Page 81 - Geochemistry of Oil Field Waters

ATOMIC ABSORPTION METHODS 69

aliquots and the 3302-3303 a doublet for the 100-mg/l aliquots. Because of
the wide .range of sodium concentrations found in brines, the higher wave-
length can be used for the lower gravity brines and the lower wavelength for
the higher gravity brines, thus avoiding making two dilutions with some of
the heavier brines. It is usually necessary to make a preliminary determina-
tion so that the correct aliquot can be used with the standard additions.

Interferences. Ionization interference is usually overcome by adding potas-
sium.

Reagents. The necessary reagents are:
(1) Potassium solution: dissolve 190.70 g of potassium chloride, KC1, in
water and dilute to 1 liter. 1 ml of this solution contains 100 mg of potas-
sium.
(2) Standard sodium solution: obtain commercially or dissolve 25.420 g
of sodium chloride in 1 liter of water. 1 ml of this solution contains 10 mg
of sodium. Dilute 10 ml of this solution to a liter. 1 ml of this solution
contains 100 pg of sodium.

Preliminary calibration. Prepare standard solutions containing 1 .O-5.0 and
100-500 pg/ml of sodium using the standard sodium solutions and 50-ml
volumetric flasks. Add to each of these, and to a blank, 0.5 ml of the potas-
sium stock solution. Aspirate these standards and blank as recommended in
the calcium method and determine the absorbance at 5890-5896 a for the
1.0-5.0 pg/ml Na solutions and at 3302-3303 a for the 100-500 pg/ml Na
solutions.

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 either about 0.05 mg or about 5 mg of sodium. Add
0.5 ml of the potassium stock solution, dilute to volume, and aspirate.
Calculate the approximate sample concentration from the preliminary cali-
bration readings. Determine the aliquot size that will contain either about
0.05 mg or 5 mg of sodium, depending on the wavelength to be used.
Transfer equal aliquots to three 50-ml volumetric flasks. For the 0.05-mg
aliquots, add no sodium standard to the first flask, 0.05 mg of sodium
standard to the second flask, and 0.10 mg to the third. For the 5-mg
aliquots, add no sodium standard to the first flask, 5 mg to the second, and
10 mg to the third. Add 0.5 ml of the potassium stock solution to each flask
and 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:
mg Na x 1,000
ml sample = mg/l Na+