Reference electrodes electrolytes

Problems with reference electrode/electrolyte. The sample solution can react with the electrolyte. The greatest source of error in biological solutions is through the formation of insoluble silver sulfide, often at the ceramic frit. A blackened spot is usually observed in a pH electrode that has been in service for a few weeks. This precipitate can impede the free flow of electrolyte and cause the probe response to become sluggish and cause large errors in the measured pH. 

An acceptable method quite frequently used in practice depends on the cell whose EMF is being measured having a liquid junction with a constant potential value. Such a situation is attained in the determination of the activity of fluoride ions, by adding a constant amount of quite concentrated buffer, for example TISAB, to the studied solution this buffer also fulfills other functions in the analysis (see p. 146). Then the liquid junction potential is a function of the composition of the reference electrode electrolyte and of the buffer composition alone, and not of the concentrations of the other components of the studied solution. 

Semiaqueous or Nonaqueous Solutions. Although the measurement of pH in mixed solvents (e.g., water/organic solvent) is not recommended, for a solution containing more than 5% water, the classical definition of a pH measurement may still apply. In nonaqueous solution, only relative pH values can be obtained. Measurements taken in nonaqueous or partly aqueous solutions require the electrode to be frequently rehydrated (i.e soaked in water or an acidic buffer). Between measurements and after use with a nonaqueous solvent (which is immiscible with water), the electrode should first be rinsed with a solvent, which is miscible with water as well as the analyte solvent, then rinsed with water. Another potential problem with this type of medium is the risk of precipitation of the KC1 electrolyte in the junction between the reference electrode and the measuring solution. To minimize this problem, the reference electrolyte and the sample solution should be matched for mobility and solubility. For example, LiCl in ethanol or LiCl in acetic acid are often used as the reference electrode electrolyte for nonaqueous measurements. 

Give some ways in which reference-electrode openings are constructed in order to minimize cross contamination between the reference-electrode electrolyte and the electrochemical-cell electrolyte. Can the openings be made too small Explain. 

The electrolyte solution of reference electrodes may serve two functions (a) to provide a constant potential of the reference electrode, and (b) to serve as the electrolyte bridge to the analyte solution, ideally with negligible diffusion potentials. The possibilities to contact the reference electrode to the adjacent solution vary greatly. The most common arrangements are shown in Fig. III.2.6. A detailed description of the diaphragms is given below. When such arrangement is used, the levels of the analyte and the reference electrode electrolytes should be balanced to prevent any contamination, either of the analyte or the reference electrode compartment. 

Figure 3.49 Double layer capacity of silver monocrystal of different orientation plotted as a function of potential measured against the saturated calomel reference electrode. Electrolyte is 0.01 M NaF (adapted from ref. [17]).

This chapter is concerned with electrodes. Section 3.1 provides information about the structure and characteristics of glass electrodes along with possible sources of error, such as sodium ion interference. This should assist in optimum selection and use of electrodes. In Section 3.2, the structure and functioning of reference electrodes are discussed. The liquid junction between the reference electrode electrolyte filling solution and the sample can introduce a junction potential, which is a major source of error in pH measurement. This subject is discussed in detail. The combination electrode is compared with an electrode pair and its advantages in small-volume samples or flat-surface measurements are discussed in Section 3.3. 

Versus Ag wire as Ag/Ag" quasi-reference electrode electrolyte Bu4N" C104 solvent Me-CN scan rate 50 mV s Solvent AI2CI2 glassy carbon electrode. 

Versus Ag wire as Ag/Ag quasi-reference electrode electrolyte Bu4N C104 solvent MeCN scan rate 20 mV s . log K = AE 0.059. Data from ref 99. Solvent CH2CI2. Measured under the same experimental conditions. 

The flow-thru cell constructions already long employed for industrial pH measurements can also be used with ion-selective electrodes. Here the reference electrode is positioned further along in the flow direction than the indicating electrode to avoid interferences from outflowing reference electrode electrolyte. A sin e reference electrode can be used with a number of different ion-selective electrodes placed in the sample channel. If the individual electrodes are placed in the center of the sample stream, quick responses to activity changes can indeed be obtained, but streaming potentials (see Chap. 1.2) of up to 1.3 V, depending on the conductivity, sample channel diameter and flow rate, must be dealt with. Van den Winkel, Mertens and... 

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