Polarography in acetonitrile of some group-IV metal halides

ELECTROANALYTICAL CHEMISTRY AND INTERFACIAL ELECTROCHEMISTRY Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

423

POLAROGRAPHY IN ACETONITRILE OF SOME GROUP-IV METAL HALIDES*

F. G~THOMAS** ANt) I. M. KOLTHOFF

School of Chemistry, University of Minnesota, Minneapolis, Minn. 55455 (U.S.A.) (Received 1st March 1971)

INTRODUCTION

In dilute aqueous solution all group-IV metals in their tetravalent state are extensively hydrolyzed unless complexing agents are present. Thus, the polarography of these tetravalent metals depends greatly on the pH of the solution and the concentration of any complexing agents 1. The IVA metals, titanium and zirconium, are reduced from the 4 + to the 3 + state in aqueous solution at the dropping electrode (DME). The waves are usually irreversible even in the presence ofcomplexing agents and at low pH. The group-IVB metals, germanium and tin, can be reduced from the 4 + state to the metal at the D M E in aqueous solution. In neutral or mildly alkaline solution, germanium(IV) is reduced to germanium(0) at the DME in various supporting electrolytes, while in acidic solutions no reduction has been observed z- 5. Tin(IV) is not reduced in alkaline solution at the D M E but is reduced in acidic solutions of certain complexing agents. In concentrated chloride solutions at low pH, where the predominant species is SnC12-, two reduction waves corresponding to the reduction of Sn(IV) to Sn(II) and Sn(0) respectively, are observed 6. Two reduction waves of equal height are observed in the reduction of tin(IV) in acid perchlorate solutions containing pyrogallol which forms complexes with tin(IV) but not with tin(II) 7. In concentrated bromide solutions of pH 1-3, tin(IV) gives a single four-electron reduction wave even in the presence of oxalate and EDTA 8"9. Because acetonitrile (AN) is a much weaker base towards the proton than water and has very little tendency to undergo protolytic dissociation1 o, the solvolysis of the group-IV metal halides does not occur in this solvent. However, AN can act as a fairly strong Lewis base and the tetrafluorides, chlorides and bromides of titanium, zirconium 11 and tin az form stable adducts of the type MX 4" 2 AN which are soluble in AN. The tetraiodides of these elements dissolve in AN without solvolysis but no stable adducts have been isolated. The germanium tetrahalides also dissolve in AN but only the tetrafluoride gives isolatable adducts 13. The polarography of the tetrachlorides of titanium, zirconium and hafnium and of titanium tetraiodide in AN has been previously reported 14-16. * This work was supported by the Directorate of Chemical Sciences Air Force Office of Scientific Research under grant AF-AFOSR-28-63 a n d b y the Hall Bequest. ** Chemistry Department, James Cook University of North Queensland, Townsville, Queensland 4810, Australia.

J. Electroanal. Chem., 31 (1971) 423-439

424

F.G. THOMAS,I. M. KOLTHOFF"

The current work is an exploratory study of the polarography of tin(IV) chloride and iodide, tin(II) chloride, germanium(IV) chloride, and zirconium(IV) chloride in AN as solvent using tetraethylammonium perchlorate (TEAP), tetraethylammonium chloride (TEAC1) and tetraethylammonium iodide (TEAI) as supporting electrolytes. Virtually all polarographic waves described in this paper are irreversible and a conclusive interpretation of the characteristics of several waves has been impossible at this time and would require the use of techniques other than d.c. polarography. In addition, the conductance of solutions of the above group-IV metal halides in AN was measured in order to obtain information on the type and magnitude of ionization that occurs in these solutions. As with titanium(IV) chloride 14, reaction between tin(IV) chloride and mercury to produce Hg2C12 was observed both in the presence and absence of air, and oxygen was found to have a pronounced effect on systems containing iodide. EXPERIMENTAL

Cells. The polarographic cell used was designed so that mercury did not remain in contact with the solution after dropping from the DME ~4. The conductance vessel used had a cell constant of 0.0366 cm- ~ and resistances of solutions in this cell were measured with an a.c. bridge to + 1%. Electrodes. The dropping mercury electrode used in this work had the following characteristics at 0.0 V vs. SCE in 0.1 M tetraethylammonium perchlorate in AN : m=1.551 mg s -~, t=4.15 s (m~t~= 1.70 mg~s -½) at a mercury height of 52,5 cm (corr.). An aqueous saturated calomel electrode (SCE) fitted with an agar-KC1 salt bridge was used as reference electrode. Polarograph. A Leeds and Northrup Electrochemograph Type E was used to record current-potential curves. Materials Acetonitrile (Eastman-Kodak practical grade) was purified by the method of Coetzee 17. The purity and the physical properties were the same as previously reported 14. Tin(IV) chloride (Matheson, Coleman and Bell purified grade) was doubly distilled under nitrogen in an all-glass apparatus. B.p. 113.3-113.8°C at 745 mm Hg. Analysis: Sn 45.38%, C1 54.20%*; calculated: Sn 45.56% and Cl 54.44%. Tin(II) chloride dihydrate (Fisher reagent grade) was heated to 150°C in a stream of dry nitrogen and hydrogen chloride to remove water and the anhydrous material sublimed in a stream of dry nitrogen at 450°C. Analysis : C1 37.30~oo ; calculated : 37.40%. Tin(I V) iodide, prepared as described in the literature ~8 was twice recrystallized from carbon tetrachloride and dried under vacuum. Analysis : Sn 18.90%, 1 80.92% ; calculated: Sn 18.95% and I 81.05%. Germanium(IV) chloride. A high-purity sample was obtained from Dr. D. * All metalswere determinedgravimetricallyas the oxide, MO2, after hydrolysisof the metal halide to give the hydratedoxidewhichwas then ignited.Halogencontentwas determinedafteracidhydrolysisby potentiometrictitration with standard 0.05M silver nitrate.

