Bases base ionization constant

A double end point, acid—base titration can be used to determine both sodium hydrosulfide and sodium sulfide content. Standardized hydrochloric acid is the titrant thymolphthalein and bromophenol blue are the indicators. Other bases having ionization constants in the ranges of the indicators used interfere with the analysis. Sodium thiosulfate and sodium thiocarbonate interfere quantitatively with the accuracy of the results. Detailed procedures to analyze sodium sulfide, sodium hydro sulfide, and sodium tetrasulfide are available (1). 

Kb is also widely called the base ionization constant. 

In the presence of a base B, water acts as an acid by donating a proton, and the equiiibrium constant for the transfer of a proton from water to a base is caiied the base ionization constant (Zjj) ... 

It is usual to discuss the ionization of a base B in terms of its conjugate acid BH+ in order to use the same set of equations for both acids and bases. The ionization constant Ka is defined by... 

In the aromatic ring of hexamethylbenzene, the electron density is so much higher than in toluene, that when it is dissolved in liquid HF (Kilpatrick and Luborsky, 1953), a proton becomes attached to the hexamethylbenzene molecule and this hydrocarbon is ionized as a relatively strong base (the ionization constants for hexamethylbenzene and toluene in HF at 20° are 5 x 10-2 and 2 x 10-7 respectively). 

In performing a calculation based on an acid or base ionization constant expression such as Eqs. (13-7) or (13-8), there are often many unknowns. Remember that in an algebraic problem involving multiple unknowns, one needs as many equations as there are unknowns. The equilibrium constant expression itself is one equation, and the Kw expression is always available. Two other types of equation are often useful equations expressing... 

The peptides, which are composed of only a few amino acids, exhibit predictable behavior in CZE in that their mobility (electrophoretic migration) can be predicted on the basis of their size (mass) and their charge characteristics. The charge of such peptides can be predicted from the pKa values of individual amino acids contained in them. For larger peptides and proteins, the calculation of charge based on ionization constant is not trivial and can not be easily calculated based on the pKas of the free amino aids. Besides the mass-to-charge ratio, the other factors that affect mobility are hydrophobic-ity, primary sequence, conformational difference, and the chirality of amino acids. This point has been demonstrated by an investigation on several nonapeptides of identical composition but with different primary sequences ... 

Titrations in concentrated salt solutions Critchfield and Johnson found that bases with ionization constants as low as about 10 can be titrated with indicator methods in aqueous solutions containing a high concentration of a neutral salt. The acidity of the strong add titrant is increased by the high salt concentration. Thus in 7 M sodium iodide the pH shortly after the equivalence point in a titration of a weak base has an effective value of about zero instead of about 2 as in the usual titration. Since the pH before the equivalence point is essentially unchanged, the pH change in the equivalence-point region is enhanced by about 2 units. 

Because [H2O] is a constant, the constants are collected on the left-hand side of the equation to derive the base ionization constant expression ... 

A strong base is a substance that reacts completely with water, so that the base ionization constant defined in Eq. (14) or (15) is effectively infinite. This situation can only be realized if the conjugate acid of the strong base is very weak. A weak base will be characterized by a base ionization constant that is considerably less than unity, so that the position of equilibrium in the reaction represented in Eq. (12) favors the existence of unreacted free base. 

Because aqueous solutions of anions are commonly prepared by the dissolution of a salt containing that anion, reactions of the type described by Eq. (16) are often termed hydrolysis reactions. Eq. (16) is necessarily characterized by its base ionization constant expression ... 

For more practice writing base ionization constant expressions, go to Supplemental Practice Problems in Appendix A. 

Base ionization constants You won t be surprised to leam that like weak acids, weak bases also form equihbrium mixtures of molecules and ions in aqueous soluhon. Therefore, the equilibrium constant provides a measure of the extent of the base s ionization. The equilibrium constant for the ionization of methylamine in water is defined by this equilibrium constant expression. 

Write ionization equations and base ionization constant expressions for the following bases. 

For weak acids and weak bases, the value of the acid or base ionization constant is a measure of the strength of the acid or base. 

Write the equation for the ionization reaction and the base ionization constant expression for ethylamine (C2H5NH2) in water. 

Write the base ionization constant expression for ammonia. 

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