Equivalent Weights of Acids and Bases

Acids. Acidic oxides, acid anhydrides. Bases, basic oxides. Neutralization—hydrogen ion plus hydroxide ion. Hydronium ion. Equivalent weights of acids and bases. Normality. Monoprotic, diprotic, and triprotic acids. Monohy-droxic, dihydroxic, and trihydroxic bases. 

By the use of indicators - compounds which change colour as their environment changes from an acidic to a basic one (or vice versa) - the precise moment of neutralization can be determined. For example, a measured quantity of acid solution of unknown concentration is placed in a flask with an indicator (e.g. litmus). A solution of base of known concentration is run into the flask until, with the addition of one excess drop of base, the colour changes. The volume of base run in has been measured and it is now possible to calculate the concentration of the acid solution. The method of calculation can be found in any elementary chemistry book and depends on knowledge of the equivalent weights of acids and bases. Titrations are used in paint chemistry to determine the acid value of a resin (Chapter 12) and it will be seen from the definition of acid value that no knowledge of equivalent weights is required for this determination it is sufficient to know the concentration of the alkali solution. 

Quantitative results of a different kind were obtained by Jeremias Richter (1762-1807). He was obsessed with obtaining mathematical relationships in chemistry, and he helped to establish the concept of equivalent or combining weight. His work was summarised in 1802 by Ernst Fischer (1754-1831), who produced a table of equivalent weights of acids and bases related to sulphuric acid having a value of 1000. On this scale, muriatic acid (HCl) had a value of 712, and soda and potash had values of 859 and 1605 respectively. This meant that 859 parts of soda or 1605 parts of potash were required to neutralise 1000 parts of sulphuric acid or 712 parts of muriatic acid. 

Equivalent weights of acids and bases. One equivalent of an acid is the quantity that transfers (gives up) 1 mole ofH (1 mole of protons). Correspondingly, 1 eq of a base is the quantity that accepts 1 mole of H. Sodium hydroxide, NaOH, can react with phosphoric acid, H3PO4, for example, in any one of three ways ... 

Equivalent Weights Acid-base titrations can be used to characterize the chemical and physical properties of matter. One simple example is the determination of the equivalent weighf of acids and bases. In this method, an accurately weighed sample of a pure acid or base is titrated to a well-defined equivalence point using a mono-protic strong acid or strong base. If we assume that the titration involves the transfer of n protons, then the moles of titrant needed to reach the equivalence point is given as... 

A known weight of salt (equivalent amounts of acid and base) is then shaken with the same quantities of water and benzene (looo c.c. and 6o c.c. respectively), and the amount of aniline in the benzene layer determined as above. Instead of using the salt, one may preferably proceed as follows Shake up looo c.c. of a solution of hydrogen chloride of... 

Perform calculations involving equivalent weights and normality of acid and base solutions... 

Because one mole of an acid does not necessarily neutralize one mole of a base, some chemists prefer a method of expressing concentration other than molarity to retain a one-to-one relationship. Concentrations of solutions of acids and bases are frequently expressed as normality (N). The normality of a solution is defined as the number of equivalent weights, or simply equivalents (eq), of solute per liter of solution. Normality may be represented symbolically as... 

Gram-Equivalent Weight of Acid, Base and Salt... 

Chemists first began to make quantitative comparisons among the amounts of acids and bases that reacted together. This procedure was then extended to reactions between acids and metals. This allowed chemists to order the metals on a numerical scale according to their equivalent weight, which, as mentioned, is just the amount of the metal that combines with a fixed amount of acid. The concept of equivalent weights is, at least in principle, an empirical one since it seems not to rest on the theoretical assumption that the elements are ultimately composed of atoms. ... 

Homberg s quantitative experiments on the neutralisation of acids and bases are the first determinations of equivalent weights. Simon Boulduc (d. Paris, 1729 demonstrator in the Jardin du Roi) tried in 1698 to find the... 

Thomson, therefore, some time before Berzelius (see Vol. IV), had extended Dalton s atomic theory to the metals and to acids and bases. W. C. Henry says his father William Henry and he were told by Dalton that the atomic theory was deduced to explain Richter s table of equivalents published in BerthoUet s Statique Chimique in 1803 (see p. 678). Dalton s early atomic weight tables, however, never mention the combining weights of acids and alkalis, the compounds investigated by Richter, whose name first occurs in Dalton s diary on 19 April 1807. Thomson, in his account of Dalton s theory in 1807, says it explains Richter s law of equivalents and it was probably Thomson who drew Dalton s attention to Richter, since Dalton was in Edinburgh when he first mentions Richter in his diary. Higgins also had developed his views in relation to gaseous compounds, as Dalton did (see p. 739). 

In the second method. Stone et al. [84] copolymerized monomer 4-2 with TFS (Fig. 2.17) by emulsion polymerization in 21% isolated yield. The optimized ratio between TFS and the dimethyl phosphonate-substimted a,, -trifluorostyrene monomer in the copolymer 4-5 was 2.4 1. The molecular weights of the resulting copolymer were 38,100 and 105,900g/mol for and respectively. Homo-polymer 4-3 (membrane A) was hydrolyzed under acidic conditions (hydrochloric acid in dioxane, 100 °C, 20 h). The yield and the equivalent weight of acid functions were 95% and 130 g/mol, respectively. Copolymer 4-4 was hydrolyzed by the authors using two processes (i) basic conditions (potassium hydroxide, 84 °C, 64 h), membrane Cl, and (ii) acidic conditions with a DMF pretreatment, membrane C3. Finally, the authors concluded that the best results were obtained with an acidic hydrolysis and that membranes based on sulfonic acid-a,, -trifluorostyrene gave better results than those obtained from the phosphonic acid homolog. 

For example, if there are 2.0 equivalents dissolved per liter, a solution would be referred to as 2.0 normal, or 2.0 N. The equivalent is either the same as the mole or some fraction of the mole, depending on the reaction involved, and the equivalent weight, or the weight of one equivalent, is either the same as the formula weight or some fraction of the formula weight. Normality is either the same as molarity or some multiple of molarity. Let us illustrate with acids and bases in acid-base neutralization reactions. 

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