Procedure to Determine the Bromine Index (BI) Using Coulometric Titration

This article describes a process to establish the bromine index (BI) using coulometric titration. This bromine index is the fraction of reactive unsaturated compounds (mostly C=C double bonds) in hydrocarbons, which are associated with the petrochemical industry. These double bonds are broken down with the attachment addition of bromine:

    R - C = C - R + Br₂ → R - CBr - CBr – R

The bromine index (BI) refers to the number of mg bromine (Br₂) bound by 100 g sample. Normally, this method is relevant to olefin-free hydrocarbons with a bromine index lower than 1000.

Products with a bromine index greater than 1000 are usually determined through potentiometric titration as the bromine number. Conversely, if longer titration times are acceptable, then coulometric analysis can also be carried out.

To determine bromine, the element is directly generated in the titration cell by applying a current to the generator electrode. This current then releases the stoichiometrically corresponding amount of bromine from the bromide-containing reagent through electrolysis. To put it simply, the coulometric determination of the bromine index is an absolute method, eliminating the need to determine a titer.

Instruments and Accessories

• 2.851.0110 851 Titrando with generator electrode without diaphragm
• 2.851.0010 851 Titrando with generator electrode with diaphragm
• 2.852.0150 852 Titrando with generator electrode without diaphragm
• 2.852.0050 852 Titrando with generator electrode with diaphragm

Balance Resolution 0.1 mg

Reagents

Generator electrode with diaphragm

The following reagents are used for titrations using the generator electrode with diaphragm:

Working medium (Anolyte)

• 260 mL methanol
• 600 mL glacial acetic acid
• Filled up to 1000 mL with aqueous potassium bromide solution, c(KBr) = 1 mol/L

Katholyte

• Aqueous potassium chloride solution, c(KCl) = 0.2 mol/L

Generator electrode without diaphragm

Only one reagent is needed for titrations using the generator electrode without diaphragm:

Working medium

• 260 mL methanol
• 600 mL glacial acetic acid
• Filled up to 1000 mL with aqueous potassium bromide solution, c(KBr) = 1 mol/L

Remark

The reagents are normally exchanged after adding about 10 mL of sample.

If the reagent turns turbid, it means it is exhausted and therefore needs to be exchanged. This can be done by discarding the contents of the titration vessel and filling it with fresh reagent. The katholyte should also be changed if the generator electrode with diaphragm is used.

Samples

BI standards (10, 100 and 1000) in accordance with ASTM 2710 (Analytical Services, Inc.)

• Toluene
• Decene
• Octene

Sample sizes

The sample size should be adapted based on the expected bromine index. The following table shows:

Table 1. Recommended sample sizes depending on the expected bromine index.

Parameters

Normally, the following parameters have been employed:

Table 2. Parameters for BI determination.

¹There might be the need to adapt the generator current and the maximal titration rate (max. rate) to the sample which is analyzed.

Calculation

The following formula is used to calculate the result of the bromine index determination in mg/100 g:

EP = bromine, which has been produced to reach the endpoint [µg]
0.1 = calculation factor for mg/100 g
Sample size = weight of used sample [g]

Filling of Titration Vessel

It must be ensured that the equipment (indicator electrode, generator electrode, titration vessel, etc) are clean. In case the generator electrode with diaphragm is used, the titration vessel is filled with about 100 mL of the working medium. The generator electrode itself is filled with c(KCl) = 0.2 mol/L until potassium chloride solution and working medium have the same level.

Only one reagent is required if the generator electrode without diaphragm is used. The titration vessel is prepared by filling it with 100 mL of the working medium. Post conditioning, one should wait until a stable drift of about 10 µg/minute is reached. The titration vessel is at last ready for sample addition and the subsequent determination of the bromine index.

Examples of Application

Different substances with different bromine indexes have been examined. Shown below are examples of titration curves as well as results of these measurements.

A generator electrode with diaphragm and a generator electrode without diaphragm were used to measure al the samples.

Bromine index standards

Three certified bromine index standard solutions have been examined. The standards included cyclohexene and toluene and had BI values of 10, 100 and 1000, respectively.

Figure 1. Titration curve of cyclohexene standard solution (BI 1000) with generator electrode with diaphragm.

Figure 2. Titration curve of cyclohexene standard solution (BI 1000) with generator electrode without diaphragm.

Table 3. Cyclohexen Standard 10, 100 and 1000.

Generator electrode without diaphragm:

Table 4. Cyclohexen Standard 10, 100 and 1000.

Generator electrode without diaphragm:

Octene

Octane was diluted with toluene to determine its BI value. 543 was the calculated bromine index of the mixture.

Figure 3. Titration curve of octene in toluene (BI 543) with generator electrode with diaphragm.

Figure 4. Titration curve of octene in toluene (BI 543) with generator electrode without diaphragm.

Table 5. Results of the octene determination.

Decene

Decene was diluted with toluene to determine its BI value. 1247 was the calculated bromine index of the mixture.

Figure 5. Titration curve of decene in toluene (BI 1247) with generator electrode with diaphragm.

Figure 6. Titration curve of decene in toluene (BI 1247) with generator electrode without diaphragm.

Table 6. Results of the decene determination.

Toluene

Toluene was directly injected into the titration vessel.

Figure 7. Titration curve of toluene with generator electrode with diaphragm.

Figure 8. Titration curve of toluene with generator electrode without diaphragm.

Table 7. Results of the toluene determination.

Conclusion

Since a generator electrode without diaphragm is easier to handle, it is recommended for the determination of bromine index. Yet, the results demonstrate that both types of generator electrodes (with or without diaphragm) can be used to determine the bromine index.

Literature

ASTM D 1492 – 08, Standard Test Method for Bromine Index of Aromatic Hydrogencarbons by Coulometric Titration

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

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