\‘()I,. I, l’~i(:l*X ii-l(i (l!J57)

Voltammetry and Amperometric at the Rotated Platinum of Manganese( II), (III), and (VII) in Pyrophosphate Medium

INTRODUCTION Se\-era1 years ago t!ie polarographie determiiiat,ioti at, the droppiiig mercury electrode of manganese(II j a.fter oxidation wit,h lead dioxide to t,he manganese(II1) ~~m~plcs in pyrophosphatc medium was in- vestigated’ and proccdurcs were gi\-w1 to make iriterfering sulkances harmlcss.2 .Also, the osidatiort potjential of the eomples mangancse- (lI)-(III) syst,cm n-as nieasurcd at a platinum elcc~trode.” Lirigane and liarplus found t,hat in the proper pll range manganrsc(I1) in t#hc presence of an excess of pyrophosphate could be osidizcd quantits- timely t’o the t’rivalent stat’c by permanganate, and they described a procedure for pot~entiornetrie d&&ion of t,hc end point. Goff art et al.5 also titrat,ed manganese(I1) with permanganak but used the amperometric method with t’\vo inclicxt,or elect’rodee --the so-called (lead stop end-point methocl-&for tdlc det~cctioii of the end point. Their proccdurc is very simple aiid can be iwed for t,ht: tlrtcrminat ion of small am0uiit.s of mauganese. 4 1. 11. I

metry of different valence forms of cerium, chromium, and vanadium in pyrophosphate medium is described and dire&ions are given for t#heelimination of the interference of these constituents in t,he various amperometric titration methods of manganese.

EXPERIMENTAL

Materials I17ater, conductivity water redistilled over potassium permanganat8r. Standard 0.01 AI permanganate prepared and standardized against sodium oxalate by the usual procedure. St,andard 0.001, 0.0005, and 0.0001 N solutions were prepared by appropriate dilution of the 0.01 Al permanganate wit,h redistilled wat(er. These dilut,e solut,ions mere prepared immediat,ely before use. A st,andard approximatlely 0.1 AI manganese(IT) solution was pre- pared in 0.01 N sulfuric acid using a Merck C.P. product of MnS04. AR20 and diluting to one liter wit,h redistilled water. The molarit,y was checked gravimet8rically by quant,itative precipitat,ion as manga- nous ammonium phosphateand weighing as pyrophosphate. Solutions of much smaller concentrations were prepared by appropriate dilution. Sodium p~rophosphate. A hot saturated soWon of a C.P. product of Xa4P204.10Hz0 was shaken for a few minutes wit#h lead dioxide and filtered. The crystals obt,ained from the filtra,te were recryst,al- lized from mater and dried at the air. We found that several C.P. products of pyrophosphate available in this laboratory contained a trace of an impurit,y which interfered with the reduction or oxidation at t,he rotated plat8inum electrode of various systems used in the pres- ent work. Although well-shaped current-voltage ~wrv~~sof Mn(III) pgrophosphate in buffers of pure pyrosphosphatc were observed at the rot,ated electrode with well-defined diffusion currents, WC,fowd rho waucs at all when using unpurijiird C.P. products 0-f pyrophosphak which apparently contained a trace of a reducing substance. The intcr- fering substance could also he removed by shaking a 0.25 AI solution of an unpurified product with lead dioxide for 15 minutes followed b? filbration t,hrough dense filter paper. T,ess concent,rated pyrophos- phate solutions were prepared by appropriate dilution wit*h redist’illed water. The pH of t,he pyrophosphate buffers was adjusted t,o the desired value by addit,ion of dilrlte sulfuric acid using a pH met,cr. Standard 0.01 A/ ferrous iron solut,iolk was prepared by dissolving the appropriate amount of Fe(~H,),(S04)5.GH90 in 0.1 S sulfuric acid. The t,iter was det,ermined hefore use wit,h a standard perman- ganate solution.

IGg. 1. C‘ttrrc,ttt.-volt:tgo curves ut 25.OO”C. OC()?-l’wt: 2.Oci X IO -5 :I/ IiRIrtO, soltttioria: (1) in 0.01 ill HiSO,; (2) in 0.1 .I1 C~lLCOOIi. (1 ‘j :~.rttl(2’) :we rc- sidttal cttrrcttts.

Apparatus A plat.iuum wire electrode (sensitjivity . 130 pa. .‘mmolc silver ,‘liter) was used in all experiments. It was rotat,ed at. 900 r.p.m. Before each experiment it was cleaued wit.h nitric acid and dist’illed water and kept iu the solution t,o hc analyzed for about ten minutes before ruw iiiug a polarogram. A 150 ml. I’ywx hcaker scwrcd as el&rolysis cell, which was closed with a ruhhcr stopper with appropriat,e holes fog t,he platinum electrode, salt bridge, aucl in- and outlet t.uhes for pure nitrogen gas. The cell was placed in a cow&ant, temperature bath at 25.00 f 0.05”C. ,111 voltammetric experiments were performed at. this tempcrat,ure. The amperometric were carried out at room temperatSurc. A saklrated calomc~l rlect,rodc (H.C.E.) served as . C:urrerlt,-roltagr curves were measured with a Sargent recording polarograph, rnodcl XXI. Whcuevrr desired the wrves wre slso mca.sured manually. RESULTS (1) of Permanganate in Pyrophosphate Buffers Some typical current-voltage curves of permanganate in 0.01 M sulfuric acid and 0.1 Ill acetic are reproduced in Figure 1. The diffusion current is well defined and proportional to concentration: the value of id/c was found to be 590 ~a./mmole/liter or 118 pa./meq./ liter. Current,-volt’age curves were also determined in pyrophosphate buffers of pH between 1.0 and 8.0 the over-all concentration of pyro- phosphate being 0.1 M. The measurements were done hot(h manu- ally and with t,he recording polarograph and t#he same results wcrc oht,ained. Air was removed by passing pure nitrogen t,hrough t,he cell for t’en mimkes before and cont’inuously over the solution during the recording of polarogram. Typical current-voltage cur~w are plot#ted in Figure 2.

t A0 102031

The reduction of permanganat,e in pyrophosphat’e buffers of pH less than 4 occurs in two steps. The first wave corresponds to a four- clect8ron reduction, the second to a one-electron reduction. In pyrophosphate buffers with pH greater than 4 only the first wave was observed. This wave corresponds to the reduction of perman- 1\IICROCIIEJIIC:.4L JOURNAL, VOL. I, ISSUE 1 ganate to Mn(II1) pyrophosphate. The second wave at pH less than I corresponds t,o t,he reaction: Mn(III) pyrophosphate + c ___f AIni II) ~‘~1~o~~lLo”~‘lL:~t~~ The reduction wave to Mn(III) shifts about, 0.09 volt, to more negative potentials with a unit increase in pII. The curren+voltage curves were also measured at’ varying pH in buffers \vith an over-all pyrophosphat,e concentration of 0.01, 0.03, 0. I, alrd 0.25 A/, rrspec- tively. The curves wcrc nearly identical wit,h those present,ed itI Figure 2. 111all solut’ions the diffusion currrllt, referred to the same number of electrons, was constjant alld the same as ill dilute sulfuric or acet)ic acid (Fig. 1), indicating that the diffusioli coefickt~ of pcr- manganate is Ilot affected by the pyrophosphatc buffers.

Fig. 3. Current-voltltgc curws of air-froo 11 n( III) l)!.r”pllosph;ltc. in 0.25 d/ pyrophosphate buffers of pH 2.0: (1) rcdual mment; (2) 6.65 X 10P1ll ;\ln(III) pyrophosphatc; (3) 1.33 X 1OP .\I 1In(III) l)!.rol,hospll:rt(~: (41 i3.325 X lO+ ,23 hln(III) pgrophosphate.

