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X-ray diffraction study on HCl aqueous solutions
1.2, number
Volume
2
CHEMICAL
X-RAY
DIFFRACTION
15 December
PHYSICS LETI’ERS
STUDY
ON HCI AQUEOUS
I971
SOLUTIONS
G_ LICHERI, G. PICCALUGA and G. PINNA Istituto
Chimico Polimttedra. Received
Universitb di Cagliari, Italy
II November
197 1
Pure water and HCI aqueous solutions 1N and 4.3N at 2S°C were investigated. The radial distribution functions (RDF) of the solutions show the same general feature as the water RDF; the first peak becomes however broader and shifts to larger R values BSexpected because of OH-Cl-interaction. The results are compared with those given
by other authors.
1. Introduction
Two X-ray diffraction solutions
have recently
studies on HCl aqueous been published by Lee and
Kaplow [l] and Terekhova [2]. The radial distribution functions (RDF) pjven by these authors are in disagreement, leading therefore to different structural interpretations. Lee and Kaplow’s RDF’s depart greatly from the pure water RDF with increasing HCl concentration; in fact, two new peaks appear, one at a shorter distance (2.56-2.75 a), and the other at a
Fig. 1.
longer distance (3.20-3.25 A) than that of the first water peak (2.8 A). The maximum at about 3.20 SL is ascribed to OH-Cl- interaction while the peak at 2.56-2.75 R is attributed to strongly bonded O-O pairs, probably ielated to the presence of excess protons; similar 0-o distances were found in HsO; ions in the crystalline structure of di- and trihydrated HCl L3]. The RDF’sreported by Terekhova, on the contrary, are very similar to the pure water RDF, no peaks appearing in the region 2.56-2.75 A; the water in the solutions is therefore supposed to retain the tetrahedral
Diagrammatic representation of tfte diffraction apparatus. 425
.Volume 12, number 2
CHEMICAL PHYsICS LE’ITERS
structure and the pro!ons to exist as H%Of ibns surrbunded by three water molecules - The structure of Hdl solutio& is of great inlerst in the work we intend to carry out on ionic aqueous solutions; accordingly ‘, we investigated
pure water, and HCI IN’and 4.3N’at
25Oc.
25 December 1971
ti [I]
0.5
reduction
2. Experimental and data
X-ray diffraction patterns were obtained with a camera for liquids diagrammatically shown in fig. 1. The samp!e is a vertical jet of liquid about 0.3 mm in diameter, falling from a glass capillary and kept in a hydrogen atmosphere. The intensity of diffracted radiation was measured in the horizontal plane with a scintillztion counter at pieSet scattering angles over the range 7’ < 28 < 141’; this corresponds to the range 0.5 < s < 7.6 A-l where s = 41~sinBIX, X being the X-ray wavelength and 26 the scattering angle. The radiation employed was Cu I&Y monochromated
0
t
in the primary beam by reflection from a quartz flat crystal. The sequence of angle values was programmed in such a way that errors due to instability of the ap-
paratus were randomly distributed over the whole angular range [4]. Experiments were repeated and intensities averaged. Intensities below 219= 7” were obtained by extrapolation. Above 28 = 50” intensity data were smoothed with a spline functions method [5] to remove some irregularities due to the higher statistical uncertainties at large angles. A 90% confidence interval was constructed about this smoothed curve: all experimental points fall inside this statistical limit. RDF’s were obtained by intensity data through the following steps: (i) correction for background, polarization and absorption; (ii) evaluation of the intensity in electron units leu; (iii) calculation of the RDF by Fourier transform:
o(R) = 4nR2po
f @R/n)
s
si(s)
sin&
ds
-9.5
-1.8
Fig. 2. d(s) functions for water and solutions.
