Page 82 - Geochemistry of Oil Field Waters

70 ANALYSIS OF OILFIELD WATERS

Precision. In a single laboratory using oilfield water samples containing con-
centrations of 22,700 and 43,200 mg Na+/l, the standard deviations were
+485 and ?1,890, respectively. The recoveries were 100.8% and 100.9%,
respectively.

Potassium

Potassium is determined at the 7664.9 A wavelength with an air-acetylene
flame.

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

Reagents. The necessary reagents are:
(1) Sodium solution: dissolve 254.20 g of sodium chloride in 1 liter of
water. 1 ml of this solution contains 100 mg of sodium.
(2) Standard potassium solution: obtain commercially or dissolve 1.907 g
of potassium chloride, KCl, in 1 liter of water. 1 ml of this solution contains
1,000 pg of potassium.

Preliminary calibration. Prepare standard solutions containing 1-5 pg/l of
potassium using the standard potassium solution and 50-ml volumetric
flasks. Add 0.5 ml of the sodium stock solution to each of these and to a
blank. Aspirate these standards and the blank as recommended in the cal-
cium method and determine the absorbance at a wavelength of 7664.9 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 0.05 mg of potassium. Add 0.5 ml of the sodium
stock solution, dilute to volume with water, and aspirate. Calculate the
approximate potassium concentration from the preliminary calibration
readings and determine the aliquot size that will contain about 0.05 mg of
potassium.
Transfer equal aliquots containing about 0.05 mg of potassium to three
50-ml volumetric flasks. Add no potassium standard to the first flask, 0.05
mg of the potassium standard to the second flask, and 0.10 mg to the third.
Add 0.5 ml of the sodium 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 K x 1,000 = mg/l K+
ml sample