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SHERMAN KOTTLEAND L. 0. MORGAN
other than the reference point and a partial reversal is found. In addition, consider the fugacity lowering curves in the concentrated region. Neglecting the cesium curve, whose behavior is practically impossible to explain, it is found that the order, a t concentrations above m’/a = 2.55, is K > Na > Li. Since the anions are considered to be identical in these solutions they cannot cause the observed differences. However, it is also impossible to attribute these differences to the cations
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present since this is in direct contradiction to the accepted theory that the amount of solvation is greater the less the size of the cation. The degrees of ionization and polymerization for lithium and sodium as indicated by the curves in the dilute solutions also point to an order in the concentrated region which is opposite t o that observed. Such behavior would seem to indicate that these solutions should nqt be regarded as simple ionic solutions in either the concentrated or dilute regions.
INTERACTION OF CHROMIUM(V1) ANIONS WITH CHROMIUM METAL SURFACES1i2 BY SHERMAN KOTTLE AND L. 0. MORGAN Contributionfrom the Departmenl of Chemistry, The University of Texas, Austin, Texas
*
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Received Jrlu 16, 1966
Experiments were carried out in which electrode potential changes were correlated with adso tion of chromium(V1) active, such as those anions on a chromium metal surface. Three classes of chromium metal surfaces were recognized: obtained upon removal of metal from a typical plating bath without subsequent treatment, (2) passive, produced by tieatment of active metal with boiling nitric acid, and (3) etched, produced from either active or passive metal by treatment with hydrochloric acid. Etched chromium was similar to reactive chromium with respect to potential behavior, but differed markedly with both reactive and passive metal in its very much larger apparent surface area. Calculations based upon the observed electrode potential change and the number of adsorbed anions led to a value of 1.40 debye for the electric moment of each adsorbed ion.
3)
Corrosion inhibition of iron by chromate, molybdate and tungstate ions has been studied by Robertson3 who concluded that those ions were chemisorbed a t the metal surface. His conclusions were based on the great similarity in the over-all behavior of the three ions despite large differences in oxidation potential. Thus, the proposal that a mixed iron-chromium oxide film is formed4 does not logically extend to a consideration of tungstate and molybdate reactions because of their very low oxidation potentials. Uhlig and Geary6 measured electrode potentials of iron immersed in chromate solutions of various concentrations and were able to correlate changes in potential with the supposed adsorption of chromate ions upon the assumption that adsorption followed a typical Langmuir isotherm. Using radioactive chromium Powers and Hackerman6 demonstrated the uptake of chromium from chromate solutions on chromium surfaces and their results followed a pattern predictable on the basis of irreversible adsorption. After due consideration of changes in ionic species in solution the results were compatible with a simple monolayer isotherm. It seems virtually certain that the primary process at both iron and chromium surfaces in chromate solutions is one of adsorption and that such adsorption is capable of inhibiting oxidation reactions a t (1) Taken from the dissertation submitted to the Graduate School of the University of Texas by Sherman Kottle in partial fulfillment of the requirements for the degree of Doctor of Philosophy, 1954. (2) This work was supported financially b y the O 5 c e of Naval Research under contract No. Nonr 375, Task Order 04. (3) W. D. Robertson, J . Electrochem. SOC.,98, 94 (1951). (4) U. R. Evans, J . Chem. SOC.,1020 (1927); T. P. Hoar and U. R. Evans, ibid., 2476 (1932); J. Mayne and M. Pryor, ibid,, 1831 (1949); Ll. R. Evans and J. Stockdale, ibid., 2651 (1929). (5) H. H. Uhlig and A. Geary, J . Electrochem. SOC.,101, 215 (1954). (6) R. A. Powers and N. Hackerman, THIS JOURNAL,67, 139 (1953).
