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Acid number determination plays a central role in petroleum quality control. This method is used to estimate the amount of additive depletion, oxidation and acidic contamination in petroleum products.

This article explores the potentiometric determination of acid number (AN) and strong acid number (SAN) in line with ASTM D664, using the example of a motor oil sample.

The acid number can be understood as the amount of base (as mg KOH/g) sample required to titrate a sample’s acidic constituents. The analysis outlined here has been undertaken using the InMotion Pro sample changer.

Cover up lid-handling has been employed in order to cover sample beakers both prior to and following titration, while the use of the SmartSample workflow facilitates the transfer of sample weight and ID from the balance to the titration system via secure wireless communication.

Image Credit: Mettler-Toledo – Titration

Approximately 80 mg of KHP was added to a glass titration beaker along with 60 mL of deionized water. Buffer potentials are saved as auxiliary values, and a new blank value is determined and stored for each new batch of solvent mixture.

In the example presented here, a series of 100 mL beakers were used on the InMotion Pro- special beakers 1-4, the rinse beaker and the conditioning beaker. These were used for rinsing and regenerating the DGi113-SC:

An optimum rinsing sequence was determined for the application after titration and before the next sample. This included:

Acid compounds as mg KOH/g sample

M=56.1 g/mol, z=1 

KOH + HA = KA + H2O

* No EQP detected for this sample

The motor oil sample’s titration curve is relatively flat, making EQP detection challenging. This particular challenge is reflected in the increased relative standard deviation (srel) of TAN EQP in relation to the srel for evaluation at fixed buffer potential pH 11.

Different sample amounts are recommended, depending on the sample’s acid number. The table below provides guidance.

The acid number represents the quantity of base as mg KOH/g sample required to titrate the sample’s acidic constituents under certain conditions, for example, for a specified endpoint and solvent.

An EQP method function is used to titrate the sample. If there is no inflection point detected, then endpoints will be taken at potential readings, which correspond to the endpoints for aqueous acidic and basic buffer solutions (pH 4: SAN, pH 11: AN).

For this sample, the SAN at pH 4 was not calculated since the initial potential was already below buffer potential 1 (pH 4).

In order to determine the blank, the solvent mixture is titrated to a buffer potential of pH 11 (B[ASTM D664 EP]).

In instances where an EQP titration does not detect an EQP, or where this only detects an arbitrary EQP, blank values may be too large. Under these circumstances, two blank values can be selected. Method B[ASTM D664 EP] was applied for this application.

It is important that the rinsing procedure employed with the non-aqueous pH-sensor is not shortened because this could reduce the sensor’s working life and its repeatability.

It is also important to note that some oils will not show any EQP, returning a very flat curve. Under these circumstances, it is only possible to perform the evaluation at the buffer potentials of pH 4 and pH 11.

This information has been sourced, reviewed and adapted from materials provided by Mettler-Toledo - Titration.

For more information on this source, please visit Mettler-Toledo - Titration.

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