Ammonium chloride salt bridge

These compounds should be dissolvable in a clean-up solution containing a quaternary organic ammonium salt, or simply ammonium chloride [1749]. The solubilities of some selected particulate bridging agents are shown in Table 9-1. A chelating agent such as citric acid or its salts is also included in the clean-up solution. 

Suppose the salt bridge of a Daniell cell contains ammonium chloride solution, NHiCliaq). As positive zinc ions are produced at the anode, negative chloride ions migrate from the salt bridge into the half-cell that contains the anode. As positive copper(II) ions are removed from solution at the cathode, positive ammonium ions migrate from the salt bridge into the half-cell that contains the cathode. 

Until recently, the most popular reference half-cell for potentiometric titrations, polarography, and even kinetic studies has been the saturated aqueous calomel electrode (SCE), connected by means of a nonaqueous salt bridge (e.g., Et4NC104) to the electrolyte under study. The choice of this particular bridge electrolyte in conjunction with the SCE is not a good one because potassium perchlorate and potassium chloride have a limited solubility in many aprotic solvents. The junction is readily clogged, which leads to erratic junction potentials. For these practical reasons, a calomel or silver-silver chloride reference electrode with an aqueous lithium chloride or quaternary ammonium chloride fill solution is preferable if an aqueous electrode is used. 

A salt bridge is a liquid junction filled with a normal solution, or better, with a saturated solution of potassium chloride where the formation of a precipitate would be the result of the contact between the solution and the electrolyte containing e. g. ions Ag+, T1+, Hg4+, a saturated solution of ammonium nitrate is applied. This junction is interposed between the electrolytes surrounding both electrodes and at the same time is an intermediary of their electric connection. In this way, instead of one interface between the electrolytes, two are formed, as is evident, e. g. from the schematic representation of this system ... 

When it is not possible to employ potassium chloride solution, e.g., if one of the junction solutions contains a soluble silver, mercurous or thallous salt, satisfactory results can be obtained with a salt bridge containing a saturated solution of ammonium nitrate the use of solutions of sodium nitrate and of lithium acetate has also been suggested. For non-aqueous solutions, sodium iodide in methyl alcohol and potassium thiocyanate in ethyl alcohol have been employed. 

Very often it is imperative to prevent any contact between the analyte solution and the reference electrolyte solution, and this can be achieved by an additional salt bridge (see Fig. III.2.7). The electrolyte solution in the salt bridge has to be chosen so that it will not influence the measurement. It must be tolerated by both the reference electrolyte solution and the analyte solution. The most important electrolytes for salt bridges are potassium nitrate, potassium chloride, sodium sulphate, or ammonium nitrate solutions. 

By using a third solution, which must not react, and in which the cation and anion have similar mobilities and diffusion characteristics, to connect the two electrode solutions the liquid junction potential may be minimized. Concentrated potassium chloride, about 4 M, is often used, since K and Cl" ions have almost the same mobility (see Topic CIO). Potassium nitrate or ammonium nitrate is also suitable if chloride ions react. Use of a salt bridge reduces the effective El to about 1 mV. 

The reaction will occur in the absence of the quaternary ammonium salt (i.e., as a two-phase reaction) but with reduced product yields. Primary alcohols are less reactive than secondary alcohols, and reaction of the latter can be carried out in the presence of the former [e.g., only the secondary alcohol of PhCH(OH)CH2CH2OH undergoes dehydrogenation]. In these reactions, the phase-transfer catalyst may convert the alcohol to the alkoxide ion and, as well, displace the bridging chloride ligand of [RhICOljCI] by hydroxide or alkoxide. 

The theoretical basis of the use of a bridge containing a concentrated salt solution to eliminate liquid junction potentials is that the ions of this salt are present in large excess at the junction, and they consequently carry almost the whole of the current across the boundary. The conditions will be somewhat similar to those existing when the electrolyte is the same on both sides of the junction. When the two ions have approximately equal conductances, i.e., when their transference numbers are both about 0.5 in the given solution, the liquid junction potential will then be small [cf. equation (36a)]. The equivalent conductances at infinite dilution of the potassium and chloride ions are 73.5 and 76.3 ohins cm. at 25, and those of the ammonium and nitrate ions are 73.4 and 71.4 ohms cm. respectively the approximate equality of the values for the cation and anion in each case accounts for the efficacy of potassium chloride and of ammonium nitrate in reducing liquid junction potentials. 

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