I. AT. KOLTHOFF AKD S. IKEDA
1020
Vol. 65
T ~ R LI1E S V M l l A R Y OF
RESTJLTS O F THE
P E R M E A B I L I T Y O F C O P P E R TO
IIYDROGWN
-
Rate,
P,
T, "e.
atm.
350 350 400 400 450 450 500 500
1.5 2.0 1.5 2.0 1.5 2.0 1.5 2.0
Rate, cm.3 hr. -1
cm.8 hr. - 1
0.00308 ,00365 ,00715 ,00826 .0138 .0160 .0217 .0251
0.00101 .00116 .00234 .00270 .00452 .00522 .00707 .00817
0.00083 .00082 ,00191 IO0191 ,00368 .00368 .00577 ,00577
Mernhrane I1 0.1284" thick 1.0000" diameter--Permeability Specific rate Cm.8 (OO. 1 atm.) Cin.8 (0'. 1 atin.)
0.00159 .00183 .00366 .00420 .00705 .00812 .0110 .0127
0.00101 .00118 .00235 .00269 .00452 .00519 ,00705 ,00811
0.00083 .00082 .00192 .00191 .00368 .00367 .00575 .00573
In this study, the downstream face was kept a t vacuum so that p z = 0, essentially. The equation may be rearranged to give
i l -.
Specific rate = Cpl'/u-Ep/RT
E
-2.0
cm.8(0°, 1 atm.)hr.-' cm.2 mrn.-'
(2)
where C = kD.
a
;
Permeability
I
E
P = Specific rate pJ2
i
-
E.
2 -3.0
2 6
.2 a, 3
-'
t , i 1.1
1.2
1.3
1.4
1.5
Graphical representation of this equation is shown in Fig. 2. The pressure dependence is computed from the data of Table I1 a t constant temperature. When the temperature is constant, equation 2 becomes
1.6
Specific rate = Clpll/a where C1 = C X e-EP/RT
103/T. Fig. Z.-Tenlperature dependence of hydrogen permeating coppel- permeability us. reciprocal temperature. p = 21.2e-12,500 f 11OIR5''. ~r
k
D A pl pz Ep L t
= quantity of gas permeating the barrier, cm.3 = = =
= = = = =
(OO,
1 atni.) solubility constant of the gas-metal system diffusion constant area of the barrier, cm.2 pressure on one face of the membrane, atm. pressure on the other face activation energy of permeation, cal./g.-atom thickness of the barrier, mm. time, hours
(4)
From equation 1, the rate of permeation should be inversely proportional to the thickness of the membrane. Equation 4 states that a t constant temperature, the specific rate should be proportional t o the square root of the pressure and that the permeability should be constant (equation 3). All these equations are confirmed by the data of Table 11, indicating the valid assumption of a diffusion controlled permeation of hydrogen atoms through the metal barrier.
POLA'ROGRAPHIC AND AClD PROPERTIES OF THORIUM PERCHLORATE IN ACETOXITRILE BY I. M. KOLTHOFF AND S. IKEDA School of Chemistry, University of Minnesota, Minneapolis, Minnesota Received December 23, 1060
Thorium perchlorate in acetonitrile (AN) behaves like a relatively strong dibasic acid with a first dissociation constant of which is slightly less than that of sulfuric acid. The second dissociation constant is very much greater than that about of sulfuric acid. Indication has been obtained that the Th(C10a)22+ion is relatively stable in AN. The neutralization product formed upon titration with di henylguanidine and the insoluble reaction product formed in electrolysis a t - 1.8 volt (us. s.c.e.) both contain about 2 C d d - per one thorium. The height of the polarographic wave of thorium perchlorate in AN is proportional to concentration. The reduction involves the evolution of hydrogen and not the formation of thorium amalgam. The value of the limiting current corresponds to the formation of a reaction product which contains about 2 C104- per one thorium.
