Page 110 - Geochemistry of Oil Field Waters

#.- 30 ANALYSIS OF OILFIELD WATERS

Calculations. Estimate the amount of copper present by using a calibration
curve prepared by using about 10-200 pg of copper:
pg Cu (from curve)
ml sample = mg/l CU+~

Nickel

Nickel forms a wine-red or brown compound with dimethylglyoxime
(Sandell, 1959, p.555). The structure of the chelate on the basis of available
evidence is:

H3C - C-C = CH3
//

/I /I
H3C - C C = CH,

Dimethylglyoxime gives a nearly specific reaction with nickel that has been
oxidized to its higher valences with an oxidizing agent such as bromine. The
wine-red compound is somewhat unstable; therefore, the absorbance mea-
surements should be made within 10 minutes after formation of the nickel
dimethylglyoximate. Cobalt and copper also give colored compounds with
dimethylglyoxime, but they can be removed by washing the chloroform
extract of nickel dimethylglyoximate with dilute ammonium hydroxide.
Iron interference is removed by extracting the nickel dimethylglyoximate
with chloroform from a solution containing citrate. Palladium, platinum, and
gold also give colored compounds when nickel dimethylglyoximate is
extracted with chloroform; however, they are removed, if present, by the
ion-exchange separation.

Reagents. Dimethylglyoxime solution: dissolve 1 g of dimethylglyoxime in
100 ml of ethyl alcohol.
Saturated bromine water.
Ammonium hydroxide solution, approximately 4N: add 200 ml of con-
centrated ammonium hydroxide to 800 ml of water.
Standard nickel solution: dissolve 0.100 g of nickel in dilute nitric acid by
heating gently. Cool, dilute with water, transfer to a 1-liter volumetric flask,
and dilute to volume. Pipet a 100-ml aliquot of this stock solution into
another 1-liter volumetric flask and dilute to volume. This solution contains
10 pg/ml of nickel.
Hydrochloric acid, approximately 6N: cautiously add 500 ml of concen-
trated hydrochloric acid to 500 ml of water.
Chloroform.