Question

Measurement of nitrogen in water in the form of nitrite NO₂^- can be achieved by diazo dye formation followed by measurement of absorbance at 543 nm. Measurement of nitrate nitrogen (NO₃^-)is achieved by reducing the nitrate to nitrite then determining total nitrite nitrogen, nitrate nitrogen is found by difference. The following calibration data were used in nitrate-and nitrite nitrogen determinations:

mg nitrogen. L-1 as nitrite	absorbance at 543nm
0.000	0.000
0.050	0.055
0.100	0.111
0.200	0.218
0.500	0.556

A undiluted water sample analysed for nitrite and nitrate nitrogen gave an absorbance of. 0.078 without reduction, and an absorbance of 0.195 with reduction. Calculate the concentration of nitrogen present as NITRATE in mg.nitrogen L^-1.

Answer 1