALOMEL HALF-CELL REFERENCE ELECTRODE... 

With the ion-selective electrode standardized at an ionic strength of 0.01, and for cells involving silver-silver chloride and saturated calomel half-cells, the residual effect of liquid-junction potential is estimated to amount to about —0.02 and —0.07 pNa units at ionic strengths of 0.1 and 1.0 for chloride, -1-0.02 and -t-0.07 pCl units at 0.1 and 1.0 for calcium, —0.03 and — 0.22pCa units at 0.1 and 1.0. 

From the following data, calculate the potential of the calomel half cell in 0.1 m KC1. Half-cell reaction wanted ... 

Effect of Temperature on the Standard Potential of Calomel Half Cell and Platinum Electrode... 

L The potential of an iron electrode when polarized as cathode at 0.001 A/cm is -0.916 V versus 1N calomel half-cell. Tlie pH of the electrolyte is 4.0. What is the value of the hydrogen overpotential ... 

Closely separated short-circuited zinc and mercury electrodes are immersed in deaerated HCl of pH = 3.5. What is the current through the cell if the total exposed area of each electrode is lOcm What is the corresponding corrosion rate of zinc in gmd (Corrosion potential of zinc versus 1N calomel half-cell is -1.03 V.)... 

Prom an "old sample" of thorium nitrate (one in which the thorlxim daughters are in equlllbrlxom), 1 gram la weighed out and dissolved in 20 to 25 ml 0-5 N hydrochloric acid. The requisite amount of standard bismuth solution (> 0.2 mg) is pipetted into the electrolysis solution which is heated in a water bath to 80-90°C. Clean platinum electrodes and a calomel half-cell are inserted and the electrolyzing current and a mechanical stirrer are switched on. The current is increased until a potential of 0.25-0.30 volte relative to the saturated calomel electrode is attained and the plating is continued for 25 minutes. 

If the KCl solution is kept saturated, all the potentials developed in the calomel half-cell will be constant. All calomel electrodes have a side arm to replenish KCl solution. 

The potential in a calomel half-cell is derived from the primary reaction... 

Calomel reference electrodes consist of mercury in contact with a solution that is saturated with mercuryfl) chloride (calomel) and that also contains a known concentration of potassium chloride. Calomel half-cells can be represented as follows ... 

As in the hydrogen half-cell, there are a number of designs for the calomel half-cell, which uses mercury and calomel (mercurous chloride) as... 

Potentiometric titration may be carried out using a general-purpose glass electrode as indicator electrode and a saturated calomel half-cell as reference electrode. 

Normally a Beckman Model G pH meter, or an instrument of similar specification, should be employed with wide range (—5 to 80 C, pH 0-11) glass electrodes and saturated calomel half-cell electrodes. Automatic reading instruments may be satisfactory for shipboard use, where a precision of about 0.03 unit is acceptable. A high-amplification pH meter such as the Beckman G.S. meter should be used for the most refined work, as in tracing marine photosynthesis, respiration, etc., where pH changes of 0.01 unit or less may be significant. Wide-mouth screw-capped 50-ml polyethylene bottles are needed, one for each sample. 

A Beckman Model G pH meter, or an instrument of similar specification, should be employed with wide-range (- 5 to 80 C, pH 0-11) glass electrodes and saturated calomel half-cell electrodes. Automatic reading Instruments are only acceptable if used with the greatest possible precision ( 0.025 unit or better). 

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