J. Electroanal. Chem.,31 (1971)423-439

425

POLAROGRAPHY OF GROUP-IV METAL HALIDES

Britton of the Inorganic Chemistry Division. Analysis: C1 66.25~o; calculated: 66.14~. Zirconium(IV) chloride (Matheson, Coleman and Bell reagent grade) was used without further purification. Analysis : Zr 39.47~o, C1 60.23~ ; calculated : Zr 39.14~o and CI 60.86~. TEAP and TEACI were prepared as previously described 19,14. TEAI (Eastman-Kodak reagent grade) was recrystaUized twice from water. All solid materials were stored over phosphorous pentoxide in a desiccator. Nitrogen (Linde 99.9~o) was used and purified as previously described t4.

Technique Current potential curves using the DME were determined as previously described 14 at 25.00 + 0.02°C. Potential data refer to the aqueous SCE and have been corrected for the i-R drop. Reported currents have been corrected for residual current. Nitrogen was pre-saturated with AN at 25°C before being passed through the polarographic cell. As a precaution against atmospheric oxidation all solutions containing iodide (either as iodide ion or as tin(IV) iodide) and/or tin(II) were preTABLE 1 MOLAR CONDUCTANCESOF GERMANIUM(IV), TIN(IV) AND TIN(U) CHLORIDES AND OF TIN(IV) IODIDE IN ACETONITRILE AT 25°C 10 a c/mol l- 1

Molar conductance/

102 ct

~ - 1 em 2 tool- 1

Germanium(IV) chloride

40.96 18.20 8.09 3.60 1.60 0.71

0.058 0.129 0.313 0.647 1.20 2.12

0.03z 0.072 0.17 0.36 0.66 1.2

Tin(IV) chloride

40.15 20.08 10.04 5.02 2.51 1.26 0.627 0.314

0.63 1.10 1.89 2.75 3.71 4.42 5.03 5.75

0.35 0.61 1.% 1.5 2.1 2.5 2.8 3.2

Tin(II) chloride

7.341 3.477 ! .647 0.580 0.275

7.44 8.24 8.93 9.73 10.52

4.13 4.58 4.96 5.41 5.84

Tin(IV) iodide

2.778 0.926 0.581 0.231 0.116

0,596 1,12 1.48 2.21 2.86

0.33 0.62 0.82 1.23 1.59

J. Electroanal. Chem., 31 (1971) 423-439

426

F . G . THOMAS, I. M. KOLTHOFF

pared using oxygen-free AN (obtained by passing nitrogen through AN for two hours prior to use) and all operations involving these solutions were performed in a nitrogen filled glovebox. RESULTS

Conductivity measurements The molar conductivities of solutions of germanium(IV) chloride, tin(IV) chloride, tin(IV) iodide and tin(II) chloride in AN are given in Table 1. It was noted that solutions of tin(IV) iodide in AN changed color over several hours from bright orange to red brown indicating that free iodine was being produced. Also the conductance of 0.231 mM SnI 4 in AN doubled in 45 minutes. This was found to be due to decomposition of the solute by both oxygen present in solution and light (cf titanium(IV) iodide14). Solutions of tin(IV) iodide made up in oxygen-free AN and stored overnight in darkened bottles showed less than a 10~ increase in conductance and no discernable color change. Because of this all measurements involving tin(IV) iodide were carried out on freshly prepared solutions made up under oxygen-free conditions in blackened vessels.

C

B

<

L)

A

0~" ~

+0.6

+

0

- 0.5

EDIviE/V(VS.SCE)

-1.0

-1.4

Fig. 1. Polarograms of tin(IV) chloride in AN with 0.1 M TEAP as supporting electrolyte. (A) 0.20 raM, (B) 1.00 mM, (C) 1.36 mM tin(IV) chloride. J. Electroanal. Chem., 31 (1971) 423439

427

POLAROGRAPHY OF GROUP-IV METAL HALIDES

Polarographic measurements Tin(IV) chloride. The current-potential curves of 0.20, 1.00 and 1.36 m M SnCl 4 in A N using 0.1 M T E A P as supporting electrolyte are shown in Fig. 1. The characteristics of the two cathodic waves are listed in Table 2. TABLE 2 CHARACTERISTICS OF CURRENT--POTENTIAL CURVES OF VARIOUS GROUP-IV HALIDES IN ACETONITRILE SOLUTIONS OF VARIOUS SUPPORTING ELECTROLYTES

Halide

Supportin9 electrolyte

Reaction at D M E

ID*/ ltA m M I mo -¢ s ~

E½*/V (vs. SCE)

Color of solution

SnC14

0.1 M TEAP

e Sn(IV) + 2e = Sn(IV) + 4e = Sn(IV) + 4e = Sn(IV) + 2e = Sn(IV) + 4e =

-3.70 ~ 10.57 10.40 4.52 10.49

+0.47 + 0.3 a - 1.06b - 1.05 ** - 1.07

Colorless

0.1 M TEAC1 0.1 M TEA1

SnI 4

0.1 M TEAP

0.1 M TEAC1 0.I M TEAl GeC14

0.l M TEAP

0.1 M TEACI 0.l M TEAl

SnCI 2

0.1 M TEAP

0.1 M TEACI 0.1 M TEA1

Sn(II) Sn(0) Sn(0) Sn(II) Sn(0)

