Determination of trace levels of nitric oxide in aqueous solution

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Anal. Chem. 1980, 52, 1662-1667

deposited films. T h e assistance of Lorraine Siperko in this work is hereby gratefully acknowledged.

LITERATURE CITED Wagner, C. D. Anal. Chem. 44, 1050 (1972). Swingle, R. S.Anal. Chem. 47, 21 (1975). Berthou, H.; Jorgensen, C. K. Anal. Chem. 47, 482 (1975). Wagner, C. D. Anal. Chem. 49, 1282 (1977). Carter, W. J.; Schweitzer. G. K.; Carlson. T. A. J . Electron Spectrosc. Relat. Phenom. 5 , 827 (1974). (6) Evans, S.; PrRchard, R. G.; Thomas, J. M. J . Electron. Spectrosc. Rebt. Phenom. 14. 341 11978). (7) Powell, C. J..Appl.‘Surf: Sci. 1, 186 (1978). (8) Powell, C. J . ASTM STP643, 5 (1978). (9) Wagner, C. D.; ASTM STP643, 31 (1978) (IO) Salvati, L.; Carter. W. J.; Hercules, D. M. ASTM STP 643 47 1978). (11) Nefedov, V. I.; Sergushin, N. P.; Band, J. M.: Trzhaskovskaya, M. B. J . Electron Spectrosc. Relat. Phenom. 2 , 383 (1973). (12) Scofield, J. H. J. .Electron. Spectrosc. Relat. Phenom. 8 , 129 1976). (13) Penn, D. R. J . .Electron. Spectrosc. Relat. Phenom. 9. 29 1976). (14) Reilman, R. F.; Msezane, A,; Manson. S. T. J . Electron. Spc trosc . Relat. Phenom. 8 , 389 (1976). (15) Palmberg, P. W. J . Vac. Sci. Techno/. 12, 379 (1975). (16) Brillson, L. J.; Ceasar, G. P. Surf. Sci. 58, 457 (1976). (17) Dreiling. M. J. Surf. Sci. 71, 231 (1978). (18) Rlzzo. F. E.: Smith. J. V. J . Phvs. Chem. 72. 485 11968). (19) Rizzo; F. E.;’Gordon, R. S.; Cutler, I.B. J . Electrochem. S i c . 116, 266 (1969).

Anal. Chem. 1980.52:1662-1667. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 09/02/15. For personal use only.

(1) (2) (3) (4) (5)

(20) Stout, D.A.; Gavelli, G.; Lumsden, J. B.; Staehle, R. W. Surf. Sci. 69, 741 (1977). (2 1) Barin, I.; Knacke, 0. “Thermochemical Properties of Inorganic Substances”, Springer-Verlag: Berlin, 1973. (22) Davis., L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G. E.; Weber, R. E. “Handbook of Auger Electron Spectroscopy”; Physical Electronics Industries, Inc.: Eden Prairie, Minn., 1972. (23) Rosencwaig, A.; Wertheim, G. K. J . Electron Spectrosc. Rebt. Phenom. 1, 493 (1972-1973). (24) Larson, P. E. J . Electron Spectrosc. Relat. Phenorn. 4, 213 (1974). (25) Schon, G. Surf. Sci. 35, 96 (1973). (26) McIntyre, N. S.; Cook, M. G. Anal. Chem. 47, 2208 (1975). (27) Schon. G. J . Ekctron Spectrosc. Rebt. Phenom. 1, 377 (1972-1973). (28) Yin, L.; Tsang, T.; Adler, I. J . Electron Spectrosc. Relat. Phenom. 9, A _ 7. I(1976)

(29) Andrews, P. T.; Weightman. P. J . Electron Spectrosc. Relat. Phenorn. 15, 133 (1979). (30) Roberts, E. D.; Weightman, P.; Johnson, C. E. J . Phys. C., Solidstate Phvs. 6. L301 (1975). (31) Anionides, E.; Janse.’E. C.; Sawatzky, G. A. Phys. Rev. 8 . 15, 1689, 4596 (1977). (32) McGuire, E. J. Phys. Rev. 6.17, 182 (1978).

