This list is based on production of 200 mL of saturated LiOBr solution, molarity about 4.4.

• Liquid (cold) bromine Br
• Lithium hydroxide LiOH powder, anhydrous or hydrated
• Purified water
• Graduated 250 mL bottle with cap
• Analytical balance
• Bucket of ice
• Plastic 2-L beaker
• Magnetic stirrer
• Graduated 100-mL cylinder
• Laminar flow hood

1) Prepare a saturated solution of lithium hydroxide from solid powder. Note that LiOH powder absorbs and reacts with CO₂ from the air, so exposure should be minimized and followed by flushing with nitrogen.

The concentration of the solution is constrained by the solubility of LiOH, which is ~10.7 g/100mL for anhydrous LiOH. If the compound is hydrated, the molecular weight of the compound must be taken into account. For ~200 mL of solution, measure out (10.7 g/100 mL) 200 mL = 21.4 g of anhydrous LiOH. Since the formula weight (FW) of LiOH is 23.95 g/M, this represents 21.4 g / (23.95 g/M) = 0.894 M.

The FW for LiOH•H₂O is 41.97, but we still need 0.894 M. In this case measure out 21.4 g (41.97 g/M) / 23.95 g/M = 37.5 g.

Dissolve the compound in the graduated bottle with 200 mL pure water. This results in a solution of molarity 0.894 M / 200 mL = 4.47 M/L.

The solution is transparent and colorless.

2) Prepare the lithium hypobromite. The reagents must be kept cold to prevent evaporation of the bromine and to facilitate the exothermic reaction.

Chill the graduated cylinder for measuring the Br in the ice bucket. Pack the 250 mL bottle with the LiOH solution in the center of the plastic beaker so that the neck of the bottle protrudes from the ice, and the contents can be stirred. Stir the LiOH packed in ice for a few minutes to thoroughly chill the solution before proceeding.

Calculate the amount of bromine needed: From the previous calculation, we need 0.894 M of bromine since the reaction requires equal molar amounts of LiOH and Br. The atomic weight of Br is 79.9 g/M (do not multiply by 2 for Br₂) so we require 0.894 M (79.9 g/M) = 71.4 g of liquid bromine. The density of liquid Br is ~3.11 g/mL so we need 71.4 g/(3.11 g/mL) = 23.0 mL.

Note that an excess of bromine will poison the solution, so somewhat less than 23 mL should be used. Measure the bromine carefully into the chilled graduated cylinder, and then add it slowly to the LiOH solution, giving it time to react before adding more.

The solution is bright yellow and transparent. Excess bromine will cause the solution to turn rose colored.

Assembly

To each of the five 60-ml syringes, a polypropylene luer female barb connector and 5 cm of Tygon transmission tubing is assembled. During the procedure, pairs of these five syringe assemblies are connected together at different times using a polypropylene barbed union as shown in Fig. 1.

Fig. 1. Apparatus for manipulating reagent solutions and gas produced at approximately atmospheric pressure, consisting of two syringe assemblies connected by a union. A total of five 60-ml syringe assemblies are required at different times during the procedure. (See diagram section for details).

Operation
General description

The two initial reagents A and B (reagents are described above) are placed separately in two syringes, eliminating all air. The reagents are mixed by connecting the two syringes together with tubing and injecting one into the other. The resulting gas is transferred through connecting tubing to a third syringe, in which it is sparged with solution C, and finally to a fourth syringe, in which it is sparged with solution D. A fifth syringe is used as the storage receptacle for the finished product. Hereafter, these syringes are designated as syringes 1–5.

Step by step operation (consult the PowerPoint presentation)

Procedure 1.

To produce 40 ml of 98% ¹⁵N₂ enriched nitrogen gas:

• Load 0.9 ml of solution A, containing 1.8 mmol of ¹⁵N, into a 1-ml syringe and inject it into syringe 1 with the barbed connector momentarily removed to allow access to the tip. The headspace and tubing air is removed, and a tubing clamp is placed at the liquid–air boundary as close to the open end of the tubing as possible, but leaving room for the barbed union.
• Syringe 2 is loaded with 20ml of solution B, again eliminating air and clamping.
• Syringe assemblies 1 and 2 are then connected with the barbed union. Both clamps are removed, and the solution in syringe 2 is pumped into syringe 1, resulting in gas immediately forming in syringe 1 and displacing the plunger. Care is taken not to allow the gas to push the plunger out of syringe 1, so that any excess gas will transfer to syringe 2.
• The syringes are shaken to ensure complete reaction of the reagents. When the reaction stops, all gas is collected in syringe 1 and all liquid is collected in syringe 2.
• The tubing of syringe 1 is clamped at the liquid–gas boundary to avoid losing gas or introducing air.
• The liquid in syringe 2 is discarded.
• Then, 20 ml of solution C is drawn up into syringe 3 and air eliminated.
• The gas is transferred to syringe 3 from syringe 1 by connecting them with the barbed union, as described above, and removing and reapplying clamps as necessary. Syringe 3 is shaken vigorously to eliminate oxides from the gas.
• Afterwards, 20 ml of solution D is loaded into syringe 4, the gas is transferred from syringe 3 to syringe 4, and syringe 4 is shaken vigorously.
• With care being taken to eliminate all liquid, the gas is transferred to syringe 5, which is used for storing and dispensing the gas.

Procedure 2.

This procedure is used to extract small volumes of gas from the “reservoir syringe 5” into a small syringe at exactly atmospheric pressure for injection into sample bottles used for a nitrogen fixation experiment.

Important: To maximize the accuracy of injection volumes, the smallest syringe that will accommodate the desired sample size should be chosen.

• The tubing of syringe 5 is punctured with the small syringe close to the tubing clamp (see Fig. 2).
• Gas is forced into the small syringe by retracting its plunger while depressing that of syringe 5. The friction between the plungers and barrels of the syringes is used to ensure pressure above atmospheric in the system.
• To ensure that the pressure remains above atmospheric, transfer more than the desired volume of gas to the small syringe, and then force the plunger of syringe 5 back with the small syringe, using the internal pressure of the system to do so, until the small syringe has the desired volume.

Important: It is essential that the pressure in the system be above atmospheric when the small syringe is withdrawn to prevent contamination of the gas with air.

• A second tubing clamp is placed on the tubing as shown in Fig.2 to isolate the puncture point and thus not lose gas from syringe 5.

Important: Make sure to isolate the puncture point as in Fig. 2 to prevent loss of gas from the reservoir syringe.

Fig. 2. Isolation of puncture point to prevent gas loss from reservoir syringe. (See diagram section for details)

• The small syringe is withdrawn from the tubing, allowing some gas to escape and equalizing the pressure to atmospheric.
• Then the gas in the small syringe is injected immediately into a sample bottle of the subject experiment or, if desired, the small syringe can be imbedded in a rubber stopper for up to several hours to prevent dilution of the gas with air.

Important: Subsequent samples are taken from the same puncture hole.