Page 117 - Geochemistry of Oil Field Waters

COLORIMETRIC METHODS 105

minutes. Discard the carbon tetrachloride phase and wash the tartaric acid
phase twice with a 3-ml portion of carbon tetrachloride. To the tartaric acid
phase add 1 ml of hydroxylamine hydrochloride solution, 5 ml of the 35%
sodium hydroxide-1% potassium cyanide solution, and 10 ml of the No. 2
dithizone solution Shake the mixture vigorously for 1 minute. Let the phases
separate and extract the dithizone phase into another 50-ml separatory
funnel. Reextract the aqueous phase with 10 ml of No.2 dithizone solution,
and add the dithizone extraction of the previous separation. Wash the
aqueous phase with 5 ml of carbon tetrachloride, extract the carbon
tetrachloride, and combine it with the two dithizone extractions. Discard the
aqueous phase. Add 15 ml of 5% sodium hydroxide solution to the com-
bined dithizone extractions, shake the mixture vigorously for 1 minute,
extract the carbon tetrachloride phase, and determine its absorbance at 620
mp in a l-cm cell with a spectrophotometer.

Calculations Run a blank and make appropriate corrections, using a cali-
bration curve prepared by using aliquots of the standard cadmium solution
containing 1-7 pg of cadmium:
pg Cd (from curve)
ml sample = mg/l Cd+2
Phosphate

Only orthophosphate will respond to the test. Polyphosphates must be
reverted to orthophosphates by boiling with acid (American Petroleum
Institute, 1968).

Interferences. Color development in the test is inhibited when the dissolved
solids content of the sample is greater than 8% (a specific gravity greater
than 1.06) or when the total iron is greater than 50 mg/l. In such cases the
sample taken for analysis must be diluted with distilled water so that these
limits are not exceeded. Sulfide interferes by giving high results, and should
be destroyed by adding potassium permanganate solution to the acidified
sample.

Reagents. Hydrochloric acid, concentrated.
Reagent No. 1 : dissolve 46 g of ammonium molybdate [ (NH), )a Mo, 02,
4H2 01 in 700 ml of distilled water. The ammonium molybdate used should
consist of white crystals without a bluish-green tinge. Add 2.5 ml of concen-
trated ammonium hydroxide to the solution and dilute to 1 liter with distil-
led water.
Amino solution: dissolve 10 g (about 1 level tablespoon) of amino powder
mixture in 100 ml of distilled water. If solution remains turbid, filter. Store
solution in a well-stoppered, brown glass bottle and prepare fresh at least
every 2 weeks.