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Electrochemical investigations in molten potassium disulphate at 430°C
J. Electroanal. Chem., 70 ( 1 9 7 6 ) 6 5 - - 7 2 © Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in T h e N e t h e r l a n d s
65
ELECTROCHEMICAL INVESTIGATIONS IN MOLTEN POTASSIUM DISULPHATE AT 430°C II. ACIDIC PROPERTIES OF PHOSPHOROUS PENTOXIDE
A L A I N D U R A N D *, G E R A R D P I C A R D a n d J A C Q U E S V E D E L
Laboratoire d'Electrochimie Analytique et Appliqude associd au CNRS, E.N.S.C.P., 11 rue Pierre et Marie Curie, 75231-Paris Cddex 05 (France) (Received 2 3 r d July 1 9 7 5 ; in revised f o r m 2 5 t h S e p t e m b e r 1 9 7 5 )
ABSTRACT In m o l t e n p o t a s s i u m d i s u l p h a t e at 4 3 0 ° C p h o s p h o r u s exists at t h e +V o x i d a t i o n state. In this m e d i u m , p h o s p h o r o u s p e n t o x i d e reacts as a w e a k acid: dissolved P 2 0 5 leads t o t h e acid PO3SO 3 following P 2 0 5 + S2 O 2 - -> 2PO3SO 3 This w e a k acid was n e u t r a l i z e d b y s u l p h a t e a n i o n s :
POaSO; + so -
+ S20 -
and the basicity constant K B of the acid--base couple P 0 3 S 0 3 / P 0 3
was determined (K B =
10--(1.4 -+0.I) mol kg--l).
INTRODUCTION
In a previous work [1] molten potassium disulphate has been considered as a solvent in which acidity can be varied. A m e t h o d of measurement of the acidity, defined as pSO4 = --log m ( S O ~ - ) / m o l kg -1 has been established using the voltammetric reduction curve of the solvent on a rotating gold micro-electrode. The solvent acidity range has been studied. It is limited by the potassium sulphate solubility to the value pSO4 = 0.6, and by the sulphur trioxide solubility to the value pSO4 = 3.2 (corresponding to the SOs pressure P(SO3) = 1 atm). This work is related to the study of the acidic properties of phosphorous pentoxide solutions. * Laboratoire de Chimie, Institut National des Sciences et Techniques Nucldaires, Centre d'etudes Nucl~aires de Saclay, P.O. Box 6, 91190-Gif sur Yvette, France.
66 Experimentally P205 dissolved in molten potassium disulphate behaves as a weak acid: p S 0 4 measurements show that the dissolution of P205 in neutral disulphate increases the acidity of the medium. Besides, it was observed that P205 can dissolve an excess of potassium sulphate according to the reaction: P205 + 2 S 0 2 - -~ 2 P 0 3 + $ 2 0 2 Also, metaphosphate anion P 0 3 might be an acid and so react with the sulphate anions: 2PO~ + 2S0~- -+ P2074- + $20~-
However, pSO 4 determinations in sodium pyrophosphate solutions showed that the P 2 0 ~ - anion was a strong base, the solvolysis reaction being of the type: p202- + $202- -+ 2PO3 + 2SO2-
