ZINC TETRO.XY CHROMATE A Rust-Inhibitive Primer Pigment W. W. KITTELBERGER The New Jersey Zinc Company (of Pa.), Palmerton, Pa.
The excellent rust-inhibitive properties of chromates are generally recognized; zinc yellow, perhaps most important of the chromate pigments, is finding extensive use in metal primers for the protection of ferrous and nonferrous metals and alloys. Efforts to overcome the effect of a relatively high content of water-soluble sulfate as well as a tendency to blister under exposure to high humidity or water have resulted in the development of a new zinc chromate pigment, ZnCr04.4Zn(OH),. This zinc tetroxy chromate is much less soluble in water than is ordinary zinc yellow and hence yields more water-resistant H E outstanding corrosion-inhibitive properties of the chromate ion are well recognized by scientists and technologists charged with the production, design, and maintenance of metals and metal structures (1, 9, 10, 11, 16, 16) although the exact nature of the protective film produced by chromates is still a point of controversy (3). The development of the stainless steels, with their relatively high content of chromium, and the increasing use of chromates in the pretreatment of metal articles (2, 8) to enhance their resistance to corrosion are but two of the many applications of the unique properties of this element and its compounds to the problem of conserving metals.
T
paint films. Since it is essentially free from corrosion-inciting ions such as sulfate, none of its water-soluble chromate is needed to counteract the effect of eelfcontained harmful ions, and all is available for combating external corrosive agents. Laboratory fresh and salt water immersion tests, tide-water-level exposures, and electrode potential measurements have demonstrated the water resistance and corrosion-inhibitive properties of priming paints containing zinc tetroxy chromate. Exterior exposures indicate excellent durability and weathering properties for zinc tetroxy chromate primers. the films were completely stripped with paint remover from the 1 X 2 inch sections marked C (Figure 2) As Figure 2 shows, the zinc dust-zinc oxide, lead chromate, and zinc yellow paints were the best of the six included in this series. One coat of the lead chromate paint permitted some rusting, but two coats afforded almost perfect protection to the metal. There was some rusting of the metal under the two coats of the red lead and the two coats of the iron oxidezinc oxide paint, and considerable rusting under the single coats of these two paints and under both one and two coats of the blue lead paint. I
Water Exposure Tests of Chromate Pigments
Atmospheric Exposure Tests on Chromate Pigments
This set of panels is typical of the good results obtained with zinc yellow and basic lead chromate paints in the protection of iron and steel under atmospheric exposure conditions. However, in underwater or extremely wet service conditions, zinc yellow and basic lead chromate primers disintegrate rapidly, although this weakness can be overcome to a considerable extent by the use of highly water-resistant synthetic resin vehicles with these primer pigments. This lack of water resistance has been demonstrated repeatedly by Florida tidewater-level exposures and laboratory immersion tests in tap water and sea salt solutions. A few characteristic results of such tests are shown in Figures 3 and 4. The iron and steel panels in Figures 3 and 4 had been primed with raw linseed oil paints containing the pigments indicated. The left half of each panel was then given a finishing coat of aluminum paint prepared with a vehicle recommended for this type of exposure. These photographs illustrate the chief weakness of the usual zinc yellow and lead chromate primers, a lack of water resistance which permits the films to blister badly and leads to corrosion of the base metal. I n the case of zinc yellow primers poor water resistance is
Paint technologists have not neglected the chromate pigments ( I ) , and their value in priming paints for metal protective service is more widely recognized today than ever before. Our own interest in the chromate pigments was stimulated by the excellent results frequently obtained during investigations of various metal-protective primers. Typical results for a representative series of such paints after 5.5-year exposure a t an angle of 45" facing south in a semiindustrial atmosphere are shown in Figures 1 and 2. The 5 X 10 inch (12.7 X 25.4 om.) panels shown in Figure 1 were cut from exposed, larger, copper-bearing black iron panels which had been finished with one coat (right half) and two coats (left half) of raw linseed oil primers containing the pigments indicated. To permit a better portrayal of the results of these tests, these sections were cut as shown in Figure 1. The small sections (1 X 2 inches, or 2.5 X 5.1 cm.) marked B were washed with soap and water to remove the dirt and grime which had accumulated during the exposure and show the condition of the paint films (Figure 2). I n order to determine to what extent the paints had protected the metal, 363
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INDUSTRIAL AND ENGINEERING CHEMISTRY
believed to be largely due to this pigment's high content of water-soluble material. For example, nine samples of domestic zinc yellow had a n average of 26.1 per cent watersoluble material. (All solubility data referred to in this paper were obtained by A. S. T. M. method D-126-36.) The relatively high sulfate content of zinc yellow is another serious handicap, since sulfate ions have a definite corrosion-inciting action which will obviously nullify the inhibitive effect of some of the soluble chromate. This effect will be illustrated later. The nine samples of zinc yellow mentioned above were found to have an average water-soluble SO8 content of 2.2 per cent, equivalent to 4.8 per cent potassium sulfate. The relatively poor showing of the scarlet basic lead chromate paints in mater immersion tests cannot be attributed to a high content of water-soluble material because this pigment is relatively insoluble. I n fact, its solubility is so low that it furnishes insufficient chromate ion to inhibit corrosion under relatively severe exposure conditions. This undoubtedly accounts for the bad rusting of the metal under this primer in the tidewater tests. The severe blistering in the water immersion tests would seein to be due to the inability of this pigment to improve the inherently poor water reaistance of the linseed oil binder.