In the present paper the results are described of an extensive study of the polarography of thorium perchlorate in acetonitrile (AX). Well-defined
reduction waves have been observed. It was suspected that the cathodic current is not due to the formation of thorium amalgam, but to a reduction
June, 1961
POLAROGRAPHIC PROPERTIES OF THORIUM PERCHLORATE IN ACETOKITRILE
of protons in perchloric acid formed by solvolysis or to a direct reduction of the proton in solvated thorium species to hydrogen. This was confirmed by electrolysis experiments at a stationary mercury cathode at potentials at which the polarographic limiting currents of thorium perchlorate were observed. For a satisfactory interpretation of the polarographic behavior of thorium perchlorate in AX it was desirable to obtain information concerning its acid properties and solvolysis in AN and its degree of dissociation. Since such information is not available in the literature, an exploratory study has been made of the solvolysis and acid properties of solutions of the salt by determining the hydrogen ion concentration with a glass electrode and also spectrophotometrically with the aid of indicators. In addition the conductance of the salt has been determined over a wide range of concentrations. Titrations with diphenylguanidine as the base and the glass electrode as indicator electrode yielded information on the strength of the thorium salt as a mono- and dibasic acid.
Experimental Materials Used. Acetonitrile.-Technical grade methyl cyanide obtained from Eastman Kodak Go. was shaken with activated alumina followed by drying over anhydrous calcium chloride and magnesium sulfate and four distillations at 81.0 f 0.5" from phosphorus pentoxide in an allglass still. The solvent was stored in a glass stoppered bottle in the dark and withdrawn-when needed-by means of an all-glass siphon provided with a drying tube containing anhydrous calcium chloride. The water content of the solvent was determined by the Karl Fischer method and found t o be 0.0025%. The conductance was 7 X ohm-'cm.-'. Thorium perchlorate was prepared by a method used in this Laboratory by Willeboordse' from thorium nitrate by adding an excess of vacuum distilled 70% perchloric acid. The mixture was well stirred and the nitric acid and moisture removed in a dry nitrogen atmosphere. The salt was recrystallized from 0.2% aqueous perchloric acid and dried to constant weight in a vacuum oven a t 70". Decomposition occurred at higher temperatures. The thorium content of the salt was determined by precipitation as oxalate in 1 N hydrochloric acid solution and weighing as oxide2 after ignition. Thorium was also determined by titration in aqueous medium with ethylenediamine tetraacetate using alizarine S as i n d i ~ a t o r . ~Perchlorate was determined by the method recommended by Kurz, et ul.,' by fusion of a mixture of the salt with sodium nitrite in a nickel crucible at 550 =k 50' for 1.5 hours. The melt was dissolved in water, the excess nitrite removed by boiling with nitric acid and chloride titrated by the Volhard method. Perchlorate was also determined by an indirect alkalimetric method by adding in a volumetric flask to an aqueous solution of the salt an excess of standard 0.1 iV sodium hydroxide. The mixture was shaken and diluted to the mark. The excess base was titrated in an aliquot of the filtrate with 0.1 N standard hydrochloric acid using methyl red as an indicator. The water content was determined by dissolving a weighed amount of the salt in anhvdrous methanol and titrating with Karl Fischer reagent .& The average of 6 determinations yielded a water content of 4.36 =t 0.04%. The results (average of 3 determinations for each method) of the thorium and perchlorate analyses were referred to the anhydrous salt Th(C104)*. Thorium content, calcd. 36.85%, (1) F. Willeboordse, Ph.D. Thesis, University of Amsterdam 19.59, p. 157. (2) W. F. Hillebrand and G. E. F. Lundell. "Applied Inorganic Analysis," 2nd ed., John \Tiley and Sons,h e . , New York. N. Y.,1953. (3) V. Suk and M. Malart, Chemist Anal.. 46, 30 (1956). (4) E. Kurs, G. Kober and M. Bier, Snal. Chem., SO, 1938 (19.58). ( 5 ) J. Blibchell, Jr.. and D. M. Smith, "Aquametry," Interscience Publishers, Inc., New Tork, N. Y., 1918.
