🧪 Olefin content in fuels
Determination of unsaturated hydrocarbon (olefin) content in fuels based on bromine number by electrometric titration method. Measure of fuel reactivity and stability.
Overview
Olefins (alkenes) are unsaturated hydrocarbons containing C=C double bonds. In motor fuels they originate mainly from fluid catalytic cracking (FCC) and thermal cracking processes. Olefin presence in gasoline favorably affects octane number (increases RON), but negatively affects oxidative stability — olefins easily polymerize and form gums, causing filter clogging, intake valve deposits and injector nozzle fouling.
Fuel Quality Directive 2009/30/EC limits olefin content in gasoline to maximum 18% v/v. Olefin content is determined by indirect methods: through bromine number (ASTM D1159) or FIA analysis — Fluorescent Indicator Adsorption (ASTM D1319). Bromine number expresses mass of bromine (in grams) reacting with 100 g sample and is a measure of unsaturation degree. FIA method separates hydrocarbons into saturated, olefinic and aromatic fractions by adsorption on silica gel with fluorescent indicators.
ASTM D1159 is preferred method due to higher accuracy, simpler procedure and lower susceptibility to systematic errors compared to FIA. Method is applicable to gasolines (including unleaded, oxygenated), naphtha, diesel oil and petroleum distillates with boiling point up to ~315°C.
Method principle
Weighed fuel sample is dissolved in solvent mixture (1,1,1-trichloroethane or dichloromethane + acetic acid). Standard bromine-bromide solution in acetic acid is added dropwise to solution. Bromine reacts with C=C double bonds of olefins in electrophilic addition reaction (Br₂ + R-CH=CH-R → R-CHBr-CHBr-R). Titration endpoint is detected electrometrically — using two platinum electrodes with applied potential. When all bromine reacts with olefins, excess free bromine causes current jump between electrodes (titration curve inflection point). Bromine number = (g Br₂ / 100 g sample). Conversion to % olefins requires knowledge of average molecular weight of olefins in sample (Annex A2 of standard).
Applications
- Quality control of motor gasolines (olefin limit ≤18% v/v)
- Assessment of fuel oxidative stability
- Control of fluid catalytic cracking (FCC) processes in refineries
- Testing of petroleum distillates — naphtha, diesel oil
- Analysis of commercial aliphatic olefins (chemical feedstocks)
- Assessment of gum formation tendency in fuels (correlation with ASTM D525)
Key parameters
| Parameter | Value |
|---|---|
| Bromine number range | 0.1–100+ g Br₂ / 100 g sample |
| Olefin limit in gasoline (EU) | ≤18% v/v (Directive 2009/30/EC) |
| Repeatability | ±(0.09 × √X) where X = bromine number |
| Endpoint detection | Electrometric (2 Pt electrodes, potential 150–200 mV) |
| Solvent | Dichloromethane + glacial acetic acid (1:1) |
Standard
- Standard number
- ASTM D1159-07(2017)
- Title (PL)
- Liczba bromowa destylatów naftowych i handlowych olefin alifatycznych metodą miareczkowania elektrometrycznego
- Title (EN)
- Standard Test Method for Bromine Numbers of Petroleum Distillates and Commercial Aliphatic Olefins by Electrometric Titration
Step-by-step procedure
1. Bromine solution standardization
Measure 25 mL bromine-bromide solution, add excess KI, titrate liberated iodine with sodium thiosulfate 0.1 mol/L (indicator: starch). Calculate exact Br₂ concentration.
2. Sample weighing
Weigh 2–20 g fuel sample (mass depends on expected bromine number — higher BN, smaller weight). Accuracy ±0.1 mg.
3. Sample dissolution
Transfer sample to beaker, add 50 mL dichloromethane/acetic acid mixture (1:1 v/v). Mix on magnetic stirrer.
4. Electrode preparation
Immerse double Pt electrode in sample solution. Connect to burette/titrator. Set polarization potential 150–200 mV DC.
5. Titration
Add bromine-bromide solution dropwise. Monitor current between electrodes. Bromine reacts with olefins — current near zero. After olefin saturation excess bromine causes current jump.
6. Endpoint
Record endpoint — current jump >50 µA sustained for 30 s. Record volume of bromine solution consumed for titration (V [mL]).
7. Bromine number calculation
BN = (V × C_Br₂ × 159.8) / (10 × m), where V = titration volume [mL], C = Br₂ concentration [mol/L], 159.8 = M(Br₂), m = sample mass [g]. Result in g Br₂ / 100 g.
8. Conversion to % olefins
According to Annex A2 ASTM D1159: % olefins ≈ BN × (M_olefin / 159.8) × 100/ρ. For gasolines typically M_olefin ~84 g/mol (hexene) is used.
9. Blank test and QC
Perform blank test (solvent only) — bromine solution consumption ≤0.05 mL. Analyze control sample every 10 samples.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Automatic burette with electrometric detection | Metrohm 905 Titrando, SI Analytics TitroLine 7000 | 25,000–60,000 PLN |
| Platinum electrodes (pair) | Metrohm 6.0338.100, double Pt electrode for bromometric titration | 2,000–5,000 PLN |
| Titration beaker 250 mL | Glass, with magnetic stirrer | 50–100 PLN |
| Magnetic stirrer | IKA RCT basic, Heidolph MR Hei-Standard | 1,500–4,000 PLN |
| Analytical balance (0.1 mg) | Mettler Toledo ME204, Radwag AS 220.R2 | 5,000–15,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Bromine-bromide solution in acetic acid | 7726-95-6 | KBr + Br₂ in glacial acetic acid, concentration ~0.25 mol/L Br₂, iodometric titration of concentration |
| Glacial acetic acid | 64-19-7 | Purity ≥99.7%, solvent and reaction medium, anhydrous |
| Dichloromethane | 75-09-2 | Analytical grade, sample solvent, co-solvent with acetic acid |
| Potassium bromide | 7758-02-3 | For preparation of titrant solution (bromine-bromide) |
| Sodium thiosulfate 0.1 mol/L | 7772-98-7 | For standardization of bromine solution (back titration with KI) |
| Potassium iodide | 7681-11-0 | For back titration — determination of bromine solution concentration |
Health and safety (OHS)
- Bromine — highly corrosive and toxic (H314, H330), work only under fume hood, bromine-resistant gloves
- Dichloromethane — suspected carcinogen (H351), narcotic, work in well-ventilated room
- Glacial acetic acid — corrosive (H314), skin burns, goggles and gloves mandatory
- Fuels — flammable, no open flame nearby
- Bromine waste — collect separately, dispose as hazardous waste