The platinum-20% rhodium electrode in potentiometry

The platinum-20% rhodium electrode in potentiometry (Received 19 January 1967. Accepted 2 June 1967) Po~~o~c technique with meta~~met~ oxide ekctrodes has been developed in great detail, but alloy electrodes have not been so thoroughly studied, The plating-rh~i~ ahoy electrode (1040% Rh) has attracted some attention, 1,s but complete information is not available. Potentiais are taken with a stainless steel electrode in some solutions,* though mostly in the study of corrosion.* With a view to studying the effect of alloying on the electrode potential, we used the platinumrhodium electrode in various redox and acid-base titrations. EXPERIMENTAL The vernier potientiometer used was manufactured by the Oriental Science Apparatus Workshop, Ambala, India and was capable of measuring lo-’ V. PIatinum and platinum-20% rhodium electrodes were prepared by fusing the 19 s.w.g. wires into 5-mm bore soft glass tubing. A dip-type 1280

II00

-

sso-

Fm. I.-Redox

titrations of O-1, @Ol and O-OOlN iron and permanganate, Pt/S.C.E. and Pt-Rh/S,C.E. electrode systems. 1333

with

% Rh

Iron(B) sulphate

Potassium permanganate

Sulphuric acid

Iron(B) sulphate

Sodium hydroxide

ALLOY

Potassium dichromate

Titrand

OF Pt-20

Iron(B) sulphate

Titrant

I.-PERFORMANCE

Cerium{IV) sulphate

.__

TABLE

0.1 0.01 0001

0.1 0.01 0001 5452 5640 -

55.38 51.82 4060

7.04 2640

25.15

o-1 0.01

5.552 4454

O+lOl

5442 56.30 -

5548 5214 41.65

7.02 26.40

2540

55.76 44.68

End-point, ml Pt/Rb Pt

o-1 o-01

Normality

TITRATIONS,

-0.10 -0.10

+0.10 $0.32 $0.05

-o*oz @OO

+0*25

+0.24 $014

Difference, ml

ELECTRODE)

AND ACID-BASE

REFERENCE

IN REDOX

(S.C.E.

ELECTRODE

Wmi

265 270

630 576 541

z

454

519 534

150 93

450

z?

93 81

I47

486 462

Wave-height, m V Pt Pt/Rh

COhiPARED

THAT

115 117

130 135 91

549 323

207

33 12

Remarks

ELECITkODR

Both waves are well formed No noticeable rise obtained with either electrodes.

Pt/Rh cnrve is deformed

Pt/Rh curve is deformed

Pt/Rh curve deformed

PLATINUM

Difference, mV

OF m!Z

1335

Short communications

saturated calomel electrode (S.C.E.) was used. The metal electrodes were cleaned ~~~~y by electrolysis of 50 % V/Y sulphuric acid at @2 A for 20 min with a platinum anode and the electrode as cathode. All the chemicals used were either Merck G.R. grade or B.D.H. AnalaR grade. Approximately O*lN solutions of iron@) ammonium sulphate, potassium permanganate, cerium(IV) sulphate, sodium hydroxide and sulphuric acid were prepared, standardized in the usual way, suitably diluted, and used in suitable combination for titrations. RESULTS

AND

DISCUSSION

Redox titrations Malmstadt and Fett used a Pt-10% Rh electrode for O*lN iromB.)-dichromate and iron(B)cerium(IV) titrations.1 They reported good results, though the end-point obtained with the alloy electrode was always later than that with the platinum electrode, by O-866 and O+%? ml respectively. We performed a wide range of redox titrations at various concentrations for the PtjS.C.E. and ~-2O%~)~S.C.E. systems, as shown in Table I. For 0.1, O-01 and O@OlN ~on~~~(~ titrations, the end-points with the alloy electrode were always later than with the platinum electrode, Deformities occur in the lower concentration curves for the alloy electrode, and the potential breaks obtained are also less than with the conventional electrode, the difference being 33,72 and 207 mV for 0.1 , 0.01 and 0.OOiN concentrations respectively. For 0.1 and 0.OliV dichromate-iron(B) titrations, the end-points are nearly the same for both the electrodes, but the potential breaks for the alloy electrode are smaller than those for platinum by 549 and 323 mV. For O-1, 0.01 and 0~OO1NironOpermanganate titrations, a similar trend of progressive diminution in potential breaks and of deformities in the lower concentration curves (Fig. 1) is obtained.

