Page 65 - Geochemistry of Oil Field Waters

FLAME SPECTROPHOTOMETRIC METHODS 53

Determination of thiosulfate, sulfate, and sulfide

Procedure. Collect a water sample as described in the dissolved oxygen
procedure. Pipet 100 ml of the sample into a 300-ml flask, and add 20 ml of
glycerol, 100 ml of an aqueous suspension of zinc carbonate, and 70 ml of
distilled water. Shake the mixture vigorously for 1 minute, filter, and discard
the precipitate.
Pipet 50 ml of the filtrate into a flask and add 5 ml of formaldehyde, and
3 ml of acetic acid, add starch indicator and titrate to the blue endpoint with
0.01N iodine. Record the amount of iodine used to calculate thiosulfate (A).
Pipet another 50-ml aliquot of the filtrate into another flask; add 0.01N
iodine until the solution remains yellow. Add starch indicator and titrate to
a colorless endpoint with 0.Ol.N sodium thiosulfate. Record the amount of
iodine used for thiosulfate plus sulfite (B).
Pipet 25 ml of water that was not treated with the zinc carbonate into a
flask and add an excess of 0.Ol.N iodine, 3 ml of acetic acid, add starch
indicator and titrate to the colorless endpoint with 0.01N sodium thiosul-
fate, sulfite, and sulfide (C).

Calculations. Milliliters iodine used in A = X ml
X ml x N x 112,000
ml sample = mg/l S2 03-2
Milliliters iodine used in A - milliliters iodine used in B = Y ml
Y ml x N x 40,000 = mg/l SO,-2
ml sample
Milliliters iodine used in C - milliliters iodine used in B = 2 ml

2 ml x N x 16,000
ml sample = mg/l S-’

FLAME SPECTROPHOTOMETRIC METHODS

When a metal salt in solution is sprayed into a flame, the solvent evapo-
rates and the salt decomposes and vaporizes, producing atoms. Some of these
atoms can be raised to an excited state by the thermal energy of the flame,
although a major portion of the atoms present in the flame remain at the
grourid state. The return of the excited atoms to the ground state results in
the emission of radiant energy characteristic of the element atomized. The
quantitative measurement of this radiation is the basis of emission flame
spectrophotometry, and the essential difference between this form of
analysis and classical arc-emission spectrography is the temperature of the
source used to excite the atoms. Because the gasair and gas-oxygen flames