Photochemistry of Environmental Aquatic Systems - American

Chapter 8 -

Reaction of Br2 Produced by Flash Photolysis of Sea Water with Components of the Dissolved Carbonate System

Downloaded by MICHIGAN STATE UNIV on March 8, 2013 | http://pubs.acs.org Publication Date: December 8, 1987 | doi: 10.1021/bk-1987-0327.ch008

Mary B. True and Oliver C. Zafiriou Department of Chemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543

-

Br2 produced by f l a s h photolysis of seawater decays by p a r a l l e l first- and second-order reactions. The en vironmentally important exponential decay i s a pseudo f i r s t - o r d e r reaction of Br2 with the carbonate/ bicarbonate system i n seawater. A chemical speciation model f o r the free ions and ion-pairs in seawater and in solutions at seawater ionic strength allowed us to measure the dependence of the pseudo f i r s t - o r d e r rate term, a , on individual CO -containing species. A pre d i c t i v e equation based on reaction of Br2 with free CO3 and the MgC03 , NaC03" and CaC03 ion pairs accounts f o r the mean seawater α at pH 8.1 within ex perimental uncertainty. The reaction product(s) are unknown. -

2

-

2-

°

°

The preceding paper (1) showed that B r 2 ~ i s produced i n the UV flash photolysis of seawater and that some previously unknown reaction be tween B r " and carbonate species i s environmentally important. The mean pseudo f i r s t - o r d e r rate f o r this reaction i n a variety of nat ural seawaters near pH 8.1 i s 2.47 +0.52 χ 1 0 s ' . The purpose of this paper i s to examine this reaction i n simple solutions to determine rates and possible reactants. In order to explore the r e l a t i v e r e a c t i v i t i e s of free CO3 ", free HCO3- and the important carbonate ion pairs towards Br2", we flashed aqueous solutions of bromide with KC1, NaCl and MgCl at seawater ionic strength over a range of carbonate concentrations and pH's. The mea sured rates f o r the reaction of the relevant carbonate species with B r 2 " i n simple solutions were combined with a model f o r the specia tion of carbonate i n seawater to see whether this reaction can ac count f o r the observed seawater k i n e t i c s . 2

3

1

2

2

Methods. We used UV flash photolysis to form Br by selective excitation of Br": Rl:

Br~ +

hv

=

Br

+ e "

0097-6156/87/0327-0106$06.00/0 © 1987 American Chemical Society

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

direct

8.

107

Reaction of Br{ Produced by Flash Photolysis

TRUE AND ZAFIRIOU

The bromine atom reacts predominantly with bromide ions in solution to y i e l d B r " (1-3). Solutions were made using a n a l y t i c a l reagent grade chemicals in M i l l i p o r e "Super-Q" water. A r t i f i c i a l seawater was prepared as pre viously described (2). We monitored the decay of Br2~ at 360 nm and f i t data from this decay to Equation 1: 2

-d [Br -]/dt

=

2

a[Br "] 2

+

k(2) [ B r " l

2

( 1 )

2

with a computer program (MIXKIN) which finds values for α and Β (B = k(2)/e where e = 7800 M" cm (M = moles/fi.) for B r ~ according to Zehavi and Rabani (4) and i n i t i a l o p t i c a l density (A ) based on an integrated form of Equation 1 ( l ) . This paper examines the environ mentally s i g n i f i c a n t exponential process involving components of the carbonate system in seawater. The second-order process i s discussed in d e t a i l in the preceding paper (1). In order to investigate the dependence of α on dissolved C0 and pH, we prepared solutions of simpler composition in which the concen trations of several important free ions and ion-pairs were maximized. In order to do t h i s , we calculated the chemical speciation of carbon ate/bicarbonate i n seawater and in solutions of KC1, NaCl, MgCl and mixtures of these salts as a function of pH in the range 6.0 to 8.9. Ion strengths were kept equivalent to those of natural seawater (v ~0.7 at S = 35°/··). For the seawater calculations, we assumed the concentrations of major cations given by Culkin (5) and a t o t a l C0 concentration of 2.333 χ 10" M. We used an i t e r a t i v e computer program to solve a series of simultaneous equations based on thermo dynamic association constants and estimated a c t i v i t y c o e f f i c i e n t s at ionic strengths of solutions for free carbonate and bicarbonate, and for the carbonate/bicarbonate ion-pairs with Mg , C a and Na (6). Our calculations did not consider the reduction in available cations due to ion-pairing with S O 4 " (7K nor any potential contributions from ion t r i p l e t s such as MgCaC03*+ or Mg C03 (8). These l i m i t a tions are not expected to have important influences on the r e s u l t s . Examples of the resulting d i s t r i b u t i o n s of the various carbonate/ bicarbonate species for seawater at pH 6.0 and pH 8.1 are given in Figure 1. The carbonate speciation of the KC1, MgCl and NaCl solutions are given in Table I. The dependence of α upon one or several major forms of bicarbonate/carbonate was investigated for each of these solutions. 1

-1

2


0

1

2

2

3

2

2+

2 +

+

2

2+

2

2

Results The decay of B r ~ in the various inorganic solutions studied follows the expected second-order kinetics when no aqueous CO2 i s present (Fig. 2, 1). In the presence of C0 , there i s a small pseudo-firstorder component (a) which can be quantified even in the presence of the much larger B term. However, the presence of multiple decay pathways results in substantial uncertainties i n the value of the α and B terms, resulting i n the rather large scatter of the data. 2

2

Note that k(2) refers to second-order

kinetics.



108

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

Figure 1. Carbonate speciation in seawater at pH 6.0 and 8.1. The percent of H2CO3 pictured for pH 6.1 includes dissolved C 0 . 2

6.0

Figure 2. [C0 ]-normalized values of α i n solutions of 0.65 M KC1 with 8 χ ΙΟ" M Br" and added K C0 /KHC0 as a function of pH. • Individual kinetic determinations; — Case 1: F i t of data i f CO3 " i s only contributor to Y i n Equation 2 in KC1 solution; Case 2: F i t of data i f Equation 2 i s : (α-C) = I k ( [ C O 3 ] + 1/30 [HCO3"]) where k was determined as i n Case 1. 2

4

2

3

3

2

y

2

y



f

0.25'

7.5 7.5 8 .0 8 .0 8 .1 8 .1 8 .1 8 .2 8 .3 5

6

6

6

5

5

6

4.24xl0" 1. 71x10-5 1.17xl0" 4.73xl0" 9.88xl02. 59xl0" 2.83xl0" 6.72xl07.88x10-5

4

4

5

4

4

4

5

2. 32xl0" 1.02xl0~ 2.60xl0" 5.15xl0" 6.H x l O 1.27xl02. 55xl0"

7.00 7.6 8 .0 8 .3 8 .3 8 .3 8 .3

0.65

4

1.40χ10~

8 .21

1.3

2

Free [CO3 -] (M)

PH

n+

[Me ] (M)

8.87X103. 57x10-3 7. 75xl0" 3.13x10-3 5.19xl0" 5. 42xl01. 49x10-3 2. 80x10-3 2. 61x10-3

1.43x10 -4 5.67x10 -4 3.94x10 -4 1.57x10 -3 3. 33x10 -4 8.72x10 -4 9.49x10 -4 2.23x10 -3 2.61x10 -3 4

4

4

4

3

2

1. 53xl01. 70xl01. 72x10-2 1. 71x10-2 2.02xl0" 4.21x10-3 8.43x10-3

2

3

0 0 0 0 0 0 0

5. 70xl0-

7.40x10 -4

1

Free [HCO3-] (M)

n

[MeC0 - ] (M) 3

n

9.24x10 -4 3.67x10 -3 8.06x10 -4 3.21x10 -3 5.41x10 -4 5.66x10 -4 1.54x10 -3 2.88x10 -3 2.68x10 -3

0 0 0 0 0 0 0

1.37x10--3

3

1

[MeHC0 - ] (M)

8

2

xlO" xlO" xlOxlO" xlO" xlO" xlO"

3

3

3

J

J

3

3

3

3

3

3

3

3

3

3

3

3

2.0 x l O " 8.0 x l O " 2.0 x l O " 8.0 x l O " 1.41x10" 2.0 x l O " 4.0 x l O " 8.0 x l O " 8.0 x l O "

17.7 17.7 17.7 17.7 2.1 4.4 8.8

8.0 x l O "

Total C0 (M)

Chemical Speciation of Carbonate and Bicarbonate as a Function of pH and Total CO2 i n Aqueous Solutions of Some Metal Ions

* Highly supersaturated MgC03 solutions are very slow to form p r e c i p i t a t e s , making i t possible to do these experiments. Thermodynamically, MgC03( ) should p r e c i p i tate.

Mg2 +

Na

n+

Metal Ion (Me )

TABLE I.


110


In seawater near pH 8.1, the α term, as determined by UV f l a s h photolysis, i s 2470 + 520 for a variety of natural seawaters. In pH 8.1 seawater free bicarbonate and bicarbonate ion pairs with magnes ium, sodium and calcium together account for 86.401 of the t o t a l d i s solved CO2. Magnesium carbonate ion pair (8.51 of t o t a l C0 ) i s the most abundant carbonate form in seawater; sodium carbonate ion pair and free carbonate together account for 3.51 of the t o t a l C0 . How ever, since for the well-documented reactions of hydroxyl r a d i c a l with the carbonate/bicarbonate system, CO3 " i s about 30 times more reactive than HCO3" (9), we expected that reactions between B r " and the carbonate system in seawater would be even more selective, hence dominated by Br ~/carbonate interactions. Since there were no published rates for the r e a c t i v i t y of carbonate ion pairs toward i n organic r a d i c a l s , we determined the r e a c t i v i t y of the major carbonate ion pairs, MgC03° and NaC03", by examining the rate of disappear ance of B r ~ as a function of added C0 i n solutions in which H g and Na were the major cations (Table I ) . We measured the r e a c t i v i t y of free carbonate and bicarbonate towards B r " in solutions i n which K was the major cation. We assumed that the r e a c t i v i t y of CaC03° was the same as that of MgC03°. The exponential decay of B r ~ i n the various s a l t solutions proved to be a function of pH-dependent and pH-independent factors. The pH dependent term varied with t o t a l C0 , while the pH independent term was constant over a wide range of t o t a l C0 concentrations. Therefore, we expressed the exponential rate, a, as in Equation 2: 2

2

2

2


2

2+

2

2

+

2

+

2

2

2

α = C

+

( 2 )

JkylY]

where Y denotes the reaction partners of B r " involving C0 containing species. The constant, C, i s of uncertain o r i g i n . In the simple case of pure solutions of 0.65 M KC1 with 8 χ 1 0 ~ M KBr near pH 8.1, added K C0 and KHCO3 are present as free HCO3- (971) and free C 0 " (31) (see Table I ) . We f i t t e d our data to two extreme cases (Fig. 2). For Case 1, we assumed that free C03 ~ was the sole species contributing to Y (Equation 2) and c a l c ulated a regression on the a s versus [CO3 "] for the individual sample determinations. From this calculation we determined that C03 ~ = M" s" , with a regression c o e f f i c i e n t , r = 0.73. For the other extreme case (Case 2), we f i t t e d the experi mental k's to Equation 2, assuming that *HC03~ * *0©3 " (a maximum based on the values for the more reactive, less selective OH r a d i c a l ) , and obtained a *α>3 ~ = 5" HC03~ 2.17 χ 1 0 M" s with a regression c o e f f i c i e n t , r = 0.60. Plot ting the log of the measured a's (corrected for the constant rate, C, measured when no C0 i s present) versus pH gives the individual points in Figure 3. The s o l i d l i n e represents a Case 1 f i t (small bicarbonate r e a c t i v i t y ) versus pH, whereas the dashed l i n e represents a Case 2 f i t (small bicarbonate contribution). At pH 8.1, either assumption f i t s the data well, while at lower pH's, Case 1 represents the data better. In Figure 4 the measured exponential rate constants for decay of B r ~ in 0.25 M MgCl are plotted versus concentration of magnesium carbonate ion pair, [MgC0 °]. Closed c i r c l e s represent points ob tained when MIXKIN was allowed to find a three-parameter f i t (α, Β 2

2

4

2

3

2

3

2

1

k

2

1 < 4 3

x

1 0 ?

1

2

1

2

s

2

5

1

_ 1

6 , 5 0

x

1 0 6

1

1 / 3 0

s _ 1

o f

a n d

2

k

2

2

2

2

3


=


8.

Reaction ofBr{ Produced by Flash Photolysis

TRUE AND ZAFIRIOU

111

[MgC0 °],mM 3

Figure 3. Dependence of α upon concentration of MgC03° in 0.25 M MgCl with 8 χ 10~ M Br" and added K C0 /KHC0 . · Three parameter ( A , α, B) f i t to i n d i v i d u a l k i n e t i c determinations. Solid l i n e i s a least-squares regression through the points, r = 0.74; ο Two parameter ( A , a) f i t to individual k i n e t i c determi nations, B = 6.40 χ 10 cm s" ; — Dashed l i n e i s a least squares regression through the points, r = 0.65. 4

2

2

3

3

0

2

0

5

1

2

Figure 4. Variation in the pseudo-first-order rate, a, for the decay of B r " in seawater as a function of pH. Lines are from our model, Equation 3, based upon ky's measured in ionic solu tions. The upper l i n e includes the mean value of C (947 s" ) found for seawater experiments with C0 removed (see text discus sion) . 2

1

2


112


and A ) t o the d a t a . F o r c i n g MIXKIN t o use the mean β from a l l t h e k i n e t i c d e t e r m i n a t i o n s w i t h a three-parameter f i t g i v e s the r a t e s shown as open c i r c l e s . The s o l i d l i n e i s a l e a s t squares f i t t o t h e c l o s e d c i r c l e s , the dashed l i n e a l e a s t squares f i t t o the open c i r c l e s . F o r c i n g MIXKIN t o use the " c o r r e c t " β value d i d not improve the q u a l i t y o f the k i n e t i c d e t e r m i n a t i o n s . T h e r e f o r e we used a k g o ° o b t a i n e d from the s o l i d l i n e o f 3.68 χ 1 0 M" s " w i t h a r e g r e s s i o n c o e f f i c i e n t , r = 0.74 f o r our seawater model. However, i n 0.25 M MgC]2 s o l u t i o n f r e e b i c a r b o n a t e and magnes ium b i c a r b o n a t e i o n p a i r account f o r a t o t a l o f 76X o f the d i s s o l v e d CO2, whereas the combined c o n c e n t r a t i o n o f MgC03° and f r e e CO3 i s o n l y 24X o f the t o t a l . We t h e r e f o r e i n v e s t i g a t e d the dependence of α on t o t a l carbonate and t o t a l b i c a r b o n a t e i n aqueous MgCl2A c a l c u l a t i o n o f α versus t o t a l added c a r b o n a t e , assuming n e g l i g i b l e b i c a r b o n a t e r e a c t i v i t y y i e l d s a k^c (TC = t o t a l c a r b o n a t e ) o f 3.57 χ 1 0 M s w i t h a r e g r e s s i o n c o e f f i c i e n t , r -0.74. Add i n g a t o t a l b i c a r b o n a t e r e a c t i v i t y term ( k ^ ) equal t o l / 3 0 t h o f t h e carbonate r e a c t i v i t y , we c a l c u l a t e d t h a t k^c = 3.30 χ 1 0 M" s and k j B = 1.10 χ 10* M s " w i t h a r e g r e s s i o n c o e f f i c i e n t o f 0.75. I n t h i s c a s e , the f i t o f the data i s very s l i g h t l y improved w i t h a s m a l l b i c a r b o n a t e c o n t r i b u t i o n . S i n c e the improvement i s marginal ( r t o 0.75 from 0.74), and s i n c e the KC1 data show t h a t b i c a r b o n a t e i s not the major r e a c t i v e s p e c i e s , we excluded a b i c a r b o n a t e term from our seawater model. I n a s i m i l a r manner we c a l c u l a t e d t h a t k j j c o ' i s 9.11 χ 1 0 M" s ( r = 0.74) i n s o l u t i o n s o f 0.54 M o r 1.3 M NaCl w i t h added c a r b o n a t e / b i c a r b o n a t e , b u t t h i s r a t e term i s based on o n l y f o u r k i n e t i c determinations. We assumed t h a t kç ço ° would be s i m i l a r i n s i z e t o 0

7

M

C

1

1

3

2


2

6

_ 1

_

1

2

6

- 1

1

_ 1

1

2

6

a

_ 1

3

2

a

*MgC03°

a

n

d

3

t h e r e f o r e set kç co ° = · a

3

6

8

x

1

q

3

6

ϋ " S"" 1

1i n

our model.

2

The r e a c t i o n o f B r " w i t h CO3 " i s not a known p r o c e s s . We s p e c u l a t e t h a t the carbonate r a d i c a l , CO3", would be the p r o d u c t : 2

R2:

Br " • 2

2

C0 " 3

·• 2 B r "

+

C 0 " (?) 3

The CO3- r a d i c a l absorbs a t 600 nm ( 1 0 ) , b u t i n seawater no c l e a r s i g n a l a t t h i s wavelength was e v i d e n t . I n MgC03° s o l u t i o n s , t h e r e i s a d e f i n i t e a b s o r p t i o n grow-in a t 600 nm t h a t accompanies the Br2~ decay a t 360 nm. However, we were unable t o q u a n t i f y the r e l a t i o n s h i p between the two a b s o r p t i o n s and so cannot prove t h a t the s t o i c h i o m e t r y o f R2 i s c o r r e c t . Q u a l i t a t i v e l y , i t appears t h a t the s i z e of the 600 nm t r a n s i e n t i s too s m a l l t o account f o r a l l o f the Br2~ d e c a y i n g by the p s e u d o - f i r s t - o r d e r pathway. We a l s o showed by p u l s e r a d i o l y s i s t h a t CO3 - a c c e l e r a t e s t h e decay o f B r 2 " (formed from OH * B r ~ ) , b u t f u r t h e r work i s r e q u i r e d to c h a r a c t e r i z e the r a t e c o n s t a n t , s t o i c h i o m e t r y , and p r o d u c t spec trum o f the Br2~ + CO3 - i n t e r a c t i o n (R2). 2

2

Discussion Our f l a s h p h o t o l y s i s s t u d i e s o f seawater and r e l a t e d s i m p l e r s o l u t i o n s show t h a t Br2~ i s produced i n these experiments ( 1 ) , and t h a t i t decays through a c o m b i n a t i o n o f f i r s t - and second-order p r o cesses ( E q u a t i o n 1 ) . I n t h i s paper we have examined a p r e v i o u s l y


8.

113

Reaction of Br{ Produced by Flash Photolysis

TRUE AND ZAFIRIOU

unknown, but environmentally important reaction between B r 2 ~ and carbonate which occurs i n seawater. In aqueous solutions of KC1, NaCl and MgCl2 we have measured the rates of reaction of B r 2 " with the relevant carbonate species. Our data show that there i s no stoichiometrically s i g n i f i c a n t reaction of 00 » H2CO3, free bicarbonate or bicarbonate ion pairs with ΒΓ2~. Therefore Equation 2 can be applied to explain the pseudo-first-order decay of B r 2 ~ i n seawater by substitution of the relevant ky's f o r the various carbonate species measured i n simple ionic solutions i n Equation 3: 2

7

2

6

k = (1.43 χ 1 0 ) [CO3 -] + (3.68 χ 1 0 ) [MgC0 °] + (3.68 χ 1 0 ) [CaC0 °] + 9.11 χ 1 0 [NaC0 -] + C p

6

3


(3)

3

6

3

The rate terms measured i n ionic solutions multiplied by the seawater concentrations of the relevant carbonate species (Table II) predict a pseudo-first-order decay rate, α « 1.69 χ 1 0 s at pH 8.1 i n sea water with a t o t a l CO2 content of 2.333 mM. In seawater, however, as in the ionic solutions studied, there i s a small pH and CO2-indepen dent term, C, contributing to a. The mean value of this term, as determined i n s i x Sargasso seawater experiments with no CO2 present, is 947 β" . When this average C i s included i n the predictive equa tion, then kp = 2.64 χ 1 0 s at pH 8.1. The mean measured sea water α at pH 8.1 l i e s between the two predicted values at 2.47 + 0.52 χ Ι Ο β" . 3

- 1

1

3

3

_ 1

1

TABLE I I . The Exponential Decay of the 360 nm Transient i n Seawater and Various Salt Solutions a

3

Solution

ΙΟ" α (s-1)

Coastal Seawater Sargasso Seawater A r t i f i c i a l Seawater 0.65 M KC1 0.25 M MgCl 0.54 M or NaCl

Rate Term k 2

C0 3

[Υ], f o r Y =

y

MgC0 ° (ΙΟ"

NaC0 ~ s" )

3

CaC0 °

3

3

3

1

2.21+0.41 2.57+0.54 2.48+0.25

b

0.40 d

0.73

c

2

0.08

0.48

e

Natural seawaters and a r t i f i c i a l seawater pH -8.1+0.1. A l l a r t i f i c i a l seawater and s a l t solutions contained 8 x l 0 ~ M KBr. a: Rate term based upon the concentration of the relevant carbonate species i n pH 8.1 seawater containing 2.333xl0" M t o t a l CO2. b: pH 7.05 - 8.3; regression c o e f f i c i e n t , r = 0.73, η = 21 c: p H = 7 . 5 - 8 . 5 ; regression c o e f f i c i e n t , r = 0.74, η = 31 d: Rate constant kç co°j assumed = k ^ g ^ e: pH 8.0 - 8.2; regression c o e f f i c i e n t , r = 0.74, η = 4. 4

3

2

2

a

2



114


A visual examination of the pH dependence of the exponential rate term (Fig. 4) shows that inclusion of the mean C in Equation 3 explains the f u l l pH range of experimental values better. The o r i gin of this constant term i s uncertain. Although i t might be due to impurities in the "pure" chemicals used to make ionic solutions, the presence of a constant term of e s s e n t i a l l y the same size in natural seawaters argues against reagent contamination as the source. We believe that this term may be an indirect result of the manner in which MIXKIN f i t s the kinetic data. Since the program forces a three parameter f i t , there i s a tendency for the f i t of the "least important parameter" to contain more error than the f i t of the parameter of interest (3). In the case where the decay of Br2~ i s almost exclusively second-order (no CO2 present), the r e l a t i v e errors in α are large at the same time that the absolute value of α i s small. Thus the true value of α in C02-stripped seawater may be close to 0 even though MIXKIN finds a mean value of 947 s . An alternative explanation for the small pH-independent portion of the seawater a's i s that some side reactions are occurring which affect the f i t t e d parameters. Thus, i n summary, most of the fast exponential decay, a, of Br2" observed in seawater i s due to an interaction with carbonate species in which CO^ ' NaC03~ and MgC03° are a l l important reactants. The reaction product i s unknown and the significance ( i f any) of the term C i s also unclear. I t i s interesting to note that in fresh water OH reacts with the carbonate/bicarbonate system d i r e c t l y (11). In seawater, OH i n i t i a l l y gives r i s e to Br2". which then reacts with carbonate spe cies in an unknown reaction. Efforts are underway to study this reaction in more d e t a i l using pulse r a d i o l y s i s , since the products are currently unknown, though CO3- seems l i k e l y . I f CO3- i s the product, the p r i n c i p a l effect of B r in seawater i s just to act as an intermediate in converting OH to CO3-. _ 1

2

t

-

Acknowledgments We g r a t e f u l l y acknowledge the assistance of Drs. E l i e Hayon and Ed Black for access to the excellent Natick f a c i l i t i e s and technical guidance; Drs. John Connolly and Jim B e l l for access to relevant computer programs for the kinetic analyses of the f l a s h results; Dr. M. A. J. Rodgers for assistance at CFKR; and Drs. Woolcott Smith and Derek Spencer for guidance in modifying the computer programs for our use at Woods Hole. This work was funded by the National Science Foundation grants GA-35401, DES 72-1553, OCE-77-09381 and OCE 84-17770. Flash photolysis studies were carried out at the U. S. Army Natick Laboratories. Pulse r a d i o l y s i s studies and their data reduction were carried out at the Center for Fast Kinetics Research. The CFKR i s supported j o i n t l y by the Biomedical Research Technology Program of the Division of Research Resources of NIH (RR 00886) and by the university of Texas at Austin. This is Contribution No. 6035 from the Woods Hole Océanographie Institution.


-

8. TRUE AND ZAFIRIOU Reaction of Br Produced by Flash Photolysis 2

115

Literature Cited


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RECEIVED June 24, 1986


Photochemistry of Environmental Aquatic Systems - American

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