Page 86 - Geochemistry of Oil Field Waters

74 ANALYSIS OF OILFIELD WATERS

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.

Calculations. See calculations under “Lithium” in the flame spectrophoto-
metric section, Fig.3.3, or Table 3.XI:
mg Ca x 1,000
ml sample = mg/l Ca+’
Precision. In a single laboratory using oilfield water samples containing con-
centrations of 17,400 and 32,500 mg Cat’ /1, the standard deviations were
k430 and +1,090, respectively. The recoveries were 103.5% and 100.3’36,
respectively.

Magnesium (2)

The following method for the determination of magnesium in an oilfield
water was supplied through courtesy of the Halliburton Company (1970),
and can be used to determine all concentrations of the magnesium ion in a
brine.

Reagents. The necessary reagents are magnesium standard solution, 1 mg/ml;
lanthanum solution, 1 g/ml; and hydrochloric acid.

Magnesium standard working so 1 u t ions

Pipet 1.0 ml of the magnesium standard stock solution into a 1 liter flask,
add 11.0 ml to a second 1-liter flask, and add 21.0 ml to a third 1-liter flask.
To each flask add 50 ml of concentrated hydrochloric acid, 10 ml of the
lanthanum stock solution, and dilute each to an overall 1,000 ml volume
with water. This yields standards of 1.0, 11.0, and 21.0 mg/l of magnesium
in the first, second, and third flasks, respectively.

Procedure. Filter the sample with the micropore filter apparatus to remove
solids and traces of hydrocarbons from the water. Transfer, by means of
“Lambda” pipet or volumetric transfer pipet, an aliquot of sample to con-
tain not more than 1.0 mg magnesium into a 100-ml volumetric flask. Add
5.0 ml hydrochloric acid, 1.0 ml lanthanum stock solution, and sufficient
water to dilute to exactly the 100-ml mark. Mix thoroughly. Aspirate the
5-mg/l standard through the burner, positioning the burner angle as neces-
sary until the recorder indicates a stable reading of about 25% absorption
using a wavelength setting of 2852 a. Record the reading and aspirate distil-
led water through the burner until the recorder returns to the original base-
line. Next, aspirate the sample through the burner until a maximum stable
reading is obtained on the recorder. Record the reading and if the sample