540-

400

400-

360

320

360-

260

300-

240

240-

200

> E

P 2 ;(Y I80 -

Pt-Rh/SCE

a IZO-

60A o6 -6050

52 I

54 I

56 I (a)

FIG. 2.-Potentiometric

I 56

I 60

62

50

ml of sulphurrc

6

IO

52

I 54

acid added

I2

14

16

54

I 58

I 60

I6 62

6 '9-C I 63

(b)

acid-base titrations, with Pt/S.C.E. and Pt-Rh/S.C.E. electrode systems. (a) OmlMNaOH vs. O*lNH,SO,. (b) O*OlIMNaOH vs. 0.OiN &SO~; curve C is for O+OOlMNaOH vs. O@OlN H$O,, with the glass eIectrode-calomel electrode system.

Short communkations

1336

Strong acid-strong base titrations using the Bt&C.E_ system have been reported,” and the ptB.h/S.C.E. system’ for higher concentrations. The end-points obtained with the alloy eheetrode are earlier than those with the platinum electrode (Table I). The potential breaks are smaller for the alloy ekctrodes as usual. No od curves were obtained for any electrode system for cencentratioxls below @OlN. Figure 2 shows tr e curves for PI and O-01Nsodium hydroxide-sulphuric acid titrations for both the electrode systems; results are more reproducible with the platirmm electrode than with the ahoy electrode. Bimetallic electrode syszem ‘l’healloy electrode is, however, found to be very useful in redox and acid-bare titrations if used m combination with a platinum electrode. This combination gives peaks which are similar to the first ditferentiai peak of the xero current potentiometric curve because the potential breaks obtained at the end-points with these two eiectrodes, used separately, are dit%reni. The peaks could afso be calculated from the difference ofpotemiai for each point in the ~~~~~~~g~ate curves shown in Fig. I, The peaks thus cakulated, as well as those actual@ obtained by using the b&eta& ekctrode system, are shown in Fig+ 3. Calculated and experimental results obtained for O-1, 0.01 and 040fN iron(~)-perman from titrations, with the bimetallic electrode system, are given in Table II. The end-points obtaine Yate the peaks coincide with the true end-points and are exactly the same for all the titrations. As the concentration decreases, the peaks become broader (the observed peaks are much broader than the calculated ones). The increase in peak height with decreasing concentration is greater for the observed 720-

660-

655

540

480-

> 420E * 360 P z 5 a

350-

485

120

60 245 : I

16 ml

Fta

of

permonganote

I

I

18

20

added

22

24 25

3.--Bimetallic electrodes (Et and Et-Bh) in the potentiometric titration of iron(B) with permanganate. Curves A, B, C are calculated from the curves in Fig* 1; carves D, E, F are those observed ex~~rn~~~y.

1337

I60 -

140 -

120 > E "z loo , O*OOlN

.D E

I

ID BOt

14

O,IN

0

1

I

I

1

I

2

4

6

6

IO ml

FOG.

I

I2

I

I

I

I

14

16

16

20

22

24

I 26

of H,SO, O-IN cdddd

&-Bimetallic electrodes (Pt and Pt-Rh) in the potentiometric titration of NaOH with H&SO‘.

~xx&sthan for the calculated peaks. This difference in behaviour may be ascribed to meter-induced polarization of the metal and alloy ekctrodes.~ For acid-base titrations however (Fig. 4, Table II) the peak heights decrease with decreasing concentration; the end-points are the true end-points. Excellent peaks were obtained for O.OOlN solutions, where the Pt/S.C.E., Pt-Rh/S.C.E. or giass/S.C.E. systems fail. IMike some bimetallic electrode systems with which an S-shaped curve is obtained,‘** this electrode system gives high peaks even at low concentrations, enabling easier true end-point detection. Smaller potential breaks are obtained with the ahoy electrode. Sometimes the end-points are later than the true end-points. In this respect, the alloy electrode is comparable to unclean and nntreated metal electrodes which always show a response lag .*J” The alloyed and unalloyed bimetallic