Development of ZTO Chromate Aware of the corrosion-inhibitive properties of the chromate ion and the excellent metal protection rendered by commercial chromate pigments under atmospheric exposure conditions, and realizing the serious weaknesses of both lead chromates and zinc yellom, this laboratory started work a number of years ago on the development of a zinc chromate pigment in which the weaknesses of zinc yellow would be eliminated without sacrificing its good properties. Our aim was to find a compound of zinc and chromium which could be produced as a pigment and would meet the following requirements: Freedom from such corrosion-inciting anions as chloride, sulfate, etc. A relatively low but uniform solubility. This pigment should not be a mixture of a number of chemical compounds of widely different solubilities, some of which would rapidly leach out of the paint film early in the exposure, resulting, perhaps, in a later deficiency in inhibitive soluble chromate. Good water resistance in the conventional type of raw linseed oil vehicle so widely used in metal primers. These requirements were met by a compound with the formula ZnCr04.4Zn(OH)z. It is a chemical individual giving a characteristic x-ray diffraction pattern which differs
Top, 100% blue lead Top, 80/20 zinc dust/zinc oxide Middle, 100% red lead Middle, 100% scarlet basic lead chromate Bottom, 85/15 iron oxide/zinc oxide Bottom, 100% zinc yellow Two coats of paint on the left half, one coat on the right half of each panel.
FIGURE1. TESTSO F RAWLINSEEDOIL
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PRIMERS O X SANDBL.4STED
BLACKIRON
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
FIQURE 2. SECTIONS OF PANELS SHOWN IN FIQUHE 1 B, small washed sections; C. sections from whioh paint ooatinga were removed.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
radically from that of commercial zinc yellow (Figure 5 ) . Following the usual system of nomenclature for zinc-chromium compounds of this type ( l a ) , we have called this compound "zinc tetra-oxy chromate", which has been shortened to zinc tetroxy chromate, or simply ZTO chromate. ZTO chromate is a golden yellow pigment of very fine, uniform particle size and a specific gravity of 4.0. It has a moisture content of about 0.5 per cent a t 110" C. When immersed in pure water, this pigment gives practically a neutral reaction (pH 6.9). I n distilled water ZTO chromate has a water-soluble CrOa content of approximately 0.2 per' cent. However, in sea water the water-soluble CrOs content increases about fivefold. The water-soluble sulfate content of this pigment is less than 0.05 per cent.
Water Immersion Tests Exhaustive tests of various types, some completed and others still in progress, have been used to determine the merits of ZTO chromate. Figure 6 shows the characteristic behavior of a group of raw linseed oil primers formulated with ZTO chromate and zinc yellow in laboratory waterimmersion tests. Duplicate sets of panels are illustrated.
