SUPPRESSION MECHANISMS OF ALKALI METAL COMPOUNDS 

Bradley A. Williams and James W. Fleming 

Chemistry Division, Code 61x5 

US Naval Research Lnhoratory 

Washington, DC 20375-5342, USA 

INTRODUCTION 

Alkali metal compounds, particularly those of sodium and potassium, are widely used as fire 

suppressants. Of particular note is that small NuHCOi particles have been found to be 2-4 times 

more effective by mass than Halon 1301 in extinguishing both eountertlow flames [ I] and cupburner flames [?]. Furthermore, studies in our laboratory have found that potassium bicarbonate 

is some 2.5 times more efficient by weight at suppression than sodium bicarhonatc. The primary 

limitation associated with the use of alkali metal compounds is dispersal. since all known 

compounds have very low volatility and must he delivered to the fire either as powders or in 

(usually aqueous) solution. 

Although powders based on alkali metals have been used for many years, their mode of effectiveness has not generally been agreed upon. Thermal effects [3], namely, the vaporization of the 

particles as well as radiative energy transfer out of the flame. and both homogeneous (gas phase) 

and heterogeneous (surface) chemistry have been postulated as mechanisms by which alkali 

metals suppress fires [4]. Complicating these issues is the fact that for powders, particle size and 

morphology have been found to affect the suppression properties significantly [I]. 

In addition to sodium and potassium, other alkali metals have been studied, albeit to a considerably lesser extent. The general finding is that the suppression effectiveness increases with atomic 

weight: potassium is more effective than sodium, which is in turn more effective than lithium 

[4]. This is true even on a mass basis in spite of the increasing atomic weight of the metallic 

element. The difference generally observed between sodium and potassium is a factor of two to 

three. Since the thermodynamics and chemical behavior of the alkali metals are fairly similar, 

such a large diffcrencc in suppression efficiency is rather surprising, and may provide clues as to 

how suppression occurs due to these elements. 

Alkali metals show completely d 

semimetals. Halogens (particularly bromine and iodine) participate in a catalytic cycle of H atom 

recombination through hydride species, HX (X=Br,I). The transition metals (Fe, Cr, Mn) and 

semimetals (P, As. Sb) possess multiple oxidation states. which may fxilitate catalytic cycles 

involving oxide and oxyhydride species. Alkali metals do not conform tn either category, since 

they do not possess multiple valences nor form stable hydrides. For this reason, this group of 

elements may provide insight into what other properties a chemical element can have to be a 

good fire suppre 

rent behavior from both halogens and transition metals and 

H:hn Oplionr Technical Working Contcrence ?7-?'J April I!JCJC) 157 
The present study investigates the possible suppression mechanisms for alkali metals, aims to 

determine possible causes for their rather high effectiveness in general, and offers explanations 

for the observed differences among elements. To this end a series of calculations was performed 

investigating the thermal effect of the addition of alkali metal compounds to flames, as well as 

kinetic modeling to investigate chemical reactions important for fire suppression and identify 

what experimental measurements would be most useful for validation of proposed kinetic 

mechanisms. 

THERMODYNAMIC DATA 

To assess the thermodynamic effect of alkali metal compounds on flames, and to perform kinetic 

calculations, thermodynamic data are required for all species of interest, both the additive itself 

and all decomposition products that might be formed in the flame. Thermodynamic data were 

compiled for the entire series of alkali metals (Li, Na, K, Rb. Cs) for the following species 

(where X represents an alkali metal atom): X, X2, XH. XO, X20, XO2, X202, XOH(gas), 

X202H2(gas), XOH(liquid), XOH(solid), XzC03(liquid), and X2C03(solid). The solid bicarbonates were also included for Na and K. The data were collected from a variety of sources [5-71. 

For some species for which published thermodynamic tables were unavailable or very uncertain, 

particularly the monoxides (XO) and the dioxides (X02). thermodynamic functions were calculated based on spectroscopic or theoretical determinations of bond dissociation energies, vibrational frequencies, and excited electronic states. The calculation assumed the harmonic oscillator 

approximation for the vibrational partition function, but allowed excited electronic states to have 

different vibrational frequencies than the ground state. From these data, polynomial fits of the 

form used by the Sandia CHEMKIN program were obtained. For the condensed phase species, 

thermodynamic functions were extrapolated above the melting or boiling point, assuming a 

constant heat capacity, so that equilibrium calculations would not spuriously predict the existence 

of condensed phases at high temperatures due to extrapolation of polynomial fits outside their 

domain of validity. 

