Reproducibility potential

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. 

It is fundamental that a reference electrode should have a stable and reproducible potential. Not all reference electrodes are suitable for all... 

Electrodes such as Cu VCu which are reversible with respect to the ions of the metal phase, are referred to as electrodes of the first kind, whereas electrodes such as Ag/AgCl, Cl" that are based on a sparingly soluble salt in equilibrium with its saturated solution are referred to as electrodes of the second kind. All reference electrodes must have reproducible potentials that are defined by the activity of the species involved in the equilibrium and the potential must remain constant during, and subsequent to, the passage of small quantities of charge during the measurement of another potential. 

Reference Electrode an equilibrium (reversible) electrochemical half-cell of reproducible potential against which an unknown electrode potential can be measured. Examples of those commonly used in corrosion are the Pt, H /H (the hydrogen electrode), Hg/Hg Clj/Cl" (the calomel electrode), Cu/CuS04/Cu, Ag/AgCl/Cl", all with fixed activities of the dissolved ions. 

The latter does not yield a completely reproducible potential equilibrium is established after about two days. Nonetheless, it is often used in research on technically important galvanic cells working in alkaline media. 

The described electrodes, and especially the silver chloride, calomel and mercurous sulphate electrodes are used as reference electrodes combined with a suitable indicator electrode. The calomel electrode is used most frequently, as it has a constant, well-reproducible potential. It is employed in variously shaped vessels and with various KC1 concentrations. Mostly a concentration of KC1 of 0.1 mol dm-3, 1 mol dm-3 or a saturated solution is used (in the latter case, a salt bridge need not be employed) sometimes 3.5 mol dm-3 KC1 is also employed. The potentials of these calomel electrodes at 25°C are as follows (according to B. E. Conway) ... 

Spore germination and protonemal growth and morphogenesis are therefore, useful systems to test in vitro, with good reproducibility, potential allelochemicals both through direct co-existence test in vitro or using the described bioassays to monitor/guide isolation, purification, characterization of chemical structure of bioactive compounds. 

The ideal reference electrode has a stable and reproducible potential versus the mobile phase, that does not vary with mobile phase changes. 

Potentiometric metal electrodes are rather simple but lack major analytical relevancy due to a number of considerable disadvantages. In particular, they are not very selective, some metals are easily oxidized and still others (Zn, Cd) can dissolve in acidic solutions. Further, certain harder metals such as iron, cobalt and nickel do not provide reproducible potentials. 

An exact potential measurement is difficult - particularly in organic electrochemistry - and probably requires very sophisticated techniques to avoid a variety of possible errors (e.g. [75]). Fortunately, for practical applications in electroorganic synthesis, it will usually be sufficient to get reproducible potentials for the current density-potential curves (see Fig. 1) as well as for the synthesis cell. A constant deviation in both measurements may be acceptable, even though the accurate value may be unknown. Some aspects will be discussed here, a more detailed overview is given, for example, in [3a]. 

There are three types of reference electrodes discussed reference electrodes of the first kind, reference electrodes of the second kind, and redox reference electrodes. The first two are used with potentiometric chemical sensors, whereas the last one helps us to get around the difficult problem of comparing potentials in different solvents. There is also a pseudo-reference electrode that does not have a stable, defined, reproducible potential. It serves only as the signal return to satisfy the condition of closing the electrical circuit (see Section 5.2). Because the liquid junction always causes some leakage of the internal solution, electrodes of the first kind are particularly affected. 

The election of the appropriate reference electrode for a given electrochemical system is conditioned by different factors (like the solvent or the temperature). The most important characteristic of a reference electrode is that it should provide a constant and reproducible potential difference when connected to the other semicell unit. For a detailed list of different reference electrodes, see [15-17]. 

