Autoprotolysis constant for water

The product of the molar concentrations (or, more accurately, the activities) of the species produced as a result of autoprotolysis. The autoprotolysis constant for water is K, equal to [H30+][0H ], or 1.0 x IQ i at 25°C. It is a temperature-dependent constant, increasing with... 

As pointed out by Mayr,28 Ritchie,15 and Hine33,34 KR also measures the relative affinities of R+ and H30+ for the hydroxide ion. It can be regarded as providing a general affinity scale applicable to electrophiles other than carbocations.33,35 It can also be factored into independent affinities of R+ and H30+ as shown in Equations (2) and (3). Such equilibrium constants have been denoted If by Hine.33 AR corresponds to the ratio of constants for reactions (2) and (3) and, in so far as Kc for H30+ is the inverse of Kw the autoprotolysis constant for water, KR = KCKW... 

Kw= autoprotolysis constant for water Kapp = apparent ionization constant required to fit data... 

As well as the inherent effects of temperature on the ionization of weakly acidic and basic functional groups, precise calculation of pK values from potenti-ometric titration requires the autoprotolysis constant for water, pKw- This quantity has been shown by careful measurements [90, 91] to be very temperature dependent, with values ranging from 14.943s at 0 °C to 13.017i at 60 °C and 12.264 at 100 °C [92-94]. 

The p-notation can be used in writing logarithmic expressions of equilibrium constants. For example, the autoprotolysis constant for water is ... 

The expression on the right side is seen to be the autoprotolysis constant for water, with value l.OO X 10 (for [H2O] taken as 1 see Section 12-2) hence we have obtained the relation... 

The autoprotolysis constant for water, K, is obtained in a similar way by setting the activity of water in eqn 4.19 to its pure value ... 

The values of pH and pOH are related. To find that relation, we start with the expression for the autoprotolysis constant of water Kw = [H3Oh [Of I ]. Then we take logarithms of both sides ... 

The calculation of pH for very dilute solutions of a weak acid HA is similar to that for strong acids in Section 10.18. It is based on the fact that, apart from water, there are four species in solution—namely, HA, A, H,0 +, and OH. Because there are four unknowns, we need four equations to find their concentrations. Two relations that we can use are the autoprotolysis constant of water and the acidity constant of the acid HA ... 

Symbol for the product of the H+ concentration (or, H3O+ concentration) and the OH concentration of an aqueous solution the autoprotolysis constant. See Water, Temperature Effects of pK, of... 

The autoprotolysis constant for H20 has the special symbol Kw, where w stands for water Autoprotolysis... 

The pH scale, however, has its limitations. First of all, it is defined for water as the medium. Furthermore, the pH range is limited from 0 (the minimum value of pH in solution) to 14 (the maximum value related to the autoprotolysis constant of water). Moreover, the pH of concentrated solutions deviates significantly from the actual acidity. Below pH values of approximately 1.5, solutions become more acidic than their concentrations would suggest, since the pH beyond this range... 

Now consider a very dilute solution of a strong base, such as NaOH. Apart from water, the species present in solution are Na+, OH, and H30+. As we did for HCl, we can write down three equations relating the concentrations of these ions by using charge balance, material balance and the autoprotolysis constant. Because the cations present are hydronium ions and sodium ions, the charge-balance relation is... 

The pK for the autoprotolysis (more precisely, the autodeuterolysis, because a deuteron is being transferred) of heavy water (D20) is 15.136 at 20.°C and 13.8330 at 30.°C. Assuming AH° for this reaction to be independent of temperature, calculate A.Sr°for the autoprotolysis reaction. Suggest an interpretation of the sign. Suggest a reason why the autoprotolysis constant of heavy water differs from that of ordinary water. 