Colorless Orange

e Sn(IV) + 2e = Sn(II) Sn(IV) + 4e = Sn(0) Sn(IV) + 4e = Sn(0) Sn(IV) + 4 e = S n ( 0 )

poorly defined 4.82 c 13.63 10.58 11,38

+0.25 + 0.11 ca. -0.4IV - 1.05 **

Bright orange

Anodic Ge(IV) + Ge(IV) + Ge(IV) + Ge(IV) + Ge(IV) +

2,10 4.78 10.94 11.54 -11.23

+0.33 -0.16 - 1.10 - 1.27 ** ca. - 1.0

Colorless

2e= 4e = 4e = 2e = 4e =

Ge(II) Ge(0) Ge(0) Ge(II) Ge(0)

Split wave See text Split reduction wave Sn(lI) + 2e = Sn(0) Sn(II) + 2e = Sn(0) Sn(II) + 2 e = Sn(0) (appears to be a split reduction wave)

1.96 c 4.62 1.34c 5.14 6.80 5.81

+0.38 +0.54 a -0.08 b - 0.723 - 0.852 ?

Pale yellow Orange-brown

Colorless Golden-yellow Colorless

Colorless Colorless

* Characteristics for 1.00 mM solutions of the group-IV halide. ** Composite with anodic halide wave. " E~ becomes more negative with increasing concentration, b E~ becomes more positive with increasing concentration. c IDincreases with increasing concentration, d ID decreases with increasing concentration, e Ill-defined anodic wave,

The second cathodic wave with a diffusion current constant It) of 10.57 corresponds to the reduction to Sn(0). This wave is welll defined and the diffusion current is proportional to concentration in the concentration range 0.068-2.00 mM. The reduction is irreversible (slope of log i/(i~- i) vs. Et)M~ plot is --0.127 V) and the halfwave potential varies from - 1.075 V for 0.068 m M SnC14 to - 1.030 V vs. SCE for 2.00 m M SnCI,. The first cathodic wave is not well defined and may consist of two waves. The current between - 0.8 V and - 0.9 V, considered to be the limiting current for the two-electron reduction of Sn(IV) to Sn(II), is not proportional to concentraJ. Electroanal. Chem., 31 (1971) 423-439

428

F . G . THOMAS, I. M. KOLTHOFF B

3.9

3.~

3.7 0

3.e

nh

",,.

A

3.5 +

5

-0.5

EDME/V(VS.SCE)

-1.0

-1.4

Fig. 2. (A) Drop time curve for 0.1 M TEA P; (B) as A with 1 mM tin (IV) chloride; (C) profiles of currenttime curves observed with solution as for B.

2C

lC

-10

t + 0.5

0

J - 0.5

EI)ME/V(vs.SCE)

~ -1.0

-1.4

Fig. 3. Effect of chloride on the polarograms of I m M tin(IV) chloride with (A) 1, (B) 2, (C) 100 mM TEACI.

tion; ID decreases from 4.10 for 0.068 mM SnCI~ to 3.53 for 2.00 mM SnCI4. Two maxima, which Occur on the plateau of the two-electron wave at - 0 , 2 7 V vs. SCE and - 0 . 6 7 V vs. SCE, respectively, complicate the interpretation of the polarogram. The height of these maxima increases with increasing SnCI~ concentration (Fig. 1, curves A, B and C). The drop time curve and current-time curves for the individual drops at various voltages are shown in Fig. 2. These indicate that absorption processes are changing the double-layer capacitance of the electrode-solution interface at the potentials where the maxima occur. The limiting currents (corrected for residual current) of the two cathodic waves appear to be diffusion controlled since the values of it(h .... )-½ (where it is the current when the drop detaches) show only a slight increase with increasing mercury height. The values for 1 mM tin(IV) chloride are for the first wave at - 0 . 9 V: 1.00 for hcorr=50.5 cm ahd 1.04 for hcorr=82.5 cm, and for the second wave at - 1.4 V: 2.77 for hco, = 50.5 cm and 2.91 for h.... = 82.5 cm. A small, ill-defined anodic wave is also observed. J. Electroanal. Chem., 31 (1971) 423-439

POLAROGRAPHY OF GROUP-IV METAL HALIDES

429

Addition of a little TEAC1 causes a rapid decrease in the limiting current of the first cathodic wave, e.9. in the presence of 2 m M TEACI (Fig. 3, curve B) the first cathodic wave of 1 m M SnC14 is only 10% of the height of the two-electron wave observed in the absence of added chloride and only one small maximum is present at - 0,3 V. In the presence of 8 m M TEAC1 only one cathodic wave is observed with E~ = - 1.04 V. The characteristics of this wave hardly change on the addition of more chloride to the system. The current-potential curve of 1 m M SnCI4 in 0.1 M TEACI as supporting electrolyte is also presented in Fig. 3, curve C. It consists of a single irreversible wave corresponding to the four-electron reduction of Sn(IV) to Sn(0). In the presence of 1 and 2 m M TEAI the maximum at -0.67 V disappears, but it becomes extremely large when 0.1 M TEAl is added. In the presence of 1 to 100 m M TEAI as supporting electrolyte there are two cathodic waves corresponding to the reduction of Sn(IV) to Sn(II) and Sn(0), respectively. In 0.1 M TEAI the first cathodic wave is composite with the anodic iodide wave (Fig. 4, curve C), the second wave is irreversible.