Therefore, an acid--base equilibrium between a form of dissolved P205 and a form of dissolved PO 3 had to be considered and was studied in what follows. EXPERIMENTAL The experimental set-up was the same as described before [1]. Baker phosphorous pentoxide was used as reagent. RESULTS AND DISCUSSION p S 0 4 variations observed during the neutralization of phosphorous pentoxide solutions with potassium sulphate for various initial P20~ concentration values, too, are reported in Table 1. The dissolved acid being a weak one, the corresponding titration curves do not present any well defined inflexion at the equivalence point (Fig. 1). The Na2S207--(NaP03)3 mixtures have been studied by Thilo and Blumenthal [2]. These authors obtained evidence of sulphatophosphate and polyphosphate bonds in complexes of the t y p e O -
O--S--O--
o
O --P--O
o
_~O S--O
-
o
but at temperatures greater than 650--700°C: the lower the temperature, the lower the n-value has been found. Nevertheless, in order to find the nature of the phosphorous(V) species involved in the so-called P205/PO ~ acid--base couple, various hypotheses were
67 TABLE 1 pSO 4 variation during the neutralization of P205 solutions I n i t i a l s o l v e n t m a s s : 0.1 kg ( 0 . 2 kg f o r t h e first t i t r a t i o n ) ; m 0 = initial P 2 0 5 c o n c e n t r a t i o n ; ( p S O 4 ) 0 = initial v a l u e o f p S O 4 b e f o r e P 2 0 5 i n t r o d u c t i o n m 0 = 0 . 1 5 4 tool kg - 1 (p804) 0 = 1.18
m 0 = 0 . 4 9 5 t o o l kg - 1 (pSO4) o = 1.56
m 0 = 0 . 5 3 4 m o l kg - 1 (pSOd) 0 = 1.55
m 0 = 0 . 3 8 8 tool kg - 1 (pS04) 0 = 0.93
[K2804] added/ 1 mol kg--
pSO 4
[K2SO 4 ] added/ t o o l kg - 1
pSO 4
[K2804 ] added/ m o l kg - 1
pSO4
[K2804] added/ tool kg - 1
pSO4
0 2.92 5.81 8.67 1.15 1.43 1.71 1.98 2.26 2.54 2.82 3.11 3.41 3.69 3.96
2.03 1.94 1.83 1.65 1.50 1.40 1.29 1.20 1.09 0.99 0.87 0.77 0.66 0.63 0.60
0 6.41 1.26 1.86 2.45 3.02 3.57 4.65 5.66 6.63 7.57 8.04 8.52 8.96 9.44 9.91 1.03 1.11 1.19
2.83 2.50 2.32 2.12 1.95 1.77 1.66 1.42 1.31 1.17 1.05 0.98 0.87 0.84 0.76 0.69 0.63 0.63 0.60
0 6.49 1.27 1.88 2.47 3.05 3.60 4.15 5.19 6.19 7.16 8.11 8.56 9.02 9.51 9.97 1.04 1.13
2.75 2.51 2.31 2.12 1.99 1.89 1.81 1.69 1.55 1.34 1.15 0.98 0.93 0.87 0.77 0.71 0.65 0.60
0 6 . 0 8 x 10 - 2 1 . 1 9 X 10 - 1 1.77 2.33 2.88 3.42 3.95 4.48 5.01 5.49 6.01 6.54 7.06 7.59
2.45 2.23 2.07 1.83 1.63 1.50 1.36 1.21 1.04 0.93 0.93 0.82 0.71 0.63 0.6
x 10 -2 x 10 - 2 X 10 - 2 X 10 - 1 X 10 - 1 X 10 - 1 X 10 - 1 X 10 -1 X 10 - 1 X 10 - 1 X 10 - 1 X 10 - 1 × 10 - 1 X 10 -1
x 10 - 2 X 10 - 1 X 10 - 1 X 10 - 1 X 10 - 1 x 10 - 1 x 10 --1 X 10 - 1 × 10 - 1 X 10 - 1 x 10 - 1 x 10 - 1 X 10 - 1 x 10 - 1 x 10 - 1
x 10 - 2 X 10 - 1 X 10 - 1 x 10 - 1 X 10 - 1 X 10 - 1 × 10 - 1 × 10 - 1 X 10 - 1 X 10 --1 X 10 - 1 X 10 - 1 X 10 - 1 X 10 - 1 x 10 - 1
71llllllllllllll 71/IIIIIIII/I/I/,PI~03~ 10h'~Yllllllll71111111111111L 3
1
-
'/////////////~ :~:~ '/4"/4~ 5~~-1"-" o: 26 "~8 ~"~'~'-%~ ~ ~..~~
Oo
I
0.25
I ....
0.50
4- . . . .
0.75
I
1.0
Fig. 1. T i t r a t i o n c u r v e o f p h o s p h o r o u s p e n t o x i d e s o l u t i o n b y p o t a s s i u m s u l p h a t e } ( o ) E x perimental values; (--) calculated curves for: (1) neutralization of the strong acid 803 by the strong base K2804; (2) neutralization of a weak acid (pK B = 1.4) by the strong base K 2 8 0 4 ; t h e s h a d e d s p a c e c o r r e s p o n d s t o t h e u n c e r t a i n t y r a n g e 1.3 < p K B < 1 . 5 ; ( 3 ) a d d i t i o n o f K 2 S O 4 in n e u t r a l s o l u t i o n .
