Page 113 - Geochemistry of Oil Field Waters

COLORIMETRIC METHODS 101

additional extractions until the dithizone-chloroform phase remains green.
Dilute the combined red dithizone-chloroform phases in the 25-ml volu-
metric flask to volume with chloroform, mix well, and determine the absor-
bance with a spectrophotometer at 510 mp.

Calculations. Prepare a calibration curve by using aliquots of the standard
lead solution containing 10-80 pg of lead:
pg Pb (from curve)
ml sample = mg/l Pb+2

zinc

Extraction of zinc with dithizone from a weakly ammoniacal solution
containing a reducing agent and citrate prevents the precipitation of iron.
Extraction of zinc at a pH of 4.75 in the presence of sodium thiosulfate
largely eliminates interference from copper, mercury, lead, and cadmium.
The zinc dithizonate complex can be broken in 0.02N hydrochloric acid,
whereas cupric dithizonate cannot. Lead and cadmium dithizonates will dis-
sociate in 0.02N hydrochloric acid, but only traces of them should be
present after the preliminary extractions.
More accurate results are obtained by applying a zincon (Platte and
Marcy, 1959) method to the zinc which has been isolated by the dithizone
extractions than by making another dithizone extraction of the isolated zinc
and using it for absorption measurements. Therefore, the following method
is a combination of the dithizone and zincon methods. Traces of any re-
maining interferences can be complexed.
Zinc reacts with dithizone to form a compound similar to:
r

Zinc reacts with zincon:
OH
1

CsHS
to form a 1:l blue complex that absorbs at a wavelength of 620 mp.