Rates Liquid junction potential

In view of the problems referred to above in connection with direct potentiometry, much attention has been directed to the procedure of potentio-metric titration as an analytical method. As the name implies, it is a titrimetric procedure in which potentiometric measurements are carried out in order to fix the end point. In this procedure we are concerned with changes in electrode potential rather than in an accurate value for the electrode potential with a given solution, and under these circumstances the effect of the liquid junction potential may be ignored. In such a titration, the change in cell e.m.f. occurs most rapidly in the neighbourhood of the end point, and as will be explained later (Section 15.18), various methods can be used to ascertain the point at which the rate of potential change is at a maximum this is at the end point of the titration. 

The foregoing text highlights the fact that at the interface between electrolytic solutions of different concentrations (or between two different electrolytes at the same concentration) there originates a liquid junction potential (also known as diffusion potential). The reason for this potential lies in the fact that the rates of diffusion of ions are a function of their type and of their concentration. For example, in the case of a junction between two concentrations of a binary electrolyte (e.g., NaOH, HC1), the two different types of ion diffuse at different rates from the stronger to the weaker solution. Hence, there arises an excess of ions of one type, and a deficit of ions of the other type on opposite sides of the liquid junction. The resultant uneven distribution of electric charges constitutes a potential difference between the two solutions, and this acts in such a way as to retard the faster ion and to accelerate the slower. In this way an equilibrium is soon reached, and a steady potential difference is set up across the boundary between the solutions. Once the steady potential difference is attained, no further net charge transfer occurs across the liquid junction and the different types of ion diffuse at the same rate. 

A liquid junction potential E-f forms when the two half-cells of a cell contain different electrolyte solutions. The magnitude of Ej depends on the concentrations (strictly, the activities) of the constituent ions in the cell, the charges of each moving ion, and on the relative rates of ionic movement across the membrane. We record a constant value of j because equilibrium forms within a few milliseconds of the two half-cells adjoining across the membrane. 

Examination of the behaviour of a dilute solution of the substrate at a small electrode is a preliminary step towards electrochemical transformation of an organic compound. The electrode potential is swept in a linear fashion and the current recorded. This experiment shows the potential range where the substrate is electroactive and information about the mechanism of the electrochemical process can be deduced from the shape of the voltammetric response curve [44]. Substrate concentrations of the order of 10 molar are used with electrodes of area 0.2 cm or less and a supporting electrolyte concentration around 0.1 molar. As the electrode potential is swept through the electroactive region, a current response of the order of microamperes is seen. The response rises and eventually reaches a maximum value. At such low substrate concentration, the rate of the surface electron transfer process eventually becomes limited by the rate of diffusion of substrate towards the electrode. The counter electrode is placed in the same reaction vessel. At these low concentrations, products formed at the counter electrode do not interfere with the working electrode process. The potential of the working electrode is controlled relative to a reference electrode. For most work, even in aprotic solvents, the reference electrode is the aqueous saturated calomel electrode. Quoted reaction potentials then include the liquid junction potential. A reference electrode, which uses the same solvent as the main electrochemical cell, is used when mechanistic conclusions are to be drawn from the experimental results. 

If two electrolyte solutions that are of different concentrations but in the same solvent contact each other at a junction, ion transfers occur across the junction (Fig. 6.3). If the rate of transfer of the cation differs from that of the anion, a charge separation occurs at the junction and a potential difference is generated. The potential difference tends to retard the ion of higher rate and accelerate the ion of lower rate. Eventually, the rates of both ions are balanced and the potential difference reaches a constant value. This potential difference is called the liquid junction potential (LJP) [10]. As for the LJP between aqueous solutions, the LJP between non-aqueous solutions can be estimated using the Henderson equation. Generally the LJP, Lj-, at the junction Ci MX(s) c2 NY(s) can be expressed by Eq. (6.1) ... 

B. Effects of specific interfacial area of a microdroplet and liquid-junction potential on the interfacial electron transfer rate 188... 

