Page 122 - Geochemistry of Oil Field Waters

110 ANALYSIS OF OILFIELD WATERS

cyanine R complex completely, and distillation is necessary to remove the
phosphate.

Reagents. Eriochrome Cyanine R stock solution: dissolve 1.80 g of
Eriochrome Cyanine R in 200 ml of distilled water.
Zirconyl nitrate stock solution: dissolve 0.40 g of zirconyl nitrate dihy-
drate in 100 ml of concentrated hydrochloric acid and dilute to 200 ml.
Fluoride indicator solution: add 20.0 ml of the Eriochrome Cyanine R
solution to about 500 ml of water, stir and add 10.0 ml of the zirconyl
nitrate solution, 75 ml of concentrated hydrochloric acid, and 4 g of barium
chloride. This mixture is stable for 4-6 months.
Thiosemicarbazide, powdered solid.

Procedure. Measure equal amounts of brine containing less than 0.03 mg of
fluoride into each of three 50-ml volumetric flasks. Add lOpg of fluoride to
one of the flasks and add 20 pg to another. Add a few milligrams of solid
thiosemicarbazide and 25 ml of fluoride indicator solution to each 50-ml
volumetric flask. If sulfate is present, it will precipitate as barium sulfate and
must be centrifuged out of suspension. Arbitrarily adjust the transmission of
the blank (25 ml of fluoride indicator solution made to 50-ml volume with
distilled water) at 540 mp to 32% and measure the transmission of the three
solutions.

Calculations. Using coordinate graph paper, plot the transmission of the
standard-addition samples on the y-axis and their concentrations in milli-
grams of fluoride per liter on the x-axis. Multiply the sample reading at 0
concentration by 2, and from this point on the y-axis, draw a line parallel to
the x-axis until it intersects the line plotted. From this point of intersection,
draw a line parallel to the y-axis until it intersects the x-axis. This value from
the x-axis multiplied by the dilution factor equals the amount of fluoride in
milligrams per liter. Fig. 3.3 illustrates this procedure.

Iodide

A rapid, accurate method for the determination of iodide suitable for field
work utilizes the principle whereby iodide is oxidized to iodine with nitrous
acid and extracted into carbon tetrachloride. Hydrogen sulfide will interfere,
but it can be removed by acidifying the sample and boiling (Collins et al.,
1961).

Reagents. Bromphenol blue: dissolve 0.1 g of bromphenol blue in 100 ml of
distilled water.
Carbon tetrachloride.
Iodide standard solution: dissolve 1.3081 g of potassium iodide in distilled
water and dilute to 1,000 ml. 1 ml contains 1 mg of iodide.