Page 52 - Geochemistry of Oil Field Waters

40 ANALYSIS OF OILFIELD WATERS

Calcium and magnesium

Probably the most common method currently used to determine calcium
and magnesium in waters is the complexometric method (American Petro-
leum Institute, 1968) which utilizes a salt such as disodium ethylenediamine-
tetraacetic acid (EDTA) or disodium 1,2-cyclohexanediaminetetraacetic
acid (CDTA) to chelate calcium or magnesium. At a pH of 10, both calcium
and magnesium are chelated, while at a pH of 12, only calcium is chelated
because magnesium will precipitate as the hydroxide.
Disodium ethylenediaminetetraacetate has the following structure
(Welcher, 1957, p.128):

0 -CO - CHI CHI -CO-0
Na’
/ ,
Na’ ‘\
H N - (CH2 ) - N H
\ / /’
0 - CO - CH, “CH, - CO - 0
Its molecular weight is 372.254, and it forms 1:l complexes with most
cations according to the following equations:
Me+’ + H2Y-’ * MeY-’ + 2H+
Me+3 + H2 Y-’ * MeY- + 2H+
Me+4 + H2 Y-’ * MeY + 2H+
where Me = the cation, H2 = EDTA, and MeY = the complex.
Y
Therefore, 1 gram-ion of EDTA reacts with 1 gram-ion of the metal,
regardless of its valence. The resulting complexes have the same composition,
differing only in the charge they carry.
A metal indicator in an EDTA titration can be represented by the follow-
ing expression :
M-In + EDTA * M-EDTA + In

where M-In = the metal indicator complex, M-EDTA = the metal-EDTA com-
plex, and In = the indicator. The metal indicator complex must be weaker
than the metal-EDTA complex. The color change occurs because the metal-
indicator complex ionizes, and the free metal is completely complexed by
the EDTA, leaving a free indicator.

Sample size

Because many petroleum-associated waters contain high concentrations of
dissolved solids including calcium and magnesium, it usually is necessary to
dilute them or to use a micropipet to obtain a small sample before
performing a complexometric titration. The dilution and subsequent aliquot