Aqueous solutions dissociation constants

For example, a monobasic reagent HR dissociates in aqueous solution (dissociation constant ) and is distributed between the organic and aqueous phases (distribution coefficient/fDR). Thus... 

The water molecule acts as a base in an aqueous solution, dissociation constant... 

A couple of years ago, Kelly et al developed an implicit solvation model for aqueous solutions, related to those we discussed above in section HE. In a more recent work, they tested the model for the calculation of aqueous acid dissociation constants, i.e., the free energy change associated with the reaction... 

Hydrogen peroxide in aqueous solution is a weak dibasic aeid the dissociation constant for H2O2 — -1- HO2 is 2.4 x 10 ... 

The strength of a weak acid is measured by its acid dissociation constant, which IS the equilibrium constant for its ionization m aqueous solution... 

This table lists the and pi (pH at the isoelectric point) values of a-amino acids commonly found in proteins along with their abbreviations. The dissociation constants refer to aqueous solutions at 25°C. 

Mahc acid is a relatively strong acid. Its dissociation constants are given in Table 1. The pH of a 0.001% aqueous solution is 3.80, that of 0.1% solution is 2.80, and that of a 1.0% solution is 2.34. Many of its physical properties are similar to those of citric acid (qv). Solubihty characteristics are shown in Figure 1 and Table 1, densities of aqueous solutions are hsted in Table 2, and pH values vs concentration are shown in Figure 2. 

The equihbrium constant of this reaction is 5.4 x 10 at 25°C, ie, iodine hydrolyzes to a much smaller extent than do the other halogens (49). The species concentrations are highly pH dependent at pH = 5, about 99% is present as elemental at pH = 7, the and HIO species are present in almost equal concentrations and at pH = 8, only 12% is present as and 88% as HIO. The dissociation constant for HIO is ca 2.3 x 10 and the pH has tittle effect on the lO ion formation. At higher pH values, the HIO converts to iodate ion. This latter species has been shown to possess no disinfection activity. An aqueous solution containing iodate, iodide, and a free iodine or triodide ion has a pH of about 7. A thorough discussion of the kinetics of iodine hydrolysis is available (49). 

Common name Formula weight Melting poiut, °C Dissociation constants ia aqueous solution, 25°C AH° at 25°C kj / mol" Solubihty, g/100 g water Reference ... 

Amino-2-hydroxybenZOiC acid. This derivative (18) more commonly known as 4-aminosa1icy1ic acid, forms white crystals from ethanol, melts with effervescence and darkens on exposure to light and air. A reddish-brown crystalline powder is obtained on recrystallization from ethanol —diethyl ether. The compound is soluble ia dilute solutioas of nitric acid and sodium hydroxide, ethanol, and acetone slightly soluble in water and diethyl ether and virtually insoluble in benzene, chloroform or carbon tetrachloride. It is unstable in aqueous solution and decarboxylates to form 3-amiaophenol. Because of the instabihty of the free acid, it is usually prepared as the hydrochloride salt, mp 224 °C (dec), dissociation constant p 3.25. 

Thiosulfuric Acid. Thiosulfuiic acid [14921 -76-7] is relatively unstable and thus cannot be recovered from aqueous solutions. In laboratory preparation, a lead thiosulfate [26265-65-6] solution is treated with H2S to precipitate PbS, or a concentrated solution of sodium thiosulfate [7772-98-7] is treated with HCl and cooled to — 10°C to crystalline NaCl. Aqueous solutions of thiosulfuric acid spontaneously decompose to yield sulfur, SO2, and polythionic acids, H2S O. Thiosulfuric acid is a strong acid comparable to sulfuric acid. Dissociation constants, = 0.25, = 0.018, have been... 

These equiUbrium constants K, K, K, and are known (10). The piC values for the four dissociation steps as well as the proportions of the species present in aqueous solution as a function of pH are shown in Eigure 3. The reaction of Na2EDTA and, represented by equation 5 and noting that... 

The oxo-anions of chlorine are weaker oxidants than the corresponding acids. Because they are also more stable, it is not too difficult to isolate certain salts of those acids that can be obtained only in aqueous solution. Hypochlorites and chlorites are hydroly2ed in aqueous solution since HOCl and HCIO2 have acid dissociation constants of 10 and 10 , respectively however, aqueous chloric and perchloric acids are hiUy iorrhed. 

G. Kortiim, W. Vogel and K. Andrussow, Dissociation Constants of Organic Acids in Aqueous Solution, Butterworths, London, 1961. 

It is known that the order of acidity of hydrogen halides (HX, where X = F, Cl, Br, I) in the gas phase can be successfully predicted by quantum chemical considerations, namely, F < Cl < Br < I. However, in aqueous solution, whereas hydrogen chloride, bromide, and iodide completely dissociate in aqueous solutions, hydrogen fluoride shows a small dissociation constant. This phenomenon is explained by studying free energy changes associated with the chemical equilibrium HX + H2O + HjO in the solu-... 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

For example, in the case of dilute solutions, the van t Hoff s equation may be used to piedict the osmotic pressure (jr = CRT) where n is the osmotic pressure of the solution, C is the molar concentration of the solute, ft is the universal gas constant and T is the absolute temperature, Fm dissociating solutes, the concentration is that of the total ions. For example, NaCI dissociates in water into two ions Na" " and Cl . Therefore, the total molar concentration of ions is hvice the molar concentration of NaCI. A useful rule of thumb for predicting osmotic pressure of aqueous solutions is 0,01 psi/ppm of solute (Weber, 1972). 

Consider a neutral base B of such strength that it can be protonated in dilute aqueous solution in the acidic range, say pH 1-2. In the conventional manner the acid dissociation constant /ibh + is defined. 

Customarily, because the term [HgO] is essentially constant in dilute aqueous solutions, it is incorporated into the equilibrium constant Alto give a new term, K, the acid dissociation constant (where K = [HgO]). Also, the term [HjO ] is often replaced by H, such that... 

Hydrogen cyanide, mp —13.3° bp 25.7°, is an extremely poisonous compound of very high dielectric constant (p. 55). It is miscible with H2O, EtOH and Et20. In aqueous solution it is an even weaker acid than HE, the dissociation constant Ka being 7.2 x 10 ° at 25°C. It was formerly produced industrially by acidifying NaCN or Ca(CN)2 but the most modem catalytic processes are based on direct reaction between... 

In dilute aqueous solution H4P2O7 is a somewhat stronger acid than H3PO4 the 4 dissociation constants at 25° are ... 

Hydrogen peroxide is a somewhat stronger acid than water, and in dilute aqueous solutions has p a(25°) = 11.65 0.02, i.e. comparable with the third dissociation constant of H3PO4 (p. 519) ... 

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