Saponification
= Saponification
{wiki}
<Saponification> is <hydrolysis> of an <ester> under basic conditions. The <conductometric saponification assay> follows <ethyl acetate> consumption by <hydroxide>, producing <ethanol> and an <acetate ion>; its equal-starting-<concentration> case provides a practical <second-order reaction> example.
<Saponification> is <ester> <hydrolysis> promoted by a <chemical base>, with net consumption of <hydroxide> to form a <carboxylate> and an <alcohol>. For a <carboxylic acid>-derived <ester>, the net <chemical equation> is
$$
\mathrm{RCOOR'}+\mathrm{OH^-}\longrightarrow\mathrm{RCOO^-}+\mathrm{R'OH}.
$$
The <hydroxide> is a <reactant>, rather than merely a regenerated <catalyst>.<Organic Chemistry by John McMurry/21.6 Chemistry of Esters>[Organic Chemistry by John McMurry; §21.6, Conversion of Esters into Carboxylic Acids: Hydrolysis; Figure 21.8 discussion of carboxylate formation]{ref}
For <ethyl acetate>, the balanced <chemical equation> becomes
$$
\mathrm{CH_3COOC_2H_5}+\mathrm{OH^-}\longrightarrow\mathrm{CH_3COO^-}+\mathrm{C_2H_5OH}.
$$
Both sides contain four <carbon> <chemical atoms>, nine <hydrogen> <chemical atoms>, three <oxygen> <chemical atoms>, and net <electric charge> $-1$. A <sodium> <ion> supplied by <sodium hydroxide> is a <spectator ion>; the <net ionic equation> omits it without losing <electric charge conservation>.
\Image[/saponification-atom-charge-ledger.png]
{title=Ethyl acetate saponification conserves atoms and charge}
{description=Original <chemical formula> ledger for one <ethyl acetate> and one <hydroxide> forming one <acetate ion> and one <ethanol>. Each column separately checks <chemical atom> counts or net <electric charge>; the arrow states <stoichiometry>, rather than a single-step <reaction mechanism>.}
In the usual <reaction mechanism>, <hydroxide> adds to the <carbonyl group>, making a <reaction intermediate>. Departure of the <alcohol>'s <conjugate base> gives a <carboxylic acid>; transfer of a <proton> then gives the <carboxylate> and the <alcohol>. Subsequent addition of a <chemical acid> gives the <carboxylic acid> by <protonation>:
$$
\mathrm{RCOO^-}+\mathrm{H_3O^+}\longrightarrow\mathrm{RCOOH}+\mathrm{H_2O}.
$$
This separate acidification changes the <reaction product>'s <protonation> state.<Organic Chemistry by John McMurry/21.6 Chemistry of Esters>[Organic Chemistry by John McMurry; §21.6, Figure 21.8 discussion: addition, elimination, deprotonation and subsequent aqueous HCl treatment]{ref}
The <conductometric saponification assay> makes <hydroxide> consumption observable. A university procedure equilibrates separate <ethyl acetate> and <sodium hydroxide> stocks in constant-<temperature> baths, then mixes them in a rinsed conductivity cell. Two <platinum> <electrodes> and an alternating-voltage <Wheatstone bridge> supply an <electrical resistance> readout. A matched <sodium hydroxide>–<water> dilution checks the initial signal; <potassium chloride> calibrates the cell constant. Replacing <hydroxide> with less mobile <acetate ions> lowers <electrical conductance>, supporting an inference about changing <concentration>. The manual supplies a procedure rather than measured outcomes. <Temperature> drift or additional <ions> can change the signal; it cannot alone identify the full <reaction mechanism>.<Manual for Advanced Physical Chemistry Laboratory-II by Nasima Arshad>[Unit 7, Experiment 1, printed p. 63: electrodes, bridge and ionic conductance; Experiment 2, printed pp. 66–67: preparation, resistance readout and controls]{ref}