Temperature dependence of (SCN)2?- in water at 25-400°C: Absorption spectrum, equilibrium constant, and decay

Article abstract of DOI:10.1021/jp0100506

The temperature dependence of the absorption spectrum of the formation and decay of (SCN)₂^?-, a well-characterized dimer anion, was investigated at temperatures from 25 to 400°C. The absorption peak was found to shift to longer wavelength with temperature (red shift), from 470 nm at 25°C to 510 nm at 400°C. The equilibrium constants K₁ and K₂ for the reactions SCNOH^?- SCN^? + OH^- and SCN^? + SCN^- ? (SCN)₂^?-, respectively, were found to decrease with temperature. Due to the considerable decrease of K₂ with temperature, a rise in temperature shifts the reaction in favor of SCN^?, so the observed yield of (SCN)₂^?- at high temperatures is strongly dependent on the SCN^- concentration. As the SCN^? concentration could be as high as or even higher than the (SCN)₂^?- concentration at high temperatures, a pseudo-first-order decay of SCN^? has to be taken into consideration to account for the overall decay of (SCN)₂^?-. Using the kinetic parameters obtained in this work and available in the literature, the decay profiles of (SCN)₂^?- can be well reproduced for any temperature and KSCN concentration considered. A combination of the simulation and the experimental results reveals a decrease of ∈_max of (SCN)₂^?- with temperature; the degree is ～30percent for a rise from 25 to 400°C.

Full text of DOI:10.1021/jp0100506

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J. Phys. Chem. A 2001, 105, 4933-4939
4933
•-
Temperature Dependence of (SCN)2 in Water at 25-400 °C: Absorption Spectrum,
Equilibrium Constant, and Decay
Guozhong Wu, Yosuke Katsumura,* Yusa Muroya, Mingzhang Lin, and Tomomi Morioka
Nuclear Engineering Research Laboratory, School of Engineering, The UniVersity of Tokyo, Shirakata Shirane
2-22, Tokai-mura, Naka-gun, Ibaraki 319-1188, Japan
ReceiVed: January 4, 2001; In Final Form: January 15, 2001
The temperature dependence of the absorption spectrum of the formation and decay of (SCN)2•-, a well-
characterized dimer anion, was investigated at temperatures from 25 to 400 °C. The absorption peak was
found to shift to longer wavelength with temperature (red shift), from 470 nm at 25 °C to 510 nm at 400 °C.
The equilibrium constants K1 and K2 for the reactions SCNOH•- a SCN• + OH- and SCN• + SCN-
a
(SCN)2•-, respectively, were found to decrease with temperature. Due to the considerable decrease of K2
•-
with temperature, a rise in temperature shifts the reaction in favor of SCN•, so the observed yield of (SCN)2
at high temperatures is strongly dependent on the SCN- concentration. As the SCN• concentration could be
as high as or even higher than the (SCN)2•- concentration at high temperatures, a pseudo-first-order decay of
SCN• has to be taken into consideration to account for the overall decay of (SCN)2•-. Using the kinetic
•-
parameters obtained in this work and available in the literature, the decay profiles of (SCN)2 can be well
reproduced for any temperature and KSCN concentration considered. A combination of the simulation and
•-
the experimental results reveals a decrease of ꢀmax of (SCN)2 with temperature; the degree is ∼30% for a
rise from 25 to 400 °C.
Introduction
as long as the newly formed radicals have optical absorption,
which is strong enough for detection.
