Acids polyprotic type

Citrus fruit derives its acidic qualities from citric acid, H3C6H5O7, a type of acid (polyprotic) discussed in Section 16-6. Another important constituent of citrus fruit is ascorbic acid, or vitamin C (page 679), a dietary requirement to prevent scurvy. 

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 ... 

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. 

Acidic salts contain anions derived from ternary polyprotic acids in which one or more acidic hydrogen atoms remain. These salts are named as if they were the usual type of ternary salt, with the word hydrogen or dihydrogen inserted after the name of the cation to show the number of acidic hydrogen atoms. 

The four possible situations which occur in polyprotic systems may be simply described in terms of the ratio of available acidity to the total anion, C /C. Methods of approaching pH problems for all these types are given in this chapter and in Chapter 6. 

Sulfuric acid is the type of polyprotic acid that can donate two protons per molecule, and it is therefore known as a diprotic acid. Ionizations of a monoprotic acid and a diprotic acid are shown in Figure 2.2. 

Phosphoric acid is the type of polyprotic add known as a triprotic acid-an acid able to donate three protons per molecule. 

Polyprotic acids are able to donate more than one proton per acid molecule, in contrast to monoprotic acids that only donate one proton per molecule. Specific types of polyprotic acids have more specific names, such as dipro-tic acid [two potential protons to donate) and triprotic acid [three potential protons to donate). 

In addition, the equilibria on the left hand side of eqn (8.20) can sometimes be defined by the dissociation constants Kdissj (j = 1, 2,. .., m), that are very familiar from acid-base reactions (the acidity constants Kgj for polyprotic acids are in fact dissociation equilibrium constants). The relationships between all these constants are given by the sets of eqn (8.21). In certain cases, it is more convenient to use one type of equilibrium constants or another, and the choice is typically made based on the simplicity of the mathematical equations describing a system of interest. Definitions and uses of all equilibrium constants in determination of concentrations of all species present in a complex system are given in many monographs on coordination chemistry. ... 

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