Determining Base Number (BN) in Oils and Lubricants

Oils and lubricants typically include essential additives to neutralize any acid formed during routine use. These additives are used with a view to increasing the period of use while simultaneously helping to prevent corrosion or machine breakdown.

Their central role means that it is important to know the amounts of basic additives present in oils and lubricants. This amount is referred to as the base number.

This article outlines an approach to the thermometric determination of the base number (BN) of oils and lubricants. In this example, BN is the amount of equivalent potassium hydroxide per gram sample. This is expressed as mg KOH/g.

The thermometric titration of BN facilitates fast, accurate analysis, and this can be automatized fully using the InMotion™ autosampler. Titration is performed using Triflic acid or perchloric acid in glacial acetic acid, and this is done to an endpoint that is detected catalytically. Here, the role of thermometric indicator is played by isobutyl vinyl.

Sample Preparation and Procedures

  1. Following METTLER-TOLEDO method M694, measure the Titer and the Blank.
  2. Use the recommended sample size. This will depend on the BN value of the sample, as per the table below:

Table 1. Source: Mettler Toledo - Titration

Base Number
[mg KOH/g]
Sample size
[g]
0.05 – 1 10 +/- 2.0
1 – 5 5.0 +/- 0.5
5 – 20 1.0 +/- 0.1
20 – 100 0.25 +/- 0.02
100 – 250 0.1 +/- 0.01

 

  1. Weigh the sample using a beaker before placing this on the sample changer.
  2. 30 mL of toluene will be added to the sample automatically.
  3. As the measurement is being performed, an instruction window will prompt the user to add 1 mL of isobutyl vinyl ether
  4. The Thermotrode is rinsed with toluene after each sample in order to clean it.

Chemistry

KOH + H+ → K+ + H2O

KOH: alkaline components

H+: acidic titrant

Determining Base Number (BN) in Oils and Lubricants

Image Credit: Mettler Toledo - Titration

Solutions

Titrant: Trifluoromethanesulfonic acid (triflic acid, CF3SO3H) = 0.1 mol/L in glacial acetic acid or perchloric acid (HClO4) = 0.1 mol/L in glacial acetic acid

Chemicals

Solvent: Toluene

Indicator: Isobutyl vinyl ether

Compound: BN as mg KOH/g, Potassium hydroxide, M = 56.11 g/mol, z = 1

Standard: Potassium hydrogen phthalate, M = 204.22 g/mol z=1 (see method M694)

Instruments and Accessories

  • Titration Excellence T5 (30252672) T7 (30252675) or T9 (30252676)
  • Titration beakers PP 100 mL (101974)
  • Rainin pipette 1 mL + pipette tips
  • Conductivity board (51109840) with FW version 1.5 or higher
  • Compact Stirrer (51109150)
  • Burette DV1010 10 mL (51107501)
  • Thermotrode (30322061)
  • XP205 Analytical balance (11106027)
  • InMotion™ Flex 100 mL (30094120)
  • SD660 membrane pump (included in InMotion™ Flex 100 mL standard delivery)
  • SP280 peristaltic pump (30094237)
  • Spatula/Syringe

Results

This approach was used to measure standard motor oil. Detailed results are included in the table below:

Table 2. Source: Mettler Toledo - Titration

  Sample mass
[g]
R2: Base Number
[mg KOH/g]
1 1.057 8.40
2 1.028 8.64
3 1.041 8.32
4 1.035 8.43
5 1.006 8.63
6 1.016 8.28
7 1.038 8.33
8 1.025 8.33
Mean
s
Srel
  8.42
0.035
1.731%

 

The results show good repeatability, particularly when considering that this titration required a relatively high dosing rate of 4 mL/min. While a slower dosing rate may have resulted in further improved repeatability, the temperature jump at the EQP would likely be less pronounced.

Remarks

Thermometric titration follows enthalpy change during a titration. It does this by measuring the slight difference in temperature triggered by chemical reactions.

When alkaline components have all been neutralized by triflic or perchloric acid, the acid excess will prompt an exothermic polymerization of isobutyl vinyl ether. The sudden temperature increase at this point will indicate the endpoint of the titration.

The titration EQP is determined through the use of a segmented evaluation (second derivative of the curve) and a constant dosing rate of 4 mL/min should be set in the Titration (Therm.) method function in order to ensure optimal results. A thermometric titration’s maximal volume is defined by the burette volume.

Suspending measurement, while titration is taking place may lead to invalid results. This is because the solvent temperature will equilibrate with the room temperature, therefore modifying the titration curve.

It should also be noted that this application method was specifically developed for the sample mentioned here. As such, the method may need to be optimized for use with other samples.

An InMotion™ autosampler was employed to automatically add solvent to the beaker throughout this application. Solvent can be added manually, however, and it is possible to perform the titration using a manual titration stand.

The Thermotrode in this application was connected to the conductivity plug on a conductivity board, while waste was disposed of as a non-halogenated organic solvent.

Waste Disposal

Dispose as non-halogenated organic solvent.

Further Information

https://www.mt.com/in/en/home/perm-lp/product-organizations/ana/ThermoTAN.html

Measured Values

Thermometric titration curve and second derivative of BN in motor oil, sample 2/8

Thermometric titration curve and second derivative of BN in motor oil, sample 2/8. Image Credit: Mettler Toledo - Titration

Table 3. Source: Mettler Toledo - Titration

Volume
[mL]
Meas. value
[°C]
Change in Temp.
[°C]
2nd deriv.
[°C/mL2]
Time
[s]
0.05375 24.1592     0.8
0.10775 24.1872 0.028   1.6
0.1615 24.2523 0.0651   2.4
0.222 24.3263 0.074   3.3
0.27575 24.4011 0.0748   4.1
0.3295 24.4798 0.0787   4.9
0.3835 24.5592 0.0794   5.7
0.43725 24.6424 0.0832 0.47 6.5
0.48425 24.7221 0.0797 0.44 7.2
0.53825 24.8123 0.0902 0.21 8
0.592 24.9004 0.0881 0.18 8.8
0.6525 24.9922 0.0918 0.11 9.7
1.77575 26.9275 0.0991 0.87 26.4
1.82975 27.0302 0.1027 1.83 27.3
1.89025 27.1429 0.1127 3.91 28.1
1.944 27.2577 0.1148 7.45 28.9
1.99775 27.3885 0.1308 12.39 29.7
2.04316 27.5268 0.1383 12.76 EQP
2.05175 27.553 0.0262 14.14 30.5
2.09875 27.7377 0.1847 9.92 31.2
2.1525 28.0493 0.3116 -1.37 32
2.20625 28.4131 0.3638   32.8
2.267 28.595 0.1819   33.7
2.32075 28.6086 0.0136   34.5
2.3745 28.5835 -0.0251   35.3
2.42825 28.5564 -0.0271   36.1

 

This information has been sourced, reviewed and adapted from materials provided by Mettler Toledo - Titration.

For more information on this source, please visit Mettler Toledo - Titration.

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