I
those surfaces although it is possible that in many cmes further reaction to form oxide films may occur and the coatings thus formed are capable of preventing corrosion. It is our purpose in this paper to demonstrate directly the correspondence between chromate adsorption and electrode potential change at the chromium metal surface. Preparation of Solutions .-The radioisotope chiefly used was chromium-51, obtained by neutron irradiation of chromium metal at the Oak Ridge National Laboratory, Oak Ridge, Tennessee. One-gram pieces of electrolyticnlly prepared chromium were irradiated for one month (about one half-life). Chromium( VI) solutions were prepared bjr electrolysis with radioactive chromium as the anode and a platinum wire as the cathode. When the solutions were to be of a concentration less than 10-6 M , distilled water was used as the electrolyte, with an anode current density of less than ten microamperes per om.* Those solutions contained essentially pure hydrogen chromate and were completely free of chromium(111) ions. Solutions of greater concentration were made by diluting a 0.01 M chromic acid stock solution prepared as follows. Chromium was electrolyzed in pH 11 to 12 sodium hydroxide solution at approximately one milliampere per cm.2. The resulting solution, containing sodium chromate and sodium chromite, was washed and treated with concentrated (30%) hydrogen peroxide. Excess peroxide was decomposed by contact with a piece of latinum foil, and the solution was purified by passing i t &rough a column, 2.5 cm. diameter and 50 cm. long, of Dowex-50 (sulfonic acid ion-exchange resin) in the hydrogen form. For solutions which were not radioactive a “dead stop” potentiometric titration method was used in which chromium(V1) was reduced to chromium(II1) with ferrous sulfate. In order to conserve radioactive chromium solutions, chromium(V1) concentrations were determined colorimetrically in those cases. Solutions were compared with known standards with a Rubicon “Evelyn” colorimeter using light of 380 mp wave length. Preparation of Chromium Samples.-Experiments were done using chromium discs 2 cm. diameter and 0.05-0.15 mm. thicE. Electrical leads, disc edges and the reverse sides of electrodes were coated with ceresin wax in order to define the conducting surface. It was determined by coprecipitation experiments that chromium( VI) was not re-
I
June, 1956
ADSORPTION OF CHROMIUM METALIN SOLUTIONS OF Cr(V1) ANIONS
duced by the wax under conditions similar to those used in the electrical measurement experiments. Chromium metal was prepared by electrodeposition from a standard plating bath.’ Three pretreatment procedures were used in the preparation of samples. (1) Chromum plates were removed from the plating bath, rinsed with distilled water, treated with cold dilute nitric acid to remove the copper basis metal, and again rinsed with distilled water. (2) The metal was removed from the plating bath, treated with boiling concentrated nitric acid, and rmsed with distilled water. (3) Chromium metal prepared in either of the preceding ways was treated with hydrochloric acid (1 :1) until hydrogen was evolved uniformly from the surface, and rinsed with distilled water. The first of these produced a surface which was distinctively different from the passive surface produced by the second procedure-measured electrode potentials were considerably more anodic, or active, and the rates of adsorption were subsequently found to be quite m e r e n t in the two cases. For the sake of brevity metal of the first kind w i l l be called “reactive” and of the second kind, “passive.” The third pretreatment process led to a very reactive chromium surface which we shall call “etched. ” Measurement of Adsorption.-Chromium uptake was determined by measurement of the radioactivity of chromium-51, which decays with the emission of vanadium Xrays, low energy electrons, and 0.32 MeV. y-rays.s Measurements were done with a scintillation counter comprising a 25 cm. diameter, thallium-activated, sodium iodide crystal optically coupled to an RCA 5819 multiplier phototube. Electrical pulses from the phototube were amplified and counted in the usual manner .D Measurement of Electrode Potentials .