Amperometry with two polarisable electrodes—III1

T8lattta.1963.Voi. 12.pp. 377to 384. Pcqmftm bar

Ltd.

AMPEROMETRY WITH TWO POLARISABLE ELECTRODES-III* CHELOMETRIC INDICATION

DETERMINATION OF IRON(II1) USING AN SYSTEM OF TWO CARBON ELECTRODES J. VORL~&K

Research Institute of Iron Mines, MnRek, Czechoslovakia

and F. VYDRA Analytical Laboratory, Polarographic Institute of J. Heyrovskf, Czechoslovak Academy of Sciences, Prague 1, Jilska 16, Czechoslovakia (Received 21 September 1964. Accepted 15 December 1964) Summary-Two carbon (graphite) electrodes have been used for indicating the end-point in the biamperometric titration of iron(II1). Satisfactory results are obtained over a wide range of applied potentials. The absolute current values are related linearly to the values of the applied potential. At a potential of 06-2.0 V the current differences

during the titration are from several ,eA to ten times these values. The influence of pH, applied potential and temperatureon the titration have been studied; the eVect of interfering elements has been investigated. Under optimal conditions the determination is highly selective and accurate. It is possible to determine l-150 mg of iron(III).

IN a preliminary electrodes

communication ,I the possibility of using two carbon (graphite) for biamperometric detection of the end-point in titrations with EDTA

was mentioned. Previously, the carbon electrode had been used as an indicator electrode in potentiometric and amperometric titration9 and also in polarography.P For biamperometric titrations with two polarisable electrodes (“dead-stop” method) an electrode system of two carbon electrodes has not yet been applkd5 In the present paper the results are reported of a study of the conditions for the biamperometric titration of iron(II1) with EDTA using an indication system of two carbon electrodes. Recently, this titration was investigated with an electrode system of two platinum electrodes.6 EXPERIMENTAL Reqents Unless otherwise stated, all solutions were prepared from reagent-grade chemicals. Fore&w ion soluths. Solutions of Ni I+, Cu’+, Th’+, Zr+, Al*+, Ti4+, UO,*+, Cop+,Zn*+, Cd’+,

Mna+,Hg*+,Pb*+,Mg’+ and Sn*+were prepared from salts soluble in water. Buffer solutions (pH 14.2). Preparedfrom 1M trichloracetic acid @H l.l-1.5), monochloracetic acid @H 2-2.8) and formic acid (pH 4.2-5.3) by neutralisation with aqueous ammonia under potentiometric control of pH with a glass electrode. Stun&d iron(II1)solutton. Prepared by dissolution of behanite (99.8 % Fe) in a minimum amount of hydrochloric acid (1: 1) and a little 30 % hydrogen peroxide. After dissolution, the hydrogen peroxide was decomposed and the cooled solution diluted with redistilled water to 1000 ml. The amount of iron in this solution was determined gravimetrically [l ml E 1.027 mg of Peck)], and the value checked regularly. * Part II: see reference 7. 377

O-05M EDTA solution. Prepared by dissolution of 33-22 g of Chelaton 3 (Lachema, Bmo, Czechoslavakia) and dilutior~ to 2tH.W ml with red&i&xl water. The titre was determined by titration against standard lead nitratG solution with Xylenol Orange as indicator.8 A&UWmS For indication of the stop” iIlstrument was used.* Two spectrograph carbon electrodes of f-mm diameter BratisWa~ C?.achoslovakia, 999% carbon) were used. They were placed in glass tubes and covered with plastic (denta@e) so that oniy 10 mm of their ksngtn remained exposed (Pig. 1). When not in usq the ekctrodes were kept immersed in waterThe pH was measured with a glass electrode and an electronic pH meter Muhoscop KII (Laboratorni potfeby, Prague, Czechoslovakia). Polarographic measurements were carried out with a polaropaph LP 55 (Laboratomi pristroje, Prague, Czechoslovakia). I%ce&re The chosen amount of iron@I) was diluted to about 150 ml with redistiIied water in a 25@ml beaker and the pH adjusted under ~tentiomet~c control with hydrochloric acid or sodium hydroxide or with 15 ml of a suitable IlId buffer solution. The titrations were carried out at pR 1-5-2-O in unbutfemd sohuion at 20” with intensive stir&g (If500rpm) using 095M EDTA sofution. Ah titra$ons were started 2-3 min after @erting the ekctrodes into the solution During this time the uilibrium” of the electrode reacuons was established. The change of current was read after each azIi tion of titrant during the titration. The stabilisation of current vahres is satisfactory mainly at the end-point and after it. All data given in this paper are the arithmetical mean of three determinations, rpuenc~

