Page 115 - Geochemistry of Oil Field Waters

COLORIMETRIC METHODS 103

taining 10 ml of 0.02N hydrochloric acid, and shake this mixture vigorously
for 2 minutes. Let the phases separate, and extract and discard the carbon
tetrachloride phase. Wash the acid phase twice with carbon tetrachloride,
extract, and discard the carbon tetrachloride. Transfer the acid phase to a
50-ml volumetric flask and make to volume with water.
Pipet 10-ml aliquots from the 50-ml volumetric flask to two 50-ml Erlen-
meyer flasks. To both flasks add 0.5 g of sodium ascorbate followed by, in
this order and with mixing, 1 ml of potassium cyanide solution, 5 ml of pH
9.0 buffer solution, and 3 ml of zincon solution. To one sample add 3 ml of
chloral hydrate solution, and to the other (which is the reference solution)
add 3 ml of water. Within 2-5 minutes after adding the last reagent, measure
the absorbance of the sample versus the reference solution at 620 mp in
1-cm cells with a spectrophotometer.

Calculations. Prepare a calibration using aliquots of the standard zinc solu-
tion containing 10-80 pg of zinc, and use the curve to calculate the amount
of zinc in the sample:
pg Zn (from curve)
ml sample = mg/l Zn+*

Cadmium

Cadmium can be extracted from aqueous solutions as cadmium dithizo-
nate into carbon tetrachloride or chloroform. Cadmium dithizonate is
extracted more readily into carbon tetrachloride than is zinc dithizonate, but
zinc dithizonate is extracted more readily into chloroform than the cadmium
compounds. Therefore, because many oilfield brines contain more zinc than
cadmium, the cadmium extraction should be made with carbon tetrachloride
to insure the best possible separation from zinc.
Although citrate and tartrate do not hinder the cadmium dithizonate
extraction, they do impede the extraction of lead and zinc. Cadmium
dithizonate can be extracted from an alkaline solution containing cyanide
and tartrate; the dithizonates of nickel, copper, silver, and tin are not extrac-
ted. Most of the interference from iron can be eliminated by oxidizing it
with peroxide and filtering.
Cadmium reacts with dithizone to form a compound of the type: