New FTIR Methods for Determining Acid and Base Numbers in Lubricants F.R. van de Voort, C. Saint-Laurent and J. Sedman McGill IR Group and D. Pinchuk Thermal-Lube Inc. Introduction

Fourier transform infrared (FTIR) spectroscopy is an instrumental technique that

allows for the rapid analysis of lubricants for condition monitoring purposes. Although

FTIR spectroscopy can be used for a wide range of specific diagnostic analyses, its most

important application has been for the qualitative monitoring of a wide variety of oil

condition parameters, ranging from soot to nitration, the analysis having been

standardized under the guise of the Joint Oil Analysis Program (JOAP) of the U.S.

Department of Defense. FTIR spectroscopy also has the potential to be used

quantitatively; however, to date no standardized methods have been developed that can

claim to successfully have achieved this goal. The McGill IR Group and Thermal-Lube

Inc. have been working for several years on the development of quantitative FTIR

methods of oil analysis, specifically acid number (AN) and base number (BN)

determinations. These methods have evolved substantially and are now based on

principles similar to those used in the ASTM methods for the determination of these

parameters, but without the analytical complications associated with the latter. This

article provides a brief overview of the ASTM methods and their limitations and

describes how the new FTIR methods provide a sound theoretical as well as practical

basis for improving AN and BN analyses.

The ASTM Methods

When one considers acid number (AN) and base number (BN) determinations of

lubricants, one is touching on two of the most fundamental measures associated with oil

quality analyses. On the other hand, one is also dealing with some very troublesome

methodology from the standpoint of both implementation and interpretation. In simple

terms, AN (expressed as mg KOH/g oil) is related to the levels of acidic constituents

present or accumulating in an oil (additives or oxidation products, respectively), while

BN (also expressed as mg KOH/g oil) is related to the amount of basic constituents

present, usually basic additives incorporated in an oil to neutralize corrosive acids which

will accumulate in the oil over time. The ASTM standard methods are all titrimetric and,

depending on the oil and its additive package, one can measure either the acidic or the

basic constituents to initially characterize a new oil or monitor the relative changes in AN

or BN over time, relating these changes to oil functionality or performance degradation.

There is, however, no simple, single generalized protocol for AN or BN determinations

per se, but rather a number of quite distinct ASTM reference methods available. Some of

the key differences between these methods are summarized in Table 1. Table 1. ASTM method parameter comparison.

Parameter Colorimetric AN/BN (KOH/HCl)

D974-92

Potentiometric AN (KOH) D664 -89

Potentiometric BN (HCl)

D4739 - 92

Potent metric BN (Perchloric)

D2896-92 Endpoint Type ♣ ♦ ♦ ♠

Titration Solvent ♣1 ♦ ♥ ♠ Electrode/Indicator ♣ ♦ ♥ ♠

Endpoints ♣ ♦ ♦ ♠ Titrant ♣ ♣ ♣ ♠

Acidic Buffer ---2 ♣ ♥ ♠ Basic Buffer --- ♣ ♣ ♠ BN (TBN) Y3 N Y Y

SBN N3 N Y N AN (TAN) Y Y N N

SAN Y Y N N Units of Measure mg KOH/g mg KOH/g mg KOH/g mg KOH/g

1Different symbols within a row indicate differences among the methods for a given parameter. 2 Not applicable; 3Yes and no, respectively. In simple terms, ASTM methods may be subdivided into colorimetric methods, which are

impractical for colored and used oils, and the more complex potentiometric methods,

which work for most oils but for all practical purposes require an autotitrator. The various

potentiometric methods use different solvent systems, electrodes, and endpoints. An

additional complication is that either inflection points or fixed mV values (if no well-

defined inflection points exist) may be used as endpoints in two of the potentiometric

methods. The specific terminology used to describe the analytical results; i.e., total acid

number (TAN), total base number (TBN), strong acid number (SAN), strong base

number (SBN), AN and BN, all expressed as mg KOH/g, is defined in relation to the

method and endpoint used. In general, the terms SAN and SBN have largely become

redundant as oils having an SAN are rarely encountered with the present use of cleaner,

low-acid fuels and accordingly fewer oils are formulated with bases sufficiently strong to

produce an SBN. The present recommended practice is to express results only in terms of