,
0
where p. is the niean number of stoichiometric units ill 1 A3, i(S) = (Icu-CiXif;z)/(~jXif;-)2, Xi being. the coefficients of the stoichiometric G-tit andfi the scattering factors. 426
9
Bol
The values of the f%used were those proposed by [6] for the HZ0 molecule considered as a quasi-
atom and those of Cromer and Waber [7]. for Cl-. me scale factor, which notiaiizes to electron units, .. was obta.@ied using the method developed by KroghMoe [S] tin< its r’eliability checked by using the Rahman [9] tesr., In cjyder to correct the red&d intensity
Volume 12, number 2
CHEMICAL PHYSICS LETTEI&
for systematic errors, which may hrise from uncertainties in the scaling factors and in the tabulated scattering amplitudes and from imperfection in the diffraction geometry, the method developed by Levy et al. [IO] was used.
I.5 December 14fL
I0ItI 15 -
3. Results and discussion The C(S) functions for water and HCl solutions =e reported in fig. 2. Fig. 3 shows the functions D(R). The RDF of pure water is in perfect agreement with that reported by Bol 161. The RDF’s of the solutions show the same general feature as the water RDF, with the same number of peaks and inflections; the first peak becomes however broader and shifts to larger R values going from pure water (2.9 a) to the most concentrated solution (3.2 A), as expected because of the OH-Cl- interaction. Therefore our RDF’s look similar to those of Terekhova and are not consistent with
10 -
5-
those of Lee and Kaplow.
The discrepancy between our RDF’s and those of Lee and Kaplow is not in our opinion ascrib;lble to our low value of s,, = 7.6 8-l ; in fact Terekhova obtained results consistent with ours using a reasonabIy high upper integration limit (s,, = 10 A-l). Moreover the andyses [ 1 1 - 131 of the cut-off effect in the Fourier transform usually indicate that a variation in the upper limit of the integral does not affect the RDF’s in their general features. As the RDF’s are also sensitive to different methods in data reduction, we tried to see if alternative procedures (e.g., using atomic scattering factors for 0 and H) could explain the discussed discrepancy; quantitatively significant differences came out in the RDF’s but they did not provide new structural details. It must be pointed out that the authors mentioned do not give any detailed information about data collection and reduction. As a consequence a complete attempt
to explain
the causes of the discrepancy
present impossible. These considerations the
precision
have persuaded us to examine
and the reliability
of the results in this interthis
of researh, before starting on structural pretations. A more complete report including
kind
.analysis is being prepared.
is at
a-
o-
Fg. 3. Radial distribution functions for water and solutions; the full link represents the function 4;rR*po. Acknowledgements
Tne authors v&h to thank Professor C. Dejak and Dr. C. Marongiu for valuable discussions and the Consiglio Nazionale delle Ricer&e for financial sup port. Numerical dalculations were performed at the Computing Centers of Cagliari University and Venice University.
427
Volume 12, number 2
CHEMiCAL’PHY~iCS
Refemncyi [I] S.Cctee’and RXapiow, Scie& 169 (1970) 477. f 21 D.S;ferr&ova, Zh, Strukt. Urn. 11(1970)‘530. 131 J.O.tundgren and I.Ofwssun, Acta Cry?. 23 (1967) 966,971. 141 W.Bol, Lab. Algemene~Chemie, Eindhove,n University of Tedmelogy, Report No. 150170. IS] LGaliigani, private communications. [6) W.Bol, f. Appl. Cry& 1 (1968) 234.
LJZTTERS
15 December
1971
171:D.T.&omer and J.T.Waber, Acta Cryst. 18 (1965) 104, {S] J.Krogf&oo, Aaa&st 9 (i956j 952. [91 A.Rahnari, 1; Chem. Phys.42 (1965) 3540. [ iU] A.H.Lcvy, M,D.Danford and A,H,Niuten, Oak Ridge Natidn;rl Laboratory,
Tennessee,
Report OR?%-3960.
[ 111 R.Kapha,
S.L.Strong and B.L.Averbach, Phys. Rev. 138 ‘. (1965) 1336. i 121 C.Dejal:, G.Lich&i &d GPiccatuga, Gazz. C&m. Ital. 101 (1971) 159.’ (13) A.H.Nerten, M.D.Danford and k.H.Levy, Oak Ridge National Laboratory,
Tennessee,
Report ORNL-3997.