(2) Voltammetry of Mn(II1) Pyrophosphate A 1.33 x IO-” A1 stock solution of RIn(III) pyrophosphatc was prepared using the procedure given by Kolthoff and Watters.* Typi- cal polarograms at pH 2.0 and different concentrations of Mn(II1) are plotted in Figure 3. Well-defined diffusion current,s proportional to 8 I. M. KOLTHCWF AND E. JACOBSEN

the concentration were observed, the value of id/c being 58.3 f 0.3 pa./mmole Mn(III)/l. This value compares with 118 for an equiv- alent solution of permanganate. The difference is attributed to a small diffusion coefficient of the Mn(II1) complex as compared to that of permanganate. The value of id/c was not affected by the over-all pyrophosphate concentrat’ion in the range between 0.25 and 0.01 Ab. Polarograms were run in the pH range between 0.0 and 8.0. From Figure 4 it is evident that no reduction occurs at pH great’er than 4.0.

Fig. 4. Influence of pH on the current-voltage curves of air-free solutions of 3.325 X 1O-4 $1 Mn( III) pyrophosphate in 0.25 M sodium pyrophosphatc. pH is indicated on curves. Broken lines are residual currents.

It is of interest to note t,hat at a pH greater than 4 permanganate gives one reduct’ion wave only to the Mn(II1) sbate, the second wave oc- curring only at a pH less than 4. The diffusion current of the Mn(III) complex is well defined at a pH less than 3. (3) Amperometric Titrations of Manganese

(a) Oxidation of Mn(ZZ) to the Mn(ZZZ) Pyrophosphate Complex with Lead Dioxide and Titration of Mn(ZZZ) with Ferrous Zron in the Absence 0.f Oxygen This amperometric tit,ration was carried out by Kolthoff and Wattersl at the dropping mercury electrode. We found that it can

MICROCHEhfICAL JOURNAL, VOL. I, ISSUE _ MANG4XESL(II), (III), ANI) (VII) IS 1’YHOl’HOSI’HATIC !I be carried out at t’he rotated platinum electrode at, much greater dilu- tions than at the dropping electrode. A practical disadvant’age of the titration is that oxygen must, be removed from the syst,em, while this is not necessary in the Mration wit,h pwmanganatc (r1.i.). The amperometric titration with ferrous iron can he carried out, at, a fairly negative potential when the diffusion current, of the Mn( II I j complex is measured or at a fairly positive potential, where tbc anodic diffusion current, of ferrous iron is ohserved. In t,he former case the current (correct#ed for dilutjion) decreases linearly to the end point with the volume of reagent, added, while in the latter case the currtnt remains equal to the residual currcnt until after the cwtl point, whcrc it in- creases linearly with excess of ferrous iron. Titrafion at -0.2 loll (P.S. S.C’.f:‘.). Solutions of manganesc(I I) in 0.25 N pyrephosphate at pH 2.0 wert oxidized with lead dioxide according t(o the procedure of Iiolthoff and Ratters. 1 .A sample of

50 ml. was placed in the titrat,ioii beaker and air was removed by passing ilitjrogen through t,he sohnioii for ten minutes before and con- t,iiiuously duriug the t,itrat,ion. 11 standard solution of 0.01 JI ferrous iron was added from a micro huret with 0.01 ml. reading. The diffu- sion currents of the RIn(I I I ) complex was measured at, an applied po- t~ernial of -0.2 volt M. S.Cr.E. with the Sargent, recording polarograph. Blanks coutaining the same amount of pyrophosphat)e pretrcatcd with lcad dioxide were also run. SOJW results are given in Tat&~ I.

TABI,l< I Tilr;ttion of Mang:tncw :lftcr Osidxtion to hIn( III) :~i, -0.2 Volt with 1.11 + IOF [II Ferrous Iron ~111t>alwn, JIol:bty 01 I3lank, AIll IhOT, mg. roln. ml. follntl, mg. ’ ; 0 293 1 Ofi x IO ’ 0.010 0 2!K< 0.0 0. 731 2.66 x 10 ’ 0.008 0. 740 -0 1 I.181 5.32 x lo- 0.0 12 I l(il +0.2 3 G53 1 33 x IO 3 0.010 3.612 -0.3

