A. T. WARDAND M. B. MYERS
1374
An Investigation of the Polymerization of Liquid Sulfur,
Sulfur-Selenium and Sulfur-Arsenic Mixtures Using Raman Spectroscopy and Scanning Differential Calorimetry by A. T. Ward and M. B. Myers Xerox Research Laboratories, Rochester, New York
(Recetved September 2 5 , 1968)
Raman spectroscopy has been used to follow the polymerization of sulfur in the pure liquid and in mixtures with arsenic or selenium. These studies have been supplemented by observations of the variation of specific heat associated with the polymerization phenomenon. The relative monomer-polymer concentrations have been determined as a function of temperature from the contribution of the monomer vibrations to the Raman scattering. In the case of liquid sulfur the contribution of polymer vibrations to the Raman scattering was also determined. The polymer concentrations deduced from both observations are in good mutual agreement but are somewhat higher than suggested by earlier work. Addition of 2 at. yo of arsenic to sulfur lowers the polymerization threshold temperature, Tg,from 159 f 3 to 122 f 4', a value which shows near invariance with further additions of arsenic up to 15y0. This lowering of the polymerization temperature is attributed to the participation of an arsenic-sulfur polymer in the propagation reaction. Selenium additions to sulfur reduce the threshold temperature uniformly throughout the experimentally observable temperature range. The lowering of Tg in this case is attributed to the introduction of monomeric species S,Ses-, which can undergo copolymerization with Ss monomer.
Introduction Spectroscopic evidence for the polymerization of sulfur in the pure liquid state and in mixtures with arsenic or selenium has been presented in an earlier paper.' The onset of polymerization was seen to be characterized by a temperature-dependent change in the relative intensities of the spectral lines in the fundamental region of the Raman spectrum. In the case of sulfur-arsenic and sulfur-selenium mixtures, additional lines attributed to distinct molecular species were also observed. This paper describes the results of a quantitative investigation of the temperature dependence of sulfur polymerization in these systems using the techniques of Raman spectroscopy and scanning differential calorimetry. The results are interpreted with respect to current models describing polymerization phenomena in these materials. Experimental Section The apparatus and experimental method for obtaining Raman spectra as a function of temperature have been described in a previous communication.' From the results of this earlier work a rough estimate of the extent of polymerization may be made by comparing the relative intensities of the spectral contributions due to monomer (SB) and polymer (S,) . The monomer contribution is proportional to the area under either of the Raman peaks at 151 and 218 cm-l due to Ss monomer vibrations belonging to the species E2 and AI, respectively. The polymer contribution is given by the area under the Raman spectrum between 400 The Journal of Phwical Chemi8tru
and 500 cm-l less the contributions in this region at 474 em-' (A1 E$) and 437 cm-' (Ea) due to Ss monomer. This latter contribution is estimated from the relative intensities of the peaks at 474, 437, 218, and 151 cm-l near the melting point (120') where no polymer is presumed to exist. The intensity ratio near the melting point is assumed to hold at all the temperatures investigated, i e . , 120-280'. The Raman scattering efficiencies in the 400-500-~m-~ region are assumed to be equal for monomer and polymer and independent of temperature. If the intensity of the peak at 151 em-' = A and the intensity of the peak a t 218 cm-' = B, the combined intensities of the peaks a t 474 and 437 cm-l = C, and the total intensity between 400 and 500 cm-l = D. Then the fraction of total sulfur present as polymer is given by $p = (D- C)/D where experimentally C = B = 2/3A. Figure 1 shows $p plotted as a function of temperature. I n this latest work the extent of polymerization has been estimated by measuring the temperature dependence of the absolute intensities of the Raman lines at 151 em-' (Ez) and 218 cm-I (AI) due to monomeric sulfur (Sa). As the temperature of the sample is raised, the absolute intensity of these lines remains constant until the onset of polymerization and/or optical absorption of the exciting radiation causes their apparent intensity to decrease. The extent of optical absorption of the exciting line has been determined in a separate experiment.
+
(1) A. T. Ward, J . Phys. Chem., 72, 4133 (1968).
POLYMERIZATION OF LIQUIDSULFUR
1375
/
-.p; ,
-20
140
160
180
I
I
,
200 220 240 TEMPERATURE O C
,
,
260
280
{
Figure 1. Temperature dependence of the polymer contribution to the Raman scattering in the 400-500-~m-~region of liquid sulfur.
