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298
FIBERS FROM GLASSDRESS. Fibers taken from the dress of the Spanish Princess Eulalia, made in 1893 and preserved in the Toledo Museum of Art, were broken in tension with the following results: Fibers Blue Bluieh green Average
Diameter In. X 100 Mm. 1.35 1.54 1.69 2.02 1.72 1.56 1.59 1.34 1.60
0.034 0.039 0.043 0.051 0.044 0.040 0.040 0.034 0.041
according to thickness. Seven fibers averaging 0.00066 inch had a strength of 92,000 pounds per square inch *13 per cent, and seven averaged 0.00078 inch with a strength of 64,000 pounds per square inch * 13 per cent.
Tensile Strength
Literature Cited
Lb./sq. in. Kg./sq. em.
11,000 4,000 8,000 5,000 9,000 7,000 12,000 11,000 8,400
770 280 560
710 630 490 770 840 590
Eight other fibers of each color were measured for diameter; the average was 0.00164 inch (42 microns). These results are appreciably lower than those with the coarsest fibers for air filters; but when it is remembered that they were obtained by pulling out softened glass rod and were probably never properly lubricated, the results seem reasonable. RAYON.A sample of 75-denier rayon, which had been spun under tension, was obtained and divided into two groups
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(1) Anderegg, F. O., Keram. Rundschau, 44, 255 (1936). (2) Bailey, James, and Lyle, A. K., paper presented before 1937 meeting of Am. Ceramic SOC. (3) Griffith, A. A., Trans. Roy. SOC.(London), A221, 163 (1920). (4) Joffe, A., Intern. Conf. Physics, London, 11, 80 (1935). (5) Karmasch, S., Mitt. gew. Ver. Hannover, 1858, 138. (6) Lillie, H. R., private communication. (7) Plummer, J. H., IND. EYG.CHEM.,30, 726 (1938). (8) Reinkober, O., Physik. Z . , 32, 243 (1931); 33, 32 (1932); 38, 112 (1937). (9) Shurkov, S., Physik. 2. Sowjetunion, 1, 123 (1932). (10) Slayter, Games, J. Am. Ceram. Soc., 19, 335 (1936). (11) Smekal, Adolf, Glastech. Ber., 13, 141, 222 (1935); Z. Physik, 91, 336 (1934); 103, 495 (1937). (12) Smekal, Adolf, J. SOC.Glass Tech., 20, 432 (1936). (13) Smekal, Adolf, 2. Physik, 103, 495 (1936). (14) Sosman, R. B., private communication. (16) Zwicky, P. W., Physik. Z.,24, 131 (1923). RECEIVBID Ootober 14, 1938.
MOLYBDENUM ORANGE PIGMENTS ARTHUR LINZ Climax Molybdenum Company, 500 Fifth Avenue, New York, N. Y.
M
OLYBDENUM. orange pigments, characterized by enormous covering power and tinting strength, bril’ liance of color, and extreme fastness to light, are becoming increasingly important in the printing ink and paint industries. Commercially these pigments are made by adding a solution of a bichromate, a sulfate, and a molybdate to a solution of a soluble lead salt. The original yellow precipitate is converted through orange to a brilliant red in successive stages under proper conditions. The properties of the pigment depend entirely upon the conditions existing in the solution during and after precipitation. Because the effects of variations are not clearly understood and have been subjects of sharp disagreement, this study was undertaken to learn what takes place during the change of the pigment from yellow to red and to determine methods which would produce uniform results in its manufacture. The first observation of the formation of red normal lead chromate was that of Schultze (12)who noticed that yellow lead ore (wulfenite) was sometimes strongly colored by the red lead ore (crocoite) found near by. Since wulfenite, or native lead molybdate, crystallizes in the tetragonal system and crocoite or lead chromate ore is monoclinic, it occurred to him that the strong red coloration might be due to crystallization of lead chromate in the tetragonal system. He conducted (and published in 1863) experiments showing that it was possible to obtain homogeneous crystals in the tetragonal form from mixtures of lead molybdate and lead chromate containing as much as 42 per cent of the latter. Such crystals were a deep, dark red in color, much darker than any crystals of pure lead chromate he had ever obtained. I n 1921 Jaeger and Germs (4,in a study of binary systems of lead sulfate, chromate, molybdate, etc., corroborated the work of Schultze and showed that lead chromate could exist
