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Reduction of indium after oxidation in fuming perchloric acid
Talanta. 1967. Vol. 14. pp. 973 to 976. Pergamon Press Ltd. Printed in Northern lreland
SHORT COMMUNICATIONS
Reduction of iridium after oxidation in fuming percbloric acid (Received 23 December 1966. Accepted 2 April 1967) RECENTpaper&* on the electrolytic reduction of iridium in perchloric acid media have contained no reference to the work carried out by Pshenitsyn et al. * These authors have suggested that the high results obtained in reduction titrations of iridium solutions which have been fumed with perchloric acid are due to reduction products of the acid remaining in combination with the iridium(IV), rather than to the existence of a higher oxidation state of iridium. They attributed the two-stage potentiometric titration curve to the reduction of one of these products followed by titration of the other simultaneously with iridium. McBryde and Cluett6 also suggested that varying amounts of oxides of chlorine remained in solution in analogous systems. The behaviour of perchlorate solutions of the platinum metals is relevant to our present study of various oxidation states of these metals in which the reduction of iridium has received particular attention.
0.4-
I 1
I 2
I 3
I 4
I 5
1 one
VOLUME
OF
HYDROQUINONE
SOLUTION
I 6 equivalent (ml
I 7
)
FIG. 1.-Effect of increasing nitrogen flushing time on the reduction
titration of iridium
after oxidation in fuming perchloric acid. (a) No nitrogen ; (b) 0.5 hr; 7
(4 1 b; (42h; 973
1 8
(e)3k
Cf,gb.
Short commumcations
974 0.67
-
O-65-
/
0.63 !
S S j
0.61
-
t 2 0.59
-
Ill a
f5 Oh w
0.57
-
0.55
-
0.531
I 0.2
I 0.4
I 0.6 Jionic
FIG. 2.-Effect
I 0.8 strength
I 1 .o
I
_
of ionic strength on the formal redox potential of the Ir(IV)-Ir(III) system in perchlorate solution.
Despite the inability of Pshenitsyn ei al. to remove the volatile products by prolonged heating of the solutions, it seemed reasonable to suppose that, if they existed, flushing of the solution with an inert gas for an extended period might accomplish this removal. EXPERIMENTAL A stock solution of chloroiridite was prepared from ammonium chloroiridate by reduction with the theoretical quantity of hydrazine hydrochloride. After boiling, the solution was diluted appropriately. The solution was standardised by the gravimetric modification of Pollard’s thiourea method**’ and was found to contain 1.10 mg of Ir/ml. Five-ml portions of this solution were evaporated to strong fumes with 1 ml of 60% perchloric acid. After dilution with 10 ml of water each solution was boiled, cooled, diluted to 100 ml, deaerated with nitrogen and titrated potentiometrically with a standard hydroquinone (0.2848 gll) or iron perchlorate solution. A platinum indicating electrode was used in conjunction with a saturated calomel electrode equipped with a lithium acetate bridge: the cell’s potential was measured with a valve voltmeter. Figure la is a typical curve and is similar to those obtained by Pshenitsyn et al. Further 5-ml portions of the chloroiridite solution were evaporated to strong fumes with perchloric acid. After dilution, each solution was transferred to a flask and a stream of nitrogen or carbon dioxide was passed through it at a flow rate of 10 l/hr, with the temperature of the solution maintained almost at boiling point. Great care was taken to avoid extraneous reduction and evaporation: an all-glass assembly was used, and the nitrogen was passed through a series of flasks containing acidified permanganate and finally a flask of water before it entered the test solution. After an appropriate time each iridium solution was titrated. Curves b-f in Fig. 1 are those obtained with increasing flushing time. When no gas was admitted the titration curve remained substantrally as Fig. la even after more than 8 hr. The two gas streams were equally effective, but use of carbon dioxide generated in situ, although showing some reduction in the titration volume, was impracticable since large quantities of carbonates had tobeaddedforacorrespondingly small reduction in the volume of titrant needed. In an analogous
Short communications
VOLUME
OF
FIG. 3.-Reduction
HYDROQUINONE
SOLUTION
975
(ml)-
titrations of distillates and fumed perchloric acid.
