Page 118 - Geochemistry of Oil Field Waters

106 ANALYSIS OF OILFIELD WATERS

The amino powder mixture is made up by adding 5 g of sodium sulfite
and 1.0 g of l-amino-2-naphthol-4-sulfonic acid to a dry mortar. Grind the
materials to a fine powder. Transfer the powder to a large wide-mouthed
bottle containing 66.5 g of sodium bisulfite (meta, powder, Naz Sz 0, ) and
35 g of sodium sulfite. Mix well by shaking. If the mix is not uniform, it
should be passed through a 20-mesh screen and again shaken in the large
bottle. Store mixture in a well-stoppered, wide-mouthed brown bottle.
Standard phosphate solution: dissolve in distilled water 0.1335 g of potas-
sium dihydrogen phosphate (KHz PO4 ) which has been dried in an oven at
105°C. Dilute to 1 liter. 1 ml of this solution is equivalent to 0.1 mg sodium
metaphosphate (NaP03 ).

Procedure. Thoroughly shake a freshly drawn sample to disperse the solids
and pipet 100-ml aliquot into each of two 250-ml beakers. If the expected
concentration of sodium metaphosphate is greater than 10 mg/l, take smaller
aliquots diluted to 100 ml with distilled water.
Note: phosphate-free glassware must be used in this determination. The
glassware should be soaked in dilute hydrochloric acid, followed by rinsing
with distilled water.
Add 7 ml of concentrated hydrochloric acid to one of the samples. If it is
suspected to contain sulfide, stir the solution vigorously for a minute to
remove as much of the sulfide as possible, then add potassium permanganate
solution (8%) dropwise until the solution just turns pink. Boil solution
vigorously for 30 minutes while maintaining the volume between 75 and 100
ml by adding distilled water. Cool sample to a temperature between 70" and
95°F and dilute to 107 ml with distilled water in a graduated cylinder
bearing a mark at the 107-ml level.
Add 7 ml of concentrated hydrochloric acid to the unboiled sample and
treat with permanganate as above if sulfide is suspected. Filter both boiled
and unboiled samples if turbid.
Add 5 ml of reagent No.1 to both samples and mix well. Add 5 ml of
amino solution to both and again mix well.
Ten minutes after the amino solution addition, measure the color with a
spectrophotometer at a wavelength of 690 mp, after adjusting the meter to
100% transmittance with a proper blank.

Calibration curve. Prepare a calibration curve by using aliquots of the stan-
dard phosphate solution containing up to 10 mg/l of sodium metaphosphate.

Calculations. Refer the spectrophotometer readings to the calibration curve
(expressed as milligrams of NaP03 versus photometer reading) to obtain the
sodium metaphosphate concentration. The results on the heated sample cor-
respond to total phosphate, whereas, those on the unheated sample cor-
respond to orthophosphate, the difference being polyphosphate, usually ex-
pressed as sodium metaphosphate (NaP03 ):