of pII

RESULTS

AND

DISCUSSLON

It is well known tlutt the chelometric titration of iron takes place in a relatively From the point of view of selectivity, the optimum condition for this course, the maximum possible acidity. The influence of pII on the biamperometric titration of iron(III) was studied in buffered Fig. 2) and unbuffered (Fig. 3) media. Titration proceeds satisfactorily at pH 1.1-2CI in the unbuffered medium and pH 1*1-4*2in the buffered solution. At other values of pH, detection of the end-point is impossible. acidic medium. titration is, of

r~~~~~~ o~~~~~~~o~~~~~ The inSuence of the applied potential was investigated over the range M-2*0 V (Fig. 4). Satisfactory results can be obtained even at an applief potential lower than 0*6V, but in this case the amperometer used must be more sensittve. This phenomenon had previously been encountered in the biamperometric titration of thallium(III).7 For measurements in the range O&2.2 V of applied potential, a microammeter with fulbscale deflection of Q-750@ was used. The mean relative error of the determinatiou of 15 mg of iron(III) was &O* f%= Z@uznceof temperaf ure Although the titration proceeds very well at room temperature, the influence of temperature was studied. It was found that at higher temperatures the stabilisation of current at the beginning of the titration is somewhat faster; no other differences were found. ‘~~~.~~~~n’~of ~~~~fr~~e~ ?‘he aim of the following experiments was to find the appropriate ir&uence of the products of the electrode reactions. These products can cover the surface of the elec= &ode and change the indicator properties of the electrode couple. From this point of

1

Determination of ironf.III)

O,OSM-EOTA,

379

ml

Fro. 2.-Influence of pH on the titration curve8 in buffered solution:I-pH 1.1; 2-15; 3-2.0; 4-2.8; 5-4.2. [Ma41mgofFeJTIQ; totalvolume: 15Oml; temperature: 20”; applied potential: l*OV.I

I

0

I 12

I

I

3

I

I’

4 5 O.OCIM- EDTA,

1

1

1

1

6

7

6

6

ml

FIG. 3.-InlIuencc of pH on the titration curves in unbuffered solution:Z-_PH 1.l ; 2-1.5; 3-2.0; 12.5. l15.41 mgof Fe(m); totalvolume: 15Oml; tempatm: #)“; applied potdid : 1.0 V.1

380

3.

VORL~CEX

and F. VYQRA

view. 50 determinations of iron were carried out under identical conditions over 24 hr. After each titration the electrodes were only washed with distilled water. Fig. 5 shows the spread of the 50 titration curves. The relative error in these titrations was not geater &an &O*2°& ~~bse~uently, attention was paid to the decrease of the current at the moment

0,05M-EDT&.

ml

4.-Influence of appliedpotential on the titration curves:A--2.0 V; 2-l .8 V; 3-1.6 V; 4-1‘4 V; S-1.2 V; 6-l.OV; 7-0.8 V; 8-@6V. (15.41 m~ofF~1~); total volume: 150 ml; pH: I.7 (uubuffer~); temperature: 2O”J. FJG.

