Page 89 - Geochemistry of Oil Field Waters

ATOMIC ABSORPTION METHODS 77

Reagents. The necessary reagents are:
(1) Lanthanum solution (same as used in the calcium standard-addition
procedure).
(2) Standard strontium solution: obtain commercially or dissolve 2.415 g
of strontium nitrate, Sr(N03)2, in 1 liter of 1% (v/v) HNO,. 1 ml of the
solution contains 1,000 pg of strontium.

Preliminary calibration. Prepare standard strontium solutions containing
1-10 pg/ml of strontium using the standard strontium stock solution and 50
ml of volumetric flasks. Add to each of these and to a blank, 5 ml of the
lanthanum stock solution. Aspirate these standards and the blank as
suggested in the calcium method and determine the absorbance of strontium
at 4607 A.

Procedure. Transfer an aliquot of brine to a 50-ml volumetric flask. The
specific gravity of the brine can be used as a guide in estimating the size of
an aliquot containing about 0.1 mg of strontium (see Fig. 3.6). Add 5 ml of
the lanthanum stock solution, dilute to volume, and aspirate. Calculate the
approximate sample concentration from the preliminary calibration readings,
and determine the aliquot size that will contain about 0.1 mg strontium.
Transfer equal aliquots containing about 0.1 mg of strontium to three
volumetric flasks. Add no strontium standard to the first flask, 0.1 mg to the
second, and 0.2 to the third. Add 5 ml of the lanthanum stock solution to
each of the three flasks and dilute to volume. Aspirate and record the absor-
bance readings for each sample.

Calculations. See calculations under “Lithium” in the flame spectrophoto-
metric section, Fig. 3.3, or Table 3.XI:

mg Sr x 1,000
ml sample = mg/l Sr+2
Precision. In a single laboratory using oilfield water samples containing con-
centrations of 840 and 2,250 mg Sr+2/1, the standard deviations were +48
and +110, respectively. The recoveries were 106.8% and 103.1%, respec-
tively.

Barium

Barium is determined at the 5336 A wavelength with an acetylene
nitrous-oxide flame.

Interferences. Ionization interference is suppressed by adding 1,000 pg/ml of
sodium.