Titration

Reagent concentration, N

Iron(U)-permanganate 8:d @OOl

Sodium hydroxidesulphuric acid

Peak height, mV * Calculated Observed True 55.38 51.82

426 482 447

End-Point, ml Calculated

Observed

5540

52.00 4040

x:;1 OGOl

489 579 600

:*!z 23.15 23.15

23.15 23.15 23.15

0.1 0.01 !Mol

146 110 56

2228 2229 22.29

2228 2229 22.30

1338

Short communications

system behaves as if it is a combination of unclean and clean platinum electrodes. It has also been observed that the untreated platinum electrode behaves like a platinum-rhodium electrode, though the potentials obtained with it are not as reproducible. V. T. ATHAVALE R. G. DHANESHWAR D. A. SARANO

Analytical Division Atomic Energy Establishment Trombay Modular Laboratories Bombay-14, India

Surnma~-The performance of a platinum-20% rhodium alloy indicator electrode is studied in a number of potentiometric redox and acid-base titrations and compared with that of the conventional platinum electrode. The potential breaks obtained with the alloy electrode are always smaller than those obtained with the platinum electrode. The end-points are also usually indicated later than the true end-points. The bimetallic electrode system comprising platinum and platinum-rhodium electrodes gives a peak at the correct end-point even with 0OOlNconcentration titrations, where the ordinary potentiometric systems do not always give good results.

Zusammenfassung-Das Verhalten einer Indikatorelektrode aus einer Legierung von Platin mit 20 % Rhodium wird bei einer Anzahl potentiometrischer Redox- und SBure-Basen-Titrationen untersucht und mit dem der gebrluchlichen Platinelektroden verglichen. Die Potentialsnrtinee mit der leaierten Elektrode sind stets kleiner als die mit der Plat&elektrode e&altenen. Die Endpunkte werden gewohnlich spater angezeigt als die wahren Endpunkte. Das Zwei-Metall-Elektrodensystem aus Platin- und Platin-Rhodium-Elektroden gibt am richtigen Endpunkt selbst bei Titrationen mit der Konzentration 0,OOlN eine Spitze, wo die gewiinnlichen potentiometrischen Systeme nicht immer gute Ergebnisse zeigen.

RBsmn&-Gn a 6tudi6 le fonctionnement dune electrode indicatrice en alliage platine-20% rhodium pour un certain nombre de titrages potentiometirques redox et acids-base et l’a compare ii celui de P&ctrode de platine usuelle. Les brisures de potentiel obtenues avec l’electrode en alliage sont toujours plus petites que celles obtenues avec l’electrode de platine. Les points de fin de dosage sont aussi indiques habituellement plus tard que les points de fin de dosage vrais. Le systeme d’electrodes bimetallique comprenant des electrodes en platine et platine-rhodium donne un pit au point de fin de dosage correct meme avec des titrages a la concentration O,OOlN, ou les systemes potentiometriques ordinaires ne donnent pas toujours de bons resultats.

REFERENCES 1. H. V. Malmstadt and E. R. Fett, Anal. Chem., 1954,26, 1348. 2. Idem, ibid., 1955,27,1757. 3. I. B. Ulanovskii, Yu. M. Korovin and Y. F. Sevast’yanov, Zh. Prikl. Khim., 1962, 35, 1066; Chem. Abstr., 1962, 57, 9569f. 4. V. A. Khitrov, N. A. Dugin and V. F. Khmel’kov, Chem. Abstr. 1963, 58, 13431~. 5. R. G. Dhaneshwar and V. P. Apte, Talanta, 1966, 13, 1595. 6. M. G. Yakubik, L. W. Safranski and J. Mitchell Jr., Anal. Chem., 1958, 30, 1791. 7. J. A. Hostetter and H. S. Roberts, J. Am. Chem. Sot., 1919, 41, 1337. 8. H. H. Willard, L. L. Merritt, Jr. and J. A. Dean, Instrumental Methods of Analysis, 3rd Ed., p. 432-433. Van Nostrand, London, 1958. 9. E. Muller and H. Kogert, Z. Physik. Chem., 1928,136,437. 10. R. G. Dhaneshwar, Ph.D. Thesis, p. 142 ff. University of Exeter, 1962.