and escape into the engine cavity in blow-by gas where they are neutralized by additives in the oil or proceed to attack the thin oil films providing lubrication for piston rings and cylinder liners. The sulphate index from infrared analysis is a measurement of the amount of sulphur based acids that have reacted with the oil and reflect the amount of sulphation that has occurred.


If fuel sulphur levels


remain constant, the sulphate index would be expected to increase continuously with use until the oil reaches the end of its useful service life, for which the sulphation level or sulphate index can be an important determinant. At normal operating temperatures, acids remain in a gaseous state in the blow by gas with minimal contact with reactive surfaces. However, when an engine experiences lower than normal operating temperatures (such as just after start up, at shutdown or when a faulty cooling system results in continuous overcooling) the acids condense and come in contact with the oil in the sump causing the oil to film on exposed metal surfaces. This places an extra burden on the lubricant because it must neutralize more acid than would be expected during normal operation. Thus, high sulphation early in the oil’s life often indicates abnormally low operating temperatures.


The measurement of the Acid Number (AN) involves a titration where the total acid content of the oil dissolved in a mixed solvent is completely neutralized by the gradual addition of an alcoholic solution of potassium hydroxide (KOH). A colorimetric method of determining the end point is effected by the use of a chemical indicator that changes colour as soon as the acid is completely neutralized. Alternatively, a potentiometric method may also be used. The AN test is performed on non engine oil samples and is used to quantify the acid build up in the oils. An increased AN is a result of oxidation of the oil, perhaps caused by overheating, overextended oil service or water


or air contamination. Components within refrigeration systems are particularly susceptible to acid attack. This can occur when air containing water vapour enters the system, or alternatively when the system is subjected to excessive heat and the refrigerant drier releases retained water. When this happens, acids created by the reaction of the air, water, refrigerant and oil cause iron components in the system to become plated with copper which can cause bearing failure due to copper plating.


In refrigerant systems, the


acid content of the oil, moisture content and copper level need to be regularly monitored to indicate incipient problems. AN limits vary enormously and depend both on OEM specifications and on the oil itself.


In some cases,


an acid number exceeding 0.05 is unacceptable while, in others, ANs of 4.00 and higher remain acceptable. As with all other readings, trend analysis is the best indication of the health of both the oil and the machine.


The measurement of the Base Number (BN) involves a complex potentiometric titration where the total alkaline reserve of one gram of oil dissolved in a mixed solvent is reacted with the gradual addition of a known excess of an acid solution. The reaction is monitored using a reference and a measuring electrode and a graph of voltage (mV) against acid added (ml) is then plotted. The end point is detected from a point of inflection on the graph or, in the case of badly degraded oils, from a predetermined millivolt reading. This test applies only to engine oil samples because such lubricants are deliberately formulated to contain a reserve alkalinity that enables them to neutralize the corrosive acidic by products of the combustion process. The BN of an oil is a direct measurement of its alkaline reserve. Every engine oil has an initial BN that gradually reduces during use. Typical starting values for compression ignition oil engine oils are between 8 and 12. A general rule of thumb


is to discard the oil when the BN drops below half its initial value. While it may seem logical to assume that oils formulated to have a high BN would be most desirable, that is not always the case because some engines may suffer burnt valves if such an oil is used. This results from the high ash content of the oil and high valve temperatures causing fusion of the valve seats. Using a lubricant specifically formulated for diesel combustion in a petrol engine could also prove detrimental, highlighting the importance of adhering to equipment manufacturers’ lubricant specifications. BN measurements are performed only on samples from infrared results flagged for analysis. A BN can be predicted through the infrared data and, where this prediction is below the specified limit, a BN test is requested to confirm the degree of degradation evident in the infrared data. All samples having a predicted BN exceeding a safe limit are reported as having a BN of +6 while the actual result is reported for samples selected for the test.


The units of BN and AN can be somewhat confusing. Although they are different tests, the results are both expressed in the same units: milligrams of potassium hydroxide per gram of oil, represented as mg KOH/g. The AN of an oil is defined as the number of milligrams of KOH needed to neutralize the acid constituents in one gram of the oil. The BN of an oil is the number of milligrams of KOH needed to neutralize the acid needed to neutralize the basic constituents in one gram of the oil.


KOH The Report • June 2017 • Issue 80 | 39


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