Reaction Rhubarb

A balanced chemical reaction indicates the quantitative relationships between the moles of reactants and products. These stoichiometric relationships provide the basis for many analytical calculations. Consider, for example, the problem of determining the amount of oxalic acid, H2C2O4, in rhubarb. One method for this analysis uses the following reaction in which we oxidize oxalic acid to CO2. 

The amount of oxalic acid in a sample of rhubarb was determined by reacting with Fe as outlined in reaction 2.2. In a typical analysis, the oxalic acid in 10.62 g of rhubarb was extracted with a suitable solvent. The complete oxidation of the oxalic acid to CO2 required 36.44 mb of 0.0130 M Fe. What is the weight percent of oxalic acid in the sample of rhubarb ... 

Mastilie, has a similar appearance, yet its chemical reactions with the tests above-montloned are very different. Should the yellow coloring matter of turmeric, saffron, or rhubarb be taken, at first sight, for that of gnmboge, the examination with acids, acetate of lead, sulphate of copper, and salts of iron successively, may be accepted as positive. 

Derivation Occurs naturally in many plants (wood sorrel, rhubarb, spinach) and can be made by alkali extraction of sawdust. Now manufactured by reaction of carbon monoxide and sodium hydroxide or of sodium formate with sodium hydroxide, followed by distillation of the resulting dihydrate crystals. 

From a knowledge of the solubility rules (see Section 4.2) and the solubility products listed in Table 16.2, we can predict whether a precipitate will form when we mix two solutions or add a soluble compound to a solution. This ability often has practical value. In industrial and laboratory preparations, we can adjust the concentrations of ions until the ion product exceeds K p in order to obtain a given compound (in the form of a precipitate). The ability to predict precipitation reactions is also useful in medicine. For example, kidney stones, which can be extremely painful, consist largely of calcium oxalate, CaC204 (K p = 2.3 X 10 ). The normal physiological concentration of calcium ions in blood plasma is about 5 mM (1 mM = 1 X 10 M). Oxalate ions ( 204 ), derived from oxalic acid present in many vegetables such as rhubarb and spinach, react with the calcium ions to form insoluble calcium oxalate, which can gradually build up in the kidneys. Proper adjustment of a patient s diet can help to reduce precipitate formation. Example 16.10 illustrates the steps involved in precipitation reactions. 

By virtue of what is called the law of mass action, the buildup of product itself works against the conversion reaction. Thus, the very buildup of lactic acid or lactate product serves as an inhibitor. (Albeit pyruvic acid is also listed as an inhibitor in the compilations, but which is a reactant for the conversion to lactic acid.) Another inhibitor listed is oxalic acid, a naturally occurring component of such vegetables as spinach and rhubarb, and which becomes toxic in large amounts. The common... 

Overdoses of Chinese rhubarb can cause a toxic reaction, especially if the fresh herb is used. Symptoms include diarrhea, nausea, vomiting, dizziness, abdominal colic, and jaundice. Long-term use may cause severe damage to the large intestine, cirrhosis of the liver, and potassium loss (Bensky et al. 2004). 

Allergic reactions to Chinese rhubarb have also been reported, usually after large doses (30 g), and include... 

To make the colors bright and fast, natural mordants (an element which aids the ehemical reaction so that the fibre absorbs the dye, preventing the colour from either fading with exposme to light or washing out) sueh as Myrballams, rhubarb leaves, oils, minerals, alum, iron Vat, etc., are used. No heavy metal mordants like copper, chrome, zinc, tin, etc., have been used. 

The acidity of food is related to the amount of undissociated and dissociated forms of carboxyKc acids and oxonium ions, respectively, that are present. The major substances that give a sour taste in foods are undissociated hydroxycarboxylic acids, citric and malic acids. Often, however, other carboxylic acids occur, such as ascorbic acid in most types of fruits, tartaric acid in grapes, isocitric acid in blackberries, oxalic acid in rhubarb, lactic acid in some dairy products (such as yoghurt), fermented cucumbers, cabbage, oKves, vinegar and propionic acid in Emmental type cheeses (see Section 8.2.6). The acidity of cola drinks is provided by phosphoric acid, sometimes accompanied by citric or other acids. Carboxylic acids also act as bacteriostatic agents and affect a number of biochemical and chemical reactions. 

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