Weak acids as indicators

FIGURE 5.2 A family of acid-base titration curves for a 0.10 M strong acid (HC1) and three weak acids, as indicated (0.10 M each), titrated with 0.10 M NaOH (strong base). HAc is a representation of acetic acid. 

Although NH4+ is a very weak acid, as indicated by its Ka value, it is stronger than H20 and thus will dominate in the production of H+. Therefore we will focus on the dissociation reaction of NH4+ to calculate the pH of this solution. We solve the weak acid problem in the usual way ... 

It is convenient to summarize the various reactions in a box diagram, such as Fig. 4.1 [17,275,280], illustrated with the equilibria of the weak base, propranolol. In Fig. 4.1 is an equation labeled pA °et. This constant refers to the octanol pKa, a term first used by Scherrer [280]. When the concentrations of the uncharged and the charged species in octanol are equal, the aqueous pH at that point defines p which is indicated for a weak acid as... 

Values of Ka and pKa = -log Ka for some typical weak acids are listed in Table 15.2. (Just as the pH is defined as -log[H+], so the pKa of an acid is defined as -log Ka.) Also included in Table 15.2 for comparison are values for HC1, a typical strong acid. As indicated by the equilibrium equation, the larger the value of Ka, the stronger the acid. Thus, methanol (Ka = 2.9 X 10-16) is the weakest of the acids listed in Table 15.2, and nitrous acid (Ka = 4.5 X 10-4) is the strongest of the weak acids. Strong acids, such as HC1, have Ka values that are much greater than 1. A more complete list of Ka values for weak acids is given in Appendix C. 

It is seen that an indicator behaves as a weak organic acid the equilibrium expression for an indicator is the same as that for an ordinary weak acid, as discussed in the following section. 

Generally, acid-base indicators are dyes that are weak acids. Because indicators are usually complex molecules, we often symbolize them as HIn. The reaction of the indicator with water can be written as... 

Similarly, bases that are very weak in aqueous solution (e.g., amines) show increased basicity in solvents of greater acidity, such as anhydrous acetic acid. Perchloric acid in acetic acid may be used as the titrant, with crystal violet in acetic acid as indicator. 

Water reacts with HCl, and the organic acid known as formic acid (HCOOH, 1) also reacts with water as a weak acid, as shown. Formic acid is a much weaker acid than HCl. When 1 reacts with water, the conjugate base is the formate anion, 2, and the conjugate acid is the hydronium ion. If 1 is a weaker acid than HCl, the equilibrium for 1 + HgO lies to the left in the reaction shown when compared to the reaction of HCl + H2O in Section 2.1. Note the (aq) term indicates solvation by the solvent water. Note also that the term reaction is used for the acid-base equilibrium. The acid-base equilibria shown for HCl and HCOOH are chemical reactions that generate two products the formate anion (HCOO , 2) and the hydronium ion (from 1) or the hydronium ion and the chloride ion (from HCl). 

Acid strength distribution has been measured by Tanabe and coworkers by means of titration with Hammett indicators as shown in Table 3.15. It can be seen that ZnO is very weak acid. As for the basicity, it was reported that the base amount was 0.05 mmole g for Ho 4.0 in the scale of the acid strength of conjugate acid (see Chapter 2). It was also reported that the acid-base properties are sensitive to pretreatment or environment. ... 

Two// scales have been constructed, one below pH 0 and one above pH 14, for measuring the pKg values of strong acids and weak acids respectively, as discussed by Paul and Long (1957). These scales are constructed by the use of optically absorbing acids as indicators, just as bases are used for the Hq scale. There is little connexion between the two scales. Thus 4N-hydrochloric acid has Ho 1 4 but is only +0 1 on the H— scale (Phillips, 1961) the difference in sulphuric acid is rather less (Boyd, 1961). For H— in highly alkaline medium, see Rochester (1966), Stewart and O Donnell (1962). In this connexion, it should be noted that the basic properties of aqueous alkalies increase greatly with concentration, in a non-linear fashion (Schwarzenbach and Sulzberger, 1944). 

The addition of even a weak acid (such as ethanoic acid) to a nitrite produces nitrous acid which readily decomposes as already indicated. Hence a nitrite is distinguished from a nitrate by the evolution of nitrous fumes when ethanoic acid is added. 

The use of methyl-orange as an indicator is based on the fact that the sodium salt in aqueous solution furnishes a yellow anion, which when treated with acids (except weak acids such as H2CO3 and H SOj) apparently gives rise to a red... 

The weakly acidic character of acycHc polyhydric alcohols increases with the number of hydroxyl groups, as indicated by the piC values in aqueous solution at 18°C (13). 

Examples include hydrochloric acid, nitric acid, and sulphuric acid. These are strong acids which are almost completely dissociated in water. Weak acids, such as hydrogen sulphide, are poorly dissociated producing low concentrations of hydrogen ions. Acids tend to be coiTosive with a sharp, sour taste and turn litmus paper red they give distinctive colour changes with other indicators. Acids dissolve metals such as copper and liberate hydrogen gas. They also react with carbonates to liberate carbon dioxide ... 

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. 

The reaction can be first or second order with respect to the H+ concentration. In weakly acidic solution it is first order in [H+], but second order in strongly acidic solution. This indicates that the monoprotonated as well as the diprotonated hydrazobenzene can undergo rearrangement. 

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

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