68
anticipated, the corresponding pS04 variations were derived and compared to the experimental results. If, for example, phosphorous pentoxide is dissolved without solvolysis (i.e., it remains in the P205 form) and if the conjugated base is POT, then the acid-base equilibrium is:
(i)
P205 + 2S042- # 2PO~ + S20~with
(i)
KB = [P2051 [S02-12/[P0312 The balance equations for P20a and K2S04 are respectively mo = [P205] + 1/2 [PO~]
(2)
m + [S02-10 = [P031 + [SO 2-1
(3)
where mo/mol kg- i is the initial (added) P205 concentration, [SO2-]o = 10 -(ps°4)0 is the i n i t i a l sulphate ion concentration, before P205 a d d i t i o n (depending on the melting conditions), [SO 2 - ] = 10 -ps°4 is the sulphate concentration, determined after addition of the quantity m of potassium sulphate.
y
~J~2
0
:, 'E" ÷m mm
~.:.'.
_I
_2 1
2
1
pS04
2
pSO4
Hyp. ]]
Hyp .i
•2
S • |
+A ~ R-
!
_I
_2 I
2
I
2
~s% Hyp. ]~
pSO4 Hyp. W
Fig. 2. Variations of the various calculated characteristic functions F (SO 2 - ) vs. pSO 4.
69
A
0
~
~
~g o~
~1~
o~O~
.,
~.~,
cq 0 ,.~ e~
o~,~
0
~
2
Z
O
O O
O9 o.1 +
+
o
e~
0
o9
e~
O
+
O
+
~g
e~ +
+
~g
0
II
O
I
I
%
0
O
+o ° I + +
¢,I r.~
~
O
O
,~
+
+
7
g
s
I e~
I
o
g O
m
~5
O
0
O
0
2
I
]
i
0
0
Io
0
I
I
0
~ 0
~~
o
O
I
0
O
2
5
70
Combining eqns. (2) and (3), eqn. (1) becomes [P205]
log
- log KB + 2pSO4 -- log
[PO~] 2
2m o - - m + [SO~-] -- [SO~-]0
(4)
2(m -- [ S O ~ - ] + [ S O ~ - ] o ) 2
Equation (4) may be considered as a characteristic function F(SO 2 - ) of the hypothesis (I). The plot of log [ P 2 0 5 ] / [ P O 3 ] 2 vs. pSO4 is reported in Fig. 2. The experimental results show no good fit with the theoretical characteristic function so that the initial hypothesis (I) was then rejected. The hypotheses concerning the solvation state of the P(V) species, and the corresponding characteristic functions are reported in Table 2. It is important to note that the formulae given for the dissolved species do not necessarily correspond to the real structure of the solute. Hence, PO~- may represent either PO~- itself or the m o n o m e r of the sulphatophosphate complex (SO3--PO3-SO4)3-, these two species being in equilibrium following to: PO3
+
S 2 0 ~ - -~ (S03 " P03
•
S04) 3-
but, the solvent activity being constant, the potentiometric determinations cannot decide between the two possibilities. Similarly, (POaSO3)- is the sim-
c.J-¢ z oo3
" o -2
0 ÷~1 & +l
•
• +
_2
+
1
I
i
•
.1
l:l
A
A
_2
2
I
2
pSO4
pSO4
(*lHyp.2
Hyp,2:[ A
7"2
S
,_oe
+
:.~ +~, %
0
A÷
• ÷m
I _I
am°
• +
_2
2 Hyp.~ B
2
p.SO4
pSO~
Hyp.~/I C
Fig. 3. V a r i a t i o n s o f t h e v a r i o u s c a l c u l a t e d c h a r a c t e r i s t i c f u n c t i o n s F ( S O 42- - ) vs. p S O 4. ( , ) ( o A ..) n = 2; ( o z~ D) n = 3.
