Monitoring the health and cleanliness of in-service lubricants is an important feature of any condition-based maintenance program. This practice helps to ensure the lubricant is performing its job of controlling the effects of harmful contaminants and preventing wear and corrosion.

Background
New motor oils, such as those used in diesel engines, are formulated with an over-base additive package to help neutralize acids such as nitric (HNO3) and sulfuric acids (H2SO4), which are produced or ingressed during service. Acids are corrosive agents that when combined with water can chemically attack critical machine surfaces leading to impaired performance, wear and motion impediment. This basic nature of a new lubricant used in engine crankcase applications is referred to as the reserve alkalinity. Over time, the reserve alkalinity of the lubricant is sacrificed or depleted, which is a normal aging process. When too little alkalinity remains, the engine's oil-wet surfaces can be attacked from acidic contaminants that failed to neutralize.

Figure 1. Titration Curve and Its Inflection Point 4

Base Number and Titration Process
Standardized testing organizations such as the American Society for Testing and Materials (ASTM), International Organization for Standardization (ISO) and the British Institute of Petroleum (IP) have developed several test standards to determine the basic concentration of new or used lubricants, thus providing a means to quantify the immunity of a lubricant against the damaging effects of acidic constituents. These test standards use a wet-chemistry method called titration which provides a common system for calculating and quantifying the basic concentration. This method is referred to as the base number (BN).

BN is defined as the quantity of acid, expressed in milligrams (mg) of potassium hydroxide (KOH, a base) per gram (gm) of a sample, that is required to titrate (or neutralize the basic portion within) a sample to a specified endpoint (the point at which all of the bases present in the sample have reacted with the acid introduced into the solution).1

Titration, in general, utilizes an acid-base neutralization reaction and an indicator whose properties change at the endpoint. The following is a list of BN test standards and their corresponding indicator:

  • ASTM D2896 and D4739 - measure the change in the solution's pH by electrical potential

  • IP 400 - measures the change in conductivity of the solution

  • ASTM D974 and D5984 - detect a change in a solution's pH by colorimetry

These methods involve adding a measured amount of acid to the oil until the reserve alkalinity has been neutralized. The BN is calculated from the amount of acid required to completely neutralize the lubricant.

The most important part when conducting titration is the determination of the endpoint, that is, the point which signals that all of the analyte present in the sample has been completely reacted by the added titrant. The endpoint can be determined by the inflection point in the titration curve (Figure 1).

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Figure 2. Typical Titration Curve for IP400 Standard5

Test Standards to Measure BN
Potentiometric Titration Methods
ASTM D2896 and ASTM D4739 both use potentiometric titration as their fundamental tool to measure BN. Potentiometric titration is a process in which the potential between two electrodes, one being the referent and the other the indicator electrode, is measured. As the acid titrant is added to the sample solution, the potential between these two electrodes will decrease. A relationship between this potential and the incremental volume of the added acid can be established to quantify the BN.

One advantage of potentiometric titration compared to manual titrations is that the endpoint, in most cases, can be determined with more accuracy and precision. Additionally, the entire process can be automated, where automated titration systems can process larger volumes of samples with minimal analyst involvement.4 However, several drawbacks are also associated with potentiometric titration.

Testing consists of preparing numerous solvents and reagents, and the maintenance and calibration of sensitive potentiometric electrodes can prove to be a challenging task.5 Typically, the electrodes need to be rinsed with water and blot-dried with soft absorbent tissue before and after each use, and soaked with distilled water refilled with electrolytes daily. Additionally, calibration with controlled titration solvent needs to be run at a weekly minimum.1,6

The main difference in test methods ASTM D2896 and ASTM D4739 is the choice of acid used and the solvent system that is employed. ASTM D2896 uses a stronger acid, perchloric acid (HClO4), and a more polar solvent system which consists of chlorobenzene (C6H5Cl) and glacial acetic acid (CH3COOH). This aggressive titrant of D2896 allows both the strong and weak base present in the oil to be titrated.

In the case of ASTM D4739, hydrochloric acid (HCl), a weaker acid used as the reagent, and a less polar solvent system is used. The solvent system consists of toluene (C6H5CH3), isopropyl alcohol (C3H7OH), chloroform (CHCl3) and a small amount of water. Based on the difference in chemical design, D4739 is recommended to measure only the strong alkaline components of the lubricant because its reagent is not always strong enough to completely titrate the weak base.