(a) Distillate trap solution; (b) Pure perchloric acid strongly fumed and diluted only; (c) Pure perchloric acid strongly fumed and degassed. case of chromium oxidized with fuming perchloric acid, in situ generation of carbon dioxide proved successful.~ RESULTS AND DISCUSSION A titration, stoichiometric for a one-electron reduction, was obtained after 8 hr or more of nitrogen or carbon dioxide treatment; further extension reduced the volume of titrant needed no further and in all cases the solution at the final intlexion had all the properties of that of iridium(III) alone. No further inflexions could be detected even with large excesses of reducing agent. The titration curve remained very flat compared with that for chloroiridate reduction, which gives a normal curve for a one-electron change, and the slope of a plot of potential vs. log [Ir(IV)]/I&o] gave an apparent electron change of 4. A further anomaly was found to be the positive shift in the E”rOrmti value with increasing ionic strength of the solution (Fig. 2), similar effects being obtained with perchloric acid and with magnesium perchlorate as added electrolyte. This is the behaviour expected of an anionic system,O whereas ion-exchange tests firmly indicated that the iridium is in the cationic form. In view of the findings of Dwyer et a1.Othat potentials determined from titration curves of chloroiridate were appreciably lower than those obtained from direct potential measurements of I&&*--IrC&*- mixtures, such errors might exist also in the case of the present system, but this should not account for the direction of change of EOrormd with ionic strength. Attempts to collect and identify chemically any volatile products proved unsuccessful even with amounts of iridium up to 50 mg. Since these products would be evolved over the 8-hr period in a volume of some 80 1. of gas their detection would be dithcult. However, the distillatetrap solutions had a highly positive potential which decreased on addition of standard hydroquinone solution. Comparison was made with potentiometric titrations of strongly fumed and diluted pure percbloric acid, with and without degassing (Fig. 3) and this suggests the presence of highly oxidizing species both in the strongly fumed perchloric acid and in the distillate-trap solution. No further oxidizing species were detected in the trap after the 8-hr flushing period.
Short communications
976
Figure 1 shows that the nitrogen treatment eliminates the first inllexion point relatively rapidly, while the position of the second remains substantially constant. Only when the first inflexion disappears does the position of the second begin slowly to decrease to give ultimately a stoichiometric one-electron reduction titre. This could suggest, if distortion is not excessive, the existence of two distinct oxidiig species, the tist of which may be. “free” and is removed relatively easily, while the second appears to be more firmly “bound” and is removed with greater difficulty and is characteristic of the presence of iridium in the solution. In general, this work supports the view of Pshenitsy+ that strongly-oxidizing volatile species, possibly Cl*0 and Cl* originating from the perchloric acid, reach high proportions by some form of association when iridium is present: one of the products gives a separate intlexion in the potential curve, the other is titrated along with the iridium(IV) to give an apparently single step. This work also indicates that a higher oxidation state of iridium is not likely to be present. Removal of the products by lengthy displacement with an inert gas is similar to what happens in the case of chromium, which is known* not to be in a higher oxidation state. Nevertheless, the removal is remarkably slow, which indicates either that the oxidizing products are rather involatile (which should have mad; them easier to detect in the exhaust gases) 06 more likely, that they are at least weakly associated with the iridium or perhaps the perchloric acid in solution. The anomalous shift in the formal electrode potential is receiving further attention in this connection. Acknowledgements-Thauks iridium salt.
are expressed
to the Sheffield Smelting Co. Ltd. for supplying
the
Department of Chembtry and Metallurgy Sheffield College of Technology Shefield, England
E.
Department of Metallurgy Imperial College London, s. w.7.
D. A. PANTONY
JACKSON
Smmna~-Evidence is given in support of the theory that high results obtained for reduction titrations of iridium solutions which have been fumed with perchloric acid are due to retention in solution of volatile reduction products of the acid. Zusammenfassung-Zu hohe Ergebnisse bei Reduktionstitrationen von mit oberchlors&re abgerauchten Iridiumlosungen kiinnten vom Verbleiben fltichtigcr Reduktionsprodukte der Sllure in der Liisung herruhren. Hinweise auf die Richtigkeit dieser Theorie werden angcgeben. R&sun&-On donne des arguments B l’appui de la th&orie suivant laquelle les r&hats 61evb obtenus dans les dosages par reduction des solutions d’iridium qui ont et6 chaulf~s avec de l’acide perchlorique sont dfis a la retention en solution de produits de reduction volatils de l’acide. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
J. A. Page and E. J. Zinser, Takmta, 1965,12,1051. G. A. Rechnitz and J. McClure, ibid., 1963, 10,417. J. A. Page. ibfd., 1962. 9, 365. N. K. P&&itsyn, S. &&burg and L. G. Salskaya, Russian J. Inorg. Chem., 1959,4,130. W. A. E. McBrvde and M. L. Cluett. Canadian J. Res., 1950.2SB. 788. W. B. Pollard, &zns. Inst. Min. Met:, 1948, 57, 377 B&l. Znsi. Mih. Met., 1948, 67, No. 497, 9. E. Jackson, Analyst, 1959,84,106. B. Bag&awe, ibid., 1959,84,475. F. P. Dwyer, H. A. McKenzie and R. S. Nyholm, J. Proc. Roy. Sot. N.S. W., 1944,78,260.
