Page 119 - Geochemistry of Oil Field Waters

COLORIMETRIC METHODS 107

where A = mg NaPO, (heated sample), and B = mg NaPO, (unheated sample).

Precision. The precision is about k3% of the amount present.

Silica

Silicon is the second most abundant element in the earth’s crust and is
found in most rocks as the oxide Si02 or as a silicate such as Mg,Si2O5
(OH), . The solubilities of silicate minerals in saline waters are a function of
temperature, pressure, pH, Eh, dissolved gases, and other ions in solution. A
limited amount of research has been done concerning silicate solubilities
(Collins, 1969) in saline solutions. Some investigators believe that most silica
exists in solution as H4Si04 (White et al., 1956); others that it exists both in
colloidal form and as H4Si04 (Krauskopf, 1956). Hydration of silica gives
the following reaction:

Si02 + 2H2 0 + Si(OH), or H4 Si04

A method developed by Schrink (1965) was used to study silicate solu-
bilities in saline waters (Collins, 1969) and it gave satisfactory results. It also
has been used to analyze some petroleum-associated waters. The method
involves adding 1 ml of a 4% ammonium molybdate solution in 0.75 molar
sulfuric acid solution to an appropriate aliquot of the water sample; add 15
ml of 4.5N sulfuric acid; extract for 1 minute with ethyl acetate; and deter-
mine the absorbance of the ester extract with a spectrophotometer at a
wavelength of 335 mp.

Nitrate nitrogen
Nitrate is the most highly oxidized form of nitrogen and is the most stable
form in an oxidizing environment. Many fertilizers contain nitrate, and
waters will leach the nitrate from soil or rock. Most rocks do not contain
much nitrate; therefore, it is unlikely that petroleum-associated waters con-
tain appreciable quantities of nitrate. The nitrate in deep waters also may be
depleted through anion exchange (George and Hastings, 1951).
Chloride is a serious interference in many of the methods used to deter-
mine nitrate nitrogen. Oxidizing or reducing agents such as ferric or ferrous
iron also interfere. The Brucine method (Fisher et al., 1958) can be applied
to a petroleum-associated water. To determine the nitrate concentration,
transfer an aliquot of the sample containing up to 15 pg of nitrate into a
50-ml Erlenmeyer flask, add 15 ml of water, 1 ml of Brucine reagent (2%