NN, AN and BN

An oil can withstand a great deal, but it should never become genuinely acidic!

If the oil becomes overloaded with acids, then a single value is already a basis for recommending an immediate oil change. Ultimately, this represents a high risk to the component to be lubricated and to the entire installation. For this reason, in our lubricant analyses, the potential acidification of the oil is meticulously investigated. In the laboratory report, according to the type of lubricant or measurement method, we provide values such as the NN (Neutralisation Number), the AN (Acid Number), or the SAN (Strong Acid Number), the i-pH value (initial or original pH-value) or the Basic Number. After this, you discover what the individual values reveal and how they are determined. Mineral oil-based or synthetic base oils are neither acid nor alkaline. With very few exceptions, they are neutral, i.e they have a pH-value which, on a scale of 0 (extremely acid) to 14 (extremely alkaline), is around 7 in most cases. The additives and active substances which are added to the basic oil nevertheless infl uence the pH value. Some compounds, such as wear and corrosion protection additives, may have a slightly acidic reaction and already cause a change in the pH value of new oil. During the practical deployment, the content of acid compounds in the oil continues to increase. On the one hand, the cause of this is the unavoidable oxidation of the basic oil itself. The oxygen which accumulates in the oil molecules makes the oil „acidic“. The longer the deployment of the oil, the higher the operating temperatures and the more impurities in the oil, the more the acid-forming oil oxidation increases. But the decomposition products of many additives also form metal salts in the desired reaction with metal surfaces, which reacidify the oil and further reduce its pH value.

A concentration of acids in the oil has disadvantageous effects. Initially, the oxidation is accelerated. By increasing the proportion of oxygen and the oxidising effect, the viscosity can increase significantly. In the extreme case, the oil which has become too thick is no longer conveyed in suffi cient quantities to the lubrication point. If free acids are present and the corrosion inhibitors are used up, this may lead to corrosion of all oil-covered surfaces. This particularly affects non-ferrous metals, such as copper and copper alloys, but also iron. The useful lives of plastics and sealants are also reduced by acidic oil. Whether and to what extent an oil has become acidic by comparison with its initial condition, and for how long it will still be fully functional can be determined during the analysis with various procedures.

Table of contents

  1. NN (neutralisation number) or AN (acid number)
    1. NN (Neutralisation number) Determination with a colour indicator 
    2. AN (acid number) Potentiometric determination with an electrode
    3. AN (acid number) Determination with thermometry – ASTM D8045
    4. AN or NN Determination with a chemometric model
  2. Engine oils BN (base number) and i-pH value
    1. BN (base number) Potentiometric determination with an electrode DIN 51639-1, DIN ISO 3771, ASTM D4739, ASTM D2896
    2. BN (base number) Determination with thermometry
    3. BN (base number) Determination with a chemometric model
    4. i-pH value (starting pH value), Determination with an electrode
  3. Determination of the AN or NN with comparable methods
  4. Determination of the BN (base number) with comparable methods

NN (neutralisation number) or AN (acid number)

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!

Determination of the AN or NN with comparable methods

Unit mg KOH/g

 

Used oil type,
different operating hours

AN, Potentiometrie

NZ, Photometrie

AN, NZ
Thermo-
metry

AN, NZ
Chemo-
metry

Mineral oil-based gas engine

1.78

1.60

1.71

1.69

Synthetic gas engine oil (PAO)

1.98

1.97

1.98

2.00

HC basis gas engine oil

2.96

2.88

2.92

2.78

HC basis gas engine oil

2.53

2.54

2.46

2.67

Mineral oil-based gas engine oil

2.75

2.78

2.61

2.87

Ester-based gas engine oil 

3.35

3.24

3.44

3.10

MO organic gear oil (PAO ester)

3.02

2.99

3.06

2.74

MO organic gear oil (PAO)

2.68

2.76

2.63

2.82

Transmission oil (PAO) with S-P 
additives

1.91

1.86

1.90

1.88

Gear oil (minimal) S-P additives 

1.39

1.36

1.39

1.42

Gear oil (minimal) S-P additives

0.98

1.00

1.03

1.08

HVLP type hydraulic oil (minimal)

0.16

0.17

0.15

0.17

HEES type bio-oil (full ester)

0.82

0.80

0.86

0.83

HL type hydraulic oil (minimal)

0.11

0.13

0.10

0.11

HVLPD type hydraulic oil (minimal)

0.53

0.66

0.62

0.58

HLPD type hydraulic oil (minimal)

1.47

1.27

1.38

1.33

Comparability pursuant to DIN/ASTM

14.1%

15%

 

 

Determination of the BN (base number) with comparable methods

Unit: mg KOH/g

Used oil type, 
different operating hours

BN, Potentiometrie

BN
Thermometry

BN
Chemometry

Synthetic gas engine oil (PAO)

8.94

9.11

9.03

Synthetic gas engine oil (PAO, ester)

3.01

3.01

3.20

Synthetic gas engine oil (PAO, ester)

1.53

1.51

1.53

Biogas engine oil (HC synthesis)

2.65

2.45

2.70

Diesel engine oil (partially synthetic)

7.46

7.18

7.44

Diesel engine oil (Low SAPPS)

7.17

6.99

7.01

Diesel engine oil (synthetic)

7.00

6.83

6.87

Petrol engine oil (synthetic) 

5.66

5.45

5.60

Comparability pursuant to DIN/ASTM

15%

   
Source:

ÖlChecker Summer 2011, page 6