V O L U M E 2 7 , NO. 6, J U N E 1 9 5 5 0.772, indicating a good correlation between these two variables. Paper produced from the pulp of highest crystallinity, cotton alpha pulp, showed the lowest tensile strength. Unfortunately, no data on tensile strength were available for paper produced from the groundwood pulps, which were characterized by markedly lower crystallinities. Considerable care is taken by paper manufacturers to maintain complete randomness of the pulp fibers during the felting process, so that the properties of the paper will be the same in the machine direction as in the cross direction. Thus, the primary valence chains of cellulose are randomly distributed in finished papers and should have little effect on the tensile strength properties. Although the exact nature of the bonding forces between the pulp fibers is not known, it would appear, from the decrease in tensile strength with increasing crystallinity observed in this investigation, that the disordered cellulose at the surface of the fiber acts as a cementing agent between adjacent fibers. An increase in the disordered fraction should increase the opportunity for interaction between the hydroxyl groups of adjacent fibers resulting in hydrogen bonding. The plot of tear strength versus per cent crystallinity (Figure 11) indicates that there is no correlation between these two variables. Tear strength of paper is considered to be a function of the strength of the individual fiber (20) rather than of the bonding strength between the fibers. The total possible relative error of the crystallinity determinations was about 3.5%, and the error of tear strength testing is considered to be still greater. These considerations preclude stating definitely that no correlation exists between these variables although the results of this study indicate that such is the case. CONCLUSIONS
The method for the quantitative determination of the crystalline content of cellulose appears to be of sufficient sensitivity to distinguish differences in crystallinity among wood pulps. The results obtained point toward the following generalizations : Mechanical grinding decreases the crystallinity of wood fibers; the bleaching process increases the crystallinity; the density of the pulp increases with increasing crystalline contents up to values of approximately 70%, beyond which the density remains con-
a95 stant; the tensile strength of paper appears to be a function of the amorphous cellulose content of the wood pulp; and there is no definite correlation between the tear strength of paper and the crystallinity of the pulp from which it was produced. ACKNOWLEDGMENT
The authors are deeply indebted to the Kimberly-Clark Corp., Neenah, Wis., especially to Homer E. Malmstrom of the research staff, for supplying the pulp samples ‘and the physical data on sheets prepared from these pulps. LITERATURE CITED
(1) Assaf, A. G., Haas, R. H., and Purves, C. B., J . Am. Chem. SOC., 66,59 (1944). (2) Clark, G. L., and Southard, J., Physics, 5 , 95 (1934). (3) Conrad, C. C., and Scroggie, A. G., I n d . Eng. Chem., 37, 592 (1945). (4) Frey-Wyssling, A., Science, 119, No. 3081, 80 (1954). (5) Frilette, V. J., Hanle, J., and Mark, H., J . Am. Chem. SOC.,70, 1107 (1948). (6) Goldfinger, G., Mark, H., and Siggia, S., I n d . Eng. Chem., 35, 1083 (1943). (7) Hengstenberg, J., and Mark, H., Z . K r i s t . , 69,271 (1928). (8) Hermans, P. H., “Contributions to the Physics of Cellulose,” Elsevier, Iiew York, 1946. (9) Hermans, P. H., and Weidinger, A., J . A p p l . Phys., 19, 491 (1948). (10) Hess, K., 2. physik. Chem., B49, 64 (1941). (11) Hess, K., Steurer, E., and Fromm, H., Kolloid-Z., 98, 148 (1942). (12) Kaufman, H. S.,J . Am. Chem. SOC.,75, 1477 (1953). (13) Mark, H., J . Phys. Chem., 44,764 (1940). (14) llathews, J. L., Peiser, H. S., and Richards, R. B., Acta Cryst., 2, 85 (1949). (15) Nelson, hl. L., and Conrad, C. ll.,Textile Research J . , 18, 149 (1948). (16) Ibid., p. 155. (17) Nickerson, R.F., Ind. Eng. Chem., 34, 85 (1942). (18) Philipp, H. J., Selson, AI. L., and Ziffle, H. If.,Textile Research J . , 17, 585 (1947). (19) Snedecor, G. W., “Statistical Methods,” Iowa State College Press, .4mes, Iowa, 1948. (20) Stephenson, J. N., “;\fanufacture of Pulp and Paper,” Vol. V, XlcGraw-Hill, New York, 1929.
