Cliffs AP Chemistry, 3rd Edition

Scenario:A student prepared ASA starting with 2.00 g of salicylic acid and 5.00 mL of acetic

anhydride (density = 1.08 g ⋅mL–1). After the product was dried, it weighed 1.90 grams. The

student then hydrolyzed the ASA with sodium hydroxide and heated the mixture to produce the
salicylate dianion:

Next, the student then acidified the mixture with a FeCl 3 −KCl−HCl solution to produce the ma-
genta complex of tetraaquosalicylatoiron (III) ion:

Since the ratio of ASA to the complex ion produced is 1:1, the concentration of the complex
ion as determined through colorimetry is the same concentration as that of the ASA. The com-
plex ion is sensitive to pH, therefore care was taken to keep the pH in the range of 0.5−2.0 to
avoid formation of di- and trisalicylate complexes of iron (III).

Before the concentration could be determined through colorimetry, the student needed to know
the wavelength of light that was most absorbed by the complex ion in order to set the spec-
trophotometer properly. The student calibrated the spectrophotometer by setting the transmit-
tance to 100% with the FeCl 3 −KCl−HCl solution as a reference. The optimal wavelength was
found to be 525 nm (see Figure 1).

O

[Fe(H 2 O) 6 ]^3 +

OH

COO−

Fe(H 2 O) 4

O

+2 H 2 O

H

O

C

2 +

+H 3 O+

O−

COO−

+H 2 O

OH

COO−

+3 OH−

OCOCH 3

COOH

+2 H 2 O+CH 3 COO−

O−

COO−

Laboratory Experiments