Page 72 - Geochemistry of Oil Field Waters

60 ANALYSIS OF OILFIELD WATERS

type 6911. Such tubes also are useful for lithium and potassium deter-
minations.
Several elements can interfere in the determination of cesium and
rubidium. However, because a solvent extraction or standard-addition tech-
nique is used most interferences are either removed or compensated (Collins,
1965).

Reagents. The necessary reagents are cesium standard solution, 0.01 mg/ml;
rubidium standard solution, 0.01 mg/ml; buffer solution, pH 6.6 (adjust the
pH of a 1M sodium citrate solution to 6.6 with 0.5M nitric acid); sodium
tetraphenylboron, 0.05M (dissolve 0.855 g of sodium tetraphenylboron in
distilled water and dilute to 50 ml - prepare a fresh solution daily);
nitroethane; hydrochloric acid, 0.1N; sodium hydroxide, 0.W; synthetic
brine solution.

Procedure. To determine the amount of rubidium and cesium in the
petroleum-associated water, transfer an aliquot of brine containing 0.005 to
0.05 mg of cesium and rubidium to a 100-ml beaker and add 25 ml of the
citrate buffer solution. Transfer the solution to a 125-ml Teflon-stoppered
separatory funnel and adjust to 100-ml volume. Add 2 ml of 0.05M sodium
tetraphenylboron aqueous solution and 10 ml of nitroethane, and shake the
mixture vigorously for 2 minutes. Allow the phases to separate for 30
minutes, after which time withdraw the aqueous phase. Centrifuge the
nitroethane phase. Determine the cesium and rubidium emission intensities
by burning the nitroethane phase in the flame spectrophotometer and
automatically scanning the 780.0 mp, 794.8 mp, and 894.4 mp lines.

Calibration curves. Prepare calibration curves by using appropriate portions
of the standard cesium and rubidium solutions. Add 5 ml of synthetic brine
solution to each standard sample before buffering and extraction. Plot the
resultant emission intensities versus milligrams of cesium or rubidium or
linear graph paper.

Calculation. Determine the milligrams of cesium or rubidium in the sample
by referring to the calibration curves. The milligrams can be converted to
mg/l by the following formula:
mgx 1,000
ml sample = mg/l Cs+ or Rb'
Fig.3.4 illustrates the relative emission intensities obtained with cesium
and rubidium in nitrobenzene, nitroethane, 1-nitropropane, and 2-nitro-
propane. 15 ml of each of these solvents.are used to extract 0.1 mg each of
cesium and rubidium tetraphenylboron from aqueous solutions. The organic
phases then are aspirated directly into the flame, and the peaks scanned
automatically. Good resolution is obtained with a 0.01 mm slit width. Amy1
alcohol gives poorer results than nitrobenzene.