Page 91 - Geochemistry of Oil Field Waters

ATOMIC ABSORPTION METHODS 79

Procedure. Transfer an aliquot containing about 0.05 mg of manganese to a
50-ml volumetric flask. Dilute to volume and aspirate. Calculate the approxi-
mate sample concentration from the preliminary calibration readings and
determine the aliquot size that will contain 0.05 mg of manganese.
Transfer equal aliquots containing about 0.05 mg of manganese to three
’
50-ml volumetric flasks. Add no manganese standard to the first flask, 0.05
mg of the manganese standard to the second flask, and 0.10 mg to the third.
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 Mn x 1,000
ml sample = mg/l Mn+’

Precision. In a single laboratory using oilfield water samples containing con-
centrations of 20 and 97 mg Mn+2/1, the standard deviations were k1 and
+3, respectively. The recoveries were 102.2% and 105.4% respectively.

Iron

Iron is determined at the 2483.2 A wavelength with an air-acetylene
flame.

Interferences. The sensitivity is reduced if nitric acid and nickel are present.
This effect can be controlled by using a very lean (hot) flame.

Reagents. The necessary reagent is a standard solution: obtain commercially
or dissolve 1.000 g of iron wire in 50 ml of (1 + 1) nitric acid and dilute to 1
liter with water. 1 ml of solution contains 1 mg of iron.

Preliminary calibration. Prepare standard iron solutions containing 1-5 Mg/ml
using standard iron solution and 50-ml volumetric flasks. Aspirate these
standards and a blank as recommended in the calcium method and determine
the absorbance at a wavelength of 2483.2 A.

Procedure. Transfer an aliquot containing about 0.05 mg of iron to a 50-ml
volumetric flask. Dilute to volume and aspirate. Calculate the approximate
sample concentration from the preliminary calibration readings and deter-
mine the aliquot size that will contain about 0.05 mg of iron.
Add no iron standard to the first flask, 0.05 mg of the iron standard to
the second flask, and 0.10 mg to the third. Dilute to volume. Aspirate and
record the absorbance readings for each sample.