Amperometry with two polarisable electrodes—VII

Tahnta, 1966. Vol. 13, pp. 439 to 443. Pergamon Press

Ltd. PrIntedin NorthernIreland

AMPEROMETRY WITH TWO POLARISABLE ELECTRODES-VII* CHELOMETRIC

DETERMINATION

OF INDIUM

F. VYDRA Polarographic Institute of Heyrovskf, Czechoslovak Academy of Sciences Analytical Laboratory, Prague 1, Jilska 16, Czechoslovakia and

J. VORLf&K Research Institute of Iron Mines, MniSek, Czechoslovakia (Received 22 February 1965. Accepted 13 October 1965) Summary-Optimum conditions have been found for the chelometric determination of indium by biamperometric titration with EDTA. Two stationary platinum electrodes (Pt-Pt) or two graphite electrodes (C-C) may be used to indicate the end-point of the titration. At pH l-l.5 the determination is highly selective. The applied potential only influences the absolute value of the current; the accuracy of the determinations remains constant over a wide interval of applied potential. At a potential of 1.0-1.7 V the change of current during a titration with 0.05M EDTA solution is of the order of 10 ,uA. Analytical applications of the method are discussed.

IN a previous

paper

in this series the universal

nature

of biamperometry

for indication

In the present paper are summarised the empirical results of a study of the biamperometric determination of indium by titration with EDTA. The course of the titration curves is of the same nature as those curves obtained for the titration of thorium with EDTA.2 Up to the equivalence point there is no change of current, but then a minimum excess of EDTA causes an increase of current because of the electrode reaction of free EDTA (probably its anodic oxidation). An exact evaluation of the titration curves will be presented subsequently. of chelometric

or all complexometric

titrations

has

been

sh0wn.l

EXPERIMENTAL Reagents O-OS&fEDTA solution. Prepared by dissolving 37.5 g of the reagent (Chelaton 3, Lachema, Prague, Czechoslovakia) and diluting to 2000 ml. The titre of this solution (OG48M) was determined against standard lead nitrate solution using Xylenol Orange as indicator. 0.02M hdrum solution. Prepared by dissolving 5.738 g of metallic indium (99.99 %) m perchlorlc acid and dilutmg to 1000 ml. Buffer solutions (pH range 145). Prepared from 1M solutions of trichloroacetlc, monochloroacetic and acetic acids by neutralisation with aqueous ammonia under potentlometrlc control using a glass electrode. All other solutions were prepared from analytical reagent grade chemicals. Apparatus For indication of the end-point identical apparatus was used to that described previously.8 Two platinum plates (7 x 5 x @2 mm) or two graphite rods (99.99 % C) impregnated with Ceresinet were * Part VI: Tahta, 1965, 12,671. i Mixture of solid saturated hydrocarbons

with higher m.p. than paraffin. 439

F. VYDRA and J. VORL~EEK

440

used as indicator electrodes. pH was measured with an electronic pH meter (Multoscop V, Lab. potkby, Prague, Czechoslovakia) and a glass electrode. Titrations were carried out with an automatic burette (motor-driven hypodermic syringe) reading to kO*OOSml. Procedure To a known amount of standard 0.02M indium solution were added 10 ml of buffer solution (or a chosen amount of 7*2M nitric acid.) When the selectivity was being studied, the appropriate solution of metal salt was added followed by dilution to about 100 ml. The resulting solution was titrated with 0*05M EDTA solution 3 min after inserting the electrodes and applying the potential. During titration the solution was stirred with a propeller (1500 ‘pm) at room temperature. After each addition of titrant the value of the current was read, stabilisation of the current being practically instantaneous. Quantitative evaluation of the titrations was carried out graphically. All results presented in this paper are an arithmetical mean of three determinations. RESULTS Influence

AND

DISCUSSION

of pH

Indium forms a stable complex with EDTA (pK = 24.9). This fact enables its chelometric determination to be made even in a relatively acidic solution. Figure 1 illustrates the course of the titration of indium in dilute nitric acid with an

10,.

tz4.

