THE OXIDISING PROPERTIES OF SULPHUR DIOXIDE. 1241 CXXXIV.-The Oxidising hoperties of Sulphur Dioxide. Part 11. Iron Phosphates. By WILLIAM WARDLAW SIDNEY RAYMOND CARTER and FRANCIS HERBERT CLEWS. THE obj& vf the experiments deearibed in the present mznmmi-clat3ion was ta determine whether the olxidising a&im of sulphur dioxide exhibited in oolnaetratd hydroohloio acid solution (Wardlaw and Clews this vol. p. 1093) was ~est~richd tol b h b add or was operaltive in ofther acid solutions. Phwphoria acid wa8 wl&d on m u a t of its noln-volatility and its mmexpdj suitlability for quantitatrive measurmentsi. EXPERIMENTAL. BxfieiGrnemt with F e v o w Pholsphta S h t w m . Solutions of f errolus phospha#b were prepared by dissolving im in phmphoria aleid (Erlenmeyer Ancnaalem 1878 194 182) and tai.ela8ting with sulphur dioxide.Essentdallly theref orel the rn0thd of procedure was practioally identical with that pemibudly employed for the investigatim of fenma uhbridei redwnb (loc. cit.). As illustrative off the met*hold used th0 following d e t d s off the1 first experiment may be given. One grah of pure iroin was dissolved in 50 U.U. of phbsphorio a,aid (I3 1-75 89 pelr centl. H,PO,) aointained in al dolsed flak a ahream of aarbon dioxide being Colntinudly passed tkotlgh the apparatus. A mloiurlessl solutibn of ferrous phosphate wm thus o;btla,ined. The evolution of hydrogen having oea+sd sulphur dioxide wm passed through the soluticm at 115O. After about ten minutes a faint milkiness was seen in the nmk of the 3mk, and in the oorursei od t l h e partides od sulphur appeared.The solution remained collolurless tjhroughout. After the sulphur dioxide had passed for four hours the gasl wit8 removed from the sollubim by the passage oif a a r h n dioxide. The femme md total iron wem detbrmined by titratioin with ptasS;um di&rmn.at.e solution. The first experiment showed that 8-1 per aeatt. of f t d a phosphah had been foirmed under the uonditims mentioned. Ih v i m of the extreme susoeptibility oif felrrous phmphatp tiol a h * sphetia d d a t i o a (Erlenmeyer Zoc. cit.) khe oxidation by d p h u r dioxide was not unexpeatad 1242 WARDLAW CARTER AND CLEWS THE OXIDISING Tempera8turel 115'. Experiments with Ferm'c Pholsphate Sollutiolns. To dehmine whether sulphur dioxide had any reducing adion on ferria phwphate 3 grams od ferria phosphata were dissolved in 50 0.0.of glaoiad phospholrio add and sulphur dioxide wa8 passed through the solutioln maintained a t 115O f o r three hours. On andysing the solution no ferrous iron muld be debated. Further elxperiments o n similar lines gave no1 evidence of reduction of the feirriu phmphate. This result s e a s to a,grea with the well-known stability of ferria phmphate in the reducing blowpipe flame. A boralx belad containing ferria iron is radily reduced whereas a, f m i o mioroawmio bead is r e d u d with diffiuulty. Temperature 150O. Experiments witit Fewow- f ewic Phosplhcat ei Sdutwws. The objwt of thew experiments was to determine whether the prolonged aution olf sulphur dioxide on ferrous phosphate would reault i n cotmplete oxidatlion to the ferria stratel.The mixtures were made by dissolving iroln in phosphoric acid excluding air during the process a,nd adding a solution of ferria phosphate in concentrated phmphoria said. Ferric iron. Per cent. I m G r T 2 a l . 0.0 8.1 12-5 15.7 21.9 24.7 25.0 28.0 39.7 -55.7 66-9 100.0 100.0 Sulphur deposit. Heavy. Moderate. Moderate. Moderate. Very light. Just noticeable. None. Ferric iron. Per cent. Ini&r%& 43.2 46.7 57.1 60.5 63.4 65.2 72.0 74.2 79.1 79.9 100.0 100.0 Sulphur deposit. Moderate. Moderate. Light. Light. Just noticeable. None. The prmnae olf the great exms of phosphoria auid teinded make tihe titration somewhat diffiault but the clharaohr of the sulphur depmit served as a goad indication of the progre&s of oxidation espeaia,lly in those case5 where1 i t wcrurred only to a slight extent.Cornpanson of the reeiults obtaJned a8t the t'wa temperaturea suggeats that felrrougi phosphate wonld be completely oxidiseld t PROPERTIES OF SULPHUR DIOXIDE. PART 11. 