Kjeldahl Distillation

A Kjeldahl distillation is a method which may follow a Kjeldahl digestion as part of a quantitative analysis. It is most commonly used to determine the nitrogen content in a material, for example in the analysis of proteins or rubber. The method works by releasing a gas from the Kjeldahl flask and the gas is recaptured in the other side of the setup. For nitrogen determination, this is carried out through the addition of base to the Kjeldahl flask from the dropping funnel which would neutralise the acid in the Kjeldahl flask, releasing any captured nitrogen as ammonia gas. The gas is driven through the distillation kit with strong heating and is recaptured in acid by forming an ammonium salt. Having a known amount of acid in the flask where the ammonia is recaptured enables the ammonia content to be determined through titration, as the ammonia will have neutralised some of the acid.

Setup

As gas will be liberated and trapped in this procedure, it is important that the glassware is set up to be air tight. In order to achieve this, all ground-glass joints should be greased and securely clipped together using Keck clips. Additionally, when a Kjeldahl distillation is running, the system is essentially closed, as the delivery tube is below the level of the liquid in the conical flask. As a consequence, gas is released from the distillation in bursts, causing the glassware to 'bump' during the distillation. This makes it even more important to ensure that everything is very securely clamped when assembling the glassware.

1. The Kjeldahl flask (containing the digestion) should be securely clamped around the neck of the flask on a tripod/gauze above a Bunsen burner.

2. A two-neck adapter is fitted into the top of the Kjeldahl flask with a dropping funnel fitted into the neck above the Kjeldahl flask and a Kjeldahl's trap fitted into the other neck.

3. The other end of Kjeldahl's trap is attached to the top of a condenser which is securely clamped to hold it in place.

4. A delivery tube is connected to the condenser.

5. A conical flask containing a known amount of solution to trap the gas is placed on a lab jack and positioned so that the delivery tube is below the level of the solution to ensure all the gas is captured.

Distillation

The specifics for carrying out the distillation will depend on the reaction being carried out, and the procedure below will need to be adapted for the the specific reaction. The procedure below assumes an acid-digest with base being added to liberate ammonia gas.

1. Ensure that the Kjeldahl flask has sufficient volume for the reaction and that the acid solution has been sufficiently diluted and mixed well so that when the base is added the acid and base solutions mix and don't form a phase separation.

2. With the tap closed, add some of the base solution to the dropping funnel. This may need to be topped up during the reaction, depending on the volumes and size of dropping funnel.

3. Check the receiving conical flask contains an accurately measured amount of acid to trap any liberated ammonia, and that the delivery tube is below the level of the liquid. As the distillation proceeds, the pressure inside the distillation will change, and this causes the liquid to be sucked up the deliver tube. The amount of water that can be sucked into the distillation is limited by the depth the delivery tube is placed below the surface of the solution. If sufficient solution is present, it is possible for liquid to be sucked back over into the Kjeldahl flask, which means the reaction is no longer quantifiable. As the distillation progresses, the volume of water in this flask will increase, so it is nescessary to lower the lab jack to keep the delivery tube within the solution, but to limit the volume that could be drawn up into the delivery tube and condenser.

4. Slowly start the addition of base into the Kjeldahl flask. Initially the acid will be neutralised and then with the excess base, become alkaline. The neutralisation process generates a lot of heat and the solution will warm up and begin to boil. It is worth leaving a small amount of base in the dropping funnel to form a seal and prevent accidentally leaving the dropping funnel tap open which would allow ammonia gas to escape, rather than being captured.

5. Once the addition of base is complete, the Kjeldahl flask is heated wiht a moderate Bunsen flame in order to ensure that all gas produced is driven across. Water will distill across which ensures that any gas is washed into the collection flask.

6. Once the distillation is deemed to be complete, lower the lab jack to remove the collection flask and turn off the Bunsen. Allow the apparatus to cool.

Quantification

The collected solution can be titrated to determine the remaining amount of acid in the flask and compared with the starting amount to determine how much gas was collected from the distillation. There are a number of options for carrying out the distillation:

• Titrating the whole amount

• Making up the solution to a known quantity in a volutmetric flask and titrating a portion.