RECEIVED for review November 26, 1954. -4ccepted February 10, 1955.
New Polarographic Electrode Employing Controlled Stirring PAUL ARTHUR, JOSEPH C. KOMYATHY1, ROY
F. MANESSZ,
and
HERMAN W. VAUGHAN3
Department o f Chemistry, Oklahoma Agricultural and Mechanical College, Stillwater, O k l a .
An apparatus is described in which the solution surrounding a stationary electrode of small diameter is stirred by means of a revolving tube, the lower end of which surrounds the electrode. With a wax-coated, mercury-filled electrode tube, excellent polarograms can be made. The curves are reproducible, smooth, and easily measurable. Large diffusion currents and very small residual currents are characteristic. With the proper techniques the curve obtained can be used to determine n values. Half-wave potentials for cathodic reactions are usually somewhat more negative than are those obtained with the dropping mercury electrode, the magnitude of this variation (0.00 to 0.075 volt) depending upon the ion studied and, in some cases, its concentration. For those ions studied, the diffusion currents varied linearly with concentration.
T
HE problem of eliminating time maxima in polarograms is
essentially one of decreasing the thickness of the effective diffusion layer around the electrode, so that equilibrium can be established very rapidly. Skobets and coworkers (6) reported that they were able to accomplish this by running the polarograms on solutions held a t 50” to 60” C. The principle of moving the electrode rapidly through the solution was used successfully by Laitinen and Nolthoff ( 2 ) in their rotating microelectrode, while Muller (I)demonstrated that excellent polarograms could be obtained by using stationary electrodes with flowing solutions. Working independently, Maness and Arthur (3) and Rogers and others ( 5 ) tested stationary electrodes with stirred solutions. Both groups found that while the time maxima disappeared, the resulting curves contained irregularities and reproducible results 1
2
8
Present address, The Texas C o . , Beacon, N. Y. Present address, Hanford Works, General Electric Co., Riohland. Wash. Present address, Dow Chemical Co., Rocky Flats, Colo.
896
ANALYTICAL CHEMISTRY
could not be obtained. I n the belief that these irregularities were caused by turbulence in the stirred solutions, Maness and Arthur ( 3 ) then tested several methods of stirring designed to reduce turbulence or to produce turbulence of such high frequency that its effect would be damped out by the polarograph itself. As a result of these experiments. it was found that excellent polarograms can be made with a stationary mercuryfilled electrode tube, if stirring is accomplished by means of a revolving stirrer tube the lower end of which surrounds the electrode. APPARATUS AND MATERIALS
All polarograms were made on a Sargent Model X X I polarograph, critical voltages being checked by means of a Leeds 8; Korthrup studenbtype potentiometer. All reagents were reagent grade. The cell and electrode assembly used in all experiments except those involving modifications in design is illustrated in Figure 1. All glass parts were made of borosilicate glass. The standard spherical joint on the side arm was used to connect the electrode compartment to a saturated calomel electrode, the corresponding side arm on the latter being fitted with a fritted-glass disk and a saturated potassium chloride-agar plug in order to minimize diffusion. The whole assembly was immersed in a constant-temperature bath held a t 25.00’ =k 0.05’ C.
