A solution containing both Fe
3+
and Al
3+
can be selectively analyzed for Fe
3+
by
buffering to a pH of 2 and titrating with EDTA. Th
e pH of the solution is then raised
to 5 and an excess of EDTA added, resulting in the
formation of the Al
3+
- EDTA
complex. The excess EDTA is then back-titrated usi
ng a standard solution of Fe
3+
,
providing an indirect analysis for Al
3+
.
a.
Show that at a pH of 2, the formation of t
he Fe
3+
- EDTA complex is
favorable, while the formation of the Al
3+
- EDTA complex is not.
The percent (% w/w) water in a freeze-dried antibiotic is determined
in the following manner. A 0.175-g sample is placed in a vial along with 4.489
g of methanol. Water in the sample extracts into the methanol, which is then
analyzed. If GC analysis of the methanol yields a peak height of 13.66, what is
the % w/w water in the antibiotic?
To show that at a pH of 2, the formation of the Fe3+-EDTA complex is favorable, while the formation of the Al3+-EDTA complex is not, we need to consider the reactions involved and the stability constants of the complexes.
The reaction between Fe3+ and EDTA can be represented as follows:
Fe3+ + EDTA ⇌ Fe(EDTA)
The equilibrium constant for this reaction is represented by K1, which is the stability constant for the Fe(EDTA) complex. At a pH of 2, the concentration of H+ ions is high, causing the solution to be acidic. The presence of H+ ions promotes the formation of the Fe(EDTA) complex, making the reaction favorable.
On the other hand, the reaction between Al3+ and EDTA can be represented as follows:
Al3+ + EDTA ⇌ Al(EDTA)
The equilibrium constant for this reaction is represented by K2, which is the stability constant for the Al(EDTA) complex. However, at a pH of 2, the concentration of H+ ions is too high, causing the solution to be extremely acidic. In such highly acidic conditions, Al3+ forms hydroxide complexes, such as Al(OH)3, rather than forming stable complexes with EDTA. Therefore, the formation of the Al(EDTA) complex is not favorable at pH 2.
In summary, at a pH of 2, the formation of the Fe3+-EDTA complex is favorable due to the presence of H+ ions, while the formation of the Al3+-EDTA complex is not favorable due to the high acidity causing the formation of hydroxide complexes.