A novel, easy to use thermometric titration method permits rapid, accurate, automated analysis of the free fatty acid (FFA) content of edible fats and oils with unprecedented precision and accuracy
The free fatty acid (FFA) content of edible fats and oils is an important indicator of rancidity.
It is used by producers to control the quality of their product, and by commercial users such as manufacturers of fried snack and convenience foods to control frying processes.
Regular monitoring of frying oils is used to control the quality of the product as well as determine replenishment and replacement schedules.
The official AOCS method involves the titration of an ethanolic solution of the fat or oil with ethanolic potassium hydroxide to a visually-determined phenolphthalein endpoint.
Titrations employing visual indicators require the analyst to be able to distinguish subtle differences in shade and colour, and to be well trained in the technique.
The task may be rendered more difficult if the color or clarity of the sample solution varies.
Differences in analytical results between analysts are common.
Automated potentiometric titration methods have been developed, with a pH electrode used to indicate the titration endpoint.
The operation of glass membrane pH electrodes in an oily, ethanolic solution requires careful monitoring, as the surface of the electrode becomes both dehydrated and fouled by the sample solution.
The junction of the reference electrode may also become fouled by the oily sample matrix.
To restore satisfactory performance, it is necessary to frequently re-hydrate the glass membrane by immersion in water. The analysis of poorly dissociated, weakly acidic species in a non-aqueous environment is not an ideal application for potentiometric titrimetry.
There is a clear need for a robust, accurate and precise automated titrimetric procedure which is substantially independent of operator technique, and where the sensor is unaffected by the nature of the sample.
Thermometric titrimetry has proved to be a robust, reliable technique for a wide variety of analytes across many industries.
A simple thermistor is used to sense the change in solution temperature which signals the titration endpoint.
Since only the change in temperature is of interest, there is no requirement to calibrate the sensor.
Unlike potentiometric probes, the thermistor may be regarded as a 'zero maintenance' sensor.
A modern thermometric titration system employs powerful algorithms to optimally condition the temperature signal and permit the computation of derivatives to accurately locate endpoints.
A requirement of many thermometric titration applications is that the temperature change at the endpoint is sufficiently great to overcome other thermal effects such as heats of mixing and loss of heat from the system.
This is often overcome by using relatively strong titrants, and commensurately higher sample amounts.
In the case of the determination of weak acids in non-aqueous media, the amounts present are relatively low and it is necessary to use a low strength titrant in order to obtain meaningful titres.
Such applications might be considered to be antipathetic to the application of thermometric titrimetry. However, Vaughan and Swithenbank discovered that it was possible to obtain highly accurate results for a range of weakly acidic species by a technique which could be described as catalysed endpoint thermometric titrimetry, or CETT.
In the determination of weakly acidic species, the sample was dissolved in anhydrous acetone and titrated with KOH dissolved in an alcohol.
After all acidic species have been reacted with the titrant, the first trace of free hydroxyl ion catalyses the exothermic condensation of acetone to diacetone alcohol.
Multitrator has found this to be a satisfactory and practical technique for the determination of phenols in coal tar distillate (the purpose for which it was originally developed), but less satisfactory for the routine determination of FFA in edible fats and oils.
Multitrator has discovered that another catalysed endpoint solvent system gives much improved analytical precision.
The base-catalysed reaction of acetone and chloroform is well known to be strongly exothermic.
In the context of the thermometric titration of weak acids, this reaction has been found to be safe and reliable.
