A spectrophotometric method using phloroglucinol is described for the deter
mination of formic acid at ppm level. The method involves the reduction of
formic acid by hydrochloric acid and magnesium powder, The formaldehyde so
formed is treated with phloroglucinol to give an orange-red coloured dye wi
th lambda(max) at 475 nm. Beer's law is obeyed in the concentration range 2
-15 mu g of formic acid per 25 ml of the final solution. The molar absorpti
vity of the colour system is 6.1 x 10(4) dm(3) mol(-1) cm(-1). The method h
as been applied to the determination of formic acid in biological samples.