RECEIVED for review March 7 , 1980. Accepted May 27, 1980. The authors gratefully acknowledge support of this work by the National Science Foundation.

Determination of Trace Levels of Nitric Oxide in Aqueous Solution Oliver C. Zafiriou” Department of Chemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543

Mack McFarland NOAAIERL Aeronomy Laboratory, Boulder, Colorado 80303

Nitric oxide at trace levels M) in aqueous solution is determlned by using a flow system to equilibrate the solution with a gas stream and measurlng NO, with a chemiiuminescence detector. A standard pNoprepared by dynamic dilution of gases is used to calibrate the detector and to test the system for adsorptlve and artifact effects. NO signals are differentiated from interferents by criteria based on the gas/solution partition coefficient of NO, its low boiling point, or reverslble formatlon of the Fe(N0)” complex. The precision of the technique at the M [NO], level is --f3% and the accuracy is estimated to be f20%; a determination requires about 2 min. The versatility of the method and its appllcability to environmental measurements are illustrated by relevant examples.

available, few data exist concerning its occurrence and behavior in the environment. Nitric oxide reacts with oxygen both in the gas-phase (7) and in solution (8) with rates proportional to [N0I2,so that concentration steps in the presence of oxygen prior to analysis lead to losses due to NO2 formation. In this paper, we describe a rapid, sensitive, and precise approach to determining traces (to M) of NO in aqueous media, including oxygenated solutions. Specificity criteria for confirming that NO causes the signal are presented. The basic approach involves equilibrating the highly insoluble NO,, in solution with a flowing gas stream (“stripping”) and measuring NO, with a sensitive, stable chemiluminescence detector. Preliminary applications to studies of trace NO, under conditions of interest in marine and natural water chemistry illustrate the versatility of the approach.

Nitric oxide (NO) is a chemically unusual and environmentally significant radical, stable to self-reaction but an excellent free radical trap (1). In the atmosphere, it is a well-known pollutant (2)and an important constituent of clean air (3, 4). Very recently it has become clear that aqueous solutions of NO may also be environmentally significant. Photochemically generated NO has been detected in seawater (5) and presumably forms in other natural waters also. Biogeochemical processes also produce NO, which is an intermediate in some denitrifications (6). However, since suitable methods for determining traces of NO,, have not been

EXPERIMENTAL Apparatus. The basic equipment consists of (A) a distribution system to provide and route gas mixtures of various compositions, including known trace NO mixtures for calibrating the detector and preparing aqueous sofutions of known NO vapor pressure, pN0,by equilibration, (B) a chamber to contain the sample and permit removal of NO,, by intimately contacting the gas and sample phases (“stripping”),and (C) a chemiluminescence NO detector. Figure 1 shows a versatile configuration for the principA components. A 300-W Xe arc UV light source and a calibrated broadband UV radiometer have been added for photochemical studies. The gas distribution system provides constant flow to the detector with mass flow controllers, yielding constant sensitivity

