Weak acid dissociation constants

Figure 2.3 shows how the chemistry of dissolved arsenious acid varies with pH. An analogous graph for arsenic acid is in Figure 2.4. As expected, protonated species of the acids are more common under low pH conditions were H+ is abundant. For both weak acids, dissociation constants (Ka values) may be derived to describe their gain or loss of H+ with changing pH conditions (Table 2.10 (Faure, 1998), 119-120). For example, the following reaction involving the dissociation of H3ASO3 in water at 25 °C and 1 bar pressure has a dissociation constant (K ) of HP9 2 (Wolthers et al., 2005), 3490 ...

The strength of a weak acid is measured by its acid dissociation constant, which IS the equilibrium constant for its ionization m aqueous solution... 

The C—H bonds of hydrocarbons show little tendency to ionize and alkanes alkenes and alkynes are all very weak acids The acid dissociation constant for methane for exam pie IS too small to be measured directly but is estimated to be about 10 ° (pK 60)... 

Monoprotic weak acids, such as acetic acid, have only a single acidic proton and a single acid dissociation constant. Some acids, such as phosphoric acid, can donate more than one proton and are called polyprotic weak acids. Polyprotic acids are described by a series of acid dissociation steps, each characterized by it own acid dissociation constant. Phosphoric acid, for example, has three acid dissociation reactions and acid dissociation constants. 

This relationship between and Kb simplifies the tabulation of acid and base dissociation constants. Acid dissociation constants for a variety of weak acids are listed in Appendix 3B. The corresponding values of Kb for their conjugate weak bases are determined using equation 6.14. 

A sample contains a weak acid analyte, HA, and a weak acid interferent, HB. The acid dissociation constants and partition coefficients for the weak acids are as follows Ra.HA = 1.0 X 10 Ra HB = 1.0 X f0 , RpjHA D,HB 500. (a) Calculate the extraction efficiency for HA and HB when 50.0 mF of sampk buffered to a pH of 7.0, is extracted with 50.0 mF of the organic solvent, (b) Which phase is enriched in the analyte (c) What are the recoveries for the analyte and interferent in this phase (d) What is the separation factor (e) A quantitative analysis is conducted on the contents of the phase enriched in analyte. What is the expected relative erroi if the selectivity coefficient, Rha.hb> is 0.500 and the initial ratio ofHB/HA was lO.O ... 

The principal limitation to using a titration curve to locate the equivalence point is that an inflection point must be present. Sometimes, however, an inflection point may be missing or difficult to detect, figure 9.9, for example, demonstrates the influence of the acid dissociation constant, iQ, on the titration curve for a weak acid with a strong base titrant. The inflection point is visible, even if barely so, for acid dissociation constants larger than 10 , but is missing when is 10 k... 

The plT at which an acid-base indicator changes color is determined by its acid dissociation constant. For an indicator that is a monoprotic weak acid, ITIn, the following dissociation reaction occurs... 

Since citric acid is a triprotic weak acid, we must first decide to which equivalence point the titration has been carried. The three acid dissociation constants are... 

Ethylenediaminetetraacetic acid, H4Y, is a tetraprotic weak acid with successive acid dissociation constants of 0.010,... 

Using its titration curve, determine the acid dissociation constant for the weak acid in problem 6. 

Both the carboxyl and the mercapto moieties of thioglycolic acid are acidic. Dissociation constants at 25°C are for pR, 3.6 pi, 10.5. ThioglycoHc acid is miscible ia water, ether, chloroform, dichloroethane and esters. It is weakly soluble ia aHphatic hydrocarbons such as heptane, hexane. Solvents such as alcohols and ketones can also react with thioglycolic acid. 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

The situation is more interesting when the solution is buffered with a weak acid, say HA. Then, if = (H )[A ]/[HA] is the apparent acid dissociation constant, we have... 

An inflection point in a pH-rate profile suggests a change in the nature of the reaction caused by a change in the pH of the medium. The usual reason for this behavior is an acid-base equilibrium of a reactant. Here we consider the simplest such system, in which the substrate is a monobasic acid (or monoacidic base). It is pertinent to consider the mathematical nature of the acid-base equilibrium. Let HS represent a weak acid. (The charge type is irrelevant.) The acid dissociation constant, = [H ][S ]/[HS], is taken to be appropriate to the conditions (temperature, ionic strength, solvent) of the kinetic experiments. The fractions of solute in the conjugate acid and base forms are given by... 