Thermal equilibrium calculations using the compiled thermodynamic data were performed for 

stoichiometric methane/air mixtures containing a 0.19% mole fraction of each of the alkali metal 

carbonates (X2CO3) in solid form. Each calculation was performed at constant enthalpy (i.e., 

calculating the adiabatic flame temperature) at one atmosphere pressure, and the results are 

summarized in Table 1. The only alkali metal containing species present in significant concentrations at equilibrium were the gas phase metal atom (X) and the gas phase hydroxide monomer 

(XOH). The monoxides (XO) were the next most significant alkali metal species, but never 

comprised more than OS% of the metal atoms. 

Two conclusions are apparent from the thermal equilibrium calculations. First, the addition of 

the carbonate powder reduces the adiabatic flame temperature by only 25-30 K. The concentrations considered here are roughly 20% of those typically required to extinguish flames. Even at a 

molar concentration of 1 % agent, the flame temperature reduction would be less than 200 K, 

much less than the decrease when N2 is added at its extinction concentration. The dominant 

mode of suppression must then be chemical, rather than thermal. Secondly, the identity of the 

metallic element has hardly any effect on the flame temperature. Thus, thermal effects do not 

158 Halon Options Technical Working Conference 27-29 April IY9Y 
TABLE I. THERMAL EQUILIBRIUM TEMPERATURES AND SPECIES 

CONCENTRATIONS FOR A STOlCHIOMETRlC METHANE/AIR 

I ATMOSPHERE AND INITIAL TEMPERATURE 300 K. 

MIXTURE CONTAINING 0. I9'% MOLE FRACTION OF X2CO?(S), AT 

Metallic Adiabatic Flame Equilibrium Mole Fractions 

Element Temperature 

(K) X(gas) XOH(gas) XO(gas) 

None 223 I 

Li 2204 4.78E-5 3.7 I E-3 4.33E-6 

N a 2200 I ME-3 1 X9E-3 I .89E-5 

K 2204 9.5SE-4 2.80E-3 9.2 I E-6 

Rb 2207 6.89E-4 3.06E-3 8.36E-6 

Cs 2207 3.52E-4 3.40E-3 6.S3E-6 

~ ~ ~ 

account for the ohscrvcd differences in suppression efficiency between clements. The primary 

difference between the different metals is the degrcc to which the hydroxide dissociates to a 

metal atom and an OH radical. The dissociation energy, smallest for sodium and largest for 

lithium. is reflected in the very low equilibrium concentration of atomic lithium compared to 

sodium. 

CHEMICAL KINETICS 

Several radical scavenging pathways have been proposed in prcvious studies of alkali metals 

in flames. The simplest, suggested by Jensen and Jones [8], involves a two-step chemical 

mechanism: 

XOH(g) + H Q H20(g) + X (1) 

X+OH+M @XOH(g)+M (2) 

where M is a third body. The kinetic cycle is carried by the two major alkali mctal species 

present in the tlame. Reaction (I) is exothermic for all the alkali metals (even though in the 

condensed phase. alkali metals react violently with liquid water), and occurs at a substantial 

fraction of the collision rate. The rate of Reaction (2) is rather uncertain. particularly at high 

tcmperaturcs. Wc performed a series of preliminary calculations of the propagation speed of a 

stoichiometric methane/air tlame inhibited by sodium and potassium hydroxide, using this twostep mechanism. The calculations demonstrated that this catalytic cycle is more than sufficient to 

explain the observed suppression effectiveness. 

