Saponification equivalents

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Dissolve approximately 3 g of potassium hydroxide in 60 mL of 95% ethanol. Allow the small amount of insoluble material, if any. to settle to the bottom of the container, and, by decantation, fill a 50-mL buret with the clear supernatant. Measure 25.0 ml of the alcoholic solution into each of two round-bottom flasks. Quantitatively transfer an accurafe/y weighed 0.3- to 0.4-g sample of pure, dry ester into one of the flasks the other basic solution is used for a blank determination. Equip each flask with a stirbar and a reflux condenser. 

Heat the stirred solutions in both flasks under gentle reflux for 1 h. When the flasks have cooled, rinse each condenser with about 10 mL of distilled water, catching the rinse water in the flask. Add a drop or two of phenolphthalein. and separately titrate the solutions in each flask with sfanc/ard/zed hydrochloric acid that is approximately 0.5 Min concentration. 

The difference in the volumes of hydrochloric acid required to neutralize the base in the flask containing the sample and in the flask containing the blank corresponds to the amount of potassium hydroxide that reacted with the ester. The volume difference, in milliliters, multiplied by the molarity of the hydrochloric acid equals the number of millimoles of potassium hydroxide consumed. Using the titration data, calculate the saponification equivalent of the unknown ester. 

The ester may not completely saponify in the allotted time, as evidenced by a nonhomogeneous solution. If this is the case, heat the mixture under reflux for a longer period of time (2-4 h). Higher temperatures may be required in some cases. If so, diethylene glycol must be used as a solvent in place o/the original 60 mL of 95% ethanol. 

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It is frequently advisable in the routine examination of an ester, and before any derivatives are considered, to determine the saponification equivalent of the ester. In order to ensure that complete hydrolysis takes place in a comparatively short time, the quantitative saponi fication is conducted with a standardised alcoholic solution of caustic alkali—preferably potassium hydroxide since the potassium salts of organic acids are usuaUy more soluble than the sodium salts. A knowledge of the b.p. and the saponification equivalent of the unknown ester would provide the basis for a fairly accurate approximation of the size of the ester molecule. It must, however, be borne in mind that certain structures may effect the values of the equivalent thus aliphatic halo genated esters may consume alkali because of hydrolysis of part of the halogen during the determination, nitro esters may be reduced by the alkaline hydrolysis medium, etc. 

The saponiflcatlon equivalent or the equivalent weight of an ester is that weight in grams of the ester from which one equivalent weight of acid is obtainable by hydrolysis, or that quantity which reacts with one equivalent of alkali. The saponification equivalent is determined in practice by treating a known weight of the ester with a known quantity of caustic alkali used in excess. The residual alkali is then readily determined by titration of the reaction mixture with a standard acid. The amount of alkafi that has reacted with the ester is thus obtained the equivalent can then be readily calculated. 

The experimental details already given for the detection and characterisation of aliphatic esters (determination of saponification equivalents h3 diolysis Section 111,106) apply equally to aromatic esters. A sfight modification in the procediu-e for isolating the products of hydrolysis is necessary for i)henolic (or phenyl) esters since the alkaline solution will contain hoth the alkali phenate and the alkali salt of the organic acid upon acidification, both the phenol and the acid will be hberated. Two methods may be used for separating the phenol and the acid ... 

The determination of the saponification equivalent of an ester by the alcohohc potassium hydroxide method is described in Section 111,106 an alternative procedure using diethylene glycol is given below. This constant should be determined if possible in the prehminary examination, since a knowledge of its value together with the boihng point provides a basis for a fairly good approximation of the size of the ester molecule. 

Determination of the Saponification Equivalent of an Ester BY THE Diethylene Glycol Method... 

Numerous methods for the deterrnination of monomer purity, including procedures for the deterrnination of saponification equivalent and bromine number, specific gravity, refractive index, and color, are available from manufacturers (68—70). Concentrations of minor components are deterrnined by iodimetry or colorimetry for HQ or MEHQ, by the Kad-Eisher method for water, and by turbidity measurements for trace amounts of polymer. 

This stoichiometry is the saponification equivalent, used to determine the equivalents of ester. 

Saponification see Hydrolysis Saponification equivalent of an ester, determination of. 392, 1065 Saturated aliphatic hydrocarbons, 233 reactions and characterisation of 234, 1058 table of, 235 ... 

Wiesenberger determined the saponification equivalent of esters by a method in which the ester is first saponified with an excess of base. The excess is then neutralized on passage through a bed of acidic resin, and the organic acid formed during saponification is titrated with standard base. In this way a relatively large excess of base can be used to ensure quantitative saponification. The determination is a direct titration therefore only one standard solution is required, rather than the two that are customary in indirect determination. 

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