Page 69 - Geochemistry of Oil Field Waters

FLAME SPECTROPHOTOMETRIC METHODS 57

TABLE 3.XI
C
Formula for standard-addition calculation C, = (rx - rb) -
where the following are true*: r-r,
. .-
Solution Concentration Reading

Unknown c, r,
c, = c, + c
Mixture . r
*C is a standard addition.

Sodium

The flame spectrophotometer offers an excellent instrumental technique
for determining sodium in a petroleum-associated water. The flames con-
taining alkali metals give strong resonance lines of these metals plus some
additional continyous radiation. The strongest line for sodium results from a
transition between the lowest excited level and the ground state. The yellow
doublet of sodium at 589.0-589.6 mp results from such a transition.

Reagents. The necessary reagents are sodium standard solutions, 1 mglml
and 0.01 mg/ml; and n-propanol.

Preliminary Calibration curves. Preliminary calibration curves similar to those
shown in Fig. 3.2 should be used to determine the approximate amount of
sodium in the sample. These curves are prepared in the same manner as the
lithium curves, except that standard sodium solutions are used; the emission
intensity of the sodium at 589 mp is determined, minus a background at
about 582 mp.
Procedure. To analyze the petroleum-associated water, transfer an aliquot of
water to a 50-ml volumetric flask, add 20 ml of n-propanol, and dilute to
volume with distilled water. (The size of the aliquot will vary with the
sample. The specific gravity can be used to help decide the aliquot size. For
a water with a specific gravity of 1.1, an aliquot of 1 ml or less probably will
be sufficient.) Aspirate the sample into the flame and record the emission in-
tensity of the background at 582 mp and sodium line at 589 mp. With these
readings, calculate approximately how much sodium is in the sample by using
the preliminary calibration curves.
Determine the aliquot size that will contain about 0.05 mg of sodium.
Transfer equal aliquots to three 50-ml volumetric flasks. Add no sodium