Morc recent studies [9,10] have indicated that the kinetic pathways of alkali iuctals in flames 

include the species XO and XO? as well, and that the recombination rcaction with molccular 

oxygen 

Halon Oplianu Tcohnical Working Cnnkrcncc 27-2') April I 'NU I59 
X + 02 + M CS XO&) + M (3) 

may be comparable in importance to that of Reaction (2) under certain conditions. A more 

comprehensive kinetic mechanism was prepared, which included these additional reactions, 

based on a recent publication of Zamansky et al. [I I], and studied the effect of sodium on postflame NOx reduction strategies. A few modifications were made to the mechanism. Since NaH 

is predicted to be one of the more significant trace species at thermal equilibrium, we added 

kinetics for this species, assuming it could be formed by a sodium atom abstracting a hydrogen 

atom from H02 or HCO. The actual importance of NaH in the kinetic pathways or in influencing 

the flame speed turned out to be minor, however. Also, the mechanism postulated [I I] that 

Na02 has a very large, temperature independent, rate coefficient for oxidation of CO: 

Na02 + CO CS NaO + CO2 (4) 

This reaction is of relatively little importance under the conditions [l I], in which there was little 

CO. In our calculations, however, this reaction was predicted to dominate CO oxidation to such 

an extent that it caused the normal CO oxidation reaction, 

CO+OH GCO2+H (3 

to run in reverse. This deficiency was addressed by adding an activation energy to the kinetic 

expression for the rate coefficient of Reaction (4), assumed to be identical to that for the reaction 

HOl + CO CS CO? + OH (6) 

Using this extended mechanism, the flame speed was calculated in a stoichiometric methandair 

mixture at atmospheric pressure, inhibited by NaOH. The computer code for calculating flame 

speeds (Sandia PREMIX) does not handle condensed phase species, so NaOH was added as a 

reactant in the form of the gaseous dimer, Na202H2. A kinetic rate was added for formation of 

the monomer with a preexponential factor approximately equal to the collision rate and an activation energy equal to the enthalpy of dimerization (49 kcal/mole). Thermal equilibrium calculations comparing the effect of adding NaOH in solid form vs. dimeric form on the adiabatic flame 

temperature showed that adding NaOH(dimer) reduced Tadlabatlc about 70% as much as adding the 

same quantity of NaOH(so1id). Thus the calculation slightly understates the thermal effect of 

NaOH addition. As shown below, however, flame inhibition by alkali metals is dominated by 

chemical, not thermal, factors. 

KINETIC MODELING RESULTS FOR SODIUM 

The addition of Na202H2, using the modified kinetic mechanism [ 1 I], caused a dramatic decrease in the predicted flame speed. The top panel of Figure l shows the flame speed as a 

function of the Na202H2 concentration. For comparison, the mole fraction of NaHC03 needed 

to extinguish a cup burner according to the study of Hamins [2] was 1.2%. Previous studies 

equating flame speed and extinction have assumed that flames having propagation speeds of 

<3-5 cm/sec will self-extinguish. Extrapolating the curve in Figure 1 and keeping in mind that 

160 Halon Options Technical Working Conference 27-29 April I999 
the mole fraction must be multiplied by two to equate to the number of sodium atoms in 

NaHCO?, the predicted inhibition cffcct is of the right magnitude. 

The adiabatic flame temperature drops by only about 70 ti over the rangc of NaOH concentrations considered in Figure I. Even taking into account the additional enthalpy of vaporization if 

NaOH(solid) were the starting material, the tenipcrnturc reduction would only be about 100 ti, 

insufficient to account for the flame spccd reduction. Furthermore. the strongly nonlinear shape 

of the flame specd curvc strongly resembles the results of Babushok et al. [ 121 for an "ideal" 

chcmical inhibitor, pointing to the chemical effect of the sodium. The concentrations of H. 0. 

and OH are plotted in the bottom panel of Figure I; chemical inhibitors generally work by reducing superequilibrium concentrations of these radicals. The calculated radical profiles reach a 

peak concentration in the flame front, then gradually decay Lo values very close to thosc predicted 

at thermal equilibrium. Therefore, the peak values of the radical profiles have been normalizcd 

by the find values at the limit of the computational domain (60 cm beyond the flame front). A 

concentration ratio of unity means that no radical overshoot occurs, implying that there is no 

morc chemical inhibition to be gained by adding additional scavenging agent. 