-Cells used in the measurement of the potentials of chromium immersed in chromate solutions were 50-ml. Pyrex beakers, fitted with holders for a saturated calomel electrode, or solution bridge and a chromium coupon. All measurements were made with the cells open to the atmosphere. A self-contained Rubicon portable potentiometerlo was used to obtain otential data. Because the calomel electrodes slowly Laked potassium chloride, they were separated from the experimental solutions by a bridge constructed of 2 mm. capillary tubing filled with cell electrolyte. The bridge ended in a tubulus which could be placed in contact with a chromium electrode. It was possible to avoid chloride contamination of the bulk solution by occasionally discarding the bridge contents. Adsorption Experiments.-Amounts of adsorbed chromium(V1) and rates of adsorption were determined in 3.3 x 10-4 M radioactive chromic acid solutions. This concentration was sufficiently great that the surfaces could be considered to be Raturated upon attainment of constant values. A t intervals the metal samples were withdrawn, washed thoroughly with distilled water and counted. The counting data were corrected for radioactive decay and amounts of chromium were calculated on the basis of comparison of observed counting rates with those of standard samples. Both rates of adsorption and total amounts varied widely depending u on the pretreatment of the metal. An example is illustrate% in Fig. 1. The bottom curve was obtamed with assive chromium, the top curve, with the same coupon aker treatment with dilute hydrochloric acid to produce reactive chromium, and the bottom curve was obtained once more with the same coupon after treatment with boiling concentrated nitric acid, which served to remove radioactivity from the surface and to repassivate the chromium. Time-adsorption curves for passive chromium are illustrated in Fig. 2. It w i l l be seen that a marked change in slope occurred at 20-30 min. as shown in the bottom curve. Another change a t a relatively much longer time may be seen in the upper curve. In solutions containing 3.3 x M chromic acid, the second change occurred approximately 60 times later than the first. Subsequent pick-up (7) George DubperneU, “Chromium Plating,” “Modern Electroplating,” The Electrochemical Society, 1942, pp. 117-143. (8) U. 5. Department of Commerce, National Bureau of Standards Circular No. 499, 1950, p. 47. (9) W. C. Elmore and M . Sands, “Electronics,” McGraw-Hill Book Co.. Inc., New York, N. Y..1949. (10) In a few cases it was necessary to use an auxiliary, highbensitivity galvanometer because of high solution resistances.
739
II
1 20
40
60
100
80
I;
Minutes. Fig. 1.-Adsorption as a function of time; chromium in 3.3 X 10-4 M chromic acid solution: 0, passive; @, etched; 0 , re-passivated.
e Hours.
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x
I
2olfl-a-
40
II
f 20
40
60
80
IO0
Minutes. Fig. 2.-Adsorption as a function of time; passive chromium in 3.3 X lod4 M chromic acid solution; A. abscissa in minutes; B, abscissa in hours.
SHERMAN KOTTLEAND L. 0. MORGAN
740
was very slow. The amount of adsorbed material continued to increase, more and more slowly, until the rate of uptake was less than the errors in observation. At maximum deposition, there was no discernible change in the microscopic ap earance of the metal surface. ‘%he same type of measurement carried out using unetched, reactive samples, yielded curves similar to those of Fig. 2 with the exception that the changes in slope occurred much sooner and were more abrupt. Time-adsorption curves obtained for etched chromium exhibited essentially the same characteristics but the total amounts of adsorption were of the order of 10 times greater. At low microscope magnifications an extensive crack system was visible in samples used for those experiments, which was in all respects similar to those observed by Hackerman and Marshall.ll After long exposure to chromium( VI) solutions, corrosion began to occur on etched surfaces. Viewed under the microscope, reduction products could be seen accumulating at crack intersections. The deposits were not removed by flowing distilled water, but were soluble in concentrated nitric acid. Because there waa no completely reliable information on the true area of the chromium surfaces, it was convenient to assume that a close-packed layer was formed in each case, and to speak of the apparent “roughness” for each mode of deposition and each type of pretreatment. If an area of 25 A.2 was assumed for each adsorbed ion, there were 4 X 10-14 per cm.2 in a monolayer. On that basis the general characteristics observed in time-adsorption curves obtained for the various surfaces may be summarized as follows Passive
Time
Roughness factor
First change in slope Second change in slope Max. adsorption
20-30 min. 20-30 hr. 250 hr.
5-7 25-30 75-80
5-10 min. 2-3 hr. 250 hr.
