Page 85 - Geochemistry of Oil Field Waters

ATOMIC ABSORPTION METHODS 73

Interferences. The chemical suppressions caused by silicon, aluminium, and
phosphate are controlled by adding lanthanum. The lanthanum also controls
a slight ionization interference. A pH above 7 causes low calcium values, so
dilute HC1 is added to standards and samples. For samples containing large
amounts of silica, it often is preferable to use the nitrous oxide-acetylene
flame. The analysis appears to be free from chemical suppressions, but a
large amount of alkali salt should be added' to control ionization inter-
ferences.

Reagents. The reagents are:
(1) Lanthanum solution: wet 58.65 g of La203 with water, add 250 ml
concentrated HC1 very slowly until the material is dissolved and dilute to 1
liter. This provides a 5% lanthanum solution in 25% (v/v) HC1.
(2) Standard calcium solution: obtain commercially or prepare by adding
50 ml of water to 0.2497 g of primary standard calcium carbonate, CaC03.
Add dropwise a minimum volume of HC1 to dissolve all of the CaCO, and
dilute to 1 liter. 1 ml of solution contains 100 pg of calcium.

Preliminary calibration. Use the standard calcium solution (1 ml-100 pg Ca)
and transfer the following amounts to six 50-ml volumetric flasks. To the
first flask add 0.5 ml, to the second 1.0 ml, to the third 1.5 ml, to the fourth
2.0 ml, and to the fifth 2.5 ml; and the sixth flask should have 0.0 ml. To
each flask add 5 ml of the lanthanum solution and sufficient distilled water
to adjust the volume to 50.0 ml. The first flask now contains 1.0 pg/ml Ca,
the fifth contains 5.0 pg/ml Ca, and the sixth is a blank. Aspirate these five
standards and the blank into an air-acetylene flame and determine the ab-
sorbance at 4226.7 A. If the atomic absorption instrument has curvature
correction controls, make the necessary adjustments to obtain a linear
relationship between absorbance and the actual concentration of the
standards. If the instrument does not have these controls, plot the results on
linear graph paper as illustrated in Fig. 3.2 by substituting absorbance for
intensity.

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 calcium. Add 5 ml of the lanthanum
stock solution, dilute to volume with water, aspirate the sample into an air-
acetylene flame, and determine the absorbance of 4226.7 A. Calculate the
approximate sample concentration from the preliminary calibration readings,
and determine the aliquot size that will contain 0.05 mg of calcium.
Transfer equal aliquots containing 0.05 mg Ca+2 to three 50-ml volu-
metric flasks. Add no calcium standard to the first flask, 0.5 mg to the
second flask, and 0.10 mg to the third.