Ion product, of water

Ion Product. The ion product of water is the product of the molality of the hydrogen and hydroxide ions, oh - temperature... 

Indeed, because most hydrogen atoms in liquid water are hydrogen-bonded to a neighboring water molecule, this protonic hydration is an instantaneous process and the ion products of water are and OH ... 

The activity of the solvent molecule HS in a single-component solvent is constant and is included in Kus. The concentration of ions is mostly quite low. For example, self-ionization occurs in water according to the equation 2H20— H30+ + OH". The conductivity of pure water at 18°C is only 3.8 X 10"8 Q"1 cm-1, yielding a degree of self-ionization of 1.4xl0"19. Thus, one H30+ or OH" ion is present for every 7.2 x 108 molecules of water. Some values of Kus are listed in Table 1.5 and the temperature dependence of the ion product of water Kw is given in Table 1.6. 

Table 1.6 Ion product of water at different temperatures. (According to B. E. Conway)... 

Figure 2. Temperature dependence of molal ion product of water (a) Fisher and Barnes (36) (b) Sweeton et at. (35) (c) Sirota and Shviraev (31) (d) Correspondence Principle estimate of Lewis (91)...

The product [H ] [OH"]—the ion product of water—is constant even when additional acid-base pairs are dissolved in the water. At 25 °C, pure water contains H and OH" at concentrations of 1 10" mol L" each it is neutral and has a pH value of exactly 7. 

Kso can be obtained from K o by replacing by K laon- (K = ion product of water) as shown in the following example involving hematite in a low ionic strength solution at 25 °C ... 

The ion product of water depends on the ionic strength of the system and on its temperature. At 25 °C and in low ionic strength solution, log = -13.99, whereas in 3 M NaC104 (the ionic medium used by Schindler et al., 1963 for solubility product determination), log = -14.22 + 0.1 the value chosen must correspond to the ionic strength of the system involved. 

The pH of pure water at 25°C is 7. Most (but not all) CD reactions take place in basic solntions at typical pH values of 9-12. Since hydroxide intermediates are often important in CD, it is worth noting that a pH of 10 is equivalent to a hydroxide ion concentration of 10 " M at 25°C (since the ion product of water, [H ][OH ], = 10 " at this temperature). As will be discussed shortly, this ion prodnct is very temperature dependent, and so the OH concentration at any par-ticnlar pH varies considerably with temperatnre. 

Based on this equilibrium, we can express [Cd ] in terms of [OH ] for the case where Cd(OH)2 will just precipitate at higher pH it will precipitate and at lower pH it will not. Since [OH ] can be converted into [H ] (and therefore pH) through the ion product of water (see Chap. 1), a graph can be made of pH vs. [Cd ] (or p[Cd ], which, analogously to pH, is equal to minus the logarithm of [Cd ]). This is the hydroxide line in Figure 3.1. Its physical meaning is that above this line, Cd(OH)2 will be present in the solution, while below it there will be no Cd(OH)2. 

Theoretical thermodynamic calculations of the conditions under which Cd(OH)2 should form were also carried out, based on the solubility product of Cd(OH)2, the stability constants of the Cd-NTA system, and the ion product of water at different temperatures. The values of Rc computed from these calculations agreed with those measured experimentally. 

What is the essence of a neutralization process How can the neutrality of a solution be determined At what hydrogen ion concentration is a reaction neutral What is the ion product of water What is the value of the ion product of water at 22 °C In what volumes... 

Kfo and Kia are quantitatively interrelated by the ionization constant of water (ion product of water), Kw ... 

As the ion product of water is constant, whenever [H+] is greater than 1 X 10-7 m, [OH-] must become less than 1 X 10-7 m, and vice versa. When [H+] is very high, as in a solution of hydrochloric acid, [OH-] must be very low. From the ion product of water we can calculate [H+] if we know [OH-], and vice versa (Box 2-2). 

The value for KeQ, determined by electrical-conductivity measurements of pure water, is 1.8 X 10-16 m at 25 °C. Substituting this value for Ke[Pg.61]

Thus the product [H+][OH-] in aqueous solutions at 25 °C always equals 1 X 10-14 m2. When there are exactly equal concentrations of H+ and OH-, as in pure water, the solution is said to be at neutral pH. At this pH, the concentration of H+ and OH- can be calculated from the ion product of water as follows ... 

The ion product of water makes it possible to calculate the concentration of H+, given the concentration of OH ", and vice versa the following problems demonstrate this. 

The value of 7 for the pH of a precisely neutral solution is not an arbitrarily chosen figure it is derived from the absolute value of the ion product of water at 25 °C, which by convenient coincidence is a round number. Solutions having a pH greater than 7 are alkaline or basic the concentration of OH is greater than that of H+. Conversely, solutions having a pH less than 7 are acidic. 

The ion product of water is the product of the molality of the hydrogen and hydroxide ions, A",. = mH >ntjn The ion product increases with temperature to 2501C and then declines. The initial increase is the temperature effect, and the later decline is on account of the decline in the dielectric constant of water. This variation means that neutral pH, which is the square root of the ion product, varies with temperature. 

---