Page 93 - Geochemistry of Oil Field Waters

ATOMIC ABSORPTION METHODS 81

Reagents. The necessary reagent is a standard zinc solution: obtain commer-
cially or dissolve 0.500 g of zinc metal in a minimum volume of (1 +1) HC1
and dilute to 1 liter with 1% (v/v) HCl. 1 ml of solution contains 500 pg of
zinc.

Preliminary calibration. Prepare standard zinc solutions containing 0.2-1.0
pg/ml using the standard zinc solution and 50-ml volumetric flasks. Aspirate
these standards and a blank as recommended in the calcium method and
determine the absorbance at a wavelength of 2138.6 a.

Procedure. Transfer an aliquot containing about 10 pg of zinc to a 50-ml
volumetric flask. Dilute to volume and aspirate. Calculate the approximate
sample concentration from the preliminary calibration readings and deter-
mine the aliquot size that will contain 10 pg of zinc.
Transfer equal aliquots containing about 10 pg of zinc to three 50-ml
volumetric flasks. Add no zinc standard to the first flask, 10 pg of the zinc
standard to the second flask, and 20 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, Fig. 3.3, or Table 3.XI:

Precision. In a single laboratory using oilfield water samples containing con-
centrations of 27 and 120 mg Zn+2 /1, the standard deviations were +1. The
recoveries were 103.5% and 102.3%, respectively.

Lead (1)

Lead is determined at the 2833.1 A wavelength with an air-acetylene
flame.

Reagents. The necessary reagent is a standard lead solution: obtain commer-
cially or dissolve 1.598 g of lead nitrate, Pb(N03)2, in 1 liter of 1% (v/v)
HN03. 1 ml of solution contains 1,000 pg of lead.

Preliminary calibration. Prepare standard lead solutions containing 2-10
pg/ml using the standard lead solution and 50-ml volumetric flasks. Aspirate
these standards and a blank as recommended in the calcium method and
determine the absorbance at a wavelength of 2833.1 8.

Procedure. Transfer an aliquot containing 100 pg of lead to a 50-ml volu-
metric flask. Dilute to volume and aspirate. Calculate the approximate
sample concentration from the preliminary calibration readings and deter-
mine the aliquot size that will contain 100 pg of lead.