Mechanism of the electrochemical phenomena occurring during “dead-stop” titrations

VOL.

4

(1950)

ANALYTICA

MECHANISM OF OCCURRING

CHIMICA

ACXA

635

THE ELECTROCHEMICAL PHENOMENA DURING “DEAD-STOP” TITRATIONS by PAUL

Dapar:msnl

of Chemistry,

Louisiana

DELAHAY Sat6 Univws

ity, Baton Rouge, La. (U.S. A.)

The “dead-stop” method of titration of FOULK AND BAWDEN~ has found put few applications, although it yields accurate results and requires only inexpensive equipments. This situation results perhaps from the fact that the electrochemical phenomena involved have never been explained in a satisfactory manned and the apparently empirical character of the method may have deterred further application. It is the purpose of the present paper to fill this gap and to explain the phenomena occurring during a “dead-stop” titration. Since polarization curves will be used in this discussion, some principles pertaining to these curves will be briefly discussed. POLARIZATION

CURVES

FOR

REVERSIBLE

AND

IRREVERSIBLE

REACTIONS

Let us call E, the reversible potential corresponding to the following Redox system x Ox+mH++ne-=y Red+ zHsO (r ) for a given PR and given values of the activities aox and aRd. A platinum electrode dipping in a solution in which equilibrium (K) is established, acquires the potential E,. When the potential E of the platinum electrode is modified by application of an external difference of potential between the platinum electrode and a reference electrode, a current will flow through the cell. Oxidation or reduction will take place at the platinum electrode depending on the value of this applied difference of potential. The variations of the current as a function of E are represented in Fig. I A. Current 1, corresponds to oxidation at the platinum electrode, current 1, to reduction. At the reversible potential E, the current is zero. If concentration polarization does not occur, the current varies linearly with E and is inversely proportional to the resistance of the cell (dotted line). If concentration polarization occurs, the current varies as shown qualitatively in Fig. I A (solid line). References

p. 640.

P.

636

DELAHAY

VOL.

4

(1950)

In the case of an irreversible system, the oxidation and the reduction processes Since the overvoltages may require the overvoltages q. and rlc respectively. be as large as I volt (Fig. I B), the potential of the platinum electrode may be modified over a large range of potentials while no current is flowing through the cell.

14

1

L

1

Fig. I A. Polarization curve for a reversible Redox system. Ia and Ic anodic ET reversible and cathodic currents, potentiel. CURRENT-VOLTAGE

CURVES

FOR

Fig. I 13.Case of an irreversible reaction_

qa and qc anodic tages.

A

PLATINUM

and cathodic

ELECTRODE

overvol-

PAIR

Two identical platinum electrodes dipped in the solution in which the reversible equilibrium (I) is established acquire the same potential. If a small external difference of potential of say 0.01 volt is applied between the platinum electrodes, a current flows through the cell. Oxidation occurs at one electrode while reduction takes place at the other electrode. If concentration polarization does not occur, the current flowing through the cell increases linearly with the difference of potential applied (Fig. z A). The cell behaves like a resistance obeying OHM’S law. _--_

i

--

Fig. z A. Current-voltage curve with a platinum electrodes pair for a rcversible Redox system. Refevemes

p.

640.

I

. TiFiO,:

B. Same system.

for

an

irreversible

VOL. 4 (x950)

DEAD-STOP

The slope of the straight line I =

g

637

TITRATIONS

is inversely proportional to the total resis-

tance of the circuit. Since the resistance is relatively low (a few hundred ohms) the current corresponding to an applied e.m.f. as low as 0.01 volt can be easily . measured with a portable galvanometer of low sensitivity. In the case of an irreversible process the difference of potential to be applied between the platinum electrodes should bc larger than r], + r), (Fig. I B) in order to observe a current (Fig. 2. B). As a result no current is observed in ,uch a case when a difference of potential of only o.or \*olt is applied between the platinum electrodes. VARIATION

Titrntion

OF

CURRENT

DURING

of mt irreversible

“DEAD-STOP”

Redox

system

END

POINT

by u reversible

TITRATIONS

Redox

system

The titration of thiosulfate by iodine is an example of such a case. The reactions involved are as follows: for the solution,

2

sO,-2

for the reagent, I, + Before

the end point

-

V

Fig. 3. Variations of current during the titlatlon of an irreversible Reclox system by a reversible Redox system. Same for the precipitation titration by a reversible Redox system as reagent and the precipitation titration in presence of an additional depolarizer.

S,0am2 + z e’

(1)

z e- ---t 2 I-

the solution contains

/d f

+

the species

(2) S,0,-2

and S,O,+.

The

I

.

v t

Fig. + Varlatlons of current: during the _ .. . titration 01 a reversItAe svstem hy an irreversible system.

system is current is practically zero because the thiosulfate-tetrathionate irreversible4(p*68) (Fig. 3). After the end point an excess of iodine is in solution arid a relatively large current is observed because the iodine-iodide system is Refevemes

p. 640.

P. DELAHAY

638

VOL. 4 (1950)

reversible4(p.56). The end point is thus indicated by a sudden increase of current. It should be pointed out that iodine is also present in solution before the end point, but only at an extremely low concentration. Therefore the current for the iodine-iodide couple is negligible before the end point.