5-6 20-25 80-100
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Three types of chromium plate were used in the potential experiments: those termed semi-bri h t reactivel2 (as contrasted with mirror-&ish plates), dulfreactive, and etched. There was usually an abrupt change in the rate of adsorption at the same time that potential equilibrium was reached. Data represented in Fig. 3 which were obtained with etched chromium show that after about eight minutes immersion in 10-8 M chromic acid solution, the potential of the chromium became essentially constant, while the adsorption curve approached linearity. When the potential change was plotted as a function of the amount of adsorption at the same time, there was a linear region over an interval of several hundred millivolts. For example, the data of Fig. 3 yielded the curve of Fig. 4 when plotted in that manner. I
1
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200-
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v;
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i
5 0
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> 7
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4100I
Reactive
First change in slope Second change in slope Max. adsorption
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Etched
First change in slope Second change in slope Max. adsorption
20-30 min. 80-100 hr. None
75-80 225-250
Potential Measurements.-With no externally applied current flowing, the potential of chromium in solutions of salts of chromic acid became more cathodic (positive with respect to the reference electrode) with increasing concentration. There were usually ronounced “memory” effects in measurement when a singk coupon was transferred from solution of one concentration to one of another. After a series of potential measurements solutions of successively greater concentration, the coupon was placed again in the most dilute solution. Invariably coupons exhibited the same potential in solutions of lower chromate concentration as they did in the moat concentrated. I
I
I
I
I
I
I
f
Minutes.
Fig. 3.-Adsorption and electrode potential as a function of time for etched chromium metal in 10-8 M chromic acid solution: 0,adsorption values; 0 , electrode potentials. (11) N. Hackerman and D. I. Marshall, Trans. EEeclrochem. Soc.. 89, 195 (1946).
1
I/ 0 :
t
0 I
IO
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I
I
20 30 40 IONS CM? x 1 0 - l ~
Fig. 4.-Potential change as a function of adsorption upon immersion of etched chromium metal in chromic acid solution. Below pH 7 there was no concentration dependence of the slopes of potential-adsorption curves over the range to M chromic acid. I n basic solutions a straight line potential-pickup relation usually appeared, but with a much greater slope. Data were obtained with difficulty; carbon dioxide, which changed the basicity, could not be completely excluded because it was necessary to open the system for each adsorption measurement. Slope of the potential-adsorption plots varied according to the type of chromium used. In units of millivolts per ion-cm.*, the slopes were: semi-bright, 2.12 X dull, 1.67 X 10-13; etched, 0.70 X l0-l8. Apparent areas were used in calculating the values given. In contrast with both active and etched chromium metal, the passive metal did not exhibit linear otential change with adsorption, any changes being irreguyar and very nearly negligible compared to the changes observed with corresponding adsorption for the other plates. Recalling the time-adsorption data of the preceding section, reactive metal behaved similarly to passive metal with regard to total adsorption while both active and etched metal exhibited linear potential-adsor tion relations, each achieving a final, steady state potentiaf, of about -0.3 volt ( v s . S. C. E.). At that potential the active metal became essentially unreactive and the etched metal did not. Electric Moment Calculations.-Data were treated in terms of the resultant total moment and an effective single (12) Betty Pegues, M.A. Thesis, The University of Texas, 1951.
1
,
THECELLULOSE-WATER-SALT SYSTEM
.Time, 1950
ionic moment was calculated from the amount of adsorbate present, using the equation for a r h n e parallel condemer AE = 4sNp
D in which AE is the change in potential, N , the number of adsorbed ions per cm.2, L) the dielectric constant, and p , the dipole moment; D was assumed to be 1. Because the adsorption data were based on unit projected area, the values so obtained were assumed to be too small by the value of the roughness factor for each surface used. IJsing values ohtained by PeguesI2 for the roughness values of the different 25; dull, 32; etched, 67 types of chromium-semi-bright, to 79-the moments were found to he 1.40, 1.41,1.39 debye, respectively.
