and treated with the Solvent-Diluent serving as Io all subsequent oils of that type (In spectra are ΔAbsn

= -Log (In /Io ), with all oils being

ratioed against a common reference oil rather than the same oil as per the manual method. Thus, in the manual method, each oil serves as its own reference, ratioing out all the common spectral features of the oil, while in the automated analytical procedure, the less rigorous use of a common representative oil is a necessity to facilitate automation. As a consequence, one requires a secondary PLS-based chemometric calibration to support the primary oleic acid calibration to account for the additional variability associated with the oil-type spectra being analysed (e.g., all mineral-based diesel engine oils). This additional PLS requirement is a substantive undertaking (2) but rewarded by high sample throughputs (>100/h) providing titrimetric quality AN results.

Analytical Benefits

Although the manual method brings compromises in terms of sample handling and analytical speed, one gains three significant benefits; (a) elimination for the need to develop a secondary PLS-based chemometric calibration, (b) independence of oil-type (e.g., universality) and (c) the ability to differentiate between weak and strong acids. These benefits all arise from the splitting the analytical samples ratioing out their common spectral features and leaving the spectral contributions of the acid-base reaction to be measured in the ΔAbs spectrum. Splitting the calibration samples also provides additional oleic acid spectral response information in relation to Carboxylic acid (COOH) concentration (Figure 1) dependence. Thus, one obtains access to both the primary response of NaPhenolate representative of the total acidity (~AN) as well as weak acids (COOH) from the oleic acid response. As such, with a sample undergoing analysis, any weak acid (COOH) measurably present can be quantified along with the total acidity or AN of an oil when mixed acid-types are present; their difference allows one to calculate the strong acid contribution:

26 LUBE MAGAZINE NO.163 JUNE 2021 for ). Here the differential

Such acid differentiation could prove useful in assessing the corrosion potential and reactivity of acids present in an oil, particularly if combined with moisture content information, moisture being a catalyst in terms of acid reactivity.


No.134 page 3 AciditySTRONG = AcidityTOTAL – AcidityCOOH [1]

Figure 1: Automated single-sample vs. the manual split-paired sample procedure modelled in spectral terms. Reagent-Diluent (RD) and/or Solvent- Diluent (SD) is added to calibration standard A to produce spectra B. Spectra C and C’ are the final Δ spectra produced by each procedure. Note spectrum C’ contains both components, while spectrum C only one. The arrows indicate the direction of absorbance change for each component as acidity rises.

Generic AN Implementation

Even with clear benefits, the general implementation of the manual stoichiometric AN method is constrained as many FTIR CM systems rely on proprietary instrumentation and software, a common problem in the spectroscopy domain. One way in which this issue has been addressed has been provided by Dr. Friedrich Menges, the developer of SpectraGryph, generic post-spectral processing software that allows the examination, processing and automatic extraction of data from spectra collected on virtually any spectrometer (10). Available as an internet download, SpectraGryph is free for

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