when the electrodes were inserted into the titration solution. In Fig. 6 is given the dependence of the current on time, where the start of the time axis is the time of inserting the electrodes. Curve 1 shows this dependence for the system of two carbon electrodes: in this case the electrodes were first soaked for 144 hr in O*lM potassium chloride solution. Curve 2 shows the dependence when the electrodes were first soaked for the same time in distilled water. These curves show that saturation of the carbon electrodes with potassium chloride has practically no influence on stabilization of the current. In both cases, after 5 min, only a small decrease of the current occurs. For analytical application it is enough to wait 2-3 min after inserting the electrodes

381

Determination of iron

O,OSM-EDT*,

ml

FIG. S.-Spread of titration curves in 50 titrations [1541 mg of Fe(M); total volume: MO-180 ml; temperature: 20”; pH: 152.0; theoretical consumption: 5.20 ml of O.OSMEDTA ; applied potential : l-0 V].

250 4

2 ‘5x3

5ofl 0

2

6

IO

1 Min

FIO. 6.-Cwent-time curves at the stabilisation of current values:ZalMKCl; 2-+&tilled water. [W41 mg of Fe@); total volume: 150 ml; pH: 1.7 (unbufked); temperature: 20’; applied potential: 1.0 V].

J. VORL~&Kand F. VYDRA

382

into the titration solution. However, when one starts the titration immediately, no great differences in the course of the titration are found. The decrease of the current just after the potential is applied is caused by polarisation of the electrodes; this does not proceed immediately under the given conditions.

The proposed end-point indication of the chelometric determination of iron(II1) permits the determination of l-150 mg of iron in a volume of 150 ml with reliable accuracy. The results of some titrations are summarised in Table I. The results from TABLEI @05M EDTA, ml

Difference

Present

Found

The&y

Found

mg

%

1.03 2.06 6.18 15.41 30.83 51.38 82.21 102.76 154.14

1.06 2.09 6.12 15.41 30.68 51.53 81.62 10340 i55.53

0.33 0.66 1.98 5.20 1040 17.35 27.70 34.68 52.03

0.34 0.67 1.96 5.20 10.35 1740 27.50 3490 52.50

+ 0.03 +0.03 -0.06 OGO -0.15 +0.15 -0.59

+2.91 +1.46 -0.97

+064 + 1.39

-8.2 +0*29 -0.73 +0.63 +o+O

this table (with the exception of the results of the titration of 1-2 mg of iron, when the error is greater) give a mean relative error of &O-87& Selectivity The selectivity of the proposed titration was verified by the titration of 15.41 mg of iron(II1) under defined conditions (pH 1.3-25; 20’; 1-OV) in the presence of a series of foreign ions. The results have shown that even a high concentration of nitrate, sulphate, phosphate, acetate, chloride and borate does not interfere. Also, many metals do not interfere, e.g., cobalt, lead, zinc, cadmium, calcium, barium, strontium, magnesium, manganese, uranium(W), iron(I1) and titanium(IV). On the other hand, even small concentrations of thallium(II1) interfere in the titration of iron(II1). Indium, gallium, zirconium, thorium and aluminium are titrated together with iron and thus interfere in the determination. Greater amounts than 50 mg of nickel and 80 mg of copper also interfere. Some metals can be masked with fluoride ions, particularly aluminium and also thorium and zirconium,1° as in the titration of iron(II1) with two platinum electrodes.6 Analytical application The described chelometric titration has a similar analytical application to the titration with an indication system of two platinum electrodes.6 The described procedure is probably best applied to the determination of iron(II1) in the presence of iron(I1) in various metallurgical materials.1° CONCLUSION

The present study has shown the possibility of replacing the two platinum electrodes with two graphite electrodes in the ,biamperometric titration of iron(II1) with