AIR VENT,
7
t
t-
GLASS ROD (FOR ADJUSTING 4)
the mercury moving upward. The final position of the meniscus is critical when diffusion currents are to be measured. With the stirrer revolving at 600 r.p.m., the polarograph is started and the run made in the usual manner. When the run has been completed, the electrode is flushed immediately by allowing 4 or 5 drops of mercury to spill into the cell. If a new solution is to be run, the cell and electrode should be rinsed before and after this flushing process. The electrode must never be allowed to become dry; otherwise it will be necessary to recondition it before it is employed again. A new electrode, or one that has stood without use too long or has become dry, must be conditioned before good polarograms can be obtained. Even with the wax coating. a t or above -1.2 volts (us. S.C.E.) moisture creeps into the electrode tube, and unless this process reaches equilibrium, the curves obtained xill be very erratic. With such electrodes, therefore, the cell should be filled with carrier electrolyte solution, the solution should be degassed, and then about - 1.4 volts should be impressed across the cell for 2 or 3 minutes. If the electrode is then flushed with 2 or 3 drops of mercury, it can be employed with good results. RESULTS
Curve Characteristics. In order to test this electrode for versatility and general applicability to polarographic problems, repeated runs aere made on solutions of ions chosen to represent cases where the reduction product is soluble in mercury (cadmium, lead, copper, and zinc), the reduction product is insoluble in mercury-e.g., nickel-the oxidized and reduced forms are both soluble in the solution (the oxalate complexes of iron), and both forms are soluble in the solution, but the process is not reversible-e.g., iodate. I n the case of every system described in Table I, excellent polarograms were obtained. As would be expected, the curves were smooth, with none of the oscillations characteristic of the dropping mercury electrode. The electrode with its greater area gave diffusion currents larger than those ordinarily obtained with the dropping mercury electrode, while, on the other hand, the residual currents were very small (often negligible), and the limiting current curves were so flat that diffusion currents could be amplified and measured with unusual ease and accuracy.
TabIe I.
Half-Wave Potentials and Diffusion Currents
Diffusion Current, E1,z U S . SCE fia./hlilliCarrier Concn., mole,’ New DhlE Ion Electrolyte mM Liter electrode value Cd 0.1.VKC1 1.99 9.27 -0.639 .0.598 1.49 9.22 -0.632 0.998 9.28 -0.623 0,699 9.04 -0,619 0.399 9.27 -0.619 0.0998 9.09 -0.61 Nit+ O.lNKC1 1.503 928 -1.05 -1.01 1.002 9.13 -1.01 i0.01 0.701 9.47 -1.03 i0.01 0.401 9.23 -1.023 i 0,006 0.1002 9.32 -0.986 Pb++ 0.1NKC1 5.01 10.18 -0.421 -0.404 0.428 10.03 -0,417 -0,404 0.107 10.23 -0.412 -0,404 Fe(C*Oa)J--- 1M KlCzOl 0.986 3.21 -0.251 -0.245 CUT+ 0 . 1 M K2SOd 1 . 0 0 6.00 -0,014 -0.02a IO*0 . 1 N HzSOa 0 . 9 6 2 24.32 -0.083 -0.075b Zn+’ 0.1SKC1 1,010 9.36 -1.072 -0.997 Carrier electrolyte, 0.1.M potassium nitrate ( 1 ) . b In 0.2.W potassium nitrate adjusted with sulfuric acid to p H of 1.15 ( 1 ) .
+’
Figure 1.
Cell, electrode, and stirring arrangement
The dimensions given in Figure 1 are typical and except where otherwise noted are those of the assembly used in this research. With this electrode, it was found necessary to coat with wax about 3 em. of the inside of the electrode tube by dipping the warmed tube into hot (preferably ceresin) wax, using filter paper to remove droplets that formed by drainage, and to “condition” the electrode (see later discussion) before using it for actual runs.
(I
PREPARATION FOR RUNS
I n preparing the apparatus (Figure 1)for use, it was found that the following sequence of steps gives the best results. The electrode tube is filled with mercury, the cell is assembled and filled with the desired solution, and then the solution is degassed. With the nitrogen still flowing, the stirrer tube is inserted and adjusted so it encloses about 5 mm. of the upper end of the electrode tube. The latter must be centered inside the stirrer, but visual estimation is adequate. Two or three drops of mercury are spilled to flush out the tip of the electrode tube; then the meniscus of the mercury is adjusted so that its highest point is exactly flush with the upper edge of the electrode tube. This adjustment must be made with
Within the limits imposed by variations in the positioning of the mercury surface in the electrode, the relationship between I d and concentration was linear for all ions tested-i.e., cadmium, lead, and nickel. When visual positioning of the meniscus was accomplished with the unaided eye, the maximum error was about 1%; when a five-power magnifier was used, the error was substantially smaller. With cadmium, the diffusion current increased an average of 2.3y0 per 1” C. rise in temperature, in the range from 20” to 35’ C. I n no case was any maximum of the type so frequently en-
89?