71 plest representation of the solvolysis product of P205 to give a m o n o m e r according to: P205
+
S 2 0 2 - -+ 2 ( P 0 3 S 0 3 ) -
The comparison between the various plots of the characteristic functions vs. pS04 (Figs. 2 and 3) seems to show that the best hypothesis for the investigated acid--base couple is the hypothesis III for which experimental points and theoretical slope are in good concordance, all the points from various runs being well distributed along the same line. The acid--base equilibrium is then: P 0 3 + $ 2 0 2 - ~ P 0 3 S 0 3 + S042with KB = [ P 0 3 S 0 3 ] [ S 0 2 - ] / [ P 0 3 ] = 10 -(1"4 ± 0.1) mol kg -1 The value of K B was obtained by extrapolation of the convenient characteristic function to pS04 = 0. Attempts were made to establish the existence of the sulphatophosphate and the polyphosphate bond. Thilo and Blumenthal showed that the Na2S2OT-(NAP03)3 mixtures, cooled from 700°C and dissolved into water undergo successively: a very f a s t h y d r o l y s i s reaction of the disulphate anion, a slow hydrolysis reaction of the sulphatophosphate bond, a very slow hydrolysis reaction of the polyphosphate bond, also obtained by boiling the sample. Solutions of base in the melt were obtained by neutralizing P20~ solutions of known concentration with potassium sulphate. The solutions were cooled, dissolved in water and titrated, after fast and total hydrolysis. The results d i d not show evidence of the existence of the sulphatophosphate bond in the melt, at 430°C, which concurs with some results of Thilo and Blumenthal obtained from determination at low temperatures. On the other hand, the hydrolysis of the disulphate anion in water promotes the formation of polyphosphoric acids in aqueous solution. Thus, the unique feature proving the monomeric nature of the phosphoric species in molten potassium disulphate is the interpretation of the pS04 variation during the neutralization of P205 solutions. ACKNOWLEDGEMENTS The authors wish to express their thanks to Prof. Tr~millon for his helpful suggestions and encouragement. They also express their thanks to the Commissariat ~ l'Energie Atomique for the financial assistance for one of them.
72 REFERENCES 1 A. Durand, G. Picard and J. Vedel, J. Electroanal. Chem., 70 (1976) 55 2 E. Thito and G. Blumenthal, Z. Anorg. Allg. Chem., 6 (1966) 169.
65
ELECTROCHEMICAL INVESTIGATIONS IN MOLTEN POTASSIUM DISULPHATE AT 430°C II. ACIDIC PROPERTIES OF PHOSPHOROUS PENTOXIDE
A L A I N D U R A N D *, G E R A R D P I C A R D a n d J A C Q U E S V E D E L
Laboratoire d'Electrochimie Analytique et Appliqude associd au CNRS, E.N.S.C.P., 11 rue Pierre et Marie Curie, 75231-Paris Cddex 05 (France) (Received 2 3 r d July 1 9 7 5 ; in revised f o r m 2 5 t h S e p t e m b e r 1 9 7 5 )
ABSTRACT In m o l t e n p o t a s s i u m d i s u l p h a t e at 4 3 0 ° C p h o s p h o r u s exists at t h e +V o x i d a t i o n state. In this m e d i u m , p h o s p h o r o u s p e n t o x i d e reacts as a w e a k acid: dissolved P 2 0 5 leads t o t h e acid PO3SO 3 following P 2 0 5 + S2 O 2 - -> 2PO3SO 3 This w e a k acid was n e u t r a l i z e d b y s u l p h a t e a n i o n s :
POaSO; + so -
+ S20 -
and the basicity constant K B of the acid--base couple P 0 3 S 0 3 / P 0 3
was determined (K B =
10--(1.4 -+0.I) mol kg--l).
INTRODUCTION
In a previous work [1] molten potassium disulphate has been considered as a solvent in which acidity can be varied. A m e t h o d of measurement of the acidity, defined as pSO4 = --log m ( S O ~ - ) / m o l kg -1 has been established using the voltammetric reduction curve of the solvent on a rotating gold micro-electrode. The solvent acidity range has been studied. It is limited by the potassium sulphate solubility to the value pSO4 = 0.6, and by the sulphur trioxide solubility to the value pSO4 = 3.2 (corresponding to the SOs pressure P(SO3) = 1 atm). This work is related to the study of the acidic properties of phosphorous pentoxide solutions. * Laboratoire de Chimie, Institut National des Sciences et Techniques Nucldaires, Centre d'etudes Nucl~aires de Saclay, P.O. Box 6, 91190-Gif sur Yvette, France.