A collimated beam of white light from a tungsten microscope lamp or red (6328 hi) light from an He-Ne laser was passed through a 1.8-cm path length Pyrex absorption cell containing either liquid sulfur, sulfurarsenic, or sulfur-selenium under an atmosphere of pure, dry nitrogen. The sample was maintained in the liquid state by heat from a cylindrical nichromeelement furnace mounted concentrically with the optic axis. Melt temperature was monitored by a sheathed copper-constantan thermocouple operating through a millivolt potentiometer. The transmitted beam was mechanically chopped and then focused onto the entrance slit of a Spex 1400 double-grating monochromator. The resulting spectrum was detected by an E.M.I. 9558B photomultiplier and P.A.R. HR-8 lock-in amplifier. The compositions studied optically included pure sulfur, prepared from Baker reagent sublimed sulfur, and the mixtures sulfur (95 at. %)-selenium ( 5 at. %), sulfur (95 at. %)-arsenic ( 5 at. %), and sulfur (85 at. %)-arsenic (15 at. %) prepared by direct synthesis from the high-purity elemental components.2 The temperature range studied extended from the melting point (120") up to 260". Some difficulty was encountered with bubbles in molten arsenic-sulfur mixtures. Unlike the sulfur and sulfur-selenium systems studied, these mixtures did not go through a mobile liquid phase between fusion and polymerization and consequently could not "outgas" readily on melting. This problem was overcome in the case of As(5%)S(95%) by preheating the samples to 300" so as to reduce the visosity sufficiently for bulk homo-
genization to occur. However, for As( 15%)S (85%) it was not possible to achieve complete homogeneity by this method. Instead, in this latter case, the effect of optical absorption on the Raman scattered intensities was estimated by following the change in intensity with temperature of the broad peak in the Raman spectrum centered near 340 em-'. This peak has been attributed to a localized vibration of arsenic trisulfide groups incorporated in an extended arsenicsulfur polymer which is present in the system over the entire temperature range investigated. Its intensity should depend only on the arsenic concentration and on the optical density of the system at the exciting wavelength. As will be shown later, this hypothesis was confirmed in the case of As (5%) S (95%) and was assumed to hold also for As (15%) S (85%). The specific heat measurements were made with a Perkin-Elmer DSC-1B scanning calorimeter. Temperatures were calibrated within &lo over the range of interest by employing gallium, benzoic acid, o-terphenyl, and indium standards. Specific heats were calibrated using the specific heat data for A120an3 The magnitude of the characteristic increase in C, in the vicinity of the polymerization threshold temperature, T$, was found to be strongly heating-rate dependent due to the kinetics of the polymerization reaction. The apparent threshold temperature, interpreted from the thermogram trace as the starting point for most rapid ascent of the C, us. T curve, also varied with heating rate. This variation was found to be small at heating rates of 2.5-5"/min. Consideration of other experimental conditions determined 5"/min to be the optimum heating rate. Actual measurement of C, indicates that the values of T$ determined using this heating rate are still 3-4" higher than those determined from the data of West4 obtained with the much slower heating rates of adiabatic calorimetry. The method of interpretation of T g used here is compatible with the analyses of Tobolsky and Eisenberg,S,6who defined this temperature as the point where a small amount of polymer is already present with a high degree of polymerization. In the case of the arsenic-sulfur system the increase in C , associated with the onset of polymerization was not sharp; hence the assignment of T$ became somewhat arbitrary.
Results The fraction q h of total sulfur present as Ss monomer at any temperature T"K is given by
dM
=
IRaman (
T)
IRarnan ( y m p )
ITrans ( T m p )
ITrsns
(T)
(1)
(2) M.B. Myers and E. J. Felty, Mater. Res. Bull., 2, 535 (1967). (3) D. 0.Ginning6 and Furukawa, J. Amer. Chem. Soc., 75, 522 (1953). (4) E. D. West, ibtd., 81, 29 (1959). (5) A. V. Tobolsky and A. Eisenberg, ibid., 81, 780 (1959). (6) A. Eisenberg and A. V. Tobolsky, J. Polym. Scl., 46, 19 (1960).