0003-2700/80/0352-1662$01 .OO/O

62 1980 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980

to allow NO-03 chemiluminescence to proceed to completion. In this mode, only slower-decaying gaseous emissions and surface reaction luminescence from the chamber (which is undiminished, as ozone is in great excess) are detected. In some experiments, elements not shown in Figure 1 were inserted between the stripper and the needle valve. A stainless steel 47-mm in-line filterholder with 3 - ~ m Fluoropore filter was used to remove aerosols. A 30-cm by 1.2-cm i.d. stainless steel “U” trap and cold bath were used to remove selected components from the gas stream. One or more gas washing bottles in series were used to react the gas stream with various solutions. The NO detector was also equipped in some experiments with a photochemical NOz NO converter (IO)and a dynamic dilution system yielding NOz standards for experiments with NO2. Reagents. “Ultra-pure” grade compressed oxygen and nitrogen or “zero” grade air was used. The standard for dynamic dilutions was Airco analyzed, 2 parts in lo6 by volume (nominal) of NO in Nz.Ordinary distilled water was provided by a stainless steel still; “ultra-pure” water was deionized and triply distilled in quartz. Reagent grade or better chemicals were used without purification. The reagent for formation of Fe(NO)*+complex was 2 M FeS04.7H20 in 0.001 M aqueous H2S04. Spectroquality organic solvents and reagent-grade acids and bases were used in cleaning procedures. Coastal seawater samples were transported in preleached polyethylene bottles. Open ocean surface seawater was sampled by plastic bucket casts, stored in the dark, air freighted to Boulder, and used within 20 days. Procedures. To minimize potential contamination and adsorption effects, the apparatus was degreased with organic solvents and rinsed copiously with methanol and distilled water. The stripper body was cleaned initially with detergent, rinsed with water, acetone, methanol, and distilled water, and then treated -30 min with dilute HC1-HF in a 70 “C ultrasonic bath, followed by distilled water rinses and a final rinse with M ammonium hydroxide. These solvents were drawn through the frit. Subsequent washings of those portions of the apparatus contacting sample solutions utilized methanol, water, acid (with ultrasonification), and dilute base rinses. The apparatus was tested for NO, signal production or consumption by comparing the signals from NO-free gas or NO calibration gas passed through the stripper with the signal from the same gas bypassing the stripper. This sequence was followed initially with a dry stripper and repeated after moistening it with a minimum amount of water. Neither NO production nor irreversible adsorptive NO losses were shown by the apparatus, either dry or wet. Solutions containing NO,, in equilibrium with a known pNO were prepared by bubbling calibration NO, mixtures through the solution for 1-10 min. The gas above and below the aqueous solution was then flushed away with NO-free carrier gas. Gas trapped below the frit was vented through a variable restriction to maintain back-pressure, preventing solution from draining through the frit. For signal peak heights equivalent to 1part NO per lo9 parts carrier gas by volume, the reproducibility of experiments involving the dissolution of calibration NO, was &20%. The amount of NO,, in solutions was determined by stripping. With photochemically generated NO,, in seawater, a total gas flow of 2.0 L of STP min-’ (SLM) and 0.5 SLM stripping gas produced a peak with a sharp leading edge ( < l o s for 90% rise) delayed about 40 s from onset of stripping (the travel time). The peak width was 25 s a t half-height, and the signal returned to within 2 % of the base line 200-250 s after stripping. By repeating the procedure with varying time delays between NO introduction or formation and stripping, kinetic studies of NO,, behavior were possible. At the 1 part NO in lo9 parts carrier level (by volume), the reproducibility of these experiments was f3%. Times 2000 s imprecise. For many studies, only precise relative NO,, measurements are required; then peak height (reported as response in kHz) was used as the measure of [NO],,. These peak p N O values may be compared for different solutions only with care, as peak shape varied as a function of ionic strength, temperature, and other variables affecting the bubble size spectrum, hence the stripping dynamics. Absolute NO,, concentrations were determined from peak areas by cutting and weighing. The peak

-


Figure 1. Apparatus for studying nitric oxide in solution. Upper left,

gas distribution system. Lower left and extreme right, photoiysis system. Lower right, stripper/sample chamber. Upper right, detector. CAL mole ratio NO/N, 10% standard. MFC = NO/N, = 1.92 X mass flow controller, MFM = mass flow meter, O3 = ozone generator, P = pressure gauge. PMT = cooled photomultiplier, AMP = amplifier-discriminator/puIse counter, REC = ratemeterhecorder (2-channel). Wavy lines symbolize photon fluxes. Stippled area represents water sample