V O L U M E 2 7 , NO. 6, J U N E 1 9 5 5
with a stirrer 7.15 111111. in inside diameter arranged to enclose the upper 5.0 mm. of the electrode tube. It had been expected that the current would be proportional to the area of exposed mercury, and therefore-since the area should lie somewhere between the area of a hemisphere and that of a flat circle-would be proportional to the radius squared. The results indicated, however, that different parts of the mercury surface were not equally effective; for the current, with electrodes from 1.79 to 3.89 mm. in inside diameter was an excellent straight-line function of the diameter. I t seems probable that inadequate stirring caused by the protective action of the walls of the tube rising around the mercury meniscus is responsible for this, this effect becoming even greater as the electrode diameter is further decreased (see Figure 3). 601
0.6
Figure 2.
VOLTS
1.2
Influence of concentration on wave form Cadmium(l1) in 0.1”V potassium chloride 1. 0.002.M 3. 0.0007M 2. 0.001M 4. 0.0001.M
countered with the dropping mercaury electrode ever observed. Owing to this it waq found possible to separate closely spaced waves-e g., the tIvo-step reduction of copper(I1) ion in 0.1N potassium chloride-where the dropping mercury electrode gave poor results. On the other hand, chromate ion in 0.1N potassium chloride, inO.1 S sodium hydroxide, and in 0,LV sodium hydroxide gave very poor waves, with strong evidence of an accumulative polarization blocking the electrode reaction. Two peculiarities are observed in curves obtained with this electrode. The first of these is the occurrence of slight dips or irregularities in the curves for cadmium, nickel, and copper, shortly before the carrier wave is reached. No other ion tested gave this dip and, as will be seen from Figure 2, the effect becomes smaller and eventually disappears entirely as the concentration of the reducible ion is decreased. The second oddity is the limited range (compared with that of the dropping mercury electrode) exhibited by these electrodes. Unwaxed electrodes gave polarograms in which the carrier wave appeared a t about -1.65 volts; with waxing and with the techniques described earlier, the range increased to about - 1.90 in neutral solutions. This limiting voltage was the same for three supporting electrolytes-Le., potassium chloride, lithium chloride, and tetramethylammonium chloride-but decreased somewhat when solutions of a p H of around 2 were used. A comparison of the half-wave potentials given in Table I shows that those ions whose reduction products are soluble in mercury-i.e., cadmium, lead, copper, and zinc-give half-wave potentials from 0.01 to 0.075 volt more negative than the corresponding dropping mercury electrode values. Although the influence of concentration was determined only with cadmium and lead, the half-wave potentials of these two shifted to more positive values as the concentration was decreased. On the other hand, the half-wave potential of the ferric oxalate complex, whose reduction is reversible and R-hose reduction products are soluble in the solution, agreed well with that obtained a t the dropping mercury electrode. For nickel, whose reduction product is soluble in neither the mercury nor the solution, the values obtained were erratic, those listed being average values taken from several runs. It was not found possible to reproduce these half-wave potentials more accurately by control of experimental conditions. Even with nickel, however, the difference is EO small that no difficulty would be encountered in identifying the wave. Electrode Dimensions. The relationship bet-iveen electrode diameter and diffusion current was studied with five thin-walled electrode tubes ranging from 1.03 to 3.89 mm. in inside diameter,
50
-
40
-
30
-
20
-
I
w
d
_-----e----’ , 0.5
IO
I 5
20
2.5
3.0
385
4.0
ELECTRODE DIAMETER I Y Y , ~
Figure 3. Diffusion current diameter
US.