Volume 79, Number 6 May lQ60
A. T. WARDAND M. B. MYERS
1376 where I R (T) is~the intensity ~ ~ of ~ a Raman peak due to monomer (SS)at temperature T"K and IRaman ( Tmp) is the intensity at the melting point (120"). ITrans( T ) is the intensity of transmission at the exciting wavelength (6328 A) at temperature T"K and ITrana( Tmp) is the intensity of transmission at the melting point. Correction for thermally induced changes in the relative populations of the ground and first vibrational levels is small compared with the experimental error and has been ignored. The fraction of polymer at temperature T"K is given by
170\
+,
= 1-
+p
4M
(2)
The results of transmission and Raman scattering experiments are plotted in Figure 2. In the case of 0% As ( L e . , pure sulfur), As(5%)S(95%), and Se(5%)S (95%), the solid lines represent the temperature dependence of the relative transmission normalized to 100% at the melting point. With the notable exception of Se (59&)S (95%) the normalized value at the melting point is also the maximum transmission. The solid line for As(5%)S(95%) also represents the temperature dependence of the intensity of the Raman peak at 340 cm-I due to arsenic-sulfur polymer vibra-
90
-
80
-
70
-
$ 60z
e w
10-
2
5J 402
20 30
IO
TEMPERATURE *C
Figure 2. Relative intensity of Raman peaks due to monomer vibrations normalized to 100% near the melting point (broken lines). Relative intensity of transmission at 6328 (solid lines) for S, S-As, and 8-Se. The Journal of Physical Chendetry
,
0
S
IO ATOMIC % A S
Figure 3. Variation of the polymerization threshold temperature, T4,in the S-As system. Circles denote calorimeter data and squares denote optical data.
tions. The solid line for 15% As represents the temperature dependence of the corresponding R,aman peak in As( 15%)S (85%) samples. Here the relative intensity has been normalized to lOOyo at the highest temperature corresponding to maximum Raman intensity of the 340-cm-' peak, in this case about 15" below the actual melting point. The broken lines represent the temperature dependence of the intensity of the characteristic 151- and 218-cm-' Raman peaks of SS monomer normalized in the same way. The temperature dependence was found to be the same for both peaks within f0.5010. This is much less than the discrepancy between consecutive runs which amounted to 5 5 % in some cases. With the exception of the experimentally indeterminate case of Se(5%)S(95%), for which the broken and solid lines coincide, a distinct threshold for polymerizsr tion is indicated at the point of divergence of the broken and solid lines. The threshold temperatures of 156" for pure sulfur and of 130" for both arsenicsulfur mixtures determined in this way are compared in Figure 3 with the values obtained by scanning calorimetry. It is seen that there is a large decrease in T+ with small additions of arsenic ( 2 at. %) and near invariance in T+ with further addition of arsenic up to 15 at. %. This variation is clearly demonstrated in the C, vs. T curves for sulfur and sulfur-arsenic mixtures shown in Figure 4. The temperature dependence of q h and 4, as calculated from eq l and 2 is shown in Figure 5. Within the experimental error of ~ 5 all % of the results for
POLYMERIZATION OF LIQUID SULFUR
1377
J
t
I
1
160
170
6*o
9q
100
110
120 I 3 0 140 150 TEMPERATURE O C
Figure 4. Specific heat data for S, 2 at. %As, and 5 at. %As in the temperature region of the polymerieation onset.
sulfur and arsenic-sulfur mixtures can be represented by the same curve appropriately shifted along the temperature axis to accommodate the change in polymerization threshold in going from pure sulfur to sulfur-arsenic. The curves for As (5%) S (95%) and As (15%)S (85%) coincide because these compositions exhibit the same threshold temperature.
-
50
-
40
-
Q,
20
IO
S
30 40 ATOMIC X Sa
SO
60
70
Figure 6. Variation of the polymerization threshold temperature, TQ,in the S-Se system. The solid line connects calorimetrically determined data, the dashed line is computed assuming Sa and Ses equilibria, and the broken line is computed assuming SSand Sese2 equilibria.
The variation in T,,+with composition in the sulfurselenium system was not observable spectroscopically because of the high optical density of the melt. The variation was, however, readily observed by the calorimetric method the results of which are shown in Figure 6. It is seen that there is a rather sharp but uniform decrease in the polymerization temperature with increasing selenium concentration.
Discussion
(ARSENIC -SULFUR) 60
0
-
-
Consideration of the temperature dependence of properties such as viscosity and specific heat of liquid sulfur and liquid selenium has stimulated a suggestion by Gee' of a dynamic equilibrium between long polymeric chains and eight-membered monomeric rings in these systems. Using the results of Gee as fundamental data, Tobolsky and Eisenberg have formalized the considerations for sulfur and applied the model to selenium. The Tobolsky and Eisenberg theory5 relates the temperature-dependent number-average chain length and monomer concentration to the initial monomer concentration and to the equilibrium constants for the initiation and propagation reactions k
SS
e
monomer
TiEMPERATURE O C
Figure 5. Temperature dependence of the sulfur monomer and polymer fractions in S and &As mixtures.
