Diprotic acid

Nucleotides are phosphoric acid esters of nucleosides Those derived from adenosine of which adenosine 5 monophosphate (AMP) is but one example are especially promi nent AMP is a weak diprotic acid with s for ionization of 3 8 and 6 2 respectively In aqueous solution at pH 7 both OH groups of the P(0)(0H)2 unit are ionized... 

Triprotic Acids and Bases, and Beyond The treatment of a diprotic acid or base is easily extended to acids and bases having three or more acid-base sites. For a triprotic weak acid such as H3PO4, for example, we can treat H3PO4 as if it was a mono-protic weak acid, H2P04 and HP04 as if they were intermediate forms of diprotic weak acids, and P04 as if it was a monoprotic weak base. 

The conclusions reached in Example 13.9 are ordinarily valid for any weak diprotic acid, H2B. 

Ascorbic acid, H2C6H606, also known as vitamin C, is present in many citrus fruits. It is a diprotic acid with the following values = 7.9 X 105 K = 1-6 X 10 12. What is the pH of a 0.63 M solution of ascorbic acid Estimate [HC6H606-] and [C4H6062-]. 

When a polyprotic acid is dissolved in water, the various hydrogen atoms undergo ionisation to different extents. For a diprotic acid H2A, the primary and secondary dissociations can be represented by the equations ... 

It can be shown that the pH at the first equivalence point for a diprotic acid is given by... 

With a knowledge of the pH at the stoichiometric point and also of the course of the neutralisation curve, it should be an easy matter to select the appropriate indicator for the titration of any diprotic acid for which K1/K2 is at least 104. For many diprotic acids, however, the two dissociation constants are too close together and it is not possible to differentiate between the two stages. If K 2 is not less than about 10 7, all the replaceable hydrogen may be titrated, e.g. sulphuric acid (primary stage — a strong acid), oxalic acid, malonic, succinic, and tartaric acids. 

For the primary stage (phosphoric) V) acid as a monoprotic acid), methyl orange, bromocresol green, or Congo red may be used as indicators. The secondary stage of phosphoric) V) acid is very weak (see acid Ka = 1 x 10 7 in Fig. 10.4) and the only suitable simple indicator is thymolphthalein (see Section 10.14) with phenolphthalein the error may be several per cent. A mixed indicator composed of phenolphthalein (3 parts) and 1-naphtholphthalein (1 part) is very satisfactory for the determination of the end point of phosphoric(V) acid as a diprotic acid (see Section 10.9). The experimental neutralisation curve of 50 mL of 0.1M phosphoric(V) acid with 0.1M potassium hydroxide, determined by potentiometric titration, is shown in Fig. 10.6. 

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. 

Neutralisation reactions. The equivalent of an acid is that mass of it which contains 1.008 (more accurately 1.0078) g of replaceable hydrogen. The equivalent of a monoprotic acid, such as hydrochloric, hydrobromic, hydriodic, nitric, perchloric, or acetic acid, is identical with the mole. A normal solution of a monoprotic acid will therefore contain 1 mole per L of solution. The equivalent of a diprotic acid (e.g. sulphuric or oxalic acid), or of a triprotic acid (e.g. phosphoric( V) acid) is likewise one-half or one-third respectively, of the mole. 

L.30 You have been given a sample of an unknown diprotic acid (a) Analysis of the acid shows that a 10.0-g sample contains 0.224 g of hydrogen, 2.67 g of carbon and the rest oxygen. Determine the empirical formula of the acid. 

STRATEGY Verify that Eq. 14 can be used by checking that S 5i> K JKal and S 5i> fC,. If so, we use Eq. 14 to determine the pH of the salts of the diprotic conjugate base (H,A ") of a triprotic acid (H SA) and the monoprotic conjugate base (HA ) of a diprotic acid (H2A). However, when the solure is a salt of an anion that has lost two protons, such as HP042-, we must adjust the expression to use the appropriate neighboring pkas. 

We have found expressions for the fractions, /, of species in a solution of carbonic acid. They are easily generalized to any diprotic acid H2A ... 

For each of the following polyprotic acids, state which species (H2A, HA, or A2 of the diprotic acid or H3A, H,A, HA2, or A for triprotic acids) you expect to be the form present in highest concentration in aqueous solution at pH = 5.0 (a) tartaric acid (b) hydrosulfuric acid (c) phosphoric acid. 