The following metals in t,en- to hundred-fold excess over mangancsc did not interfere: iron, copper, cadmium, zinc, cwhalt,, and Cckel. The interference by chrmnium, cerium, and vanadimn was climinat,ed following the procedure by Wat,ters and Kolthotf,’ who remove chro- mate, vanadate, and ceric cerium in tht: presence of much ferric iron with pyridine (v.i.). Pyrophosphatc is added to thr filtrate, the pH 10 I. M. KOLTHOE’E AND 1:. JBCOBW_U adjusted to 2, and the manganese oxidized with lead dioxide. Even with only 0.3 mg. manganese Dhe error was less than 0.5%. Titration at +I.10 volt (21s.S.C.E7.). Current-voltage curves of ferrous iron in 0.1 M pyrophosphate buffers in the pH range between 0 and 8 were run. Waves of normal appearance with well-defined diffusion currents were observed at pH 0 and 1, but in the pH range between 1.6 and 8.0 the residual currents were the same as in the pres- enre of 2 X lo-* M ferrous iron. Some current,-voltage curves at pH 1.0 in 0.1 31 pyrophosphate at different iron concentrat,ions are shown in Figure 5. For comparison a curve is added of aquo ferrous iron in 0.05 111sulfuric acid which is pract’ically identical with that of

Fig. 5. Current-voltage cwves of Fe(I1) in 0.05 M H&O, (CWVC!1) and in 0.1 .I/ pyrophosphate buffer of pH 1.0 (curves 1, 2, and 3): (1) 5.55 X 1O-5 bf Fe(H); (2) 1.11 X 10e4 .V Fe(I1); (3) :tnd (4) 2.22 X 1O--4M Fe(II). Broken cwvcs rcprescnt residual currents. the iron ill the pyrophosphate buffer. :Ipparent,ly the ferrous iron is not, complexed at pH 1.0 in 0.1 M pyrophosphate. The diffusion current was found proportional to concentration, &/c being equal to 79.0 f I pa./mmole Fe(TI)/l. This value is considerably larger than t,hat, of the manganese(Il1) complex, also indicating that t,he ferrous iron is not complesed in t)he pyrophosphate buffers of pH 1.O. Titrat,ions at 1.10 volt (vs. S.C.E.) of 50 ml. samples of manganese after oxidation with lead dioxide in 0.1 JI pyrophosphate buffer at

IWCKOCHEJIICAL JO6KNAL, VOL. I, ISSUE 1 MANC.4NlSWQI), (III), AND (VII) IS I’YI~OI’~IOSI’H~~TI’ 11

pH 2.0 and adjust’ment of the pH of t’he filtrates to O.Tj gave results which were accurate and precise within 0.25% with amount,s of man- ganese varying lx&-ecu 0.3 and 3.5 mg.

According to IArgane and liarpI& pcrmanganat,e rcack with manganesc(I1) in pyrophosphnte l)iiffrrs of the proper pTT according to the equat’ion :

-IAIII++ + IIrrO~ + 8H’ + 15H,l’,O, = 51\IIl( H”l’?o,):~ + 1H,O

From Figure 4 it is seen t)hat the manganese(I11) complex does not yield any reduction wave at pH greater than 4, while t’he diffusion current of permanganate is found bet’ween f0.2 and -0.G volt (Fig. 2). Consequent,ly it, should he possible t,o titrate manganesr(I1) directly with permanganat’e wit,hout applying any voltage when a saturated calomel electrode is used as reference electrode. Princi- pally t’he method is t,he samr as t’hat’ of Goffart,,5 who used an internal silver electrode as reference electrode. The pH of solutions of mangancse(I1) in 0.25 A/ pyrophosphate was adjusted t)o 1.5 to 8.0 and the solutiotis t#itrat#ed at, a11applied potcn- t#ial of 0 volt, with very dilute stlandard permanganatc. The molar concent~ration of pyrophosphate can 1~ varied within wide limits. ‘Iit,rations of 1.5 mg. of manganese wre carried out in 50 ml. 0.25 to 0.01 31 pyrophosphatc buffers of pH 6. The result’s were accurate wit,hin 0.2rj0. l’he main reason for using 0.25 dl pyro- phosphate is that, the t,itrat.ion of manganese can 1~ carried out everi in t,hc presence of ferric iron in amounts 150 times greakr t,han t,he amount, of mangsnesc. The ahsolut,e accuracy of the tit,rat,ion was checked in t,he same n-a? as by T,ingane.4 h given volume of standard pcrmanganate was re- duced to manganese(I1) and titrated with the same st,andard solut.ion. In three det,erminat,ions using 0.28 mg. of mariga~~csc an accuracy of 0.2% was found. The resulhs obtained wit*h amounts of manganese varying hetwccn 2.9 and 0.007 mg. in 50 ml. solmion are report’ed in Table IT. It is seen that as lit,tle as 0.01 mg. mangaiirse can hc tit8ratetl with :ur ac- curacv of 0.:“~~’/O. 12 I. hf. KOLTHOFF ANI) Ii. JACOE31S?J

