Page 75 - Geochemistry of Oil Field Waters

FLAME SPECTROPHOTOMETRIC METHODS 63

and calcium without several preliminary separations. Precipitations as the
carbonate or oxalate have the same disadvantages, and precipitation as the
nitrate and subsequent solvent extraction of calcium with butylcellosolve
still leaves barium in the precipitate. The use of a standard addition flame
photometric method gives reproducible results without the necessity of
several separations.

Reagents. The necessary reagents are standard strontium solution, 1 mg/ml;
and n-propanol.

Preliminary calibration curves. To determine approximately how much
strontium is present in the samples, it is advantageous to prepare preliminary
calibration curves. A procedure similar to that used in the lithium method
can be used, except that the strontium emission should be determined at 680
mp with a background reading at 690 mp. The data are plotted in a manner
similar to Fig. 3.3.

Procedure. To determine the amount of strontium, transfer an aliquot of
brine to a 50-ml volumetric flask, add 20 ml of n-propanol, and dilute to
volume with distilled water. Aspirate the sample into the flame and read;
record the emission intensity of the background at 690 mp and the stron-
tium line at 680 mp. With these readings and the preliminary calibration
curves, calculate approximately how much strontium is in the sample.
Determine an aliquot size that will contain about 1.0 mg of strontium.
Transfer equal aliquots to three 50-ml volumetric flasks. Add no strontium
standard to the first flask, 1.0 mg to the second flask, and 2.0 mg to the
third flask. Add 20 ml of n-propanol to each flask and dilute to volume with
distilled water. Aspirate and record the background at 690 mp and the
emission intensity of each sample at 680 mp.

Calculation. A graph can be used in the calculation as illustrated in Fig.3.3.
The value obtained in milligrams can be converted to milligrams per liter by
the following formula:
mg Sr x 1,000
ml sample = mg/l Sr+*

The formula, shown in Table 3.X1, can be used to calculate the amount of
strontium in the sample using the flame spectrophotometric readings in lieu
of the graph method.

Barium

A flame spectrophotometric method was developed which utilizes the
chromate precipation followed by dissolution in nitric acid, mixing with an
alcohol, and burning in the flame (Collins, 1962). The flame method is