The study of solute-solute and solute-solvent interactions
in sub- and supercritical water is important for the application
of supercritical water as a medium for chemical processing such
as waste oxidation and hydrolysis as well as for the understand-
ing of temperature effects on radical reactions in the field of
water chemistry. Chemistry and kinetics of reactions in super-
critical water may change considerably with even a small
variation in temperature or pressure because water properties,
for example, H-bonding and dielectric constant, are very
sensitive to such variations. Spectroscopic methods are widely
used to investigate the change of water properties and its
subsequent effect on solvation processes of solutes. UV-vis
spectroscopy,1,2 Raman spectroscopy,3 NMR,4 and X-ray ab-
sorption fine structure5 have been applied by many researchers
in the study of supercritical water. Time-resolved spectroscopy
is a more powerful technique because it enables one to follow
rapid formation or decay processes of transient species on the
time scale of nanoseconds to milliseconds, providing more
detailed insights into chemical reactions. 2-Naphthol was used6
as one probe molecule for the study of solute-solvent interac-
tions. For this purpose its excited-state deprotonation in super-
critical water is followed with time-resolved fluorescence
spectroscopy. More recently, flash laser photolysis7 and pulse
In our previous work10 the temperature dependence of eaq
-
was studied over a wide temperature range. It was revealed that
-
the absorption peak of eaq shifted substantially to longer
wavelength with temperature, and the spectrum was much
-
broadened in supercritical water. Being the opposite of eaq
,
the OH• radical is a strong oxidizing species, and its direct
observation is not convenient because its absorption band is in
the ultraviolet region and the absorption coefficient is small (540
M-1 cm-1 at 188 nm12) at room temperature. The role of OH•
is important in water radiolysis, and it raises a corrosion problem
in nuclear reactors. The OH• can be converted to another type
of oxidizing radical that is convenient for optical determination
via the reaction with some inorganic ions, for instance, SCN-
and CO32-. The oxidation of SCN- by OH• forms a dimer anion,
(SCN)2•-. At room temperature, this radical has a strong and
broad absorption band centered at 472 nm and its lifetime is
long. Because the absorbance of (SCN)2•- can be conveniently
and accurately determined, N2O-saturated 0.01 M KSCN
aqueous solution is usually used as a standard reference in pulse
•-
radiolysis. Because the (SCN)2 is an active species, rate
constants for its reactions with many substances have been
reported. Unlike the eaq-, the behavior of which at elevated
temperatures has been extensively studied, there was only one
radiolysis8-11 techniques have been applied to the study of
•-
paper by Elliot and Sopchyshyn13 on the decay of (SCN)2
-
radical reactions in supercritical water. Hydrated electrons (eaq
)
over the temperature range of 15-90 °C. They found that the
and OH• generated from water radiolysis are active with many
substances and secondary radicals are easily formed, so it is
possible to study the solvent effect on radical reactions in detail
rate of decay was a function of temperature and SCN-
•-
concentration and that the absorption coefficient of (SCN)2
was independent of temperature over the temperature range of
interest.
In this work, the behavior of (SCN)2•- was investigated over
the temperature range of 25-400 °C covering the supercritical
conditions. Temperature effects on the spectra, equilibrium
* Address correspondence to this author at the Nuclear Engineering
Research Laboratory, School of Engineering, The University of Tokyo,
Hongo 7-3-1, Bunkyo-ku, Tokyo 113, Japan (fax/telephone +81-3-5841-
8624; e-mail katsu@q.t.u-tokyo.ac.jp).
10.1021/jp0100506 CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/26/2001

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4934 J. Phys. Chem. A, Vol. 105, No. 20, 2001
Wu et al.
•-
constants K1 and K2, and decay rates of (SCN)2 were
•-
examined. The SCN- concentration dependence of (SCN)2
was observed to be more substantial at higher temperatures. The
•-
decay profiles of (SCN)2 were reproduced well for any
conditions using the known and some assumed kinetic param-
eters. A pseudo-first-order decay of the SCN• radical is
considered and included in the reaction scheme; its role is
important at high temperatures or even at room temperature with
very low SCN- concentration. This work may also give a clue
to the understanding of behavior of other dimer anions such as
Cl2•-, Br2•-, and I2•- in high-temperature and supercritical water.
Experimental Section
KSCN and other chemicals were of reagent grade and used
as supplied. Distilled water further filtered from a Millipore
system was used for preparation of aqueous solutions. All of
the KSCN solutions (1 × 10-4-0.2 m) were at natural pH
except that in some cases 0.01 m NaOH was added for the study
of pH dependence. The KSCN solution was deaerated by
-
bubbling N2O so that the eaq was rapidly converted to OH•.
The solution was loaded to the irradiation cell by an HPLC
pump with a flow rate of 3-4 mL/min, corresponding to a
residence time of 6-8 s in the cell.
•-
Figure 1. Temporal profiles of (SCN)2 in 0.01 m KSCN solution,
N2O-saturated, dose ) 35 Gy pulse-1. Conditions: 25-200 °C, 200
atm; 300 °C, 250 atm; 350 °C, 300 atm; 400 °C, 350 atm.
Details of the apparatus for pulse radiolysis were described
elsewhere.10 The high-temperature cell made of Hasteloy C273
can withstand temperatures up to 500 °C and pressures up to
500 atm, well beyond the critical point of water. The pulsed
electron beam has an energy of 35 MeV, and its width was 50
ns. A pulsed xenon lamp was used in the optical detection
system to generate light with strong intensity, and the optical
length was 1.5 cm. The analyzing light exiting from the cell
was focused and passed through a monochromator and then
detected by a photodiode. The time-dependent signal was
recorded with a digital oscilloscope. The dose per pulse at room
temperature ranged from 25 to 50 Gy, determined using N2O-
saturated 0.01 M KSCN solution and a Gꢀ{(SCN)2•-} of 5.2 ×
10-2 m2/J at 472 nm.14 Absorbed energy was corrected for
density variation of water with temperature and pressure. The
water density was taken from the equation of Hill.15 The
pressure-volume-temperature relationship of the KSCN solu-
tions was assumed to be the same as that of pure water, and
some consequent errors were allowed. The molal concentration
(moles per kilogram) was used in the discussion of temperature
effects, but in other cases the molar concentration (moles per
liter) was used for convenience.
