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98
OF AVERAGE REFLECTIVITY TO FERRIC TABLEIV. RELATION OXIDECONTEXT
Group
Av. Reflectivity R
Ferric Oxide, ’%
32.3 29.0 27.4 26.3 25.2 23.5 23.0 22.2 20.6 17.4
2.3 2.4 2.8 2.6 2.6 2.9 2.9 3.2 3.3 3.5
Sp. Surface Sq. Cm./G: 1842 1728 1718 1728 1709 1742 1720 1748 1751 1737
in surface of 100 sq. cm. per gram. The color percentages remain essentially constant, despite the change in surface, which indicates that the main effect of surface change is apparently one of lightness rather than hue. Based on these observations, it is indicated that small variations in ferric oxide content are more effective in changing the average reflectivity, or lightness, of a cement than relatively large changes in specific surface.
Vol. 33, No. 1
Aclrnowledgment The authors express their gratitude to D. I. Elder and other members of the staff of the Universal Atlas Cement Company’s Research Laboratory for their helpful cooperation. Thanks are due W. V. Friedlaender of the research staff for his help and comments, and t o L. L. Huspek, of the company’s Central Laboratory for several of the specific surface determinations.
Bibliography (1) Duff, W. A., Textbook of Physics, Philadelphia, P. Blakiston’s Son and Go., 1926. (2) Dunagan, W. M., Iowa Eng. Expt. Sta., Bull. 139 (1938). (3) Fairbanks, E. E., “Litboratory Investigation of Ores”, 1 s t ed.
(Chap. IV, “Practical Photomicrography”, Loveland and Trivelli), New York, McGraw-Hill Book Co., 1928. (4) Hodgman, C. D., Handbook of Chemistry and Physics, 20th ed., Cleveland, Chemical Rubber Publishing Co., 1925. (5) Munsell, A. H., “Munsell Book of Color”, Baltimore, Munsell Color Co., Inc., 1929. (6) Spinney, L. B., Textbook of Physics, 4th ed., New York, Macmillan Go., 1931. (7) Watson, W., Textbook of Physics, London, Longmans, Green and Co., 1927.
Application of the System Sodium Chloride-Sodium Nitrite-Sodium Nitrate to Meat Curing X-Ray and Microscopic Studies GEORGE L. CLARK
LLOYD A. HALL
University of Illinois, Urbana, 111.
I
N SPITE of the fact that three very common sodium salts
are involved, little work was reported in the literature concerned with studies of the three-component system sodium chloridesodium nitrite-sodium nitrate. There should be considerable interest in the fundamental study of such a system on its own account, but great weight is added t o the desirability of knowledge of the system in view of the fact that these three salts, either in simple solution, simple solid mixture, or specially blended, are used for the curing of meats. Thousands of tons of these salts in various proportions, either as a physical mixture or as an intimately deposited crystalline mass from solution or from melts, and in solution and as a dry cure are used each year in the meat-packing industry. A thorough search has failed to reveal in the literature any report of an analysis of the ternary system. There are a few scattered observations on binary systems involving the three salts, but the general supposition seems to be that no chemical compound formation and no solid solubility are involved. Several x-ray studies, particularly by Thomas and Wood (S), have demonstrated that binary mixtures-for example, sodium chloride and potassium nitrate, or vice versa-when melted together bring about rearrangement of ions, and x-ray patterns disclose the presence, in addition to the t v o original
The Griffith Laboratories, Chicago, Ill.
