Total base number

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Total Base Number (TBN) is a measurement of basicity that is expressed in terms of the equivalent number of milligrams of potassium hydroxide per gram of oil sample (mg KOH/g) that is required to titrate and neutralise the acidic components in a sample. TBN is an important measurement in petroleum products, and the value varies depending on its application. TBN generally ranges from 6–80 mg KOH/g in modern lubricants, 7–10 mg KOH/g for general automotive use and 10–15 mg KOH/g for diesel operations. TBN is typically higher for marine grade lubricants, approximately 15-80mgKOH/g, as the higher TBN values are designed to increase the operating period under harsh operating conditions, before the lubricant requires replacement.[1] Basic additives are introduced into the oil formulation, which can react and neutralise acids in fresh and used oil formulations [2]. Therefore, it prevents damage of engine parts, which includes corrosion of metal surfaces and damage to gaskets and other seals in an engine, by acidic components in the oil. The constituents that may be considered to have basic characteristics include organic and inorganic bases, amino compounds, salts of weak acids (soaps), basic salts of polyacidic bases, and salts of heavy metals. [2]. Weak organic and strong inorganic acids are formed by oxidation of the lubricant or from the combustion of trace impurities such as sulphur. The TBN value of oil will gradually drop over time as an engine is operated, so routine testing for TBN can provide useful information about serviceability of the oil and an early warning of developing corrosion problems [3]. TBN can also be a particularly useful tool in assessing the combustion efficiency of an engine, if rapid depletion is observed [4] . Fuels containing a higher amount of impurities such as sulphur have a greater potential of forming sulphuric acid, which would decrease the TBN sooner [2].

Basic constituents in petroleum products, lubricants, biodiesel and blends of biodiesel can be determined with a range of methods. A common method is using an automated potentiometric titration. This test method is applicable to both fresh oils and used oils, where the only difference is the mixtures of solvents used to pre-dissolve the oil sample, and the types of titrant used to neutralise the acid.

Potentiometric Determination[edit]

Although, IP Standard test methods exist the more common methods for TBN are ASTM standardised, such as the potentiometric titration for fresh oils (Test method TBN ASTM D2896). A sample is typically dissolved in a pre-mixed solvent of chlorobenzene and acetic acid and titrated with standardised perchloric acid in glacial acetic acid for fresh oil samples. [2] The end point is detected using a glass electrode which is immersed in an aqueous solution containing the sample, and connected to a voltmeter/potentiometer. This causes an ion exchange in the outer solvated layer at the glass membrane, so a change in potential is generated which can be measured by the electrode. [5] When the end point of the chemical reaction is reached, which is shown by an inflection point on the titration curve using a specified detection system, the amount of titrant required is used to generate a result which is reported in milligrams of potassium hydroxide equivalent per gram of sample (mg of KOH/g).[2] Potentiometric titration for used oils (Test method TBN ASTM D4739): a sample is dissolved in a solvent mixture of Toluene/ Propan-2-ol /Chloroform with 0.5% deionised water and then titrated with standardised alcoholic hydrochloric acid. The detection system is equivalent to the fresh oil method. The used oil method uses a less polar solvent and weaker titrant, which will not dissolve the wear metals produced during operation, hence it is more suitable to analyse used oils. [6]

Photometric Determination[edit]

A colour-indicator titration, for example using test method ASTM D 974, can be carried out to indicate relative changes that occur in an oil sample during its use under oxidising conditions. A sample is dissolved in a solvent mixture of Toluene/ Propan-2-ol with 0.5% deionised water. A methyl orange indicator is added and the solution is titrated using alcoholic potassium hydroxide. The end point is indicated by a colour change from orange to green. [7]

Thermometric Determination[edit]

In thermometric titrations, a constant addition rate of titrant equates to a constant amount of heat being given out or consumed, and hence a more or less constant temperature change up to the endpoint. In a titration, the titrant reacts with the analyte in the sample either exothermically or endothermically. The thermoprobe measures the temperature of the titrating solution. When all of the analyte in the sample has reacted with the titrant, the temperature of the solution will change, and the endpoint of the titration is revealed by an inflection in the temperature curve. [8]

An appropriate aliquot of the sample is pipetted directly into the titration vessel, and isobutyl vinyl ether and toluene solvent are added. The solution is then titrated with Trifluoro methane sulfonic acid (TFMSA) to a single thermometric endpoint. [8]

Conductometric Determination[edit]

A conductometric titration method can also be used for the determination of the base number of petroleum products. A conductivity sensor is used to measure the conductivity of the analyte which allows the endpoint to be detected. It is suitable for both new and used products having base numbers from 1 mg to 40 mg KOH/g. A sample is dissolved in a solvent mixture of Toluene/ Propan-2-ol with 0.5% deionised water. A conductivity cell is placed in the titration vessel. The sample solution is titrated with alcoholic hydrochloric acid. [8]

Spectroscopic Determination[edit]

Mid-FTIR spectroscopy can be used to rapidly and quantitatively determine the TBN of hydrocarbon lubricating oils by spectroscopically measuring the carboxylate (COO-) functional group of the salt produced when trifluoroacetic acid (TFA) reacts with basic constituents present in an oil sample. [3]

See also[edit]

References[edit]

  1. ^ http://www.labinspection.com/total%20base%20number.html

[2]ASTM D2896-11, Standard Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration, ASTM International, West Conshohocken, PA, 2011, www.astm.org

[3] J. Dong, F. Van de Voort, V. Yaylayan and A. Ismail, Determination of total base number (TBN) in lubricating oils by mid-FTIR spectroscopy, Journal of the Society of Tribologists and Lubrication Engineers, 1999

[4] http://www.cleanoilservices.com/oil_analysis.html

[5] Metrohm, Basics of Potentiometry: http://www.metrohm.com/en/applications/%7BBEA83F6B-1BC5-474A-822D-9D6E71BFB388%7D

[6] ASTM D4739-11, Standard Test Method for Base Number Determination by Potentiometric Hydrochloric Acid Titration, ASTM International, West Conshohocken, PA, 2011, www.astm.org

[7] ASTM D974-14e1, Standard Test Method for Acid and Base Number by Color-Indicator Titration, ASTM International, West Conshohocken, PA, 2014, www.astm.org

[8] Application Bulletin AB-405_1_EN Determination of the total base number in petroleum products