Finding a Better Base Number

Editor’s Note:
The information for this article was derived from the paper, “Performance of the IP400 Base Number Method for New and Used Lubricants,” by D. A. Armitage, M. F. Fox and S. M. Pickering of De Montfort University, Leicester, UK, and from information supplied by advisory editors, Clifford Mansfield, Equillon LLC and Mervin Jones, University of Wales Swansea.

The Challenge of Ion Measurement
The measurement of lubricant acid or base number can be a daunting task. Technicians performing the standard titration tests such as ASTM D-664 and ASTM D-2896 are faced with the preparation of a number of different solvents and reagents, and the maintenance and calibration of sensitive potentiometric electrodes. Typically low repeatability and reproducibility statistics reflect these difficulties, and illustrate the frustrations that the user community faces.

In theory, determining base concentrations in an oil should be a straightforward procedure of titration with an acid, but this approach sets up the problem, because it assumes the well-behaved and predictable results of a simple water-based acid/base titration. This is often not the case. The electrodes themselves are designed to work best in an aqueous (water-based) fluid environment. When ions dissolved in water are near the electrode’s porous glass surface, optimum conditions exist for ion interaction at the interface allowing these electrodes to measure the electrical potential of the solution.

Because the typical mineral or synthetic based oil is the medium to be measured, you must first condition the sample to allow the potentiometric electrode to function. This is accomplished in such tests as ASTM D-664 by the use of a carrier solution that bridges the gap between the aqueous and non-aqueous worlds. A toluene-isopropanol solvent system has proven to be fairly effective at both solvating the non-aqueous oil and then introducing water to the solution. Isopropanol is both an organic solvent, and water miscible. The combination allows basic or acidic substances in the oil sample to be introduced to, and ionized in, the water present in the carrier solution. This situation sets up the conditions that allow measurement of solution potential by potentiometric electrodes to determine a base or acid number.

While traditional standard tests for measurement of acid and base content of oil are adequate in providing a mechanism for such measurement, practice has shown that the methods are challenging and difficult to perform consistently and repeatedly. Great care must be exercised in the handling and maintenance of the Saturated Calomel Electrode (SCE), which is used in the D-664 and D-2896 tests. Typically, the SCE’s must be rinsed between each titration, cleaned thoroughly at the end of each day, soaked in distilled water overnight, and refilled with electrolyte each morning. Control samples need to be run each day to verify that the electrodes are functioning properly. Even then, electrodes are known to progressively degrade with time. The life of the electrode is typically most affected by the degree of degradation of the oil sample being tested.

IP400 to the Rescue
The British Institute of Petroleum maintains standards on petroleum product testing, much like the American Society of Testing and Materials. In fact, many of the ASTM lubricant tests can be cross- referenced to analogous IP standards. For example, ASTM D-664 and D-2896 correspond to IP177 and IP276, respectively. Recently, an IP standard was developed that does not have an ASTM equivalent, and in it we may have a way to address some of the issues that make D-664 and D-2896 so challenging.

The IP400 standard uses exactly the same chemical systems as those specified for the IP177 (ASTM D-664) and IP276 (ASTM D-2896). The key difference is that the titration is measured using conductimetric electrodes in place of the troublesome potentiometric electrodes. When performed under IP400, the end points for both the hydrochloric acid and glacial acetic acid systems are clear and do not degrade in quality even with contaminated used oil samples. Figure 1 shows the typical titration curve obtained using the conductimetric electrodes of the IP400 standard. There is a clear equivalence point at the intersection of the two straight-lines. The marked difference in conductance changes, before and after equivalence, replaces the sometimes ambiguous curve inflections used to indicate endpoints in the more common potentiometric methods (Figure 2). The ambiguity is usually greatest with the most aged and contaminated samples, making the determination of end-point more difficult.

The electrodes used in the IP400 procedure are simple platinum squares, same as for standard aqueous conductivity measurements. The conductivity equilibration time after adding a titrant aliquot is very rapid, as quick as achieving thorough mixing, as observed by the disappearance of refractive index differences within the liquid. The electrodes do not require ‘conditioning’ and do not progressively degrade. Indeed, dry conductimetric electrodes, which have rested on the shelf for several months can be used almost
immediately for the titration of used lubricant samples.

