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Daniele Naviglio » 10.Alkalimetry


Alkalimetry

The term alkalimetry refers to that part of volumetric chemical analysis which enables us to work out the concentration of an acid solution using an alkaline solution at a known concentration and a suitable indicator.

Translated from: Treccani online dictionary


Alkalimetry (cont.)

Alkalimetry is used in the determination of many different inorganic, organic and biological substances which have acid properties. Some examples are:

  • determining acidity of oil;
  • determining acidity of milk;
  • determining volatile acidity in wine.

Olive oil

The chemical composition of olive oil can vary depending on factors like: parasite attacks, latitude, type of soil, plant variety, way olives are stored, etc. Oil is mainly made up of triglycerides, esters derived from glycerol with three fatty acids, and oleic acid (C18:1_9) is the most common fatty acid. If there is water present, lipases which are normally contained in olives can act on the oil to hydrolise the triglycerides releasing fatty acids and thus mono and diglycerides and glycerine are formed.

The determination of acidity in oil, therefore, measures the degree of hydrolytic rancidification.

Main constituents of Olive oil

Main constituents of Olive oil


Average acid composition of oil


European community classification of olive oil (Reg. CE 1513/2001)


Determining acidity of oil

A sample of about 5g of oil is taken for analysis, and 80 mL  of a mixture of ethanol and ethylic ether (at a ratio of 1:2) and a few drops of 1% phenolphthalein are then addes. Titration using a 0.1 M or 0.5 M KOH is continued until the solution turns pink.

Calculation to determine acidity of oil through alkalimetry  titration

Calculation to determine acidity of oil through alkalimetry titration


To recap …


Practical example

Imagine we titrated 5.12g of olive oil with a 0.1 M KOH solution and that the burette was not put to zero, but we know the original volume was 12.05 mL. If the final volume (read off correctly) is 16.20 mL, what is the acidity expressed in g% of oleic acid of the olive oil under analysis?

Calculation for acidity of oil as in the example

Calculation for acidity of oil as in the example


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