Polyprotic acids salts

Buffer mixtures are not confined to mixtures of monoprotic acids or monoacid bases and their salts. We may employ a mixture of salts of a polyprotic acid, e.g. NaH2P04 and Na2HP04. The salt NaH2P04 is completely dissociated ... 

The principal difference between a polyprotic acid and a monoprotic acid is that a polyprotic acid donates protons in a succession of deprotonation steps. For example, a carbonic acid molecule can lose one proton to form I ICO , and then that ion can donate the remaining proton to form CO, 2. We shall see how to take this succession of deprotonations into account when assessing the pH of the solution of a polyprotic acid or one of its salts. In addition, we shall see how the relative concentrations of the ions in solution, such as P043, HP042, and H,P04 depend on the pH of the solution. 

The parent acids of common polyprotic acids other than sulfuric are weak and the acidity constants of successive deprotonation steps are normally widely different. As a result, except for sulfuric acid, we can treat a polyprotic acid or the salt of any anion derived from it as the only significant species in solution. This approximation leads to a major simplification to calculate the pH of a polyprotic acid, we just use Kal and take only the first deprotonation into account that is, we treat the acid as a monoprotic weak acid (see Toolbox 10.1). Subsequent deprotonations do take place, but provided Kal is less than about fCal/1000, they do not affect the pH significantly and can be ignored. 

The pH of the aqueous solution of an amphiprotic salt is equal to the average of the pKlts of the salt and its conjugate acid. The pH of a solution of a salt of the final conjugate base of a polyprotic acid is found from the reaction of the anion with water. 

The above derivations assume the presence of a single competing ion in the mobile phase. For ion chromatography mixtures of polyprotic acids and tfteir salts are commonly used, for... 

An acidic salt is a salt that contains an ionizable hydrogen atom. It is the product formed when less than the amount of base required for complete neutralization (reaction) reacts with a polyprotic acid ... 

The pH of a solution of a salt of the final conjugate base of a polyprotic acid is found from the reaction of the anion with water. 

The titration of a polyprotic acid has a stoichiometric point corresponding to the removal of each acidic hydrogen atom. The product at each stoichiometric point is the salt of a conjugate base of the acid or one of its hydrogen-bearing anions. 

First stoichiometric point The first stoichiometric point is reached when the number of moles of base added equals the number of moles of acid originally present. At this point, the main species in solution is the salt consisting of the cation from the base and the conjugate base of the polyprotic acid (for example,... 

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... 

Buffer solutions can be prepared with polyprotic acids, just as with mono-protic acids. If any acid and its conjugate, which differs by one proton, are together in the same solution, they form a buffer solution. Instead, if an acid and its salt are combined, they will react to produce one or more acid salt(s). 

The common ion effect is also important in solutions of polyprotic acids. The production of protons by the first dissociation step greatly inhibits the succeeding dissociation steps, which also produce protons, the common ion in this case. We will see later in this chapter that the common ion effect is also important in dealing with the solubility of salts. 

We see that the pH of a solution of a normal salt (Na PO, NaCgHgOo, (NH4)oS04, NH CN) is determined by the extent of hydrolysis, which depends on the dissociation constants of the acid and base (the last constant only for polyprotic acids e.g., = AThpo4—... 

AAfPHOT RJC SALTS—INTERMEDIATE IONS OF POLYPROTIC ACIDS... 

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. 

If less than stoichiometric amounts of bases react with polyprotic acids, the resulting salts are known as acidic salts because they are still capable of neutralizing bases. 

E4.19 (a) H3PO4 and Na2HPO4 H3PO4 is a polyprotic acid and Na2HPO4 is its acidic salt. Considering that for a... 

We saw in the following section how polyprotic acids are capable of providing more H ions in several steps and we saw how pH ay be calculated in solutions of polyprotic acids. In connection with this we looked briefly at acid and base properties of salt and on how pH may be calculated in such salt solutions. In connection with the influence of foreign ions on the equilibrium conditions in chapter 4 we looked briefly at ion effects and its influence on pH conditions. 

The acidity or basicity of a solution is frequently an important factor in chemical reactions. The use of buffers of a given pH to maintain the solution pH at a desired level is very important. In addition, fundamental acid-base equihbria are important in understanding acid-base titrations and the effects of acids on chemical species and reactions, for example, the effects of complexation or precipitation. In Chapter 6, we described the fundamental concept of equilibrium constants. In this chapter, we consider in more detail various acid-base equilibrium calculations, including weak acids and bases, hydrolysis, of salts of weak acids and bases, buffers, polyprotic acids and their salts, and physiological buffers. Acid-base theories and the basic pH concept are reviewed first. 

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