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d Cosmochimica
Acln Vol. 57. p. II57
Copyright 0 1993 Pergamon
Press
Ltd.Pnnted
in U.S.A
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REPLY
Reply to the Comment on “Speciation of aqueous palladium(I1) chloride solutions
using optical spectroscopies”
C. DREW TAIT and DAVID R. JANECKY
Los Alamos National Laboratory, INC Division, MS-J514, Los Alamos, NM 87545, USA
(Received September 8, 1992; accepted in revised form September 25, 1992 )
neutral chloride solutions, but for concentrated Pd (3 mM).
THE THOUGHTFULCRITICALComment of zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
BYRNE and KUMP
Furthermore, they report a chloride-free Pd precipitate, in( 1993 ) presents different opinions/ interpretations to two
dicating continued loss of Cl- after the formation of Pdprevious works on palladium speciation, namely TAIT et al.
C&OH *- . Therefore, the reaction associated with the observed
( 199 1) and WOOD ( 199 1) . In essence it argues that the exslow kinetics cannot be proven to be polymerization or further
perimental data in each of these papers should be taken more
hydrolysis. Even if the reaction is polymerization, it may
at face value than the original authors did. BYRNEand KUMP
suggest that the mixed chlorohydroxy palladium species are
( 1993) present arguments to show that the presence of mixed
not stable, with the chlorides ultimately being replaced by
ligand chlorohydroxypalladium (II) complexes extends to low
(bridging) oxygens.
concentrations relevant to seawater (in contrast to a concluThe identity of the slow reaction beyond the formation of
sion of TAIT et al., ( 199 1) and that the hydrolysis constants
PdC&OH *- remains an important but unanswered question.
measured by WOOD ( 1991) are in fact not too small and
should not be rationalized away. The self-consistency of the
Even if colloid is forming under the conditions which have
equilibrium values presented in the BYRNEand KUMP ( 1993)
been examined experimentally, it may still represent the end
comment is very alluring, especially when they are achieved
of a reaction in which the chlorides are replaced by hydroxide
from both statistical predictions and linear free energy relabefore polymerization, and hence call into question what actions. Several interesting and valid points are made by BYRNE
tually happens under the lower Pd concentrations of seawater.
and KUMP (1993), but perhaps some counterpoints should
Indeed, the Comment of BYRNE and KUMP (1993) notes
be considered. We would like to respond to the comments
the possibility of unusually slow kinetics for Pd(OH)* formade on our previous paper.
mation, although they do not favor this explanation. This
As noted in their Comment BYRNE and KUMP (1993),
Reply basically attempts to underline this possibility. Ultiwe dutifully reported mixed Pd-Cl-OH complexes under the
mately, as noted by KUMP and BYRNE (1989), inorganic
given experimental conditions. In fact, our Fig. 4a gives direct
palladium speciation may be eclipsed by organic complexspectral evidence for the mixed complexes and strongly
ation in some and perhaps many natural environments, inbacks up the inferred evidence from curve fitting of KUMP
cluding in seawater. The case for hydroxy-pseudocolloids
and BYRNE ( 1989). However, we closely observed two dif(Pd( OH), attachment to organic colloids) may also be made.
ferent palladium concentrations (124 PM and 3 PM) and
Thus, while we appreciate the self-consistency from different
observed less evidence for the mixed complexes at lower conapproaches and the detailed analyses of BYRNE and KUMP
centration. We, therefore, felt that extrapolation to even lower
( 1993), we feel there is still some room to suspect that Pd
Pd concentrations relevant to seawater was not warranted.
behavior and complexation is not quite as neat as presented
BYRNE and KUMP ( 1993) are correct in pointing out that
by BYRNEand KUMP ( 1993), and that extrapolation to seaOH- is not a thermodynamically limiting reagent (with OHwater systems may still present some difficulties.
Editorial handling: G. Faure
being available from water from hydrolysis reaction with the
metal), but in our view it could be a kinetically limiting
REFERENCES
factor.
We (as well as others: e.g., KAZAKOVAand ~ITSYN, 1967;
BYRNER. H. and KUMPL. R. ( 1993) Comment on “Speciation of
NABIVANETSand KALABINA,1970) reported a slow chemical
aqueous palladium (II) chloride solutions using optical spectroscopies” by C. D. TAIT, D. R. JANECKY, and P. S. Z. ROGERS.
reaction from pH changes, absorbance changes, and the
Geochim. Cosmochim. Acta 57, 115 1-l 156 (this issue).
appearance of precipitate at higher Pd concentrations.
KMAKO~A V. I. and PTITSYN B. V. ( 1967) Hydrolysis of halogenoWe interpreted this slow reaction as continued hydrolysis
complexes of palladium. Russ. .I. Inorg. Chem. 12, 323-326.
beyond the quickly forming PdC130H2- (i.e., PdC130H2KUMP L. R. and BYRNE R. H. ( 1989) Palladium chemistry in seawater. Environ. Sci. Tech. 23, 663-665.
+ OH- - + Pd(OH)2 + 3Cl-), while BYRNEand KUMP
NABIVANETS B. I. and KALABINA L. V. ( 1970) State of palladium
( 1993 ) assign it to a polymerization reaction that would not
(II) ion in perchlorate solutions. Russ. J. Inorg. Chem. 15, 8 18occur at the low Pd concentrations found in seawater and
821.
hence would not further affect the PdC130H2- initially
TAIT C. D., JANECKY D. R., and ROGERS P. S. Z. ( 199 1) Smiation
of aqueous palladium (II) chloride solutions using optical specformed. While BYRNE and KUMP ( 1993) cite NABIVANETS
troscopies. Geochim. Cosmochim. Acta 55, 1253-1264.
WOOD S. A. ( 199 1) Experimental determination of the hydrolysis
constants of Pt2+ and Pd2+ at 25°C from the solubility of Pt and
Pd in aqueous hydroxide solutions. Geochim. Cosmochim. Acta
55, 1759-1767.
and KALABINA ( 1970) as a reference to polymerization,
NABIVANETS and KALABINA ( 1970) only showed polymerization
for acidic, non-Cl_ solutions with [ Pd] > 10 PM. KAZAKOVA
and PTITSYN ( 1967) also mention polymerization for near1157