Ammonium hexanitratocerate(iv) as an oxidising agent

Ammonium hexanitratocerate(iv) as an oxidising agent

Talanta. 1962. VoI 9, pp 835 to 839 AMMONIUM TITRATION Pergamon Press Ltd Printedm Northern Ireland HEXANITRATOCERATE(IV) OXIDISING AGENT OF O...

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1962. VoI

9, pp

835 to 839




Press Ltd Printedm Northern Ireland






of Chemistry, Andhra University, Waltalr, S India

(Recewed 27 March 1962 Accepted June 1962) Summary-The use of ammomum hexamtratocerate(IV) In oxldlmetry has been very little studied, although the salt can be obtamed In a high degree of purity Condltlons have now been established for the tItratIon of oxahc and mandehc acids with this reagent at room temperature In a nitric or hydrochloric acid medium usmg ferrom as Indicator The procedures developed give precise results and they constitute definite Improvements over those mvolvmg cermmIV sulphate

SMITH, Sullivan and Frank1 described the preparation of ammonium hexamtratocerate(IV) m a high degree of purity and studied its stab&y m aqueous solution acidified with nitric or sulphuric acid. Subsequently, Smith and Fly2 described the preparation of ammomum hexamtratocerate(IV) m a sufficient degree of purity to be used as a primary standard. They claimed several advantages for the use of this salt : (1) high equivalent weight, (2) high degree if stability m the crystalline condition,* and (3) the attainment of a high oxldatlon potential m perchlonc acid (1*70_1*87v from l-Slv) or m nitric acid (1.61-1.56~ from l-Slv) media. Smith, Sullivan and Frank1 determined the purity of the salt by dlssolvmg a weighed sample m sulphurlc acid (1: 10) and titrating with standard iron” sulphate solution, using ferrom as indicator. Smith and Fly2 determined the purity of the salt by dlssolvmg weighed samples m 70% perchlorlc acid and titrating with a standard solution of sodmm oxalate m perchlorlc acid solution, using mtroferrom as indicator. This reverse titration 1s preferred to the usual direct titration for reasons fully discussed m their original paper. The use of ammonium hexamtratocerate(IV) in a mtnc acid medium does not appear to have been studied although Smith, Sullivan and Frank stated that “The nitrate ion had no influence upon the determination of ferrous non, the most commonly determined element. It is not expected to interfere m most cases. The value of the proposed new standard need not be in the least mmimlsed by such conslderFrom a solution of known weight of ammonium hexamtratocerate in ations sulphurlc acid, a solution of cerlc ammonium sulphate 1s easily obtained free from nitric acid by the simple process of digestion. All question of the possible presence of the undesirable nitrate ion is thus eliminated.” The process of digestion with sulphuric acid to eliminate the nitrate ion 1s to be

* The reagent IS somewhat unstable m solution and requires frequent standardsatlon. 835







