Page 56 - Geochemistry of Oil Field Waters

44 ANALYSIS OF OILFIELD WATERS

Using an expanded-scale pH meter, adjust the pH of the sample to 7.0
with 0.1N NaOH, add 5 ml of 37 wt.% formaldehyde solution, and heat the
mixture to 4OoC. (Do not exceed this temperature.) Cool immediately to
ambient temperature using an ice water bath, and titrate the sample with
0.02N NaOH to pH 8.6 using an expanded-scale pH meter to detect the
endpoint .
The weak hydroxide titrant must be protected from atmospheric carbon
dioxide, and a reagent blank must be determined because formaldehyde
contains formic acid.
The reactions are :
6HCHO + 4NH4 C1 --f (CH2 )6 N4 + 4HC1+ 6H2 0
HC1+ NaOH + NaCl + H2 0

Calculation:

(ml NaOH x N used for sample) -
(ml NaOH x N used for reagent blank) x 14,007
ml sample = mg/l NH4N

Chloride

A modification of the Mohr method (Furman, 1962) is satisfactory for
the determination of chloride in petroleum-associated waters. Common
interferences are bromide, iodide, sulfide, and iron. Sulfide can be removed
by acidifying the sample with nitric acid and boiling. Iron can be removed by
ion exchange or precipitation with sodium hydroxide or sodium peroxide
followed by filtration.
Because most petroleum-associated waters contain high concentrations of
chloride, it usually is necessary to dilute the sample before titrating with
silver nitrate, because the voluminous precipitate masks the endpoint. About
50 mg of chloride is maximum for a satisfactory titration. The indicator
usually is potassium chromate or sodium chromate, and at the endpoint the
chromate ion combines with excess silver to form the slightly soluble red
silver chromate:
Ag+ + C1- + AgCl
ZAg+ + --f Ag2Cr04

The specific gravity of the sample can be used to estimate the correct
aliquot size. Table 3.X indicates aliquot sizes that will contain less than 50
mg of chloride. The micropipet can be used as demonstrated in the
calcium-magnesium procedure and Table 3.IX.