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The one group was photographed after the painted steel panels had been immersed in fresh tap water for 1000 hours, the other after an immersion of 650 hours in a 3 per cent solution of sea salt. The good water resistance of the ZTO chromate primers and their superiority over the corresponding paints containing zinc yellow, are clearly indicated by the relative blistering of the films. The differences are most easily seen in the case of the two-coat systems exposed in sea water. A further demonstration of the excellent water resistance of primers pigmented with ZTO chromate is afforded by Figure 7. These are photomicrographs of raw linseed oil primers pigmented with 100 per cent ZTO chromate and with 100 per cent zinc yellow after a condensation type of water resistance test. The panels of aluminum alloy and steel were primed with the chromate paints and given a finish coat of aluminum paint. They were tested by exposure on a watercooled brass plate inclined at an angle of 30" to the horizontal in a high-humidity cabinet (practically 100 per cent relative humidity a t 110" F.). Under these conditions the test films were continually wet with condensing water. On completion of the tests, the coat of aluminum paint was carefully removed from the primer film with a small, sharp, chisel-shaped in-
March, 1942
INDUSTRIAL
strument. Figure 7 shows the porous and pitted zinc yellow fiJm contrasted with the dense firm ZTO chromate film on the aluminum alloy panels. The steel panel tests (Figure 7) showed the same porosity and pitting of the zinc yellow primer and, in addition, rusting of the metal; the ZTO primer was practically unaffected by this exposure. A paint film protects a metal article against corrosion either by preventing, by mechanical means, contact of the corrosive agents with the metal surface or by inhibiting corrosion chemically. In most practical cases a paint film will perform both of these functions more or less effectively. The preceding paragraphs have shown that zinc yellow paints exercise good chemical protection but fail under certain service or exposure conditions because of poor mechanical resistance t o water or moisture. Since ZTO chromate paints have the desired water and blister resistance, it was of interest to determine whether this had been achieved by sacrificing the chemical protection generally attributed to chromate pigments.
Electrode Potential Measurements We first considered the idea of measuring or merely observing the corrosion of small steel strips immersed in water slurries of various pigments, or of using the so-called ferroxyl agar jelly test to indicate corrosion or prevention of corrosion. However, it was felt that a test which could be made on paintcoated panels rather than on the pigments themselves would have greater practical significance. This point of view is also expressed by Burns et d.(6, 7), who indicate that corrosion tests with aqueous suspensions of pigments may be misleading.
EMISTRY
367
Several investigators have used potentiometric measurements for studying the corrosion inhibition exercised by the so-called inhibitive pigments and paints (4,6,l a ) . This same principle and a somewhat modified technique were used in our studies of the metal protective properties of raw linseed oil p r i m e r s p i g - , mented with ZTO chromate: The panels were of S. A. E. specification 1010 cold-rolled steel strip, 4 X 6 X 0.035 inch (10.2 X 15.2 X 0.089 om.), and were carefully selected for flatness. The various paints were applied to the thoroughly cleaned steel panels by means of a steel spreader having a clearance of 0.005 inch (0.127 mm.).
Commercial zino yellow
FIWJRE5.
BTO ohromate
X-RAY DIFFRACTION PATTERNS
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INDUSTRIAL AND ENGINEERING CHEMISTRY
65/35 ZTO chromate/iron oxide
1.
100~o ZTO chromate
3.
2.
100% zinc yellow
4. 65/35 zinc yellow/iron oxide
Upper row, salt water: lower row, fresh water.
5. 6.
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35/35/30 ZTO chromate/iron oxide/asbestine 35/35/30 zinc yellow/iron oxide/asbestine
Left half of each panel, primer plus aluminum finish coat; right half, primer only.
FIGURE 6. BLISTERING OF ZTO CHROMATE AND ZINC YELLOW PRIMERS ON STEELPANELS
After drying, the films were in the neighborhood of 0.0025 inch (0.0635 mm.) thick. The paints were allowed to age 10 days before glass cylinders (open at top and bottom) were cemented to the coatings with paraffin (Figure 8). Duplicate tests were carried out on each film. Fifty milliliters of the corrosive solution (distilled water or a 3 per cent solution of sea salt in distilled water) Tyere placed in each cell just a few seconds before the first potential measurement was made. The other half of each corrosion cell consisted of a commercial calomel electrode (National Technical Laboratories No. 1170) which was merely immersed in the solution to be tested. An alternating-cur r e n t, compensated, thermionic electrometer resembling the circuits of Barth ( 5 ) and Penick ( 1 4 ) , in combination with a potentiometer, was employed to measure the voltages of the corrosion cells. This type of instrument was used because it permits determination of the potentials with such extremely small current flow that the danger of polarization of the cell is practically eliminated. All electrode potential values discussed in this paper are referred to a scale on which the normal hydrogen electrode has a potential of zero.