and K. S


avoided for three reasons: (1) it IS tedious, (2) it deprives the advantage of this complex salt as a primary standard, and (3) the conversion of the ammonmm hexamtratocerate(IV) to ceriumIv ammomum sulphate as proposed by Smith, Sullivan and Frank” will remove its advantages as a reagent of high oxidation potential. Further, according to Smtth and FIY,~ the preparatron of a solutron of ammonium hexanltratocerate(IV) m a sulphunc acid medium requires great care, otherwise mixed msoluble mtratosulphatocerates may be precipitated. Although a higher oxidation potential of the ammonmm hexanitratocerate(IV) can be utihsed by working with solutions of the salt m a perchlonc acid medmm, there are some ddliculties associated with this reagent as discussed fully by Smith and Flyea A survey of the hterature has shown that so far no one has extensively studied the reagent in a nitric acid medmm,usmg ferrom as indicator. We have now made a study of this reagent both m nitric acid and hydrochloric acid media. Our results are somewhat at variance with the statement of Smith, Sulhvan and Frank about the non-Interference of mtnc acid. We have ascertained the con~entratlons of nitric acid under which ironn and oxalate can be titrated accurately with the cerate solution at room temperature. EXPERIMENTAL Reagents Ammomum ~e~anltratocerate~IV~ soiutton The crystalkne complex salt is prepared aeeordrngto the method of Smith et at 1 A known wereht of the solid thus obtained IS drssolved m mtrrc acid and the solutton made up to 1 htre wrth thevrequWe amount of dilute mtrlc acid so that the overall concentratron of nitric acid 1s 1 ON Although Smith et al experienced some dtfficulty m the preparation of clear soluttons of ammonmm hexamtratoeerate(IV) m 1 ON perchlortc actd because of the preerprtatzon of complex ~rc~o~tonitrat~rates, no such d&culty has been experienced by us m the preparatron of a mtrrc acid solutron of thts reagent The solution is stored m an amber coloured Jena glass bottle The solutron 1s standardtsed agamst a sofutron of Mohr’s salt m a sulphurtc acid medium, usmg ferrom as mdtcator, the Mom’s salt solutton havmg been previously standardtsed against a standard cermmn’ sulphate solutron which m its turn has been standardtsed agamst a standard solutton of reagent grade sodium oxalate wrth a potentrometrtc end-point All other reagents employed m thts mvesttgation were of analytical reagent grade Xztratron ofiron” Tltratlon m a sulphurtc acrd medwn From 5 O-10 0 ml of an approx 0 05N norm solutton (prepared by dissolvmg non wrre of AnalaR grade m sulphurtc acid and adlustmg the overall sulphurrc acid concentratron to about 1N) were taken m an Erlenmeyer flask and treated with 20 ml of 2N sulphurtc acid and 20 ml of dtstrlled water The mixture was treated with 0 5 ml of 0 OOIM ferroln then titrated wrth the ~onl~ hexanitrato~rat~1~ solutton with constant sttrrmg untd a permanent pale blue colour persrsted The mdtcator correctron IS neghgrble m titrations with 0 1-O 05N soluttons of the cerate, but m trtrattons with 0 02-O OlN soluttons a correction IS necessary dependmg on the quantrty of ferrom, added. Accordmgly, tt 1s convement to dilute 0 01M ferrom solutton 10 tnnes and use 0 5 ml of thus solutton each tune as mdtcator The mdtcator corm&on can then be precrsely applied. Tztrat~o~ m a nrhrc actd medrum. Iron” can be titrated m 1 ON mtrrc acid usmg ferrobr as mdtcator, provided the tronr* solution IS not kept for more than 5-10 mm after the addition of mtrrc acid If the mixture IS allowed to stand longer, the tronn IS oxtdtsed slowly by the mtrrc acrd, leadmg to low tttres and sluggish end-pomts. Thrs ddIiculty 1s enhanced by mcreasmg the concentratron of mtrtc acid. A closer study of the matter revealed that dttIiculty concernmg the mdrcator action IS traceable to the nrtrous acid formed as a result of the reactron between nor?* and mtrtc actd Nttrous acid ts capable of slowly reducmg ferrun to ferrom, this reductron occurrmg only when all of the rronrr 1s oxrdlsed It IS, therefore, desnable to add mtrtc actd last to an tronn solution, Ee after the addrtton of water and the mdrcator solution In no case should rntrrc acrd of concentratton greater than 2 ON be added to the nonI* solutton Using these precauttons it IS possible to carry out accurately the tttratlon of even a 0 01 N solutton of ironI wtth 0 01N ammomum hexamtratocerate(IV) The reverse trtratron of ammornum hexarntratocerate(IV) m mtrtc acrd wrth nonrl sulphate solution ean also be carrred out w&out ditIicuIty usmg ferrom as mdrcator Evidently fernin 1s



as an oxidismg agent

qmte stable m contact with an excess of ammomum hexamtratocerate(IV) acid without losmg its reversibihty as an indicator


