Water, ionic product

Table 2.4 Thermodynamic constant of water ionic product (-temperature range 0-800 C and pressme range 0.1-500 MPa Shvarov, 1992).

The precipitation depends on and the pH since the water ionic product is obeyed ... 

The laboratory temperature during titration is rarely equal to the ambient temperature or process temperature. The sample pH will change with temperature since the dissociation constants and water ionic product change with temperature. This is a change in actual solution pH and is not to be confused with the change in millivolts generated by the glass electrode per the Nemst Equation in Chapter 4. Conventional temperature compensators use a temperature sensor embedded inside the electrode to correct for the Nemst effect. 

Water, ionic product, 103 Worked examples (Tables)... 

In the first publication describing the preparative use of an enzymatic reaction in ionic liquids, Erbeldinger et al. reported the use of the protease thermolysin for the synthesis of the dipeptide Z-aspartame (Entry 6) [34]. The reaction rates were comparable to those found in conventional organic solvents such as ethyl acetate. Additionally, the enzyme stability was increased in the ionic liquid. The ionic liquid was recycled several times after the removal of non-converted substrates by extraction with water and product precipitation. Recycling of the enzyme has not been reported. It should be noted, however, that according to the log P concept described in the previous section, ethyl acetate - with a value of 0.68 - may interfere with the pro-... 

Because the ionic product of water = [H ] [OH ] = 1.04 x 10" at 25°C, it follows that pH = 14 - pOH. Thus, a neutral solution (e.g., pure water at 25°C) in which [H j = [OH ] has a pH = pOH = 7. Acids show a lower pH and bases a higher pH than this neutral value of 7. The hydrogen ion concentrations can cover a wide range, from -1 g-ion/liter or more in acidic solutions to -lO" " g-ion/liter or less in alkaline solutions [53, p. 545]. Buffer action refers to the property of a solution in resisting change of pH upon addition of an acid or a base. Buffer solutions usually consist of a mixture of a weak acid and its salt (conjugate base) or of a weak base and its salt (conjugate acid). 

The typical strong acid of the water system is the hydrated proton H30+, and the role of the conjugate base is minor if it is a sufficiently weak base, e.g. Cl-, Br-, and C104. The conjugate bases have strengths that vary inversely as the strengths of the respective acids. It can easily be shown that the basic ionisation constant of the conjugate base KR canj is equal to Kw/KA conj, where Kw is the ionic product of water. 

The ionic product varies with the temperature, but under ordinary experimental conditions (at about 25 °C) its value may be taken as 1 x 10 14 with concentrations expressed in molL-1. This is sensibly constant in dilute aqueous solutions. If the product of [H + ] and [OH-] in aqueous solution momentarily exceeds this value, the excess ions will immediately combine to form water. Similarly, if the product of the two ionic concentrations is momentarily less than 10-14, more water molecules will dissociate until the equilibrium value is attained. 

Table 2.1 Ionic product of water at various temperatures...

The hydrolysis constant is thus related to the ionic product of water and the ionisation constant of the acid. Since Ka varies slightly and Kw varies considerably with temperature, Kh and consequently the degree of hydrolysis will be largely influenced by changes of temperature. 

Wash solutions for precipitates, 426 Washing of precipitates 118, 426 by decantation. 119 solubility losses in, 119, 427 Washing soda D. of sodium carbonate in, 295 Water absorbents for, 477 ammonia-free, 679 deionised, 90 D. of hardness, 332 D. of total cation concentration, 210 D. with Karl Fischer reagent 637 distilled, 90 high purity, 91 ionic product of, 36 types and standards for, (T) 90 volume of 1 g at various temperatures, (T)87... 

In aqueous solutions, H+ and OH ions are present, owing to the dissociation of water molecules. In dilnte solntions, the activity of water is constant. Hence, for the activities of these ions an eqnation of the type (3.17) is obeyed, too. The ionic product... 

Kw ionic product of water Ka = dissolution constant of weak acid ... 

As the pure solvent is only slightly ionized, both the activity coefficients and the concentration of the non-ionized solvent molecule may be regarded as unity, and one prefers to use Kw = [H30+][0H ], the so-called ionic product of water. It was determined for the first time by Kohlrausch and Heydweiller at 18° C from the conductivity, k = 0.0384 10"6 (cf., Ch. 2), which is given by... 

As in water, neutralization in all amphiprotic solvents represents the backward reaction of self-dissociation down to the equilibrium level of the ionic product in the pure solvent. 

The solvent dependence of the reaction rate is also consistent with this mechanistic scheme. Comparison of the rate constants for isomerizations of PCMT in chloroform and in nitrobenzene shows a small (ca. 40%) rate enhancement in the latter solvent. Simple electrostatic theory predicts that nucleophilic substitutions in which neutral reactants are converted to ionic products should be accelerated in polar solvents (23), so that a rate increase in nitrobenzene is to be expected. In fact, this effect is often very small (24). For example, Parker and co-workers (25) report that the S 2 reaction of methyl bromide and dimethyl sulfide is accelerated by only 50% on changing the solvent from 88% (w/w) methanol-water to N,N-dimethylacetamide (DMAc) at low ionic strength this is a far greater change in solvent properties than that investigated in the present work. Thus a small, positive dependence of reaction rate on solvent polarity is implicit in the sulfonium ion mechanism. 

Yakolev, Y. B. Kul ba,F. Y. Zenchenko, D. A., Potentiometric measurement of the ionic products of water in water-dimetylsulphoxide, water-acetonitrile and water-dioxane mixtures, Russ. J. Inorg. Chem. 20, 975-976 (1975). 

Lee et al.97 investigated the HF, HC1, and H2S dissociation reactions in water clusters. A good agreement between the DFT(BLYP) and the MP2 results was reported for binding energies (within 2 kcal/mol) and geometries. The studies showed also that at least four water molecules are needed for ionic products of dissociation to coexist. 

RhHI 3 (L=P(i-Pr)3) is capable of forming the water adduct (21). The water addition to RhHL3 takes place in pyridine readily at room temperature to give an ionic product [RhH2(py)2L2]0H (eq. 17), which can be isolated as its BPh salt (v(Rh-H) 2076, 2112... 

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