•-
Figure 2. Normalized absorption spectra of (SCN)2 at different
temperatures. Conditions: [KSCN] ) 0.01 m; 25-200 °C, 200 atm;
400 °C, 350 atm.
difference of 0.196 eV. The peak shift was ascertained by a
number of repeated experiments. It is also noted that the
absorption band of (SCN)2•- broadens with increasing temper-
ature. The spectral change with temperature for (SCN)2•- is very
Results and Discussion
Temperature Dependence of Absorption Spectra. Figure
1 shows time profiles of (SCN)2•- observed at 480 nm for 0.01
m KSCN solution under various conditions. Absorption of
(SCN)2•- was clearly observed up to the supercritical conditions,
-
similar to that for eaq-, although the change for eaq is more
•-
-
remarkable.10,16-18 The red shift of (SCN)2 and eaq with
temperature is likely to be related to the decrease of polarity,
H-bonding, and dielectric constant of water at high temperatures.
Temperature Dependence of GEmax for 0.01 m KSCN
Solution. Because the temperature dependence of the G value
and ꢀmax has not been separately established, the product Gꢀmax
was used to evaluate the radiation effect at high temperatures
by taking a 0.01 m KSCN solution as an example. As shown
•-
for example, 400 °C and 350 atm. The decay of (SCN)2 is
slow at room temperature and significantly enhanced by
increasing the temperature. The lifetime in supercritical water
is shorter by 2 orders of magnitude than in room temperature
water. Also indicated is a decrease of the initial absorbance of
(SCN)2•- with temperature due to the decrease of water density
and changes of some relevant parameters concerned with
•-
•-
(SCN)2 formation, as will be discussed later in the text.
in Figure 3, the Gꢀmax of (SCN)2 increases with temperature
Figure 2 shows the temperature dependence of the absorption
spectrum taken at the end of the pulse. As can be seen, the
absorption peak shifts gradually to longer wavelength with
temperature. The peak shifts from 470 nm at 25 °C (200 atm)
to 510 nm at 400 °C (350 atm), corresponding to an energy
up to 140 °C and then decreases continuously as the temperature
is further increased. The value of Gꢀmax in supercritical water
(400 °C, 350 atm) is less than one-fifth of that in room
temperature water. The Gꢀmax in alkaline solution was also
determined for comparison. With the addition of 0.01 m NaOH,

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•-
Temperature Dependence of (SCN)2 in Water
J. Phys. Chem. A, Vol. 105, No. 20, 2001 4935
Figure 3. Plots of Gꢀmax as a function of temperature for (SCN)2•- in
N2O-saturated (b) 0.01 m KSCN and (9) 0.01 m KSCN plus 0.01 m
NaOH solutions.
the Gꢀmax at room temperature is increased by ∼10% due to
the conversion of H atom to eaq- via the reaction OH- + H f
Figure 4. Concentration dependence of the absorption maximum for
(SCN)2 at various temperatures. Conditions are identical with those
in Figure 1.
•-
H2O + eaq and the subsequent conversion of eaq to OH•
through the reaction with N2O. The result is consistent with
G(OH•) ) 0.6319 (N2O saturation) and G(H) ) 0.06 µmol/J in
water. The Gꢀmax in alkaline solution is nearly constant up to
100 °C and then decreases with temperature. At temperatures
>120 °C, the Gꢀmax in 0.01 m KSCN plus 0.01 m NaOH
solution is always smaller than that in 0.01 m KSCN solution
under otherwise identical conditions. Additionally, the decay
-
-
should be pointed out that the use of concentrated KSCN in
supercritical water causes severe corrosion of the reactor wall.
Ions of Mo, Ni, and W with unusually high concentrations were
detected by inductively coupled plasma (ICP) spectroscopy in
the collected effluent from a test using 0.1 m KSCN at 400 °C
and 350 atm.
