How to answer the three most common Kjeldahl questions

By Bob Applequist, Senior Application Specialist

On April 22, 2014 in Articles

As the Product Manager for Labconco’s Kjeldahl products for 20 plus years, I have answered these three questions many times. Now I’m giving the answers to you so that you can be a Kjeldahl expert too!

Someone who is setting up their new Kjeldahl system will say…

Question: “What is a Water Ejector, and why do I need it for the Fume Removal System?”

Answer: The most common name for this device is Water Aspirator. But they have also been call Water Pumps, Water Ejectors, Vacuum Aspirators, and Filter Pump Lab Aspirators.

No matter the name, anyone who has performed a vacuum filtration with an Erlenmeyer Vacuum Flask, a funnel and filter paper in their high school chemistry class and, has used one. These plumbing fixtures are simply screwed onto a gooseneck faucet and are generally made of nickel-plated brass. Aspirators often have a serrated tail and a 3/8" NPT tapered sidearm for vacuum tube connections.

The function is simple; as water flows down through the vertical tube, which has a narrow hole inside, the restriction causes the water speed to increase. After the restriction, due to the Venturi effect, pressure decreases, creating a vacuum at the side arm location.

Water Aspirators are ideal for vacuum filtrations because the adjustable water flow creates adjustable vacuum pressure. They are also ideal for removing acid fumes that readily dissolve in water. The flow or vacuum flow control is used to promote the refluxing of acid fumes during a Kjeldahl digestion procedure.

Very large water aspirators require larger volumes of water to work well.

Question: “During high temperature Kjeldahl digestions, my sample flasks bump and fall off the heating block. I am even using glass boiling beads to help the boiling process, but nothing seems to work. The flasks just will not boil smoothly. Should I try a lower heat setting initially, or do I need to have the flasks at a specific angle on the heating elements?”

Answer: Round glass boiling beads do not work at high digestion temperatures. You should use a boiling stone or chip with an irregular surface. Even small pieces of broken lab glass will work better.

Initial lower digestion temperatures are generally only required for water or sludge samples. Samples with high water content often benefit from a 10-15 minute pre-boil step at lower temperatures.

With Macro and Micro Kjeldahl systems, the fume removal manifold requires the flasks to lean over at a 45-degree angle for connection. This angle helps increase the cooling surface area where the acid fumes can condense and drip back down to continue the digestion chemistry. Without this refluxing of the acid fumes, digestions can boil dry over the 45 minute to 1 hour digestion time.

Question: “I recently purchased the Labconco Rapid Digestor-4, and the fume removal system. I need a length of tubing to connect to a water ejector… what type of tubing do you recommend, and where can I find it?”

Answer: Any type of polyvinyl chloride (PVC) tubing with the right diameter will do. Yes, it will age, become brittle and need to be replaced, but so will any type of tubing when exposed to heat and acids. Silicone tubing can last longer, but it will cost more.

Please remember to keep the overall length of the tubing run as short as possible. Avoid sharp bends and dips in the tubing run from the fume removal manifold to the water ejector. Sharp bends will reduce the amount of vacuum, and dips in the tubing run will collect liquid sulfuric acid.