salts, of sodium nitrate and potassium chloride. The three salts under consideration crystallize in three different systems and, under normal conditions, would be expected t o show only faint relations t o one another; or indeed it might be expected that they would be essentially incompatible. The essential data on crystal structures are as follows: Salt Crystal System Unit Cell Dimensions. i. NaCl NaN09 NaNOs
Cubic Orthorhombic Rhombohedral
a0
a a
-
= 5.628 E
3.55, b 6.32, OL
-
E
5.56, c 47’1.5’
-
5.38
As a matter of fact, however, the three structures are more closely related than a t first appears. Both the planar anions NOT and K O p can be considered t o replace C1- in the NaCl lattice with some distortion resulting in the orthorhombic and rhombohedral structures. From a melt or concentrated solution of these three salts crystallizes a solid material, the identification and properties of which are the subject of this investigation in relation to powders formed from mixing or grinding together in a physical mixture the three individual salts. There are a t least two fundamental methods of approach to the identification of the mixed crystals-namely, that of x-ray diffraction analysis of the powders and a microscopic study, particularly in polarized
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INDUSTRIAL AND ENGINEERING CHEMISTRY
light, since sodium chloride is isotropic and the other two salts possess definite anisotropy. That some differences between the two types of samples are possible is indicated by the fact that with corresponding grain sizes following careful sifting through 200- or 300-mesh sieves the velocity of solution of the mixture cocrystallized from solution or from the melt is ten to fifteen times greater than that of the physical mixture. This was experimentally demonstrated in the following manner: I n two beakers, containing equal volumes of water a t the same temperature and stirred with identical electric stirrers, were placed simultaneously equal weights of (a) the cocrystallized blend of sodium chloride, nitrite, and nitrate, and (b) a physical mixture of powders of the three separate salts in the same proportion as the blend. Two stop watches started simultaneously were used to determine the time required in each instance for the complete solutions of the solid crystals. I n a large number of trials with different amounts of samples, a t different temperatures, the great bulk of the blend sample invariably dissolved far more rapidly in the ratios indicated above. While each crystal grain of the mixture represented a single salt, a t least an overwhelming preponderance of the blend grains were complex, though each grain did not necessarily have the composition of the over-all mass. It follows that the heat of solution of the blend should be higher than that of the mixture. This was actually observed from thermometer readings, though no attempt was made to measure the heat effects quantitatively since there was no measurement of grain size distribution. There are several ways in which the salt blends may be cocrystallized, but we are particularly concerned with the salt blend formed when a steam-heated, chrome-plated drum rotates in a 25 per cent aqueous solution of chloride, nitrite, and nitrate together in various proportions, and water on the heated surface is driven out of a fluid film causing deposition of the dried salt on the surface of the drum, which is then scraped off, before the particular area on the surface of the
99
In the first of a series of investigations dealing with the effect of some common salts in the curing of meat, results are reported on the x-ray diffraction analysis and microscopic study of mechanically powdered mixtures and cocrystallized blends of sodium chloride, nitrite, and nitrate. Special attention is given to the blend prepared by drying aqueous solutions of the salts on the heated surface of a rotating drum in a large-scale manufacturing unit. The x-ray results indicate that for the blend containing not more than a total of 10 to 12 per cent sodium nitrite and sodium nitrate, there is a very slight indication of solid solution, and except for faint traces, only the characteristic lines of sodium chloride appear; this indicates a nearly complete inclusion of nitrite and nitrate within sodium chloride crystals. This is further directly proved by microscopic examination in polarized light. For nineteen blends an estimation is made microscopically of the percentage inclusion of nitrite and nitrate within chloride crystals, the values varying from 100 to 0 per cent. The growth of crystal planes of one salt on planes of another is discussed in terms of spacing tolerance and in strain which manifests itself in a marked increase in rate of solution and heat of solution in water as compared with analogous physical mixtures.
drum is again immersed in the solution of the three salts cooled through a louvre dryer with air a t 20" C., and collected. The temperature of the salt solution is 82' C. and that of the surface of the rolls 140-150O C. A tswical large-scale manufacturing u& is shown in Figure 1.
X-Ray Diffraction Analyses
Courtesy, Grifltith Laboratories, Inc.
FIGURE 1. (Left) CHROMIUM-PLATED DRYINGROLLS,SPLASHFEED,WITH CHROMIUM STEELKNIFESCRAPER, CONVEYOR TO LOUVRE DRYER.(Right)DUSTCOLLECTOR AND WEIGHING SCALE Cold dry air from ice machine is fed into louvre dryer. Capacity 25,000 pounds per 24-hour day.
The powder diffraction method must be used since the grain size is too small under the rapid conditions of deposition and drying to permit growth of sufficiently large single crystals. The dried salt sample is mounted in the wedge sample holder in the cylindrical camera commonly used in the x-ray laboratory a t the University of Illinois. Figure 2 shows the powder diffraction patterns. For the physical mixture the pattern, d, is exactly what is expected, corresponding to the sequence of lines characteristic of the three compounds. The presence of 4 or 5 per cent of sodium nitrite and nitrate is sufficient to produce the characteristic lines of each in spite of the overwhelming predominance of sodium chIoride. For the cocrystallized salt, as obtained and without grinding, a remarkable difference is observed. I n twenty or more patterns, even for mixtures containing larger percentages of sodium nitrite and nitrate, exposed under precisely the same conditions as for the powdered mix-
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
100
FIQURE a. b. c.
Pure sodium chloride P u r e sodium nitrite Pure sodium nitrate
FIGURE 3. ( b ) GROWTH OF CUBEFACE OF SODIUM CHLORIDE ON THE RHOMBOHEDRAL FACE O F SODIUM N I T R A T E (a)
A FIGURE 4.
2.
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POWDER DIFFRACTION PATTERNS d. e.
