Malhotra, K. C.

Puri, J. K.

English

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Indian Journal of ChemistryVol. 14A, June 1976, pp. 384-387Behaviour of Boron Trichloride & Tribromide, Boric Anhydride &Boric Acid in Disulphuric Acidx. C. MALHOTRA'" & J. K. PURlChemistry Department, Panjab University, Chandigarh 14Received 23 June 1975; accepted 1 September 1975Boron trichloride, boron trtbromlde, boric anhydr-ide and boric acid form condensedstructures having brid~in~ sulphate groups with the overall composition, H[B(HSO.),] whendissolved in disulphuric acid. Acid-base titrations have been carried out to show thatH[B(HSO.).l is a strong acid of the system. Cryoscopic stu~ies show that at hi~h~r concen-trations H[B(HSO.).] polymerises to form condensed borate rmgs through B-O-B Iinkagc.CRYOSCOPIC and conductometric measure-ments on dilute solutions of boric a:id al~dboric anhydride in 100% sulphunc acidand dilute oleum have been interpreted in tenusof the formation of tetrahydrogensulphato boricacid1,2 which has been found to be fairly strongacid of the solvo system sulphuric acid". Arotskyand Symons' studied the behaviour o~ boron saltsin 65 % oleum and suggested the reactions:BCl3+3H2S207+3S03-~B(HS207h+3HS03ClB(HS207)3 + HS20,-+ B (HS207)4As it has been established by Gillespie and co-workersv" and also by Paul and coworkerstt? thatmost labile anion in disulphuric acid is HS30iorather than HS20" it was, therefore, of interest toinvestigate the behaviour of ,boron .compoun?s,viz. boron trichloride and tribromide, boric anhydndeand boric acid in disulphuric acid. Presently wehave carried out conductometric and cryoscopicstudies of some boron compounds in disulphuricacid to explore the possibility of formation of thesolvo acid of this solvent.Materials and MethodsThe design of the conductance c~ll and the cryos-copic set up, the method to obtain parameters Y,v and c have been discussed earlierS,7,ll. Thesolvent of stoichiometric composition was preparedusing indicators, its freezing point in cr.r:0scopicexperiments and its specific conductance III con-ductometric experiments. Boric acid and boricanhydride were BDH (AR) reagents and were usedas such. A slow stream of boron trichloride waspassed through a weighed c;mount of dis~lphuricacid for 2 hr and then weighed. The differencein the weight gave the amount of boron trichloridedissolved in known weight of disulphuric acid.A known weight of this solution was then addedto the solvent for conductance and cryoscopicstudies.*Present address: Department of Chemistry, HimachalPradesh University, Simla 1.384Results and DiscussionBoron trichloride is highly soluble, to the extentof 15%, in disulphuric acid and solutions up tothis concentration range could be easily preparedwithout any precipitation. At low concentrationof the solute, the values of Y and v are low butincrease rapidly at higher concentrations. Fromthe values, it is evident that boron trichloride under-goes solvolytic reaction in it. Va.rious possiblemodes of the solvolytic reactions are:BCI3+3H2S207--+B(HS04b+3HS03CI ... (1)BCla+6H2S207--+B(HS207)a+3HS03Cl+3H2S04 ••• (2)BCla+9H2S207--+B(HS301oh+3HSOaCl+6H2S04 ••. (3)As the solutions are fairly conducting, the solvolyticreaction cannot be represented by any of the aboveequations. When potassium hydrogen sulphate (astrong base) is added to the above solution, thereis a decrease in the conductance of the solutionsuggesting that the solvolysed product behavesas an acid of the system. Possible reaction involvingthe formation of HfS04 ions in the solution maybe represented asBCla+ 7H2S207--+H;S04 +B(HS04l4 +3HS03CI+2H2S3010 •.