The recent introduction of non-aqueous media extends the applicability of CE. Different selectivity, enhanced efficiency, reduced analysis time, lower Joule heating, and better solubility or stability of some compounds in organic solvent than in water are the main reasons for the success of non-aqueous capillary electrophoresis (NACE). Several solvent properties must be considered in selecting the appropriate separation medium (see Chapter 2) dielectric constant, viscosity, dissociation constant, polarity, autoprotolysis constant, electrical conductivity, volatility, and solvation ability. Commonly used solvents in NACE separations include acetonitrile (ACN) short-chain alcohols such as methanol (MeOH), ethanol (EtOH), isopropanol (i-PrOH) amides [formamide (FA), N-methylformamide (NMF), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA)] and dimethylsulfoxide (DMSO). Since NACE—UV may present a lack of sensitivity due to the strong UV absorbance of some solvents at low wavelengths (e.g., formamides), the on-line coupling of NACE... 

Strategy Acetic acid is a weak acid consequently, we expect the molarity of H30+ ions to be less than 0.10 moI-L-1 and, therefore, its pH to be greater than 1.0. To find the actual value, we set up an equilibrium table S with the initial molarity of acid equal to 0.10 mol-L 1 and allow the molarity of acid to decrease by x mol-L1 to reach equilibrium. Assume that the presence of acid dominates the pH and therefore that the autoprotolysis of 5 water need not be considered. We assume x is less than about 5% of the ini-j rial molarity of acid and simplify the expression for the equilibrium constant f by ignoring x relative to the initial molarity of the acid. This assumption i must be verified at the end of the calculation. 

H20(1) H3CT(aq) + OH-(aq). autoprotolysis constant The equilibrium constant for an autoprotolysis reaction. Example For water, Km with Kw = [H30+][0H-]. 

For example, comparing ethanol and water as acids in isopropanol solution, Hine and Hine found ethanol to be 0.95 as acidic as water, and Kolthoff found ethanol to be 0.002S as basic as water. Accordingly, considering acidic and basic properties alone, we might expect the autoprotolysis constant of ethanol to be 0.95 x 0.0025 x 10 or 2.4 X 10" . The actual value is smaller, 3 x 10" , evidently in part because the low dielectric constant of 25 causes more association than expected. In addition, the associated structure of water and alcohols in the liquid state complicates estimates of this type. 

Glacial acetic acid, though like water in being classed as amphiprotic, represents a solvent type distinctly different from water in that it is a much weaker base. This weak basicity makes it a useful solvent for the titration of weakly basic substances. As mentioned in Section 4-3, the autoprotolysis constant has a p T h value of 14.45 for the reaction... 

Ethylenediamine (en), NH2C2H4NH2, a strongly basic substance, may be considered to represent solvents that are weakly acidic compared with water. Ethylenediamine is therefore useful as a solvent for the titration of weakly acidic substances. It is a leveling solvent for adds whose ionization constants are larger than about 10 in water thus acetic add and hydrochloric acid are leveled to about equal strength. The titrant base normally used in en is sodium ethanolamine. The autoprotolysis constant of en is 5 x 10" for the equilibrium... 

Although acid-base titrations in alcohol-water mixtures have been studied extensively, we do not consider them in detail since titration curves and indicator equilibria in ethanol-water and methanol-water mixtures can be calculated in the same way as in water. Values of the autoprotolysis constants of the mixtures, are close to for mixtures containing only a moderate amount of alcohol. On the other hand, even a trace of water in ethanol causes a large increase in J SH According to Gutbezahl and Grunwald, pXsH is 14.33, 14.88, 15.91, and 19.5 for ethanol-water mixtures containing 20, 50, 80, and 100 wt % ethanol. 

The extent of the autoprotolysis is a measure of both the acidic and basic strengths of the solvent and is given by the autoprotolysis constant or ionic product for example, for water K p = [H3O+] [OH ] = lO" (25°) and for sulfuric acid Kpp = [H3SOi+] [HSO4-] = 1.7 X 10- (10°). The autoprotolysis constant of sulfuric acid is greater than that for any other solvent that has been studied. Such a large value implies that, in spite of its very high acidity, sulfuric acid must also be appreciably basic. 

---