AN and BN and to drop any reference to SAN and SBN as well as TAN and TBN. It is,

however, useful to provide a context for these titrimetric terms, and thus Figure 1

presents a composite "titration" plot of oils containing either acids (curve A) or bases

(curves B and C) summarizing most of the endpoints, both potentiometric and

colorimetric, differentiated by appropriate subscripts.

Figure 1. Idealized titration curves for the titration of oils with acid (curve A) or base (curves B and C) in accordance with the respective ASTM methods indicating their endpoints and associated terminology, with the subscript "c" referring to a colorimetric endpoint and subscripts "ps" and "pi" representing standardized and inflection-based potentiometric endpoints, respectively.

As illustrated in Figure 1, the various ASTM methods may produce different

results for the same oil sample. Hence, not only is the methodology confusing, but so is

the presentation and interpretation of the results. Beyond these issues are the shortcuts

that are frequently taken by oil analysis laboratories in the routine implementation of the

ASTM potentiometric methods for reasons of convenience or speed of analysis. In short,

there are many problems associated with ASTM AN and BN analyses - not only in terms

of methodology and defining and interpreting results, but also in terms of costs,

implementation, and disposal of reagents. Recognition of these problems provided the

impetus to work on developing quantitative FTIR AN and BN methods, the principles of

which are described below together with their advantages in relation to the ASTM

methods.

The FTIR Methods

The FTIR spectra of lubricants are generally extremely complex, exhibiting

numerous bands, many of which overlap extensively. Thus, among the wide variety of

acidic or basic constituents that may be present in a lubricant, few will give rise to readily

identifiable bands in its spectrum, precluding. the direct determination of AN or BN from

the FTIR spectra of lubricants. Hence, an indirect approach based on concepts of signal

transduction, differential spectroscopy and back-titration is required. The application of

these concepts in the determination of AN by FTIR spectroscopy is illustrated in Figure

1, the principles employed for BN determination being analogous.

Figure 2. Schematic diagram illustrating the concept of signal transduction in relation to the determination of AN by FTIR spectroscopy

The first step in Figure 2 involves the addition of a fixed amount of a strong base

(alcoholic KOH) to an oil containing undefined acids (RH) which has been diluted with

propanol. The acids present in the oil are rapidly converted to their corresponding salts,

consuming a proportionate amount of the KOH added. The residual KOH is then

subsequently reacted with a fixed, excess amount of a "weak" carboxylic acid (COOH;

pKa ~6.5) to form its conjugate base (COO-). At the conclusion of this step, which is

equivalent to having back-titrated the sample, the amount of residual carboxylic acid is

equal to the sum of the amounts of all the original "undefined" (RH) acids in the oil that

contribute to AN. The residual carboxylic acid can be readily quantitated by FTIR

spectroscopy owing to its intense absorption around 1700 cm-1. Conceptually, one can

state that the sequence of events illustrated in Figure 2 involves the "transduction" of the

IR signals of the various undefined acids present in the sample into a single IR

measurable carboxylic acid signal. The accurate measurement of this signal requires the

use of differential spectroscopy to eliminate the spectral contributions of the lubricant

formulation, some of which may occur in the region of measurement. This requires that

the neutralized sample be split, with one half being treated with the reactive carboxylic

acid reagent and the other half with a blank reagent as outlined in the analytical protocol

shown in Figure 3.

Figure 3. Analytical protocol for the determination of AN by FTIR spectroscopy.

