Page 54 - Geochemistry of Oil Field Waters

42 ANALYSIS OF OILFIELD WATERS

to 1 liter. (To standardize, dissolve 2.4971 g calcium carbonate in the smallest
amount of hydrochloric acid possible and dilute to 1 liter with water (0.025M
Ca solution). Pipet triplicate 10-ml aliquots into flasks and dilute to approxi-
mately 50 ml with water. Begin at paragraph 2 of the procedure, and carry
the standard solutions through all the steps of that paragraph. Calculate the
molarity using the following equation:
10.0 x 0.025 0.25
M= or -
V V
where V = volume of CDTA required to titrate 10 ml of 0.025M calcium
solution.
Sodium hydroxide: dissolve 320 g sodium hydroxide pellets in water and
dilute to 1 liter.
Ammonium chlorideammonium hydroxide buffer solution : dissolve
67.5 g ammonium chloride in approximately 200 ml of water. Add 570 ml
concentrated ammonium hydroxide and dilute to 1 liter with water.
Eriochrome Black T indicator (sodium 1-( l-hydroxy-2-naphthylaz0)-6-
nitro-2-naphtol-4-sulfonate): dissolve 0.5 g of the indicator and 4.5 g of
hydroxylamine hydrochloride in 100 ml of water.
Calcon: dissolve 0.4 g sodium 1-( 2-hydroxy-l-naphthylazo)-2-naphthol-4-
sulfonate in 100 ml of methanol.
Triethanolamine: dilute 30 ml triethanolamine to 100 ml with water.

Procedure. Filter the sample to remove undissolved solids and traces of oil
from the water. Transfer, by means of “Lambda” pipet or volumetric trans-
fer pipet, an aliquot of sample containing not more than 10 mg of calcium
into an Erlenmeyer flask. Dilute to approximately 50 ml with water. (No
more than 10 ml of standard CDTA are to be used in a titration for either
calcium or magnesium.)
Add two to three drops of triethanolamine solution and approximately 4
ml of sodium hydroxide solution. The pH of the solution at this point
should be 12.0-12.5. Add six drops of calcon indicator and titrate with
standard CDTA solution until the indicator blue endpoint is reached. Record
the volume of CDTA titrant used to titrate calcium.
Using the same pipet, pipet another aliquot into another Erlenmeyer flask
and dilute to 50 ml with water. Add two to three drops of triethanolamine
solution, 3-5 ml of the ammonium chloride ammonium hydroxide buffer
solution, and three to four drops of Eriochrome Black T indicator. Refill the
buret with the same standard CDTA titrant and titrate the sample until the
color changes from wine to pure blue. This endpoint is sometimes delayed,
so proceed cautiously with the titration near the endpoint. Record the
volume of CDTA used to titrate calcium and magnesium, or hardness (Ca +
Mg).