Question: At which laboratory was Carsten Olsen employed as an assistant to the Danish chemist who introduced the concept of pH?
Context: Christen Thomsen Barfoed (June 16, 1815 – April 30, 1899) was a Danish chemist who devised a way to detect monosaccharide sugars in a solution, now known as the Barfoed's test.  Barfoed is also credited with having introduced systematic chemical analyses in Danish agricultural sciences. Søren Peder Lauritz Sørensen (9 January 1868 – 12 February 1939) was a Danish chemist, famous for the introduction of the concept of pH, a scale for measuring acidity and alkalinity.  He was born in Havrebjerg, Denmark. Carsten Erik Olsen (March 1, 1891 – August 19, 1974) was a Danish plant ecologist and plant physiologist, who pioneered the study of plant nutrition in soils of different pH.  He was born in Copenhagen and began studies of botany at the University of Copenhagen in 1910, at first with professor Eugenius Warming, then with professor Christen Raunkiær.  His doctoral dissertation (1921) was on the influence of soil pH on the natural distribution of plants.  He was then employed by the Carlsberg Laboratory as an assistant to the chemist S. P. L. Sørensen, later in his own lab.  There, he worked on plant uptake of ions, especially iron, nitrogen fixation and calcicolous plants. Holger F. Struer was a Danish chemist and founder of "H. Struers Chemiske Laboratorium" (In Danish: "Struers Kemiske Laboratorium") in 1875 at Skindergade 38, the centre of Copenhagen.  Struers introduced in 1943 Micropol, a new principle for electrolytic polishing which made the preparation process within metallography more controlled in order to achieve better preparation results. Haldor Frederik Axel Topsøe (29 April 1842 in Skælskør, Slagelse Municipality, Denmark – 31 December 1935 in Frederiksberg, Denmark) was a Danish chemist and crystallographer.  He is grandfather of the engineer Haldor Topsøe (1913–2013) who has got his name from his grandfather, and great-grandfather of the mathematician Flemming Topsøe (born 25 August 1938) and the engineer Henrik Topsøe (born 10 August 1944). Carl Andreas Koefoed (known in Russian as Андре́й Андре́евич Кофо́д , "Andrey Andreyevich Kofod"; 16 October 1855, Skanderborg, Denmark – 7 February 1948, Copenhagen) was a Danish agronomist active in the Russian Empire in the early 20th century.  He was the brother of Danish chemist Emil Koefoed. In thermochemistry, the Thomsen–Berthelot principle is a hypothesis in the history of chemistry which argued that all chemical changes are accompanied by the production of heat and that processes which occur will be ones in which the most heat is produced.  This principle was formulated in slightly different versions by the Danish chemist Julius Thomsen in 1854 and by the French chemist Marcellin Berthelot in 1864.  This early postulate in classical thermochemistry became the controversial foundation of a research program that would last three decades. Paul Delos Boyer (born July 31, 1918) is an American biochemist, analytical chemist, and a professor of chemistry at University of California Los Angeles (UCLA) .  He shared the 1997 Nobel Prize in Chemistry for research on the "enzymatic mechanism underlying the biosynthesis of adenosine triphosphate (ATP)" (ATP synthase) with John E. Walker, making Boyer the only Utah-born Nobel laureate; the remainder of the Prize in that year was awarded to Danish chemist Jens Christian Skou for his discovery of the Na+/K+-ATPase.  He is the oldest living Nobel laureate at age 99 . Johan Gustav Christoffer Thorsager Kjeldahl (] 16 August 1849 – 18 July 1900), 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. Bergnart (Bernhard) Carl Lewy was a Danish chemist; born in Copenhagen on 5 July 1817.  He died there on 1 January 1863.  He obtained the degree of graduate of pharmacy in 1835, and then studied chemistry for three years at the polytechnic school.  In 1839 he studied in Berlin (Ph.D.), and spent the winter of 1839-40 in Rome.  He then obtained a position as assistant in the private laboratory of J. B. Dumas in Paris.
Answer: Carlsberg Laboratory