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Chapter 2 Basic Tools of Analytical Chemistry 23

of the protons are capable of reacting with the chosen base. In the reaction be-
tween H 3 PO 4 and NH 3
+
–
H 3 PO 4 (aq) +2NH 3 (aq) t HPO 4 (aq) +2NH 4 (aq)
a conservation of protons requires that
2 ´moles H 3 PO 4 =moles of NH 3

2C. 4 Conservation of Electron Pairs

In a complexation reaction, the reaction unit is an electron pair. For the metal, the
number of reaction units is the number of coordination sites available for binding
ligands. For the ligand, the number of reaction units is equivalent to the number of
electron pairs that can be donated to the metal. One of the most important analyti-
cal complexation reactions is that between the ligand ethylenediaminetetracetic acid
(EDTA), which can donate 6 electron pairs and 6 coordinate metal ions, such as
2+
Cu ; thus
2+
6 ´mole Cu = 6 ´moles EDTA
2C. 5 Conservation of Electrons
In a redox reaction, the reaction unit is an electron transferred from a reducing
agent to an oxidizing agent. The number of reaction units for a reducing agent is
equal to the number of electrons released during its oxidation. For an oxidizing
agent, the number of reaction units is given by the number of electrons needed to
cause its reduction. In the reaction between Fe 3+ and oxalic acid (reaction 2.2), for
3+
example, Fe undergoes a 1-electron reduction. Each carbon atom in oxalic acid is
initially present in a +3 oxidation state, whereas the carbon atom in CO 2 is in a +4
oxidation state. Thus, we can write
3+
1 ´moles Fe =2 ´moles of H 2 C 2 O 4
Note that the moles of oxalic acid are multiplied by 2 since there are two carbon
atoms, each of which undergoes a 1-electron oxidation.

2C.6 Using Conservation Principles in Stoichiometry Problems
As shown in the following examples, the application of conservation principles sim-
plifies stoichiometric calculations.

EXAMPLE 2. 7

Rework Example 2.6 using conservation principles.
SOLUTION
Conservation of electrons for this redox reaction requires that

3+
moles Fe =2 ´moles H 2 C 2 O 4
which can be transformed by writing moles as the product of molarity and
volume or as grams per formula weight.

2 ´ g H 2 C O 4
2
M Fe 3+ ´V Fe 3 + =
FW H 2 C O 4
2