*

despite unavoidable pressure surges during operation. It also provides precise, resettable flows of dynamically diluted NO, standards. The valving passes part of the gas around the sample at all times; the rest of the flow is switched around or passed through the sample. The best compromise between minimal foaming vs. short apparatus response time can be selected. The small volume above the gas-liquid interface avoids peak spreading in the gas phase but is sufficient to allow foam to break rather than being entrained and contaminating the detector. The regulators are ultra-high-purity quality for low contamination and the low dead volume plumbing is stainless steel passivated with nitric acid. With these features, response times, hysteresis effects, and “ghost” peaks were minimal and instrumental response was determined by gas-phase flushing times rather than adsorptiondesorption effects. The aqueous sample ( - 1L) is contained in a borosilicate glass “stripper” of 80-mm diameter, 1.3-L volume, underlain by a medium-porosity glass frit. The stripper is connected to the gas distribution system with ‘/,-inch stainless steel fittings and PTFE ferrules. Additional ports (Figure 1)permit adding or withdrawing liquids or gasses by syringe through septa or by pipetting. Draining and/or purging the space below the frit is also possible. The borosilicate glass transmits UV light >310 nm. The chemiluminescence detector (NO + OJ is detailed elsewhere (9, I O ) . Briefly, the sample gas mixes with ozone in a mirrored reaction chamber, where

-

NO

+ O3 3

NOz* + O2

(R1)

+ h~

(R2)

NO** 3 NO2

yield red chemiluminescence monitored a t -660-960 nm by a photon counting system (Figure 1). Our detector has a sensitivity of -4 X 10” Hz atm-’ NO when operated at 4 L S T P min-’ air flow and pumped at 400 L/min (nominal), equivalent to a detection limit of -1 part NO per 10” parts gas. The detector withstood the input of gases and aerosols from water, seawater, and related aqueous solutions for 100 h without cleaning with little deterioration in performance. The sensitivity factor remained nearly constant and the background was low and drift-free. The sensitivity in the presence of 100% relative humidity at 23 “C was 10-15% lower than to NO in dry gas, owing to physical quenching of NOz* by water vapor. The apparatus operates as a closed system. The stripper effluent passes through a needle valve that regulates the pressure upstream and then enters the detector. The stripper pressure was 660 i 100 Torr for most experiments. A major detector feature allows rapid, routine background signal measurements by routing the reaction mixture through a prereactor (Figure 1)

-

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ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980

weight-NO,, calibration was determined from the steady-state signal of a calibration gas passed through the same solution. Absolute NO,, values could then be obtained for related experiments from peak height-peak area ratios. Photochemical experiments were conducted in two modes: "pulse" and "steady state". Pulse mode experiments consisted of irradiating the sample with light pulses of various durations and intensities, followed by stripping after known, variable time delays. Steady-state experiments consisted of simultaneously irradiating and stripping a solution, usually resulting in a constant signal aft& a short time. The steady-state modepermits more reproducible measurements for short time delays or short NO lifetimes. Since these irradiated samples scatter light, the absorbed light doses are unknown and steady-state results are not directly comparable to pulse experiments. Procedures were developed to determine whether a given signal was due to NO,,: (A) the peak shape defines the water solubility of the signal-generating gas, (B) filtering and cryogenic scrubbing of the stripper effluent requires that the signal be due to extremely low-boiling component(s), (C) reaction of the gas from the stripper with an aqueous solution of ferrous sulfate demonstrates (reversible) formation of Fe(N0)2+complex. These procedures may be applied empirically by comparing NO,, standards with the signal of interest. Methods A and C were also treated theoretically (see Results and Discussion). To compare these calculations of stripping behavior with the experimental signals, it is necessary to take into account the efficiency of equilibration and the signal spreading in the gas phase during transport to the detector. To measure this spreading with or without additional gas washing bottles processing the effluent from the stripper, we injected pulses of NO, with a syringe into the foam at the top of a sample while stripping. The peakspreading in the absence of complexing for method C was estimated by replacing the Fez+solution in the gas washing bottle with the same volume of dilute sulfuric acid. The efficiency of the stripper was taken as the ratio of the measured removal rate of dissolved oxygen from seawater by a nitrogen stream to the rate predicted from Equation 2 (see Results and Discussion) with c = 1. This rate ratio yielded eo2 = 0.24 for seawater at 7 "C and a seawater salinity of 31 parts per thousand (w/w) under conditions used in the stripping experiment.