1021
found 36.98% (gravimetric) and 37.27% (titrimetric). Perchlorate content, calcd. 63.15%, found 62.80.%{Volhard) and 62.95% (alkalimetric). From the analysis it appears that the over-all composition of the salt is Th(ClO,),. 1.7Hz0. Solutions in AN up to a concentration of 1 x M were clear and remained so on standing, 2 X 10-2 M solutions became turbid soon after the preparation as a result of solvolysis (vide infra). Anhydrous Perchloric Acid.-Much of the work on the effect of perchloric acid on the polarography and on the hydrogen ion concentration of thorium perchlorate was carried out with a solution of the acid in anhydrous acetic acid6 until it was discovered that acetic acid reacts with thorium perchlorate (vide infra)with the liberation of hydrogen ions. All the experiments were than repeated by synthesizing anhydrous perchloric acid solutions from barium perchlorate and 1 0 0 ~ sulfuric o acid prepared from oleum and 96% acid. The barium salt was recrystallized from water and dried to constant weight in a vacuum oven a t 100". I n a 10-ml. volumetric flask of 0.42 g. of the barium salt was dissolved in BN, 0.0800 ml. of 1 0 0 ~ sulfuric o acid was added from an ultramicroburet, and the volume adjusted to 10 ml. After thorough mixing the mixture was centrifuged for 10 minutes at 3500 r.p.m. The clear supernatant liquid was 0.25 M in perchloric acid and 0.0770 in water. The anhydrous solution of the acid was prepared on the day when it was used. Tetraethylammonium Perchlorate.-This salt was prepared and purified as described in a previous paper.6 o-Nitroaniline and p-chloro-o-nitroaniline, used as indicator bases, were Eastman Kodali products and recrystallized from alcohol. Techniques. Polarographic cell.-This cell was the same as that used by Knecht? for polarographic measurements in N-methylacetamide as a solvent. All polarograms were determined in the absence of oxygen by passing high-purity Linde nitrogen through and over the solution. Because of the relatively high vapor pressure of AN (80 mm. a t 25') the gas was presaturated with AN by passing it through two wash-bottles containing the solvent. All experiments were carried out in a thermostat at 25.00'. A saturated calomel electrode (s.c.e.) in aqueous medium was used as reference electrode. Electrolytic contact with the solution in the cell was made in the way described by Coetzce? The salt bridge was inserted into the polarographic cell only Tvheii the polarogram was run. The dropping mercury electrode had the following characteristics in 0.1 JI tetraethylammonium perchlorate solution: m = 1.12 mg./sec.; t = 5.6 sec. a t 0 volt us. s.c.e., 4.6 sec. a t -1.0 volt and 3.5 sec. a t -1.7 volt. Potential values reported were corrected for the iR drop across the cell. The resistance of the cell containing 0.1 Af tetraalkyl perchlorate was 1505 & 50 ohms as measured with an Industrial Instruments Inc. KO. RC-IB conductance bridge. Reported values of limiting or diffusion currents have been corrected for the residual current. The electrical conductivity of solutions of thorium pcrchlorate in AN a t 25 5 0.02" was determined with the above conductance bridge. The constants of the two conductance cells used were 0.1957 and 0.03763, respectively. The hydrogen ion concentration was determined spectrophotometrically Tvith o-nitroaniline and p-chloro-o-nitroaniline as indicators by measuring the absorption of the alkaline forms in Pyrex cells with a path length of 1.8 cin. in a Beckman D U spectrophotometer a t a wave length of 410 m r a t 25'. The pH was also determined potentiometrically with a Beckman pH meter. The glass electrode was soaked in the solvent before use. A 0.1 .Ifsolution of sodium perchlorate in AX was used as a salt bridge to make contact with the saturated calomel electrode. Coiitrollrd potential electrolysis was carried out a t a potential of -1.8 to -1.85 volt (us. s.c.e.) in a nitrogen atmosphere iii a cell containing 25 ml. of thorium perchlorate solution which was 0.1 h ' in tetraethylammonium perchlorate. A layer of mercury was used as the cathodc. A glass rod with blades placed in the mercury served as a stirrer during electrolysis. The cell was provided with 2 glass tubes separated from the cell by sintered glass disks. One sidearm contained a platinum foil electrode as anode placed in the same solution as contained in the electrolysis cell. The other side tube also (6) See I. M . Kolthoff and J. F. Coetzee. J . Am. Chem. Soc., 79, 870, 1852, 6110 (1957). ( 7 ) L. A. Knecht. Ph.D. Thesis, University of Minnesota, 1958.