The linear relationship existing between adsorption of chromium(VT) anions and the observed change in electrode potential of the surface was obtained over the first portion of adsorption curve for reactive and for etched chromium. If all of the surfaces had the same roughness the slopes of the adsorption-potential curves obtained for each would have been the same. Because the surfaces were not equivalent and exhibited roughness depending upon the conditions of deposition and type of pretreatment the net effective moment of the adsorbed ions varied in the several experiments. The effective moment of each ion was the component of the total moment normal to the projected surface. It was to be expected, therefore, that the greater the roughness of the surface the smaller was the effective moment exhibited by each adsorbed ion. Correction for the predicted roughnesses of these separate surfaces did, in fact, lead to a constant value for the total electric moment per adsorbed ion of 1.40 X lo-’* e.s.u. Reactive and passive chromium differed from each other with respect to the rate of adsorption
74 1
and the electrical behavior upon the adsorption of the chromium(V1) anions. The two types of surface were the same with respect to the total amount of chromium adsorbed. It is apparent that the nitric acid passivation of the metal polarized it to about the same extent as did adsorption of chromate ions. We suggest that the passive surface was covered with a layer of polar molecules, which were replaced only slowly by chromium(V1) anions, and that reactive surfaces contained a layer of chemisorbed non-polar oxygen atoms which were replaced more rapidly upon immersion in chromium(VI) solutions. I n each the total surface eventually covered by chromate ions was the same. The total area exhibited by etched chromium was much greater than in either of the other two cases. The roughness facbor of 75 suggests that an extensive interstitial crack system was made available to the solution, probably through dissolution of chromium(II1) oxideLswith hydrochloric acid, so that areas normally blocked off became available to adsorption. Pick-up of radioactive chromium continued a t a slower rate aftm the etched surface was covered with a monolayer of adsorbed ions. The additional pick-up was apparently associated with reduction of chromium(V1) and the formation of oxides.14 Those conclusions were suggested by the formation of visible deposits a t the crack intersections upon long immersion of the etched chromium metal. (13) J. B. Cohen, Trans. Electrochem. Soc.. 8 6 , 441 (1944). (14) Oxidation-reduction occurring under these circumstances may be attributed to local cells set up as a result of depletion of dissolved oxygen a t the bottoms of the cracks or through establishment of concentration gradients within the cracks. Further consideration of the effect was not within the scope of this work.
THE CELLULOSE-M’ATER-SALT SYSTEM BY S.M.NEALE AND G. R. WILLIAMSON FacrrllU of ‘l‘echnology, Uniilersity of Jlanchesler, dlanchesler, England Recewed Augual 17. 1956
The specific volume of cellulose in aqueous solutions of various inorganic and organic salts has been measured. The ppecific volume of cellulose in sodium chloride and in potassium chloride solutions passes through a maximum wit,h increasing solute concentration. A qualitative explanation is offered on the basis of a cellulose-salt linkage. The conditions for the validity of specific volume measurements for the determination of “bound water” in cellulose are also considered. The abtjorption of water from unsaturated vapor by cellophane sheets a t 25 and 40’ impregnated with sodium chloride and potassium chloride, has been studied. Analysis of the isotherms shows that the water uptake of the cellulose-salt system is governed by the quantity of solute directly adsorbed onto the cellulose surface, and is virtually independent of the presence of excess macrocrystalline salt. It is suggested that at low humidities the water molecules are in competition with the salt ions for the artive sites on the cellulose surface, and the adsorption of water is then reduced owing to the presence of the salt. A t a fairly critical activity of water in the system, the adsorbed salt is displaced from the cellulose surface, and both cellulose and salt then absorb watei , so that the absorption at the higher humidities is greater than in the case of cbellulose alone.
Part I. The Specific Volume of Cellulose in Aqueous Solutions Introduction Various methods have been used during the last century in attempts t o obtain accurate and reproducible results for the specific volumes of cellulose in a variety of media, but it was not until 1927 that the first reliable data were published. In this year Davidson] (1927) determined the specific vol(1) C . F. Davidson, J . Terl. I n s l . , 18, T.175 (1927).
urne of cotton and regenerated cellulose in water, toluene and helium gas, and found in every case the specific volume in helium was about 1% lower, and the water value 3+y0 lower, than the value obtained in toluene. The variation of the specific volume of a colloid with the liquid immersion medium used in the determination is well known, and has been attributed to two factors: (1) a compression of the liquid a t the surface of the colloid; (2) A varying degree of
