Page 74 - Geochemistry of Oil Field Waters

62 ANALYSIS OF OILFIELD WATERS

1-liter flask, add 10 ml of hydrochloric acid, and dilute to volume with
distilled water; (1 ml of this solution contains 10 pg of manganese); chloro-
form solution of 8-hydroxyquinoline (dissolve 1 .O g of 8-hydroxyquinoline
in 100 ml of chloroform); hydrogen peroxide (3% solution); ammonium
hydroxide (3N); sodium potassium tartrate (10% solution); ammonium
fluoride (5% solution); n-propanol; and chloroform.

Procedure. Transfer an aliquot of brine containing up to 150 pg of manga-
nese to a 100-ml beaker; add 1 ml of hydrogen peroxide, 5 ml of ammonium
fluoride, and 10 ml of sodium potassium tartrate; and adjust the pH of the
mixture to 9.0 with ammonium hydroxide. Transfer the solution to a 125-ml
Teflon-stoppered separatory funnel, add 10 ml of 8-hydroxyquinoline
chloroform solution, and bring the mixture to equilibrium by shaking it
vigorously for 1 minute.
Draw the chloroform phase off into a 100-ml beaker and strip the aqueous
phase by an additional extraction with chloroform. Evaporate the combined
chloroform extracts to dryness over a hotplate, taking care to prevent the
residue from charring. Dissolve the residue in n-propanol and make to 50 ml
volume with n-propanol. Aspirate the n-propanol solution directly into the
flame and determine the net emission by subtracting the background emission
at 400 mp.
Calculate the amount of manganese in the sample from a calibration curve
prepared by adding known amounts of manganese to a synthetic brine solu-
tion. The calibration curve should be linear for up to 150 pg of manganese
when the emission intensity is plotted versus micrograms of manganese on
linear graph paper.

Calculation :
pg Mn (from curve) = mg/l Mn +*
ml sample
The intensity of the emission of manganese in a flame spectrophotometer
is enhanced by a factor of 16 by using n-propyl alcohol rather than water as
the solvent. With this increased intensity, the sensitivity of the method is
about 1 mg/l, although additional sensitivity is attainable by concentrating
the brine by evaporation. The precision of the method is about 3%, and the
accuracy is about 6% of the amount present.

Strontium

Several flame photometric methods are available for determining stron-
tium in oilfield brines; a standard curie may be unreliable if there are
instrument changes, such as a slightly plugged burner, change of resistance in
the amplifying circuit, or other variables. Chemical precipitation of stron-
tium as the sulfate does not satisfactorily separate strontium from barium