Potassium chloride salt bridge

With any endpoint detection system several practical considerations are important for reliable results. For example, the indicator electrode should be placed in close proximity to the flow pattern from the burette, so that a degree of anticipation is provided to avoid overrunning the endpoint. Another important factor is that the indicator electrode be as inert and nonreactive as possible to avoid contamination and erratic response from attack by the titration solution. A third and frequently overlooked consideration is the makeup of the reference electrode and, in particular, its salt bridge. For example, a salt-bridge system that contains potassium chloride can cause extremely erratic behavior of any electrochemical system if the titrant solution contains perchlorate ion (because of the precipitation of potassium perchlorate at the salt-bridge titrant-solution interface). Likewise, a potassium chloride salt bridge in a potentiometric titra-... 

A quinhydrone electrode in a solution of unknown pH was combined with a KC1(0.1 n), Hg2Cl2(s), Hg electrode through a saturated potassium chloride salt bridge the e.m.f. of the resulting cell was — 0.3394 volt at 30°. Calculate the pH of the solution. 

It is clear from equation 38 that the potential of the reference electrode is a function of the chloride ion concentration. In order to maintain a constant chloride ion concentration in varying conditions of humidity, a saturated solution is used. If the relative humidity decreases and evaporation of the reference electrode occurs, the excess chloride precipitates out of solution. Conversely, with high humidity the volume of solution increases slightly and additional potassium chloride dissolves. It is assumed, of course, that the temperature is constant. Electrical contact between the reference electrode and the solution being tested is maintained by means of a potassium chloride salt bridge. This junction is made through a fibrous or ceramic membrane (see Figure 1-5C, B) embedded in the bottom of the reference electrode or the side of a combination electrode. 

Experimentally, the hydrogen electrode is immersed in a series of solutions containing weak acids and their salts, the true dissociation constants of the acids being known. Connection of the chosen reference electrode to this solution is achieved by means of a saturated potassium chloride salt bridge. When the left-hand side of Equation (6.56) is plotted as a function of ionic strength, a reassessed value of E fgf is obtained as intercept on the n axis. No account has been taken of the liquid junction potential which exists between the salt bridge and the electrolyte solution and whose value varies with both the nature of the buffer used and its concentration. Such variations only produce uncertainties of the order of tenths of millivolts in the value of obtained and the scale of pH values thus obtained constitutes the conventional pH scale. 

Reference Electrodes and Liquid Junctions. The electrical cincuit of the pH ceU is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride [7447-40-7]. The solution makes contact at one end with the test solution and at the other with a reference electrode of constant potential. The Hquid junction is formed at the area of contact between the salt bridge and the test solution. The mercury—mercurous chloride electrode, the calomel electrode, provides a highly reproducible potential in the potassium chloride bridge solution and is the most widely used reference electrode. However, mercurous chloride is converted readily into mercuric ion and mercury when in contact with concentrated potassium chloride solutions above 80°C. This disproportionation reaction causes an unstable potential with calomel electrodes. Therefore, the silver—silver chloride electrode and the thallium amalgam—thallous chloride electrode often are preferred for measurements above 80°C. However, because silver chloride is relatively soluble in concentrated solutions of potassium chloride, the solution in the electrode chamber must be saturated with silver chloride. 

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. 

In the presence of bromide ions the electrode was subject to a drop in potential, (e.g., 1.5 to 5.7 mV at a Br iCl ratio of 2000 3) and to delayed response. A considerable hysteresis effect is also observed in concentrated solutions of chloride when the electrode is used in a 1M chloride solution and then dipped in one that is 0.02 M. Equilibrium is reached only after 10 min. The junction potential is minimised by diluting the test solution with the salt-bridge solution (10% aq. potassium nitrate). 

The outer tube has a porous fiber tip, which acts as the salt bridge to the analyte solution and the other half-cell. A saturated solution of potassium chloride is in the outer tube. The saturation is evidenced by the fact that there is some undissolved KC1 present. Within the inner tube is mercury metal and a paste-like material known as calomel. Calomel is made by thoroughly mixing mercury metal (Hg) with mercurous chloride (Hg2Cl2), a white solid. When in use, the following half-cell reaction occurs ... 

It consists of two beakers in which the two half-reactions can occur, one containing copper(II) sulfate solution and a copper rod as the positive electrode, the other containing zinc sulfate solution with a zinc rod as the negative electrode. The two solutions are brought into electrical contact with a salt bridge - a tube containing an agar gel and potassium chloride. 

For most potentiometric measurements, either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few mV. The silver/silver chloride electrode also finds application in non-aqueous solutions, although some solvents cause the silver chloride film to become soluble. Some experiments have utilised reference electrodes in non-aqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for non-aqueous systems as are any of the prototypes that have been developed to date. When there is a need to exclude water rigorously, double-salt bridges (aqueous/non-aqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the non-aqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence, the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause dramatic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

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. 

Finally, one should note that the mobilities of K+ and Cl are almost equal. It is for this reason that potassium chloride is frequently used in salt bridges in an attempt to avoid the contribution of liquid junction potentials to the cell potential. 

Minimization of the liquid junction potential is commonly carried out using a salt bridge in which the ions have almost equal mobilities. One example is potassium chloride (t+ = 0.49 and t =0.51) and another is potassium nitrate (t+ = 0.51 and / = 0.49). If a large concentration of electrolyte is used in the salt bridge this dominates the ion transport through the junctions such that the two values of have the same magnitude but opposing polarities. The result is that they annul each other. In this way values of E can be reduced to 1-2 mV. 

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 the measurement is finished, the selector must be switched to zero position, and the electrodes rinsed with distilled water and stored away. The glass electrode must be kept in water or dilute hydrochloric acid, while the salt bridge of the calomel electrode should be left dipped into concentrated potassium chloride. When finishing for the day, the pH-meter is switched off, otherwise it should be left on, rather than switched on and off frequently. 

In many instances, however, it has not yet been found possible to avoid a junction involving different electrolytes. If it is required to know the e.m.f. of the cell exclusive of the liquid junction potential, two alternatives are available either the junction may be set up in a reproducible manner and its potential calculated, approximately, by one of the methods already described, or an attempt may be made to eliminate entirely, or at least to minimize, the liquid junction potential. In order to achieve the latter objective, it is the general practice to place a salt bridge, consisting usually of a saturated solution of potassium chloride, between the two solutions that w ould normally constitute the junction (Fig. 70). An indication of the efficacy of potassium chloride in reducing the magnitude of the liquid junction potential is provided by thf. data in Table XLVII 3 the values iucorded are the e.m.f.of the cell, with free diffusion junctions,... 

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. 

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. 

This method of expressing the results is of little value for practical purposes the particular reference electrode and salt bridge employed should be standardized by means of equation (2) using one of the reference solutions in Table LXII. If the reference electrode is a calomel electrode with 0.1 N potassium chloride, and a liridge of a saturated solution of this electrolyte is employed, it has been found possible to express the experimental data by means of the equation... 

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