British Journal of Anaesthesia 1996;77:534–536
Extraction of nitric oxide and nitrogen dioxide from an oxygen carrier
using molecular sieve 5A
B. B. POULTON, L. FOUBERT, J. KLINOWSKI, R. D. LATIMER, P. R. KNOWLES AND A. VUYLSTEKE
Summary
Nitric oxide (NO) is effective in the management
of pulmonary hypertension and shunt-related
hypoxia. Nitrogen dioxide (NO2) is formed when
the gas is delivered with oxygen. Both oxides of
nitrogen have well recognized adverse effects.
The scavenging properties of several forms of
soda lime have been investigated. A gas flow
containing NO 70 ppm and NO2 5 ppm in oxygen
was
introduced
into
a
vertically
mounted
Waters’ canister containing: (i) 125 g of molecular sieve 5A (a calcium aluminosilicate zeolite)
and (ii) 135 g of soda lime containing a
potassium permanganate marker. NO and NO2
concentrations were measured at hourly intervals at the entry and exit points using an electrochemical analyser. Extraction ratios (gradient/
inlet 100) were calculated for a 24-h period.
High extraction ratios (in excess of 90%) of NO
and NO2 were observed with both compounds
for up to 1 h but these declined rapidly after this
time with soda lime. In contrast, the molecular
sieve produced extraction ratios in excess of
98% for both gases over the 24-h period. We
conclude that the molecular sieve 5A is a highly
effective scavenger of NO and NO2. (Br. J.
Anaesth. 1996;77:534–536)
Key words
Equipment, scavenging devices. Gases non-anaesthetic,
nitric oxide. Gases non-anaesthetic, nitrogen dioxide.
Methods
A vertically mounted Waters’ canister (id 6.5 cm,
length 10 cm) was filled with: (1) molecular sieve 5A,
pellet size 1.5 mm, mass 125 g (BDH Chemicals,
Poole, UK) or (2) soda lime, green-to-brown, 8–12
mesh, mass 135 g (Sofnolime, Molecular Products,
Thaxted, Essex, UK). As the intra-crystalline space
of the zeolite is normally filled with water, before use
the material was dehydrated by heating at 400 ⬚C for
24 h. A gas flow containing NO 70 ppm and NO2
5 ppm in oxygen was introduced at a flow rate of
10 litre min91. A calibrated Bedfont EC 90 electrochemical analyser (Bedfont Scientific Limited, Upchurch, Kent, UK) was used to assess concentrations
of NO and NO2 at the entry and exit points.
Measurements were recorded hourly for 24 h with
each absorber. Calibration of the electrochemical
analyser was verified on completion of each set of
readings. Gas pressure at the entry and exit points
was recorded for each compound; the gradient
remained at less than 1.5 cm H2O throughout.
Extraction ratios of NO and NO2 for each compound
were calculated for the 24-h period.
Results
High extraction ratios (in excess of 90%) of NO and
NO2 were observed with both compounds for up to
1 h. Subsequent observations revealed a rapid
decline in the soda lime extraction of both NO and
NO2. This was more marked for NO. In contrast, the
molecular sieve gave extraction ratios in excess of
98% for both gases over the 24-h period. Complete
extraction of nitrogen dioxide was observed at 24 h
(figs 1, 2).
Discussion
Therapeutic administration of nitric oxide (NO) is
complicated by the formation of nitrogen dioxide
(NO2). Extraction of NO and NO2 by several forms
of soda lime has been investigated but these failed to
demonstrate high extraction ratios for both oxides of
nitrogen over prolonged periods under conditions
which are relevant to the clinical situation (i.e. with
an oxygen-rich carrier gas and an appropriate
NO:NO2 ratio)1–4.
We have investigated the extraction properties of
zeolite Ca-A (known commercially as molecular
sieve 5A because of its 5 Å channel aperture) under
such conditions. A comparison was made with soda
lime containing a potassium permanganate indicator
which was shown by Pickett and colleagues to have
higher extraction ratios than other forms of soda
lime1 2.
Zeolitic molecular sieves are aluminosilicates built
from corner-sharing SiO4 and AlO4 tetrahedra
linked by the apical oxygen atoms to form frameworks of high internal surface area with regular
channels and cavities of molecular dimensions5–7.
The channel systems, which may be one-, two- or
three-dimensional, may occupy more than 50% of
B. B. POULTON, FRCA, R. D. LATIMER, FRCA, P. R. KNOWLES,
FRCA, A. VUYLSTEKE, MD, Department of Anaesthesia, Papworth Hospital, Papworth Everard, Cambridge, CB3 8RE.
