The ionic product of water

Kohlrausch and Heydweiller (1894) found that the most highly purified water that can be obtained possesses a small but definite conductivity. Water must 

Applying the Law of Mass Action to this equation, we obtain, for any given temperature  

Since water is only slightly ionised, the ionic concentrations will be small, and their activity coefficients may be regarded as unity the activity of the un-ionised molecules may also be taken as unity. The expression thus becomes  

In pure water or in dilute aqueous solutions, the concentration of the undissociated water may be considered constant. Hence  

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. 

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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 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. 

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). 

Thus [H-i]=[OH ] and fioo-i=Kw, the ionic product of water. Refer also to the last column in the Model matrix. 

This equilibrium constant or dissociation constant for the ionisation of water is known as the ionic product of water and is given the symbol K. As is an equilibrium constant, its value is dependent on temperature. At 24°C the value of is approximately 1 x 10 T... 

C) which he derived from the ionic product of water (Kw = 10 14 mol x dm 3). Some years later, Lewis introduced the concept of activity, and in 1923 Debye and Hiickel published their theory for strong electrolyte solutions. On the basis of this knowledge, Soerensen and Linderstroem-Lang [2] suggested a new pH definition in terms of the relative activity of hydrogen ions in solution ... 

Fig. 4. knjkh for spontaneous reactions. (Open circles are experimental points for nitramide, filled circles experimental points for acetic anhydride. The respective curves are calculated from equation (165) with jS=0-75 and from equation (166) with a=0-5. The bottom curve reproduces equation (131) for the ionic product of water.)... 

The other application explained here is the ionic product of water, pKw [11], which is essentially the equilibrium constant of the auto-ionization process of water (Kw =... 

The constant KV) which is called the ionic product of water, may be computed from equations (V-44) and (V-46), if partial molal free energies (potentials) of formation of all the reaction components in the corresponding standard states are known. For such standard states we select both the state of a hypothetical ideal solution with molal concentration of hydrogen and hydroxyl ions equalling unity and the state of hypothetical, absolutely undissociatcd pure water. Since in actual diluted solutions the activity of undissociated water hardly differs from the activity in its standard state, aji2 in the equation (V-51) may be considered as equalling unity so that then Km = K . The following expression is valid for a temperature of 25° C ... 

From accurate electrochemical measurements the ionic product, of water will result Kv = 1. 008 x 10-u. 

If we take into account the ionic product of water h+- oh 10-14 this equation can be transformed in the following manner ... 

Since, for dilute solutions, the activity a (Chap. 10) of water is considered to be constant and very close to 1.0, and the activities of the solutes may be represented by their concentrations, we can define a practical constant, Kw, called the ionic product of water ... 

The product of these two concentrations is known as the ionization constant of water, Kyf (or as the ionic product of water, or maybe sometimes as the autoprotolysis constant, Kap)... 

Just as in the acid pKa scale, the lower the pKa the stronger the acid, In the basic p/fe scale, the lower the pKb, the stronger the base. The two scales are related the product of the equilibrium constants simply equals the ionic product of water. 

KR4 is related to the association constant K of Eq. (1) through the ionic product of water Kw by K = KR+/KW. Although the symbol KROH has occasionally been used instead of Kku the latter is preferable since it stresses the similarity of the heterocyclic cation-pseudobase equilibrium and the carbonium ion-carbinol equilibrium for which KR+ was originally introduced by Deno et al,71... 

Table 1.8 The ionic product of water at various temperatures...

The importance of the ionic product of water lies in the fact that its value can be regarded as constant not only in pure water, but also in diluted aqueous solutions, such as occur in the course of qualitative inorganic analysis. This means that if, for example, an acid is dissolved in water, (which, when dissociating, produces hydrogen ions), the concentration of hydrogen ions can increase only at the expense of hydroxyl-ion concentration. If, on the other hand, a base is dissolved, the hydroxyl-ion concentration increases and hydrogen-ion concentration decreases. 

Finally, we know that in any aqueous solution the ionic product of water (cf. Section 1.18) ... 

For most purposes Ush may be replaced by the molecular concentration of solvent molecules in the pure solvent with water, for example, the concentration of water molecules in moles per liter is 1000/18, i.e., 55.5, so that the dissociation constant of H2O as an acid or base is equal to the ionic product of water divided by 55.5. 

The Ionic Product of Water.—An ionic product of particular interest is that of water the autoprotolytic equilibrium is... 

The equilibrium between H3O+ and OH ions will exist in pure water and in all aqueous solutions if the ionic strength of the medium is low, the ionic activity coefficients may be taken as unity, and hence the ionic product of water, now represented by is given by... 

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