Page 80 - Geochemistry of Oil Field Waters

68 ANALYSIS OF OILFIELD WATERS

Lithium

Lithium is determined at the 6707.8 A wavelength with an air-acetylene
flame.

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

Reagents. The necessary reagents are:
(1) Potassium solution: see reagents preparation under “Sodium”.
(2) Standard lithium solution: obtain commercially or dissolve 5.324 g of
lithium carbonate, Liz CO, , in a minimum volume of one part Hz 0 to one
part of HC1 (1 + 1). Dilute to 1 liter with water. 1 ml of this solution contains
1,000 pg of lithium.

Preliminary calibration. Prepare standard lithium solutions containing 1-5
pg/l of lithium using the standard lithium solution and 50-ml volumetric
flasks. Add to each of these and to a blank, 0.5 ml of the potassium stock
solution. Aspirate these standards and the blank as recommended in the
calcium method and determine the absorbance at a wavelength of 6707.8 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 lithium. Add 0.5 ml of the potassium
stock solution, dilute to volume with water, and aspirate. Calculate the
approximate sample concentration from the preliminary calibration readings
and determine the aliquot size that will contain about 0.05 mg of lithium.
Transfer equal aliquots containing about 0.05 mg of lithium to three
50-ml volumetric flasks. Add no lithium standard to the first flask, 0.05 mg
of the lithium standard to the second flask, and 0.10 mg to the third. Add
0.5 ml of the potassium stock solution to each of the three flasks 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:
mgLix 1,000
ml sample = mg/l Li+

Precision. In a single laboratory using oilfield water samples containing con-
centrations of 90 and 190 mg Li+/l, the standard deviations were k3 and +5,
respectively. The recoveries were 100.6% and 92.996, respectively.

Sodium

Two wavelengths are used: the 5890-5896 A doublet for the 1-mg/l