RESULTS AND DISCUSSION I n initial experiments, nitrite-free distilled water and seawater samples were stripped in the dark. No signals above the background were detected. A variety of inorganic salt solutions used in t h e preparation of artificial seawater also showed this behavior. We were unable to detect a p N o 2 of lo+' a t m after passing the mixture through water in the stripper, as might be anticipated from the reactivity and high water solubility expected for this gas. Specificity for NO,. T h e sensitive chemiluminescence detector is not necessarily specific for NO. However, no stable small molecule besides NO is known to be effective; many have been explicitly excluded by tests. Atmospheric studies establish that the detector is insensitive to common trace gases. McFarland et al. (4) found a signal equivalent to 5 X atm NO, in marine air at night. This low signal and t h e composition of maritime air require that t h e instrument be >lo5 times more sensitive to NO than to N2, 02, H 2 0 , COz, CHI, CO, H2,and 03.Tests showed a sensitivity factor for ethylene and @pinene l o 3 times smaller than for NO. With a clean system, t h e instrument is insensitive to NOz. Since it is essential t o verify that trace signals are caused by NO,, we have developed several confirmation methods of varying convenience and rigor. These all basically constrain t h e possible physical and chemical properties of the signalcausing gas severely and allow direct empirical comparison of t h e signal with calibration NO,,. T h e first method is to compare the peak shape with that given by NO,,. T h e rate of stripping of the signal-causing gas from aqueous solution is related to its vapor pressure over water. Since NO is among the least water-soluble gases, this constraint is significant. To

react rapidly with ozone, a gas must possess unsaturated or strongly polar groups (e.g., sulfides, amines), requirements that Seem incompatible in small molecules (except NO) with low water solubility. For example, the approximate partition coefficients H (molecules per unit volume gas/molecules per unit in water near l5 O C Of a variety Of gases that react with ozone are: NO, -20; ethylene, -7; propylene, -2; HgO, -0.5; dimethyl sulfide, -0.3; sulfur dioxide, -0.01 (11);and hydrogen sulfide, -0.3 (12, given erroneously in Ref. 11). We found no difference in peak widths or shapes (*lo%) between the signals from calibration NO,, in seawater compared to the signal from photolyzed nitrite-containing seawater with air or nitrogen stripping gas. A few seawater samples containing added nitrite and EDTA showed a slow-decaying component when stripped with air. T h e peak shape expected theoretically for the stripper (Figure 1) can also be approximated by treating the stripper as a stirred reactor. Assume the NO, concentration entering the solution is zero, the liquid has a volume VI (cm3), the NO, concentration leaving the solution is determined by the solubility equilibrium ([NO], = H[NO],,), the solution is well mixed, and NO is neither produced nor consumed during stripping. At steady state, stripping gas enters and leaves the solution a t a volume flow rate F. Then, PNO

0 -HFt/V1

= PNO e

(1)

where pNoo is the initial vapor pressure of NO above the solution and t is time. Using Equation 1and H = 33 (see next section), V , = 1100 cm3 and F = 1.7 cm3 s-l STP = 2.09 cm3 s-l (at 660 Torr and 23 "C), we calculate that t h e NO signal should decay with a half-life of 11 s. However, some gas phase spreading of the NO peaks was observed. When the measured gas phase peak spreading function and the calculated 11-shalf-life are convoluted, a half-life of 14 s is predicted for stripping NO from seawater. Measured values under these conditions were 32 and 34 s for stripping photochemically generated NO from seawater. T h e stripping is considerably slower than predicted by Equation 1,corrected for peak spreading. Nonideal stripper behavior probably explains this departure. Gas-solution equilibrium is not achieved, resulting in an effective partition coefficient, H', that is kinetically controlled (H' < H).Thus, if we define the actual stripping efficiency as t (H' = E H ) , Equation 1 becomes: -tHFtl V I

