Page 88 - Geochemistry of Oil Field Waters

76 ANALYSIS OF OILFIELD WATERS

with water. This yields standards of 1.0, 11.0, and 21.0 mg/l of calcium in
the first, second, and third flasks, respectively.

Procedure. Filter the sample through the micropore filter apparatus to remove
solids and traces of hydrocarbons from the water. Transfer, by means of
micropipet or volumetric transfer pipet, an aliquot of sample containing not
more than 2.0 mg calcium into a 100-ml volumetric flask. Add 5.0 ml hy-
drochloric acid, 1.0 ml lanthanum stock solution, and sufficient water to di-
lute to exactly the 100-ml mark and mix thoroughly. Aspirate the ll mg/l
standard through the burner, positioning the burner angle as necessary until
the recorder reaches a maximum stable reading of about 22% absorption using
a wavelength setting of 4227 A. Record the reading and aspirate distilled water
through the burner until the recorder returns to the original baseline. Re-
move and aspirate the sample through the burner until a maximum stable
reading is obtained on the recorder. Record the reading and aspirate distilled
water through the burner until the recorder returns to the original baseline.
If the sample reading on the recorder is greater than the 11 mg/l standard,
aspirate the 21 mg/l standard through the burner until a maximum stable
reading is obtained. Record the reading and if the sample reading on the
recorder is less than the 11 mg/l standard, aspirate the 1 mg/l standard
through the burner until a maximum, stable reading is obtained. Record the
reading.

Calculations:
(%A ,--%A 2 )
10 + mg/12 x DF = mg/l Ca+2
%A 1 -76 2
where %A = percent absorption of high standard; %A2 = percent absorption
of low standard; %A, = percent absorption of sample; mg/ll = mg Ca+2/l of
high standard; mg/12 = mg Ca+2/l of low standard; mg/l, = mg Ca+?/l of
sample; and DF = dilution factor of sample (100/ml sample).

Strontium

Strontium is determined at the 4607 A wavelength with an air-acetylene
flame.
Interferences. The chemical suppression caused by silicon, aluminum, and
phosphate is controlled by adding lanthanum. The lanthanum also controls
ionization interference. The nitrous oxide-acetylene flame can be used to
control chemical interferences, but a large excess of alkali salt should be
added to control ionization.