SHORT COMMUNICATIONS
Reduction of iridium after oxidation in fuming percbloric acid (Received 23 December 1966. Accepted 2 April 1967) RECENTpaper&* on the electrolytic reduction of iridium in perchloric acid media have contained no reference to the work carried out by Pshenitsyn et al. * These authors have suggested that the high results obtained in reduction titrations of iridium solutions which have been fumed with perchloric acid are due to reduction products of the acid remaining in combination with the iridium(IV), rather than to the existence of a higher oxidation state of iridium. They attributed the two-stage potentiometric titration curve to the reduction of one of these products followed by titration of the other simultaneously with iridium. McBryde and Cluett6 also suggested that varying amounts of oxides of chlorine remained in solution in analogous systems. The behaviour of perchlorate solutions of the platinum metals is relevant to our present study of various oxidation states of these metals in which the reduction of iridium has received particular attention.
0.4-
I 1
I 2
I 3
I 4
I 5
1 one
VOLUME
OF
HYDROQUINONE
SOLUTION
I 6 equivalent (ml
I 7
)
FIG. 1.-Effect of increasing nitrogen flushing time on the reduction
titration of iridium
after oxidation in fuming perchloric acid. (a) No nitrogen ; (b) 0.5 hr; 7
(4 1 b; (42h; 973
1 8
(e)3k
Cf,gb.
Short commumcations
974 0.67
-
O-65-
/
0.63 !
S S j
0.61
-
t 2 0.59
-
Ill a
f5 Oh w
0.57
-
0.55
-
0.531
I 0.2
I 0.4
I 0.6 Jionic
FIG. 2.-Effect
I 0.8 strength
I 1 .o
I
_
of ionic strength on the formal redox potential of the Ir(IV)-Ir(III) system in perchlorate solution.
Despite the inability of Pshenitsyn ei al. to remove the volatile products by prolonged heating of the solutions, it seemed reasonable to suppose that, if they existed, flushing of the solution with an inert gas for an extended period might accomplish this removal. EXPERIMENTAL A stock solution of chloroiridite was prepared from ammonium chloroiridate by reduction with the theoretical quantity of hydrazine hydrochloride. After boiling, the solution was diluted appropriately. The solution was standardised by the gravimetric modification of Pollard’s thiourea method**’ and was found to contain 1.10 mg of Ir/ml. Five-ml portions of this solution were evaporated to strong fumes with 1 ml of 60% perchloric acid. After dilution with 10 ml of water each solution was boiled, cooled, diluted to 100 ml, deaerated with nitrogen and titrated potentiometrically with a standard hydroquinone (0.2848 gll) or iron perchlorate solution. A platinum indicating electrode was used in conjunction with a saturated calomel electrode equipped with a lithium acetate bridge: the cell’s potential was measured with a valve voltmeter. Figure la is a typical curve and is similar to those obtained by Pshenitsyn et al. Further 5-ml portions of the chloroiridite solution were evaporated to strong fumes with perchloric acid. After dilution, each solution was transferred to a flask and a stream of nitrogen or carbon dioxide was passed through it at a flow rate of 10 l/hr, with the temperature of the solution maintained almost at boiling point. Great care was taken to avoid extraneous reduction and evaporation: an all-glass assembly was used, and the nitrogen was passed through a series of flasks containing acidified permanganate and finally a flask of water before it entered the test solution. After an appropriate time each iridium solution was titrated. Curves b-f in Fig. 1 are those obtained with increasing flushing time. When no gas was admitted the titration curve remained substantrally as Fig. la even after more than 8 hr. The two gas streams were equally effective, but use of carbon dioxide generated in situ, although showing some reduction in the titration volume, was impracticable since large quantities of carbonates had tobeaddedforacorrespondingly small reduction in the volume of titrant needed. In an analogous
Short communications
VOLUME
OF
FIG. 3.-Reduction
HYDROQUINONE
SOLUTION
975
(ml)-
titrations of distillates and fumed perchloric acid.