electrode tube
That such protective action exists was shown also when successive runs were made on cadmium solutions without flushing the surface mercury from the electrode. A s would be expected, at the beginning of the second run in each of such series an anodic wave, due to dissolution of cadmium metal, appeared; in addition, however, the cathodic wave itself had a small rounded hump a t the place where it should have leveled off into the limiting current. Beyond this hump the wave was normal in all respects. On succeeding runs both the anodic wave and the hcmp became progressively larger. When, with a later modification of the electrode, a platinum bead was used to support the mercury so it protruded above the electrode tube and was thus all exposed to the action of the stirrer, such successive runs yielded an anodic wave but gave no hump. In explaining these humps, therefore, it n-as felt that when the cadmium formed in earlier runs was anodically destroyed, the cadmium ions formed were not efficiently swept out of the space where the mercury curved downward to meet the electrode tube walls. An unusually high concentration of cadmium ions was thus available for the cathodic reaction, this causing the current to rise above the normal limiting current, then fall off as the excess was depleted. Such an effect would be somewhat cumulative and such also was the observed result. With the bead arrangement, the anodic wave observed would be expected, but no hump would form since anodically produced cadmium ions would be readily swept away. Stirrer Diameter and Positioning. In this part of the investigation, six stirrers Kith diameters ranging from 5.00 to 18.6 mni. were employed, first with an electrode 1.79 mm. in diameter and then with various electrodes from 1.03 to 3.89 mm. in diameter. The results pointed rather definitely to two facts: For a given electrode diameter, the smaller the diameter of the stirrer (allowing. however, for free passage of solution), the greater the diffusion current will be; and stirrer tubes 11 mm. or more in internal diameter permit undesirable turbulence and consequently
ANALYTICAL CHEMISTRY
898 yield erratic curves. The internal diameters of both tubes, therefore, influence greatly the magnitude of the diffusion current, an electrode 2.71 mm. in diameter, for example, giving, with stirrers 7.15 and 7.78 mm. in inside diameter, diffusion currents of 26.0 and 16.4 p a , respectively, for 0.001M cadmium in 0.l.V potassium chloride. Of the combinations tested, a 3.89-mm. electrode with a 7.15-mm. stirrer gave the largest currents. S o efforts were made to go beyond this electrode size. To determine how far the electrode should extend upward into the tube, an electrode 1.79 mm. in diameter was used with stirrers 7.15 and 11.26 mm. in diameter, with the electrode extending 12.0, 9.0, 6.0, and 3.0 mm. into the stirrer, and also with the lower end of the stirrer level with the top of the electrode, 3.0 mm. above it, and 6.0 mm. above it. Even with the stirrer above the electrode good curves could be obtained, an occasional one, however, being erratic, especially when the larger stirrers were employed. Marked differences in the diffusion currents ohtained were observed, however, the currents for 0.001.V cadmium when the i.15-mm. stirrer was employed being (for the positions mentioned above, and in that order) 9.17, 7.70, 6.50, 5.90, 3.20, 2.30, and 3.10 pa., respectively. These results show that vertical positioning of the stirrer must be duplicated closely when accurate diffusion current measurements are desired. When closely fitting electrode and stirrer combinations were used, the electrode was found t o operate most smoothly and reproducibly with 3.0 to 6.0 mm. of its tip inside the stirrer. Greater lengths frequently gave curves whose limiting currents sloped downward as the voltage increased, indicating the probabilitx that the transfer of solution t o and from within the stirrer was restricted resulting in a significant decrease in the concentration of available reducible ion. Rate of Stirring. Studies of the influence of stirring rate upon diffusion current were made at rates varying, in steps of 100 r.p.m., from 400 to 1100 r.p.m. Some evidences of stirrer vibration a t 1000 t o 1100 r.p.m. made values obtained a t these speeds somewhat questionable. From 400 to 800 r.p.m., however, good results were obtained, the current increasing about i%over this range. The least change occurred in the range of 600 to 800 r.p.m., this amounting to only 1.77,. As this represents a change of 33% in rate, the results would indicate that, for this electrode, nonsynchronous motors map be emploved for ordinary purposes. THEORETICAL
Although diffusion currents and electrode dimensions can be related only empirically, a study of equations developed ( 3 ) for the dropping mercury electrode shows that, for certain classes of electrode reaction, some of these equations might be expected t o apply equally well to this new electrode. Thus for cases in which the reduction product is soluble either in mercury or in
i
water, the graph of E us. the function logZ=
should be a straight
line with a slope numerically equal to 0.059/n; while for cases where the product is a metal insoluble in mercury the slope of the graph E us. log ( i d - i) is equal to 0.059/n. This new electrode differs from the dropping mercury electrode in one very important respect-Le., the fact that the latter has a constantly self-renewing surface. TVith the dropping mercury electrode, therefore, any amalgams formed must always be very dilute, and any accumulation of insoluble metal-e.g., iron-will be swept away with each falling drop. For those cases where the product dissolves in the electrolyte solution, self-renewal of the surface should offer no advantage. Consequently, it would be expected that where the product is watersoluble, the slope of the graph E
1s.
log &should
equal 0.059;n
for the new electrode as well as for the dropping mercury electrode. This same relationship should also hold when the product is a metal soluble in mercury, but only for very dilute solutions, as only then will the amalgam formed be sufficiently dilute.