The BN for used oils as determined by D4739, provides the value for basic components that would contribute to the preservation of the oil. The BN for used oil as determined by D2896 may provide a higher value than the base that is actually available to the oil for mitigating acids.8

Conductimetric Titration Method
IP400 was developed by The British Institute of Petroleum in 1996, and it does not have an ASTM standard equivalence. The other standards, ASTM D2896 and ASTM D4739, are equivalent to IP 276 and IP 177, respectively.5 The conductimetric titration method used in IP400 employs the same titrant and solvent system as the ASTM D4739. However, the change in conductivity of the sample solution is monitored instead of the potential difference between the referent and indicator electrode. Conductance is the reciprocal of resistance, and is measured in siemens (S).

According to research on the IP400's performance conducted by De Montfort University, IP400 is believed to be superior to the current potentiometric methods because of its choice of the electrode system. This electrode system is reported to require much less effort to operate and maintain. In addition, the endpoint in the IP400 method is determined by the intersection of two straight lines, while in the potentiometric curve, it is the occasional indistinguishable inflection point. The ease in identifying the endpoint comes with greater repeatability and reproducibility of the test results. Figure 2 illustrates how to obtain the endpoint for the IP400 test method.

Colorimetric Titration Methods
ASTM D974 and ASTM D5984 are methods that use color-indicator titration to determine the basic constituents in petroleum products and lubricants. Colorimetric titration takes advantage of the visual changes of a chemical compound when its environment shifts from acidic to alkaline. In other words, the color of this indicator will change at the pH corresponding to the inflection point. The titration technique is the same as potentiometric, with an acid being added to the oil, and its added volume monitored.

In principle, D5984 is different from D974 because it employs the back titration technique (to be discussed later). Its titrant, HCl, is added in excess, and NaOH is used afterward to back titrate this overadded amount of acid. Methyl red is used as the indicator, changing its color from magenta to yellow at the pH corresponding to the inflection point. In ASTM D974, similar to D4739, hydrochloric acid is used as the titrant; a mixture of toluene and isopropyl alcohol containing a small amount of water is used as the solvent system and p-naphtholbenzein is used as the color indicator, which is orange in acid and green-brown in base.

One apparent weakness of the colorimetric titration is that dark-colored oils - such as cutting oils, diesel engine oils and other severely darkening used oils - cannot be analyzed due to obscurity of the color-indicator endpoint.3

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Figure 3. Titration Curves to Illustrate Selection of Endpoints1

A Time-saving Alternative
Back Titration
Back titration (BT) can be used as a time-saving alternative for measuring BN. The titration technique depends on the indication of the endpoint to determine the concentration of the analyte (in this case, the amount of basic constituents) within a sample. Other methods besides ASTM D974 encounter difficulties in testing dark-colored or heavily used oil because of the obscure color change, and potentiometric titration methods such as ASTM D2896 and ASTM D4739 are often challenged due to a vague or nonexistent inflection point provided in the titration curve. The more aged and contaminated the oil sample, the more ambiguous the endpoint might be.5 In Figure 3, an example of an unclear titration endpoint is shown.

Figure 3 illustrates the following guidelines to determine the endpoint of the titration, according to ASTM D4739:

  • Titration curve A - no inflections, take the endpoint at the buffer potential

  • Titration curve B - inflection within prescribed window, take inflection as the endpoint

  • Titration curve C - inflection prior to buffer potential but not in prescribed window, take the endpoint at buffer potential

BT often is regarded as an effective means to cope with these situations, and it is also reported to be a more resourceful method, saving operational time and cost for the analysis laboratories.7, 8, 9 With BT, the endpoint is more recognizable because it is a process of titrating strong acid with strong base. This usually gives a more standard sigmoidal curve and therefore a more defined inflection point.

This involves adding an excessive amount of acid to the oil sample. Potassium hydroxide, which is a strong base, is then used to titrate back the leftover amount of acid in the sample. This process is a reaction of a straight-forward and fast-reacting strong acid and strong base neutralization. Because the concentration and volume of the acid is known, the amount of the bases originally present in the oil can be calculated, which leads to the calculation of the BN.

Forward Titration
In forward titration (FT), the acid is gradually added with the time between each increment fixed at a predetermined value. The basic constituents in the sample of various sources, types and strengths are allowed to react with the acid slowly as more acid is introduced. This gradual process presents a significant time commitment for the operator, even when the process is automated.