(2) = pNOo e For comparison we measured to2 = 0.24 a t 7 O C . At this temperature, Ho,= 30, while we estimate t h a t " 0 is very similar, -33, a t 23 f 2 O C . Taking t N O = 0.24 and the / NO ~ boundary conditions cited above, we calculate T ~ for stripping -46 s, or -47 s after convolution with the peak spreading function. T h e agreement of experiment with t h e idealized model is reasonable given the uncertainties in input parameters. However, even if t = 1,the results compared to predictions from Equation 2 put firm lower limits on H of the signalcausing gas at 23 f 2 "C: H > 12 (taking into account the measured gas-phase peak spreading). Thus, peak shape alone confirms that the major component is due to a gas with H > 12. We are unaware of any gas other than NO with Hsw a t -23 "C > 8 (e.g., ethylene) that reacts rapidly with ozone. T h e peak identity was also tested by inserting a cold trap between the stripper and the detector to ensure t h a t only small, low-boiling molecules need be considered as signal sources. This method has been used for pyrolytically produced NO in conjunction with a NO-03 detector (13). Table I compares the behaviors of NO and the signal from photolysis PNO

ANALYTICAL CHEMISTRY, VOL. 52, NO 11, SEPTEMBER 1980

'7-

Table I. Cold Trap Experiments

peak origin

NO/N, in air (12.5x 10-9 atm/atm) Sargasso seawater photolysisb,c Sargasso seawater + MNO; photolysis

-114 -196 -196 -114 -196 -114 -196 -196

% peak 10ssa

631

4c

82+ 5 94 * 2 O k 3 63 r 1 5

,

-

A

103


?

(R3)

At 25 OC and ionic strength of 0.54 M, the equilibrium constant K 3 = 480 M-' and the rate constant k3 = 6.2 X lo5 M-l s-l (14). For 1M Fe2+,the extremely fast reaction R3 reduces PNO by a factor of 480. Used as a scrubber, R3 shows that the signal-generating gas reacts rapidly with Fe2+,a reductant. Since the detection reaction R1 is a fast oxidation, confirmation and detection require contrasting molecular properties. T h e combination should be highly specific. Rigorous confirmation is possible when the signal-containing gas is available continuously, as in steady-state photochemical experiments. With appropriate flow rates, trap size, and Fez+ concentration, the following behavior is expected when NO,, initially bypassing the Fez+scrubber, is passed through it. The signal first drops nearly to base line and then rises to a steady value equal to that observed with the trap bypassed. The trap capacity is being saturated; its pN0 eventually equals that of incoming gas. Upon purging the trap with NO-free gas, the signal decays to the base line in the mirror-image exponential curve as NO,, in equilibrium with F e ( N 0 ) 2 +is swept from solution. These kinetics should follow a modification of Equation 1,with pNO= the NO partial pressure over the Fez+,, and the gas solubility H" = H (1 + K[Fe2+])-'. The equation relating (the PNO a t the exit of the Fez+ scrubber) to t is

(3) where pNOin is the pN0going into the scrubber. Figure 2A shows a semilogarithmic plot of grow-in and strip-out of NO from calibration gas. T h e reaction is clearly reversible. On this longer time scale, gas equilibration rates ( t 1 because k, is large) and gas phase mixing distortions are unimportant, and the kinetics are close t o exponential. Figure 2B shows the same experiment with the signal from continuous UV photolysis of Sargasso seawater containing 10 pM nitrite. Probably a steady state is never achieved owing to significant nitrite depletion and alteration of scavengers in the seawater during the slow buildup of Fe(N0)2+in the trap, accounting for the curvature. However, the NO evolution curve obeys the expected kinetics and has within experimental error the same rate constant as authentic NO, (Figure 2A).

