Page 94 - Geochemistry of Oil Field Waters

82 ANALYSIS OF OILFIELD WATERS

Transfer equal aliquots containing about 100 pg of lead to the three 50-ml
volumetric flasks. Add no lead standard to the first flask, 100 pg of lead
standard to the second flask, and 200 pg to the third. Dilute to volume.
Aspirate and record the absorbance readings for each sample.

Calculations. See calculations under “Lithium” in the flame spectrophoto-
metric section, and Fig. 3.3, or Table 3.XI:
mg Pb x 1,000
ml sample = mg/l Pb+’

Precision. In a single laboratory using an oilfield water sample containing a
concentration of 16 mg Pb+*/l, the standard deviation was k2.6. The recovery
was 74.8%.

Lead (2)

Lead is determined by chelating with ammonium pyrollidine dithiocar-
bamate (APDC) and extracting with methyl isobutyl ketone (MIBK) (Brooks
et al., 1967). The organic extract is analyzed by means of atomic-absorption
spectrophotometry. Interferences have not been observed in the air-
acetylene flame.

Reagents. The necessary reagents are methyl isobutyl ketone (MIBK); 0.3M
hydrochloric acid; ammonium pyrollidine dithiocarbamate (APDC) (dissolve
1.0 g of APDC in 100 ml of distilled water); bromphenol blue indicator
solution (dissolve 0.1 g bromphenol blue in 100 ml of 50% ethanol); 2.5M
sodium hydroxide; and lead standard solution. The latter can be bought
commercially or made from lead nitrate. The presence of 0.5% nitric acid in
the lead standards of low concentrations retards the plating of the lead on
the sides of the container.

Procedure. Pipet the sample into a 200-ml volumetric flask and adjust the
volume to approximately 100 ml with distilled water. Add two drops of the
bromphenol blue indicator solution. Adjust the pH by adding 2.5M NaOH
by drops until a blue color persists. Add 0.3M HC1 until the blue color
disappears. Add 2.0 ml of HC1 in excess. The pH should be 2.4. Add 2.5 ml
of the APDC solution and mix. Add 10 ml of MIBK and shake vigorously for
1 minute. Allow the layer to separate and add distilled water until the
ketone layer is in the neck of the flask. Aspirate the ketone layer for lead
content. Prepare a calibration curve by adding known amounts of lead to a
synthetic brine solution.
Calculations:
mg Pb (from curve) -
- mg/l Pb’ *
ml sample