Page 125 - Geochemistry of Oil Field Waters

COLORIMETRIC METHODS 113

pulled through the precipitate until the entire filtration and washing process
is complete.
Transfer the filter and precipitate back to the 150-ml beaker and add 5 ml
of a 1:l mixture of hydrochloric acid and nitric acid. Heat the mixture to
near boiling for a few minutes, taking care not to let the mixture boil
violently or to dryness. Examine the mixture carefully to make sure that all
of the selenium has dissolved. Place the beaker containing the mixture in a
vacuum desiccator over anhydrous magnesium perchlorate and sodium
hydroxide and let the mixture evaporate to dryness. Dissolve the residue in 5
ml of concentrated hydrochloric acid and heat the mixture to near boiling
for a few minutes.
Cool the mixture, add 20 ml of water, and filter it through Whatman No.4
filter paper into a 100-ml volumetric flask. Adjust the volume to 100 ml and
pipet an aliquot containing 1-100 pg of selenium (IV) from the 100-ml flask
into a 100-ml beaker. Add 5 ml of 0.1M EDTA and 2 ml of 2.5M formic
acid and adjust the pH to 1.5 with hydrochloric acid. Adjust the volume to
about 50 ml with water, add 2 ml of 3,3'-diaminobenzidine solution, mix,
and let stand for 30 minutes. Adjust the pH to 8 and transfer the solution to
a 1254 Teflon-stoppered separatory funnel containing 10.0 ml of toluene.
Shake this mixture vigorously for 2 minutes and let the phases separate.
Extract the toluene phase, which now contains the monopiazselenol, into a
centrifuge tube. Centrifuge briefly to clear the toluene of water droplets. If a
centrifuge is not available, the organic phase can be filtered through a dry
filter paper to which has been added 100 mg of anhydrous sodium sulfate.
Determine the absorbance of the toluene phase at 420 mp versus a reagent
blank. 1-cm cells are used; however, longer path length cells will increase
sensitivity.

Calculation. Prepare a calibration curve by plotting log I,JI, which is the
extinction or optical density of the solution versus the concentration, using
solutions containing known amounts of selenium and treated as previously
described. Estimate the amount of selenium from this curve, and calculate as
follows:

'g Se = mg/l Se-*
ml sample

Semiquantitative determination of selenium

Pipet a 20- to 50-ml aliquot of brine into a 100-ml volumetric flask and
dilute to volume with concentrated hydrochloric acid. (To increase the
detection limit, the brine can first be concentrated by careful evaporation
after acidifying it to pH 2 with hydrochloric acid.) Mix the solution and
allow it to stand until most of the sodium chloride precipitates.
Withdraw an aliquot of the supernatant clear liquor into a small beaker,