Electrode Potential-Time Tests
100% Z T O chromate
1 0 0 7 , zinc yellow
FIGURE 7. PHOTOMICROGRAPHS ( X 2 5 ) OF RAWLINSEED OIL PRIMERFILMS AFTER A CONDENSATION TYPEOB WATERRESISTAKCE TESTON ALWMIKEM ALLOYPANELS (above) AND STEELPANELS (bebw)
To obtain some idea of the relation between corrosion and electrode potential values, bare steel panels were tested in contact with several different aqueous media. Figure 9 shows the behavior of uncoated steel in distilled water, a solution of 3 per cent sea salt in distilled water, and a potassium dichromate solution
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
FIGURE 8. CORROSION TEST CELLSFOR ELECTRODE POTENTIAL MEASUREMBNTS
369
a constant negative value, are generally associated with corrosion of the metal. Actually, however, this phenomenon is much more complicated, especially if all possible types of exposure conditions are to be considered, The measured electrode potential in a corrosion cell of this type is a resultant value which depends upon the anode and cathode potentials and resistances of the shortcircuited cells on the metal panel surface. The only true measure of the rate of corrosion is the current which flows between these anodic and cathodic areas; this of, course, is greatly influenced by polarization, either anodic or cathodic, or both. The conditions under which we measured the electrode potentials of painted steel panels were designed to simulate what might be termed “normal” under water exposure for painted steel structures. That is, the corrosion medium contained sufficient oxygen to prevent cathodic polarization from controlling the rate of corrosion. The electrode potential-time curves for five painted steel panels appear in Figure 11; photographs of the test areas of metal after removal of the coatings on completion of the experiment are presented in Figure 12. That there is a t least a rough correlation between the curves and the degree of rusting of the metal is evident. However, it should be pointed out that while panel 3 (Figure 12) appears to be the most severely corroded because of the uniform deposit of rust over the entire area, panels 4 and 5 showed numerous points of concentrated attack (pitting) so that 5, a t least, undoubtedly represents more corrosion than 3. Following the preliminary experiments just described, electrode potential measurements were made on steel panels coated with raw linseed oil paints pigmented with ZTO chromate and zinc yellow, and 65-35 combinations of these pigments and iron oxide. Electrode potential-time curves for four painted steel panels in contact with a 3 per cent solution of sea salt are presented in Figure 13. During the 1000-hour test period there was a gradual decrease in the electrode potential values for all four paints. This change was much greater for the zinc yellow than for the ZTO chromate films. The condition of the metal under these coatings after 1000-hour contact with synthetic sea water is shown in Figure 14. There was considerably more corrosion under the zinc yellow than under the ZTO chromate primers. Similar tests were carried out with these same paints in fresh water and with other zinc yellow and ZTO chromate primers in both fresh and salt water; in all cases the results were essentially the same as those pictured in Figure 14. The results of these electrode potential measurements indicate that in producing ZTO chromate there has been no
containing 0.02 gram of CrOa per liter. The electrode potential of the metal in contact with distilled water snd with sea water dropped sharply soon after the start of the test, and the metal rusted rapidly. I n contact with the dichromate solation, the potential first rose slightly and then remained approximately constant a t $0.33 volt. The metal remained bright and uncorroded. The effect of sulfate ion on the concentration of chromate ion required to inhibit the corrosion of steel is shown in Figure 10. The electrode potentials were measured for steel in contact with a series of potassium dichromate-distilled water solutions, and a series of solutions containing 0.435 gram potassium sulfate (0.2 gram SO8) per liter and increasing concentrations of potassium dichromate. The potentials after 72 hours are plotted against the CrOs content of the solutions. An examination of the steel panels showed corrosion only in those cases in which the potentials were negative. No corroded areas could be detected in the positive potential cells. These curves clearly indicate the corrosive effect of soluble sulfates, and show how much more chromate ion is needed to inhibit rusting in their presence, Earlier investigators in this field appear to have come to the conclusion that a careful interpretation of elect r o d e potential-time curves for painted steel panels would indicate the tendency of a metal to corrode under the test conditions. Thus a rising or a relatively constant positive potential would appear Bo indicate inhibition of corrosion, T I M E IN H O U R S whereas a falling potenFIGURE 9 tial, and possibly also
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-0.8
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CROs CONTENT I N GRAMS PER L I T E R
FIGURE10
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INDUSTRIAL AND ENGINEERING CHEMISTRY
390
sacrifice in rust-inhibitive properties as compared with zinc yellow, in spite of the much lower soluble chromate content of the former.
