Acids for weak

As the amines become more weakly basic, the normal method of diazotization becomes progressively more difficult. The equilibrium between amine and ammonium salt increasingly favors the former which, usually because of its poor solubility in water, is prevented from taking part in the reaction. Research into the mechanism of diazotization has demonstrated that the important step is the addition of the nitrosating agent to the base of the amine. Thus, the acidity for each diazotization should be so chosen that the equilibrium concentration of base corresponds to that of its saturated solution. This rule leads to the use of higer concentrations of aqueous mineral acid for weakly basic amines. 

As shown above (Section 14.3.3) the proton accepting ability of the hydride ligand (expressed as the Ej factor) determines the thermodynamic drive toward proton transfer from different acids. For weak basicity hydrides (Ej < 0.8), proton transfer is possible only from strong acids like HBF4 or CF3COOH, while those... 

For bases, e.g. penta- to heptasufonic acid for weak bases and octa- to dodecasulfonic acid for strong bases, pH 3.5. 

Infinite dilution. It is often said that a weak electrolyte becomes completely ionized at infinite dilution, (a) In an aqueous solution, what is [H ] equal to at infinite dilution (in pure water) (b) For a weak acid, HA, the degree of ionization is a = [A ]/c, where c = [HA] + (A j. Write the equilibrium condition for the ionization of HA in terms of a, c, and [H ]. (c) Substitute the answer to (a) into the equation in (b). Does c drop out of the equation Is the equation valid in the limit c -> 0 (d) Solve for a at infinite dilution in terms of K, and Kw. (e) Calculate a at infinite dilution at 25°C for acetic acid and far hydrocyanic acid, (f) Is the statement at the begitming of this problem correct for weak acids for weak bases for other weak electrolytes, such as HgCU ... 

Values of are small for weak acids and they range very widely (Table 4.1). It is common practice to quote values as the negative logarithm to the base ten, i.e. — logjo K.. since such numbers are less cumbersome and positive when Aj < 1. The symbol for -logio is by convention "p/ fhus -logjo becomes pK,. Table 4.1 shows some typical pAg values. 

Separations based upon differences in the chemical properties of the components. Thus a mixture of toluene and anihne may be separated by extraction with dilute hydrochloric acid the aniline passes into the aqueous layer in the form of the salt, anihne hydrochloride, and may be recovered by neutralisation. Similarly, a mixture of phenol and toluene may be separated by treatment with dilute sodium hydroxide. The above examples are, of comse, simple apphcations of the fact that the various components fah into different solubihty groups (compare Section XI,5). Another example is the separation of a mixture of di-n-butyl ether and chlorobenzene concentrated sulphuric acid dissolves only the w-butyl other and it may be recovered from solution by dilution with water. With some classes of compounds, e.g., unsaturated compounds, concentrated sulphuric acid leads to polymerisation, sulphona-tion, etc., so that the original component cannot be recovered unchanged this solvent, therefore, possesses hmited apphcation. Phenols may be separated from acids (for example, o-cresol from benzoic acid) by a dilute solution of sodium bicarbonate the weakly acidic phenols (and also enols) are not converted into salts by this reagent and may be removed by ether extraction or by other means the acids pass into solution as the sodium salts and may be recovered after acidification. Aldehydes, e.g., benzaldehyde, may be separated from liquid hydrocarbons and other neutral, water-insoluble hquid compounds by shaking with a solution of sodium bisulphite the aldehyde forms a sohd bisulphite compound, which may be filtered off and decomposed with dilute acid or with sodium bicarbonate solution in order to recover the aldehyde. 

Carboxylic acids are the most acidic class of compounds that contain only carbon hydro gen and oxygen With s of about 5 they are much stronger acids than water and alcohols The case should not be overstated however Carboxylic acids are weak acids a 0 1 M solution of acetic acid m water for example is only 1 3% ionized... 

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. 

Besides equilibrium constant equations, two other types of equations are used in the systematic approach to solving equilibrium problems. The first of these is a mass balance equation, which is simply a statement of the conservation of matter. In a solution of a monoprotic weak acid, for example, the combined concentrations of the conjugate weak acid, HA, and the conjugate weak base, A , must equal the weak acid s initial concentration, Cha- ... 

Equation 6.44 is written in terms of the concentrations of CH3COOH and CH3COO- at equilibrium. A more useful relationship relates the buffer s pH to the initial concentrations of weak acid and weak base. A general buffer equation can be derived by considering the following reactions for a weak acid, HA, and the salt of its conjugate weak base, NaA. 

It has been shown that for most acid-base titrations the inflection point, which corresponds to the greatest slope in the titration curve, very nearly coincides with the equivalence point. The inflection point actually precedes the equivalence point, with the error approaching 0.1% for weak acids or weak bases with dissociation constants smaller than 10 , or for very dilute solutions. Equivalence points determined in this fashion are indicated on the titration curves in figure 9.8. 

The pH range of an indicator does not have to be equally distributed on either side of the indicator s piQ. For some indicators only the weak acid or weak base is colored. For other indicators both the weak acid and weak base are colored, but one form may be easier to see. In either case, the pH range is skewed toward those pH levels for which the less colored form of the indicator is present in higher concentration. 

Potentiometric titration curves are used to determine the molecular weight and fQ or for weak acid or weak base analytes. The analysis is accomplished using a nonlinear least squares fit to the potentiometric curve. The appropriate master equation can be provided, or its derivation can be left as a challenge. 

The acidity of a water sample is determined by titrating to fixed end points of 3.7 and 8.3, with the former providing a measure of the concentration of strong acid, and the latter a measure of the combined concentrations of strong acid and weak acid. Sketch a titration curve for a mixture of 0.10 M HCl and 0.10 M H2CO3 with 0.20 M strong base, and use it to justify the choice of these end points. 

For weakly basic amines, ie, those containing several electron-withdrawing groups, nitrosyl sulfuric acid (NO HSO is used as the nitrosating species in sulfuric acid, which may be diluted with phosphoric, acetic, or propionic acid. 

Ion-exchange resins swell in water to an extent which depends on the amount of crosslinking in the polymer, so that columns should be prepared from the wet material by adding it as a suspension in water to a tube already partially filled with water. (This also avoids trapping air bubbles.) The exchange capacity of a resin is commonly expressed as mg equiv./mL of wet resin. This quantity is pH-dependent for weak-acid or weak-base resins but is constant at about 0.6-2 for most strong-acid or strong-base types. 

It has been found that there is often a correlation between the rate of deprotonation (kinetic acidity) and the thermodynamic stability of the carbanion (thermodynamic acidity). Because of this relationship, kinetic measurements can be used to construct orders of hydrocarbon acidities. These kinetic measurements have the advantage of not requiring the presence of a measurable concentration of the carbanion at any time instead, the relative ease of carbanion formation is judged from the rate at which exchange occurs. This method is therefore applicable to very weak acids, for which no suitable base will generate a measurable carbanion concentration. 

PK. — the negative logarithm of the equilibrium constant for acids or bases. This parameter is an indicator of the strength of an acid or base. Strong acids, such as H2SO4, and HCl, have low pK s (i.e., -1.0) while strong bases such as KOH and NaOH, have pK s close to 14.0. Weak acids and weak bases fall in the intermediate range. 

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