1243 the ferrio state but tha,t at the temperatures used in thew ex@-m0nb the velmity of reaotion is belcoming slow wit'h increasing proportion of ferria salt. Hence during the four holurs very little oxidation is deteloteld with the higher propoctions of ferrio iron. Limit of Oxidation. of Ferrous Phosphate. Expmirnentg to determine the limit of olxidation were oarried out in a spmial apparatus primarily designed for obtaining the velcrcrity of reactlion.The relactioln vewell colnsisbd of a wide-neaked flask of about 600 0.0. capacity fitted with a rubber stopper, through which pas& the bearing for a stirrelr a syphoa sampling-t u b made of uapillary tube of fairly wide bolre an inlet tube for the sulphur dioxide and al wa,ter-uololed exit tube. The syphon sampling-tube wits connected with a water-ooolled 10 0.0. buratb, whioh in turn was colnnelotleid with a suatioln pump. The stirrer was fitted with it mercury seal and clmd a t the end so that sulphur dioxide w d d be maintaineld at a slight pressure in the1 flask without leakage. A simple device was used to colntrol the pressure of sulphur dioxide in the flask. The gas issuing from a sulphur dioxide syphon was divideid by a T-piecel one arm being oonnmhd with the reactlion flask a8nd the oather connected witlh a gauge-tube containing cmmntrated sulphuria acid.By allowing the sulphur dioxide t o bubble velry dolw1y through the sulphurio atid the prwure in the reaotioln flask was maintained a t a value greater than atmospheriu the ex- pressure being proportional ta the height of the mlphurio acid column. Method of Procedure.-To 250 0.c. of phosphorio acid contained in the flask wibs addeld a definitei welight of pure iron. A ourrent of oarboa dioxide wae. then passed through the apparatus and on warming the iron dissolved. When solution was complete( the temperature of the theirmostat was adjustad tlo 150° and the stream of oarboln dioxide replaced by one of sulphur dioxide.By closing the exit tube the pressure of sulphur dioxide was adjusted. Samples of solution were periodically drawn off into the burette, allowed to awl t o the ordinary temperature the volume adjusted, and run into a flask filled with carboln dioxide. The solution was then diluted with 30 per cent. phosphofria acid and the sulphur dioxide driv0a off. The volume was made up to 100 C.O. aad titrated with N / ZO-potassium psrmanganata The titre was assumed ta be proportional to the conmntrahition of ferrous iron in the reaotion flask at the moment od sampling. Froan experiments conduotd in this way a coastlant ferric content was found to be e&ablisheld at the end of from two to three1 days and the1 following were the results obtained 1244 WA~DLAW CARTER AND CLEWS THE OXIDISfNG (a) A t a aolnae4tiratioln of 7-35 grams of iron per 250 0.0.olf plhosphoria add the aixture aotntained 36 per oecnt. of flerria irofi. ( 6 ) At a corllaentratioln od 3-65 grams per 250 u.o. the mixture mntaJnd 45 per oentl. of ferric iron. These results differ distinctly from those obtained from the synthetio mixtureis of felrrous and ferric phosphahes. This differ-ence d g h t conceivably be due to the sulphur formed in the sdu-tions ccmtlaining initia(1ly ody ferrous phosphate whiuh is absmt in the case od the synt(he1tia mixt,ures. Expelriments with synthetic mixturm of felrrolua and ferria phmphatles ta whiuh had been added finely divided or co3loidad sulphur showed that this had no observ-able deut on the degree od olxidatJoln by sulphur dioxide.In addition sulphur wm found to ha,ve no appreciable reducing power on ferria phospha’h in concentrated phosphoilic acid solution. Maquenne (BUZZ. Bolc. chim. 1890 [iii] 3 401) has shown that aqueous solutions af phosphorous acid ar0 oxidised by sdphm dioxide cmpleitely to phosphoric a,oid (compare also W Ohler, A d e m 1841 89 252 and Caxazzi Gaazetto 1886 16 169). The posaibility of the reaction 2E3P03+ S02=2H,P04 + S beJng reversible was not overlooked but experiments did not mnfim this oolnjeoture. Velocity nindl Order of Reactim.