0.e I W
0.4 I-
$ >
0.2
Mean-Tide and Exterior Exposure Tests
3 . 0
Water immersion and mean-tide level exposure tests in Miami, Fla., verified in principle the conclusions arrived at on the basis of the electrode potential studies. Figures 15 and 16 illustrate the results obtained in two representative tests of this type. At the conclusion of the water immersion tests, the panels portrayed in Figure 6 were stripped. The bare panels are shown in Figure 15. T h e superiority of ZTO chromate over
d
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ELECTRODE POTENTIAL-TIME CURVES FOR PAINTED STEEL PANELS IN DISTILLED WATER
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indicatcci the diifcrences in rusting. In every CXX, for I)otli fresh untl xilt water, t h e was colisiderably more rusting under t h e zinc yelluw tlinn wider the c o rr e?1)o nd i n g 2 T 0 chroninte pnints. (:onsitlcring t l i o s c portions of the p n e l s . . . having a finish coat of
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4 5 FIGURE 12. TESTAREASOF PAINTED STEELPANELS EXPOSED TO DIBTILLED WATER,AFTER REMOVAL OF COATINGS (see FIGURE 11)
1
2
600
400
TIME
IN
1000
800
HOURS
FIGURE 13 1. 100% ZTO chromate 2. 100% zinc yellow 3. 65/38 ZTO chromate/iron oxide 4. 05/35 zinc yellow/iron oxide
3
4
FIGURE 14. TESTAREAS OF PAINTED STEELPANELS EXPOSED TO SEAWATER, AFTER REMOVAL-OF COATINGS (see FIGURE 13)
ZTO chromate ;: El9Upper ainc yellow row, salt water; '
3. 65/35 ZTO chromate/iron oxide 5. 35/35/30 ZTO chromate/iron oxide/asbestine 4. 65/36 zino yellow/iron oxide 6. 35/35/30 zinc yellow/iron oxide/asbestine lower row, fresh water. Left half, primer plus aluminum finish ooat; right half, primer only.
OF FIGURE 15. RUSTING
1. 100% zinc yellow 2. 65/35 zinc yellow/iron oxide
FIGURE 16.
S T E E L P A N E L S UNDER
ZTO
CHROMATE AND Z I N C YELLOW PRIMERS
3. 35/35/30 zinc yellow/iron oxide/asbestine 5. 65/35 ZTO chromate/iro? oxide 4. 100% ZTO chromate 6. 35/35/30 ZTO chromate/iron oxidejasbestine Left half, primer plus aluminum finish coat: right half, primer only.
COMPARISON OF Z I N C YELLOW AND
ZTO CHROMATE PRIMERS ON SANDBLASTED BLACK I R O N P A N E L S AFTER % M O N T H EXPOSURE AT MEAN-TIDE LEVEL 371
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INDUSTRIAL AND ENGINEERING CHEMISTRY
aluminum paint, it will be noted that every zinc yellow panel is rusted, whereas every ZTO chromate panel except one is free from corrosion. Similar results were obtained in exposures a t mean-tide level in Miami. These panels, like the former Florida set, were of sandblasted, noncopper-bearing, black iron. After the entire surface was primed, the left half of each panel was given a finish coat of aluminum paint. The results after 3-month exposure and pertinent formulation data are shown in Figure 16. The superiority of ZTO chromate over commercial zinc yellow is again indicated and is most evident in the twocoat sections of the panels. These show considerable blistering and rusting for the three zinc yellow tests, while the corresponding ZTO chromate panels show only slight failures. It is interesting to note that under these extremely severe exposure conditions, the single-coat tests showed improvement with increasing ZTO chromate content, whereas with zinc yellow the results became poorer. I n addition, exhaustive exterior exposure tests are under way under a variety of service conditions, including industrial atmospheres. Although some of these tests have been under exposure for about three years, failures have not yet progressed to a point where reliable conclusions can be drawn. However, all of the ZTO chromate films are still in perfect condition, and it seems apparent that satisfactory metal protection can be obtained by the use of paint systems comprising suitably formulated ZTO chromate primers and a finishing paint adapted to the particular service conditions. This new chromate pigment makes possible the formulation of metal protective paints having the outstanding rust inhibiting characteristics of zinc yellow without the necessity of resorting to synthetic resin vehicles to obtain films of satisfactory water resistance. Because of the good water
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resistance that can be obtained with ZTO chromate primers prepared with ordinary linseed oil vehicles, the superior corrosion-inhibiting action associated with chromates can now be taken advantage of more generally in the field of metal finishing. Fortunately the pigment possesses a low specific gravity, so that ZTO chromate primers do not add unnecessarily to the dead load of large structures such as bridges and ships.