As seen in Figure I, the H atom concentration exhibits the most overshoot in thc uninhibited 

flame, and the OH concentration has far less overshoot than either H atom or 0 atom. Also 

plotted is the geometric mean of the normalized concentrations of the three radical species 

(x~"Xo,,*Xo)"~ where X denotes the nonnalized mole fraction. This quantity is intended as an 

overall measure of how much chemical inhibition can be obtained. By the time 05% Na?O?H? is 

added, no overshoot occurs for OH or 0, and the H atom overshoot is only about 30%). compared 

to a factor of nearly 20 for the uninhibited flame. If thc kinetics are reasonably correct, the 

addition of sodium compounds beyond this quantity will only produce thermal effects on the 

tlamc, since the chemical inhibition effect is nearly saturated. 

Analysis of the kinetic pathways indicates that the Reactions (I) and (2) are indeed primarily 

responsible for flame inhibition. The recombination of Na + 02 is of minor importance. and 

occurs primarily at low temperature. due to the relatively low dissociation energy of NaO?. The 

finding of Hynes et al. [9] that this reaction was the dominant recombination process may have 

bccn a consequence of their investigation of lean hydrogen/oxygen flames, at low temperatures 

and high 02 concentrations. After Na and NaOH, thc next most important species in the kinetic 

pathways is NaO, whose dominant reactions are 

NaOH + OH 

NaO + 0 

=> NLIO + H2O 

=> NLI + 02 

NaO + H => Na + OH 

The number of sodium atoms participating in these reactions, however, is only aboul one-third 

that of Reactions (1) and (2). A simplified kinctic schcme for sodium appropriate to the 

conditions considered here should include at least the species Na, NuOH, and NaO. and 

Reactions (I). (2), (7), (X), and (9). 

Halon Options Technical Working Cunl'crcncc 27-2Y April 1999 16 I 
45 

40 

35 

._ 2 30 

3 25 

1 ln 

v 

V 

0 

- 0 

Is) 

.& 20 E 

3 

U 

a, 

-0 

15 

.u 10 

2 

a5 

c 

0 

20 

18 

16 

F 14 

a, 

0 

._ - 

z 

z 12 

._ 2 10 

0 

- 

U 

m 

U 

a, 

N 

u8 

._ -

m6 

E 

04 Z 

2 

0 

. 

1.0 0.1 0.2 0.3 0.4 0.5 

(NaOH), Concentration (Mole YO) 

Figure 1. Predicted flame speed (top) and radical concentration overshoot (bottom) for 

methane/air mixtures inhibited by Na20zH2. 

162 Halon Options Tcchnical Working Conference 27-29 April lYY9 
EXTENSION TO LIlHIUM AND I'OIASSIUM 

For other alkali metals. the chemical reactions relevant to suppression are even less well studied 

than for sodium. Nevertheless, calculations analogous to those for sodium for flames inhibited 

by Li?OZH? and KzOZH~ were performed. As a starting point, it was assumed that the kinetic rate 

coefficienls of the forward reactions (written in thc exothermic direction in the kinetic mechanism hut assumed to be reversible) are identical to the riltes for the analogous sodium reactions. 

The one exception was that the conversion of the hydroxide dimer to moiioincr was given an 

activation energy equal to the dissociation energy for the particular element (S8 kcal/ mol for 

lithium and 45 kcal/mol for potassium. compared to 49 kcal/mol for sodium). The preexponenti31 factor in the Arrhenius rate coefficient for this reaction was not changed between elements. 

Calculations were performed for a stoichiometric CHJair mixture containing 0.2% mole fraction 

of (XOH)?. These calculations were meant to determine how thermodynamics influences the 

efficiency of chemical suppression. Must sodium, potassium, and lithium have different kinetics, 

or can differences in their thermochemistry account for the observed trends in suppression 

efficiency'! 