B. Effects of Specific Interfacial Area of a Microdroplet and Liquid-Junction Potential on the Interfacial Electron Transfer Rate... 

The logarithmic response of ISEs can cause major accuracy problems. Very small uncertainties in the measured cell potential can thus cause large errors. (Recall that an uncertainty of 1 mV corresponds to a relative error of 4% in the concentration of a monovalent ion.) Since potential measurements are seldom better than 0.1 mV uncertainty, best measurements of monovalent ions are limited to about 0.4% relative concentration error. In many practical situations, the error is significantly larger. The main source of error in potentio-metric measurements is actually not the ISE, but rather changes in the reference electrode junction potential, namely, the potential difference generated between the reference electrolyte and sample solution. The junction potential is caused by an unequal distribution of anions and cations across the boundary between two dissimilar electrolyte solutions (which results in ion movement at different rates). When the two solutions differ only in the electrolyte concentration, such liquid junction potential is proportional to the difference in transference numbers of the positive and negative ions and to the log of the ratio of the ions on both sides of the junction ... 

This liquid junction potential or diffusion potential is caused by slight separation of ionic charges that results from the tendencies of the various ions to diffuse at unequal rates across the boundary of the solutions. The effects of the liquid junction potential can be minimized by using a... 

Morse (21) recently reported near-equilibrium pH-stat rates in sea water at 25°C and 10 atm CO2. His rates are all less than 8 mg cm yt . Table III compares our calculations (using equation 15 and assuming surface PCO2 is equal to the bulk fluid value) of a set of Morse s rates near equilibrium, and shows generally poor agreement. In calculation of Q and aH (s), no correction for liquid junction potential error in measured pH was necessary, as in our earlier calculations for pseudo-sea water. 

When immersed in solution, the reference electrode in Fig. 10.5 usually fulfills its role of simply completing the electrical circuit. In a colloidal suspension, however, the colloid may cause K+ and CP to diffuse at different rates. Because of attraction or repulsion by the charged colloid, one ion moves ahead of the other. Ion separation at the junction between the electrode solution and the suspension produces a charge separation or electrical potential, the liquid-liquid junction potential ( j). Accurate... 

The diffusion potential results from a tendency of the protons in the inner part of the gel layer to diffuse Toward the dry membrane, which contains —SiO Na, and a tendency of the sodium ions in the dry membrane to diffuse to the hydrated layer. The ions migrate at a different rate, creating a type of liquid-junction potential. But a similar phenomenon occurs on the other side of the membrane, only in the opposite direction. These in effect cancel each other, and so the potential of the membrane is determined largely by the boundary potential. (Small differences in boundary potentials may occur due to differences in the glass across the membrane—these represent a part of the asymmetry potential.)... 

Liquid junctions. When two different liquids are in contact, usually through a porous separation, it is called a liquid junction. The concentrations of ions on either side of a liquid junction are generally not equal, hence there is a diffusional flow of ions. If the rates of flow of the different ions are unequal, a potential difference will be generated across the liquid junction. Such a potential is called the liquid junction potential. The liquid junction potential may be reduced by the use of a salt bridge, in which the flow of the positive and negative ions are nearly equal. 

Another factor which may influence the liquid junction potential is termed the "suspension effect" in which the presence of colloids or suspended particles, e.g., red blood cells, produce an anomalous liquid junction potential. It has been suggested that this phenomenon is caused by the effect of colloidal particles on the relative rates of diffusion, i.e., transference numbers, of the salt bridge electrolyte. Another possibility is that colloids with ion-exchange properties give rise to a Donnan potential across the suspension/supernatant liquid interface. Whatever the cause, the effect may be significant and must be avoided in accurate studies with electrodes. 

A liquid junction potential arises at the interface because in the case that r+ r, the different rates of diffusion of the two ions from high to low concentration generates a charge separation at the interface and hence a potential difference known as a liquid junction potential, ljp- According to the classical theory of liquid junction potentials [P. Henderson, Z. PhysiL Chem. 59 (1907) 118], for a salt and X , this has a limiting magnitude of... 

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