Metrohm describes a method for determination of low-level total acid number in mineral oils and biodiesel and low-level free fatty acid content in edible fats and oils using thermometric analysis.
In a trans-esterification reaction that is a catalysed by a base, acid or enzyme, biodiesel is produced by chemically reacting a vegetable oil or fat with methanol to yield fatty acid methyl esters.
Depending on production and storage conditions, biodiesel can contain small amounts of detrimental free fatty acids (FFA) that cause corrosion and deposits in the engine.
The FFA content as the per cent by mass of oleic acid in the sample, and the total acid number (TAN), expressed in mg KOH required to neutralise 1g of FAME, are used as reliable monitoring tools.
Both determinations are non-aqueous acid-base titrations using a strong base dissolved in alcohol.
Endpoint recognition, unlike that applied in EN 14104 and ASTM D 664, is performed thermometrically using a very sensitive temperature sensor.
By means of a straightforward thermometric titration procedure using a catalytically enhanced indication reaction, several solid palm oil and biodiesel samples are analysed in terms of FFA content and TAN, respectively.
In thermometric titration, each chemical reaction is associated with a change in enthalpy that causes a temperature change, which, when plotted versus volume of titrant, can be used to monitor the course of the reaction and thus to detect the titration endpoint.
For a simple reaction this means that the increase (exothermic reaction) or reduction (endothermic reaction) in temperature depends on the amount of substance converted.
In a thermometric titration, reagent solution (titrant) is added to the sample at a constant rate until the endpoint is reached.
The latter can be recognised by a break in the titration curve obtained by plotting the amount of titrant added to the sample versus the temperature.
Since the temperature sensor has a response time of 0.3s and a resolution of 10-5K, even small changes are reliably monitored.
A new thermometric titration procedure overcomes inherent problems with the current manual and potentiometric methods.