Volume
2
CHEMICAL
X-RAY
DIFFRACTION
15 December
PHYSICS LETI’ERS
STUDY
ON HCI AQUEOUS
I971
SOLUTIONS
G_ LICHERI, G. PICCALUGA and G. PINNA Istituto
Chimico Polimttedra. Received
Universitb di Cagliari, Italy
II November
197 1
Pure water and HCI aqueous solutions 1N and 4.3N at 2S°C were investigated. The radial distribution functions (RDF) of the solutions show the same general feature as the water RDF; the first peak becomes however broader and shifts to larger R values BSexpected because of OH-Cl-interaction. The results are compared with those given
by other authors.
1. Introduction
Two X-ray diffraction solutions
have recently
studies on HCl aqueous been published by Lee and
Kaplow [l] and Terekhova [2]. The radial distribution functions (RDF) pjven by these authors are in disagreement, leading therefore to different structural interpretations. Lee and Kaplow’s RDF’s depart greatly from the pure water RDF with increasing HCl concentration; in fact, two new peaks appear, one at a shorter distance (2.56-2.75 a), and the other at a
Fig. 1.
longer distance (3.20-3.25 A) than that of the first water peak (2.8 A). The maximum at about 3.20 SL is ascribed to OH-Cl- interaction while the peak at 2.56-2.75 R is attributed to strongly bonded O-O pairs, probably ielated to the presence of excess protons; similar 0-o distances were found in HsO; ions in the crystalline structure of di- and trihydrated HCl L3]. The RDF’sreported by Terekhova, on the contrary, are very similar to the pure water RDF, no peaks appearing in the region 2.56-2.75 A; the water in the solutions is therefore supposed to retain the tetrahedral
Diagrammatic representation of tfte diffraction apparatus. 425
.Volume 12, number 2
CHEMICAL PHYsICS LE’ITERS
structure and the pro!ons to exist as H%Of ibns surrbunded by three water molecules - The structure of Hdl solutio& is of great inlerst in the work we intend to carry out on ionic aqueous solutions; accordingly ‘, we investigated
pure water, and HCI IN’and 4.3N’at
25Oc.
25 December 1971
ti [I]
0.5
reduction
2. Experimental and data
X-ray diffraction patterns were obtained with a camera for liquids diagrammatically shown in fig. 1. The samp!e is a vertical jet of liquid about 0.3 mm in diameter, falling from a glass capillary and kept in a hydrogen atmosphere. The intensity of diffracted radiation was measured in the horizontal plane with a scintillztion counter at pieSet scattering angles over the range 7’ < 28 < 141’; this corresponds to the range 0.5 < s < 7.6 A-l where s = 41~sinBIX, X being the X-ray wavelength and 26 the scattering angle. The radiation employed was Cu I&Y monochromated
0
t
in the primary beam by reflection from a quartz flat crystal. The sequence of angle values was programmed in such a way that errors due to instability of the ap-
paratus were randomly distributed over the whole angular range [4]. Experiments were repeated and intensities averaged. Intensities below 219= 7” were obtained by extrapolation. Above 28 = 50” intensity data were smoothed with a spline functions method [5] to remove some irregularities due to the higher statistical uncertainties at large angles. A 90% confidence interval was constructed about this smoothed curve: all experimental points fall inside this statistical limit. RDF’s were obtained by intensity data through the following steps: (i) correction for background, polarization and absorption; (ii) evaluation of the intensity in electron units leu; (iii) calculation of the RDF by Fourier transform:
o(R) = 4nR2po
f @R/n)
s
si(s)
sin&
ds
-9.5
-1.8
Fig. 2. d(s) functions for water and solutions.