[°5/ v

6

-B

EDME/V (VS.SCE)

Fig. 4. Effect of iodide on the polarograms of I m M tin(IV) chloride with (A) 1, (B) 2, (C) t00 m M TEA1.

Tin(1V) iodide. In 0.1 M TEAP supporting electrolyte one anodic and two cathodic waves are observed. These are shown in Fig. 5, curves A and B, and the characteristics for 1 m M SnI4 are listed in Table 2. The anodic wave is poorly defined for tin(IV) iodide concentrations above 0.3 raM. Although the height of this wave is only about 6 0 ~ of the height of the four-electron cathodic wave, it may be due to a four-electron oxidation of the type14: SnI 4 + 4 Hg = Sn(IV) + 2 Hg2I 2 + 4e-

(1)

the current being affected by Hg212 film formation. The first cathodic wave is irreversible and not composite with the anodic wave. The height of this wave is not proportional to concentration, the value of 1D varying from 4.06 in 0.27 m M SnI4 to 4.82 in J. Electroanal. Chem., 31 (1971) 423-439

430

F . G . THOMAS, I. M. KOLTHOFF

4C

\

B D

20

10

:2.

~o 8 -10

-2T,

V

, o 4.5 EDME/V(V$, SCE)

20

-1.4

Fig. 5. Polarograms of tin(IV) iodide in various supporting electrolytes. The symbol + indicates zero current and potential (A) 0.27 mM SnI4, 0:1 M TEAP; (B) 1 mM SnI4, 0.1 M TEAP; (C) 1 mM SnI4, 0.1 M TEAC1; (D) 1 mM Snip, 0.1 M TEAI.

1.0 m M SnI4. The second cathodic wave is well developed after a small maximum and the current at - 0 . 8 V is proportional to concentration. The effect of adding small amounts of TEAC1 to a 1.00 m M solution of SnI4 in 0.1 M TEAP supporting electrolyte is shown in Fig. 6. At chloride concentrations of 1 to 4 m M tile two cathodic waves of SnI 4 with E½ values of +0.11 V and - 0 . 4 0 V, respectively, are decreased, and the development of a new cathodic wave with an E½ of - 1.05 V,is apparent. In 0.1 M TEAC1 as supporting electrolyte a single 4-electron reduction Wave is also observed at - 1.05 V (Fig. 5, curve C). The characteristics of this wave are similar to those observed for 1.0 m M SnCI4 in the same supporting electrolyte. The addition of small amounts of TEAI (Fig. 6, curve D) results in the successive decrease of the first cathodic wave until it becomes an anodic wave at iodide concentrations of 2 m M or more. The 4-electron reduction wave is hardly changed except for a slight reduction in the final limiting current. The changes in the anodic portion of the current-potential curve are similar to those observed on the addition of small amounts of chloride. In 0.1 M TEAI as supporting electrolyte only a single four-electron reduction wave, composite with the anodic iodide wave is observed. (Fig. 5, curve D), The lo for this wave is some J. Electroanal. Chem., 31 (1971) 423-439

431

POLAROGRAPHY OF GROUP-IV "METAL HALIDES

3°I

tI

. . . . . . . .

D

/ i

/

9(3 •

A

<

u

-3C _351 i -I-0.5

!/

I OEDME/V

I (VS. S C E )

-os

-;.o

-';.4'

Fig. 6. Effect of small concentrations of TEAC] and TEA1 on polarograms of tin (IV) iodide in 0.] M TEAP.

(A) 1, (B) 2, (C) 4 mM chloride, respectively; (D) 1 mM iodide.

1 5 ~ smaller than that ini0.1 M TEAP. Germanium(IV) chloride. In ffl M TEAP solution, three well-defined waves are observed, two cathodic and one anodic as shown in Fig. 7, curve A. The characteristics of these waves ar~listed in Table 2. The limiting currents for all three waves are proportional to con~ntration over the concentration range 0.195 to 1.30 mM. The electrode reaction which occurs on the anodic wave is the subject of further study. The second cathodic wave is extremely drawn out and may consist of several waves. Successive addition of chloride to the system results in the gradual elimination: of the first cathodic wave and a shift in the half-wave potentials of the second cathodic wave to slightly' more negative potentials. In 0.1 M TEACI supporting electrolyte (Fig. 7, curve B) the first cathodic wave is composite with the anodic chloride wave and is very small. The second wave is well developed, and though still irreversible, is nowhere near as drawn out as in perchlorate supporting electrolyte. The addition of small amounts of iodide has little effect (Fig. 7, curve D) on the polarogram, except for producing a well-defined anodic iodide wave, which is composite with the first reduction wave. In 0.1 M TEAI solution two cathodic waves are J. Electroanal. Chem., 31 (1971) 423439

432

F.G. THOMAS, I. M. KOLTHOFF

25 B

2O

B

-10

= DJ +0.5

w 0

C

-(~.5

-1~0

-115

EDME/V(VS. SCE)

-2.0

Fig. 7. Polarograms of 1 mM germanium(IV) chloride with various supporting electrolytes. (A) 0.1 M TEAP, (B) 0.1 M TEACI, (C) 0.1 M TEAl, (D) as A with 1 mM TEAl added.

