The 859 Titrotherm from Metrohm can be used to determine the low-level total acid number in mineral oils and biodiesel and low-level free fatty acid content in edible fats and oils.
In a trans-esterification reaction that is 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, defined as the percent 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.
Manual titration with a phenolphthalein indicator according to EN 14104 is slow and the endpoint can be difficult to detect, especially with coloured solutions and when using weak titrants.
In potentiometric titration according to ASTM D 664, in a non-aqueous environment, glass membrane becomes rapidly dehydrated and therefore requires frequent regeneration.
Poor electrical conductivity of the titrating medium can lead to imprecise results.
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 D664, 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.
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 enthalpy changes are reliably monitored.
The 859 Titrotherm from Metrohm can perform this thermometric titration.
The 859 is a PC-controlled thermometric titrator with up to four 800 Dosino dosing systems, two thermometric measuring interfaces for Thermoprobe, and the optional use of potentiometric electrodes (via dongle), and USB connection.
Advantages include: fast titrations that take around 10-60 seconds to complete; a reliable thermoprobe that if stored dry, never needs regenerating or recalibrating; works with highly colored and turbid samples; it is carried out in normal titration vessels; and it can be automated with a sample processor.
