Field of the Invention
This invention relates to the testing for basic species present in oil, and more specifically to a method and apparatus for determining the total base number of a sample of oil.
Generally, additives such as anti-oxidants are often incorporated by the manufacturer into lubricating oils to extend the useful life of the oils. These additives are commonly alkaline, i.e., basic, chemical species (sometimes herein referred to as "basic species") which serve to alleviate the undesired effects of acidic species which may form under the operating conditions of the engine or other device lubricated by the oil. Over the course of extended use, the basic additives can be consumed or degraded, resulting in the build-up of acidic contaminants. Of course, lubricating oils which contain no or very little akaline additives are also subject to the build-up of acidic contaminants therein. The continued use of lubricating oil containing significant concentrations of such acidic contaminants can cause corrosive damage to the engine or other mechanism it is intended to lubricate. Accordingly, it is often desired to examine the condition of lubricating oil periodically to determine whether the oil is spent and needs replacement.
Gasoline, diesel oil and other liquid hydrocarbon fuels contain varying levels of sulfur or sulfur compounds, from negligible quantities to 5 percent by weight or higher. Accordingly, one of the products of combustion in the normal operation of, e.g., internal combustion engines powered by combusting such fuels, is sulfur dioxide which is usually exhausted from the cylinders in a gaseous state. However, when sulfur dioxide reacts with water vapor present in the combustion gases, it will form sulfurous acid which readily becomes oxidized to sulfuric acid, a corrosive by-product. Therefore, basic species additives such as calcium salicylate are often added to engine lubricating oils to combat the corrosive characteristics of sulfuric acid. The sulfur level in the fuel will usually determine the usable lifetime of the lubricating oil. That is, a high sulfur content of the fuel can destroy almost all of the base additives in relatively few hours of use. Consequently, tests for the total base number ("TBN") value of lubricating oils have been devised in order to assess the corrosiveness of the oil. Thus, in the case of oils which contain basic species additives, it is desired to measure the reserve alkalinity, i.e., to measure the quantity of the basic species additives in, or remaining in, the oil. For oils which are not formulated with or no longer contain basic species additives, it is desired to measure the quantity of corrosive acidic species contained in the oil to assess the amount of corrosives build-up. Consequently, tests for the total acid number ("TAN") value of lubricating oils have been devised in order to assess the corrosiveness of such oils. Usually, the TAN is of concern with respect to oils which are employed as lubricants in devices other than internal combustion engines. Accordingly, it is sound practice to periodically check the TBN of lubricating oils used in equipment, especially engines, such as the internal combustion engines of construction equipment, trucks, ships, etc., and to check the TAN of lubricating oils used in equipment other than internal combustion engines, such as compressors and gear boxes, to determine when the oil needs to be changed. The need for an oil change is indicated by a low TBN or a high TAN. The TBN is a number scale giving the quantity of acid, expressed in terms of mg of KOH equivalents per gram of oil sample; the TAN is a number scale giving the quantity of base, also expressed in terms of mg of KOH equivalents per gram of oil sample. In order to avoid damage to engines or other lubricated devices due to deterioration of the lubricating oil, it is highly preferable to be able to chemically determine the condition of the lubricating oil before degradation of the oil is visually evident. The alternative is the wasteful, and expensive, procedure of replacing lubricating oils at short intervals based on worst-case operation scenarios or placing the equipment at risk due to corrosion. The above discussion applies as well to oils used for purposes other than lubrication, so long as the corrosiveness of the oil is of concern.
Related Art
European Patent Publication 170,466, published Feb. 5, 1986, discloses a method and test kit for determining the alkalinity of oil. The method is colorimetric and is practiced by placing a fixed volume of an indicator solution comprising halogenated hydrocarbons in a titration container with a known amount of sample oil. The two are mixed together by shaking, and aqueous acid is added in fixed aliquots until a specific color change is brought about in the aqueous phase. The halogenated hydrocarbons in the indicator solutions make the density of the oil phase greater than that of water, so that the aqueous phase separates as the top layer.
U.S. Pat. No. 4,203,725 to Snowden, Jr. et al dated May 20, 1980, discloses a method and test kit for detecting the presence of contaminants in oil. This is accomplished by taking a sample of oil and adding a solvent-reagent. In the case of determining acidic or basic contaminants, the solvent is water (see column 8, lines 44-45) and the reagent is a pH indicator (see column 2, lines 48-56). The solvent-reagent solution is added to and mixed with the oil sample, and the mixture is allowed to separate. The color of the indicator in the lower, aqueous layer is observed and may be compared to a color scale to determine the contaminant content of the sample.
Several current Test Methods of the American Society for Testing and Materials ("ASTM"), Test Methods D2896-88, D974-87, D4739-87 and D664-89, disclose methods for the determination of TBN and TAN in petroleum products by forward and backward titrations with various acids. The D2896-88 procedure involves the use of a potentiometric titration system, the use of powerful and dangerous reagents including perchloric acid, glacial acetic acid, sodium perchlorate and chlorobenzene, and is designed to be carried out in a laboratory environment. The D664-89 procedure is a method for determining total acid number ("TAN") and the D4739-87 procedure is a TBN technique utilizing an electrode based titration. The D974-87 method uses less hazardous chemicals, but is a colorimetric test that cannot be used on dark colored oils which constitute the majority of used lubricating oils. More specifically,
ASTM D974-87 discloses a titration method for determining the TBN or TAN of a sample of oil. Part 11 of this procedure, relating to a determination of strong acid number, teaches the taking of a fixed volume of sample, adding water, mixing the water with the oil sample, removing the water, adding an indicator (methyl orange) and titrating the water with a basic solution to determine the acid number. This test is limited to use with petroleum products which are soluble or nearly soluble in mixtures of toluene and isopropyl alcohol and is not suitable for measuring the basic constituents of many basic additive-type lubricating oils (see section 1.1, and note 2). This reference also teaches the determination of acid and base numbers by titrating a single phase comprising the sample and a "titration solvent" comprising toluene, isopropyl alcohol and about 5% water. See sections 9, 7.7, and 10.
U.S. Pat. No. 3,510,260 to Krawetz et al dated May 5, 1970, discloses a method for determining acid content of an oil sample comprising taking a sample of the oil and adding a pre-determined amount of base dissolved in an immiscible solvent (see column 2, lines 25-31). An oil diluent is also added, and the three components are mixed and allowed to separate. An indicator solution is added, and the color of the separated, immiscible layer is observed. This Patent teaches that the process may be reversed to test for basic species (see column 2, lines 44-52) and that by providing an immiscible layer in which to observe the color of the indicator, the difficulty of observing the color of darkened oil is avoided (see column 2, lines 53 through column 3, line 5 and column 3, lines 29-39).
A portable TBN test offered by Mobil Corporation is presently commercially available and involves the use of glacial acetic acid and petroleum distillates. It is designed to cover a limited TBN range and requires a predetermined estimate of the expected TBN value of the oil in order for the colorimetric endpoint to work effectively. Determination of the endpoint is difficult when dirty or contaminated oils are used and the instructional procedures recommend that a flashlight be used to determine the colorimetric endpoint. It is not designed to be operated or handled by relatively untrained persons. For obvious reasons of time and economy, it is desirable to be able to have relatively untrained persons field test the TBN of any lubricating oil at its point of use.