Titration

of a reversible

Redox

system

by an irreversible

Redox system

The titration of a solution of iodine by thiosulfate is an example of this type of reaction. Before the end point I, and I- are present and a large current is observed since the system 1,/I- is reversible (Fig. 4). Just before the end point the iodine concentration decreases rapidly and the current decreases accordingly. At the end point the iodine concentration is extremely low and the current is practically zero. After the end point the current remains very small since the %Os-z/s,O,-e couple is irreversible. Preci$itation

litration

with a reversible

Redox system

as reagent

This case is encountered for example in the titration of a zinc salt by potassium ferrocyanide. This titration was suggested by CLIPPINCER AND FOULK~. The technique of the titration has been described by SWINEHAR+. The composition of the precipitate K,Zn,[Fe(CN),], is constant provided an excess of ammonium sulfate is present as in the case of the potentiometric determination’. The ferricyanide may be added in small quantity but in general this is not necessary because the ferrocyanide solution contains a small amount of ferricyanide resulting from the oxidation of ferrocyanide by air. Before the end point the concentration of ferrocyanide in solution is extremely after the end point the low and the current is practically zero. Immediately concentration of ferrocyanide ion increases rapidly and a large increase of current is observed because the ferrocyanide-ferricyanide couple is reversible4(p~2’2). The current thus varies as shown in Fig. 3. Zinc ion does not interfere because there is no substance in solution which could be oxidized at the potential at which the discharge of Zn+s ion takes place (E, = -o.762 volt,4 (P* ls5)).

Pveci@?ation

titration

in #wesence of an additional

depolarizer

It has been reported that the “dead-stop” end point method is applicable to the titration of halides by silver nitrate in presence of sodium nitrite”. The curve

obtained is similar to Fig. 3. The mechanism of the end point detection is the following. . Before the end point the current is practically zero because the reaction NO,is not reversible

Refsvences

although

p. 640.

+

HsO + the

NO,- +

anodic

2 Hf

overvoltage

+ 2 e’ is not very

(2)

Iarge4(P*86). Imme-

DEAD-STOP

VOL. 4 (x950)

TITRATIONS

639

diately after the end point the concentration of silver ion increases suddenly. Ag+ ion is reduced at one electrode while NO,- is oxidized at the other electrode. As a result the current increases rapidly after the end point. The reversible potential for the couple NO,-/NO,is a E = 0.84 - 0.059 px + 0.0295 log 2 If one assumes that the titration is carried out at pH y_and &at the contribution aNO,’ of the term in log--

aNO,-

is negligible, the rev&

%$potential

for reaction

(2)

is 0.43 volt. The standard potential for the Ag+/Ag couple is 0.80 volt4(m.176)and therefore the reduction of Ag+ ion occurs at a potential at which the oxidation of NO,- takes place even for low Ag+ concentrations. .PossibLe role of dissolved oxygen It is sometimes assumed that oxygen is playing a role in the mechanism accounting for the variations of current during a “dead-stop” titration. This assumption, however, is usually not justified because the O&-&O couple is very irreversible in both direction&pJs). Oxygen could bc of importance only when the reagent used in the titration is oxidized at potentials at which oxygen is actually reduced on a platinum electrode, i.e. approximately + 0.2 volt.

ACKNOWLEDGEMENT

The author is indebted to Professor P. VAN RYSSELBERGHE of the University for valuable comments on this paper.

Oregon

of

SUMMARY titrations are discussed. The causes of current variations for various “dead-stop” Redox titrations are possible provided that either the rea ent or the analyzed met %od cannot be applied solution is an irreversible Redox couple. The “dead-stop” to a Redox titration for which both the reagent and the analyzed solution are reversible or irreversible Redox couples. Precipitation titrations m which the reagent is a reversible Redox system are possible. Precipitation titrations in presence of an additional depolarizer are also possible.

L’ffuteur discute les ph&nom&nes dlectrochimiques Q la base des titragcs “deadsto Les titrages Redox pduvent etre exCcut& B condition que le reactif ou P* a solution B analyser soit un couple Redox irr&ersible. La m&hode ne peut pas etre appli u&elorsque le rkactif et la solution h analyser sont tous deux des La mdthode dst 6galemBnt applicable aux couples rkversl% les ou u-reversibles. titrages par prdcipitation dans les cas oh le reactif est un couple Redox r4vcrsible. rklpitation peuvent Bgalement Qtre exkutks apt&s addition Les titrages par d’une substance B kpolarisante. Re/erenccs

p.

640.

640

P.

VOL.

DElaAHAY

4 (1950)

ZUSAMMENFASSUNG Die Ursachen dcr Stromschwankungcn bei verschicdenen “dead ato “-Titrationen I? werdcn cr&tert. Redox-Titrationen srnd unter dcr Bedingung mijgrch, dass entwedcr das Rcagens oder die zu analysierende Lbsung ein n-reversibles RedoxMcthode kann nicht angewendct werden. SysCem darstellen. Die “dead-stop”wenn sowohl das Reagens als due XII analysierencle LBsung reversible oder irreversible Rcdoxpaare slnd. Titrationen clurch Fallung, bei denen das Reagcns tin eines reversibles Redox-System iat, sind mcjglich. hlan kann such in Gegenwart Depolarisiermittels durch 16911ung titriercn.

REFERENCES C. W. I;ouI,K AND

A -1. P,AWDEN, J. AU?. Chwz. SOL. 48 (1926) 2045. N. H. FURMAN AND E. J3. \VILSON, if&.. 50 (1928) 277. W. B&~TGER AND I-I.. 15. FOHCHE, Milz~oc/rcmzc, 30 (1942) 138. Tire Oxidalion States 01 tire E’lenle9ats and their Potottlals 291 W. M. LAT~MIZR. Aqrcoous Solztftons, Prentice Hall, Inc., New York (1938). D. R. CLIPPINGJW AND C. W. FOULK, Zwd. Enx. Chew And. G-Z., I I (1939) 21G. pa er presented at the 115th National Mcetmg of the American D. F. SWINEHART, Chemical Socmty in g an Brancisco (March 1949). I. M. I
Tdrntrons,

Received

Aprrl

p. 270,

~2th.

Wiley,

I g5o