•-
Temperature effects on the KSCN dependence of (SCN)2
•-
of (SCN)2 appears to be faster in the alkaline solution over
can be explained by the variation of kinetic parameters involving
the temperature range studied. However, no obvious spectral
difference between the two systems was found.
the (SCN)2•- formation. In N2O-saturated SCN- solutions, the
•-
(SCN)2 formation processes can be illustrated as follows:
KSCN Concentration Dependence of (SCN)2•- and Tem-
•-
perature Dependence of K2. At room temperature, the (SCN)2
•
H2O Df eaq-, OH•, H, HO2 , ...
(1)
(2)
(3)
(4)
yield was found20,21 to increase with SCN- concentration in
the pulse radiolysis experiments due to the higher OH scaveng-
ing capacity of concentrated SCN- solutions. However, this
concentration dependence is not significant. It was shown20 that
the (SCN)2•- yield at 1.0 × 10-4 M SCN- is nearly half of that
at 0.01 M SCN-, and the yield levels off at concentrations of
0.01-1.0 M. As already noted in Figure 3, the Gꢀmax for 0.01
m KSCN solution decreases considerably with temperature,
indicating lower (SCN)2•- yield at high temperatures. However,
the use of high SCN- may result in a higher observable
eaq- + N2O f N2 + O•-
O
•- + H2O f OH- + OH•
SCN- + OH• f SCNOH•-
K1
{\
SCNOH•-
} SCN• + OH-
(5)
(6)
•-
(SCN)2 yield because the concentration dependence is prob-
SCN• + SCN- {K2
\} (SCN)2
•-
ably more substantial in the high-temperature region. To confirm
this speculation, the concentration dependence of (SCN)2•- was
investigated in detail by varying the KSCN concentration.
Figure 4 shows the absorbance maximum of (SCN)2•- at 25-
400 °C with KSCN concentrations ranging from 1 × 10-4 to
0.2 m. As is seen, the concentration dependence is actually more
substantial at high temperatures than at room temperature.
Rate constants for reactions 1-4 are very fast, and reaction 5
lies predominantly to the right in acidic and neutral solutions,
•-
so the determining step for the (SCN)2 formation is reaction
6. An attempt was made to derive the equilibrium constant, K2,
at high temperatures from the experimental results.
•-
According to Elliot et al.,13 K2 as a function of temperature
Leveling off of the (SCN)2 yield at high concentrations is
•-
observed at 25 and 100 °C but is not observed at and above
200 °C. At lower temperatures, the (SCN)2•- yield is relatively
large even at KSCN concentrations as low as 1 × 10-4 m; at
higher temperatures, however, the (SCN)2•- is nearly impossible
to measure at such a low concentration. Although the solubility
of KSCN in supercritical water at 400 °C and 350 atm is
unknown, it must be >0.01 m because the absorbance of
for (SCN)2 can be investigated on the basis of the equation
A∞/A ) 1 + {1/(K2[SCN-])}
(7)
•-
where A∞ is the absorbance per unit dose for (SCN)2 when
all of the SCN• radicals are converted to (SCN)2•- and A is the
absorbance of unit dose at a certain concentration of SCN-.
For the sake of simplicity, the absorbance at 0.1 m KSCN at
each temperature is taken as the A∞. Because the highest KSCN
concentration applied at 400 °C is 0.05 m, the A∞ is obtained
by extrapolating the concentration to 0.1 m.
•-
(SCN)2 at 0.05 m KSCN is much higher than at 0.01 m. By
assuming the same solubility for KNO3 and KSCN and using
the solubility-water density correlation22 for KNO3 at 450-
525 °C, the solubility of KSCN at 400 °C and 350 atm would
be close to 0.1 m. More evidence is that the solubility of KOH
is 1.06 × 10-2 m at 450 °C and 30.4 MPa.23 The electrolyte
solubility has been found to be correlated with density and not
As the dissociation constant of electrolytes decreases with
temperature, the actual SCN- concentration must be smaller
than the added KSCN concentration at high temperatures,
especially in supercritical water. The free SCN- concentration
22
with temperature for both KNO3 and KOH.23 However, it

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4936 J. Phys. Chem. A, Vol. 105, No. 20, 2001
Wu et al.
constant of reaction 5, K1, is expected to vary with temperature.
K1 was reported25 to be 3.2 × 10-2 M at room temperature. In
the present work the temperature dependence of K1 was studied
over the temperature range of 25-400 °C.
When the equilibrium is reached for both reactions 5 and 6,
we get
•-
K1[SCNOH•-] [(SCN)2
]
[SCN•] )
)
(9)
[OH-]
K2[SCN-]
Of the species indicated in eq 9, only (SCN)2•- was optically
determined. Under conditions at which reactions 5 and 6 lie
entirely to the right, concentrations of SCNOH•- and SCN• are
close to zero and (SCN)2•- is at the highest concentration, C∞.