Literature Cited (1) Am. SOC.for Testing Materials, Proceedings, 15, I, 214 (1915). ( 2 ) Anonymous, Steel, 98,43 (June 1, 1936). (3) Bancroft, W. D., and Porter, J. D., J . Phys. Chem., 40, 37 (1936). (4) Bannister, L. C., and Evans, U. R., J. Chem. SOC.,1930, I, 1m i
Barti,,‘G.,2.Physik, 87,399 (1934). Burns, R. M.,and Haring, H. E., Trans. Electrochem. Soc.,
(11) (12) (13) (14) (15) (16)
69, 169 (1936). Burns, R. M., and Schuh, A. E., “Protective Coatings for Metals”, A. C. S. Monograph 79, pp. 306-12, Yew York, Reinhold Pub. Corp., 1939. Edwards, J. D. (to Aluminum Co. of Am.), U. S. Patents 1,946,151-2 (Feb. 6, 1934). ENG. CHEM.,27, 1145 Edwards, J. D., and Wray, R. I., IND. (1935). ENG.CHEM., ANAL.ED.,7, Edwards, J. D., and Wray, R. I., IND. 5 (1935). McCloud, J. L., IND. ENG.CHEM.,23,1334 (1931). May, R., J. Inst. Metals, 40, 141 (1928). Mellor, J. W., “Comprehensive Treatise on Inorganic and Theoretical Chemistry”, Vol. XI, p. 279 (1931). Penick, D. B., Rm. Sci.Instruments, 6, 115 (1935). ENG.CHEM.,30, 1152 (1938). Speller, F. N., IND. Winston, A. W., Reid, J. B., and Gross, W. H., Ibid., 27, 1333 (1935).
PRESENTED before the Division of Paint, Varnish, and Plastics Chemistry a t the 102nd Meeting of the AMERICANCHEMICAL SOCIETY, Atlantia City, N.J.
Molecular Volume of Liquid Alkanes at Corresponding Temperatures
GUSTAV EGLOFF AND ROBERT C. KUDER
Universal Oil Products Company, Chicago, Ill.
I
N ATTEMPTS to find comparative conditions such that
the molecular volume in homologous series is an additive function, the boiling Point (7, 10) and the melting Point (8, 11) have been used as temperatures of comparison- The use of these temperatures derives from the fact that for the members of a series either temperature is approximately a constant fraction of the respective critical temperatures, the desirability of comparison a t equal reduced temperatures being recognized from the theory Of states* The best data show, however, that in the case Of hydrocarbons a t their boiling points (7) the molecular volume of homologs is not a strictly additive function, and that in the case of aliphatic hydrocarbons at their melting points (8) the situation is complicated by the effectsof molecular and alternating melting points.
The failure of the boiling point or the melting point as a comparison temperature in the quest of additive molecular volume may be ascribed t o a t least two factors. The first factor is the approximation involved in the Guldberg relation between boiling points and critical temporetures; the inconstancy of the ratio of boiling point t o critical temperature is As Table I readily discerned from the work of Young shows,the melting point is a variable reduced temperature in the case of normal The second factor is the failure of the theory of corresponding states itself to m7i1son and Bahlke ( l a )pointed out that the Plot of reduced volume (in the liquid state) against the reduced temperature “does not give one line for all the normal paraffin hydrocarbons, but . . . a t any one ‘reduced tempera-