-

1 .

" 8

. . 3

z z t 'II-

90 + 4

+ NO = F e ( N 0 ) 2 +

^.

3.

I c ! !

0 + 3 79

of seawater containing nitrite. The signals behaved identically within experimental error. NO was only partially trapped at t h e lowest temperatures; a t these partial pressures, it is not liquified but trapped by adsorption and/or dissolution in small amounts of condensed air. The third confirmation technique involved complexing NO with ferrous ion. Gas from the stripper passed through a gas washing bottle containing acidic 1 M Fez+,,. NO, dissolved via the "brown ring" reaction:

0. r .

0 + 3

1.5 1.5 0.5 1.5 1.5 1.5 1.5 0.5

Relative to same gas stream bypassing cold trap. Gas was humidified to R H = 100%before entering the detector to eliminate water vapor effect on sensitivity. b 1 0 mW/cmz, 310-400 nm UV. This sample yielded a very weak ( < atm P N ) signal ~ without any added NO,-.

Fez+

A

8C

trap bath flow rate, temperature, L STP "C min-'

1665

^.

z.

" - 0

r12c'

1:. c .

,re

B

eCC

60-

0

00

8

43

a

0

.

O O

C

a a

2o

1

e

.b-04 4-

... .. $5

C 0

23

2

25

riME : M ~ ~ T E s J

Figure 2. Semilogarithmic plot of reversible complexation of NO by Fez+. (A) 12.8 X lo-' atm NO, in air. (0)NO, uptake by solution. ( 0 )NO, flushout from solution. Solid line slope corresponds to T,/* = 7.0 min. Broken line = flushing response of scrubber. (B) Signal from UV-irradiated nitrite-containing Sargasso seawater. Open circles = NO, uptake. ( 0 )NO, flushout from solution. Solid line slope corresponds to T , / ~= 5.3 min

Furthermore, the half-equilibration time calculated from Equation 3 for H" = 480Hsw is 5.4 min, in good agreement with the data. The conditions were not optimized in this preliminary experiment. R3 leads t o a pNOdependence of -7% per "C. Clearly, the scrubber must be thermostated if accurate agreement is expected; ours was not. By varying flow rates, solution volumes, and ferrous concentrations, a better compromise between speed and ideal scrubber behavior should be achievable. NO Solubility. Since calibration standards are gases, accurate measurement of NO,, requires knowledge of the partition coefficient, HNo. The solubility of nitric oxide in water has been measured at high PNO (15),but solubilities in seawater and in water at low p N O could be significantly enhanced. For example, the solubility of CO at very low pco may be up to eight times that a t high pco, presumably due to strong binding to trace impurities (16). Since NO is also a versatile ligand (13,we measured its solubility in distilled water and seawater at trace levels by equilibrating 38 f 4 X atm NO, in N2 (total pressure = 0.87 0.07 atm) with a known volume of aqueous phase, flushing away excess NO, with NO-free carrier gas, and then measuring NO,. Flushing was complete. NO was not reacting irreversibly in solution, since solubilities were not a function of time after equilibration (Figure 3). The NO partition coefficient, H , between Nzgand H201at 24 OC was 31 f 6.3 (rsd = 19%; n = 5 ) . Between nitrogen and seawater, H = 33 f 7.3 (rsd = 22%; n = 6). The partition coefficient calculated from the solubility of pure NO in water (15) assuming ideal-gas behavior is 21.0 at 25 "C. This value and the salinity dependence of solubility