The results are summarized in Table 2. For lithium, considerably less flame speed reduclion 

occurs than for sodium. even with identical kinetics. This appears to be due to two factors. The 

dissociation energy ofthe LiOH dimer is much higher than for sodium, so the species Li and 

LiOH are not produced until much later in the flame, where scavenging of radicals is less effective in reducing the flame speed. Secondly. in the equilibrium calculations (Table I). the conccntration of Li is some 30 times less than for sodium, when the two elements are added in identical 

concentrations. The two species responsible for the catalytic cycle (Li and LiOH) are present in 

veiy unequal concentrations. In the tlame calculation, LiOH is present in 10 tinies the concentration of Li. Since the Li + OH + M reaction is likely to he the rate limiting step in the kinetic 

cycle, the low concentration of Li dramatically slows down the whole process. Interestingly, in 

the sensitivity calculations shown below, the Li + 02 + M Q LiOz + M reaction is rather 

important, unlike the case for sodium. This may be ;i consequence of the higher dissociation 

energy for LiO2, reducing the importance of the reverse (dissociation) reaction. 

TABLE 2. PREDICTED FLAME SPEEDS OF STOICHIOMETRIC METHANEiAIR 

MIXTURES INHIBITED BY 0.2% DIMER OF Li202H2. Na20ZH2. and K2OZH2. 

Agent Flame speed (cmisec) R. 'I d' IC~ .. Overshoot Factor' 

None 39.4 8.2 

Li202H2 30.7" 7.0 

NLI~O~H~ 17.0 I .8 

K202H~ 13.2 2.4 h 

"kinetics assumed identical to sodium 

'geometric mean of peak H, OH, and 0 concentration normalized by postllame concentration 

modified kinetic rate coefficients for KOH + H. K + OH + M. and K + 02 + M h 
For potassium, use of the same kinetic parameters as for sodium results in the prediction of a 

slightly higher flame speed than for sodium, indicating less efficient suppression. The predictions are at odds with the observed behavior, indicating that differences in thermochemistry 

cannot be responsible for the greater suppression efficiency of potassium. To investigate what 

kinetic modifications would lead to better suppression for potassium, the rates for the following 

reactions were modified: 

KOH+H GK+H20 (10) 

K+OH+M GKOH+M (11) 

K+02+M @K02+M (12) 

The rates for (IO) and (12) were taken from Slack et al. [IO], while that of (I 1) was from Husain 

et al. [13]. Modification of these rates did lead to the prediction of more efficient suppression by 

potassium, although the difference between the two elements appears to be somewhat less than 

that observed experimentally. Table 3 shows sensitivity coefficients of predicted flame speeds to 

kinetic rates of individual reactions for Li, Na, or K. One noteworthy trend is that the sensitivity 

coefficient of the reaction converting hydroxide dimer to monomer (necessary before any radical 

scavenging reactions can occur) decreases along with the enthalpy of dimerization from lithium 

to potassium. For lithium, breaking of the dimer is predicted to be a rate-limiting step in the 

inhibition process, although it is not for sodium or potassium. In Table 3, sensitivity coefficients 

have been normalized with respect to those of the H+OzGOH+O reaction. A negative number 

indicates that increasing the kinetic rate will decrease the flame speed. The larger the absolute 

value of the number, the more important the reaction is to inhibition. 

One interesting comparison between sodium and potassium is that according to Table 2, the 

flame inhibited by potassium has a lower predicted flame speed than the flame inhibited by 

sodium, even though it has higher peak radical concentrations. This indicates that other factors 

must determine flame inhibition in addition to the reduction in the peak radical concentrations. 

Figure 2 shows profiles of temperature, major radical species (H, 0, and OH), and alkali metal 

species for the two flames. The position of the origin on the horizontal axis is arbitrary; the 

positions in the two calculations are made to coincide at the point where the temperature reaches 

600 K. The potassium-inhibited flame has a slower temperature rise, even though the final flame 

temperatures are virtually identical. The peak mole fractions of H, OH, and 0 are all higher in 

the potassium-doped flame than in the sodium-doped flame. The locations of the peaks, however, occur later. at a location where the temperature is about 100 K higher than at the location of 

the peaks in the sodium-inhibited flame. The concentrations of H, 0, and OH early in the flame 

are lower in the potassium-inhibited flame. This implies that whew the radicals are depleted is 

critical to inhibition, as well as how nzuch depletion occurs. 

164 Halon Options Technical Working Conference 27-2Y Apnl lY9Y 
TABLE 3. SENSITIVITY COEFFICIENTS OF ALKALI METAL REACTIONS FOR FLAME 

SPEED IN STOICHIOMETRIC METHANE/AIR MIXTURES CONTAINING 

0.2%) Li202H2, 0.2% Na202H2. OR 0.2% K202H2. 