In titrations of very dilute solutions it is very essential to run blanks and to use freshly dilut,ed permanganat’e solutions.

TABLE II Titration of Manganese in 0.25 M Pyrophosphntc Ruffcrs of pH 1.5--8.0 with Pcrm:tngsnitt~e at 0.0 Volt, vs. S.C. I(;. ,2In Slol:trit~~ Rlolnrit) Blnnk, MI1 Ihor, taken, mg. of solr1. of KMnO, ml. founti, mg. $;, 2.922 1.06 x 10-3 I .0x x 10 2 0 00:s 2.916 -0.2 1.461 5.32 x 10-a ic ci 1 ,460 0.0 0 292 1.06 X 1O-4 I .0:3 x IO-” 0 .080 0 .29:3 +o :I 0.146 5.32 x lo-” I‘ “ 0.14i +0.6 0 on0 2.66 X lo-” 5.15 x 10-4 0 ,062 0, oi29 0 0 0.0365 I.:<:3 x 10-5 I .o:< x 10 -.I 0 .:310 0.0367 +o 5 0. 01 Ifi ,5.:32 X 1W6 ‘i ‘, 0.0147 to.6 0 0073 2.66 X IOP 5..51 x 10 3 0.620 0. ooi3 0 .o

Interferences. In titrations of 0.15 mg. manganese it was found in agreement mit)h Lingane” and Goffart5 that the results are not affected by ferric iron, chloride, nitrate, and ammonium in amount’s 150 times great.er t,han t,he amount of manganese, and by cadmium, copper, magnesium, cobalt, nickel, zinc, aluminum, and chromium(II1) in amount,s 50 times great,er t’han the amount of manganese. ‘I%(~ effect, of larger amounts was not t’ested. Cerium. GoffartG uses the same procedure for the tit’ration of ce- rous to cleric cerium as for manganese in the pH range from 6-8. We confirmed the results of Goffart by working with 1O-3 ill cerous cerium in 0.25 M pyrophosphate buffers of pH 5-8. The results were accurate wit,hin 0.25%. Current,--voltage curves of ceric and cerous cerium in 0.25 AI pyrophosphat,e of pH 2 to 8 were run. The same curves were obtained a,s in absence of cerium, indicating that neither the cerium(IV) nor the c*erium(IIT) is elect,roactive at the rotatIed platinum electrode in this medium. From this behavior it cannot be inferred that the cerium- (TV) pyrophosphat,e complex is a weaker oxidizing agent than the manganic c*omplex. As a matter of fact, when a solution which was 0.001 dl in manganese(I1) and 0.005 M in cerium(IV) in 0.25 dl pyrophospha,te of pH C was allowed to stand, the color of the man-

MICKOCHEhII(‘AI, .JOUHNAI,, VOI,. I, IRSI’E I XIANGANESE(II), (III), ANI) (YII) I?; I’YROI’HOSPHATI~ 13 ganic complex developed slowly. The eqnilibrium :

Mn(I1) pyrophosphate + Cc(JV) pyrophosphat~~ d Mn(TII) pyrophosphatr + Ce( III) pgrophosphxte is est,ablished slowly. Sinw t,he cerous cerinm formed in t,he above reaction is reoxidized by pcrmanganate, it is evident, that) ceric curium should not interfere in the titrat,ion of mangallexe(II). This is con- firmed by t’he result’s in Table III which show that nlallganese (*all 1~ Mrated in t’he presence of at least, 50 times t,hc amount of ceric vxiunl. Also, the curren&voltage curves of manganese(lII) pyrophosphate arc not affected by c&r cerium.

