PH measurements nonaqueous

This brings up the question about nonaqueous pH measurements which are covered in more detail in Chapter 6. Most often an aqueous pH buffer solution is used to standardize the pH measuring system. If the measurement is to be made on a nonaqueous sample, the correlation between the activity of hydrogen ion in an aqueous standard and the activity in a nonaqueous sample is not valid. If, however, the pH value obtained is stable and can be correlated to some results, the hydrogen ion activity need not be known. The relative pH value can be used as an indicator to alter... 

If the electrode is allowed to dry, it may partially lose its pH function. Therefore it is imperative that a glass electrode which is used for nonaqueous pH measurements be soaked periodically in water to rejuvenate it. Even with partial dehydration, glass electrodes function properly for a moderate time period in nonaqueous solutions having a dielectric constant as low as 2.3 (see Table A.2). 

The reason potassium chloride is typically used as a filling solution is that it best meets all of the requirements for most applications. This filling solution, however, should not be the only one considered. In fact, in some applications this composition must be avoided, or perhaps another composition will provide better stability. This is particularly true when taking nonaqueous pH measurements. Thus the composition of the salt bridge may vary with the application. The conditions which a filling solution should meet are ... 

Hydrogen-iodine reaction, 13 770 Hydrogen-ion activity, 14 23-34 nonaqueous solvents, 14 32 pH determination, 14 24-27 pH measurement systems, 14 27-31 Hydrogen ion concentration (total acidity), 14 23... 

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. 

When the acid solution is highly diluted in water, the pH measurement is convenient, but it becomes critical when the acid concentration increases and, even more so, if nonaqueous media are employed. Since areference cell is used with aliquid junction, the potential at the liquid junction also has to be known. The hydrogen ion activity cannot be measured independently, and for this reason the equality of Eq. (1.9) cannot be definitely established for any solution. 

The solution chemistry of nonaqueous solvents is very different from that of water-rich mixed solvents. pH measurement in nonaqueous solvents is difficult or impossible. Salts often show a limited degree of dissociation and limited solubility (see [132] for solubility of salts in organic solvents). Ions that adsorb nonspecifically from water may adsorb specifically from nonaqueous solvents, and vice versa. Therefore, the approach used for water and water-rich mixed solvents is not applicable for nonaqueous solvents, with a few exceptions (heavy water and short-chain alcohols). The potential is practically the only experimentally accessible quantity characterizing surface charging behavior. The physical properties of solvents may be very different from those of water, and have to be taken into account in the interpretation of results. For example, the Smoluchowski equation, which is often valid for aqueous systems, is not recommended for estimation of the potential in a pure nonaqueous solvent. Surface charging and related phenomena in nonaqueous solvents are reviewed in [3120-3127], Low-temperature ionic liquids are very different from other nonaqueous solvents, in that they consist of ions. Surface charging in low-temperature ionic liquids was studied in [3128-3132]. 

Where potentiometric titrations, other than those involving pH measurement are concerned, such as precipitation, complexation, oxidation-reduction, nonaqueous media, etc., the data obtained will be in the form of E versus V. All of the titration theory, and that of titration curves, will apply to such titrations, as will the general methods of endpoint location. 

There are a number of buffer standards which have become common for a specific pH measurement application. These include photographic developer, heavy water (D2O), nonaqueous, seawater, and biological pH measurements. 

There are several difficulties which are inherent with pH measurements in mixed solvents. The objective of this section is to explain briefly the concept of pH in nonaqueous solvents and to discuss minimization of the difficulties. 

Added to the medium effect on activity is the hindering of the pH glass functioning by the solvent dehydrating the glass, by high sample resistance, and by large liquid junction potential developed at the reference electrode. These factors make nonaqueous pH difficult to measure and interpret. 

One reason for the observation of drift when making a nonaque-ous pH measurement is that the thickness of the gel layer, which surrounds a glass bulb, is changing due to dehydration. When the hydration rate equals the dissolution rate of the outer glass layer, equilibrium is established. 

The choice of pH meter can also help provide better stability when making pH measurements in nonaqueous solvents. A meter... 

The objective of this brief discussion on titrations is to relate the parameters discussed in other chapters to the problem associated with this technique. In other words, an aqueous or nonaqueous acid-base titration is nothing more than a series of pH measurements. All of the precautions, techniques, and equipment previ-... 

Despite its early date of pubhcation, one of Schwabe s most important publications till today concerns pH measurements under extreme conditions. It discusses extremely acidic and extremely alkaline solutions. It goes into measurements at high and low temperatures and tmder increased pressure. Finally nonaqueous solutions and the use of metal electrodes are discussed. For certain technical applications, particularly with extremely alkaline solutions, glass electrodes are not suitable as the glass surface is too severely affected this led to Schwabe s successful investigations into the use of bismuth electrodes for pH measurement. 

A third dominant field in Schwabe s work concerns his investigations of the behavior of concentrated electrolytic solutions, on acidity measurement, and on the activity of single ions. Some of the applications and results in this area are to be found again in his corrosion research and research into pH measurement for example, the investigations of corrosion in nonaqueous media and in trifluoroacetic acid. Schwabe s main message was the conclusion that in principle no single ion activities (beyond the Debye-Hiickel range) can be determined or calculated. In a number of more theoretical publications, Schwabe demonstrates that the solvation... 

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