(b) Preparation of calibration curve. Transfer 0.5, 1.0, 1.5,
2.1, 2.5, and 3.0 ml, of standard iron solution to 100 ml. volumetric
flasks and add 35 ml. of water.
Carry along a reagent blank. Add 10 ml.
of hydroxylamine hydrochloride solution (5 percent) and allow to stand
for 15 minutes.
Add 10 ml. of ortho-phenanthroline solution (0.2 percent)
and 15 ml. of sodium citrate solution (20 percent) and dilute to the mark.
Allow to stand for 1 hour and measure the transmittance at 510 millimicrons
with a spectrophotometer that has been set to 100 percent transmittance
with the reagent blank.
Plot mg. of iron against percent transmittance on