66 Experimentally P205 dissolved in molten potassium disulphate behaves as a weak acid: p S 0 4 measurements show that the dissolution of P205 in neutral disulphate increases the acidity of the medium. Besides, it was observed that P205 can dissolve an excess of potassium sulphate according to the reaction: P205 + 2 S 0 2 - -~ 2 P 0 3 + $ 2 0 2 Also, metaphosphate anion P 0 3 might be an acid and so react with the sulphate anions: 2PO~ + 2S0~- -+ P2074- + $20~-
However, pSO 4 determinations in sodium pyrophosphate solutions showed that the P 2 0 ~ - anion was a strong base, the solvolysis reaction being of the type: p202- + $202- -+ 2PO3 + 2SO2-
Therefore, an acid--base equilibrium between a form of dissolved P205 and a form of dissolved PO 3 had to be considered and was studied in what follows. EXPERIMENTAL The experimental set-up was the same as described before [1]. Baker phosphorous pentoxide was used as reagent. RESULTS AND DISCUSSION p S 0 4 variations observed during the neutralization of phosphorous pentoxide solutions with potassium sulphate for various initial P20~ concentration values, too, are reported in Table 1. The dissolved acid being a weak one, the corresponding titration curves do not present any well defined inflexion at the equivalence point (Fig. 1). The Na2S207--(NaP03)3 mixtures have been studied by Thilo and Blumenthal [2]. These authors obtained evidence of sulphatophosphate and polyphosphate bonds in complexes of the t y p e O -
O--S--O--
o
O --P--O
o
_~O S--O
-
o
but at temperatures greater than 650--700°C: the lower the temperature, the lower the n-value has been found. Nevertheless, in order to find the nature of the phosphorous(V) species involved in the so-called P205/PO ~ acid--base couple, various hypotheses were
67 TABLE 1 pSO 4 variation during the neutralization of P205 solutions I n i t i a l s o l v e n t m a s s : 0.1 kg ( 0 . 2 kg f o r t h e first t i t r a t i o n ) ; m 0 = initial P 2 0 5 c o n c e n t r a t i o n ; ( p S O 4 ) 0 = initial v a l u e o f p S O 4 b e f o r e P 2 0 5 i n t r o d u c t i o n m 0 = 0 . 1 5 4 tool kg - 1 (p804) 0 = 1.18
m 0 = 0 . 4 9 5 t o o l kg - 1 (pSO4) o = 1.56
m 0 = 0 . 5 3 4 m o l kg - 1 (pSOd) 0 = 1.55
m 0 = 0 . 3 8 8 tool kg - 1 (pS04) 0 = 0.93
[K2804] added/ 1 mol kg--
pSO 4
[K2SO 4 ] added/ t o o l kg - 1
pSO 4
[K2804 ] added/ m o l kg - 1
pSO4
[K2804] added/ tool kg - 1
pSO4
0 2.92 5.81 8.67 1.15 1.43 1.71 1.98 2.26 2.54 2.82 3.11 3.41 3.69 3.96
2.03 1.94 1.83 1.65 1.50 1.40 1.29 1.20 1.09 0.99 0.87 0.77 0.66 0.63 0.60
0 6.41 1.26 1.86 2.45 3.02 3.57 4.65 5.66 6.63 7.57 8.04 8.52 8.96 9.44 9.91 1.03 1.11 1.19
