Page 83 - Geochemistry of Oil Field Waters

ATOMIC ABSORPTION METHODS 71

Precision. In a single laboratory using oilfield water samples containing con-
centrations of 456 and 5,680 mg K+/1, the standard deviations were *25 and
+325, respectively. The recoveries were 93.7% and 97.8%, respectively.

Magnesium (1)

Magnesium is determined at the 2852.1 A wavelength with an air-
acetylene flame.

Interferences. The silicon and aluminum suppression of the magnesium
absorption is generally removed by the addition of lanthanum or by the use
of a nitrous oxide-acetylene flame.

Reagents. The reagents are:
(1) Lanthanum solution (same as used in the calcium procedure).
(2) Standard magnesium solution: obtain commercially or dissolve 1 .OOO g
of magnesium ribbon in a minimum of (1 + 1) HC1, and dilute to 1 liter
with 1% (v/v) HC1. 1 ml of this solution contains 1,000 pg of magnesium per
ml and should be made up daily to use for the standard additions.

Preliminary calibration. Prepare standard solutions containing 0.1-0.5 pg/l
of magnesium using the standard magnesium solution and 50-ml volumetric
flasks. Add to each of these and to a blank 5 ml of the stock lanthanum
solution. Aspirate as suggested in the calcium method and determine the
absorbance at 2852.1 A.

Procedure. Transfer an aliquot of brine to a 50-ml volumetric flask. The
specific gravity of the brine often can be used as a guide in estimating the
size of an aliquot containing about 0.005 mg of magnesium. Add 5 ml of the
lanthanum stock solution, dilute to volume with water, and aspirate. Calcu-
late the approximate sample concentration from the preliminary calibration
readings, and determine the aliquot size that will contain about 0.005 mg
magnesium.
Fig. 3.6 illustrates a plot of the concentration of magnesium found in
some oilfield brines compared to their specific gravity. This figure cannot
necessarily be applied to all oilfield brines, however, because some will
contain more and some less. The concentrations of magnesium in brines
from the same formation at about the same depth often are similar.
Transfer equal aliquots containing about 0.005 mg magnesium to three
50-ml volumetric flasks. Add no magnesium standard to the first flask, 0.005
mg to the second flask, and 0.010 mg 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 absorbance readings for each sample.