Moderately weak acids

Derivatives of alkaline, alkaline earth metals and phenols, naphtols, an-troles, etc. (pK> 10). These compounds are salts in their nature and, as the salts of strong bases and moderately weak acids, can exist in water solutions. They are soluble only in polar solvents (water, liquid ammonia), are prone to form adducts with phenols, water, etc., have high thermal stability and cannot be transferred into the gas phase. 

COSMO models [27-30] were compared with the SM approach [22] by Klamt [43] and by Cramer and Truhlar [44]. A very recent paper by Klamt and coworkers [45] shows that improved calculated pKa values are obtained for the limited domain of strong to moderately weak acids by a cluster-continuum method in which the acid and conjugate base are each associated with one or a few solvent molecules and this cluster is then continuum-calculated with COSMO-RS. The authors point out, however, that for the calculation of pKa a consistent and generally applicable method is still lacking . This paper clarifies the problem raised in [41]. The matter is under study.1... 

Although nitrous acid has never been isolated as a pure compound, its aqueous solution is a widely used reagent. Nitrous acid is a moderately weak acid with p/fa = 3.35 at 291 K. In the gaseous state, it adopts the trans-planar structure, as shown in Fig. 15.2.2(c). 

Nitrous acid is a moderately weak acid (p/fa = -3.35 at 291 K). Aqueous solutions of nitrite salts contain nitrous acid and are therefore not very stable. Nitrous acid is an extremely weak base ... 

If the acid is very weak, e.g., phenol or boric acid, or a very dilute solution of a moderately weak acid is employed, the initial conductance is extremely small and the addition of alkali is not accompanied by any decrease of conductance, such as is shown in Fig. 25. The conductance of the solution increases from the commencement of the neutralization as the very weak acid is replaced by its salt which is a strong electrolyte. After the equivalence-point the conductance shows a further increase if a strong base is used, and so the end-point can be found in the usual manner. Owing to the extensive hydrolysis of the salt of a weak base and a very weak acid, even when excess of acid is still present, the titration by a weak base cannot be employed to give a conductometric endpoint. 

The procedure described here may be regarded as typical of that adopted for any moderately weak acid, i.e., of dissociation constant 10 to 10 for weaker acids, however, some modification is necessary. In addition to the acid dissociation... 

If a is the initial concentration (molality) of the weak or moderately weak acid HA, and h is the amount of strong, monoacid base MOH added at any instant, then h is also equal to mM > the molality of ions at that instant, since the salt MA produced on neutralization may be taken as being completely dissociated. The acid HA is only partially neutralized to form A ions, and so... 

In conductometric titrations, a change in conductance is caused after the endpoint owing to the increase in concentration of the mobile hydroxide ion. Very weak acids (such as boric acid) and moderately weak acids (such as acetic acid) can be titrated using conductometric titration. 

A direct electrochemical measurement of the reduction potential for the O2/O2 - couple in aqueous solutions is complicated by the rapid proton-induced disproportionation reactions for O2 -. The perhydroxyl radical (HOO ) is a moderately weak acid (roughly equivalent to acetic acid)24... 

Because HOO- in water is a moderately weak acid (pf HA/ -9)/ at pH 1 about 1% of the biogenerated O2 - exists as HOO-. The latter species is an effective initiator for the auto-oxidation of lipids via their allylic carbons (see discussion in Chapter 5), and probably represents the primary hazard of superoxide production in biological matrices. 

The above technique was applied by Harned and his colleagues to determine acidity constants for a variety of weak acids in both water and in water-non-aqueous solvent mixtures [3]. It may also be used to determine the self-dissociation constant of water. In the case of moderately weak acids the extrapolation procedure requires a more careful consideration of the contribution of H to the ionic strength. More details can be found in the monograph by Harned and Owen [3]. 

The mucosa of the stomach is also damaged by the action of aspirin, the chemical name of which is acetylsalicylic acid. Aspirin is itself a moderately weak acid ... 

If the pH Of the solution and the concentrations of HB and B are measured, pKa can be readily calculated. Spectrophotometry is employed in determining the ratio (chb/cb ). Since the conjugate acid and base forms of a substance will be in equilibrium, the relative concentration of each form will depend on the pH of the solution. For moderately weak acids or bases, the pH of the solution may be adjusted so as to make one of the forms predominant. For example, the spectrum of an acid HB in alkaline solution will correspond to the absorption of the anion ( >b ) and in acid solution, to that of the undissociated acid (Dhb). At intermediate pH values, however, both forms will be present and the observed absorption (D) will be a linear combination of those of the two forms (provided Beer s law holds). Since the optical densities (jDb- and Dhb) of the two forms (for a fixed concentration of the solute) are known, the relative concentrations of the two forms at any intermediate pH may be calculated. A similar spectrophotometric procedure may be applied to bases. The acidity function, H0, which is a measure of the proton-donating tendency of a medium towards a neutral base, has also been determined by spectrophotometry85. 

We can see from these equilibria that L-ascorbic acid is a moderately weak acid in the loss of the first hydrogen ion and a very weak acid in the loss of the second. The pA"a for dehydroascorbic acid is only a rough value. The radical, HA, however, is a strong acid, comparable with the mineral acids. Of course all these species have their own redox behaviour values of the reduction potentials which have been determined or estimated for these moieties are shown in equations... 

Sodium hydroxide is usually standardized by titrating a weighed quantity of primary standard potassium acid phthalate (KHP), which is a moderately weak acid Ka = 4 X 10 ), approximately like acetic acid a phenolphthalein end point is used. The sodium hydroxide solution should be stored in a plastic bottle to prevent absorption of CO2 from the air. If the bottle must be open (e.g., a siphon bottle), the opening is protected with an Ascarite (asbestos impregnated with NaOH) tube. 

Compartment Extremely acid Strongly acid Moderately + weakly acid Neutral Moderately + weakly alkaline Strongly alkaline Extremely alkaline... 

N and (c) 0.0001 N (titrated with NaOH) and (d) 0.001 N acetic acid (titrated with NH OH or KOH). B Various moderately weak acids with NaOH. 

When no linear region prior to the endpoint can be easily obtained, one can add alcohol or some other water-soluble organic compound to reduce the dissociation of the acid so that it behaves more like a very weak acid. This often may be the easiest approach for titrating weak and moderately weak acids. If these techniques fail, then potentiometric methods should be tried. 

In all proton containing solvents acid-base phenomena can be described in terms of the Bronsted-Lowby theory. All of these solvents have the solvated proton in common as the solvent cation, and this determines to a considerable extent the chemistry in their solutions. Bronsted acids are usually characterized by their acidic strength in water, e.g. by the acidity constant in this solvent. Thus acetic acid and hydrofluoric acid both behave as moderately weak acids in water with at room temperature. When acetic acid is dissolved in liquid hydrogen fluoride, the former is successfully competing for the protons, so that acetic acid acts as a base ( acetic-base ) in this medium just as it does in nitric acid ... 

In titrating a weak acid with a strong base) or a weak base with a strong acid) greater care is needed in the selection of an indicator. Let us consider the titration of 0.2 N acetic acid, a moderately weak acid with = L80 X 10 , with 0.2 N sodium hydroxide. When an amount of the alkali equivalent to that of the acid has been added, the resultant solution is the same as would be obtained by dissolving 0.1 mole of the... 

The pH of the solution of sodium acetate is hence 8.87. By reference to Figure 12-1 we see that plienolphtlialein, with pK = 9, is the best indicator to use for titrating a moderately weak acid such as acetic acid. 

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