•-
In this work, the (SCN)2 concentration observed in 0.1 m
KSCN solution is assumed to be C∞, being equivalent to A∞/ꢀl,
where ꢀ is the molar absorption coefficient and l is the optical
path. Although the SCN• concentration in 0.1 m KSCN at 300
and 400 °C is not close to zero, it is much smaller than the
(SCN)2•- concentration. Therefore, the assumption will not make
a large error in the conclusion. The (SCN)2 concentration in
0.01 m KSCN solution is taken as A/ꢀl. According to the
material balance
Figure 5. Van’t Hoff plot of K2 over 15-90 °C (O, Elliot et al.13) and
100-400 °C (b, this work). Water densities are 0.967 (100 °C), 0.878
(200 °C), 0.743 (300 °C), and 0.475 (400 °C) g/cm3. Inset: Plots of
•-
•-
A∞/A as a function of [SCN-] for (SCN)2 at various temperatures.
[SCNOH•-] + [SCN•] + [(SCN)2•-] ) C∞
and [SCNOH•-] can be written as
[SCNOH•-] ) A∞/ꢀl - A/ꢀl - [SCN•]
(10)
was calibrated by assuming the same dissociation constant (KDm
)
as for NaCl. The logarithm of the molal dissociation constant
for NaCl as a function of temperature and of the logarithm of
the water density up to 600 °C is given by Ho et al.24
(11)
log KDm
)
-0.997 + 650.07/T + (10.420 - 2600.5/T) log Fw (8)
By substituting eq 11 into eq 9 and rearranging the equation,
we get
where T is the Kelvin temperature and Fw is the water density.
This assumption should make little error in consequence because
both NaCl and KSCN are very soluble in water and dissociated
into only monovalent ions. The correction is minor for tem-
peratures <300 °C but large at higher temperatures, and it is
also very dependent on the KSCN concentration and the water
density. For instance, the percentages of KSCN dissociated into
free SCN- are 96 and 76% for solutions with 0.01 and 0.1 m
KSCN at 300 °C and 250 atm, respectively, but they are changed
to 55 and 31% for solutions with 0.01 and 0.05 m KSCN at
400 °C and 350 atm.
K1(A∞/ꢀl - A/ꢀl)
K1 + [OH-]
[SCN•] )
(12)
If the temperature, pressure, and SCN- concentration are fixed
•-
but the pH is varied, the ratio of (SCN)2 concentrations
determined at two different pH values ([OH-]1 and [OH-]2)
will be
(A∞ - A1)/(K1 + [OH-]1) A1
)
(13)
(A∞ - A2)/(K1 + [OH-]2)
A2
The inset of Figure 5 shows plots of A∞/A as a function of
the reciprocal of [SCN-]. There is a good linearity for all of
the lines, being consistent with eq 7. K2 values at various
temperatures are obtained from these curves. K2 at room
temperature was not determined in this work because it has been
previously reported,13 and the determination requires the use
of very low KSCN concentration. Figure 5 shows the van’t Hoff
plot of K2 using the values obtained in this work in combination
with those previous data. Our data over 100-400 °C (with
densities of 0.967-0.475 g/cm3) agree well with those of Elliot
et al.13 over the range 15-90 °C. The enthalpy and entropy
In the N2O-saturated alkaline solutions, the H atom is first
-
converted to eaq by the reaction with OH- and further
transformed to OH• by reacting toward N2O, so the H atom is
•-
an additional (SCN)2 source. In ambient liquid water the G
-
value of H is ∼10% of the sum of G values of eaq and OH•,
so the contribution of the H atom was assumed to be temperature
independent and corrected for KSCN solutions with the addition
of 0.01 m NaOH. Letting
A1 A∞ - A
A2 A∞ - A1
obtained from the plot are -36 kJ mol-1 and 23 J mol-1 K-1
,
2 ) q
(14)
respectively. Because K2 decreases rapidly with temperature,
the (SCN)2-• yield for a fixed KSCN concentration is decreased.
This is why we observed a rapid decrease of Gꢀmax with
temperature for 0.01 m KSCN solution.
then we get
[OH-]2 - q[OH-]1
•-
pH Dependence of (SCN)2 and Temperature Depen-
K1 )
≈
dence of K1. As already seen in Figure 3, the addition of NaOH
q - 1
•-
to the KSCN solution leads to a decrease in Gꢀmax of (SCN)2
.
[OH-]2
(when [OH-]2 . [OH-]1) (15)
This indicates that the backward process of reaction 5 can no
longer be neglected at high OH- concentration. The equilibrium
q - 1