solution m l*ONmtrtc acid

Determrnatron of oxalate Kirk and Tompkm9 carried out the microtitration of oxalate with ammomum hexamtratocerate(IV) both m nitric actd and m perchloric acid media These authors stated that the results obtained m a nitric acid medium “are somewhat misleading m that the reaction is too slow for convenience and the end-point was difficult to read with ortho-phenanthrolme ferrous sulphate particularly m presence of asbestos” Consequently, the authors did not recommend this procedure but preferred the back-titration usmg cermmrv sulphate We have found that the microtitration of oxalate is fairly satisfactory if the mitral concentration of nitric acid m the titration mixture IS0 1-O 2N and If the titrant IS added slowly If the concentration of nitric acid m the titration rmxture exceeds 0 2N the end-point becomes difficult to recognne, the dtthculty mcreasmg with mcreasmg concentration of the acid Our experiments show that this diIIiculty m detectmg the end-pomt arises from two factors * (1) the reduction of the light blue ferriin by oxalate to the red ferrom is slow; (2) as the titration progresses there IS a gradual decrease m the mtensity of the ferrom red colour, with the result that the red colour reaches only a small fraction of its origmal mtensity near the end-point even after waiting for some time &ously enough this difficultv is enhanced when the sneed of addition of the cermmrv salt is increased For these reasons the transition from the very light red colour to the very light blue colour is ditlicult to notice m macrotitrations. Sigmficant observations made durmg attempted macrotitrations of oxalate with ammomum hexamtratocerate(IV) m a nitric acid medium are summarised as follows (1) Durmg the early stages of the titration, on the addition of each portion of the cerate solution the ferroin red colour changes to the yellow colour of the cermmlV which gradually changes to red, through orange, m about 68 set, the mtensity of the restored red colour is somewhat less than that at the start of the titration (2) The intensity of the restored red colour, as stated above, becomes less and less as the equivalence point IS approached (3) The loss m mtensity of the ferrom red colour mcreases as the mtric acid concentration of the titration rmxture is increased, so that m titrations carried out m media of mtric acid concentrations greater than 0 2N the colour transition at the equivalence point is difficult to discern (4) Macrotitrations of oxalate are not possible, even in 0 1-O 2N ndric acid media, because of the difficulties pointed out above We have made the interesting observation that the addition of a little potassium iodide or iodate (1 ml of lo-*M solution per 50 ml of titration mixture) eliminates these difficulties; both the micro and macrotitrations can then be accurately carried out in mtric acid media rangmg from 0 1 to 0 8N When potassmm iodide is used m the mentioned amount, an extra consumption of cerate (0 1 ml of 0 OlN solution) is required and a correspondmgly lesser amount of cerate when potassium iodate is used It has also been noticed that the correction is negligible when a mixture of potassmm iodide and iodate to give a concentration of iodme equivalent to that of potassium iodide used above is employed When titrations are made using any of these catalysts, the ferrom red colour remams almost the same m mtensity throughout the course of the titration Hence the transitron from red to light blue can be sharply perceived at the equivalence point, both m micro and macro titrations when the nitric acid concentration lies between 0 1 and 0 8N However, when the acid concentration is raised to 1 ON or above, the fading of the ferrom colour again makes itself felt, and the titrations become difficult even m the presence of the above mentioned catalysts In our opmton the action of the catalyst is more concerned with the functionmg of the indicator rather than with the reaction of cerate with oxalate Further work IS m progress to elucidate the mechamsm of the action of the catalyst Recommendedprocedure for the tltrahon of oxalate m a mtrtc acid medrum Take 5-10 ml of the oxalate solution m a lOO-ml Erlenmeyer flask and treat with 40 ml of 0 5-O 8N nitric acid, 0 5 ml of 0 OOlMferrom and 1 ml of a mixture of potassium iodide and potassmm iodate (equivalent to 10-SN iodme) Titrate with a solution of ammonmm hexamtratocerate(IV) till the ferrom red colour lust changes to very pale blue If the blue colour reverts to a pale red tinge m l&15 set, a fraction of a drop of the cerate solution may be added to obtain a permanent pale blue colour A large number of micro and macro titrations have been made according to this procedure Some typical results are given m Table I Tztratron of oxalate in a hydrochlorzc acldmedmm. Oxalate can be conveniently titrated in 0 2-O 5N hydrochloric acid at room temperature with ammomum hexamtratocerate(IV) using ferrom as indicator. The fading of the ferrom colour noticed durmg titrations m a nitric acid medium IS not so much m evidence here Accurate results have been obtained both m titrations with 0 05N and 0 OlN