Tit,r:ttion of 11ixturr of 1I~tng:tnese(II) :md Crric Crrium in 0.25 ‘11 I’yrophosphntr Buffrr of pH G with 1.03 X 10 -s .I/ I+rm:~ngnn:ttc at 0 Volt, vs. 8.C.K. Time of MIl(11) Ce(lV) st,anding Color of Mn(11) ICrror, takrn, mg. present, mg. of mix. soln. fOlllld, mg. ‘,‘b

0.7305 7 10 min. Colorless 0.729 -0 2 0.7305 T 1 hr. J’ale red 0.731 0 0.7305 T 2 hr. Pale red o.i30 0 0.7305 5 3 hr. l’de red 0.732 +(I.2 0.292 7 I hr. Pale rrd 0.292 0 o.z!n 14 1 hr. I’alr red 0.201 -0.3 Vanadil~m. We confirmed the st,atement, by Lingane and Karplns” that vanadyl is quantit,at’ively oxidized 1)~ permangannt.e tjo vanadat,e in pyrophosphat#e buffers of pH 5-8. Current-voltage curves of vanadium(V) in 0.25 31 pyrophosphat,e buffer of pH 4-8 were run. The curves were the same as in the va- nadium-free buffer, indicating that \-anadium(V) is not clectroactire nnder the experimental condit,ions. Amperometric titrat,ions of varying amounts of manganese(II) iI1 the presence of large amounts of vanadium(\‘) were carried ant in t#hepH range 5-8. In agreement with Goffart,j we found that amount,s of vanadate more t’han 60 times as large as the amomlt of manganese did not, int,erfere. The behavior of vanadium(i’) is very similar to that of cerium(IV). Some of the manganese(I1) may be oxidized to manganese(II1) by vanadate, but the vanadinm(IV) formed is reoxidized by permanga- nate dnring the titrabion. r 14 I. PI. ROT,TtIOFF AN) E. JhCX)RSI’N

Chromium. As has been stated already by I,ingane4 and Goffart,s chromium(I11) was not found to interfere. Current-volt,age curves of chromium(V1) in 0.25 AI pyrophosphate of pH 5-8 were ident,ical with the residual current, indicating that the chromate is not reduced under these condit’iollx. However, like ce- rium(IV) and vanadate, chromium(1’1) was found able t,o oxidize manganese(I1) in pyrophosphate buffers of pH 5-8. When a mixture containing 3 mg. of manganese(l1) and 30 mg. of cAhromium(VI) in 50 ml. 0.25 111pyrophosphaCe of pH 5-8 was allowed t,o stand for one hour, no permanganate was used in the titration. Evidently under the above cnondit,ions chromate oxidizes manganese- (IT) quantitatively ho t,he manganese(II1) csomplex. The inter- ferencae by cshromium(V1) is eliminat’ed by reducing it, Do chromium- (III). For the sake of interest it may be added that chromium(V1) also oxidizes cerous cerium quantitatively in pyrophosphat,e buffers of pH 5 to 8. Analytical applications of this behavior are evident. Iron. Large amounts of ferric iron do not int)erfere. Ferrous iron is made harmless by oxidation to ferric iron. If nitric acid is used for t)he oxidation, the oxides of nitrogen can be removed with urea as recommended by Lingane and liarplus It appears that ferrous iron, vanadium(IT’), and cerium(II1) are t’itrated t,ogct’her with manganese(I1). Iron and vanadium can easily be made harmless by oxidation, but for the determination of man- gancse(IJ) in the presence of cerium(II1) a separation is necessary. Solutjions containing 6 mg. ferric iron, 2 mg. each of chromium(VI), vanadium(V), and caerium(IV), and 0.36, 0.072, and 0.036 mg. man- ganese(II), respectively, were treated wi-ith pyridine according to the procedure given by Kolthoff and Watters.? The filtrate was added to a pyrophosphat,e buffer of pH (i and the mixture titrated with per- manganate. The results obt,ained were accurate within 0.2y0,.