15F

c

EDME//V(vS. SCE)

Fig. 8. Polarograms of 1 mM tin(II) chloride with various supporting electrolytes. (A) 0.1 M TEAP, (B) as A with 1 mM TEAC1, (C) 0.1 M TEAC1, (D) as A with 1 mM TEAI, (E) 0.1 M TEAI.

observed (Fig. 7, curve C). The first is composite with the anodic iodide wave and has a large maximum. The second wave is rather drawn out but has a well-developed plateau current. Tin(II) chloride. Polarograms of tin(II) chloride with various supporting electrolytes are reproduced in Fig. 8, with 0.1 M TEAP. The characteristics of the two anodic and two cathodic waves are listed in Table 2. All four waves are irreversible. The values of both the total anodic current at + 0.6 V and the total cathodic current at - 1.0 V are proportional to concentration over the range 0.2 m M to 1.5 m M . J. Electroanal. Chem., 31 (1971) 423-439

POLAROGRAPHYOF GROUP-IVMETALHALIDES

433

The anodic wave at +0.38 V and the first cathodic wave are not proportional to concentration, the ID values for both waves decrease with decreasing concentration of tin dichloride. The addition of 1 m M chloride to 1 m M tin(II) chloride solution in 0.1 M T E A P results in the disappearance of the first cathodic wave and only a singly two-electron reduction wave is observed at - 0 . 7 3 V (Fig. 8, curve B). Again two anodic waves at +0.32 V and +0.5 V are observed. These correspond to the two anodic waves in the absence of chloride but are increased in magnitude. Further addition of chloride causes the half-wave potential of the single cathodic wave to be shifted to more negative potentials. This wave becomes more reversible on increasing the chloride ion concentration and the ID value does not change significantly at concentrations of chloride greater than 2 m M (see Table 3). In 0.1 M TEAC1 the slope TABLE 3 EFFECT OF

TEACI

ON THE CHARACTERISTICS OF THE CATHODIC WAVE OF TIN(II) CHLORIDE

lO3[C1-]/ mol l- 1

103 [ClO~.]/ mol l- 1

ID/Iza mM -~ I mg-~s ½

E½/V (vs. SCE)

Slope of E/V vs. tog i/(id-i) plot

1 2 4 100

99 98 96 0

6.50 6.72 6.75 6.80

-

-

0.735 0.753 0.773 0.852

0.078 0.063 0.049 0.032

of the log i / ( i d - i) vs. E plot virtually corresponds to a reversible two-electron reduction. (Fig. 8, curve C, and Table 3). In the presence of small amounts of iodide two cathodic waves are observed (Fig. 8, curve D). The fu'st wave is composite with the anodic iodide wave. In 0.1 M TEAl supporting electrolyte (Fig. 8, curve E) two cathodic waves are still observed. The first, composite with the anodic iodide wave, has a maximum and the second wave is well de,~eloped and occurs at about the same potential as the second cathodic wave in perchlorate supporting electrolyte. Z i r c o n i u m ( I V ) chloride. The polarography of zirconium(IV) chloride was investigated using both the dropping mercury electrode and the rotated platinum electrode (RPtE). The current-potential curves obtained in 0.1 M T E A P supporting electrolyte were similar to those reported by Olver and Ross 1s. Three cathodic waves were observed at - 0 . 9 4 , - 1.71 and - 1.97 V with ID values of 1.71, 3.68, and 7.92, respectively. These waves correspond to the successive reduction of Zr(IV) to Zr(III), Zr(II) and Zr(0). The E~ values quoted by Olver and Ross for these three waves were -0.82, - 1.61 and - 1.88 V (vs. SCE), respectively 15,16. It should be pointed out that these authors used a HgffEABr(s)/TEABr(sat.), T E A P (1 M) (solvent not specified but presumably AN) electrode as reference electrode and state that it "varied less than 50 mV over a two-month period". They converted their experimental E½ values to volts vs. SCE by using the Na ÷ wave in 0.1 M T E A P as an internal comparison wave (E~r~Na+)= --1.85 V vs. SCE t9 and - 1 . 5 4 vs. Olver and Ross's reference electrode). In addition to the three cathodic waves reported by Olver and Ross we observed a well-defined anodic wave with E½= +0.38 V vs. SCE and an Io value of 10.5. This wave is probably due to an electrode reaction of the type ZrCI~+4 Hg = Z r ( I V ) + 2 Hg2C12 + 4 e J. Electroanal. Chem., 31 (1971)423-439

434

F . G . THOMAS, I. M. KOLTHOFF

similar to that observed for titanium(IV) chloride in this supporting electrolyte 14. Addition of as little as 4 m M chloride to this system results in a single four-electron reduction wave with E~ = -2.25 V and an ID value of 7.83. The half-wave potential hardly changes on increasing the chloride ion concentration and is -2.28 V in 0.1 M TEAC1 supporting electrolyte. (Olver and Bessette x6 report -2.14 V for this wave in the presence of 0.01 M chloride.) In a 0.05 M TEAI+ 0.05 M TEAP supporting electrolyte the in'st reduction wave is split, the first of the split waves being composite with the anodic iodide wave and the second having an E½ of -1.20 V. The value of ID for the first reduction process (sum of the split waves) is the same as that of the first wave in perchlorate supporting electrolyte. The second wave (for the reduction of Zr(IV) to Zr(II)) is the same as in perchlorate medium, while the third wave (corresponding to reduction to Zr(0)) is also split. The first of the latter two waves occurs at the same potential as the single wave in the absence of iodide while the small second wave of this pair has an E½ value of -2.28 V. The total ID value at -2.45 V is 8.22. Olver and Ross ~5 reported that solutions of zirconium tetrachloride and iodide in AN quickly turn yellow from production of free iodine, this being accompanied by the reduction of Zr(IV) to Zr(III). As in the case of titanium(IV) chloride with iodide ion in AN 14 we found this reaction only occurred in the air-containing solutions and that if oxygen was removed from both the zirconium solution and the iodide solution before they were mixed no such reaction was observed. These observations were confirmed by determining the current-potential curves of zirconium tetrachloride solutions in AN containing iodide (prepared under oxygen-free conditions) with a RPtE. No reduction wave could be identified as arising from the reduction of iodine. It would appear that the production of iodine in these solutions is due to reaction between oxygen and iodide being promoted by the multivalent zirconium. A possible overall reaction is ZrC14 + ½ 02 + 2 1 - = Zr O C I 2 + 12 + 2 C1-