and K S








Oxalate taken, mmoles

Oxalate found, mmoles

Oxalate taken, mmoles

Oxalate found, mmoles

2 3000 1 2000 0 5600 0 4500 0 3400 0 2300

2 2900 12010 0 5610 0 4508 0 3401 0 2295

0 1600 0 1200 0 0860 0 0590 0 0350 0 0240

0 1601 0 1202 0 0859 0 0588 0 0351 0 0241

cerate solutions When the concentration of hydrochloric acid 1sraised to 1 ONor above, an mcreasmg fadmg of the ferrom colour 1s notlced, and the tltratlon becomes almost nnposslble m a 2.ON hydrochloric acid medmm The use of iodide-lodate mixture, however, enables the titration to be made with the usual facdlty, even m 2 ON hydrochloric acid The titration5 of oxahc acid with cermmIv sulphate at room temperature m the presence of barium ion as a scavenger for sulphate ion is subject to the hmitatlon that the colour of the ferrom mdlcator 1s somewhat masked by the preclpltated barnun sulphate The present method, involving the use of ammomum hexamtratocerate(IV) m a nitric acid or hydrochloric acid medium, does not suffer from this disadvantage. Determnatlon of mandelrc acrd Titration of mandehc acid 1s not possible m either nitric acid or hydrochloric acid media, even of very low concentration, wlthout the use of the catalyst This ISbecause the ferrom red colour does not return about 0 2-O 3 ml before the eqmvalence pomt, even after waltmg for lo-15 mm If the catalyst 1s added at the stage when the ferrom colour fades very much (about 0 4-O 5 ml before the equivalence pomt), however, the red colour 1s regained m good mtenslty and the tltratlon can be carried out wlthout any further difficulty In view of these findings, the followmg procedure 1s recommended for the tltrunetrlc determmatlon of mandehc acid Take S-10 ml of mandehc acid solution (0 01-O OSN) m an Erlenmeyer flask, then add 4&45 ml of 0 2N nitric acid (or hydrochloric acid) and 0 5 ml of 0 OOlM ferrom Titrate with a solution of Near the equivalence pomt when the ammomum hexamtratocerate(IV) of appropriate strength ferrom red colour has faded very much, add 0 5 ml of potassmm lodlde-lodate mixture (eqmvalent to 10-3N iodine) and contmue the titration dropwlse until a permanent light blue colour 1s obtamed A large number of tltratlons have been carried out as described above Some typical results are gnen m the Table II TABLE










Mandehc acid, mmoles Taken


0 6800 0 6000 0 5500 0 5300 0 4300 0 2300 0 1300 0 0965 0 0595

0 0 0 0 0 0 0 0 0

6805 5980 5507 5285 4310 2290 1290 0966 0595

The direct tltrlmetrlc procedure now developed for the determmatlon of mandehc acid 1s an improvement over the procedure of Verma and Paul,* which consists of addmg an excess of cermmlv sulphate to an ahquot of the mandehc acid solution, allowmg to react for 5 mm, then titratmg the excess cermmIV with standard KonII sulphate solution



as an oxtdtsing agent

Zusammenfassung-Obwohl das Salz m hohem Remhettsgrad erhaltlich 1st wurde die Anwendung von Ammonmm hexamtrato cerate m der Oxydtmetrte nur wemg &tdtert. Bedmgungen warden ausgearbeltet urn Oxalsaure und Mandelsaure mtt dem Reagens bet Znnmertemperatur m salpeter- oder salsaurem Medmm zu tuneren. Ferrom wlrd als Indtcator verwendet. Dte neu entwickelte Methode gtbt genaue Resultate Die Methode hat Vortetle gegenuber der Verwendung von Cersulfat. R&n&--L’utihsatton d’hexamtratocerate d’ammomum en oxydimetrte a et& t&s peu Ctudtee, been que le se1 ptusse dtre obtenu avec une t&s grande pure6 Les condtttons de tttrage par ce r6actd des actdes oxahque et mandehque ont ett6 etabhes 51la temperature ambiante en nuheu actde nitrtque ou chlorhydrtque en utlhsant la ferroine comme mdtcateur inteme. Les m&odes ainst muses au pomt docent des r&.tltats pr&cis. Elles constttuent de nettes amehorattons sur les methodes mettant en leu le sulfate de c&mm(N) comme &a&f REFERENCES 1G e G. aP 4M LV

F. Smtth, V. R. Sullivan and G. Frank, Ind Eng. Chem, Analyt , 1936,8,449. F. Srmth and W. H Fly, Anulyt. Chem , 1949,21,1233. L. Irk and P. C. Tompkins, Ind. Eng. Chem , Analyt., 1941,13,277. R. Verma and S. D. Paul, J. Sci. Zrzd.Res., Zndza, 1953,12B, 178; 1954,13B, 346. P. Rao and G G Rao, Talanta, 1959,2, 370