(a) Distillate trap solution; (b) Pure perchloric acid strongly fumed and diluted only; (c) Pure perchloric acid strongly fumed and degassed. case of chromium oxidized with fuming perchloric acid, in situ generation of carbon dioxide proved successful.~ RESULTS AND DISCUSSION A titration, stoichiometric for a one-electron reduction, was obtained after 8 hr or more of nitrogen or carbon dioxide treatment; further extension reduced the volume of titrant needed no further and in all cases the solution at the final intlexion had all the properties of that of iridium(III) alone. No further inflexions could be detected even with large excesses of reducing agent. The titration curve remained very flat compared with that for chloroiridate reduction, which gives a normal curve for a one-electron change, and the slope of a plot of potential vs. log [Ir(IV)]/I&o] gave an apparent electron change of 4. A further anomaly was found to be the positive shift in the E”rOrmti value with increasing ionic strength of the solution (Fig. 2), similar effects being obtained with perchloric acid and with magnesium perchlorate as added electrolyte. This is the behaviour expected of an anionic system,O whereas ion-exchange tests firmly indicated that the iridium is in the cationic form. In view of the findings of Dwyer et a1.Othat potentials determined from titration curves of chloroiridate were appreciably lower than those obtained from direct potential measurements of I&&*--IrC&*- mixtures, such errors might exist also in the case of the present system, but this should not account for the direction of change of EOrormd with ionic strength. Attempts to collect and identify chemically any volatile products proved unsuccessful even with amounts of iridium up to 50 mg. Since these products would be evolved over the 8-hr period in a volume of some 80 1. of gas their detection would be dithcult. However, the distillatetrap solutions had a highly positive potential which decreased on addition of standard hydroquinone solution. Comparison was made with potentiometric titrations of strongly fumed and diluted pure percbloric acid, with and without degassing (Fig. 3) and this suggests the presence of highly oxidizing species both in the strongly fumed perchloric acid and in the distillate-trap solution. No further oxidizing species were detected in the trap after the 8-hr flushing period.
Short communications
976
Figure 1 shows that the nitrogen treatment eliminates the first inllexion point relatively rapidly, while the position of the second remains substantially constant. Only when the first inflexion disappears does the position of the second begin slowly to decrease to give ultimately a stoichiometric one-electron reduction titre. This could suggest, if distortion is not excessive, the existence of two distinct oxidiig species, the tist of which may be. “free” and is removed relatively easily, while the second appears to be more firmly “bound” and is removed with greater difficulty and is characteristic of the presence of iridium in the solution. In general, this work supports the view of Pshenitsy+ that strongly-oxidizing volatile species, possibly Cl*0 and Cl* originating from the perchloric acid, reach high proportions by some form of association when iridium is present: one of the products gives a separate intlexion in the potential curve, the other is titrated along with the iridium(IV) to give an apparently single step. This work also indicates that a higher oxidation state of iridium is not likely to be present. Removal of the products by lengthy displacement with an inert gas is similar to what happens in the case of chromium, which is known* not to be in a higher oxidation state. Nevertheless, the removal is remarkably slow, which indicates either that the oxidizing products are rather involatile (which should have mad; them easier to detect in the exhaust gases) 06 more likely, that they are at least weakly associated with the iridium or perhaps the perchloric acid in solution. The anomalous shift in the formal electrode potential is receiving further attention in this connection. Acknowledgements-Thauks iridium salt.
are expressed
to the Sheffield Smelting Co. Ltd. for supplying
the
Department of Chembtry and Metallurgy Sheffield College of Technology Shefield, England
E.
Department of Metallurgy Imperial College London, s. w.7.
D. A. PANTONY
JACKSON
Smmna~-Evidence is given in support of the theory that high results obtained for reduction titrations of iridium solutions which have been fumed with perchloric acid are due to retention in solution of volatile reduction products of the acid. Zusammenfassung-Zu hohe Ergebnisse bei Reduktionstitrationen von mit oberchlors&re abgerauchten Iridiumlosungen kiinnten vom Verbleiben fltichtigcr Reduktionsprodukte der Sllure in der Liisung herruhren. Hinweise auf die Richtigkeit dieser Theorie werden angcgeben. R&sun&-On donne des arguments B l’appui de la th&orie suivant laquelle les r&hats 61evb obtenus dans les dosages par reduction des solutions d’iridium qui ont et6 chaulf~s avec de l’acide perchlorique sont dfis a la retention en solution de produits de reduction volatils de l’acide. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
J. A. Page and E. J. Zinser, Takmta, 1965,12,1051. G. A. Rechnitz and J. McClure, ibid., 1963, 10,417. J. A. Page. ibfd., 1962. 9, 365. N. K. P&&itsyn, S. &&burg and L. G. Salskaya, Russian J. Inorg. Chem., 1959,4,130. W. A. E. McBrvde and M. L. Cluett. Canadian J. Res., 1950.2SB. 788. W. B. Pollard, &zns. Inst. Min. Met:, 1948, 57, 377 B&l. Znsi. Mih. Met., 1948, 67, No. 497, 9. E. Jackson, Analyst, 1959,84,106. B. Bag&awe, ibid., 1959,84,475. F. P. Dwyer, H. A. McKenzie and R. S. Nyholm, J. Proc. Roy. Sot. N.S. W., 1944,78,260.