Today is the birthday of Johan Gustav Christoffer Thorsager Kjeldahl (August 16, 1849-July 18, 1900) He was a Danish chemist who developed a method for determining the amount of nitrogen in certain organic compounds using a laboratory technique which was named the Kjeldahl method after him.

Kjeldahl worked in Copenhagen at the Carlsberg Laboratory, associated with Carlsberg Brewery, where he was head of the Chemistry department from 1876 to 1900.

He was given the job to determine the amount of protein in the grain used in the malt industry. Less protein meant more beer. Kjeldahl found the answer was in developing a technique to determine nitrogen with accuracy but existing methods in analytical chemistry related to proteins and biochemistry at the time were far from accurate.

A painting by Otto Haslund of Johan Kjeldahl.

His discovery became known as the Kjeldahl Method

The method consists of heating a substance with sulphuric acid, which decomposes the organic substance by oxidation to liberate the reduced nitrogen as ammonium sulphate. In this step potassium sulphate is added to increase the boiling point of the medium (from 337 °C to 373 °C) . Chemical decomposition of the sample is complete when the initially very dark-coloured medium has become clear and colourless.

The solution is then distilled with a small quantity of sodium hydroxide, which converts the ammonium salt to ammonia. The amount of ammonia present, and thus the amount of nitrogen present in the sample, is determined by back titration. The end of the condenser is dipped into a solution of boric acid. The ammonia reacts with the acid and the remainder of the acid is then titrated with a sodium carbonate solution by way of a methyl orange pH indicator.

In practice, this analysis is largely automated; specific catalysts accelerate the decomposition. Originally, the catalyst of choice was mercuric oxide. However, while it was very effective, health concerns resulted in it being replaced by cupric sulfate. Cupric sulfate was not as efficient as mercuric oxide, and yielded lower protein results. It was soon supplemented with titanium dioxide, which is currently the approved catalyst in all of the methods of analysis for protein in the Official Methods and Recommended Practices of AOAC International.

And Velp Scientifica also has an explanation of his method, which is still in use today.

Kjeldahl (center) in his laboratory.