I. M. KOLTHOFF AND S. IKEDA
1022
Vol. 65
i,. Thorium perchlorate yields a reduction wave with a well-defined limiting current, which was found proportional to the thorium concentration 6 (see Table I). At concentrations greater than 1.5 X M thorium perchlorate irregular results were obtained and the polarogram had an appearance of three waves, the last one with a slight maximum a t - 1.85 volt, the diffusion current becoming constant a t -2.15 volts. Apparently these irregularities are caused by the separation of a slightly soluble basic salt interfering with the M evolution of hydrogen a t the electrode6 (vide infra). Fig. 1-Effect of water on polarograin of 0.5 X thorium perchlorate in 0.1 M tetraethylammonium per- The waves are drawn out indicating irreversible chlorate. Concentration of water: A, 0.00570; B, 170; reduction. The half wave potential a t about - 1.2 C, 5%; D, loi&; E, 20%; F, 50%. volt was found to shift slightly to more negative
t
TABLE I POLAROGRAPHY OF THORIUM PERCHLORATE I N PUREAN
J
l4 12
Concn. Th(ClOd4, M x 108
0 2 0 5 1 0 1.5
E I ~(us. Z s.o.e.1 - I 18 -1 18 -1 20 -1 22
id, pa.
2 2 5 3 10.4 15 R
Id
8 8 8 7
42 16 02 96
TABLE I1 THORIVM hRCHLOR4TE
COXDUCTANCE O F
Fig. 2.-Effect of acetic acid on polarogram of 1 x 10-3 M thorium perchlorate. A, no acetic acid added; B, with 1.0 x 10-2 M acetic acid.
u A.'6 \ , 4
C) -07
-09
-11 VOL'
-13
-I5
-17
-1.9
vs S C E
Fig. 3.--Polarograms of mixtures of perchloric acid and thorium perchlorate. A, 0.001 M HCIOl; B, 0.0005 M Th(C10&; C, mixture of 0.001 M HC10, and 0.0005 M Th( C104)d; D, summation of curves A and B. contained the same solution as contained in the cell; into it was placed a saturated potassium chloride salt bridge in agar gel to make contact with a saturated calomel electrode. The level of the solutions in the side tubes was lower than in the cell t o prevent entering of solution from the side tubes into the cell during electrolysis. The salt bridge was placed in the side tube only when the potential of the mercury pool m s checked during the electrolysis. The potential of the mercury could be kept constant during the electrolysis with the aid of a Fisher controlled potential electroanalyzer Sitrogen was passed through during electrolysis. The bottom of the cell was provided with a three-way stoprock which allowed collection of the mercury in a nitrogen atmosphere after the electrolysis and also of the aqueous layer in the cell.
Experimental Results Polarography of Thorium Perchlorate.-Unless otherwise stated all figures refer to freshly prepared solutions, 0.1 M tetraethylammonium perchlorate being the supporting electrolyte. The residual current 2, varied betwen -0.2 p a , at +0.4 volt (us. s.c.e.) to +0.5 pa. a t -2.8 volts, indicating high purity of d v e i i t and supporting electrolyte. ReDorted diffusion currents havr brei1 corrected for
b
Aac
4 x 10-2 37 75 38.25 2 x 10-2 44 70 4 x 10-3 61 50 63.50 2 x 10-3 71 50 4 x 10-4 100 00 104 5 a 1 M = 4 S. Subscript f refers to fresh solutions. c Subscript a refers to solutions aged for a week.
A&yl
2
-05
Normalitya
potentials with increasing concentration of thorium salt which is probably caused by the water content of the salt. Addition of water made the waves less drawn out and displaced them to more negative potentials (Fig. 1). It is remarkable that even in the presence of 50 vol./vol. yoof water the limiting current remained unchanged. Irregularities in the polarogram of 2 X 10-3 111 thorium perchlorate disappeared in the presence of 1% water, and the limiting current was twice the value found in 1 x M solution. Aging of the thorium perchlorate solution for two days hardly affected the shape of the polarograms and the limiting currents remained unchanged. In a study of the effect of perchloric acid on the polarograms a solution of the acid in acetic acid was used first. Only one wave was observed in the mixtures, the wave observed in thorium perchlorate solution alone being displaced to more positive potentials. This effect must be attributed to an interaction between thorium perchlorate and acetic acid with the formation of perchloric acid. As is evident from Fig. 2 , addition of acetic acid alone to the thorium solution displaces the wave to more poiitive potentials, while the limiting current increases only a few per cent. In separate experiments it v a s shown that addition of acetic acid greatly increases the hydroPen concentration of thorium Derchlorate solutions. In order to eliminate the aceti'c acid effect solutions of perchloric acid prepared by metathesis from barium perchlorate and siilfuri~acid n w e uwd. +kt ,
