Bromine Number Determination in Petrol via ASTM D1159

The term ‘bromine number’ refers to an analytical procedure that determines the degree of unsaturation, hydrocarbons or content of reactive C=C double bonds. These double bonds are found in a range of hydrocarbons commonly present in petroleum products.

This article investigates and showcases the fully automated determination of the bromine number in petroleum products, achieving this via titration. The process involves a bromide-bromate solution, the InMotion water bath rack and a thermostat.

Titration is carefully monitored using a platinum double pin electrode DM143 operating at a fixed polarization current (voltametric indication).

Sample Preparation and Procedures

Bromine Number Solvent Preparation:

A total of 714 mL of glacial acetic acid, 134 mL of 1,1,1- trichloroethane, 134 mL of methanol and 18 mL of H2SO4 (16% v/v) were mixed together in a 1 L volumetric flask.

Next, a blank value was determined. This was stored as blank value B in line with ASTM D1159 and was determined for each new batch of solvent mixture.

Titrant Preparation:

A total of 10.2 g KBr and 2.78 g of KBrO3 were dissolved with deionized water into a 1 L volumetric flask before this was filled up to the mark. The salts were initially dried at 105 °C (220 F) for a 30 minute period.

This titrant can also be purchased on a commercial basis, with the commercially available product suitable for use with no need for additional standardization.

The conditioning beaker was filled with Glacial acetic acid, while special beaker 1 was filled with ASTM D1159 solvent.

Starting the Method:

A total of 60 mL of solvent was added to the titration beaker and allowed to cool to 5 °C. Finally, the sample was added using a dosing unit to allow titration to take place.

Bromine Number Determination in Petrol via ASTM D1159-01

Image Credit: Mettler-Toledo - Titration

Chemistry

BrO3- + 5 Br- + 6 H+ → 3 Br2 + 3 H2O

R1-CH=CH-R2+Br2 → R1-CHBr-CHBr-R2

Solutions

  • Titrant: Bromide-bromate solution, Br-/BrO3-c(Br2) = 0.05 mol/L
  • Chemicals: 50 mL ASTM D1159-01 solvent (see Preparation and Procedures)
  • Sample: Petrol

Instruments and Accessories

  • Titration Excellence T7(30252675) / T9 (30252676)
  • InMotion Pro Base (30094122)
  • Water Bath Rack InMotion Pro (30094139)
  • Lauda Eco Silver Thermostat
  • 2 additional dosing units (51109030)
  • XPE205 Balance (30087653)
  • LabX Titration SW
  • PP Titration beakers, 100 mL (51109388)
  • Burette DV1010 10 mL (51107501)

Optional:

  • Heat exchanger for thermostating (23834)
  • DM40(51337000) / DM50(51337002)

Results

Table 1. Source: Mettler-Toledo - Titration

Sample R1: Temperature [°C] R2: Bromine Number [g/100 g]
1
2
3
4
5
6
7
8
9
10
5
5
5
5
5
5
5
5
5
5
21.04698
20.18857
20.33925
20.07898
20.40203
20.34039
20.85292
20.93853
20.59152
20.72622
Mean
s
srel


 
20.49539
0.29820
1.455%

 

Remarks

The Br2-number indicates the quantity of bromine-reactive constituents present in a sample. In this context and when used under standard test method conditions, it represents the amount (in grams) of Br2 which reacts with 100 g of sample.

It is important to note that these method parameters have been optimized for the sample discussed here, and it may be necessary to adapt the method to accommodate a different sample. It is also important to note that this application does not supersede the ASTM standard.

In this method, only the bromine reaction occurs because the temperature is maintained between 0 °C and 5 °C. Substitution and oxidation are significantly reduced via this method.

When working with a sample size for bromine number <50, direct titration is required. Here, an appropriate amount of sample should be injected into the titration beaker directly.

Table 2. Source: Mettler-Toledo - Titration

Estimated Bromine number Sample size [g]
1 – 5 6.0 – 1.2
5 – 10 1.0 – 0.6
10 – 20 0.5 – 0.3
20 – 50 0.25 – 0.1

 

Table 3. Source: Mettler-Toledo - Titration

Estimated Bromine number Sample size [g]
50 – 100 1.0 – 1.6
100 – 200 0.5 – 0.4
200 – 500 0.35 – 0.15

 

When working with a sample size for bromine number >50, dilution of 10 mL of 1,1,1-trichloroethane in a 50 mL flask is recommended. Under these circumstances, a test specimen is added according to the following steps.

  1. Obtaining sample mass (m’) by back-weighing
  2. Filling the flask to the mark with 1,1,1- trichloroethane before mixing this
  3. Injecting a 5 mL aliquot of the resulting solution into the beaker along with 60 mL of solvent
  4. Taking extra care to enter m’/10 as the sample mass

A dosing unit was used to fully automate sample addition, with sample weight determined by measuring the density was with a DM40 density meter beforehand. This confirmed that: d(Petrol) = 0.7002 g/mL.

InMotionPro with water bath rack and Cover Up lid handling

InMotionPro with water bath rack and Cover Up lid handling. Image Credit: Mettler-Toledo - Titration

Lauda Eco Silver thermostat

Lauda Eco Silver thermostat. Image Credit: Mettler-Toledo - Titration

Cooling Procedure

The cooling procedure saw 60 mL of solvent dispensed using a dosing unit before this was allowed to cool to 5 °C. Petrol was then added using a second dosing unit, with a further temperature measurement taken to ensure that the temperature returned to below 5 °C before titration.

Employing this procedure ensured that no sample was permitted to evaporate ahead of titration. Further lid handling can also limit sample evaporation while lowering the amount of sample and solvent vapor entering the immediate environment.

It is important to ensure that the sample is not cooled below 0.0 °C, due to this resulting in water condensation on the water bath rack, which could enter the InMotion autosampler’s electronic components.

The setup outlined here was tested at 20 °C and at a relative humidity of ~50%.

In settings where it is not possible to guarantee these conditions – particularly where there is higher humidity and higher temperatures – it is advisable to employ a glass cool finger connected to a thermostat. Two glass fingers can be used simultaneously to ensure yet faster cooling.

Manual titration stand with one glass cool finger

Manual titration stand with one glass cool finger. Image Credit: Mettler-Toledo - Titration

References

ASTM D1159-01, “Standard Test Method for Bromine Numbers of Petroleum Distillates and Commercial Aliphatic Olefins by Electrometric Titration”, www.astm.org

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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