The spectra of the two portions are then recorded and a differential spectrum is obtained

by subtracting the spectrum of the unreacted sample from that of the reacted sample, such

that only the spectral changes associated with the back-titration step are recorded. Figure

4 presents an overlay plot of the differential spectra obtained when a series of acid-spiked

samples of polyalphaolefin (PAO) are analyzed by the protocol illustrated in Figure 3.

This figure shows the increase in the intensity of the COOH band of the carboxylic acid

reagent with increasing AN, together with the corresponding proportional decrease in the

intensity of the carboxylate (COO-) band due to conversion of the carboxylic acid reagent

to its salt by reaction with the residual KOH.

Figure 4. Differential spectra generated for a series of standards prepared by spiking PAO with p-toluenesulfonic acid and analyzed according to the protocol illustrated in Figure 3.

A standard curve can be generated by plotting AN vs. the intensity of either the COOH or

the COO- band, and typically a linear regression of such data yields an SD of <0.07 mg

KOH/g oil, substantially better than anything that can be obtained by the ASTM methods.

The standard curve generated is used to predict the TAN values of "unknown" oil

samples and the data obtained are reproducible to ±0.10 AN. This method has been

shown to produce consistent results for a wide range of new and used hydrocarbon-based

oils as long as their ester content is less than 10% (or the oils have an absorbance of <1.0

over the 1700-1600 cm-1 region). The concepts presented above have also been

developed and implemented in a parallel manner for the determination of BN, and this

parallel method has also been shown to produce very reliable and consistent results.

1850 1800 1750 1700 1650 1600 1550

0.00

0.05

0.10

0.15

0.20

0.25 AN = 0

IncreasingAN

IncreasingAN

Abso

rban

ce

Wavenumber (cm-1)

Implementation and Advantages of the FTIR Methods

The AN and BN FTIR methods have been implemented as pre-programmed

procedures on a Thermal-Lube COAT FTIR analyzer to automate sample analysis after

reagent addition. The analytical protocol for the two methods is the same in all respects

other than the reagents employed, which simplifies their implementation in a commercial

laboratory and streamlines sample preparation and analysis. The analytical protocol is

simple and innocuous (Figure 4), with the use of reagents and solvents minimal and the

analysis time in the order of 5 minutes per sample. Once reacted, the samples are stable

and can be batched for analysis whenever it is convenient, making the FTIR methods

amenable to the use of an autosampler for high-volume work.

These new FTIR AN and BN methods have the potential to provide oil analysis

laboratories with substantial savings in terms of labor, time, and reagent and disposal

costs. These savings come largely from circumventing the slow titration process and

large solvent volumes inherent to the ASTM methods, the FTIR methods requiring only

small volumes of sample and reagent. While these FTIR methods adhere to the

fundamental tenets of the ASTM methods, they improve on their execution and should

lead to more uniform inter-laboratory test results. In short, FTIR spectroscopy provides a

viable alternative to titrimetric methods leading to improvements in analytical

performance, speed and accuracy, important to commercial and high-volume laboratories.

Conclusion

The new FTIR AN and BN methods represent a significant advance over the

ASTM wet chemical methods. However, these new methods will only prove themselves

by undergoing extensive testing and evaluation in condition monitoring laboratories, and

participants and queries are welcomed**. Should such methods prove their mettle in the

field, these new quantitative methods will complement the inroads FTIR spectroscopy

has made in lubricant analysis and further demonstrate the value of investing in FTIR

technology for condition monitoring.

**Please contact D. Pinchuk, President, Thermal-Lube Inc.

Further Reading

Dong, J., van de Voort, F.R., Yaylayan, Y. and Ismail, A.A. (2001) Journal of the Society of Tribologists and Lubrication Engineers. 3:23-29 van de Voort, F.R., Pinchuk, D. and Pinchuk J. Proceedings of the Society of Tribologists and Lubrication Engineers Condition Monitoring Conference (2001). Paper #7 CD ROM. Dong, J., van de Voort, F.R., Ismail, A.A., Akochi-Koblé, E. and Pinchuk. D. (2000) Lubrication Engineering 6:12-20.