Page 61 - Geochemistry of Oil Field Waters

TITRIMETRIC METHODS 49

Equipment. The necessary equipment includes glass-stoppered bottles,
pipets, flasks, and microburets.

Sa mp 1 ing
Care must be taken to obtain uncontaminated samples of water for deter-
mining dissolved gases. Glass-stoppered bottles should be used for sample
containers. To determine dissolved oxygen in water, 300-ml bottles with
pointed, ground-glass stoppers and overflow lips of the type used for bio-
chemical oxygen-demand tests are particularly suitable. These bottles are so
designed that samples may be obtained without contamination by atmo-
spheric oxygen and so the necessary chemical reagents may be introduced
during the analysis without excessive overflow from the lip of the bottle.
Before a sample is taken, rinse the bottle three times with the water to be
sampled and fill through a rubber tube extending to the bottom of the bottle.
A quantity of water equal to at least three times the capacity of the bottle
should be allowed to overflow the bottle, and the rubber tube should be
withdrawn slowly so that the space in the bottle occupied by the tube is
filled simultaneously with water. The glass stopper, when placed in the
mouth of the bottle, will displace all excess water. If any bubbles are seen,
the sample is immediately analyzed. If the temperature of the water taken
for analysis of dissolved gases is above 2OoC, a cooling coil should be used to
cool the sample before the water enters the bottle. It is important that the
samples contain no included atmospheric oxygen or carbon dioxide, as errors
may be introduced into many of the analyses if extraneous oxygen or carbon
dioxide is present in the water.

Procedure. All reagents in the following steps 1 through 8 should be added
slowly and carefully under the surface of the water near the bottom of the
bottle, using pipets, permitting the displaced water to overflow the top of
the bottle. The quantities of reagents added should be recorded for use in
the final calculation. After each reagent is added, the stopper should be
carefully replaced and the bottle inverted gently several times so as not to
introduce air into the bottle while adding and mixing reagents.
Collect the sample as described previously. Add excess 0.5N iodine solu-
tion to give the sample a yellow color and let stand 5 minutes. Add saturated
hydrogen sulfide water until the sample is a very light straw-yellow, and 1 ml
of starch solution as an indicator. Add dilute hydrogen sulfide water until
the blue color just disappears and then add, drop by drop, 0.1N iodine
solution until a faint blue color persists. Add 1 ml of manganous sulfate
solution, 1 ml of alkaline iodide solution, and 1 ml of concentrated sulfuric
acid, letting it run down the neck of the bottle.
Transfer 200 ml of the solution by pipet from the sample bottle to a
500-ml Erlenmeyer flask. Titrate the 200-ml sample in the Erlenmeyer flask
with 0.025N sodium thiosulfate solution. The starch indicator should be