Naturally occurring weak organic acids

Organic acids occur naturally in a variety of foods as a result of (1) normal biochemical metabolic processes, (2) direct addition as acidulants, (3) hydrolysis, or (4) bacterial growth (Gomis, 1992). In Table 7.1 some of the most abundantly found organic acids are described. 

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The equivalence point, as we have seen, is the point at which the number of moles of OH ions added to a solution is equal to the number of moles of ions originally present. To determine the equivalence point in a titration, then, we must know exactly how much volume of a base to add from a buret to an acid in a flask. One way to achieve this goal is to add a few drops of an acid-base indicator to the acid solution at the start of the titration. You will recall from Chapter 4 that an indicator is usually a weak organic acid or base that has distinctly different colors in its nonionized and ionized forms. These two forms are related to the pH of the solution in which the indicator is dissolved. The end point of a titration occurs when the indicator changes color. However, not all indicators change color at the same pH, so the choice of indicator for a particular titration depends on the nature of the acid and base used in the titration (that is, whether they are strong or weak). By choosing the proper indicator for a titration, we can use the end point to determine the equivalence point, as we will see below. 

With very few exceptions, naturally occurring acids and alkalies are weak. All acids known in antiquity were of organic origin some occur in fruits, especially in unripe fruitjuices. Most ancient alkalies were derived from the ash of plants such as barilla, Salsola soda and Salsola kali (Russian thistle), and kelp. 

Carboxylic acids are a large group of naturally occurring compounds such as fatty acids, prostaglandins, bile acids and other organic acids all of which have relatively weak UV and visible absorption. 

Many weak acids occur naturally and are organic in origin. For instance ... 

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