in the tetragonal form a t room temperature in the presence of lead molybdate. On August 30, 1930, Lederle (6) applied for a patent in Germany, covering a yellow to red pigment. In this patent Lederle cIaims “as a ‘new’ article of manufacture, mixed crystals suitable as yellow to red pigment coloring matters comprising lead chromate and a t least one salt of lead with an acid selected from the group consisting of molybdic and tungstic acids.” The remainder of the claims cover mixed crystals of lead molybdate, lead chromate, and lead sulfate or tungstate with or without the use of barium or strontium chromates, molybdates, or sulfates. On August 2,1932, Wagner, Haug, and Zipfel(l4) submitted for publication a short survey of their work on the modifications of lead chromate in which they discuss, briefly, tetragonal lead chromate. They refer to the work of Jaeger and Germs in producing this form by the pyrogenic method (S), but no reference is made to the pertinent work (4) by these same authors published in the same periodical in the same year, or to that of Schultze ( I d ) . They state: “One obtains a red tetragonal mixed crystal modification if one also adds PbMoOd to the chromate in addition to PbSOc by the simultaneous use of ammonium molybdate.” They also give the crystal lattice of a “compound,” 5PbCr04.3PbMo04.10PbSO4. The impression which they seem to be trying to produce is that red tetragonal lead chromate is stable a t room temperature only in the presence of both molybdate and sulfate, which supports Lederle’s patent. Lederle’s second patent (6) was applied for in Germany on August 30, 1932, just 28 days after the article by Wagner, Haug, and Zipfel was received for publication. This is a process patent claiming methods for production of mixed crystals of lead chromate, molybdate, and sulfate in acid
MARCH, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
solution in which redder pigments are obtained than when basic or neutral solutions are used. On December 15, 1932, Lederle applied for his third patent (7) on this subject, which claims mixed crystals of basic lead chromate and basic lead molybdate or tungstate, with or without basic lead sulfate and with or without alkaline earth metal chromates, sulfates, molybdates, or tungstates, together with processes for producing them. I n 1937 Linz took out a Belgian patent (8) claiming the production of orange to red mixed crystal pigments of lead chromate, lead molybdate, and normal or basic lead carbonate or lead phosphate. A survey of the literature shows that there are two opposing views on the subject: 1. The orange-red pigment commonly called molybdenum orange is a mixed crystal pigment consisting of lead chromate, lead molybdate, and lead sulfate or at least one third component. 2. Based upon the preliminary work of Schultze and of Jaeger and Germs, the red product is simply the third modification of lead chromate (tetragonal) which is formed in connection with molybdate. The sulfate may also enter into combination but is not a necessary constituent.
At the outset it was decided to follow the assumption of color manufacturers (working from Lederle's patents) that the sulfate was a necessary constituent, and to determine, if possible, a straightforward method for preparing molybdenum orange which would give good, reproducible results. Such a procedure should clarify the situation considerably since, as before mentioned, information now generally available is inadequate and often unreliable and confusing. Preliminary attempts to prepare molybdenum orange according to formulas given in the patents failed for lack of sufficient data. Consequently it was necessary to piece together the information available from various sources and, using this as a basis, work out the details of procedure. Wagner (14) gives something of the characteristics and methods of preparation of the three modifications of lead chromate: RHOMBIC.Light yellow granular crystals. Prepared by adding a dilute solution of chromate and sulfate to a soluble lead salt. Stable only at low temperature and in presence of excess lead salt. Can be rotected from transition to the monoclinic to some extent by tEe use of protective colloids or by adding aluminum sulfate and precipitating aluminum hydroxide on the crystals as a film. MONOCLINIC.Darker colored than rhombic and crystallizes in needle crystals. Always present to some extent on precipitation of the rhombic modification. Formed on long standing of rhombic crystals or more quickly at tem eratures above 50" C. and at higher concentrations. Formed a k o s t exclusively in the presence of excess chromate. TETRAGONAL. May be obtained pyrogenetically but is stable only above 750" C. Red mixed crystals may be obtained by precipitation of lead chromate in the presence of both sulfate and molybdate. No details are given. Obviously, since we are interested in an orange to red pigment, the tetragonal modification is the one to be produced and stabilized. Unfortunately there seems to be little additional information in the literature, and the patents give only enough to try to ensure coverage of the subject.
