Citric acid ionization

Citric acid ionization, 465 metal complexes, 476, 963 crystal structure, 476 NMR, 467... 

TABLE 10.1. Citric Acid Ionization Constants from Selected Literature Sources ... 

Ionization of citric acid, C6H807 or C3H50(C00H)3 ... 

The enzyme isocitrate dehydrogenase is one of the enzymes of the Krebs or citric acid cycle, a major feature in carbohydrate metabolism (see Section 15.3). This enzyme has two functions, the major one being the dehydrogenation (oxidation) of the secondary alcohol group in isocitric acid to a ketone, forming oxalosuccinic acid. This requires the cofactor NAD+ (see Section 11.2). For convenience, we are showing non-ionized acids here, e.g. isocitric acid, rather than anions, e.g. isocitrate. 

Ionized and non-ionized forms of many compounds are regarded as synonymous in the text, thus citrate/citric acid, acetate/acetic acid or others may be used according to the author s whim and context, and should not be considered as having any especial relevance. 

Citric acid is a weak acid and loses hydrogen ions from its three carboxyl groups (COOH) in solution. The loss of a hydrogen ion from each group in the molecule results in the citrate ion, C3H50(C00)33. A citric acid molecule also forms intermediate ions when one or two hydrogen atoms in the carboxyl groups ionize. The citrate ion combines with metals to form salts, the most common of which is calcium citrate. Citric acid forms esters to produce various citrates, for example trimethyl citrate and triethyl citrate. 

The ionization of citric acid has been studied in detail by several authors using distinct techniques.30-36 More recently, the protonation of citrate, tartrate and malate have been studied using potentiometric and calorimetric methods to obtain protonation constants and other thermodynamic data for a wide range of ionic strengths and two different temperatures (25 and 37 °C).37 The complexation of Na+ by those ligands37 and of Li+, Na+, K+, Rb+, Cs+ and NH by citrate ions38 has also been investigated. The latter results show how the cation of the ionic medium can influence the ionization equilibria and should be taken into account in corrections for this effect. 

Figure 4.8 Intermediates in the citric acid cycle shown in the ionized forms in which they exist at physiological pH values. The final oxaloacetate product reacts with acetyl-CoA from glycolysis to start the cycle over again.

In manufacturing a fruit drink, citric acid is added to water in the amount of 0.1 mol/liter. What is the pH of the resulting solution Use an acid dissociation constant of 8.4 X 10 4 for the ionization of citric acid (assume only the first deprotonation reaction occurs). 

As is the case with sorbic acid, benzoic acid penetrates the cell wall in the undissociated form. As a consequence, it is active at lower pH values only (pKa at 25°C = 4.19) and therefore serves as a preservative for sour products such as fruit juices and jams. In shrimp preservation it is applied as a powder that is spread over the shrimps, passes cell walls, and then ionizes in the intracellular fluid to yield protons that acidify the alkaline interior of the cell. The main cause of its activity, however, is biochemical effects (Eklund, 1980) such as inhibition of oxidative phosphorylation and of enzymes from the citric acid cycle (Chipley, 1983). In mayonnaise preserved by benzoic acid, the undissociated acid is mainly present in the lipid phase, which can be considered as a reservoir for the aqueous phase. 

Write the chemical equations for the stepwise ionization of citric acid, C3H50(C00H)3, a triprotic acid. 

The pH of the medium is vital for a good yield of citric acid. Ideally, the pH should fall below 2 within a few hours after the initiation of the spore inoculation. At high pH, A. niger tends to accumulate gluconic acid. This is due to the activation of mycelial-bound glucose oxidase at high pH while at low pH (< 2), this enzyme is inactive [47]. The pH of the medium will also affect the ionization of citric acid. At pH values of about 2, most of the citrate will be present as either citrate or citrate, whereas at an internal pH of about 7, the citrate will be present mainly as citrate. It has been suggested that only citrate ions can be transported out of the mycelium easily [48]. 

The ionization of citric acid moieties may be described by the general dissociation reaction... 

Citric acid, HjCsHgOy, and vitamin C (ascorbic acid), H2C6H6O6, are polyprotic acids found in citrus fruits. Why are these acids classified as polyprotic acids Write balanced equations that show the stepwise ionization of these acids in water. 

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