(c) A mperomctric Titration of Permanganate in Pyrophosphate l!l~dium~ with Standard Manganese(II) The reduc*tion wave of permanganate was found unaffect,ed by cleric caerium and vanadate in pyrophosphat,e buffers in the pH range 5-8. In the presence of chromate a film is formed on the electrode which decreases the sensitivity of the electrode. The diffusion current, of permanganate, however, was still proportional t’o its concentration. The film is easily removed by treating the electrode with 4 N potas- RlICROCHEhfIC.41, JOURNAL, VOL. I, ISSUE 1 M4NGANESIi(II), (III), ANI) (VII) I?J PYROI’HOSl’lI~~TI~: 15

sium hydroxide followed by dilut#e sulfuric acid and distilled wat,er. In the previous section it was stated t’hat manganese(I1) is slowly oxidized by chromat,e, ceric cerium, and vanadat’e. In the titration of permanganate with manganese(I1) in t,he presence of these ions no int,erference occurs because the reaction between permanganate and manganese(I1) is wry rapid in the pH rangcl 5% and occurs before manganese(T1) can react, with t’he other ions. Rio& convenient,ly the titration is carried out by short-circuit~illg tjhr plat~inum electrode with t#he wlomel clertrodc. The pyrophos- phat,e solution was pretreated with lead dioxide and it yielded a zero blank. From the results in Table TV it appears t,hat t#he t,itration gives ex- cellent results even in the presence of relatively large amounts of vana- date, wric~ crrium, and cahromatc.

TABLE 11: Titration of Pwmangnnnte in 50 Ml. 0.25 ill Pyrophosphat,e B~lfft=r of pH 5 -8 with 1.33 X IO-* M hlangancse( II) at, 0 Volt us. S.E.C. M1104- talu.w, Other ions I\lnOd- found, I’rror, mmolo X 1W :tddtd, mmole mmole X 10” (‘C

This t)itrat,ion can also be carried out potentiometrically but, the amperomekic method mnkcs it, possi blc to det,crminr mu(41 smaller quantities of manganese.

References

iI) I. RI. Kolt~hoff and J. I. bVatters, Id Eng. f”he,n., ilnal. Ed., 15, 8 (1943). (2) J. I. Watters and I. &I. Kolthoff, ibid., 16, 187 (1944). (3) J. I. \VVC’nttersand I. M. Kolthoff, .I. Am. Chem. SW., 70,2455 (1948). (4) J. .J. Lingane and R. Karplus, Id. Eng. Chem., Anal. Ed., 18, 191 (1946). (5) G. Goffart,, G. Michel, and Th. Pit,nner, Anal. Chim. Acta. 1, 393 (1947). (6) G. Goffnrt, ibid.. 2, 140 11948). 16 I. nf. KomHoFF ANI) E. JACOBSEN

Synopsis At a pH smaller than 3, permanganate in pyrophosphate buffers gives two waves at the rotated platinum wire electrode, the first wave corresponding to a four- electron reduction to Mn(II1) and the second wave to a one-electron reduction. At a pH between 4 and 8 only the four-electron reduction wave is observed. Man- ganese(II1) in pyrophosphate buffers of pH 3 or smaller than 3 yields a well- defined one-electron reduction wave, no reduction being observed at a pH of 4 or greater. Manganese(I1) can be determined by oxidation with lead dioxide in a pyrophosphate buffer and by titration of the manganese(II1) in the filtrate with ferrous iron at a pH of 0.5 in the absence of air at a potential of -0.2 volt (US. S.C.E.) where Mn(II1) yields a diffusion current or at +l.lO volt, when the dif- fusion current of the excess of iron is measured. More convenient is the direct titration of manganese(I1) with permanganate in a pyrophosphate buffer with pH between 4 and 8 in the presence of air by short-circuiting the indicator electrode with the S.C.E. As little as 0.01 mg. of manganese in a volume of 50 ml. was tit,rated with an accuracy of 0.5%. Cerous cerium and vanadium(IV) can be titrated in the same way and are separated from manganese using the pyridinc procedure. Permanganat,e can be titrated in pyrophosphate butlers of pH 5-8 with manganese(II). Even large amounts of vanadate, ceric cerium, and chro- mate did not interfere in the titration of 0.06 mg. manganese (VII).

MICROCHEhlICAL JOURNAL. VOL. I. ISSUE 1