Cohen studied chemistry under Svante Arrhenius in Stockholm, Henri Moissan at Paris, and Jacobus van't Hoff at Amsterdam.

In 1893 he became Van't Hoff's assistant and in 1902 he became professor of Physical Chemistry at the University of Utrecht, a position which he held until his retirement in 1939.

Cohen's areas of research included polymorphism of both elements and compounds, photographic chemistry, electrochemistry, pizeochemistry, and the history of science.

[1] Following the 29 April 1942 decree that Dutch Jews wear the yellow badge, he was arrested by Nazi police for non-compliance[3] and forced to resign.

[4] According to Margit Szöllösi-Janze, in her book, Science in the Third Reich, Cohen "put great efforts into restoring the relationships of Western European scientists with their German colleagues after the First World War.