2.83 2.50 2.32 2.12 1.95 1.77 1.66 1.42 1.31 1.17 1.05 0.98 0.87 0.84 0.76 0.69 0.63 0.63 0.60
0 6.49 1.27 1.88 2.47 3.05 3.60 4.15 5.19 6.19 7.16 8.11 8.56 9.02 9.51 9.97 1.04 1.13
2.75 2.51 2.31 2.12 1.99 1.89 1.81 1.69 1.55 1.34 1.15 0.98 0.93 0.87 0.77 0.71 0.65 0.60
0 6 . 0 8 x 10 - 2 1 . 1 9 X 10 - 1 1.77 2.33 2.88 3.42 3.95 4.48 5.01 5.49 6.01 6.54 7.06 7.59
2.45 2.23 2.07 1.83 1.63 1.50 1.36 1.21 1.04 0.93 0.93 0.82 0.71 0.63 0.6
x 10 -2 x 10 - 2 X 10 - 2 X 10 - 1 X 10 - 1 X 10 - 1 X 10 - 1 X 10 -1 X 10 - 1 X 10 - 1 X 10 - 1 X 10 - 1 × 10 - 1 X 10 -1
x 10 - 2 X 10 - 1 X 10 - 1 X 10 - 1 X 10 - 1 x 10 - 1 x 10 --1 X 10 - 1 × 10 - 1 X 10 - 1 x 10 - 1 x 10 - 1 X 10 - 1 x 10 - 1 x 10 - 1
x 10 - 2 X 10 - 1 X 10 - 1 x 10 - 1 X 10 - 1 X 10 - 1 × 10 - 1 × 10 - 1 X 10 - 1 X 10 --1 X 10 - 1 X 10 - 1 X 10 - 1 X 10 - 1 x 10 - 1
71llllllllllllll 71/IIIIIIII/I/I/,PI~03~ 10h'~Yllllllll71111111111111L 3
1
-
'/////////////~ :~:~ '/4"/4~ 5~~-1"-" o: 26 "~8 ~"~'~'-%~ ~ ~..~~
Oo
I
0.25
I ....
0.50
4- . . . .
0.75
I
1.0
Fig. 1. T i t r a t i o n c u r v e o f p h o s p h o r o u s p e n t o x i d e s o l u t i o n b y p o t a s s i u m s u l p h a t e } ( o ) E x perimental values; (--) calculated curves for: (1) neutralization of the strong acid 803 by the strong base K2804; (2) neutralization of a weak acid (pK B = 1.4) by the strong base K 2 8 0 4 ; t h e s h a d e d s p a c e c o r r e s p o n d s t o t h e u n c e r t a i n t y r a n g e 1.3 < p K B < 1 . 5 ; ( 3 ) a d d i t i o n o f K 2 S O 4 in n e u t r a l s o l u t i o n .
68
anticipated, the corresponding pS04 variations were derived and compared to the experimental results. If, for example, phosphorous pentoxide is dissolved without solvolysis (i.e., it remains in the P205 form) and if the conjugated base is POT, then the acid-base equilibrium is:
(i)
P205 + 2S042- # 2PO~ + S20~with
(i)
KB = [P2051 [S02-12/[P0312 The balance equations for P20a and K2S04 are respectively mo = [P205] + 1/2 [PO~]
(2)
m + [S02-10 = [P031 + [SO 2-1
(3)
where mo/mol kg- i is the initial (added) P205 concentration, [SO2-]o = 10 -(ps°4)0 is the i n i t i a l sulphate ion concentration, before P205 a d d i t i o n (depending on the melting conditions), [SO 2 - ] = 10 -ps°4 is the sulphate concentration, determined after addition of the quantity m of potassium sulphate.
y
~J~2
0
:, 'E" ÷m mm
~.:.'.
_I
_2 1
2
1
pS04
2
pSO4
Hyp. ]]
Hyp .i
•2
S • |
+A ~ R-
!
_I
_2 I
2
I
2
~s% Hyp. ]~
pSO4 Hyp. W
Fig. 2. Variations of the various calculated characteristic functions F (SO 2 - ) vs. pSO 4.