:F t

0 0

4

2

3& O,OSM-EDTA

of pH on titration of indium (Pt electrodes) : 1. 0*35M HNOI, 3. pH 1.35, 2. O.llM HNO,,_. 4. nH 1.95,

FIG. 1.-Influence

I

[2*5 ml of O-OSMIn(ClO&;

HNOI or?02 if2l M buffer solution; 1.5 V; total volume: 100 ml]

applied potential:

indicator system of two platinum electrodes. Satisfactory detection of the end-point with an error under O-2% rel. is possible in O*lM nitric acid and at pH 1.3-3.5. Above pH 3.5 the titration curves are deformed and stabilisation of the current takes much longer. Similar results to those with platinum electrodes have been found with the use of two graphite electrodes. These latter electrodes are more convenient in a strongly acidic medium (see curves 1 and 2, Fig. 2), in which the current at the beginning of the titration is relatively smaller but the equivalence point is more pronounced. When

Amperometry with two polarisable electrodes-VII

441

using graphite electrodes for analysis there is no need to make a graphical extrapola-

tion of the equivelence point. Infruence of applied potential As in the chelometric titration of other metals the applied potential has practically no influence on the biamperometric titration of indium, which means that the accuracy of the determination is unaffected by the applied potential. At an applied potential tl V the current changes are, however, smaller. By using a sufficiently sensitive indicator instrument (ammeter, galvanometer) the described technique can be successfully applied.* It is , of course, more convenient to use an applied potential > 1 V.

OO. 0,05M-EDTA

44

of pH on titration of indium (C electrodes) : 1. 0.35M HNOII, 3. pH 1.5, 2. 0*075M HNoa, 4. pH 3.4,

FIG. 2.--Influence

[2 ml of 005&f In(ClOJ,;

HNOt or :b k?ik%f buffer solution; applied potential: 1.3 V; total volume: 100 ml]

Infruence of temperature Fig. 3 shows titration curves obtained at different temperatures and illustrates the influence of temperature on the absolute value of the current (curves 2 and 3). Even under these conditions detection of the end-point is accurate enough, and the error is not greater than f0.2 % rel.

Passivation of electrodes After repeated chelometric titrations of indium with platinum or graphite electrodes no accumulation of the reaction products occurs on the surface of the electrodes. Therefore, it is quite sufficient to wash the electrodes with water after each titration. When the electrodes are not in use they should be immersed in distilled water. When the potential is applied and the solution intensively stirred, stabilisation of the current occurs after 3 min. Accuracy and concentration range Using the described procedure it is possible to determine 2.5-50 mg of indium with a mean error of 0.3 % rel. The determination of lesser amounts of indium using more dilute EDTA solutions will be described in a further paper. 8

F. VYDRA and J. VoaLfEk

442

Selectivity

By the described biamperometric method it is possible to determine indium in a strongly acidic medium (O*lM nitric acid-about pH 3.5). Under these conditions several metals do not form complexes with EDTA or only weak complexes are formed.

01 0

f

34

2 405M-EDTA

FIG. 3.-Influence of temperature on titration of iudium : 1. 250, 2. 500, 3. 80” [2.5 ml of 0*05&f In(ClO&; pH 1.5; applied potential: 1.5 V; R electrodes; volume : 100 ml]

"Ol-t---5--3c;-l

total

0,05M-EDTA

FIG. 4.-Influence of cobalt and nickel on titration of indium: 1. 250 mg of Co, 2. 177 mg of Ni [2 ml of O*OSM In(ClOJ,; pH 1.5; applied potential: 1.5 V; C electrodes; volume : 100 ml]

total

Thus, practically all bivalent metals (with the exception of copper) do not interfere with the determination of indium. Experiments have shown that 6.9 mg of indium can be determined at pH l-l.3 in the presence of an excess of Mg, Sr, Ba, Ca, Be, Ag, U, Al, Ti, Zn, Cd, Pb and Co, with a maximum error of 0.3 % rel. More than 200 mg of nickel and 10 mg of cobalt interfere with the determination. Fig. 4 illustrates the influence of nickel and cobalt on the determination of indium (under identical conditions). Small amounts of Fe(III), Bi, Ga, Th, Zr and rare earths also interfere with the

