1
EXPERIMENT 6
ASSAY FOR PHENOL
Phenol is an important industrial chemical. In 1865 this compound, then known as carbolic
acid, was first employed as a surgical antiseptic by Dr. Lister. (His name survives in the trade name
"Listerine" and many of his patients survived too.) Phenol has current medical application as a mild
disinfectant (for example in Chloroseptic, a sore throat spray) but its main use is in plastics
manufacture.
You will no doubt recall from your study of Organic Chemistry that the hydroxy substituent
strongly activates the phenyl ring and is an ortho-, para- director. Thus, phenol reacts quantitatively
with three mol Br2/mol phenol to form 2,4,6-tribromophenol.
Equation (1)
OH
OH
Br
Br
Br
+
3Br2
+
3HBr
This reaction may be used as a basis for phenol analysis. However, the use of
bromine in this determination presents some practical difficulties. Bromine is quite volatile even at
room temperatures so that standardization and storage of bromine solutions is impractical. Instead,
accurately known quantities of Br2 may be introduced by reaction of BrO3- (bromate) with Br- in
acid media.
Equation (2)
BrO3- + 5 Br- + 6 H+ = 3 Br2 + 3 H2O
Pure NaBrO3 and KBrO3 are readily available and their solutions may be accurately standardized
and are quite stable.
The present phenol analysis employs a measured quantity of BrO3- solution to generate an
excess of Br2 which reacts with phenol. The excess of Br2 is determined by a technique called
"iodimetry". After completion of the Br2, phenol reaction, excess I- is added to the mixture. This
reacts with any Br2 still present to form I2.
2
Equation (3)
Br2 + 2 I- = 2 Br- + I2
The quantity of I2 formed in this way is determined by titration with thiosulfate, S2O32-. The
endpoint is signaled by the disappearance of the blue-black starch-iodine complex.
Equation (4)
2 S2O32- + I2 = S4O62- + 2 I-