-Expdments made with the appamtius described above with a+ v i m tol dekmina the rate and order olf reactlioln gave no vary definite results prob’ably awing tlo the disturbing aotim of sulphur.L k i t i m g Comcmtratmom of Pholsphov-ic A&.-This has been determined b t w e m narrow limits for ferrous phoephate at looo. The p r d u r e followed was similar to that used in the uase of hydrolahloriu acid solutions (Wardlaw and Clews this vol. p. 1097). One graa of iron was dissolved in 50 a.a. of phmphmia mid of different mncentratioins kh0 values folr which were obtained by measuring the spwific gravity oif the phosphoric aoid and aahu-laling the colllaentrations from the tablea (Hager ‘‘ fiommelntar ziir Phasmaqija Germanisai ” ; Gmdin-Kraut’e “ Handbuah,” Val. I, Pt. 3). A gravhetric estimation was made on the solution in expt. 6 maording t a Finkmar’s melthold (Ber. 1878 11 1640) axld it was shown tlol colntain 400.7 grams of phosphoris acid per litrel, whilst 32.83 p w cent.phoaphoriu acid (elxpt. 6) contains 397.2 grams per litre. Assuming the iroln t a be in cambination as Fel(H,PO,) (Erlenmeyer Zolc. cit.) the frm aoid in 1 litre of the ferroas phosphate solution was 400.7 - 68.6 =332*1 grams. The raults are shown in table 111 PROPERTIES OF SULPHUR DIOXIDE. TABLE In. ConcentratJon of i r o n 1 gram per 50 0.0. Temperature looo. Durahicm of experiment8s foiur hours. Concentration of HsP04 per cent. .............................. 76.27 45-93 Observations ..................... sulphur. sulphur. Concentration of HSPO4 per cent. .............................. 34.38 32.83 Observations ..................... Minute Very Experiment 1. 2. Oxidation per cent. ............ 2.1 1.3 Experiment 6.6. - I Oxidation per cent. ............ amounts. minute deposit. PART 11, 3. 40.87 1.0 sulphur. 7. 31-80 N O deposit. -1246 4. 37-17 sulphur. 8. 20.02 N O deposit. --Ib is aonaluded thelrefow that oxida4tion of felrrcnxs phosphate by sulphur dioxide is just pelrceptible in a solution cmtaining 332.1 grams olf ( ‘ f r e ~ ” phwpholria acid per litre at 100’. Discussim of R esuEts. The reaoticms of sulphur dioxide with iron phmphah in the preaehae of concentrated phosphorio acid differ in several ways from tlhme with iron ohlolrides in concentrated hydrochloric acid sdutioa. I n the first plaoel there! is no1 elvidelnm tqhatl sulphur dioxide1 clan act as a reducing agelnt in the presence oC concentrated phosphoric acid folr ferria pholsphate unlike feInic ohloridel in the preseam olf the comwponding concentrated acid is not affected by the a,&ioln of sulphur diolxidei.The relaation betlwelen sulphur dioxide and iron phosphatlee in concentrated phosphoric aoid solu-tion is theiredore sollelly one olf oxidation and may be e x p r d by the equation 4Fel(HzP0,) + 4H,PO + SO = 4Fe(H2P04) + 2H,O + S. The question now arise6 as to whether the1 reachion is a revelrsible onei like1 the mrraponding felrroas chloride relaction or whether it proceeds to m p l e t i o a . The faat thab mixed ferrous-ferric phosphate sollutions gave evidence of olxidatim with relatively large perrcrentlagea olf ferric salt rather swms to indioate that the readion would ultimately proceied to completion.This idea also seems to be supported by the non-raduclibility of ferria phosphate by sulphur. Against this conception however must be plaaed the limited yield od ferrio salt from an initially pure ferrous phosphate solution and the favourable influam of the oonaentrakd phos-pholrio said on the axidaltion. Both these fa& wmld be deduce 1246 WARDLAW CARTER AND CLEWS THE OXIDISINC; from the la4w of mass action if a,pplield t o the above equation, tlaking it as relpresentjing a re8vewible reaction Undoubtedly the complimting fea,ture in tlhel raotion is t'he f ormattion of the st,able aomplelx compoand whic;h fe,rricl pholsphafe is known to form with phoepholric; a.cid. This