Table 11. Number of Electrons Per Ion Ion Cd++
P b +’ CU++ I03 Fe(CzOd--ZntT
Concn., Millimoles/ Liter 1.99 1.49 0.998 0,699 0.399 0.0998 5 01 0 428 0 107 1 00 0 962 0 986 1 01
Calrd.
Carrier Electrolyte 0 . 1 N KC1 0.I N KC1 0. I N KC1 0.1NKC1 0 . 1 N KC1 0.l.V KC1 0 1,V KC1 0 l.V KC1 0 1 N KC1 0 llii K2SOd 0 1.V Hi304 1 O M KzCzOd 0 l.V KCl
n
1.2 1.4 1.5
1.6 1.7 2.0 1 4 1 4 1 9 1 5 0 63 1 0 0 9
The truth of this reasoning was shown when graphs of both relationships were made of results obtained with several ions
i
(see Table 11). In the first place E cs. log -gave straight Zd-
Z
lines for all except nickel; the other relationship gave badly curved lines for all. Secondly, the n values for the various ions, as calculated from the slopes of the graphs, varied in the expected manner. With cadmium, lead, copper, and zinc the calculated values were too small when concentrations from 0.3 to 5.0mM were employed; but with the two ions tested-Le., cadmium and lead-as the concentrations was decreased the slope changed and the n values approached the theoretical. The ferric oxalato complex, whose reduction product remains in solution, gave the theoretical value even in 1 m X solutions. The low results obtained Tyith iodate are comparable to those reported for the dropping mercury electrode and are generally attributed t o the irreversibility of the reaction (3). CONCLUSIONS
Slthough the fact that the surface of the new electrode is not self-renewing will probably make the electrode less versatile than the dropping mercury electrode, it should be superior in the accuracy obtainable in certain quantitative analyses, in extending the application of polarography to lower ranges of concentration, and in making it easier to use oscillographic and differential polarography. For the latter purpose, it should be easy to duplicate electrodes within very small limits of error. By using solutions of very low concentration, half-wave potentials comparable to those obtained with the dropping mercury electrode should be obtainable. ACKNOWLEDGMENT
The authors wish to express their appreciation for the assistance rendered in the form of a grant [ilEC Project (11-1)-71, Project KO.31 from the Atomic Energy Commission through the Research Foundation of Oklahoma X&M College. LITERATURE CITED
Kolthoff, I. 31.,and Lingane. J. J., “Polarography,” Interscience New York, 1946. Laitinen, H. A , , and Kolthoff, I. AI., J . Phys. Chem., 45, 1079 (1941). Rlaness, R. F., “Investigation of Polarographic Electrodes,” unpublished L1.S. thesis, Oklahoma -1.& Rl. College, 1950. lIuller, 0. H., J . Am. Chem. Soc., 69, 2992 (1947). Rogers, L. B., hliller, H. H.. Goodrich, R. B., and Stehney, A. F., ASAL.CHEM.,21, 777 (1949). Skobets, E. 3T.,Torov, P. P., and Ryaboken, V. D., Zatodslzaya Lab., 14, 131 (1948). RECEIVED for review August 28, 1952. Accepted February 21, 1955. Presented in part a t the Seventh Southwest Regional Meeting of the AWERICAN CHEMICAL SOCIETY, Austin, Tex., December 6 t o 8, 1951. Portions taken from theses presented t o the Graduate School of Oklahoma Agricultural and Mechanical College by Herman n’. Vaughn and Roy F. Maness in partial fulfillment of t h e requirements for the MS. degree and by Joseph C . Komyathy in partial fulfillment of the requirements for t h e M.S. and Ph.D. degrees.
