Page 66 - Geochemistry of Oil Field Waters

54 ANALYSIS OF OILFIELD WATERS

are much cooler than the spark and arc sources used in spectrography, analy-
sis by emission flame spectrophotometry is usually limited to the more easily
excitable elements - lithium, sodium, and potassium.
Instrumentation requirements include:
(1) A method of introducing the sample into the flame for vaporization.
(2) A method of detecting and recording the radiation intensity emitted.
(3) A method of selecting the correct wavelength, ordinarily a variable
monochromator.
A more complete discussion of the theory and instrumentation can be
found in books by Burriel-Marti and Ramirez-Munoz (1957) and Dean
(1960), as well as in publications of commercial instrument manufacturers.

Lithium

Lithium usually is calculated as a part of the sodium content in reporting
the results of oilfield water analyses rather than being determined and
reported separately. One of the more accurate methods to determine lithium
in petroleum-associated waters is the flame spectrophotometric method
(Collins, 1962).

Reagents. The reagents are lithium, standard solutions, 0.1 mg/ml and 0.01
mg/ml; and n-propanol.

Equipment. The necessary equipment includes a flame spectrophotometer,
10-ml 'microburets, and volumetric flasks.

Preliminary calibration curves. Preliminary calibration curves are useful in
determining approximately how much lithium is in the sample and in deter-
mining the optimum amount of standard lithium solution to use in the
analysis. Because n-propanol is easier to work with, it usually is used; how-
ever, if additional sensitivity is needed, the acetone-n-amyl alcohol mixture
can be used (Collins, 1965).
To prepare the preliminary calibration curves, transfer to 50-ml volumet-
ric flasks aliquots of diluted standard lithium solution containing the follow-
ing amounts of lithium: 0.01 mg, 0.05 mg, 0.1 mg, 0.15 mg, and 0.2 mg.
Add 20 ml of n-propanol to each flask and dilute to volume with distilled
water. Aspirate, burn, and record the emission intensity of each of these five
standards at 670.8 mp and their background at about 665 mp. Record
several peaks for each standard at various sensitivity levels and slit widths.
Plot the results on linear graph paper by plotting milligrams of lithium
versus intensity. Prepare a curve for each sensitivity level and slit width used,
as illustrated in Fig. 3.2. The sensitivity of the instrument will determine the
optimum concentrations of lithium and this will require some experimenta-
tion.
The analyst may find it convenient to scan all the emission lines of