Biomedical Engineering Reference
In-Depth Information
the same way as the reactions occurring on the anode and cathode for the electrochemical
reactions. The coupled reactions are commonly used in describing metabolic pathways as
we will learn later in this topic.
The coupled reaction concept can be applied in searching for application of redox pairs,
for example. When we design a convenient way for chemical analysis, color changes of
certain substance pairs are usually explored as spectrophotometer can be brought in to
quantitatively detect the color change. Consider the (incomplete) redox reaction:
NO
2
NH
2
NO
2
HO
HO
NO
2
(3.116)
COOH
COOH
3,5-dinitrosalicylic acid
3-amino-5-nitrosalicylic acid
(yellow)
(orange-red)
that change one nitro group into one amino group. DNS (3,5-dinitrosalicylic acid) is an
oxidant and is yellow in aqueous solution. On the other hand, the reductant 3-amino-5-
nitrosalicylic acid is orange-red in aqueous solution. Examining the two substances, we
found that (1) the reaction is only complete if two O are removed and two H are added in
the above reaction; (2) the change in the degree of reduction:
Dg
DR
¼ g
DR 3,5-dinitrosalicylic acid
g
DR 3-amino-5-nitrosalicylic acid
¼
6, which represents an oxidation power of
six electrons. DNS is found to be able to oxidize the aldehyde end groups (R-CHO) to
corresponding carboxylic acid groups (R-COOH). Each sugar (as one can recall from
Chapter 2) molecule contains one aldehyde group. Therefore, DNS has been devised to esti-
mate sugar concentrations (R. Hu, L. Lin, T. Liu, P. Ouyang, B. He, S. Liu 2008 “Reducing
sugar content in hemicellulose hydrolysate by DNS method: A revist”, JBMBE, 2: 156
e
161).
For each molecule of aldehyde converted to carboxylic acid, there is only a net one
O consumed. The oxidation power needed for oxidizing sugar to carboxylic acid is two
electrons. Therefore, the DNS sugar pair works, by coupling the DNS (incomplete) redox
reaction with
2
g
DR, O
2
g
DR, H
¼
3
R
e
CHO
þ
H
O
/
3R
e
COOH
(3.117)
2
as H
2
O is abundant in an aqueous solution.
3.12. REACTOR MASS BALANCES
We need reaction rate expressions to insert into species mass balance equations for
a particular reactor. These are the equations from which we can obtain compositions
and other quantities that we need to describe a chemical process. In introductory chem-
istry courses, students are introduced to first-order irreversible reactions in the batch
reactor, and the impression is sometimes left that this is the only mass balance that is
important in chemical reactions. In practical situations, the mass balance becomes more
complicated.
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