SS (7) G .
+ S,,-S*
sS*
(initiation)
(3)
(propagation)
(4)
dirad ioa 1
ks
aSa*
Gee, Trans. Faraday SOC.,48, 515 (1952). Volume YS, Number 6 Mau 1969
A. T. WARDAND M. B. MYERS
1378 The results of the present study are interpreted in terms of this model of homopolymerization and in terms of *the0 related model of copolymerization developed by Tobolsky and Owenas A complete summary of the relevant formulations, derivations, and original references can be found in the review by Tobolsky and MacKnight , According to the theory of homopolymzeriation, at temperatures a t and above T+, where the degree of polymerization is large, the equilibrium concentration is expressed by
[Sa]
1/Ka
0.505 * 0
0.45
-
OAO
-
0.35
-
0.30
(5)
-
[S8] is the monomer concentration in moles per kilogram, and K3 is the equilibrium constant for reaction 4 which is expressed as K 3 = exp [ ( AC\S3/R)- ( A H 3 / R T ) ] . The standard state of [S,] = 3.90 mol/kg corresponds to unit mole fraction of monomer + M = 1 (at T 5 T $ ) , It follows that the temperature dependence of + M is given by log h
i
=
(Ha/R) C(l/T> - (1/7'4)1
1
\
1
1
I
-$!&
SLOPE-
AH 8 4.6 kcal .mole -1 l.NTERCEPT = IO5/ T# T#
0
-
424.S0 K
(6)
loa QC 1.8
8C
1.9
2.0
2.1
2.2 2,3 1031~
2.4
23
2.6
Figure 8. Graph of log (monomer fraction) against the reciprocal of the absolute temperature.
7c
.e" Z
ec
6C
where T+ is the polymerization temperature and A H 8 is the heat of reaction 4. The temperature variation of +p has been independently estimated by Hammick, Cousins, and Langford'O using melt-quenching .and solvent-extraction techniques and by Poulis, Massen, and Leedenl' using a high-temperature static susceptibility method. The experimental and theoretical results are summarized in Figure 7. The values of +p obtained here from Raman spectroscopy lie near the upper limit of the previous results. Since the threshold temperature T+ is in good agreement with previous work, the A H a derived from analysis of the spectroscopic results will be somewhat higher than estimated by other workers. A plot of log + M 08. 1/T, shown in Figure 8, yields the value A H 8 = 4.6 kcal mol-'; cf. AH8 = 3.2 kcal mol-' (Tobolsky and Eisenberg).6
0'
2 !fi 8
5c
4c
3c
2c
I(
C
140
160
200 220 240 TEMPERATURE O C
180
260
280
Figure 7q..Temperature dependence of the polymer fraction in liquid sulfur according to: 1, this work (Raman spectroscopy); 2, Gee' (interpretation of soivent extraction data of Hammick, et al.) ;la 3, Poulis, Massen, and Leeden" (static susceptibility); 4, Tobolsky and Eisenberg6 (theoretical); and 5, Fa,irbrother, Gee, and MerralP (interpretation of heat capacity data of B.Braune and 0. Moller, 2.Naturforsch., 9a, 210 (1954)). The Journal of Physical Chsmiatry
(8) A. V. Tobolsky and G. D. T. Owen, J. Polym. Sci., 59, 329 (1962). (9) A. V. Tobolsky and W. J. MacKnight, "Polymeric Sulfur and
Related Polymers," Interscience Publishers, Inc., New York, N. Y., 1965.
(10) D.L. Hammick, W. R. Cousins, and E. J. Langford, J. Chem. Soc., 797 (1928).
(11) J. A. Poulis, 0. H. Massen, and D. Soc., 58, 474 (1962).
