Areas of decreased density producing dips in the density-concentration curves
made with molybdenum blue may be found and are reproducible about 50% of
the time. Such dips take place at all points of the molybdenum blue spectra
between 400 and 800 mp; Under
the conditions of these experiments the dips appear above 0.14 mg. of phosphorus
(in 100.0 ml. of solution), but are not associated with specific changes in redox
potential. The dip phenomenon represents a possible source of error when the
molybdenum blue reaction is used for the determination of phosphorus with
chlorostannous acid as the reducing reagent.