Bases polyprotic

Polyprotic bases, like polyprotic acids, also have more than one base dissociation reaction and base dissociation constant. 

Many acids can donate more than one proton. Examples are H2CO3, H3PO4, and [Al(H20)6l. These acids are referred to as polyprotic acids. Similarly, bases that can accept more than one proton, for example, OH , C03, a.nd NH, are polyprotic bases. Many important substances, for example, proteins or poly aery lie acids, so-called polyelectrolytic acids or bases, contain a large number of acidic or basic groups. 

Polyprotic bases (species that can accept more than one proton) also exist. 

Frassineti, C. Ghelli, S. Gans. P. Sabatini, A. Moruzzi, M.S. Vacca, A. Nuclear magnetic resonance as a tool for determining protonation constants of natural polyprotic bases in solution. Anal. Biochem. 1995, 231. 374-382. 

There are also basic compounds that can accept more than one proton in a stepwise process. For example, each nitrogen atom in hydrazine, H2NNH2, has an unshared pair of electrons available for bonding with a proton. As is typical of all such polyprotic BASES, the ionization constant for the second step is much smaller than that for the first step ... 

Calculations involving the ionization of polyprotic acids and bases present no great difficulties, provided that one makes a simplifying assumption, Because the second ionization constant is usually much less than the first, we can assume that the concentration of A , which is produced in the second ionization, is much less than the concentration of HA , which is produced in the first ionization. Another way of looking at it is that the first ionization accounts for most of the H+, only a little more being added by the second ionization. Calculations involving the first ionization can then be performed as if the acid were monoprotic, and the results of this calculation can then be used to find [A ]. Corresponding remarks apply to ionization of polyprotic bases. 

Weltin, E. Galculating Equilibrium Goncentrations for Stepwise Binding of Ligands and Polyprotic Acid-Base Systems, ... 

This approach can be used to sketch titration curves for other acid-base titrations including those involving polyprotic weak acids and bases or mixtures of weak acids and bases (Figure 9.8). Figure 9.8a, for example, shows the titration curve when titrating a diprotic weak acid, H2A, with a strong base. Since the analyte is... 

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 ... 

Polyprotic acids (or mixtures of acids, with dissociation constants AT, K2, and AT3) and strong bases. The first stoichiometric end point is given approximately... 

The theory of titrations between weak acids and strong bases is dealt with in Section 10.13, and is usually applicable to both monoprotic and polyprotic acids (Section 10.16). But for determinations carried out in aqueous solutions it is not normally possible to differentiate easily between the end points for the individual carboxylic acid groups in diprotic acids, such as succinic acid, as the dissociation constants are too close together. In these cases the end points for titrations with sodium hydroxide correspond to neutralisation of all the acidic groups. As some organic acids can be obtained in very high states of purity, sufficiently sharp end points can be obtained to justify their use as standards, e.g. benzoic acid and succinic acid (Section 10.28). The titration procedure described in this section can be used to determine the relative molecular mass (R.M.M.) of a pure carboxylic acid (if the number of acidic groups is known) or the purity of an acid of known R.M.M. 

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... 

The conjugate base of a polyprotic acid is amphiprotic it can act as either an acid or a base because it can either donate its remaining acidic hydrogen atom or accept an acidic hydrogen atom and revert to the original acid. For example, a hydrogen sulfide ion, HS-, in water acts as both an acid and a base ... 

Suppose we need to estimate the pH of an aqueous solution of a fully deproto-nated polyprotic acid molecule. An example is a solution of sodium sulfide, in which sulfide ions, S2-, are present another example is a solution of potassium phosphate, which contains P04 ions. In such a solution, the anion acts as a base it accepts protons from water. For such a solution, we can use the techniques for calculating the pH of a basic anion illustrated in Example 10.11. The K, to use in the calculation is for the deprotonation that produces the ion being studied. For S2, we would use Ki2 for H2S and, for P043-, we would use Kai for H3P04. 

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. 

We can predict the pH at any point in the titration of a polyprotic acid with a strong base by using the reaction stoichiometry to recognize what stage we have reached in the titration. We then identify the principal solute species at that point and the principal proton transfer equilibrium that determines the pH. 

H2 CO3] =0.050 M [H3 0+]= 1.5 X IO M [HCO3] = 1.5 x lO" M What happens if we add a base to this solution The strongest acid donates protons preferentially, but the concentration of hydronium ions is so small that this ion is rapidly consumed. Then the next strongest acid, H2 CO3, reacts with added base. Generalizing, when a base is added to a solution that contains both a polyprotic acid and its anion, the base accepts protons preferentially from the neutral acid. Only after the neutral acid has been consumed completely does the anion participate significantly in proton transfer. Example provides molecular pictures of this feature. 

Any anion of a weak acid, including the anions of polyprotic acids, is a weak base. The acid-base properties of monoanions of polyprotic acids are complicated, however, because the monoanion is simultaneously the conjugate base of the parent acid and an acid in its own right. For example, hydrogen carbonate anions undergo two proton-transfer reactions with water ... 

In the titration of a polyprotic acid, the added base reacts first with the more acidic hydrogen atoms of the neutral acid. For example, the titration of maleic acid takes place in two steps. For removal of one acidic hydrogen atom of maleic acid, p = 1.82 (. al = 1-5 X 10 ) ... 

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