ASTM D2710 for Bromine Index of Petroleum Hydrocarbons by Electrometric Titration
6. Apparatus
6.1 Electrometric End Point Titration Apparatus - Any apparatus designed to perform titrations to pre-set end points (see Note 4) may be used in conjunction with a high-resistance polarizing current supply capable of maintaining approximately 0.8 V across two platinum electrodes and with a sensitivity such that a voltage change of approximately 50 mV at these electrodes is sufficient to indicate the end point. Other types of commercially available electric titrimeters, including certain pH meters, have also been found to be suitable.
NOTE 4 - Pre-set end point indicated with polarized electrodes provides a detection technique similar to the dead stop technique specified in previous versions of this test method.
6.2 Titration Vessel - A jacketed glass vessel of approximately 150-mL capacity of such a form that can be conveniently maintained at 0 to 5°C (32 to 41°F). A pair of platinum electrodes spaced not more than 5 mm apart shall be mounted to extend well below the liquid level. Stirring shall be by a mechanical or electromagnetic stirrer and shall be rapid, but not so vigorous as to draw air bubbles down to the electrodes.
6.3 Burets, 10 and 50-mL capacity.
6.4 Iodine Number Flasks, glass-stoppered, 500-mL capacity.
7. Reagents
7.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the American Chemical Society where such specifications are available. Other grades may be used, providing it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
7.2 Purity of Water - Unless otherwise indicated, references to water shall be understood to mean distilled water, or water of equivalent purity.
7.3 Preparation and Standardization:
7.3.1 Bromide-Bromate Standard Solution (0.05 N) - Dissolve 5.1 g of potassium bromide (KBr) and 1.4 g potassium bromate (KBrO3) in water and dilute to 1 L. Standardize to four significant figures as follows: Place 50 mL of glacial acetic acid (Warning - Poison. Combustible. May be fatal if swallowed. Causes severe burns. Harmful if inhaled) and 1 mL of concentrated hydrochloric acid (HCl, sp gr 1.19) (Warning - Poison. Corrosive. May be fatal if swallowed. Liquid and vapor cause severe burns. Harmful if inhaled.) in a 500-mL iodine number flask. Chill the solution in an ice bath for approximately 10 min, and with constant swirling of the flask, add from a 50-mL buret 40 to 45 mL of bromide-bromate solution, estimated to the nearest 0.01 mL, at a rate such that the addition takes between 90 and 120 s. Stopper the flask immediately, shake the contents, place it again in the ice bath, and add 5 mL of potassium iodide (KI) solution in the lip of the flask. After 5 min, remove the flask from the ice bath and allow the KI solution to flow into the flask by slowly removing the stopper. Shake vigorously, add 100 mL of water in such a manner as to rinse the stopper, lip, and walls of the flask, and titrate promptly with the standard sodium thiosulfate (Na2S2O3) solution. Near the end of the titration, add 1 mL of starch indicator solution and titrate slowly to the disappearance of the blue color. Calculate the normality of the bromide-bromate solution as follows:
N1 = A2N2/A1
N1 = normality of the bromide-bromate solution,
A1 = millilitres of the bromide-bromate solution,
N2 = normality of the Na2S2O3 solution, and
A2 = millilitres of the Na2S2O3 solution required for titration of the bromide-bromate solution.
7.3.2 Potassium Iodide Solution (150 g/L) - Dissolve 150 g of KI in water and dilute to 1 L.
7.3.3 Sodium Thiosulfate, Standard Solution (0.05 N) - Dissolve 12.5 g of sodium thiosulfate pentahydrate (Na2S2O3•5H2O) in water and add 0.01 g of sodium carbonate (Na2CO3) to stabilize the solution. Dilute to 1 L and mix thoroughly by shaking. Standardize by any accepted procedure that determines the normality with an error not greater than more or less 0.0002. Restandardize at intervals frequent enough to detect changes of 0.0005 in normality.
7.3.4 Starch Indicator Solution - Mix 5 g of soluble starch with approximately 3 to 5 mL of water. If desired, add about 0.65 g salicylic acid as preservative. Add the slurry to 500 mL of boiling water and continue boiling for 5 to 10 min. Allow to cool and decant the supernatant liquid into glass bottles and seal well. Starch solutions (some preserved with salicylic acid) are also commercially available and may be substituted.
7.3.5 Sulfuric Acid (1+5) - Carefully add 1 volume of concentrated sulfuric acid (H2SO4, sp gr 1.84) to 5 volumes of water and thoroughly mix. (Warning - Poison. Corrosive. Strong oxidizer. Contact with organic material may cause fire. May be fatal if swallowed.)
7.3.6 Titration Solvent - Prepare 1 L of titration solvent by mixing the following volumes of materials: 714 mL of glacial acetic acid, 134 mL of 1,1,1-trichloroethane or dichloromethane, 134 mL of methanol, and 18 mL of H2SO4 (1+5) (see 7.3.5).
7.4 Solvents:
7.4.1 Acetic Acid, glacial. (Warning - Poison. Combustible. May be fatal if swallowed. Causes severe burns. Harmful if inhaled.)
7.4.2 Methanol (Warning - Flammable. Vapor harmful. May be fatal if swallowed. Causes severe burns. Harmful if inhaled.)
7.4.3 1,1,1-Trichloroethane (Warning - Harmful if inhaled. High concentrations may cause unconsciousness or death. Contact may cause skin irritation and dermatitis.)
7.4.4 Dichloromethane (Warning - Harmful if inhaled. High concentrations may cause unconsciousness or death. Contact may cause skin irritation and dermatitis.)
NOTE 5 - The replacement of 1,1,1-trichloroethane, an ozone-depleting chemical, is necessary because its manufacture and import has been discontinued. Dichloromethane is temporarily being allowed as an alternative to 1,1,1-trichloroethane until a permanent replacement can be identified and adopted by ASTM.
NOTE 6 - The user of this test method may choose to use either 1,1,1-trichloroethane or dichloromethane to the exclusion of the other solvent. The selected solvent is to be used for all operations, including the preparation of the titration solvent and the dilution of samples, if applicable.
NOTE 7 - Commercially available reagents can be used in place of laboratory preparations.