Like sulfuric acid, a certain diprotic acid, H,A, is a strong acid in its first deprotonation and a weak acid in its second deprotonation. A solution that is 0.020 M H,A(aq) has a pH of 1.66. What is the value of fCa2 for this acid ... 

Figure 11.14 shows the pH curve of a diprotic acid, such as oxalic acid, H2C204. There are two stoichiometric points (B and D) and two buffer regions (A and C). The major species present in solution at each point are indicated. Note that it takes twice as much base to reach the second stoichiometric point as it does to reach the first. 

FIGURE 11.14 The variation of the pH of the analyte solution during the titration ot a diprotic acid (oxalic acidl and the major species present in solution at the two stoichiometric points (B and D) and at points when half the titrant required to reach a stoichiometric point has been added (A and C). Compare this diagram with Fig. 10.20. The labels SP1 and SP2 denote the volumes of base required to reach the two stoichiometric points. 

Hydrogen sulfide dissolves in water to give a solution of hydrosulfuric acid that, as a result of its oxidation by dissolved air, slowly becomes cloudy as S8 molecules form and then coagulate. Hydrosulfuric acid is a weak diprotic acid and the parent acid of the hydrogen sulfides (which contain the HS ion) and the sulfides (which contain the S2 ion). The sulfides of the s-block elements are moderately soluble, whereas the sulfides of the heavy p- and d-block metals are generally very insoluble. 

Statistical Effects. In a symmetrical diprotic acid, the first dissociation constant is twice as large as expected since there are two equivalent ionizable... 

Carbonated water contains carbonic acid, a diprotic acid that forms when carbon dioxide dissolves in water C02(g) +H2 0(/) H2 C03(c2 q) A typical carbonated beverage contains 0.050 M H2 CO3. Determine the concentrations of the ions present in this solution. 

The problem describes a weak diprotic acid and asks for ion concentrations. To determine concentrations of all ions, we need to consider more than one equilibrium. This is done in stages, starting with the dominant equilibrium. We apply the seven-step strategy. The problem asks us for the concentrations of the ions in carbonated water, in which the major species are H2 CO3 and H2 O. 

C17-0095. Deeimine the concentrations of the Ionic species present in a 0.355 M solution of the diprotic acid H2 SO3. (See Table IM for K values.)... 

The titration of a diprotic acid, H2 A, can be divided into six regions that differ in the major species present in solution. Shown here is the titration curve for maleic acid (50.0 mL, 0.10 M) titrated with 1.0 M NaOH. 

At the beginning of the titration, the diprotic acid (represented by H2 A) and H2 O are the only major species in the solution. As we describe in Chapter 17, the hydronium ion concentration can be calculated from the... 

At the first stoichiometric point of the titration, aii the diprotic acid has been converted to its conjugate base, H A. This amphiprotic anion can react with itseif, analogous to the self-ionization of water ... 

By recognizing species in solution and their dominant equilibrium, we can construct titration curves for other diprotic acids. Example shows how this is done for sulfurous acid. 

The starting pH of the solution is calculated using i and the initial molarity of the diprotic acid. We use the standard approach to a weak acid equilibrium ... 

The calculated curve shows the general features of the pH titration curve for a diprotic acid. The pH of the solution is acidic at the first stoichiometric point (major species = weak acid HA ) and basic at the second (major species =... 

C18-0142. The amine group of an amino acid readily accepts a proton, and the protonated form of an amino acid can be viewed as a diprotic acid. The p Zg values for serine (H2 NCHRCO2 H, i = CH2 OH) are p ra(H3 N"") =9.1 and p (002 H) - 2.2. (a) What is the chemical formula of the species that forms when serine dissolves in pure water (b) If this species is titrated with strong acid, what reaction occurs (c) 10.00 mL of 1.00 M HCl is added to 200. mL of 0.0500 M serine solution. This mixture is then titrated with 0.500 M NaOH. Draw the titration curve, indicating the pH at various stages of this titration. 

Equations have been published [16] which relate pKa and p0Ka values to partition coefficient (P) values for monoprotic acids and bases, and diprotic acids, bases and ampholytes. For example, P1 for a monoprotic acid is calculated from... 

M. H., Determination of acidity constants of monoprotic and diprotic acids by capillary electrophoresis,... 

Although the formula is sometimes written as H3P03, it is a diprotic acid that has the structure... 

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