(2)

DISCUSSION

Conductance The conductance data show that all four halides, SnC14, SnI 4, GeC14, and SnC12 are weak electrolytes in acetonitrile. The following values for the molar conductances (f~- 1 cm 2 mol- 1) in 1 mM solution were found by interpolation of the data in Table 1 : SnC14 4.62 ; SnI~ 1.07 ; GeC141.60; and SnC12 9.33. These can be compared with the value of 158.3 ~ - 1 cm 2 mol- x for the molar conductance ofa 1 mM solution of TEAC1 in AN 2°. The simplest type of ionic dissociation that these solutes can undergo (ignoring solvation) is MX. ~ {MXt._ 1)) + + X -

(3)

the ionization constant, K a, being given by Ka = [{MX~n- x)}+] [ X - ] / [ M X , ] = ~2c/(1 - e)

(4)

where e = Ac/Ao = At/180. The value of Ao for these ions at infinite dilution was taken as being of the same order as Ao for TEAC1 in AN 2°. From the data in Table 1 it is J. Electroanal. Chem., 31 (1971) 423-439

POLAROGRAPHY OF GROUP-IV METAL HALIDES

seen that K d calculated using eqn. (4) is reasonably constant for tin(IV) chloride (K d = 8 __+4 x 10- 7) and tin (IV) iodide (K d = 3.5 _ 0.4 x 10- s ), indicating that the simple ionization (eqn. 3) predominates in these solutions over the concentration range studied. In addition to this simple ionization the following reaction may also occur

MX.+X-

(5)

which, in conjuction with eqn. 3 gives the overall disproportionation reaction 2 M X , ~ {MX~,_ 1)} + + {MX~,+ 1)}-

(6)

If MX, has a much greater affinity for the chloride ion than for AN, then reaction 6 predominates and the molar conductivity of the solution does not change on dilution, i.e. ~ remains constant. This is found for TIC1, la. In the ease of tin(II) chloride, the value of at only increases slightly with dilution (Table 1) and so, both simple ionization (eqn. 3) and disproportionation (eqn. 6) occur with the latter predominating. This is analogous to the homoconjugation that occurs with proton acids such as H2SO4 in AN z 1. Using the procedure of Kolthoff and Chantooni 2 ~, the values of the equilibrium constants for the three reactions 3, 5 and 6 were found to be K d = [SnC1 +] [CI-]/[SnCI2] = 1.2_+0.4 x 10 -6 K 3 =

[SnCI~]/[SnCI2] [CI-] = 3.6 +_0.4 x 102

K2(MX,) = [SnC13] [SnCI+]/[SnC12] 2 = 4 + 2 x l0 -4 The value of ~2c/(1 - ~ ) for germanium(IV) chloride increases greatly with dilution. In all cases the specific conductance x is very low and in fact is only 10 times that of the solvent used in the most concentrated solution studied. Also, x varies only slightly with concentration, indicating that impurities in the solvent may be the main cause of the very small conductance observed. Because the amount of ionization is only about 1% in 1 mM solution, the major species in solution is expected to be the undissociated solute and the polarography of this solute will be interpreted in terms of undissociated germanium(IV) chloride.

Polarography Tin(IV) halides. With perchlorate as supporting electrolyte SnI 4 and SnCI 4 both give current-potential curves with two cathodic waves whose diffusion current constants are in the ratio of 1 : 2.83 and 1:2.85, respectively, in 1 mM solution. With both compounds the first wave is not proportional to concentration; for SnI4 the value of the diffusion current constant increases with increasing concentration, while for SnC1, it decreases with increasing concentration. The first cathodic waves in both cases are considered to be due to the reduction of Sn(IV) to Sn(II) since the values of the diffusion current constants for these waves are in the range previously reported for two-electron reductions in A N 19. It may be noted that the "suppression" of the first wave is analogous to what is observed in acidic aqueous chloride solutions of Sn(IV) 1. The second cathodic waves of these two tin(IV) halides in perchlorate media correspond to the reduction of Sn(IV) to Sn(0). The.higher value in 0.1 M TEAP of 13,63 for the diffusion current constant of the second cathodic wave of SnI4 relative to that of 10.57 for SnC14 (Table 2) can J. Electroanal. Chem., 31 (1971) 423-439

