The picric acid test for reducing sugar is a method for detecting the presence of certain kinds of sugar in a solution. Picric acid is added to potash in one-to-one proportions to produce picrate of potassium, which changes color in the presence of glucose and inverted cane sugar.
The picric acid test was first developed by a chemist named Braun. Doctor George Johnson, an English physician working in the 1880s, reintroduced the picric acid test and developed a method for determining the amount of sugar in a solution using it. Johnson's method involved boiling the solution after the sugar had been added, then diluting it until it reached a certain color. From the amount of liquid needed to dilute the solution, the amount of sugar could be calculated. Johnson also used picric acid to test for the presence of blood plasma in urine.
Picric acid testing is no longer used, perhaps because picric acid is extremely toxic and a particularly dangerous explosive.