V. D. Leeden, Trans. Faraday
POLYMERIZATION OF LIQUID SULFUR
1379
-
The experimental error in (pp considered to vary from =t4% near T+ to &lo% at T - T+ 100". The resulting uncertainty in AH3 is of the order of fl kcal- mol-1, a value which embraces many of the previous experimental and theoretical data. The results of this investigation can therefore be considered as additional support for the equilibrium polymerization model. Arsenic addition to sulfur is known to cause initial removal of some of the sulfur during the formation of an arsenic-sulfur polymer' consisting of a network of sulfur chains branched by trivalent arsenic atoms. The effect of the presence of arsenic in an arsenicsulfur mixture below the sulfur polymerization threshold is, therefore, a reduction in the fraction of S S monomer per unit volume. This monomer fraction remains essentially constant up to the sulfur polymerization threshold ( 120-130" in the concentration range of interest). Beyond this point the monomer fraction undergoes an equilibrium polymerization which, except for the lowering of the threshold temperature, is identical with that characteristic of pure which represulfur; ie., the ratio [sf$]T-Tg/[S6]T+, sents the relative decrease in the monomer fraction occurring when the temperature is raised from the polymerization threshold, T4, to some higher temperature T , is independent of arsenic concentration, up to 15 at. % of arsenic, when the appropriate value of Tg is used. This implies that the arsenic present is not involved directly in the chemical processes occurring above T+. Accordingly, all of the arsenic atoms introduced can be considered as being permanently bound in the arsenic-sulfur polymer. h'evertheless, the arsenic-sulfur polymer does not act as an inert diluent. It has been shown theoretically by Tobolsky and Eisenberg12 and experimentally by Fairbrother, Gee, and MerralP that addition of inert diluents raises rather than lowers the sulfur polymerization temperature. It follows that above the polymerization threshold the sulfur chains present in the arsenic-sulfur polymer react with free S8 monomer according to the propagation step 86 f ASS,*
(7) where as indicated by the above comparison of [s6]!?-T+/[s6]!?+, the enthalpy of propagation for this reaction will be the same as in the pure sulfur reaction (4). However, the entropy of propagation will be higher as indicated by consideration of the expression S! ASSn+8*
(ma/!!'+) - R In [SS]O
(8) where [S& is the initial monomer concentration below T+. Both [s6]0 and T g are lower in the arsenic-sulfur system than in the pure sulfur system causing AS3 to be higher in the arsenic-sulfur case. It is possible that the increasein assfor reaction 7 Over reaction 4 is due to the greater availability of reaction sites in a ASa
=
trifunctionally branched arsenic-sulfur polymer as compared with a linearly propagating sulfur chain polymer containing the same number of SS monomer units. It would be helpful to know the relative concentrations of arsenic-sulfur polymer and sulfur rings in equilibrium below the transition temperature. This would require a comparison of the absolute Raman intensities due to monomer in equal excitation volumes of pure sulfur and sulfur-arsenic glass. While relative intensities can be measured reliably for any one system, a comparison of absolute intensities for two different systems cannot be made with the necessary precision at this time. Tobolsky and Owens have developed a theory of copolymerization for sulfur-selenium mixtures which indicates that the addition of selenium to sulfur should decrease the polymerization threshold temperature. This finding is in qualitative agreement with the viscosity data of Schenk14and the observations of the present study. According to the copolymerization theory the threshold temperature is defined by the condition K~(A)[MA]o
f
&(B)[MB]O
= 1
(9)
where MA]^ and [M,],, are the initial monomer concentrations of the reacting species and &(A) and K 3 ( B ) are the equilibrium constants for the respective homopolymerization propagation reactions. Equation 9 is the copolymerization theory analog of the homopolymerization theory equation ( 5 ) . In the TobolskyOwens treatment of the sulfur-selenium system the monomer species were assumed to be 88 and Ses and the Ks values were calculated from the thermodynamic parameters for the appropriate homopolymer systems M3(sulfur) = 3170 cal mol-'; ASa(su1fur) =
4.63 cal d e g ' mol-'
A H 3 (selenium) = 2270 cal mol-'; AS3(selenium) = 5.47 cal deg-' mol-'
The polymerization threshold temperatures computed for the compositions of interest using these data and assumptions are indicated by the dashed line in Figure 6. It is seen that the measured T+ values decrease with increasing selenium content much more markedly than predicted by the Tobolsky-Owens treatment. This discrepancy is explained by the observation' that the selenium is not present in the form of Sea rings but enters sulfur rings substitutionally to give species of the type SZSe8-$ where 8 1 x 2 4. Such monomer species, even with only one or two substitu(12) A.
V. Tobolsky and A. Eisenberg, J . Colloid Sei., 17, 49 (1962). Q. T. Merrall. J . Poltm. S c L
(13) F. Fairborther. G. Gee, and 16, 459 (1955).
(14) J. Schenk, Phystca, 23, 325 (1957).