• (4)A lower value of v in dilute solutions is due to thefact that trisulphuric acid formed is one of theproducts of molecular self-dissociation of the solventand does not contribute to 'I value of one. Athigher concentrations, a lower value of then expectedfrom Eq. (4) is due to polymerization of B(HS04)4to form long chain molecules. The tendency ofB(H~04)4 .to polymerize has already been reportedby Gillespie and coworkers'>. Cryoscopic titrationsagainst sulphur trioxide rule out the formation ofany sulphuric acid in this solvolytic reaction; rathertitrations against sulphuric acid show the presenceof two moles of sulphur trioxide (H2SaOlO) permole of boron trichloride providing support to thereaction represented by Eq. (4). The presentcryoscopic studies do not support the presence ofl\L\.LHOTRA & PUIU: BEHAVIOUR OF SOME BORO~ COMPOUNDS IN DISULPHURIC ACID10.910.1810,)(, )Co:6 :0 BBr'59.31 ------- Bel,I -t:J B H'5B 03- I7 a.s!Ev7.7l'E.s:::0I'-'•...Q)( 6.9ciz0u~ 6·1IJ)5.34.520 24 28MOLALITY~IOl. 40Fig. 1 - Specific conductance of some boron compounds in H2S207 at 25°the ions of the type H~S207 or H~S3010 or B(HS207)4:or B(HS3010):i, etc., as these species would haveproduced different number of particles. The con-ductometric and cryoscopic data are graphicallypresented in Figs. 1 and 2.In the case of boron tribrornide, bromine is evolvedin the conductance cell and the conductivity set up.Unlike hydrochloric acid, hydrobromic acid pro-duced as a result of solvolysis is oxidized to bromine.From the values of Y, v and c, the possible solvolyticreaction may be postulated as:2BBr3 +8H2S207--+2H;S04 +213(HS04).i + 3Br2+3S02+H2S3010 ... (5)Both sulphur dioxide and bromine are known tobehave as non-electrolytes in disulphuric acid and,therefore, contribute a value of one to the v factor.UV spectrum of the solution shows an absorptionband at 290 urn confirming the presence of sulphurdioxide'! in the solution, thereby supporting theabove reaction.Conductometric titrations between boron tri-chloride solutions and some bases have been carriedout to ascertain whether these solutions are acidicin nature or not. A decrease in the conductanceof the solution on the addition of a base is due tothe removal of H;S04 ions from the solution asH;S04 + HS30io--+2H2S207' The conductance-molar ratio curves pa"s through a minima (sharp)which occurs at base/acid molar ratio of 1: 1 sug-gesting that tetrahydrogensulphatoboron behavesas a strong acid of the system. The conductanceof the solution at the maximum has the same valueas that of the pure solvent. It suggests that theions non-specific to the autoionization of the solventcontribute very little to the conductance of thesolution. This seems reasonable as the processof diffusion is quite negligible in a solvent of highviscosity.Boric anhydride dissolves very slowly in disul-phuric acid. Its solubility increases at highertemperature and the solutions up to 0·5 molal con-centration can be prepared but the conductanceof the solution does not change much. At lowconcentration, the value of v is only 3'1 but increasesconsiderably at higher concentration. Cryoscopicand conductance data support the reaction:B203+ 11H2S207...