Consider the ionization of some weak acid, HA, occurring with an acid dissociation constant, Ai,. Then,... 

Acid Dissociation Constants and piC, Values for Some Weak Electrolyt (at 25°C) es... 

The Henderson-Hasselbalch equation provides a general solution to the quantitative treatment of acid-base equilibria in biological systems. Table 2.4 gives the acid dissociation constants and values for some weak electrolytes of biochemical interest. 

More specifically, the pi of any amino acid is the average of the two acid-dissociation constants that involve the neutral zwitterion. For the 13 amino acids with a neutral side chain, pi is the average of pKal and p/amino acids with either a strongly or weakly acidic side chain, pi is the average of the two lowest pKa values. For the three amino acids with a basic side chain, pi is the average of the two highest pKz values. 

The strength of weak acids is expressed by pA, the negative log of the acid dissociation constant. Strong acids have low pA values and weak acids have high pAl, values. 

The presentation finally descends on the very last of the listed chemical precipitation processes, and this pertains to precipitation implemented by carbonate anions (CO-). The dissolution of carbon dioxide takes place with the formation of the weak diabasic acid, H2C03. The overall dissociation of this acid (dissociation constant, KD) is represented by ... 

Formazans behave as weak acids as well as weak bases. Salts of formazans have been isolated.26,334,335 The acid dissociation constants of some substituted formazans have been determined from their solution spectra.336... 

Recall that, because of the strong H-F bond, hydrofluoric acid is a weak acid with a small acid dissociation constant Ka of 6.8 x 10-4. In contrast, the other binary acids of the halogen family—HC1, HBr, and HI—are strong acids that completely dissociate in water.) The fluorosilicic acid produced, H2SiF6, is a water-soluble substance with a structure as in Fig. 6.3.1. 

Alternatively one could suggest that a micellized sulfuric or sulfonic acid is not strong. For example, apparent acid dissociation constants of weak acids decrease when the acids are bound to anionic micelles (Hartley, 1948), and the rapid hydrolysis of micellized alkyl sulfates at low pH is consistent with... 

The 13C chemical shift pH dependence for both the Mo(IV) and W(IV) systems is similar to that observed in Fig. 6 for the Re(V), with the exception that only one protonation step is observed in weak to mild basic solutions (there is a large difference in the pKal and pKa2 values for both the Mo(IV) and W(IV) systems, see Table II). The corresponding acid dissociation constants were similarly determined as in the case of Re(V) complexes. 

Consider how a weak electrolyte is distributed across the gastric mucosa between plasma (pH 7.4) and gastric fluid (pH 1.0). In each compartment, the Henderson-Hasselbalch equation gives the ratio of acid-base concentrations. The negative logarithm of the acid dissociation constant is designated here by the symbol pAa rather than the more precisely correct pK1. 

Since this is the equilibrium constant associated with a weak acid dissociation, this particular Kc is the weak add dissociation constant, Ka. The IQ expression is ... 

The equilibrium for a weak acid is described by K, the acid dissociation constant. 

You can determine the value of for a particular acid by measuring the pH of a solution. In the following investigation, you will add sodium hydroxide to acetic acid, which is a weak acid. (See Figure 8.8.) By graphing pH against the volume of sodium hydroxide that you added, you will be able to calculate the concentration of the acetic acid. Then you will be able to determine the acid dissociation constant, Ka, for this acid. 

Table 8.2 lists the acid dissociation constants for selected acids at 25°C. Notice that weak acids have values that are between 1 and about 1 X 10 . Very weak acids have Ka values that are less than 1 x 10 . 

Table 8.2 Some Acid Dissociation Constants for Weak Acids at 25°C...

Propanoic acid, CH3CH2COOH, is a weak monoprotic acid that is used to inhibit mould formation in bread. A student prepared a 0.10 mol/L solution of propanoic acid and found that the pH was 2.96. What is the acid dissociation constant for propanoic acid What percent of its molecules were dissociated in the solution ... 

ACID DISSOCIATION CONSTANT, K CALCULATING THE pH OF A WEAK ACID... 

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