In general a necessary part of a potentiometric measurement is the coupling of a reference electrode to the indicating electrode. The ideal reference electrode has a number of important characteristics (1) a reproducible potential, (2) a low-temperature coefficient, (3) the capacity to remain unpolarized when small currents are drawn, and (4) inertness to the sample solution. If the reference electrode must be prepared in the laboratory, a convenient and reproducible system is desirable. 

Compared with the hydrogen electrode reference electrodes are more advantageous as they are easy to prepare, easier to work with, and give perfectly reproducible potentials their electrolytes generally have so small liquid junction potentials that they can be disregarded. Since their potentials, compared with the standard hydrogen electrode are exactly known, the in-... 

Calomel electrode — is an - electrode of the second kind. It was introduced in 1890 by Ostwald, F.W. Asa- reference electrode of fixed, well-known, and very reproducible -+potential, it is still a commonly used reference electrode in electrochemistry [i—iii]. It consists of mercury, sparingly soluble mercurous chloride (calomel), and a chloride-containing solution. The electrode net reaction can be formulated in the following way ... 

The behaviour of the glass electrode has also been examined.42-43 The glass-calomel electrode system yields stable and reproducible potentials which vary in the normal way with changes in hydrogen ion concentration. However, the EMF of the couple shifts several hundred millivolts as the solution composition changes from water to hydrogen peroxide. Table 1.2 summarizes the apparent and true pH of aqueous solutions of hydrogen peroxide. 

Therefore the development of synthetic phases that can offer similar recognition properties seems desirable. One promising way to introduce selectivity in chemical analysis is the use of molecularly imprinted polymers (MIPs) [14-16]. These can in favourable cases recognise small molecules with affinities and selectivites exceeding that of antibody-antigen and have, due to their robustness, capacity and reproducibility, potential as reusable adsorbents in assays or sample pretreatment. 

To be considered a suitable reference electrode, an electrode must have a known and reproducible potential versus the NHE. This potential must be nearly an invariant of the current flowing through the electrode, which implies that the electrode reaction is extremely fast and the electrode reactants are extremely concentrated (see Secs. III.C.3 and IV.B.2). It should also have a small temperature coefficient and should be easily constructed in a reproducible way. A large variety of electrodes meeting these requirements to different degrees have been devised Table 2 presents a few of the most frequently used. 

The indicator electrode must have a stable and reproducible potential for a course of measurement and should be able to respond in a Nernstian manner to varying conditions in the high-temperature aqueous environment. In other words, the activity of the dissolved species, a,-, and the standard open-circuit potential, E°, should be, in principle, definable by measuring the open-circuit potential between the indicator and reference electrodes and applying the Nemst equation (17). 

As far as detection limits, selectivity, and Nemstian response ranges, the performance of CWEs is essentially analogous to that of conventional polymer membrane electrodes. However, one major drawback to their use appears to be the lack of reproducible potentials. Even within a single day, cell potentials can vary substantially for the same standard solution. As a result, CWEs must be recalibrated often, or better yet, standard addition or titration techniques must be utilized to ensure accurate results. Obviously, such analytical techniques could not be employed if in situ determinations are desired. 

The corrosion chemical inhibitor must meet certain requirements for each specific application such as stability against temperature, time, and exposure to the corrosive environment. It must function at low concentration and be easy to apply. Solubility characteristics must be designed for each application, and the inhibitor must be pumpable at the system temperature. It must be compatible with other chemicals in use and must meet performance specifications. It must inhibit corrosion. It must also be compatible with the system in which it is used and not cause system upsets. It cannot be too toxic and the flash point must be within specifications. Raw materials must be readily available and not too expensive, and manufacturing processes must be capable of control and reproducibility. Potential savings for each of those goals must be evaluated to determine if a program of corrosion inhibition will be economical. Because costs are sometimes difficult to estimate, the best method is to obtain data on maintenance, replacement, and so forth, from past history of the system to be protected or from a similar system. Literature on the economics of inhibition is a tremendous aid in estimating costs. 

---