woald accolunt for the greater yield of felrric sa,lt( from a pure1 felrroius phospha,tel solution than from a pure ferrous chloride solution owing to1 tlhe fact that the ferricl; sa,lt formeld would be removed from the1 reaction in the form of the stab,le complelx oompwnd and henoel favour the degree of oxida-tion.This forma.tion of the1 c,omplelx sa.lt may also a,cclount fomr the non-reducibility olf t'hel felrrio phosphate in concentrated phosphoria acid sollutlioln bly tlhe action of sulphur. It is quite conaeiva4b3e t8ha,t sulphur ma'y reiduclel felrric phospha,te when not in t'he form od the clolmplex compolund. Such ccmdit'ions a,re probably prelselnt wheln sulphur diolxide is passed into1 a pure ferrous phos-p h a b wlutXion. 'Thet sulphur libelraked is in a very a,ctivs state and may rea'ch with t'he ferric; phosphah genera'teld before itl is t.aken up in the folrm of the1 complex.A considera,tioln of t h w fa.ottolrs indinel the1 authofrs to the idela that the reladion of sulphur dioxide1 wit.h ferrous phosphate is a reversibmle one simi1a.r to that of sulphur diolxide with felrrous chloride in the prmence of their corresponding oonaelntlra,tletd acids but tha.t itl is modified in the case od the1 fesrous phmpha,te by the forma,tbon of the cl~nplex stable1 oo,mpound which felrric; phosphatei forms with phmphorio acid, The faclts that sulphur diolxide redurn most reladily in a veiry dilute a,cid medium and tchalt it oaidises most readily in a strong acid meidium ma4y be correlated i f oxidation a,nd relduotion a,re explained an an ionic b'asis oxidahion being retpreseated by the surrender of poaitive chasgesl and reduotioa by the trmsferenw of nelga,tive uharges.Sulphur dioxide in a,queo,us solut'ioln is genetrally regarded as a modeirately weak aaid ionising principally inta H* HSO,' and SO," ions. Itl is in this condit'ion that it reaots as a reducing agent. Thus: 2Fe'"+ SO," + H20=2Fe" +SO,"+ 2H'. I n strongly a,&d solution containing a large number of hydrions, t,he mnaelnt'ra;tioln of SO," ions will be r e d u d and on the1 above assumption its powelr of reduoing should be diminished. This is in amolrdance with expeirimental results. Nolw let it be assumed that sulphur dioixide is oapable of ionising tlo an ext#remeily minute e x t a t als a b,a,se yielding a correspond-ingly minutxet amount of sulphur ions.Itl has been shown that the sulpholxidet3 the orgvio analogue6 of sulphur dioxide hav PROPERTIES OF SULPHUR DIOXIDE. PART 11. 1247 basia propelrtiee ( F r m and Raiziss Amnalen 1910 374 90; F r o m ;bid. 1913 396 75). This tlendelncy will be all the g r a t e r tfhe larrger the number of hydrions present in solution. Thus : SO ,? OS(OH) G? SO" + 2(OH)', SO" &? S"" + 2(OH)'. In view of the large number od hydrions present in the solution, tlhe oonmntratioln of hydrolxyl ions would be reduced to a very low value and the reaction towa]rds the rightl fa,voured. Oxidation is now represented : H2O HZ* S"" + 4Fe" -+ 4Fe'" + S. I f oxidation takea place due to the ion SO", 2SO" + 4Fe" + 4Fa"' + 250, 250 + H20 + H2S,0,, which represents an intmmdiate stage in t'he reduction of sulphur dioxide to1 sulphur. Thiosulphuria acid would break u p into sulphur dioxide and sulphur. Itt may be observed thaf the latter hypotheeis is in many respects a restatement olf t#hel thionyl chloride hypot-hesis (Wardlaw and Clelws this vol. p. 1103) applield ioniclally and more generally. The oxidising prolpertiee of arsenio acid uranyl ohloride per-manganic acid and chromio acids have been explained by Stieglitz (" Qualitative Chemical Analysis," Voll. I p. 282 et s e q . ) on a similar assumption. There appears to be room for investigation of these mactions fmm the point of view o'f delelcrtlriric;al potentials when some of the assumptiolnns matdo in the1 absove coald be teateld and it is hoped that efxprimelnts nolw in progrea may throw some further light-on this hypothesis. CHEMISTRY DEPARTMENT, THE UNIVERSITY BIRMINGHAM. [Received Auqwt 27th 1920.