L. FOUBERT, MD, Department of Anaesthesia, University
Hospital Gent, Belgium. J. KLINOWSKI, PHD, Department of
Chemistry, University of Cambridge, Lensfield Road,
Cambridge CB2 1EW. Accepted for publication: June 17,
1996.
Correspondence to B. B. P.
Extraction of NO and NO2
Figure 1 Extraction ratio of nitric oxide from an oxygen carrier
using molecular sieve 5 (!) and soda lime (").
Figure 2 Extraction ratio of nitrogen dioxide from an oxygen
carrier using molecular sieve 5 (!) and soda lime (").
crystal volume. Their microporous structure enables
zeolites to be used as molecular sieves as they can
only adsorb molecules of certain size. The net negative charge of the framework, equal to the number of
the constituent aluminium atoms, is balanced by
exchangeable cations, Mn;, located in the channels
which normally also contain water. The general oxide
formula of zeolite 5A is:
Ca 6 [Al12 Si12O 48 ]× 27 H 2O
There are approximately 40 identified zeolite minerals and at least 140 synthetic species with a wide
range of compositions. The most important properties of zeolites are their ability to adsorb organic and
inorganic substances and to act as cation exchangers
and catalysts. Depending on pore diameter and
molecular dimensions, species such as gaseous
elements, ammonia, alkali metal vapours, hydrocarbons and many other organic and inorganic species
may be accommodated in the intra-crystalline space
of dehydrated zeolites. This process, known as
molecular sieving, is a powerful method for the resolution of mixtures. Commercial applications include
the drying of organics, separation of hydrocarbons
and of N2 and O2 in air, and removal of NH3 and
CS2 from industrial gases. Cations neutralizing the
electrical charge of the aluminosilicate framework
can be exchanged for other cations from solution.
Zeolites often possess selectivities for certain cations,
and this is used for their isolation and concentration.
Molecular sieving properties of zeolites are further
modified by ion exchange. Thus sodium aluminosilicate zeolite absorbs both N2 and O2 while calcium
aluminosilicate zeolite (molecular sieve 5A) absorbs
nitrogen preferentially to oxygen. We have demonstrated slightly preferential uptake for NO2 over NO
with 5A. Uptake of both oxides of nitrogen by 5A is
marginally reduced when nitrogen is used as the
carrier gas (data not shown), but more efficient
535
extraction from an oxygen-rich carrier is desirable if
the compound is to be used as a scavenger. A reduction in efficiency of uptake can also be expected with
the introduction of carbon dioxide and water to the
carrier gas. Water vapour is a particular problem as it
reacts with nitrogen dioxide to form nitric acid which
leads to gradual degradation of the sieve. This problem has been encountered in the commercial
production of nitric acid which involves the catalysed
oxidation of ammonia. Degradation can be delayed
substantially by the interposition of a bed of silica gel
(US patent specification 3674429). Other properties
which make the molecular sieve 5A suitable for use as
a scavenger are its low cost and lack of toxicity6.
Green-to-brown soda lime uptake of NO and NO2
for the first hour was similar to that reported by
Pickett and colleagues who showed green-to-brown
Sofnolime to have a higher extraction ratio for NO
than that of pink-to-white or violet-to-white Sofnolime2. Using a ratio of 8.6 : 1 of NO : NO2, Ishibe and
colleagues reported an extraction ratio of 11.9% NO
for Sodasorb lime3.
Uptake of NO appears to depend on the presence
of NO2 (indirectly proportional to the NO : NO2
ratio). This observation is consistent with the
reactions suggested by Ishibe and colleagues4 and
Kain8:
2NO;2NO2;2NaOH;Ca(OH)2!
2NaNO2;Ca(NO2)2;2H2O
4NO2;2NaOH;Ca(OH)2!
2NaNO2;Ca(NO3)2;2H2O
The higher NO extraction ratio observed with soda
lime containing a potassium permanganate marker is
explained by the powerful oxidizing properties of this
material. The reaction proceeds as follows9:
NO;KMnO4!KNO3;MnO2
The contribution of potassium permanganate to
NO uptake is supported by our observation that the
rapid reduction in NO extraction coincided with the
colour change from green to brown of the soda lime.
Prolonged efficiency of any form of soda lime as a
scavenger of both oxides of nitrogen has yet to be
demonstrated. Ishibe and colleagues reported a 5%
breakout time of 30 days for NO2 using high purity
nitrogen as the carrier gas and a flow rate of 1 litre
min91.
We conclude that the molecular sieve 5A was a
highly effective scavenger of NO and NO2. We have
demonstrated this property with a clinically relevant
gas composition over a 24-h period. Advantages over
soda lime include lower cost and greater efficiency.
References
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