Physical mixture of.9070 NaC1, 6% NaNOz, 4% NaNOa Three salts codeposited on a heated d r u m from a solution in same proportions as d
ture, there appear only faint traces of a few nitrite and nitrate lines, in addition to those which may be attributed to sodium chloride; they include @-lines or faint lines due t o tungsten radiation (from the evaporation of the filament of the x-ray tube onto the target). I n t h e second place, these sod i u m chloride
lines, especially a t greater angles, show a slight shift toward smaller angles; in other words, the sodium chloride spacings are slightly expanded by limited solid solution in it, primarily of sodium nitrite whose lattice is more closely related to that of sodium chloride. The almost complete disappearance of lines for sodium nitrate and nitrite is thus due in small part to solid solution, but more particularly t o the fact that for certain ranges of composition sodium chloride preferentially surrounds every crystal nucleus of the nitrate and nitrite. Thus any diffracted x-rays from these must pass through intimately surrounding layers of sodium chloride. This fact is proved by microscopic observation in polarized light, as demonstrated in Figures 5 , 6, and 7 . Another important factor is involved in this system, to which further intensive investigation is being given. This is the remarkable and unpredicted ability of one crystal to grow upon the face of another, even though the crystallographic system and the angle of the lattice net differ, as long as there
B PHOTOZIICROGRAPH O F PROCESSED h l I X T U R E S NaCl 7% NaNOs % NaNOs A 90 6 4 R 80 12 8
( x 91)
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101
A B MIXTURBI OF SODIUM CHLORIDESODIUM NITRITE-SODIUM NITRATE, SAMEFIELD FIGURE5. MECHANICAL A . Ordinary light ( X 85). B . Crossed niools ( X 30). Light areas identify NaNOa or NaNOa; NaCl crystals in A do not appear in B.
A B FIQURE 6. PROCESSED BLENDOF SODIUM CHLORIDE-~~ODIIJM NITRITE-~~ODIUM NITRATE COCRYETALLIZED FROM SOLUTION (0.09 PERCENTWATER) A. B.
Ordinary light ( X 106). Crossed nicols ( X 90). The widely disseminated NaNO2 and NaNOs crystals appear as white inclusions.
is a compatibility within tolerance limits in planar spacings. The first investigations in this field were in 1937 by Seifert (8) and by Heintze (1). Thus sodium chloride displays the ability to grow easily and rapidly on the rhombohedral faces of sodium nitrate. The lattice constant along the rhombohedral edge is 6.47 A. and the angle is 102" 42', corresponding to 90" in sodium chloride. Figure 3 illustrates the growth of the cube face of sodium chloride on the rhombohedral face of sodium nitrate. Here the spacing betweenooppositely charged ions is Na+-C1- 2.81, Ka+-NOa- 3.24 A., a difference of 0.43 A. or a tolerance value of 13.2 per cent. Sodium iodids, another alkali halide, has a corresponding spacing of 3.23 A. or only 0.3 per cent difference. It is a more favorable case for growth under comparable conditions. Other directions are less favorable (octahedral face 21.3 per cent on first diagonal, or only 1.4 per cent on the second). The actual growth is a function of concentration, temperature, and impurities. Because sodium chloride is a limiting case, it tends to form skeletal rosettes on sodium nitrate so that the actual orientation is difficult to discover. However, solid solution of sodium nitrite in sodium chloride increases the spacings so that the discrepancy is smaller; it also serves t o condense and orient regularly in the diagonal position the sodium chloride
as it grows on sodium nitrate. It is interesting also to note that alcohol added to the aqueous solution also increases the tendency to a preferential diagonal selection. Such crystals are obviously under strain; it is remarkable that salts with different spacings and with planes a t different angles will grow a t all upon one another. It is this transition zone under strain which leads t o unusual properties, such as abnormally high rate of solution. Water, corn sugar, glycerol, etc., in small amounts must also have a profound effect on the mechanism of growth of one crystal on the other and the grain size which is developed. The latter two are actually added t o stabilize the strained crystals in humid atmospheres, partly by preferential absorption of water. Further microscopic studies of these growth phenomena in the ternary system are in progress.
Microscopic Studies During the research development of new curing materials for meat, it was necessary to make detailed microscopic studies of certain combinations of sodium chloride, sodium nitrite, and sodium nitrate. It has been found that when these ingredients are melted together or dissolved in mater, crystals are formed of sodium chloride having a heartlike center of
102
INDUSTRIAL AND ENGINEERING CHEMISTRY
FIGURE
7. A. B.
PROCESSED
A BLENDOF
Vol. 33, No. 1
B NITRITE-SODIUMS I T R A T E (2 PERCENTWATER)
SODIUM CHLORIDE-SODIUM
COCRYSTALLIZED FROM SOLUTION
Ordinary light ( X 105). Crossed nicols ( X 90). T h e NaNOz and NaNOs crystals are larger and less widely disseminated t h a n in Figure 6.