69
A
0
~
~
~g o~
~1~
o~O~
.,
~.~,
cq 0 ,.~ e~
o~,~
0
~
2
Z
O
O O
O9 o.1 +
+
o
e~
0
o9
e~
O
+
O
+
~g
e~ +
+
~g
0
II
O
I
I
%
0
O
+o ° I + +
¢,I r.~
~
O
O
,~
+
+
7
g
s
I e~
I
o
g O
m
~5
O
0
O
0
2
I
]
i
0
0
Io
0
I
I
0
~ 0
~~
o
O
I
0
O
2
5
70
Combining eqns. (2) and (3), eqn. (1) becomes [P205]
log
- log KB + 2pSO4 -- log
[PO~] 2
2m o - - m + [SO~-] -- [SO~-]0
(4)
2(m -- [ S O ~ - ] + [ S O ~ - ] o ) 2
Equation (4) may be considered as a characteristic function F(SO 2 - ) of the hypothesis (I). The plot of log [ P 2 0 5 ] / [ P O 3 ] 2 vs. pSO4 is reported in Fig. 2. The experimental results show no good fit with the theoretical characteristic function so that the initial hypothesis (I) was then rejected. The hypotheses concerning the solvation state of the P(V) species, and the corresponding characteristic functions are reported in Table 2. It is important to note that the formulae given for the dissolved species do not necessarily correspond to the real structure of the solute. Hence, PO~- may represent either PO~- itself or the m o n o m e r of the sulphatophosphate complex (SO3--PO3-SO4)3-, these two species being in equilibrium following to: PO3
+
S 2 0 ~ - -~ (S03 " P03
•
S04) 3-
but, the solvent activity being constant, the potentiometric determinations cannot decide between the two possibilities. Similarly, (POaSO3)- is the sim-
c.J-¢ z oo3
" o -2
0 ÷~1 & +l
•
• +
_2
+
1
I
i
•
.1
l:l
A
A
_2
2
I
2
pSO4
pSO4
(*lHyp.2
Hyp,2:[ A
7"2
S
,_oe
+
:.~ +~, %
0
A÷
• ÷m
I _I
am°
• +
_2
2 Hyp.~ B
2
p.SO4
pSO~
Hyp.~/I C
Fig. 3. V a r i a t i o n s o f t h e v a r i o u s c a l c u l a t e d c h a r a c t e r i s t i c f u n c t i o n s F ( S O 42- - ) vs. p S O 4. ( , ) ( o A ..) n = 2; ( o z~ D) n = 3.
71 plest representation of the solvolysis product of P205 to give a m o n o m e r according to: P205
+
S 2 0 2 - -+ 2 ( P 0 3 S 0 3 ) -
The comparison between the various plots of the characteristic functions vs. pS04 (Figs. 2 and 3) seems to show that the best hypothesis for the investigated acid--base couple is the hypothesis III for which experimental points and theoretical slope are in good concordance, all the points from various runs being well distributed along the same line. The acid--base equilibrium is then: P 0 3 + $ 2 0 2 - ~ P 0 3 S 0 3 + S042with KB = [ P 0 3 S 0 3 ] [ S 0 2 - ] / [ P 0 3 ] = 10 -(1"4 ± 0.1) mol kg -1 The value of K B was obtained by extrapolation of the convenient characteristic function to pS04 = 0. Attempts were made to establish the existence of the sulphatophosphate and the polyphosphate bond. Thilo and Blumenthal showed that the Na2S2OT-(NAP03)3 mixtures, cooled from 700°C and dissolved into water undergo successively: a very f a s t h y d r o l y s i s reaction of the disulphate anion, a slow hydrolysis reaction of the sulphatophosphate bond, a very slow hydrolysis reaction of the polyphosphate bond, also obtained by boiling the sample. Solutions of base in the melt were obtained by neutralizing P20~ solutions of known concentration with potassium sulphate. The solutions were cooled, dissolved in water and titrated, after fast and total hydrolysis. The results d i d not show evidence of the existence of the sulphatophosphate bond in the melt, at 430°C, which concurs with some results of Thilo and Blumenthal obtained from determination at low temperatures. On the other hand, the hydrolysis of the disulphate anion in water promotes the formation of polyphosphoric acids in aqueous solution. Thus, the unique feature proving the monomeric nature of the phosphoric species in molten potassium disulphate is the interpretation of the pS04 variation during the neutralization of P205 solutions. ACKNOWLEDGEMENTS The authors wish to express their thanks to Prof. Tr~millon for his helpful suggestions and encouragement. They also express their thanks to the Commissariat ~ l'Energie Atomique for the financial assistance for one of them.
72 REFERENCES 1 A. Durand, G. Picard and J. Vedel, J. Electroanal. Chem., 70 (1976) 55 2 E. Thito and G. Blumenthal, Z. Anorg. Allg. Chem., 6 (1966) 169.