The definition of the acid component is an important parameter in assessing all types of waste oils. According to the type of oil, different analytical methods have been established. The defi nition procedure is always very similar. A sample of lubricant oil with a weight (2-20 g), which is dependent on the expected result, is intensively agitated with a solvent with a low water content. In so doing, the acids present in the oil are „washed“ into the water component of the solvent. These may then be detected in titrations. In this way potassium hydroxide (KOH), used as a strong base, is added to the sample a drop at a time until the oil becomes „neutral“. When all of the acids have been neutralised by the potassium hydroxide, the next added drop of base causes a sudden increase in the pH value. The acid content of the sample can then be calculated from the consumption of KOH until this „transition point“ and stated in mg of KOH/g of oil.

NN (Neutralisation number) Determination with a colour indicator 

DIN 51558, ASTM D974, DIN ISO 6618

The oldest technique for the determination of acids in oil is the NN (neutralisation number). In addition to the water-solvent mixture, a colour indicator is also added to the sample. This changes colour at the precise transition point. The NN is determined manually pursuant to the standard. OELCHECK has automated the procedure. A phototrode is incorporated into the titrator, which recognises the transition point better than the human eye. In general, the colour transition may only be observed with bright or transparent hydraulic, gear or turbine oils. For almost completely black samples from diesel or gas engines, no change in colour of the indicator can be observed.

AN (acid number) Potentiometric determination with an electrode

DIN EN 12634, ASTM D664

For dark oil samples, potassium hydroxide is added in small steps via a burette to the same solvent mixture in a beaker, albeit without an indicator, until an electrode which permanently records the pH value indicates the transition point. The result is indicated as the AN (acid number). The procedure can be applied to all oils and to many greases. pH electrodes filled with an electrolyte are very sensitive. These must be cleaned after every sample and then regenerated. Moreover, these always react with a slight delay to the change of the pH value in the sample. The base must therefore be added in particularly small steps and with pauses between every individual step. The oil determination for the same assertion therefore takes significantly longer for the AN than the NN.

AN (acid number) Determination with thermometry – ASTM D8045

With a third, still very new method, the advantages of the two previous procedures may be combined. For thermometry, a procedure presented by OELCHECK for the DIN and the ASTM as a draft standard, a special indicator is added to the solvent- sample mixture before the start of the titration. At the neutral point, however, this indicator does not change its colour but reacts with vigorous release of heat. An especially sensitive temperature sensor records this jump in temperature. The result is analogous to the change in colour and also to the electrode AN. The temperature sensor responds with similar speed to the change in colour. It does not require any elaborate monitoring or care of electrodes. The same jump in temperature occurs with all oils and is not limited to bright and transparent oils, as is the case for NN. For all three methods of acid determination, the same solvent mixture and the same potassium hydroxide are used in the same quantities. The same chemical reaction occurs for all of the procedures. The results and their interpretation are hence comparable. The difference between the procedures lies in the recognition of the transition point and hence of the endpoint of the reaction.

AN or NN Determination with a chemometric model

The acid content of oil can nevertheless be determined without the reaction with a base. Provided that suffi cient titrations are carried out for one type of oil, i.e. normally more than 5,000 conventionally executed titrations and detailed infrared spectra are taken at the same time, a so-called chemometric model can be correlated from this data. The widest variety of acids are created in the oil through the aging, oxidation and degradation of additives. All of these change the infrared spectrum of an oil sample. This change cannot be measured, however, as e.g. for the determination of oxidation, by a very specific band of the spectrum, but affects wide ranges. For a chemometric calculation of the AN or NN, all variable parts of the spectrum are determined in the first step. After this, the changes are calibrated against the acid numbers determined with the conventional titration. This model (statistical calculation formula) can then be used to determine the acid number from the infrared spectrum for the types of oil for which the model was developed.

The advantage lies in the considerably simpler execution on the basis of an IR spectrum that is in any case available. Several thousand reliably executed titrations and the associated infrared spectra are nevertheless a precondition for the preparation of the sustainable model. Since the infrared spectra are slightly different for all of the oils, even if they have the same application, a separate model must also be drawn up for each type of oil.

Which values for which oils

• AN (acid number) or NN (neutralisation number) for all oils and fluids.

The higher the neutralisation number in comparison to the new oil, the worse the oil is.

• BN (base number) for diesel, petrol, and natural gas engines.

The greater the fall in the base number in comparison to the new oil, the worse the engine oil is.

• i-pH value, BN (base number) and AN (acid number) for all gas engines.

The interplay of these three values implies impurities in the fuel gas.

• SAN (Strong Acid Number) for oils in special gas engines.

These show exceedingly aggressive acids, occurring for pH values of below 4. Immediate oil change!