the sodium nitrite and sodium nitrate or of the mixed nitrite and nitrate. These crystals actually must be classed, of course, not as a unique new chemical combination, but as a type of physical mixture in which one salt “grows” on another. Many examples are known. Numerous combinations of sodium chloride, nitrite, and nitrate (with or without water to spray) have been studied. The results here reported are for blends prepared by drying solutions on heated drums as already described. Microscopic studies were made with a petrographic microscope containing a polarizer and a red and green selenite plate. Light oil of high refractive index, such as a high-boiling kerosene oil, was used as the mounting liquid. Under such conditions the sodium chloride granules appeared green with a definite outline of their cubical shape, and the nitrite and nitrate grains appeared pinkish red. These colors were obtained regardless of whether the metallic ion was sodium or potassium. For the samples prepared by spontaneous crystailization of chloride, nitrate, and nitrite, it is a t once apparent that sodium chloride crystals tend to grow around a nucleus of nitrate or nitrite or combination of the two. With an ocular micrometer it was possible to measure approximately the amount of inclusion of the nitrate and nitrite within the sodium chloride crystals. Table I for three series of samples gives an indication of this inclusion on a percentage basis. The following conclusions may be drawn from the observations on these cocrystallized samples: 1. Far greater percentages of inclusions are observed for sodium nitrate and nitrite to ether than for either separately; for example, for 24 per cent of sodium nitrate alone there is no evidence of inclusion in sodium chloride crptala, whereas in the presence of as much as 25 per cent sodium nitrite and 25 per cent sodium nitrate there is at least 50 per cent inclusion. 2. Sodium nitrite seems to be more effective in aiding inclusion than sodium nitrate in corresponding percentages; and it has a fairly constant effect, independent of its actual percentage, with increasing sodium nitrate content, since there is a linear relation from 6 to 24 per cent sodium nitrate in spite of varying sodium nitrite; 12 per cent sodium nitrite and 8 per cent sodium nitrate are 100 per cent included, while 8 per cent sodium nitrite and 12 per cent sodium nitrate are only 80 per cent included. However, 18 per cent sodium nitrate is 20 per cent included in the presence of 4 per cent sodium nitrite and 80 per cent in the presence of 12 per cent sodium nitrite. 3. One hundred per cent inclusion is noted for seven of the experimental mixtures with a maximum of 12 per cent sodium nitrite and 10 per cent sodium nitrate. Thus a product of hornogeneous character is limited within these ranges.
TABLEI. NITRITE-NITRATEINCLUSIOXS IN CHLORIDECRYSTALS ARRANGED IN DESCENDING ORDER No. 1 2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17
1s 19
% NaNOz
% Neb%
% NaCl
1.0 1.5 4.0 6.0 8.0 8.0 12.0 8.0 12.0 16.0 18.0 6.0 20.0 30.0 8.0 4.0 3.0 1.5 0.0
0.0 0.0 6.0 4.0 6.0 10.0 8.0 12.0 18.0 12.0
99.0 98.5 90.0 90.0
100 100 100 100
82.0 80.0 80.0 80.0 72.0 70.0 74.0
100
12.0 20.0
25.0 20.0 16.0
18.0 20.0 22.0
24.0
80.0
50.0 50.0 76.0 78.0
77.0
76.5
76.0
% Inoluaion
100 100 80 80 70 70
66.6 60
45
40 20 10 0 0
Approximately 75 per cent inclusion (70-80 per cent) is found in five samples with a maximum of 18 per cent of sodium nitrite or nitrate. The microphotographs (Figures 4 to 7 ) show clearly the degree of dissemination of nitrate and nitrite in the crystals of the processed material compared with the mechanically mixed material. The marked effect of water content is shown in Figures 6 and 7 for samples otherwise identical in oomposition and processing. When the crystals contain 2 per cent water, grain size is larger and the dissemination and inclusion of nitrate-nitrite are less uniform than in samples with lower water content. However, the free moisture content, determined a t 105” C. of all the samples which show the most marked nitrite-nitrate inclusion, averages about 1.2 per cent. This microscopic study indicates that when a large amount of sodium chloride is present in a solution which contains the nitrite and nitrate, processing is effective in preventing the nitrite-nitrate combination from forming a cake, and in cauaing tiny particles or nuclei to be surrounded with a mass of substantially pure sodium chloride and thus building crystals with heartlike centers.
Literature Cited (1) Heintze, w., 2. KTiSt., 97,241 (1937). (2) Seifert, H., Ibid., 96, 111 (1937). (3) Thomas, E. B.,and Wood, L. J., J. Am. Chem. Soc., 56,92 (1934).