Determination of iron

383

EDTA. The obtained results are of the same accuracy and of the same selectivity. In an unbuffered medium the determination can be carried out at pH l-l-2.5 and in a buffered medium at pH 1.142. If a microammeter is used for indicating the end-point, a higher applied potential than 0.6 V should be used. The graphite electrodes are much cheaper than the platinum ones, which is an advantage in routine analysis. Another advantage is the possibility of determining iron(II1) in an acidic medium at room temperature, which was impossible using two platinum electrodes. Future papers in this series will present the utilisation of amperometry with two identical electrodes for the chelometric determination of some other metals which can be titrated with EDTA, e.g., thorium, indium, gallium, zirconium, copper and rare earths. As in the case of the titration of iron(II1) with EDTA using two platinum indicator electrodes, it is not yet possible to give an exact explanation of the electrode reactions involved in the titration with two carbon electrodes. In particular, the decrease of current during the titration before the equivalence point cannot yet be explained satisfactorily. An increase of current after the equivalence point was found in the chelometric titration of all metals, provided that indication with two identical electrodes of platinum or of carbon was used and, of course, the pH was kept at the optimum value for each titration. This phenomenon extends the previous presumption that the described biamperometric titrations with EDTA are possible only with metals forming redox systems. Metals which do not form redox systems have a different character in their titration curves. Because the metal ion and also its complex do not give the electrode reaction over a wide potential interval, there are no changes of current before the equivance point. Immediately after the equivalence point the presence of a minimum amount of free EDTA (which takes part in the electrode reaction) causes an increase in the current. Zusammenhssung--Zur Anmige der biamperometrischen Titration von Eisen(II1) mit EDTA wurden xwei Kohle-(Graphit-) elektroden verwendet. Refriedigende Ergebnisse wurden in einem weiten Bereich der angelegten Sparmung erhalten. Die absoluten Stromwerte hiingen linear von der anaeleaten Spannung ab. Rei 0,6 bis 2,0 V gehen die Stromdiffemnxen -w&rend her T&&ion von einigen PA-bis xum Zehnfacheti davon. Der EinSti von DH. Suanmmg und Temneratur auf die Titration wurden untersucht iowie’der Eit%uD von Stiirela menten. Unter optimalen Redingungen ist die Restimmung sehr selektiv und genau. Man kann l-150 mg Eisen(II1) bestimmen. R&a~~~&-Pottr l’indication du dosage biamperometrique du fer(III) au mo en d’EDTA, on a utilid deux &ctrodes de carbone (graphite). Des 2 ultats satisfaisants ont 6te obtenus dans un large domaine de potentiels appliques. Les valeurs absolues du courant sont Ii&s lineairement aux valeurs du potentiel applique. A un potentiel de 0,6-2,0 V, lea differences de courant durant le dosage vont de plusieurs CA a dix fois ces valeurs. On a 6tudi6 les influences du pH, du potentiel annliouc et de la temperature sur le dosage; on a examint l’infhtence dd &ments interf&ts. Darts des conditions optimalea, le dosage est hautement dlectif et r&is. 11 est possible de doser l-150 me de Fe(II1). REFERENCES * F. Vydra and J. Vorlllkk, Tuluntu, 1964,11,655. * J. Ciallk, Potenciometrie. NCSAV, Praha, 1962.

384

J. VoRLiErurand F. VYDRA

wtih the Gqhite k&cat@ Ekctro& in Analytical ‘P. J.EJvingtsndD. L.Smi&, -yysis Chat&try 1962, lRe Roceehgs c-f the buemational Syqwsinm, Ehwr Publishing Comphy, Am&dam, 1963, p. 204. ’ V. 0. Barikov and 0. A. Songina, Zacodrkcrya Lab., 1964,30,5. ‘ a. patent pfihl. No. 530264. @J. VorWck and F. Vydra, Coil. Czech. Chem. Comm., in press. ’ I&m, Tolonto, 1%5.12,139. ’ IL Pfibil, Ko*I. SNTL Pmha$ 1957, ’ AlwiwMet~ for chemicd Anaiysis, ASTM, Pkli!ikphia, 1960, p. 26. lo F. Vydra and J. VorlWc, ChemWAnalyst, 1964,53,103.