Determining the acid component is an important parameter in assessing all types of waste oils. Although different analytical methods have been established according to the type of oil involved, the determination procedure is always largely the same. A lubricant oil sample weighing between 2 and 20 g (according to the expected result) is intensively agitated with a solvent mixture containing a very small quantity of water. Acid components from the oil are transferred into the water during this process, which is necessary because the acids cannot be detected directly in the oil itself; it has to be detected in this „aqueous phase“ by means of titration. Used as a strong base, potassium hydroxide (KOH) is added to the sample one drop at a time until the oil becomes „neutral“. When all of the acids have been neutralised by the potassium hydroxide, the next drop of base added causes a sudden increase in the pH value. The acid content of the sample can then be calculated from the KOH consumed up to this „transition point“ and stated in mgKOH/g of oil. NN (Neutralisation Number) and AN (Acid Number) are determined using the same principle. When determining the NN, a colour indicator is added to the water-solvent mixture which changes colour exactly at the transition point - a change that the laboratory technician is trained to recognise. The AN, which requires more resources to determine, is usually obtained only if a sample is too dark for the transition point to be observed. The sample is agitated with the same solvent used to determine the NN, only minus the indicator. The titrant (KOH) is then added – usually automatically – in small steps with a burette until an electrode that continuously registers the pH value indicates the transition point. The KOH used up to this point is indicated as the AN in mgKOH/g. Not every titration curve runs in a way that allows an unambiguous transition point to be defined, however. To ensure that the acid component in the oil can still be determined in spite of this, the AN is obtained by titrating the titration up to a potential that has previously been measured in an alkaline buffer. For many years, under the „old“ AN determination procedure (ASTM D664) this titration was carried out to a pH value of 11 (-240 mV). Under the ASTM D664 standard that applies today, however, titration is only carried out up to a „buffer“, or a pH value of 10 (-180 mV). This change in the standard means that KOH consumption is lower, with pH 10 having to be achieved instead of the previous 11. It also means that the AN value provided in OELCHECK lab reports has been systematically lower since the entry into force of the new AN standard than it was when pH 11 was the buffer (as trend analyses in particular reveal).
This is because the formula for obtaining the AN is as follows: