Page 107 - Geochemistry of Oil Field Waters

COLORIMETRIC METHODS 95

O-phenanthroline or 2,2‘-bipyridine (either reagent can be used, how ever,
2,2’-bipyridine is subject to less interferences): dissolve 0.5 g of either re-
agent in 100 ml of distilled water. The solution can be warmed to 60°C to
effect more rapid dissolution.
Sulfuric acid, approximately 9N (441.36 g per liter): cautiously pour 270
ml of pure concentrated sulfuric acid into 650 ml of distilled water. Care-
fully mix the solution, cool, and dilute to 1 liter with distilled water.
Spectrophotometer capable of measurements at 508 mp or 522 mp, glass-
electrode pH meter, 100-ml volumetric flasks, 10-ml microburet, and pipets.

Procedure. Prepare a calibration curve by transferring aliquots of the stan-
dard iron solution, containing from 0.02 mg to 0.20 mg of iron, to 100-ml
volumetric flasks. To separate aliquots, add 5 ml of the sodium citrate solu-
tion and determine how much sulfuric acid is necessary to adjust the pH to
3.5. Add this amount to the aliquots in the volumetric flasks. Add reagents
in the following order: 5 ml of hydroquinone solution, 5 ml of
2,2’-bipyridine or O-phenanthroline solution, and 5 ml of sodium citrate.
The citrate must always be added last. Convert to volume with distilled
water, mix well, and let stand for 1 hour. Prepare a reagent blank in the same
manner.
Determine the absorbance at 522 mp if 2,2’-bipyridine is used or 508 mp
if O-phenanthroline is used. Plot the absorption versus iron concentration on
coordinate graph paper. The resulting curve should be linear, as shown in
Fig. 3.11.
Obtain a clean sample of brine, free of oil. Determine ferrous iron, by
following the above procedure, but omit the addition of hydroquinone. To
determine dissolved iron, filter the sample and follow the above procedure.
To determine total iron, do not filter the sample. The amount of ferric iron
can be calculated from the difference.

Calculations:
1,000 x mg iron from curve
sample volume = mg/l Fe+2 or Fe+’

Concentrating copper, iron, lead, and nickel by ion exchange

To determine accurately, using colorimetric methods, copper, nickel, lead,
zinc, and cadmium in oilfield brines, they should be separated from inter-
fering ions. Many oilfield brines contain metals in such minute amounts that
they must be concentrated before analyses can be made. Concentration
methods investigated were ion exchange, electro-deposition, solvent extrac-
tion, and evaporation. An ion-exchange method proved to be the most
practical for concentrating copper, nickel, and lead, because it is less time
consuming and requires less expensive equipment than any of the other
methods studied.