Volume 79,Number 6 May 1060
THOMAS I. CROWELL AND MARIE G. HANKINS
1380
tions, will be characterized by enthalpies of propagation much lower than that for sulfur on account of the lower bond energy of a sulfur-selenium or selenium-selenium linkage. Furthermore, the abundance of "mixed" rings will be relatively large even for small additions of selenium. The effect of these factors on the variation of T+can be demonstrated by considering a TobolskyOwens type of copolymerization involving S S and Sese2 monomer species with the latter characterized by the same entropy of propagation as Ss but with an enthalpy of propagation the same as that for Se8. As indicated in Figure 6, the composition dependence of T+ calculated in this way is in closer agreement with experimental observation. The consumption of S S rings in such a copolymerization should be detectable spectroscopically wit,h sufficiently sensitive techniques. I n the Raman experiment performed here the threshold for removal of Ss rings from the system was obscured by the high optical density of the melt a t the temperatures of interest. Note on Resonance Enhancement of Raman Scattering. The phenomenon of resonance enhancement of Raman scattering occurs when the energy of the exciting radi-
ation approaches the energy of a fundamental electronic transition of the scattering species. The close agreement between the results of the two types of Raman scattering experiment described here implies that the resonance enhancement phenomenon does not play a significant role in this case. Either the principal contribution to the optical absorption must occur in a species which does not contribute greatly to the Raman scattering of interest or the absorbing center must be localized within the vibrating species in such a way as to produce minimal coupling between the electronic and vibrational transitions. The latter situation probably holds in the case of the polymeric chains S, for which the electronic transition of interest is probably associated with the presence of an unpaired electron localized a t each of the chain ends while the vibrational transitions of interest are associated with the normal modes of the entire molecule. Acknowledgment. The authors wish to acknowledge the benefit of helpful discussions with D. Olechna and R. C. Keezer and the assistance of J. O'Neill in the experimental work.
The Hydrolysis of Thiocyanic Acid. I. Dependence of Rate on Acidity Function by Thomas I. Crowell and Marie G. Hankins Department of Chemistry, UnWersitg of Virginia, Charlottesville, Virginia
28901
(Received September 3 0 , 1 9 6 8 )
The rate of formation of ammonia, hydrogen sulfide, and carbon dioxide from potassium thiocyanate, in 0.0511.0 M hydrochloric, phosphoric, and sulfuric acids, measured at 13.5-90.1", shows a linear correlation of log k with Ho. The slope is 1.0 at high acidities where the predominant species is HNCS and about 1.4 below Ho = -2. The pK* of thiocyanic acid is -2.3 to -2.0 at 25", depending on the estimated H- of hydrochloric acid solutions. There is a solvent isotope effect on the absorption spectrum of thiocyanate ion in DgO. The hydrolysis rates are correlated by log IC = [h-/(K* -k h-)] (IC1 k h ) , an equation consistent with concurrent mechanisms involving thiocyanic acid and one of its conjugate acids.
+
Thiocyanic acid is a volatile compound in which the molecular structure HN=C=S predominates over HSC=N.l It readily polymerizes, and forms a variety of heterocyclic compounds.2 Aqueous solutions are stable enough for measurements of the dissociation constant of thiocyanic acid,s which is quite strong, PKAbeing about -2. Hydrolysis occurs, however, in acid solution, according to the equation4
H'
+ HNCS + 2H20
--+
H2S
+ C02 +
4'"
(1)
Because i t is necessary to know the rate of this reaction if the dissociation constant of the acid is to be determined accurately and if any reactions of thiocyaThe Journal of Physical Chemistry
nate ion are to be quantitatively investigated at low pH, we have undertaken a kinetic study of reaction 1.
Results and Discussion Most of the rates reported at 90.1"were measured by the volume of carbon dioxide evolved. Several runs (1) 0. I. Beard and B. P. Dailey, J . Chem. Phys., 18, 1437 (1950). (2) U. Rtick and H . Steinmetz, 2. Anorg. Chem., 7 7 , 51 (1912); W. A. Sherman, "Heterocyclic Compounds," Vol. 7. R . (3. Elderfleld, Ed., John Wiley & Sons, Inc., New York, N. Y., 1961, p 569. (3) (a) T . D. B . Morgan, (3. Stedman, and P. A. E. Whincup. J . Chem. SOC.,4813 (1965); (b) 9. Tribalat and J. M. Caldero, BulZ. SOC.Chim. Fr., 774 (1966). (4) C. J. Hansen, French Patent 661,508, Oct 5, 1928; Chem. Abstr., 24, 473' (1930).