-+2H;S04+2B(HS04)4+4H2S3010 ••. (6)The existence of the species B(HS207)4 or B(HS30lO)4:does not seem to be possible as the acid H[B(HS04)4Jformed is far stronger than H2S30lO or H2S207and, therefore, would not be solvolysed. The slopeof the conductance curve for B203 is concave inthe low concentration range which shows thatbecause of the extensive self-dissociation and ion-ization of the solvent the formation of H;S04 issufficiently, suppressed. .Here again the tendencyof B(HS04i4: to polymenze at higher concentrationis quite evident as at low concentration the valueof v is 3·2, and at 0·2 molal concentration the valueof v is 5·8: After that the valu.e of v starts decreasing.Cryoscopic and conductornetric data are graphically385INDIAN J. CHEM., VOL. 14A, JUNE 197634.834.6,34.4f34.2,...0°'-'~34.00Il.~ ss.azN&&J&&J33.6a:~3'3.433.233.032.832.620 4)( M B 8Y'!.8 B sci,ts :0 B1O~• • H3 B03~--~--~----~6----8~--~IO~--~IZ~--~14~--~16~~~~~20MOLALlTYXIO'l.Fig. 2 - Freezing points of some boron compounds in H2S20,presented in Figs. 1 and 2 respectively. Similarly,boric acid undergoes solvolytic reaction in disul-phuric acid. From the value of 'Y, 'Y and c, thepossible solvolytic reaction may be represented asB(OH)a+3H202S7-+H;S04+B(HSO,)4+H2S04 .. ·(7)Conductornetric acid-base tit rations against nitro-methane, potassium hydrogen sulphate, etc., havebeen carried out to ascertain its acidic character.The trends of these tit rations are the same as hasbeen discussed in the case of boron trichloride solu-tions.It is observed that for all these solutes, the valueof v decreases at higher concentrations of the solute.Possibly the tetrahydrogensulphate boron anionpolymerizes to give condensed structures havingbridging sulphate groups. These structures maybe similar to the structures of hydrated polyborateswherein the basic structure is a six-membered ringwherein one or at the most two of the boron atomshave a tetrahedral four coordinated configuration386while the remainder have a trigonal three coordi-nated configuration15,16. It is also pos- ible thatthere may be more than one type of the condensedpolymer" present in the solution with the overallcomposition H[B(HS04l4J.References1. FLOWERS, R H., GILLESPIE, R. J. & OUBRIDGE, J. V.,J. chem . Soc., (1956), 1925.2. GILLESPIE, R. J. & ROBINSON, E. A., Non-aqueoussolvents, edited by T. C. Waddington (Academic Press,New York), 1965, 115.3. FLOWERS, R H., GILLESPIE, R. J. & ROBINSON, E. A.,Can. J. Chem., 38 (1960), 1363.4. AROTSKY, J. & SYMONS, M. C. R, Trans., Faraday Soc.,56 (1960), 1426.5. GILLESPIE, R. J. & MALHOTRA, K. C., J. chem, Soc., A(1968), 1933.6. GILLESPIE, R. J. & MALHOTRA, K. c., Inorg. cu«; 8(1969),1751-7. PAUL, R C., PURl, J. K. & MALHOTF:A, K. C. cse«;Cornrnun. (1970), 776.MALHOTRA & PURl: BEHAVIOUR OF SOME BORON COMPOUNDS IN DISULPHURIC ACID8. PAUL, R. C., KAPILA, V. P. & MALHOTRA, K. C., J.chem. Soc., (A), (1970), 2267.9. PAUL, R. C., PURl, J. K. & MALHOTRA, K. c., J., inorg,nucl. Chem., 33 (1971), 2459.10. PAUL, R. C., KAPILA, V. P., PURl, J. K. & MALHOTRAK. C., Indian J. Chem., 9 (1971), 469.11. PAUL, R. C., KAPILA, V. P., PURl, J. K. & MALHOTRA,K. C., J. chern. Soc., (A), (1971), 2127.12. GILLESPIE, R. J. & ROBINSON, E. A., Can. J. Chem.,40 (1962), 1009.13. MALHOTRA, K. C. & SHARMA, R. D., Indian. J. Chem.,10 (1972). 430.14. GOLD, V. & TVE, F. L., J. chem, Soc. (1952), 2172.15. EDWARD, J. O. & Ross, V., J. inorg, nucl, Chern.,15 (1960), 326.16. DALE, J., J. chem, soc., (1961), 922.387

Behaviour of Boron Trichloride & Tribromide, Boric Anhydride & Boric Acid in Disulphuric Acid

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Behaviour of Boron Trichloride & Tribromide, Boric Anhydride & Boric Acid in Disulphuric Acid

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