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Ionization of Some Inorganic Halides in Chlorosulphuric Acid Solvent System-Redox Reactions of Phosphorus, Arsenic & Antimony Halides in Chlorosulphuric Acid

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Ionization of Some Inorganic Halides in Chlorosulphuric Acid Solvent System-Redox Reactions of Phosphorus, Arsenic & Antimony Halides in Chlorosulphuric Acid Indian Journal of Chemistry Vol. 20A, August 1981, pp. 773-776 Ionization of Some Inorganic Halides in Chlorosulphuric Acid Solvent System-Redox Reactions of Phosphorus, Arsenic & Antimony Halides in Chlorosulphuric Acid Z. A. SIDDIQI·, MOHAMMAD ASLAM, N. A. ANSARI, M. SHAKIR & S. A. A. ZAIDI Chemistry Department, Aligarh Muslim University, Aligarh 202 001 Received 13 October 1980; revised and accepted 29 November 1980 The behaviour of solutes, viz. phosphorus trichloride, tribromide, pentachloride, pentabromide and hexachloro- iodide in HS03CI has been investigated conductometrically. While the pentahalides and bexacbloroiodide ionise pro- ducing the stable cationic species PX~, tbe trihalides are oxidized producing ultimately the conjugate acid POH+X. in tbis solvent, The conductometric redox titrations of phosphorus trihalides with the appropriate halogens and inter- halogen (ICI) compounds indicate the formation of cationic species PCI3Br+ and PBr.CI+ as stable entities in solutions. Solutes like AsCI•• SbF. and SbCI. are only partially ionised whereas SbCI •• BiCI. and VCla remain insoluble in this solvent. The basic ionization constants ofthe partially ionizing solutes have also been evaluated. Tis quite well known- that the interaction of the sulphuric acid19 give fairly conducting solutions stoichiometric amount of the appropriate halogen (Fig. 1) in chlorosulphuric acid; the conductivity Iwith a few binary phosphorus(III) halides in gradually increases with time and attains a stable non-polar solvent like carbon tetrachloride increases value on standing for a few minutes. The average the oxidation state of phosphorus resulting in the y-values of the stable solutions have been found to formation of the corresponding mixed phosphorus(V) be 0.20 and 0.80 respectively indicating the formation halides. However, recently chlorosulphuric acid, to a considerable extent of free SOsCI- ion (res- a non-aqueous, ionizing, strong acidic solvent2-15, ponsible" for conducting almost all the current) in has been considerably exploited as a suitable medium solutions. These solutes possibly ionise either by a to carry out redox reactions8-10,15 for the formation simple protonation (Eq. 1) or are incompletely and stabilization of various cationic species which ionised (Eq. 2) or undergo gradual oxidation to under normal conditions are unstable. It W2.S there- form phosphoryl halides (POXa) which are ultimately fore considered of interest to study the behaviour of protonated? to give the conjugate acids (POH+Xa) in a few halides of group (V) elements in chlorosulphuric solutions (Eq. 3). acid with a view to investigating their modes of ionization as well as to examine the possible forma- PXa + HSOaCl = HPXj + SOsCI- .. (I) tion and stabilization of'heterocationic species, e.g. PXs + HSOaCI = PX~ + SOaCl-+ HX ... (2) PClaBr+, PBr:lCl+, PClaI+ and PBrzI+ in HSOaCI solvent system. PXa + HSOaCI = POX~ + S02 + HCl POXa + HSOsCI = POH+X2 + SOaCl- ...(3) Materials and Methods X = Cl, Br Chlorosulphuric acid (Riedeljhaving sp. conducti- vity 4.37 x 10-4 mhos was used. Al the materials Due to the solvent cut-off for the acid at about were commercially pure samples and were used 2.S 290 nm where S02 is also reported'" to show a received excepting phosphorus trichloride, phosphoryl strong absorption, it has not been possible to obtain chloride and iodine which were purified by established UV-spectroscopic evidence for the existence of SOil methods'P'!". Phosphorus pentabrornide, phosphorus in solution of phosphorus trichloride and phos- hexachloroiodide and arsenic trichloride were synthe- phorus tribromide in chlorosulphuric acid. It is, sized according to the publishe:i methods-'"!". however, noteworthy that specific c~>nductance- The design of the conductivity cell, meaning and concentration profile of the stable SOJUtlO~Sof PCla significance of the notations y and wand the deter- lies point by point to that of POCla (ref. 7) III ~h1oro- mination of y have been described else where 5-7013 sulphuric acid (Fig. I), consequently favouring the A Systronic type 302-S.R. No. 306 conductivity mode given by Eq. (3), which also seems quite bridge thcrrnostated at 25° ± 0.1 °C was used for reasonable with the oxidizing behaviour of chlorosul- conductance measurements. phuric acid8-l0,15. In disulphuric acid " =. forma- tion of the same fully protonated species POH+X3 Results and Discussion has been reported. However, in HSOsCl the observed The phosphorus trichloride and phosphorus tri- average y-values (Table 1) are consistent with only a bromide, reported to be non-electrolytes in fluoro- partial protonation. The basic ionization constants 773 INDIAN J. CHEM .• VOL. 20A. AUGUST 1981 14.0 12.0 .... o 10.0 z <-•... I V E ~.:!8.0 0 I Z E o~ v 0 vI'> 6.0 -... 