Page 121 - Geochemistry of Oil Field Waters

COLORIMETRIC METHODS 109

10 ml of 1OM NaOH and dilute to 1 liter with distilled water. 1 ml of this
solution contains 1.00 mg of A% 03. Dilute this stock standard solution as
required.
Hydrochloric acid, concentrated, analytical-grade.
Lead acetate solution: dissolve 10 g of Pb(C2H302)2 *3H20 in distilled
water and dilute to 100 ml.
Potassium iodide solution: dissolve 15 g of KI in distilled water and dilute
to 100 ml. Store in an amber colored bottle.
Silver diethyldithiocarbamate solution: dissolve 1 g of AgS[SN(C2 H5 )2 ]
in 200 ml of pyridine. Store in an amber colored bottle.
Stannous chloride solution: dissolve 40 g of arsenic-free SnC12 *2H2 0 in
1:3 HC1 and dilute to 100 ml with the same acid.
Zinc, 20 mesh, arsenic-free.

Procedure. Place a 25-ml sample, or suitable aliquot, containing less than 20
pg of arsenic in a Gutzeit generator.
Add to the flask successively, 5 ml of concentrated HCl, 2 ml of KI
solution, and eight drops of SnC12 solution. Thoroughly mix after each
addition. Allow 15 minutes for reduction of the arsenic to the tervalent
state.
Insert a plug of glass wool that has been impregnated with the lead acetate
solution into the scrubber. Assemble the generating apparatus and add 4 mi
of the silver diethyldithiocarbamate solution to the absorber. Glass beads
should be added to the absorber until the liquid just covers them.
Add 3 g of zinc to the generator and reconnect immediately. Allow 30
minutes for complete evolution of the arsine. Warm the generating flask
gently to assure complete evolution of the arsine and then pour the solution
from the absorber directly into the spectrophotometer cells. Make the deter-
minations within 30 minutes as the color developed is not permanent.

CuZcuZutions. The quantity of arsenic in the sample is determined from a plot
of absorbances of the standards:
pg As (from curve) = mg/l As
ml sample

Fluoride

Because of interferences from large amounts of chloride present in
petroleum-associated waters, a standard addition method was developed
which is accurate in the presence of large amounts of chloride and sulfate
and is more rapid than methods requiring distillation (Collins et al., 1961).
Up to 0.01 mg of phosphate in the aliquots taken for analysis can be toler-
ated. Larger amounts of phosphate than this decolorize the zirconium