Optimum Conditions for Preparation RATIOOF CONSTITUENTS. A check on molecular ratios given in the patents showed that many variations were possible with resultant variation in properties of the products. The most commonly used was 7PbCr04.2PbS04.1PbMo04. Analyses of two commercial molybdenum oranges gave the following percentages: Sample 1
Sample 2
67.71% 12.04 11.58
66.90% 12.51 7.66
299
Calculating the molecular ratios from these results gives the following: Sample 1: 21PbCr01.3.5PbSO4.3PbMoO4 Sample 2: 21PbCr04.2.5PbS04.3.5PbMoOa Standard solutions of Pb(NO&, NazCrz0~.2Hz0,Nar MoO4.2H20,and NazSOr were made up to contain one gram atom of lead, chromium, molybdenum, or sulfur per liter; they were thus made equivalent so that 1 cc. of either of the other solutions would precipitate the lead contained in 1 cc. of the lead nitrate. Stoichiometric molybdenum oranges were then made in approximately the proportions given by analysis of commercial product 1, adopted as standard. Preliminary runs were made by taking 100 cc. of the standard lead nitrate solution [containing 33.1 grams of Pb(N03)2], diluting it to a definite volume (usually 1 liter), adding a solution made up by mixing a total of 100 cc. of dichromate, molybdate, and sulfate standard solutions, and diluting to the same volume; excesses of either lead or precipitants were thus avoided. When the chromate-sulfatemolybdate mixture was added to the lead nitrate, a finely divided yellow precipitate formed. If the pH was between 1.0 and 4.0, these yellow crystals changed to red, the speed of this change depending on concentration, temperature, and pH of the solution. When stirred further, after a certain maximum redness had been attained, the color seemed to reverse itself; it became more orange in hue while the precipitate became more bulky. After being stirred overnight, the precipitate was orange-yellow and very bulky and puttylike in appearance. When mixed, crystals of rhombic lead chromate apparently are formed; they change over to the tetragonal form and subsequently to the monoclinic or needle form, which accounts for the bulkiness of the final precipitate. Such a supposition is in accord with all the observed facts and the available literature on the subject. The sensitivity of these changes to solution conditions accounts for the difficulty of stabilizing molybdenum orange in commercial manufacture. PH. The question of pH was most interesting, since even slight differences gave quite varied products. I n the preliminary work a considerable drop in pH occurred when the solutions were mixed; this occasioned some surprise at first. However, it was easily explainable on the basis of using sodium dichromate to precipitate the lead monochromate; nitric acid was formed as one of the products according to the following equation: NazCrZO?
+ 2Pb(NO& + H?O-+ ZPbCrO4 + 2NaN08 + 2HNOs
(1)
It was decided to prepare a series of colors, identical in all respects except pH; the pH of both the lead nitrate and chromate mixture was to be recorded before precipitation and that of the final solution after precipitation since these values might show something of interest. Accordingly 400 cc. of the standard molar lead nitrate (331 grams per liter) were diluted to 4 liters, which were divided into eight 500-cc. portions. Then 280 cc. of 0.5 molar sodium dichromate (149.02 grams Na&rz0~.2Hz0per liter), 80 cc. of molar sodium sulfate (142.06 grams per liter), and 40 cc. of molar sodium molybdate (242 grams NazMo04.2H~Oper liter) were mixed, diluted to 4 liters, and divided into eight 500-cc. portions. The pH of each solution was adjusted by adding increasing amounts of 4 molar sodium hydroxide (160 grams per liter). The amount of sodium hydroxide added to each solution, resultant pH, pH of the mixture after precipitation, and characteristics of the prgcipitate and solution after stirring for half an hour are given in Table I.
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VOL. 31, NO. 3
CONCENTRATIOX. Preliminary work also showed that an increase in concentration caused h-anF1 an increase in the speed of reaction. Best colors _.I-__ NaOH Added to pH of pH after have been produced a t a concentration of about Sample Added to pH of Chromate Chromate PrecipiCharacteristics after Pb(N0a)i Pb(N0a)z iMixt. No. Mixture tation Stirring 30 Min. 0.1 molar or 33.1 grams of lead nitrate per liter cc . cc. with the chromate, molybdate, and sulfate mix32 0 4.7 0 5.4 1.5 Orange ppt., soh. ture a t about the same concentration. This oolorless represents approximately 1 part of lead nitrate 33 1 5.3 2 6.05 1.8 Ppt. redder than -foregoing to 30 parts of water. Doubling this concentra34 1.5 .. 3 6.25 1.9 Put. redder than -foregoing tion does not seriously affect the color although 35 2.0 *. 4 6.4 2.05 Ppt. redder than the brilliance is somewhat impaired. f oregoin,g 36 2.5 .. 5 6.56 2 4-. -Pnt, 3st of all BUFFERS. I n commercial work it is often 27 xn fi7 -. 