Amperometrywith two polarisableelectrodes-VII

443

determination of indium. Because the determination of indium can be carried out in the presence of fluoride ions it is possible to mask some metals (thorium and rare earths) with an excess of fluoride ions. Practical

applications

Practical utilisation of the described method has been verified by the determination of indium in solid solutions with antimony and zinc or cadmium. By means of repeated evaporations with hydrobromic acid antimony was separated. Zinc and cadmium, even when present in large concentrations, do not interfere with the described determination of indium. Procedure. 0.5 g of the powdered alloy was dissolvedwith a mixture of 3 ml of sulphuric acid (1 + 9) and 5 ml of 40 % hydrobromic acid in a porcelain basin placed on a sand bath. Subsequently, the solution was evaporated with 10 ml of hydrobromic acid and this operation repeated five times. The residue from these evaporations was then taken to dryness with 5 ml of hydrochloric acid (1 + 1) when the tinal residue should be white. This residue was dissolved in 5 ml of cu. 1M nitric acid and after dilution to 50 ml the indium determined by biamperometric titration with O.OSMEDTA solution at an applied potential of 15 V.

In Table I the results obtained by the above procedure (Method B) are compared with those from an EDTA titration of indium in an alkaline medium carried out after partial separation of zinc or cadmium using an ion-exchange technique (Method A),5 TABLE I

Alloy Cd-%-In Zn-Sb-In

Indium found, * % Method B Method A 1.00 1.03 0.64 0.65

* Average of 3 determinations. Acknowledgment-The authors wish to express their thanks to the Institute of Solid State Physics, Czechoslovak Academy of Sciences, for kindly providing the samples of indium alloys. Zusammenfassung-Die besten Arbeitsbedingungen xur chelometristhen Bestimmung von Indium durch biamperometrische Titration mit EDTA wurden ermittelt. Zwei feststehende Platinelektroden (Pt-Pt) oder xwei Graphitelektroden (C-C) konnen zur Anzeige des Titrationsendpunktes verwendet werden. Bei pH l-1,5 ist die Bestimmung hochselektiv. Die angelegte Spannung beeinflult mu den Absolutwert des Stromes; die Genauigkeit der Bestimmungen bleibt tiber einen weiten Bereich angelegter Sparmungen konstant. Bei einer Spannung von l&1,7 V liegen die Stromlnderungen wiihrend einer Titration mit 0,05M EDTA-Liisung in der Grijfienordnung von 10 ,uA. Analytische Anwendungen der Methode werden diskutiert. Rt%um~On a trouvb les conditions optimales de dosage chelatometrique de l’indium par titrage biamp&ometrique a I’EDTA. On peut utiliser deux electrodes stationnaires en platine (Pt-Pt) ou deux electrodes en graphite (C-C) pour indi uer le point de fin de dosage. A pH l-1,5, le dosage est hautement s%lectif. Le potentiel applique n’intluence que la valeur absolue de l’intensitt du courant; la precision des dosages reste constante dans un large intervalle de potentiel applique. Pour un potentiel de l,O-1,7 V, les variations d’intensitb durant un dosage au moyen dune solution d’EDTA 0,05M sont de l’ordre de IO PA. On discute des applications analytiques de la methode. REFERENCES 1 J. VorliEek and F. Vydra, Tahmta, 1965, 12, 671. * Idem. Collection Czech. Chem. Commun., in press. a AS. ?.M. Methods for Chemical Analysis, p.i6. A.S.T.M., Philadelphia, 1960. b F. Vydra and J. Vorlicek, Talanta, 1965, 12, 139. 6 K. &rrdik, Hutnick& Listy, 1965, 20, 287.