,
0
where p. is the niean number of stoichiometric units ill 1 A3, i(S) = (Icu-CiXif;z)/(~jXif;-)2, Xi being. the coefficients of the stoichiometric G-tit andfi the scattering factors. 426
9
Bol
The values of the f%used were those proposed by [6] for the HZ0 molecule considered as a quasi-
atom and those of Cromer and Waber [7]. for Cl-. me scale factor, which notiaiizes to electron units, .. was obta.@ied using the method developed by KroghMoe [S] tin< its r’eliability checked by using the Rahman [9] tesr., In cjyder to correct the red&d intensity
Volume 12, number 2
CHEMICAL PHYSICS LETTEI&
for systematic errors, which may hrise from uncertainties in the scaling factors and in the tabulated scattering amplitudes and from imperfection in the diffraction geometry, the method developed by Levy et al. [IO] was used.
I.5 December 14fL
I0ItI 15 -
3. Results and discussion The C(S) functions for water and HCl solutions =e reported in fig. 2. Fig. 3 shows the functions D(R). The RDF of pure water is in perfect agreement with that reported by Bol 161. The RDF’s of the solutions show the same general feature as the water RDF, with the same number of peaks and inflections; the first peak becomes however broader and shifts to larger R values going from pure water (2.9 a) to the most concentrated solution (3.2 A), as expected because of the OH-Cl- interaction. Therefore our RDF’s look similar to those of Terekhova and are not consistent with
10 -
5-
those of Lee and Kaplow.
The discrepancy between our RDF’s and those of Lee and Kaplow is not in our opinion ascrib;lble to our low value of s,, = 7.6 8-l ; in fact Terekhova obtained results consistent with ours using a reasonabIy high upper integration limit (s,, = 10 A-l). Moreover the andyses [ 1 1 - 131 of the cut-off effect in the Fourier transform usually indicate that a variation in the upper limit of the integral does not affect the RDF’s in their general features. As the RDF’s are also sensitive to different methods in data reduction, we tried to see if alternative procedures (e.g., using atomic scattering factors for 0 and H) could explain the discussed discrepancy; quantitatively significant differences came out in the RDF’s but they did not provide new structural details. It must be pointed out that the authors mentioned do not give any detailed information about data collection and reduction. As a consequence a complete attempt
to explain
the causes of the discrepancy
present impossible. These considerations the
precision
have persuaded us to examine
and the reliability
of the results in this interthis
of researh, before starting on structural pretations. A more complete report including
kind
.analysis is being prepared.
is at
a-
o-
Fg. 3. Radial distribution functions for water and solutions; the full link represents the function 4;rR*po. Acknowledgements
Tne authors v&h to thank Professor C. Dejak and Dr. C. Marongiu for valuable discussions and the Consiglio Nazionale delle Ricer&e for financial sup port. Numerical dalculations were performed at the Computing Centers of Cagliari University and Venice University.
427
Volume 12, number 2
CHEMiCAL’PHY~iCS
Refemncyi [I] S.Cctee’and RXapiow, Scie& 169 (1970) 477. f 21 D.S;ferr&ova, Zh, Strukt. Urn. 11(1970)‘530. 131 J.O.tundgren and I.Ofwssun, Acta Cry?. 23 (1967) 966,971. 141 W.Bol, Lab. Algemene~Chemie, Eindhove,n University of Tedmelogy, Report No. 150170. IS] LGaliigani, private communications. [6) W.Bol, f. Appl. Cry& 1 (1968) 234.
LJZTTERS
15 December
1971
171:D.T.&omer and J.T.Waber, Acta Cryst. 18 (1965) 104, {S] J.Krogf&oo, Aaa&st 9 (i956j 952. [91 A.Rahnari, 1; Chem. Phys.42 (1965) 3540. [ iU] A.H.Lcvy, M,D.Danford and A,H,Niuten, Oak Ridge Natidn;rl Laboratory,
Tennessee,
Report OR?%-3960.
[ 111 R.Kapha,
S.L.Strong and B.L.Averbach, Phys. Rev. 138 ‘. (1965) 1336. i 121 C.Dejal:, G.Lich&i &d GPiccatuga, Gazz. C&m. Ital. 101 (1971) 159.’ (13) A.H.Nerten, M.D.Danford and k.H.Levy, Oak Ridge National Laboratory,
Tennessee,
Report ORNL-3997.