436

F. G. THOMAS, I. M. KOLTHOFF

probably be attributed to the difference in the Lewis acidities of the two halides. SnC14 forms a stable adduct 12"z2 with two molecules of AN: SnC14(CH3CN)2, which has been isolated as a white solid, whereas no such adduct appears to form with SnI412'22. In fact, we observed that SnI4 dissolves slowly and has a relatively low solubility in AN (5.3 × 10-3 M at 25°C). On cooling hot saturated solutions of SnI, in AN the unchanged solute crystallizes from solution. With SnCI 4, however, solution is rapid and exothermic (cf TiCI4 and AN14). Thus it is reasonable to assume that the major species in SnC14 solutions is SnCI4(CH3CN)2 and in SnI, solutions is the simple unsolvated solute molecule. Since SnC14(CH3CN)2 is a considerably larger species than SnI4, a lower diffusion current constant is expected for the former species. In the presence of 0.1 M TEAC1 both tin(IV) halides give (within experimental error) identical four-electron reduction waves with practically identical diffusion current constants (Table 2), indicating that the same tin(IV) species is present in both solutions, The disappearance of the first cathodic waves indicates the presence of a new species in both cases. This is probably the SnCI62- ion with SnCI 4 and is presumably the major species present with SnI4, although the pale yellow color of the solution would indicate the presence of some iodide containing species such as SnlC12- which must be reduced at the same potential as SnCI~-. The ease of halogen interchange that occurs with group-IVB tetrahalides 23 is demonstrated by the addition of chloride to SnI4 and of iodide to SnCI4. A solution of SnI4 in AN is bright orange but on addition of chloride the color fades until it is only pale yellow in 0.1 M chloride solutions. This ease of halogen interchange is also evident from the polarograms of tin(IV) iodide in the presence of chloride (Fig. 5, curve C and Fig. 6). The addition of small amounts of chloride to tin(IV) iodide solutions had a marked effect since the wave at - 1.05 V is apparent even in the presence of only 1 mM chloride (Fig. 6, curve A). All polarograms of tin(IV) chloride in the presence of various concentrations of iodide (1 m M 0.1 M) exhibit two cathodic waves. Although the polarograms cannot be readily interpreted in terms of the species present, the development of color in the solutions from pale yellow with 1 mM I - to orange with 0.1 M iodide indicates that some halogen exchange and/or addition has occurred to give species of the type SnCI~I~x÷y- 4)where x + y is most probably 6. It should be noted that in all systems containing chloride (SnC14, SnCI4 plus CI-, SnCI4 plus I - , and SnI, plus C1-) the reduction to Sn(0) occurs at approximately the same potentials ( E , = - 1.05 V to - 1.07 V), i:e~ species such as SnCI4(CH3CN)2, SnC12- and SnlxCltrx+ r-4)- are all reduced to Sn (0) at similar potentials. The single four-electron wave of tin(IV) iodide in the presence of 0.1 M iodide is composite with the anodic iodide wave which occurs at more negative potentials than either of the two reduction waves of this solute in 0.1 M perchlorate solutions. Thus nothing definite can be said of the species present in this system. A striking feature of the current-potential curves of tin(IV) chloride in 0.1 M TEAP solutions is the presence of two maxima in the plateau region of the first cathodic wave (Sn(IV) to Sn(II) reduction). Decreasing the tin(IV) chloride concentration causes the maxima to decrease and small amounts of iodide (1 mM) (Fig. 4, curve A) and of chloride (2 mM) added to 1 mM tin(IV) chloride in 0.1 M TEAP cause the maximum at -0.67 V to disappear. The maximum at -0.27 V, however, persists although not as pronounced on addition of chloride (Fig. 3, curves A and B) to the solution until the first cathodic wave disappears (at 8 mM C1-). On addition J. Electroanal. Chem., 31 (1971) 423-439

POLAROGRAPHY OF GROUP-IV METAL HALIDES

437

of iodide (Fig. 4) the maximum at - 0.27 V shifts to more negative voltage, becoming a "normal" maximum at the head of the first cathodic wave. The droptime curve and the shape of the current-time curves for each drop (Fig. 2) indicate that the maxima are associated with changes in the double-layer capacitance. Such changes in double-layer capacitance are probably due to adsorption-desorption phenomena involving the neutral molecule SnCI4(CH3CN)2 and/or the Sn(II) species produced by the electrode reaction. Germanium(IV) chloride. The general feature of the current-potential curves of germanium(IV) chloride in AN are similar to those of tin(IV) chloride in this solvent with the reduction potentials being more negative for germanium than for tin. In 0.1 M TEAP supporting electrolyte two two-electron waves are observed, the second being extremely irreversible and the ratio of the diffusion current constants for these two waves is 2.29 : 1. Unlike tin(IV) chloride, there are no unusual maxima on the plateau of the first cathodic wave and the anodic and first cathodic waves are both well defined and their heights are proportional to concentration. The similar magnitude of the diffusion current constant for the four-electron wave in perchlorate for tin(IV) chloride (10.57) and germanium(IV) chloride (10.94) is surprising in view of the fact that no adduct of GeCI~ and AN has been isolated ~3'24. It would have been expected that the small GeC14 molecule would have a significantly higher diffusion coefficient than SnC14(CH3CN)2. This may indicate that a labile adduct of the type GeC14(CH3CN)2 is present in AN solution which is not stable enough to be isolated. This conclusion is supported by the fact that the diffusion current constant of GeCI 4 is larger in 0.1 M TEACI than in 0.1 M TEAP (Table 2). In the presence of 0.1 M TEACI as supporting electrolyte a small pre-wave is present which is composite with the anodic chloride wave. This pre-wave is only about 2 ~ of the height of the main four-electron wave and indicates that some uncomplexed germanium(IV) chloride may still be present in solution. This is in accord with the weaker Lewis acidity of germanium(IV) chloride (cf tin(IV) chloride 13) which would require a much larger excess of chloride for the complete conversion of germanium(IV) chloride to the complex GeC12- ion. Current-potential curves indicate that the predominant species present in 0.1 M chloride solution are chloro complexes such as GeC15 and GeC12-. If the pre-wave is due to uncomplexed GeC14 then the major species would be GeCI~ ; alternatively, if uncomplexed GeCI~ is insignificant, then the pre-wave should be due to GeCI~ and the major species is GeC12-. Addition of iodide to the system again shows the ease of halogen exchange with group-IVB halides, the solution becoming golden-yellow in color due to the formation of GeClxly species. With iodide alone it is unlikely that x + y is greater than 4 because of steric factors. Also in common with most iodide containing solutions of group-IV halides a maximum appears on the first cathodic wave. It has been noted that in all these cases the magnitude of the maxima depends to some extent on deoxygenation of the solutions prior to mixing the iodide and group-IV halide, and to the precautions taken to keep oxygen out of the system after mixing. Tin(ll) chloride. The total currents at +0.6 V and at - 0 . 9 V vs. SCE in the current-potential curves of tin(II) chloride in 0.1 M TEAP (Fig. 8, curve A) are proportional to concentration. -The total cathodic current corresponds to the twoelectron reduction of Sn (II) to Sn(0). However, the reason for the split in the reduction wave is not clear. Conductance results show that at these concentrations only about J. ElectroanaL Chem., 31 (1971)423-439

