WPI APOLLO 4000, Instruction Manual

Page: 37 / 74

APOLLO 4000
NITRIC OXIDE DETECTION
WORLD PRECISION INSTRUMENTS C-13
Predicting the level of detectable NO according to the
molar ratio of SNAP in the presence of catalyst
(Method II)
Experiments have shown that SNAP is decomposed instantaneously under the
following set of experimental conditions:
Temperature 25
°
C
Catalyst solution 0.1M copper sulfate
SNAP WPI, 98% purity. Fresh stock solution with
5 mg/250 mL solution EDTA added.
Copper sulfate is at equilibrium with ambient air (aerobic conditions).
SNAP (RSNO) decomposes to NO and a disulfide byproduct according to the
following equation:
2RSNO
→
2NO + RS-SR
Theoretically, the concentration of generated NO should be equal to the final
concentration of SNAP in the copper sulfate solution in the calibration vial if the
decomposition goes to completion and if the generated NO is detected quickly
before it is oxidized to nitrite and nitrate.
However, it is expected that the level of detectable NO will be below the theoretical
value because the copper sulfate solution was at equilibrium with ambient air, and
consequently a portion of the generated NO would have been immediately oxidized
to nitrite and nitrate before it was measured by the NO sensor. In addition, it is
possible that decomposition of SNAP does not go to completion even in the
presence of a catalyst. Results on the kinetics of SNAP decomposition in the
presence of a catalyst in an anaerobic environment are published elsewhere (Zhang
et al., “Novel Calibration Method for Nitric Oxide Microsensors by Stoichiometrical
Generation of Nitric Oxide from SNAP”,
Electroanalysis, 2000, 12 : 6).
Our experimental data indicates a conversion efficiency of SNAP to NO of
approximately 0.6 ( i.e., 60%). This result is only applicable for calibration of a NO
sensor in a solution, which is at equilibrium with ambient air and at the
experimental conditions described above. Hence for each mole of SNAP, 0.6 mole
of NO is liberated under the proposed set of parameters. It is assumed the other
40% of SNAP is either not decomposed or a proportion that is decomposed to NO