2 - x V lo: '"Q. 4.0 .,oJ) v s . Bri!' v s . Ic I. v s, rct O.O~--~----~--------~--------~~------~ 1.0 2.0 3.0 4.0 MOlON RATIO Fig. 1 - Specific conductance-concentration curves of the various solutes in chlorosulphuric acid at 25° KBH+ of the conjugate acids POH+X3 have also been The observed lower i-value of 0.60 from that expect- evaluated ". The average KBH+ values obtained are ed (i.e., i = 1.0) from Eq. (5), however, indicates 4.74 X 10-3 and 6.48 x 10-2 mol kg-l for the that unlike phosphorus pentachloride the phosphorus chloro and bromo analogues respectively. The pentabromide is incompletely ionized in solution. relative higher value for latter species is quite in It may presumably be due to the less polar nature of agreement with its expected more basic nature. the P-Br bond in PBrs as compared to P-Cl in PCls' The phosphorus pentachloride dissolves extensively The basic ionization constant, Kb has been evaluated in chlorosulphuric acid with the evolution of HCI and its average magnitude is given in Table 1. gas producing a highly conducting solution. The Phosphorus hexachloroiodide (PCI61), hitherto not conductance of the solution remains fairly stable and investigated in any of the known strong acidic the average i-value has been found to be 1.05. The media, produces a highly conducting solution in mode of ionization consistent with the above observa- chlorosulphuric acid (Fig. 1) having an average tions may be expressed by Eq. (4). y-value of 1.25. It has been shown= that in non- polar CCI , the dissolution of the solute is accom- PCl, + HSOaCI = PClt + S03Cl- + HCl .. (4) 4 panied by its decomposition into the constituent mole- The cation PClt is known to exist as a stable species cules, i.e. PCls and ICI. However, the absence of in non-aqueous acidic media19,20,22. However, phos- variation in the observed conductivity with time rules ,phorus pentah-ornide which produces a yellow out any probable decomposition of PCl61 in HSOaCl. coloured conducting solution has a i-value of 0.6 Furthermore, the solution acquires an intense brown only. The conductance of the solution r~mains fairly colour due to the liberation of iodine", The mode stable on standing, consequently, rulmg out any of ionization compatible with these observations possibility of its decomposition unlike to that reported may therefore be written as : in H.SZ07 (ref. 20). The mode of ionization of phos- phorus pentabromide in HSOaCl is, therefore, reasonably expressed by Eq. (5). TABLE 1 - AVERAGE MAG !TUDES OF Y AND Kb OF THE SOLUTES IN CHLOROSULPHURIC ACID AT 25°C PBrs + HSOaCl = PBrt + HBr + SOaCl- .. (5) The existence of PBrt as a stable cationic species Compound 'Y x; x 10' in the solution has recently'" been indicated from (mol kg-I) 3lp NMR spectroscopic studies also. The yellow coloration of the solution might be PCI3 0.20 due to bromine formed as a result of oxidation of PBr3 0.80 PCl. 1.05 HBr by chlorosulphuric acid without causing any PBr. 0.60 5.063 appreciable variation in the conductance of the solu- PClsI 1.25 .tion, i.e. AsCI. 0.22 2.140 SbF, 0.060 0.164 2HBr + 2HS03Cl Br2 + S02 + 2HCI seer, 0.047 0.093 + H2S04 .. (6) 774 SIDDIQI et al .. : IONIZATION OF INORGANIC HALIDES IN HSOaCl The reaction of phosphorus trichloride with iodine 2PCl61 + 3HSOaCI = 2PClt + 2S0aCI- + 12+S03 + 3HCI + Cl, .. (7) monochloride in chlorosulphuric acid can be re- presented by either of the following two possible The products S03 and Cl2 are non-electrolytes.' modes (9) or (10). .. whereas 12 and HCI are reported to act as weak bases3'15 of this solvent system. PCl3 + ICI + HS03CI = PClaI+ + SOaCI- The conductometric redox titrations of PCl3 and +HCI .. (9) PBr3 with the appropriate halogens (Brz ~nd IJ a~d interhalogen (LCl) have also been carried out In PCI3 + ICI + HS03CI = PClt + HI chlorosulphuric acid with a view to examining t~e + SOaCI- .. (10) possibility of formation of a few heterocatioruc 25 species '26, viz. PClaBr+, PBr3CI+, PCI3I+ and The solution, however, shows liberation of iodine as PBr31+ as stable entities in solution. evident from its violet coloration, which might be When phosphorus trichloride is added to solutions due to HI oxidation and therefore, favours Eq. (10). of bromine and of iodine monochloride in HS03CI, In fluorosulphuric acid 26 also it has been shown that a sharp increase in the conductivity occurs as com- the titration of phosphorus trichloride with iodine pared to that of the solute in HSOaCl. A similar monochloride results in the formation of PClt behaviour has also been noted in the case of phos- cation as a stable entity in solution rather than the phorus tribromide in HSOaCI solution of iodine aforesaid PClaI+ cation. monochloride. These observations suggest inter- The observed sharp break at mole ratio PBra/ICI = actions involving an increase in oxidation state of 3.0 (Fig. 2), having a y-value of 0.8 and the liberation central phosphorus atom in the solutes by the of iodine for the conductometric titration of PBr3 appropriate reagents yielding highly conducting with ICI are compatible with the formation of the species other than that produced in their neat solu- cationic species PBraCl+ in solution according to the tions.
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