Determination of the index of saponification

The index of saponification corresponds to the mass of KOH out of Mg necessary to neutralize the free Fatty-acids and for Saponifier the fatty-acids combined in a Gram of body gras.
The quantity of KOH varies with the molar mass of the fatty-acids. The higher the molar mass is, the more the index of saponification is low: the index of saponification is thus an indirect measurement of the molar mass of the fatty-acids.

Principle

It is about a back titration. One makes hot react a solution of fatty-acid with an excess of KOH. This excess is then proportioned by a hydrochloric acid solution.

If one carries to boiling a greasy substance in the presence of KOH, the fatty-acids are saponified; it is a slow total reaction to room temperature but which takes between 40 to 60 minutes by heating by soft boiling. The potash reacts with the fatty-acids released to form Savon.

Protocol

To calibrate the solution of HCl given then to carry out tests and witness.

Material

beakers equipped with stoppers
oil-can
pipette measured of 10 ml
pipette measured of 25 ml
bath Marie

Solutions

alcoholic Potash of concentration 0,5 mol/L
Hydrochloric acid of exactly known concentration 0,5 mol/L
Phenolphthalein
ethanol
Oxide of étyle

Stages

Preparation of the solution of greasy substance

The greasy substances being insoluble in water, they should be dissolved in a solvent.

To start by weighing a mass known and close to 4 G in a beaker,
To add 100 ml of contitué solvent of ethyl ethanol and oxide in proportions of volume to volume,
To agitate to dissolve the greasy substance.

Proportioning of the acid value

To introduce into a beaker 10 ml of solution of greasy substance,
To add 25 ml of alcoholic potash of concentration 0,5 mol/L,
To put at the Marie bath boiling during 45 to 60 minutes,
To add 2 to 3 phenolphthalein drops,
To proportion the potash excess by hydrochloric acid of concentration 0,5 mol/L while agitating constantly until the turn with colorless of phenolphthalein,
To carry out two tests.

Realization of the witnesses

As the concentration of alcoholic potash is not exactly known, it is necessary to determine it by a witness.

To introduce into a beaker 25 ml of alcoholic potash and 10 ml of solvent
To treat under the same operating conditions as the tests (Marie bath)
To add 2-3 drops of Phenolphthalein.
To proportion until the turn with colorless of phenolphthalein.

Equation of the reaction

$CH_2- (OOC-R) - CH (OOC-R) - CH_2- (OOC-R) + 3 NaOH --> CH_2OH - CHOH - CH_2OH + 3 R-CO _2-Na$

to see Saponification for more details.

Calculations

Calculation of the concentration of alcoholic potash

A mole of

OH^-

reacts with a mole of

H_3O^+

$n \ OH^- = N \ H_3O^+$ .

$C_ {KOH} \ times V \ {OH^-} = C _ {HCl} \ times V \ H_3O^+$ .

$C_ {KOH} = \ frac {C _ {HCl} \ times V \ H_3O^+} {V \ {OH^-}}$ .

Calculation of the index of saponification

$IS = \ frac {(VT - VE) \ times C_ {HCl} \ times M_ {KOH}} {m}$

IS : Index of saponification
VT: Volume poured with the witness in ml
VE: Volume of the test in ml
$C_ {HCl}$ : concentration of the hydrochloric acid solution in mol/L
$M_ {KOH}$ : mass molar KOH in g/mol
m: mass of oil weighed exactly in G

Precise details/Safety

nothing to throw in the wash-hand basin , the organic solvents is dangerous for the Environnement, to recover them in containers so that they are treated. Moreover, to handle solvents under Hood ventilated with adapted protection (gloves and safety goggles). For more detail on safety, to refer to the labelling of the products. the solution of fatty-acid for the Détermination of the acid value can be preserved.

Some Indices of saponifications

See too

Random links: