1647
Exchange of Oxygen between Carbon Dioxide and Carbonate
Kinetics of the Exchange of Oxygen between Carbon Dioxide and Carbonate in Aqueous Solution C. K. Tu and D. N. Silverman" Department of Pharmacology and Therapeutics, College of Medicine, University of Florida, Gainesville, Florida 326 10 (Received September 12, 1974; Revised Manuscript Received May 1, 1975)
A kinetic analysis of the exchange of oxygen between carbon dioxide and carbonate ion in alkaline, aqueous solutions is presented. The exchange was observed by placing ls0-labeled carbonate, not enriched in 13C, into solution with 13C-enriched carbonate, not enriched in I80. The rate of depletion of lSO from the 12Ccontaining species and the rate of appearance of l80in the 13C-containing species was measured by mass spectrometry. From these data, the second-order rate constant for the reaction between carbon dioxide and carbonate which results in the exchange of oxygen a t 25' is 114 f 11M-l sec-l. I t is emphasized that this exchange of oxygen between species of C02 in solution must be recognized in studies using oxygen-18 labels to determine the fate of COz in biochemical and physiological processes.
Introduction Much information concerning the reactions of C02 in aqueous solutions has been obtained by studies based on the use of isotopes. Mills and Ureyl determined the rate constant for hydration of COz in acid media with oxygen-18 labels. Using similar techniques, Poulton and Baldwin2 showed that bicarbonate and C032- could catalyze the hydration-dehydration reactions of COz. In certain cases the use of isotopic labels led to results a t variance with kinetic constants obtained by other techniques. For example, Himmelblau and Babb3 reported experiments in which 14C-labeled bicarbonate was placed into bicarbonate solutions. They found that the rate of appearance of 14C02 in the solutions was 50 to 130 times faster than could be accounted for in terms of the accepted bicarbonate dehydration rate. The unusual results of Himmelblau and Babb3 have stimulated speculations on the nature of the intermolecular interactions of species of COP in aqueous solution^.^,^ Furthermore, Gerster et aL6 have obtained evidence that there is an exchange of oxygen between COz and C032- in aqueous solution, a fact which qualitatively explains the results using carbon-14 labels. The exchange was observed by placing 180-labeled carbonate not enriched in 13C into aqueous solutions a t alkaline p H containing 13C-labeled carbonate not enriched in lSO. The detection by mass spectrometry of species such as l3ClSO2from these solutions was taken as evidence of intermolecular exchange of oxygen. By varying p H and reactant concentrations, Gerster e t aL6 established the following reaction as responsible for the observed lSO exchange.
The purpose of this report is to expand the initial work of Gerster e t aL6 by deriving a kinetic expression for the isotope exchange rate. From this kinetic expression we are able to determine the second-order rate constant for the oxygen exchange reaction between COz and C032- using mass spectrometric techniques.
Experimental Section A. Materials. Oxygen-18 enriched mixtures of potassium carbonate and potassium bicarbonate were prepared by dissolving these compounds in Is0-enriched water (Miles
Laboratories, normalized 60 atom % initially), letting the resulting solution stand overnight, and then distilling off the water. Carbon-13 containing potassium carbonate solutions were prepared by the following procedure. 13C02was generated by adding 4 N phosphoric acid to 13C-enriched barium carbonate (Merck, 90 atom %). The COz generated was then absorbed into a small volume of 1 N NaOH solution within a vacuum system. The resulting solution was neutralized with HC1. B. Procedure. A 5-ml glass syringe fitted with a water circulation jacket was used as the reaction vessel. The temperature was maintained a t 25 f 0.1' by circulation of fluid using a Haake Type F constant temperature circulator. The solution in the vessel was in contact with no gas phase from which C02 could be absorbed or into which C02 could escape. A volume of solution containing 13C-enrichedcarbonates not enriched in lSO was adjusted to a selected p H with NaOH or HC1 and introduced into the reaction vessel. Following this, a t zero time, a solution containing lS0-enriched carbonates not enriched in 13C, adjusted to the same pH, was rapidly mixed with the 13C-containing solution. The final total volume in the reaction vessel was 5 ml and the concentration of all carbon-containing species was 50 mM. Aliquots of 0.3 to 0.5 ml in volume were removed periodically from the reaction vessel by injection through a serum stopper into an evacuated flask containing 9 M sulfuric acid. This rapidly stopped the exchange processes and released COz from all carbonate species in solution. The C02 liberated was passed through a trap immersed in a Dry Iceacetone bath and collected in vials. Enrichments of the COz thus collected were measured on a Finnigan 3000 quadrupole mass spectrometer a t an ionizing voltage of 70 eV. The following atom fractions of ISO in COz were measured; the numbers in parentheses on the right-hand sides refer to the heights of the corresponding mass peaks.
'/,147)
(I3)' = (45)
' = (44)
1/2(46) (45)
+
+
+ (47)
(49) + (49)
+ 1/2(47) + (48) + (49) + (46) + (47) + (48) +
(49)
The Journal of Physical Chemistry, Vol. 79, No. 16, 1975
C. K. Tu and D. N. Silverman
1648
The standard deviations in four measurements of the above ratios were 1.5%or less on any COz sample.
O'
31
Kinetic Method Upon placing lsO-labeled carbonate not enriched in 13C in an aqueous solution with 13C-labeled carbonate not enriched in l80,following the procedures given in the previous section, one observes a simple first-order decay of the total IsO content T in the COz released by acid as shown in Figure 1. The kinetics and mechanism of this exchange of l80 between carbonate species and water is well worked o ~ t . l JThe rate constant for this exchange is simply related t o the rate constant for the dehydration of bicarbonate k' and, since this is an equilibrium experiment, is also related to the rate constant for hydration of COz, k 8 The hydration-dehydration reaction a t equilibrium can be simply described as: k(CO2) = k'(HCO,-)
(3)
where k and k' are pseudo-first-order rate constants and vary with P H . ~ The possible reactions of 180-labeled bicarbonate and carbon dioxide resulting in the exchange of l80 with hydroxide or water are described by the following (in which C is either 12Cor 13C).
5
~~180180180.
~180180
40
20
60
80
100
TIME (MINUTES)
Figure 1. The points represent atom fractions of oxygen-18 found in 13C-containing species (I%-, and the 12C-containing species (%, overall oxygen-18 content of both "C- and '%-containing species r as a function of time during the course of isotope exchan e. The temperature was 25', pH 8.12, and the concentration of Q2C-containing carbonate species in solution was 28 m M and 13C-containing carbonate species in solution was 22 mM.
+ 1 8 0 ~ -
Carbon labeling allows us to follow not only total l80 content but also the l80 content of the 12C- and 13C-containing species in solution. The rates of change of (12)7and (13)T are shown in Figure 1. Following Gerster et aL6 we assume a mechanism for exchange involving the interaction between COz and carbonate. The possible reactions which result in an intermolecular exchange of oxygen are described below with h2 representing the second-order rate constant for intermolecular exchange. 13~1600'60
0.01
+
not written out here, must be considered; namely, the equations for which 13C160180 and 13C180180are reactants. Furthermore, an entire set of exchange reactions must be considered analogous to those above but with l2C and 13C interchanged. All of these possible exchange reactions are used in the subsequent derivation. Before writing the rate equations for each of the labeled carbonate species, a simplification is introduced. Since the rate of proton transfer between HC03- and C032- is nearly instantaneous compared to the lSO exchange rates, and since the identity of C032- and HC03- is lost in the final measurements of enrichment of COz, it is convenient to consider the combined concentrations of carbonate and bicarbonate and to represent them as follows: (COOO) = (COCO2*) + (HCOOO?*
- 13~1601800'602-
3 13~160160
+
2 !Zcho180 + !1?c1"160 3
3
When it is necessary to refer to the carbonate or bicarbonate concentration, they are written as (C032-) = F,(COOO) and (HC03-) = F b ( C 0 0 0 ) with F, = (co32-)/[(co32-) f (HC03-)] and a similar expression for F b . In this kinetic analysis, the rates of change of the ISO content of the 12C-containingspecies are used. In all of the equations which follow, C and 0 refer to 12Cand l60unless otherwise indicated. Furthermore, to condense the equations we write:
+ i Z c i F 0 1 6 ~ 1 ~ ~ 2 ~ 13~1601C01802-
+ 12
c 16ol b o
These are only the reactions in which the reactant, I3CO2, is not labeled with lsO. Additional equations derived by the straightforward application of statistical factors, although The Journal of Physical Chemistry, Vol. 79, No. 16, 1975
~
(
1
3
= ~
0( ' 3~ ~ 0 0 )
+ (13c0180) + (13ci80180)
C ( C o o 0 ) = (CooO) +
(cooi80)+
( ~ 0 ~ ~+ 0(c*80i80180~ ~ ~ 0 ) (6)
The appropriate rate expressions are:
1649
Exchange of Oxygen between Carbon Dioxide and Carbonate
In the experimental method, enrichment measurements are taken a t times early in the exchange reaction; that is, the measurements are only made when y' is small (i.e., y' less than 5% of 37). At pH 8.12, measurements are considered in the first 15 min of the experiment. Hence, only the first four points of Figure 1 are taken for the calculation of 122. This approximation is supported using data in the Results and Discussion. Consequently, in eq 10, y' is neglected with respect to 3y to give 2 -3 dt = k'Fb ( y - CY) +kE
(3y
~CY)C('~CO~)
-
(11) Equations 9 and 11 can be solved simultaneously. The solution for y, which is of most interest in this work, has the following form: y =
+
aie-@lt
a2e-@2t
(12)
The expressions for B1 and 02 are the roots of the characteristic equation obtained from eq 9 and 11. 24, = k
+
Fhkt
z('3cooo)] f
+
+
k,Fc[C('3C02)
{ [k
F&'
~ ( ' 3 c o ~ ) ] -] 2'[Fhk' 3
+
k2Fc[C(13C02) f k,F~C('~C02)][k+
k2FcC('3C000)]}'/2 (13)
23 (%X-J'80'80) +
('3C'80'80'80)]}
conThe atom fraction a , given below, describes the tent of 12C-containing CO,; and y describes the l80content of 12C-containingcarbonate and bicarbonate.
The part of these roots which is in the form ( x 2 - y)lI2 can be approximated by 1 x - y/2x1, an excellent approximation since x2/y > 75 in the pH range 7.5 to 10, the pH region investigated in this study. Using this approximation, the roots are expressed as follows: 281.2 = k
+
Fbk'
k2F,[C('3C02)
C('3CooO)] f ~,F,[c('~co~)
The identical expressions for 13C-containing species are a' and y'. The rates of change of the atom fractions a! and y are derived from the previous expressions for the rates of change of concentrations of COZ and COOO.
+
{K
+
F,k'
+
C ( ' 3 ~ ~ -~ ~ ) ]
One of these roots (designated 8 2 ) is very large; as a result, the second term on the right in eq 1 2 is unimportant compared to the first term. The important root is obtained from eq 14.
e,
=
'l,[F,k' + k,F,C('3CO2,][k + k , F c ~ ( ' 3 C 0 0 0 ) ] k -k F,k' + k,Fc[~(13C02) x ( ' 3 c o o o ) ] (15)
81 is the initial slope of a plot of In y vs. time. When the concentration of 13C-containing species is set equal to zero, The Journal of Physical Chemistry, VoL 79, No. 16, 1975
C. K. Tu and D. N. Silverman
1650
TABLE I: Rate Constant for the Exchange of 1 8 0 between Carbon Dioxide and Carbonate at pH 8.6 and25”
r
lool
60-1
40i 38 49 54 65 74
i
1.96 2.05 2.24 1.86 1.96
/f
6
is {[(45)+ (47) + (49)]/[(44) + (45) + (46) + (47) + (48) + (49)]}X 100%. Calculated from experimental data using eq. 17. The total carbonate concentration is 50 mM.
/
a %I3C
I
i
J
-0 01
0.41
+
10.04 Fc
I
eq 15 gives the expression for the rate constant of the ls0 exchange between bicarbonate and water.7
Recognizing that Fbk’ + F , k ~ Z ( ~ ~ c 0=2 (Fbh’/h)(h ) F,hzZ(13C000)), eq 15 is solved for F,k2.
10.2
0 2 i 0
7
-0.006
’./
-0 004
/
-0 002
10 001
1 8
9
10
PH
The atom fractions 01 and y are now related to the experimentally measured quantity (12bof eq 2.
In the range of pH 8 to 10, the fraction of all carbonate species existing as COZ is small (2% a t pH 8, 0.02% a t pH 10). Hence, it is a good approximation to set y = (12)7and neglect the COZ fraction. At pH 7.5 the fraction of C02 is 6% and this approximation is less acceptable.
Results and Discussion Values of F,kz are calculated by use of eq 17 which is based on the assumption that the neglect of y’ in eq 10 is valid. One support for this assumption can be made by examining experimental values of F,k2 a t different 13C enrichments. If the percent 13C is small in solution and the percent 12C-containing species enriched in lSO is large (total carbonate concentration is always 50 mM), then the rate of increase of y’ will be faster than if percent 13C is large. Table I presents values of F,k2 at pH 8.6 and at five different 13C enrichments. The small variations in F,hz support the assumption that y’ is negligible in the initial phase of the intermolecular exchange. This small variation is more difficult to observe at lower values of pH because the exchange rates with the oxygen of water are more rapid and the pH is more difficult to control in this pH region. The values of F,kz obtained from several experiments a t different values of pH and calculated from the data using eq 17 are plotted in Figure 2. The values of F , = (co32-)/ [(C03z-) (HC03-)] are indicated by the solid line. In these calculations the two ionization constants for carbonic and 4.7 X M.9 (Using acid are taken as 1.7 X these values of the ionization constants, we obtained k’, the rate constant for dehydration, using eq 16. The values of h’ determined from our Is0-exchange data agreed to within 5% with the values of h’ given by Gibbons and Edsall.lo The contribution to the ionic strength of carbonate and bicarbonate has been neglected.) The good fit of the data to
+
The Journal of Physical Chemistry, Vol. 79, No. 16, 1975
Figure 2. A comparison at 250 of the rate constants Fck2 with Fc at different values of pH. Fc = (C032-)/[(C032-) 4- (HCO3-)]. The variation of Fc with pH is shown as the solid line with corresponding values of Fc given on the right-hand coordinate. The experimental points represent Fck2 with values given on the left-hand coordinate. Fck2 is obtained from the experimental data using eq 17.
the solid line in Figure 2 confirms the conclusion of Gerster et a1.6 that the intermolecular oxygen exchange in an alkaline pH region occurs between Cog and C032-. Bicarbonate ion is not involved in this exchange, but is involved in the exchange of oxygen with water. Dividing the experimental values in Figure 2 by F, yields as a mean value of h2 (and standard deviation): h2 = 114 f 11M-l sec-l. Kinetic isotope effects are neglected. Comparison of this rate may be made with the hydration ~ the reaction of COz and OH-, having a rates of C O Z . Only rate constant of 8.5 X lo3 M-l sec-l, occurs more efficiently. In contrast with the value of h2, the rate constants for the catalytic effect of C032- and HC03- on the reaction of COz with hydroxide ion are quite small.2 An awareness of and accounting for this exchange of ls0 in aqueous solutions is important in measurements which rely on labeling carbonate species with oxygen-18. This is especially true in studies using I 8 0 labels to determine the fate of CO2 in biochemical and physiological processes and in studies based on the distribution of l80among carbonate species in solution.ll In an alkaline pH region, ls0 can be depleted from labeled COz by intermolecular exchange with carbonate. Furthermore, it has been determined that the zinc-containing enzyme carbonic anhydrase catalyses the exchange of oxygen between species of COZ in solution.6,12 Acknowledgment. The authors thank Professor Thomas
H. Maren for his encouragement and support of this work and Dr. Richard Gerster for many helpful discussions and suggestions. The technical assistance of Mr. George C. Wynns is gratefully acknowledged. This work was supported by United States Public Health Service Grant No. GM 16934.
Pulse Radiolysis of Liquid Ammonia
References and Notes (1) G. A. Mills and H. C. Urey, J. Am. Chem. SOC.,62, 1019 (1940). (2)D. J. Poulton and H. W. Baldwin, Can. J. Chem., 45, 1045 (1967). (3)D. M. Himmelblau and A. L. Babb. Am. inst. Chem. fng. J., 4, 143 (1958). (4)P. V. Danckwerts and K. A. Melkerson, Trans Faraday SOC.,58, 1832 (1962). ( 5 ) J. Koefoed and K. Engel, Acta Chem. Scand., 15, 1319 (1961).
