Reference electrolyte

Reference Electrolyte and conditions Breakdown potential of commercially pure titanium (V)... 

A representative ISE is shown schematically in Fig. 1. The electrode consists of a membrane, an internal reference electrolyte of fixed activity, (ai)i , ai and an internal reference electrode. The ISE is immersed in sample solution that contains analyte of some activity, (ajXampie and into which an external reference electrode is also immersed. The potential measured by the pH/mV meter (Eoe,) is equal to the difference in potential between the internal (Eraf.int) and external (Eref.ext) reference electrodes, plus the membrane potential (E emb), plus the liquid junction potential... 

E0 of Compound ferrocene moieties A ° Reference Electrolyte electrode"1 Ref. 

Glass electrode [see Fig 2.10 (1)J. The pH glass electrode, as the most important representative of the glass electrodes, will be the first subject to be treated, and especially in its application to aqueous solutions. Attached to the stem of high-resistance glass, the electrode proper consists of a pH-sensitive glass bulb that acts as a membrane between an inner reference electrolyte and an outer... 

Challenges remain in the development of lab-on-a-chip sensing systems. The overall lifetime of a sensor chip is always determined by the sensor with the shortest lifetime, which in most cases is the depletion of reference electrolytes. Measures to minimize cross-talking among sensors, especially when biosensors are integrated in the system, also should be implemented [122], The development of compatible deposition methods of various polymeric membranes on the same chip is another key step in the realization of multisensing devices. 

The differences in the solvation abilities of ions by various solvents are seen, in principle, when the corresponding values of As ivG° of the ions are compared. However, such differences are brought out better by a consideration of the standard molar Gibbs energies of transfer, AtG° of the ions from a reference solvent into the solvents in question (see further section 2.6.1). In view of the extensive information shown in Table 2.4, it is natural that water is selected as the reference solvent. The TATB reference electrolyte is again employed to split experimental values of AtG° of electrolytes into the values for individual ions. Tables of such values have been published [5-7], but are outside the scope of this text. The notion of the standard molar Gibbs energy of transfer is not limited to electrolytes or ions and can be applied to other kinds of solutes as well. This is further discussed in connection with solubilities in section 2.7. 

Poorly Conducting Samples. Samples with low ion concentrations (less than a few mM) will result in poor conductivity of the sample solution. Low conductivity not only causes a slow response time from the electrode but also creates a diffusion potential between the reference electrolyte and the measuring solution which results in an inaccurate pH reading. Using circular ground junctions that create optimal contact between the reference electrolyte and measuring solution can eliminate the problem. Adding conductive-free ions such as a few drops of... 

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. 

Protein-Rich Solution. When analyzing a sample with a high protein concentration, interference between the protein and the reference electrolyte may be a problem. Use of a suitable electrolyte is critical for accurate measurement. 

Sensitive to handling Reference electrode is not filled/Top up with electrolyte solution, free of air bubbles. Reference electrodes tilled with the wrong solution/Empty and refill the reference electrolyte. Diaphragm clogged/Clean diaphragm. Measurement of poorly conductive solutions/Measure with different amplifier or add supporting electrolyte. 

The limiting molar conductivity, A°°, for a reference electrolyte [(i-Am)4N+B(i-Am)4, (i-Am)3BuN+BPh4, etc.] is determined experimentally and t100/2 is considered to be the limiting conductivity of the constituent ions. 

Reynolds and Kraus (17) obtained conductance for 14 salts in acetone at 25°C, and used the Fuoss method to calculate their equivalent conductances at infinite dilution. Among the salts were tetra-n-butylammonium fluorotri-phenylborate, tetra-n-butylammonium picrate, lithium picrate, and tetra-n-butylammonium bromide. They then derived ionic equivalent conductances at infinite dilution by the method of Fowler (18) using tetra-n-butylammonium fluorotriphenylborate as the reference electrolyte and obtained a value of 188.7 12 1 cm2 eq-1 for Aq for lithium bromide. 

One final and useful correlation can be obtained from an investigation of the salt tri-isoamyl-n-butyl ammonium tetraphenylborate, and application of Stoke s Law (46). Coplan and Fuoss (47) have shown that the single ion conductances Xq and Aq for this salt are equal (to within 1 %), i.e. AJ = Aq = A0/2. Thus in principle it is possible to obtain single ion conductances for any species using this as a reference electrolyte [e.g. see Ref. (26)]. Furthermore, the corresponding Stoke s radii can then be estimated,... 

Reference fill hole Used to replace the reference electrolyte solution. 

The thermodynamic functions of transfer of individual ions cannot, of course, be studied experimentally, since only complete electrolytes are thermodynamic components, so that an extrathermodynamic assumption is needed in order to split the measurable quantity into the contributions from the individual ions. A commonly employed assumption is that for a reference electrolyte with large, univalent, nearly spherical cation and anion of nearly equal sizes the measured... 

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 ... 

Spoilage of the reference electrolyte is one of the major problems during long-term cultivations. Monzambe et al. [292] and Buhler (personal communication) have reported discrepancies of one pH unit between in situ on-line and off-line measurements which were caused by black clogging of the porous diaphragm. Either acidification or pressurization of the electrolyte was suitable to restrain this. 

The use of the word electrolyte is common in the battery industry to mean salt + solvent. We will use that convention here, realizing that, rigorously, such as in the more academic references, electrolyte refers to the salt alone. The salt that dissociates to provide the ionic conductivity of the electrolyte is formally known as the supporting electrolyte or solute. 

Recently reference electrolytes have been used to estimate medium effects. The assumption is based on a simple picture, Le., the ionic free energies of transfer of an uni-univalent electrolyte which is composed of large ions of equal size and similar surface, are the same for cation and anion. A reference electrolyte was used for the first time by Grunwald et al. namely, tetraphenylphosphonium-tetraphenyl-... 

Analogously, the following extrathermodynamic reference electrolyte assumptions are widely used ... 

The most convenient and generally accepted extrathermodynamie assumption is that using Ph4As Ph4B as the reference electrolyte (Ph = CgHs). 

The third approach may be called the reference electrolyte assumption. It is based on the assumption that the thermodynamic parameters of the electrolyte composed of a large and symmetrical anion and a similarly shaped cation of a 1 1 type can be divided into two equal components. This approach is due to Born [21] but was further developed by other workers who, as a reference electrolyte, suggested tetra-phenylphosphonium tetraphenylborate [28], tris(isoamyl)butylammonium tetra-phenylborate [29] and tetraphenylarsonium tetraphenylborate [30-33]. 

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