Hydrogen acid constant

The rate of saponification of ethyl 2-thenoate, in contrast to ethyl 3-thenoate, was found to be considerably slower than predicted from the pKa of the acid, showing that the reactivities of thiophenes do not parallel those of benzene. The first explanation, that this was produced by a steric effect of the ring sulfur similar to the case in or /lo-substituted benzenes and in ethyl 1-naphthoate, could not be upheld when the same effect was found in ethyl 2-furoate. It was later ascribed to a stereospecific acid strengthening factor, involving the proper relation of the carboxylic hydrogen and the heteroatom, as the rate of saponification of 2-thienylacrylic acid was in agreement with that predicted from the acid constants. ... 

It is worth mentioning that an attempt was made by Tsao and Willmarth to determine the acid dissociation constant of HO2. The reaction between hydrogen peroxide and peroxydisulphate was used for the generation of the HO2 radical. However, these experiments, like others where the HO2 radical is studied under steady-state conditions, could yield only a value of acidity constant multiplied by a coefficient consisting of a ratio of kinetic parameters. Unfortunately, in this case there are no independent data for the kinetic coefficient, and the value of cannot be evaluated. Considering the kinetic analogue of the titration curve it can be stated only that ionization of HO2 becomes important in the pH range from 4.5-6.5. The value of acidity constant of HO2 obtained by Czapski and Dorfman is (3.5 + 2.0)x 10 mole.l. . ... 

The representation of surface groups as diprotic weak acids is appealing because it includes a modest degree of complexity (two acidity constants), allows convenient representation of the condition of zero surface excess of hydrogen ion, and is still quite manageable mathematically. However, it must be borne in mind that this model is still a grossly simplified representation of the actual surface. It remains to be shown that this simplification is significantly better than any other simplification. 

Cyclic voltammograms of PtSn microelectrodes in 0.5 M sulfuric acid solution are shown in Fig. 15.6. The potential range was -200 to 800 mV (vs. SCE) and the scan rate was 100 mV/s. It can be seen clearly that hydrogen desorption from the PtSn-2 electrode is seriously inhibited compared with that from the PtSn-1 electrode. From the hydrogen desorption peak areas in the CV curves and the Pt single crystallite hydrogen desorption constant of 210 /xC/cm Pt, the electrochemical surface areas (ESA) for PtSn-1 and PtSn-2 were calculated to be 391 and 49 cm /mg, respectively. However, it is evident from XRD and TEM results that the two catalysts have similar particle size and so they should possess the similar physical surface area. The difference... 

When determining Kia values of organic acids, one generally uses techniques by which the hydrogen ion activity [pH = -log (Ya+ [H+])] is measured, while HA and A are determined as molar concentrations. Thus, many acidity constants reported in the literature are so-called mixed acidity constants which are operationally defined for a given aqueous medium (e.g., 0.05 - 0.1 M salt solution) ... 

Figure 8.5 Examples of proximity effects on acidity constants (a) hydrogen bonding and (b) steric interactions.

In the second part of Table 9-3, the class of weak acids with first acid constant about 10 y there are several acids in which the number of hydrogen atoms is one less than the number of oxygen atoms. These include acids such as chlorous acid, CIO(OH) sulfurous acid, SO(OH phosphoric acid, PO(OH)8 and periodic acid, IO(OH)5. 

Also given in this class are two acids, phosphorous acid and hypo-phosphorous acid, which seem to be out of place, inasmuch as their formulas, H8POs and H PO, seem not to put them in this class. Their acid constants, l.fi X 10 2 and 1 X 10 2, respectively, are, however, appropriate to the class, and an explanation must be sought for the apparent abnormality. The explanation is that one of the hydrogen atoms in phosphorous acid is bonded directly to the phosphorus atom, and two of the hydrogen atoms in hypophosphorous acid are bonded... 

One important application of the Nernst equation is the measurement of pH (and, through pH, acidity constants). The pH of a solution can be measured electrochemically with a device called a pH meter. The technique makes use of a cell in which one electrode is sensitive to the H30+ concentration and the second electrode serves as a reference. An electrode sensitive to the concentration of a particular ion is called an ion-selective electrode. One combination is a hydrogen electrode connected through a salt bridge to a calomel electrode. The reduction half-reaction for the calomel electrode is... 

The acidity scale in anhydrous hydrogen fluoride has been the subject of electrochemical investigations by Tremillon and coworkers48 and is presented in Figure 1.9. The figure also indicates the acidity constants of various Lewis acids allowed to buffer the medium to a pH value as calculated by Eq. (1.33), or in dilute solution by Eq. (1.34). 

Table 5 shows the rate constants for hydrogen exchange in the CH3 group of quinaldine with alcohols at 120° (fc120). In addition, values of the relative acidity constants (Kf) are also given. The latter were determined by the indicator method (Hine and Hine, 1952). The... 

Early workers in the field of H20-D20 systems recognized that it was possible to define different acidity constants for the isotopic hydrogen ions, that for an isotopically mixed ion it was necessary to distinguish between proton acidity and deuteron acidity and that it... 

We can define six acidity constants and, as shown in equations (58) to (63), substitute in the defining equations for the concentrations of water and hydrogen ion isotopes [see equations (18) to (24) and (27)]. The superscript and subscript respectively denote the hydrogen isotope to which the particular acidity constant refers and the isotopic hydrogen ion concerned. 

Acidity constant — Kan (sometimes also called acid dissociation constant , or acid-ionization constant ) is an equilibrium constant indicating the extent of the dissociation of the nth hydrogen ion from a charged or uncharged acid [i] ... 

The illustrated compound, l-phenyl-2-propenone, doesn t yield an anion when treated with base because the hydrogen on the a carbon is vinylic and isn t acidic (check Table 22.1 for acidity constants). 

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