*

1666

ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980 I 10

I 0 09

08 h

$

0 7

36

T4,2-23

2

0

TIME

3

4

6

IMINUJTESJ

Figure 4. Semilogarithmic plot of dark decay kinetics of NO,, generated by nitrite photolysis; hand-fitted lines. (A)200 pM NO,- in distilled water, (0)= Sargasso seawater 0.1 pM NO,?, (0) = Sargasso seawater 1.0 pM NO,-. Signals have been normalized to t = 0 value

+


+

1

I

,

I

C

4

8

'2

16

TIM€ DELAY

20

24

NO,, does not build up and nitrite is not consumed because of subsequent thermal reactions in solution:

20

IMINUTESJ

Figure 3. Semilogarithmic NO,, dark reaction kinetics. Samples were: ( I ) , distilled water, N, flushed; (2) = (1) 4- 30 p M NO,-, 20-inch UV light pulses; (3)= water triply distilled in quartz, N, flushed; (4) = (3), air flushed; (5) = (4) + 20 pM NaOH; (6) = (5),photolyzed 8 h with full UV light, then N, flushed; (7) = (6),air flushed; (8) = (7). O2flushed; (9) = Sargasso seawater, N, flushed; (10) = (9), air flushed; (11) = ( l o ) , 0,flushed. Data from samples ( l ) ,(2), (3),(6),and (9) fell in the "N," region. Data from samples (4), (5),(7), and (10) fell in the "air" region. Data from samples (8) and (11) fell in the "0,"region for the similar gases, oxygen and nitrogen (18),yield an expected HNOa t 25 "C of 26.0 for seawater. In both cases, our measured values suggest that NO is less soluble than estimated; however, the differences are not statistically significant a t the 95% confidence level. The scatter in these experiments was large, since it was difficult to vent the subfrit area without losing sample or accidentally forcing some gas through the liquid while replacing the NO/N2 with pure N2. Premature NO,, stripping losses may contribute to our high values; the deviations are in the opposite direction from any effect of complex formation. Reaction of NO,, with Dissolved Oxygen. The recovery of NO,, was not a function of time in nitrogen-degassed water, but when oxygen was present NO,, recoveries decreased markedly with time for a variety of samples, as shown in Figure 3. The decrease was more rapid in oxygen-saturated than in air-saturated solutions, suggesting the involvement of oxygen as a reactant in the rate-determining step of a chemical reaction consuming NO,,. Since trace impurities in the system or sample may catalyze the NO-Os reaction, leading to erratic rates, detailed kinetic studies were not performed. The data in Figure 3 only illustrate the effect of oxygen on NO,, reaction rate in water and seawater and are not intended to imply that the reaction is strictly first order or to establish rate constants. According to Pogrebnaya et al. ( 8 ) ,the kinetics of the reaction of NO,, with 02,,at high concentrations follow: - d [ N O ] / d t = 9 X l o 6 M-2 s - ~[N01:,[021,,

(4)

An estimation of this rate under the initial conditions of our M and [O,],, = 2.5 X M) is experiment ([NO], = M s-l. The NO,, half-life is greater than d[NO]/dt = 2 X one month. Clearly, we measure much shorter half-lives and are observing a reaction path with a different mechanism. It may be homogeneously or heterogeneously catalyzed. NO Formation in Nitrite Photolysis. According to Treinin and Hayon (19): NOz-

+ HOH + hrl

4

NO,,

+ OH + OH-

(R4)

+ NO2- = NOz + OHNO2 + NO = NzO, N203 + HOH = 2N02- + 2H+ OH

(R5) (R6) (R7)

However, the evidence that NO is a primary photoproduct is indirect: OH trapping products have been detected. Furthermore, efforts to substantiate schemes R4 to R7 by flash photolysis and pulse radiolysis experiments were only partially successful (19). From Treinin and Hayon's work, we estimated that at steady state, [NO],, should exceed the detection limit under our conditions, even after >10 s of decay in the dark. Preliminary stripping of distilled water showed an easily detectable NO peak at extremely low UV light intensities and nitrite concentrations W/cm2;

Determination of trace levels of nitric oxide in aqueous solution

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