Page 70 - Geochemistry of Oil Field Waters

58 ANALYSIS OF OILFIELD WATERS

standard to the first flask, 0.05 mg to the second flask, and 0.1 mg to the
third flask. Add 20 ml of n-propanol to each flask and dilute to volume with
water. Aspirate and record the emission intensity of each sample at 589 mp
and its background at 582 mp.

Calculation. Use the graph or formula illustrated in the lithium method. The
value obtained in milligrams can be converted to milligrams per liter by the
following formula:
mg Na x 1,000
ml sample = mg/l Na'
The precision and accuracy of the method are approximately 3% and 6%,
respectively, of the amount of sodium present. Some elements, when present
in the solution being analyzed, will cause a change in the emission intensity
of the sodium. The use of a standard addition technique largely compensates
for these interferences.

Potassium

Potassium usually is included with sodium without any differentiation in
reporting the results of brine analyses, although potassium is known to be
present in many oilfield brines. Potassium compounds often are dissolved
before sodium compounds; however, they do not remain dissolved as readily
because they are readily adsorbed and enriched in clays. In sea water and
oilfield brines, only a small part of the originally dissolved potassium remains
in solution. The fact that many oilfield brines are low in potassium with
respect to sodium, whereas surface waters and young volcanic waters are
enriched in potassium with respect to sodium, is an important criterion in
identifying the sources of brines.
The flame spectrophotometer provides a sensitive method for the determi-
nation of potassium. The strongest lines for potassium detection in a flame
are the doublet at 766.5 and 769.9 mp.

Reagents. The necessary reagents are potassium standard solution, 0.1
mg/ml; and n-propanol.

Preliminary calibration curves. Preliminary calibration curves are useful in
determining the approximate amount of potassium in the sample, so that the
optimum sample size for standard addition can be selected for the analysis.
These curves can be prepared in the same manner used in the preparation of
the lithium preliminary calibration curves (Fig.3.2) except that standard
potassium solutions are used. The emission intensity of the potassium line at
766.5 mp minus the background at about 750 mp can be used in preparing
the curves.