1651 (6) R . H. Gerster, T. H. Maren, and D. N. Silverman, Proceedings of the First International Conference on Stable Isotopes in Chemistry, Biology and Medicine, May 9-1 1, 1973,Argonne National Laboratory, p 219. (7)R . Gerster, int. J. Appi. Radiat. isotopes, 22, 339 (1971). (8)D. M. Kern, J. Chem. fduc., 37, 14 (1960). (9)H. S. Harned and B. B. Owen, "The Physical Chemistry of Electrolytic Solutions", Reinhold, New York, N.Y., 1958 pp 690-694,755. (IO) B. H. Gibbons and J. T. Edsall, J. Bioi. Chem., 238, 3502 (1963). (1 1) D. N. Silverman, Mol. Pharmacoi., I O , 820 (1974). (12)D. N. Silverman and C. K. Tu, to be submitted for publication.
Absorption Spectrum, Yield, and Decay Kinetics of the Solvated Electron in Pulse Radiolysis of Liquid Ammonia at Various Temperatures Farhataziz"' and Lewis M. Perkey Radiation Laboratory,* University of Notre Dame, Notre Dame, indiana 46556 (Received February 12, 1975) Publication costs assisted by the U.S. Energy Research and Development Administration
The transient absorption spectrum of earn- induced in neat liquid ammonia by nanosecond pulse radiolysis shifts to higher energies as temperature decreases. The transition energy a t the absorption maximum increases from 0.67 to 0.88 eV as temperature decreases from 23 to -75'. The portion of bandwidth a t halfmaximum measured from the absorption maximum to the high-energy side of the spectrum is constant over the temperature range from 23 to -75'. For the same temperature range, the primary yield of earn- is constant with a value and average deviation of 3.1 f 0.3. The mechanism of decay of earn- in pulse radiolysis of neat liquid ammonia is complex.
Introduction Various investigator^^-^ have reported a measured 100eV yield of solvated electrons G (earn-) in pulse radiolysis of liquid ammonia under conditions for which G(earn-) can be identified with the primary yield g(earn-) of electrons that survive a period of nonhomogeneous kinetics3 At 23, -15, and -48', reported values of g(earn-) are 3.1,3 3.17,5 and 3.0,4 respectively. For liquid deuterated ammonia, Seddon et al.5 report that the primary yield is invariant over a temperature range from -15 to -50' with a value of g(earn-) = 3.6 f 0.2. An interpretation for the anomalously large g(earn-) is that the reaction of earn- with NH4+ is very S ~ O W ~ compared - ~ to a diffusion-controlled r e a ~ t i o nconse;~ quently, the loss of earn- by the geminate neutralization reaction during the period of nonhomogeneous kinetics is r e d u ~ e d . ~ . ~Such J O an interpretation is supported by the reported decrease of g(earn-) a t 23' with increase in pressure up to 6.7 kbars.ll A systematic determination of g(earn-) for a wide temperature range will contribute to development of a theoretical understanding of earn- and the radiolysis of liquid ammonia. Such studies require knowledge of the absorption spectrum of earn- as a function of temperature. Spectra are available for solutions of alkali metals in liquid ammonia.12-17 However, the absorption spectrum obtained for earn- in solutions of alkali metals in liquid ammonia can be c0mp1ex.l~Pulse radiolysis of neat liquid ammonia provides an alternative technique for determination of the absorption spectrum of earn-.ll In this paper we report determination of the absorption spectrum, yield, and decay ki-
netics of earn- for temperatures in the range from 23 to -75' by study of the pulse radiolysis of neat liquid ammonia.
Experimental Section The purification of ammonia, the irradiation cell, and the pulse-radiolysis system and split-beam technique for the measurement of transient absorption spectra are described el~ewhere.~Jl For irradiation, the cell containing the sample was held in the center hole of an aluminum cell holder that was placed inside a quartz dewar with three Suprasil windows (Figure 1). The aluminum cell holder was designed to accommodate five cells. Only the center hole could be used for irradiation. The four holes around the center hole were shallower than the center hole, and thereby were not in the path of electron beam from the Linac. In one of these holes, an irradiation cell filled with methanol was placed for a continuous monitoring of temperature during irradiation. The remaining three holes were used to cool extra samples. The temperature of the sample was fixed by continuous flow through the quartz dewar of a stream of nitrogen that was obtained by continuous boiling of liquid nitrogen with an immersion electric heater and was brought to the desired temperature by flow through a thermostatically controlled heater. Sample temperature was measured by a copper-constantan thermocouple and was read on a digital thermocouple thermometer (Model DS-100-T3 of Doric Scientific Corp.). The temperature was controlled within f0.2'. For determination of g(earn-), a liquid ammonia sample The Journal of Physical Chemistry, Voi. 79. No. 16, 1975
