Helpful tips to effectively measure nitrogen levels using Kjeldahl analysis.
Nitrogen is everywhere. It’s not just in the air we breathe but involved in countless natural and manmade processes. So it isn’t surprising that many industries require precise tests to determine the concentration of elemental nitrogen.
In the food industry, for example, nitrogen testing helps determine the quantity of protein in food for quality control and labeling purposes. ISO standards that specify Kjeldahl analysis include the testing of general food products, cereals and pulses, milk products, oilseeds and animal feeding stuffs.
Many wastewater treatment plants must also check nitrogen levels in water before discharge. Excess nitrogen in surface water can lead to eutrophication, the enrichment of surface water causing excessive growth of algae and plants. International standards, such as ISO 22447:2019 and EU directive 91/271/EEC, regulate nitrogen discharge.
Nitrogen gas (N2) makes up around 80% of the atmosphere, but when analyzing solid or liquid samples, nitrogen is often present as ammonia (NH3), ammonium (NH4+), nitrate (NO3-), nitrite (NO2-), and organic nitrogen. Which of these is measured depends on the specific application.
What is the Kjeldahl method?
Total Kjeldahl nitrogen is a commonly used and prescribed standard for nitrogen measurement in many industries. It measures the combined level of organic nitrogen, nitrogen in ammonia and ammonium. The limitations of this method is the measuring of the nitrogen in nitro and azo groups as well as the nitrogen present in heterocyclic aromatic organic compounds.
The analysis involves the following steps:
- Digesting a sample with concentrated sulfuric acid to convert organic nitrogen to ammonium.
- Adding sodium hydroxide after the sample has ‘cleared’ to liberate the nitrogen as ammonia gas (NH3).
- Leading the gas into a solution of boric acid, which converts the gas back to ammonium.
- Titration, for example with sulfuric acid, to determine the amount of ammonium—and therefore the total amount of organic nitrogen and ammonium—in the original sample.
As the method is standardized, much of it can be automated and analysis completed within a few minutes. This leaves sample collection and preparation as the most significant bottleneck and source of variability.
Collecting and preparing a sample
As with all analytical methods that rely on sampling, the validity of the measurement depends on whether the sample is representative of the bulk material, be it a foodstuff or a water sample. Collecting many, more representative samples helps maximize the reliability and reproducibility of nitrogen analysis.
Contamination, adsorption, and biological activity can all lead to changes in the measurable nitrogen in a sample. So, samples should be stored in a way that doesn’t affect their nitrogen content.
Take aqueous samples, for example. Adding a small amount of sulfuric acid directly after collection and storing at 4˚C improves sample preservation. When these samples contain solids, it’s necessary to homogenize them to maximize the reliability of measurement and digestion speed.
Transferring a sample using Kjeldahl weighing paper
Within the Kjeldahl sample preparation stage, the collection and transfer are an integral step. Any errors made during this step either due to sample error or contamination can impact the final accuracy of the results. Additionally, this can be a hazardous step due to the risks involved with the acid solution. Kjeldahl weighing paper is a fast and reliable method to safely transfer samples prior to Kjeldahl analysis.
Cytiva provides a wide range of filtration devices and filter papers for laboratory use, including low nitrogen Kjeldahl weighing papers. Whatman™ Grade B-2 Kjeldahl analysis weighing paper is a fast and reliable way of both weighing then transfer to the acid solution by folding the paper around the sample. Whatman™ Kjeldahl weighing boats are a prefolded option and is a fast and reliable way to safely transfer samples for Kjeldahl analysis
Due to its construction from low nitrogen parchment paper with no glue or chemical additives, Whatman Kjeldahl analysis papers dissolve residue-free in the digestion solution without influencing analytical results and can be a loss free alternative to the handing and transfer of samples.
Our team of experts are familiar with a wide range of filtration applications and can advise on which filtration devices are most suitable for you. Contact your local Cytiva laboratory filtration specialist to discuss your needs, use the methods and standards search tool to determine the best filter for your Kjeldahl application or try our filter selector to find your filter fast.