Bromine Number of Petroleum Products According to ASTM D1159-01

This application note (download below) provides the detailed method and procedure tips for automated bromine number determination of petroleum distillates and aliphatic olefins based on the standard method ASTM D1159. The temperature of the sample is kept under 5 °C during the titration to minimize the side-reactions (oxidation and substitution) that can occur with bromine present. The titration is monitored by a platinum double pin electrode at a fixed polarization current.

Background

Bromine number, or bromine index, is a parameter used to estimate the amount of unsaturated aliphatic groups (olefins) in a hydrocarbon. The double C=C bonds can crosslink or react with oxygen to form epoxides, which are highly reactive. The crosslinking causes the hydrocarbon to form long chain molecules (polymers) which are no longer liquid but solid and are termed gums or varnishes. These form deposits in a fuel system or in a carburetor and can clog up both filters and the narrow tubing that supplies fuel to the internal combustion engine. The reduced fuel flow results in a decrease of engine power and might even prevent the engine from starting.

Bromine number is expressed as grams of bromine reacted with 100 g of a sample and it can be measured by titration of a sample with a bromide-bromate solution as titrant. During the titration, bromine is produced in situ by a redox reaction before undergoing an addition reaction with unsaturated hydrocarbons.

Reaction:

BrO₃^- + 5 Br^- + 6 H⁺ → 3 Br₂ + 3 H₂O

R₁-CH=CH-R₂ + Br₂ → R₁-CHBr-CHBr-R₂

For more applications related to petroleum product analysis see the acid number measurement according to ASTM D664 and determination of hydrogen sulfide and mercaptans according to UOP 163-10.

Figure: Experimental setup for automated bromine number determination