Page 114 - Geochemistry of Oil Field Waters

102 ANALYSIS OF OILFIELD WATERS

Reagents. Standard zinc solution: dissolve 1.00 g of zinc metal in hydrochloric
acid and dilute to 1 liter with water. Dilute 10 ml of the stock solution to
1 liter to prepare a standard containing 10 pg/ml of zinc.
Sodium citrate solution: dissolve 10 g of sodium citrate in water and
dilute to 100 ml.
Hydroxylamine hydrochloride solution : dissolve 10 g of hydroxylamine
hydrochloride in water and dilute to 100 ml.
Buffer solution, pH 4.75: dissolve 130 g of sodium acetate and 57 ml of
glacial acetic acid in water and dilute to 1 liter.
Dithizone solution: dissolve 0.1 g of dithizone in a liter of alcohol-free
carbon tetrachloride. Extract any alcohol from the carbon tetrachloride by
shaking it with distilled water. Keep a water blanket on the extracted carbon
tetrachloride when storing it.
Potassium cyanide solution: dissolve 1.0 g of potassium cyanide in water
and dilute to 100 ml.
Buffer solution, pH 9.0: dilute 213 ml of lN sodium hydroxide to 600 ml
with water. Dissolve 37.3 g of potassium chloride and 31.0 g of boric acid in
water, mix with the sodium hydroxide, and dilute to 1 liter.
Zincon solution: dissolve 0.13 g of zincon in 2 ml of 1N sodium
hydroxide and dilute to 100 ml with water.
Chloral hydrate solution: dissolve 10 g of chloral hydrate in water and
dilute to 100 ml.
Hydrochloric acid, 0.02N: add 1.7 ml of concentrated hydrochloric acid
to water and dilute to 1 liter.
Ammonium hydroxide.
Sodium ascorbate.
Sodium thiosulfate solution: dissolve 25 g of sodium thiosulfate in water
and dilute to 100 ml.

Procedure. Add 10 ml of the hydroxylamine hydrochloride solution to an
aliquot of brine containing up to 200 pg of zinc, mix, add 10 ml of sodium
citrate solution, and adjust the pH to 8.3 with ammonium hydroxide.
Transfer the sample to a separatory funnel, add 3 ml of 0.01% dithizone
solution, and shake the mixture vigorously for 1 minute. Let the phases
separate and note the color of the dithizone phase. If any zinc is present, the
dithizone phase will be red or violet, but not green. If the dithizone phase is
green, take a larger aliquot of brine. If the dithizone phase is red or violet,
extract it into another separatory funnel containing 1 ml of sodium thiosul-
fate solution and 10 ml of pH 4.75 buffer solution. Make additional
extractions of the brine solution with dithizone solution until the dithizone
remains green, which indicates that all the zinc has been extracted. This is
important because the final dithizone phase must be green, not violet.
Discard the brine solution and wash the combined dithizone extracts by
mixing them vigorously for 1 minute with the buffer solution. Let the phases
separate, extract the dithizone phase into another separatory funnel con-