Using conductimetric electrodes, both the IP177 (D664) and IP276 (D2896) titrants have been extensively and successfully tested with large numbers of determinations drawn from fresh, used and extensively degraded samples, from a wide range of original lubricant formulations and from gas and diesel engines, marine diesel engines, and concentrated additives. The results of the two method’s are well within the IP requirements for reproducibility and repeatability.

It’s All About the Electrode
Repeatability and reproducibility are measures of consistency for a given analytical test. Repeatability refers to duplicate results by the same operator. Reproducibility compares the results from two different laboratories evaluating the same sample in a round-robin. Standards suggest that results should be considered suspect if they differ by more than the stated percentage of the mean base number result, given in Table 1. The “back titration” figures are given for samples whose endpoint is sufficiently vague as to require a back titration with potassium hydroxide to determine the endpoint.

For IP400, the solvent and titrant chemistries of previous standards have been retained, but the difference is in the conductimetric detection method. Otherwise, the determination method and calculation is exactly the same. Over 5,000 lubricant samples have been analyzed at De Montfort University, in Leicester, England, using the IP400 method. Note that with conductimetric titration on used oil samples the repeatability and reproducibility is improved to roughly half of the potentiometric results. From this it becomes clear that the previous problems of determining a base number are primarily due to the choice of electrode system. Note that the back titration criteria are no longer required because of the definitive endpoint provided by the conductimetric method.

Benefits at a Glance

Ability to Work With Small Samples
Base number values for the De Montfort study set out in Table 1 were readily achievable on sample sizes down to 0.1 g. Whereas the IP177/276 systems require a minimum sample of 5 g, the IP400 system has provision for using samples of less than 0.2 g.

Extension into an Automatic Analytical System
The physical properties and performance of the conductimetric IP400 system is very suitable for development into an automatic analysis system. These titration methods have given a new impetus to automating base number determinations for high throughput commercial laboratories with greater precision and significantly smaller volumes of sample.

Two automated methods have been developed for base number determination of lubricating oil samples. The first is for a multi-sampler, which is completely automated after sample loading with a throughput of 10-15 samples/hr, and a sample mass of 0.2 g.

The second system uses manual introduction via an electronic top-pan balance, followed by an automated titration. This system was developed for intermittent sample throughput and uses an electronic balance for the sample mass in a beaker, followed by pre-calibrated pipette addition of the solvent and automated titration.

Extension into Simultaneous Base and Acid Number Determinations
Acid number determination was developed as a simple, sequential extension of the base number determination of the same sample in the same reaction vessel. The base number is determined first, followed by a back titration to determine acid number. This means that these values can be determined sequentially on the same small sample in the same reaction vessel.

Cost Advantages
Laboratories and their customers can receive the cost benefits of:

• increased overall accuracy, leading to a more informed decision process for the timed replacement of lubricant,
• reduced sample sizes, in turn requiring much less titration solvent to purchase and dispose of, and
• the ability to perform simultaneous acid/base number titrations for used engine oils.

Work at De Montfort University and independent laboratory work done in the U.S. identify the IP400 method as potentially superior to the previous standards for base number determination. In sum the reasons are:

• the clear and unequivocal break, or intersection, in the end point,
• the continued quality of the end point for new, used and severely used lubricant samples from both gasoline and diesel engines, with no need for a back titration,
• enhanced repeatability and reproducibility, compared to the other methods,
• reduced analytical time for each determination from the rapid equilibration for each added aliquot, compared to the previous methods,
• elimination of electrode contamination and replacement problem,
• small sample sizes, ~ 0.2 g, and
• suitability for automation.

These advantages warrant giving consideration to the IP400 standard for those technicians and managers tasked with providing base number analyses of used lubricating oil. The method may provide a simpler, more cost effective and more accurate alternative to the IP177/276 (ASTM D-664/D-2896) standards. And, with the ability to back-titrate engine oils to obtain their acid number as well, it is possible to get a 2-for-1 special; that kind of bargain would be hard to pass up.

References
M. F. Fox, Z. Pawlak and D. J. Picken, Tribology International, 24, 335 (1991).

Z. Pawlak, Chemia Anal., (PL), 25, 711 (1980): M F Fox, E Gierz and Z Pawlak, , Chemia Anal., (PL), 30, 841 (1985).

Bibliography IP400/96, IP Standards for Petroleum and Its Products, Institute of Petroleum. London, Pt. I, 1996.

Annual Book of ASTM Standards, Petroleum Products, Fuels, Solvents, Burner Fuels, Oils, Lubricating Oils, Cutting Oils, for the appropriate year.