438

F . G . THOMAS, I. M. KOLTHOFF

5 ~ of the solute exists as ions. It is possible that the first reduction wave is partly kinetic and partly diffusion controlled and is to be attributed to two-electron reduction of SnC1 +. This is substantiated by the fact that the pre-wave disappears even in the presence of only 1 mM of chloride to 1 mM of SnC12 (Fig. 8, curve B), the chloride apparently virtually completely converting the Sn(II) species to SnC13. Conductance data indicate that this is a favored reaction (K3 = 360). Addition of more chloride causes the cathodic wave to move to more negative potentials, the waves only approaching reversibility at higher chloride ion concentrations. It should be noted that E~ for SnCl4+4e is - 1.06 V in 0.1 M TEAP and - 1.05 V in 0.1 M TEAC1, whereas for SnC12+2e E ½ = - 0 . 7 2 and -0.85 V, respectively, indicating the pronounced irreversibility of the SnCI4 reduction. The split anodic curve in perchlorate media, however, remains split on addition of little chloride (curve B, Fig. 8). It is of interest to note that the total diffusion current at +0.6 V is well defined (compare with Fig. 1) and that in the presence of 1 mM TEAC1 this current is twice as large as that of 1 mM SnC12 in the absence of added chloride. The overall electrode reactions cannot be written with certainty, but the fact that this increase occurs tends to indicate that the main reactions are: SnC12 + 2 Hg ~ Sn(II) + Hg2C12 + 2e

(7a)

2 SnCl3 + 6 Hg ~ 2 Sn(II) + 3 Hg2CI 2 + 6e

(7b)

In addition there is indication that some oxidation of Sn(II) to Sn(IV) occurs. The large increase in I o on addition of 1 mM TEAC1 to 1 mM SnC12 indicates that the oxidation of Sn(II) to Sn(IV) is enhanced by the formation of SnCl3. Addition of a small amount of iodide gives a composite iodide oxidation and Sn(II) reduction wave (Fig. 8, curve D), the mixed potential (in curve D) being -0.36 V. At - 0.15 V the observed anodic current of 1.8 pA is equal to the difference between the cathodic current of 2.7/tA for the Sn(II) reduction and an anodic current of 4.5/zA (expected for 1 mM iodide). This indicates that there is little tendency for iodide to complex with stannous chloride. Also, in 0.1 M iodide solution the existence of two cathodic waves, with the second wave being similar to the second cathodic wave in 0.1 M TEAP solution, supports the view that iodide does not complex to any great extent with stannous chloride in AN. SUMMARY

The polarography and the conductance of solutions in acetonitrile of tin(IV) chloride and iodide, tin(II) chloride, and germanium(IV) and zirconium(IV) chlorides have been investigated. Tetraethylammonium perchlorate, chloride and iodide have been used as supporting electrolytes. With perchlorate the tetrahalides of tin and germanium yield two reduction waves, the first one corresponding to a two-electron reduction and the second wave to reduction to the metal. With chloride as supporting electrolyte only one four-electron reduction wave is observed, but two two-electron waves in iodide with tin(IV) and germanium(IV) chlorides. Tin(IV) iodide with tetraethylammonium iodide as supporting electrolyte yields a single, four-electron reduction wave, composite with the iodide wave. All compounds yield anodic waves which are poorly developed, probably because of film formation. Tin(II) chloride yields in J. Electroanal. Chem., 31 (1971) 423-439

POLAROGRAPHY OF GROUP-IV METAL HALIDES

439

the three supporting electrolytes an overall two-electron reduction wave. All waves appear to be irreversible and the anodic waves are of a complex nature. Contrary to the literature, zirconium tetrachloride in the presence of iodide and oxygen yields iodine, without reduction of zirconium(IV). Tin(IV) iodide reacts with chloride, the chloride replacing the iodide. From conductance data it is concluded that tin(IV) chloride and iodide dissociate into SnX~- and X - ions, the ionization constants being 8 x 10-7 and 3.5 x 10-8, respectively. On the other hand, the dissociation of SnCI2 occurs mainly by the reaction: 2 SnCI2 ~ SnC1÷ +SnC13 and to a lesser extent by the reaction SnC12 ~ SnC1 + + C1-. The equilibrium constants for these two reactions were estimated to be 4 x 10 -4 and 1.2 x 10-6, respectively.

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J. Electroanal. Chem., 31 (1971) 423-439