Strong acids polyprotic

Diphenylcarbazide as adsorption indicator, 358 as colorimetric reagent, 687 Diphenylthiocarbazone see Dithizone Direct reading emission spectrometer 775 Dispensers (liquid) 84 Displacement titrations 278 borate ion with a strong acid, 278 carbonate ion with a strong acid, 278 choice of indicators for, 279, 280 Dissociation (ionisation) constant 23, 31 calculations involving, 34 D. of for a complex ion, (v) 602 for an indicator, (s) 718 of polyprotic acids, 33 values for acids and bases in water, (T) 832 true or thermodynamic, 23 Distribution coefficient 162, 195 and per cent extraction, 165 Distribution ratio 162 Dithiol 693, 695, 697 Dithizone 171, 178... 

Estimate the pH of a polyprotic acid for which all deprotonations are weak by using only the first deprotonation equilibrium and assuming that further deprotonation is insignificant. An exception is sulfuric acid, the only common polyprotic acid that is a strong acid in its first deprotonation. 

The definition of pH is pH = —log[H+] (which will be modified to include activity later). Ka is the equilibrium constant for the dissociation of an acid HA + H20 H30+ + A-. Kb is the base hydrolysis constant for the reaction B + H20 BH+ + OH. When either Ka or Kb is large, the acid or base is said to be strong otherwise, the acid or base is weak. Common strong acids and bases are listed in Table 6-2, which you should memorize. The most common weak acids are carboxylic acids (RC02H), and the most common weak bases are amines (R3N ). Carboxylate anions (RC02) are weak bases, and ammonium ions (R3NH+) are weak acids. Metal cations also are weak acids. For a conjugate acid-base pair in water, Ka- Kb = Kw. For polyprotic acids, we denote the successive acid dissociation constants as Kal, K, K, , or just Aj, K2, A"3, . For polybasic species, we denote successive hydrolysis constants Kbi, Kb2, A"h3, . For a diprotic system, the relations between successive acid and base equilibrium constants are Afa Kb2 — Kw and K.a Kbl = A w. For a triprotic system the relations are A al KM = ATW, K.d2 Kb2 = ATW, and Ka2 Kb, = Kw. 

We can calculate pH titration curves using the principles of aqueous solution equilibria. To understand why titration curves have certain characteristic shapes, let s calculate these curves for four important types of titration (1) strong acid-strong base, (2) weak acid-strong base, (3) weak base-strong acid, and (4) polyprotic acid-strong base. For convenience, we ll express amounts of solute in millimoles (mmol) and solution volumes in milliliters (mL). Molar concentration can thus be expressed in mmol/mL, a unit that is equivalent to mol/L ... 

Just as in the ionization of polyprotic acids, so in the hydrolysis of their salts, the reaction proceeds in successive stages. The extent of the second stage is generally very small compared with the first. This is particularly true in this case, where H2Cr04 is quite a strong acid with respect to its first ionization and much weaker in the second ionization. The equation of interest is... 

Definition of Acids and Bases 222 Conjugate Acid-Base Pairs 222 Amphoteric Species 224 Strong Acids 225 Strong Bases 225 Weak Acids 226 Weak Bases 226 Polyprotic Acids 227 Acid and Base Strength Ka and Kb 228 Acid/Base Strength of Conjugate Acid-Base Pairs 230 Acid-Base Reactions 231... 

Conjugate base of polyprotic acid titrated with strong acid... 

There are a few main types of titrations a strong acid titrated with a strong base (or a strong base titrated with a strong acid) a weak acid titrated with a strong base a weak base titrated with a strong acid and a polyprotic acid titrated with a strong base. Each one of these produces characteristic results and will need to be discussed separately. For the solutions of weak acids and bases, the process is complicated by the common-ion effect. 

Drugs may also have more than two pKa values, such as polyprotic or polybasic compounds (e.g., minocycline), and such drugs exhibit a complex pH solubility profile. It is essential to know per se pH of the drug solution during preformulation studies. The pH is measured or theoretically calculated if the pKa and drug concentration C are known. The pH of a weak acid or the salt of a weak base and a strong acid can be calculated using the equation... 

Equation 20-8 permits the calculation of all points on a titration curve by means of a single equation. As written, it handles strong acids, weak acids or mixtures, and it is readily expanded to handle mixtures of polyprotic acids. 

There is a close similarity between the working of Example 16.8 and an acid-base calculation. The first step (the assumption that the reaction goes to completion and is followed by a small amount of back dissociation) is analogous to the procedure for dealing with the addition of a small amount of a strong acid to a solution of a weak base. The subsequent calculation of the successive dissociation steps resembles the calculation of polyprotic acid equilibria in Example 15.12. The only difference is that in complex-ion equilibria it is conventional to work with formation constants, which are the inverse of the dissociation constants used in acid-base equilibria. 

Sulfuric acid is called a polyprotic acid because it has more than one ionizable hydrogen atom per molecule. It is the only common polyprotic acid that is also a strong acid. 

Calculating acid-base titration curves Strong acids, strong bases (Table 8.1), p. 266 Spreadsheet calculations, p. 269 Weak acids, weak bases (Table 8.2), p. 272 Spreadsheet calculations, p. 277 Indicators (key equations 8.4, 8.5), p. 270 Titration of Na2C03, p. 280 Titration of polyprotic acids (Table 8.3), p. 281 Titration of amino acids, p. 286... 

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). 

Several common acids have more than one ionizable hydrogen ion (Table 1). Each successive hydrogen ion in these polyprotic acids ionizes less readily. For example, sulfuric acid is a strong acid because of the complete ionization of the first hydrogen ion. 

Table 14.4 lists the stepwise values for some polyprotic acids. What is the difference between a monoprotic acid, a diprotic acid, and a triprotic acid Most polyprotic acids are weak acids the major exception is H2SO4. To solve for the pH of a solution of H2SO4, you must generally solve a strong acid problem as well as a weak acid problem. Explain. Write out the reactions that refer to and for H2SO4. 

---