Danish chemist who in 1909 introduced the concept of using the pH scale as a measure of the acidity of a solution. On Sørensen's scale, still used today, a pH of 7 is neutral; higher numbers represent alkalinity, and lower numbers acidity.
Red and white are the colours of Bohemia, dating back to the 13th century. Blue represents Moravia. Unlike that of the Slovak Republic, the Czech flag is not based on the pan-Slav colours. Effective date: 1 January 1993.