Chemistry Reference
In-Depth Information
COOH
HOOC
N
N
COO
-
Na
+
Na
+
-
OOC
Figure 6.5
The structure of disodium edetate.
The concentration of metal ions in electrolyte preparations is often
stated in millimoles per litre or sometimes millimoles per mL, where a
millimole is simply one thousandth of a mole. This means that the method
of deriving the equivalent relationship needs to be altered slightly from that
previously stated. Using calcium ions as an example:
1 mole Ca
2
ions
1 mole disodium edetate
1 mole Ca
2
ions
1000 mL 1
M
disodium edetate
1 millimole Ca
2
ions
1 mL 1
M
disodium edetate
0.02 millimole Ca
2
ions
1 mL 0.02
M
disodium edetate
This implies that for every 1 mL of titrant added from the burette, 0.02
millimoles of calcium will be complexed. The relationship is called a
millimolar equivalent
.
Older readers may remember the use of milliequivalents per litre as a
means of describing electrolyte concentrations. Derivation of milliequiva-
lents relies on calculation of the
equivalent weight
of the sample. For metal
ions, the equivalent weight is found by dividing the relative atomic mass of
the ion in question by its valency. In the case of monovalent ions such as
Na
and K
this is straightforward, since the relative atomic mass and the
equivalent weight are the same. For divalent ions such as Ca
2
and Mg
2
the
equivalent weight is half the relative atomic mass, while for trivalent ions
(e.g. Al
3
) the equivalent weight is a third of the relative atomic mass. The
use of equivalent weights was discarded in pharmacy some years ago
but, unfortunately, some physicians still prescribe injections and infusion
solutions in terms of milliequivalents of ion per litre.
The indicators used in compleximetric titrations are usually them-
selves complexing agents, which form weak complexes with the metal ion
when added initially. As the edetate solution is titrated, the weak complex
is displaced by the stronger edetate complex to reveal the free colour of the
indicator. The most commonly used indicator is known by the sinister name
of
mordant black
. This indicator forms wine-red complexes with metal
ions, but changes to a dark blue colour at the end point when the edetate
has displaced all of the metal ions from the indicator complex.