For example, in ASTM D4739, the neutralization's slow reaction rate can result in the process time of up to 60 minutes without cleaning and conditioning time commitments.7 BT, on the other hand, requires much less time because the presence of the excess of acid in the solution allows a more rapid and complete neutralization. In fact, a duration of 18 to 20 minutes titration time, including cleaning and conditioning of the electrodes, has been recorded by various analytical labs conducting BT.7, 8, 9

Recent Work to Develop BT Method
ASTM International subcommittee D02.06 has recently pursued the possibility of a back titration ASTM standard.8 According to the report presented in the subcommittee meeting in Orlando in 2006, the presence of predose of the acid enhances ionic conductivity, which improves the electrode response. The data for this report is based on the work performed by two leading instrumentation laboratories, Mettler and Brinkmann. Additionally, the excess of acid combined with effective stirring can ensure a thorough reaction between the acid and the alkaline constituents within the oil sample.

Table 1 shows the BN values (in mg KOH) obtained for various fresh and used oil samples from Mettler (in blue) and Brinkmann (in green). For each type of oil, two runs were performed for BT and another two for FT, for comparison. The amount of KOH in mg equivalent to the acid used to titrate the samples to their inflection point is recorded for back titration (BT Inf) and for forward titration (FT Inf).

The percent standard deviation is then calculated for values obtained within a lab to determine repeatability, and across different labs to determine reproducibility. In the ASTM standards, these terms are defined as followed: Repeatability is the difference between two test results, obtained by the same operator with the same apparatus under constant operating conditions on identical test material. Reproducibility is the difference between two single and independent results obtained by different operators working in different laboratories on identical test material.

The percent standard deviations for BT test results suggest that this is a viable method for determining BN. However, because this is only preliminary data, further testing on a wider range of new and used lubricants with different additive concentrations is needed for a final conclusion to be drawn.

In addition, the BN values observed for BT are consistently higher than those for FT, according to Table 1. This suggests that BT (in the specific procedure carried out by Mettler and Brinkmann) recognizes more basic constituents in the original sample than does FT. Possible explanation could be that the excess of acid might concentrate the sample in a more aggressive environment than expected, assisted by surplus stirring time, causing a more complete neutralization of both strong and weak alkaline constituents. The results show a better overall reproducibility for testing used oil with BT.

Table 1. ASTM D4739 Back Titration and Forward Titration Results9

Conclusion
The BT results acquired by Mettler and Brinkmann have introduced a potentially time-saving operational method for measuring BN, considering that the total process time is reduced by more than one-third when using BT rather than FT.

Studies have shown that the data obtained is most dependable when the required amount of KOH titrant solution is between 10 to 80 percent of the predose acid amount. A study that is currently of interest is one that assesses the required mixing times in BT procedure and then evaluates its effectiveness against the reliability of the data achieved. This study can justify the reduction of unnecessary sample mixing to save even more time without losing accuracy.

Future Work
Back titration has recently made its place in the ASTM D4739 standard; however, it is used only when the forward titration has offered no positive indication of an endpoint. The evidence presented in this article suggests that more studies and research should be conducted regarding adopting BT as a practical procedure for measuring BN in used oil analysis.

It is noted in the ASTM D2 meeting that testing and feedback from end users through the ASTM committee could advance the possibilities of making BT a separate ASTM method or a modification of D4739. Interested parties who would like to participate in developing the method are encouraged to visit www.astm.org or call the D02 staff manager, David Bradley, at 610-832-9681.

References

1 ASTM D4739-96: Standard Test Method for Base Number Determination by Potentiometric Titration.

2. W. Van Dam. "Measuring Reserve Alkalinity." Practicing Oil Analysis magazine, July 2002.

3. ASTM D974-97: Standard Test Method for Acid and Base Number by Color-indicator Titration.

4. Sabrina Godfrey Novick. "Chapter 6 - Common Ion Effect." General Chemistry 4A - Online Lecture: http://www.geocities.com/CapeCanaveral/Launchpad/1309/c4lec6.html.

5. D. Armitage, M. Fox and S. Pickering. "Finding a Better Base Number." Practicing Oil Analysis magazine, July 2000.

6. ASTM D2896-98: Standard Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration.

7. Tore Fossum. "Report on D4739 Back Titration Method." ASTM D2 Meeting. Orlando, 2007.

8. Personal Communication, Janet Lane. ExxonMobil Research and Engineering. Analytical Sciences Laboratory. November 14, 2007.

9. ASTM D02.06 Preliminary Back Titration Data based on work by Mettler and Brinkmann laboratories, June 2007