Rcact ion Sensitivity Coefficient 

Li202H. + M Q 2LiOH + M 

Li + 0: + M Q LiO2 + M 

LiOH + H Q Li + HzO 

LiO2 + OH Q LiOH + 02 

Li + OH + M Q LiOH + M 

LiO +OH Q Li + HO! 

-0.25 

-0.24 

-0.06 

-0.02 

-0.02 

0.02 

NaOH + H Q NLI + HzO 

Na+ OH + M Q NaOH + M 

NaO+ H Q Na+ OH 

N~1202H2 + M Q 2NaOH + M 

NaO + 0 Q NLI + 02 

NaO + OH Q NLI + HO: 

NaO + Hz Q Na + H20 

NaO2 + OH Q NaOH + 02 

NaOH + H Q NLI + H20 

Na+ OH + M Q NaOH +M 

Na0 + H Q Na +OH 

Na202H~ + M Q 2NaOH + M 

NaO + 0 Q NZI + 02 

NLIO + OH Q N~I + HO! 

NaO + H2 Q Na + Hz0 

NaO2 + OH Q NaOH + 02 

-0.32 

-0. I2 

-0.1 1 

-0.07 

-0.04 

0.03 

-0.0 I 

-0.0 1 

-0.32 

-0. I2 

-0.1 I 

-0.07 

-0.04 

0.03 

-0.01 

-0.0 I 

KO? + OH Q KOH + 02 -0.36 

K+OH+MQ KOH+M -0.27 

KOH + H Q K+ H2O -0.09 

K202H2 + M Q 2KOH + M -0.08 

KOz+HQKOH+O -0.04 

KO+OQK+Oz 0.02 

KOz+H Q KO+OH -0.0 I 

K+HCO@KH+CO -0.0 I 

K+02+MQKO:+M -0.0 I 

Halon Oplions Technica Workins Conference 27-20 April I900 165 
2200 " ' '1 " 

2000 1 - CH./air/0.2% K,O,H, I " I' ' " 1' " " " 

1800 

- 1600 

3 1400 ?? 

9 1200 

- 

a, 

1000 

+ aI 800 

600 

400 

~ 

L) ZLL 

. CH,/air/0.2% Na,O,H, 

1.0 - 

h 

I 0.8 - 

5 0.6 

0 

7 

v 

._ - 

0 

9 

= 0.2 1 

U 0.4 - 

a, 

0 

- 

0.0 

..o.. - OH/lO(KOH) 

0 (NaOH) 

*.. - O(K0H) 

H (NaOH) 

4.. H (KOH) 

I h 0 T 2.0 f' 

::e 

= 0.5 LU flL 

x. 

( 

c 

._ 0 

G 1.5 

9 

LL 

- 1.0 

0 

0.0 

-0.05 0.00 1.05 0.10 0.15 0.20 0.25 0.30 0.35 

Position (crn) 

Figure 2. Predicted profiles of temperature (top), key flame radicals (middle), and alkali 

metal species (bottom) in methane/air flames inhibited by sodium and potassium 

hydroxide. 

166 Halon Options Technical Working Conference 27-29 April IYYY 
DISCUSSION 

The mechanism of flame suppression by alkali metals is dominated by scavcnging of the major 

sodium can he explained by d 

does not mean lhat the dominant effect of alkali metals on flames is thermal. Rather, it demonstrates how thermodynamic considerations can significantly influence the efl‘ectiveness of the 

catalytic suppression cycles. For potassium, on the othcr hand, the greater effectiveness relativc 

to sodium must be due to kinetic differences. At present, kinetic parameters for the relevant 

reactions are not sufficiently well known to definitively conclude exactly how the kinetics must 

r. One possible explanation involves the greater atomic mass and size of potassium and its 

pi-obable effect on the rate of the recombination reaction X + OH + M. The same considerations 

apply to other recombination reactions such as X + 02 + M, as well. 

Two effects can increase the likelihood of stabilization of a collision complex by a third body. 

