Page 346 - Modern Analytical Chemistry

1400-CH09 9/9/99 2:13 PM Page 329

Chapter 9 Titrimetric Methods of Analysis 329

The principle of the conservation of electron pairs is easily extended to other com-
plexation reactions, as shown in the following example.

EXAMPLE .9
9
–
The concentration of Cl in a 100.0-mL sample of water drawn from a fresh
water acquifer suffering from encroachment of sea water, was determined by
titrating with 0.0516 M Hg(NO 3 ) 2 . The sample was acidified and titrated to the
diphenylcarbazone end point, requiring 6.18 mL of the titrant. Report the
–
concentration of Cl in parts per million.
SOLUTION
Conservation of electron pairs requires that
–
Moles Cl =2 ´moles Hg 2+
–
Making appropriate substitutions for the moles of Cl and Hg 2+
g Cl -
= 2 ´M Hg ´V Hg
AW Cl
and rearranging leaves us with
–
g Cl =2 ´M Hg ´V Hg ´AW Cl
Substituting known values and solving gives

2 ´0.0516 M ´0.00618 L ´35.453 g/mol = 0.0226 g Cl –
–
The concentration of Cl in parts per million, therefore, is
mg Cl - 22 6 mg
.
= =226 ppm
liter 0.1000 L

Finally, quantitative problems involving multiple analytes and back titrations also
can be solved by applying the principle of conservation of electron pairs.

9
EXAMPLE .10
An alloy of chromel containing Ni, Fe, and Cr was analyzed by a
complexation titration using EDTA as the titrant. A 0.7176-g sample of the
alloy was dissolved in HNO 3 and diluted to 250 mL in a volumetric flask. A
50.00-mL aliquot of the sample, treated with pyrophosphate to mask the Fe
and Cr, required 26.14 mL of 0.05831 M EDTA to reach the murexide end
point. A second 50.00-mL aliquot was treated with hexamethylenetetramine
to mask the Cr. Titrating with 0.05831 M EDTA required 35.43 mL to reach
the murexide end point. Finally, a third 50.00-mL aliquot was treated with
50.00 mL of 0.05831 M EDTA, and back titrated to the murexide end point
2+
with 6.21 mL of 0.06316 M Cu . Report the weight percents of Ni, Fe, and
Cr in the alloy.