Polyprotic acid charge

B We know Kd = [doubly charged anion] for a polyprotic acid. Thus Kiy = 5.3xl0-5 = [ C2042 J... 

Sulfuric acid is a far stronger acid than the hydrogen sulfate ion, because much more energy is required to remove a proton from a negatively charged ion. The strength of a polyprotic acid decreases as the number of hydrogen atoms that have dissociated increases. 

The isoelectric point (or isoelectric pH) is the pH at which the average charge of the polyprotic acid is 0. Most of the molecules are in the uncharged form HA, and the concentrations of H2A+ and A- are equal to each other. There is always some H2A+ and some A in equilibrium with HA. 

Isoelectric pH is the pH at which average charge of the polyprotic acid is 0. 

From an acid-base titration curve, we can deduce the quantities and pK.d values of acidic and basic substances in a mixture. In medicinal chemistry, the pATa and lipophilicity of a candidate drug predict how easily it will cross cell membranes. We saw in Chapter 10 that from pKa and pH, we can compute the charge of a polyprotic acid. Usually, the more highly charged a drug, the harder it is to cross a cell membrane. In this chapter, we learn how to predict the shapes of titration curves and how to find end points with electrodes or indicators. 

Citric acid is a polyprotic acid with pK, pK2, and pKj equal to 3.15, 4.77, and 6.39, respectively. Calculate the concentrations of H+, the singly charged anion, the doubly charged anion, and the triply charged anion in 0.0100 M citric acid. 

The titration curve of a polyprotic acid can be obtained by using the same approach as for monoprotic weak acids or weak bases. The mass and charge balance equations... 

Acids that contain more than one ionizable hydrogen atom per molecule are called polyprotic acids. These acids ionize in steps. The second (or third) proton has a much lower dissociation constant than does the prior proton because it is harder to remove a hydrogen ion the more negatively charged the Brpnsted acid (Table 19.4). Also, the prior ionization produces hydronium ions that repress the further ionization, in accord with LeChatelier s principle. Any acid ionizes less in the presence of a stronger acid (see Example 19.20). Thus, the hydronium ion in a solution of a polyprotic acid comes mainly from the first step in the ionization. 

Polyprotic acids (Section 19.5) ionize stepwise, and the hydronium ion from each step represses the ionization of later steps. The second (and third) steps are essentially weaker because it is harder to remove a proton from a negatively charged species than from a neutral one. However, polyprotic acids follow the usual rules of equilibrium. For example, LeChatelier s principle can be used to predict and explain their behavior. 

The first step in the ionization of H2SO4 is complete in dilute aqueous solution. The second step is nearly complete in very dilute aqueous solutions. The first step in the ionization of a polyprotic acid always occurs to a greater extent than the second step, because it is easier to remove a proton from a neutral acid molecule than from a negatively charged anion. 

The result of this charging effect on inhibiting dissociation is that the apparent pAa of polyprotic organic acids can only be compared directly with the pA, of monoprotic organic acids when the polyprotic acid is undissociated (a = 0). Thus, the pKa value at a = 0 is called the intrinsic pKa of the humic material. The relationship of the apparent pA to the intrinsic pKa is given by... 

Table 15.5 shows the ionization constants of several diprotic acids and one polyprotic acid. For a given acid, the first ionization constant is much larger than the second ionization constant, and so on. This trend is reasonable because it is easier to remove a ion from a neutral molecule than to remove another H+ from a negatively charged ion derived from the molecule. 

The stepwise K values of polyprotic acids get progressively smaller as the increased negative charge makes... 

Plan We examine the formulas to determine the cations and anions. Depending on the nature of these ions, the solution will be neutral (strong-acid anion and strong-base cation), acidic (weak-base cation and strong-acid anion, highly charged metal cation, or first anion of a polyprotic acid), or basic (weak-acid anion and strong-base cation). 

Note that Ka2 for sulfurous add is much smaller than fC . Because of electrostatic attractions, we would expect a positively charged proton to be lost more readily from the neutral H2SO3 molecule than from the negatively charged HSO3 ion. This observation is general It is always easier to remove the first proton from a polyprotic acid than to remove the second. Similarly, for an add with three ionizable protons, it is easier to remove the second proton than the third. Thus, the values become successively smaller as successive protons are removed. 

Up to now you ve seen that cations of weak bases (such as NH4 ) are acidic, anions of weak acids (such as CN ) are basic, anions of polyprotic acids (such as H2P04 ) are often acidic, and small, highly charged metal cations (such as are acidic. Therefore, when salts containing these ions dissolve in water, the pH of the solution is affected. You can predict the relative acidity of a salt solution frran the relative ability of the cation and/or anion to react with water. 

Another area of practical importance is in controlling (or altering) the stability of colloid dispersions by organic acids. In analogy to hydrolyzed cations, polyprotic acids that react with the particles may either promote coagulation or inhibit it. The effect that prevails depends on the charge and the adsorptivity of anions in question. As example, the stability domain for finely dispersed hematite as a... 

We conclude that the acid strength of a polyprotic acid and its anions decreases with increasing negative charge (see Table 16.2). 

---