First, as the physical size of the alkali metal increases, there is a greater cross section for collision 

by a third body to stabilize the XOH collision complex. Secondly, as the mass increases. the 

collision occurs more slowly. Assuming identical potential energy surfaces. the time for a 

collision to occur is expected to scale as the square root of the reduced mass, p, given by 

I_. ‘I d’ ical . species present in the flame. The lower suppression effectiveness of lithium relative to 

renccs in thermochemistry between the two elements. This 

!-l= MxMoti/(Mx+MoH) 

where Mx and MOH are the masses of the alkali metal, and the OH molecule, respectively. In the 

simplest approximation (treating X-OH as a two-body, one-dimensional system), lhe third body 

collision must occur within this time interval for the XOH complex to be stabilized. The product 

Amp”’ , where A, is the solid angle-averaged projected area ofthe XOH molecule, should correlate with the likelihood of stabilization of XOH by ii third body during the collision. For all the 

alkali metals. this product is much larger than for hydrogen (Table 4). providing :I possible 

explanation as to why these elements enhance radical recombination. The value for potassium is 

roughly twice as large as for sodium, which in turn is about twice as large as for lithium. 

TABLE 4. PREDICTED SIZE EFFECT ON X + OH RECOMBINATION RATES 

Element Reduced Mass Atomic Radius Stabilization Cross Section 

p(amu) R (IO-“’ m) ~,*pl” (relative to H) 

H 0.94 0.30 I .o 

Li 4.96 1.23 8.7 

Na 9.78 1.54 17.7 

K 11.80 2.03 3 1.6 

Rb 14.20 2.16 38.8 

CC 15.10 2.35 46.7 

Halon Oplions Technical Working Conterence 27-29 April 149V 67 
Patrick and Golden [I41 published a theoretical study of recombination of Li, Na, and K with OH 

and 02. They predicted that potassium should have a rate 2-3 times faster than sodium with 02. 

and about 30% faster than sodium with OH. Lithium was predicted to have a kinetic rate faster 

than sodium, but slower than potassium, for both reactions. These results are consistent with the 

scaling relations stated here for sodium and potassium, but not lithium. For lithium, other factors 

(vibrational frequencies, dissociation energy), which can influence the stabilization rate, are quite 

different from either sodium or potassium. In Reference 14, the authors commented that the lack 

of reliable experimental data, particularly on the temperature dependence of the rate, made it difficult to assess the validity of the approximations made in their derivation. If the scaling relationship in Table 4 is valid for the heavier alkali metals, differences in kinetic rates should result 

from physical considerations on the atomic scale, rather than differences in chemical bonding. 

Previous experimental studies on the flame chemistry of alkali metals have generally monitored 

the metal atom, as well as the OH radical, by optical techniques. Kinetics were inferred based on 

changes in OH radical concentrations as the alkali metal compound was added to the flame. 

Unfortunately, the OH concentration is much less sensitive to the chemical suppression than 

either H or 0 concentrations. Data on either of these latter two species are likely to be more 

definitive in providing experimental validation for kinetic mechanisms. The observation by 

Slack et al. [IO] that there was no discemable difference in the OH profiles between sodium or 

potassium addition may be a result of the low sensitivity of OH to kinetic differences. 

In terms of observations applicable to flame inhibitors in general, it appears that a chemical 

catalytic cycle is most efficient (in a premixed flame) when the scavenging species are created 

early in the flame zone. In a nonpremixed flame, the condition is likely to be that the scavenger 

must be present at the location where the H + O2 reaction has the largest flux, typically at a 

moderate temperature on the air side of the flame. A catalytic cycle for radical recombination is 

likely to be most efficient when all species involved in the cycle (two in the case of alkali metals) 

have comparable concentrations at thermal equilibrium. Finally, it appears that there may be a 

size effect in cases where a third-body mediated recombination reaction is needed for a catalytic 

cycle to exist. All other things being equal, a larger and heavier scavenging element may be 

more efficient, since there will be a longer time and a larger target area for a collision with a third 

body to stabilize the collision complex. 

ACKNOWLEDGMENTS 

We thank Valeri Babushok for helpful discussions. This work was funded by the SERDP NextGeneration Fire Suppression Program. 

168 Halon Options Technical Working Conference 27-29 April 149Y 
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