4.5 Orange ppt. 38 3.5 5: 35 7 6.86 6.4 Yellow ppt. necessary to adopt procedures which simplify 39 5.0 5.5 10 11.5 9.1 Oranae-yellow uut.: control of a process. Thus, the addition of a soh. was yellow buffer often takes the place of a careful adjustment of pH since more acid or base is required to produce a given change of pH in a buffered Although 1 cc. of sodium hydroxide raised the pH of the solution than in an unbuffered one. For this reason there is lead nitrate solution from 4.7 to 5.3, 4 cc. more raised it only less chance of overrunning the end point with a consequent to 5.5. I n this range where lead hydroxide is formed, spoiling of the batch. The use of buffers is quite advantageous hydroxyl ions are eliminated from solution by precipitation in controlling pH in the preparation of molybdenum orange. according to the equation: If, for example, sodium acetate is used in place of sodium hydroxide for raising the pH of the chromate solution or if some Pb(N0& 2NaOH -+. Pb(0H)z 2NaNOs (2) lead nitrate is replaced by lead acetate, the pH is much less sensitive to additions of acid or base with a resultant greater I n the case of the chromate solution a similar range is ease in control. However, there is some question as to the encountered over which the addition of relatively large effect of such buffers on the properties of the finished product. amounts of base produce little change in pH. Thus the To check this point, two colors were prepared under as addition of 7 cc. of sodium hydroxide raised the pH from 5.4 nearly identical conditions as possible; sodium hydroxide to 6.8; 10 cc. added to a similar solution at 5.4 raised the pH was used in one case and sodium acetate in the other for to 11.5, the 10 cc. being a slight excess over the amount reraising the pH of the chromate solution. The resultant quired to convert all the dichromate to monochromate accolors were rubbed out, 20-to-1 reductions with zinc white cording to the equation: were made, and color scratch-outs were prcpared from them. These seemed to show that the pigment produced without 2NaOH Na&rpO? -+2Na&rO, HzO (3) buffer is slightly superior in color and strength to the one in which the buffer was used. It therefore becomes a quesConsequently, there are grounds for the contention of tion of whether the color maker prefers to be more painstakcolor manufacturers that they cannot control the product by ing in adjustment of pH, with more chance of overrunning relying on the pH of solutions in this range since there is the desired value, or to use a buffer to make pH adjustment little doubt that a measurement of the pH of either solution simpler, with a resultant slight decrease in desirable properbefore mixing will not give a reliable indication of what the ties. final pH will be. WASHINGPRECIPITATES. If the color is merely allowed This is best shown by a comparison of the results. The to develop during stirring, and then the precipitate is filpH's of the lead nitrate solutions used differ only by 0.05 tered off and dried a t 100" C., practically all of the red and those of the chromate solutions by only 0.8, whereas color is lost. This is probably due to the fact that a transthe pH's after mixing differ by 2.6. The pH of the final formation goes on during the drying period which finally solution seems to be the important factor. yields monoclinic lead chromate. If the precipitate is Colors produced a t low pH go through a transformation thoroughly washed until the wash water is neutral, no such from yellow to red and back to yellow again within the 30difficulty is encountered. It was later noted that almost as minute period; those at higher pH undergo no color transgood results couId be obtained if the p H of the solution was formation and remain yellow from the start. It is evident raised to about 7.0 by the addition of sodium hydroxide. that a pH either too high or too low yields a yellower and less The excess acid apparently assists in the loss of color on desirable color. Best pH is apparently somewhere about 2.4. drying. If the solution is neutralized, the value of the washA further consideration of these data will be found in the ing operation is questionable. I n fact, some colors actually discussion of theory. lost brilliance by being washed. TEMPERATURE. Preliminary experiments showed that an DRYINGTEMPERATURE. The lowest practicable temperaincrease in temperature increased the speed with which ture should be employed. During the first part of the the transformation from. yellow to red crystals took place; drying operation, the wet color is still very sensitive to inthe same depth of color develops in 2 or 3 minutes a t 50" C. creased temperature, probably changing over to the monoas would be developed at 0" C. in an hour. Unfortunately clinic form. The use of vacuum dryers, as recommended by an increase in temperature also increases the speed of the Sapgir, Rassudova, and Kvitner (11) for drying light yellow secondary change, which results in a loss of color. Therefore chrome yellow, would probably ensure least transformation a t elevated temperatures we obtain the formation of the of these colors. desired red color and then a transformation to the unwanted ADDITIONAL AGENTS. Many producers of molybdenum secondary yellow color within a very few minutes; on a orange are said to add various materials, such as oxalic acid, plant scale, by the time all the precipitating agent has been sugar, alum, blackstrap molasses, sulfonated oils, etc., in added, the color will have gone almost completely over to the order to stabilize their products to the action of light and to yellow form. Therefore, it was found advantageous to work prevent discoloration during drying, as well as to prevent at room temperature or lower, the solutions in most cases transformation to the yellow color after the desired red being between 15" and 20" C. FPrlrl' r__
'
+
+
+
+
INKMIL]; GRINDING COLORED INK
tate a film of aluminum hydrate on the particles, as recommended by Wagner (IS) for stabilization of the rhombic form, decreases oil absorption and increases the brilliancy of the pigment. Therefore, the procedure was adopted of stirring the mixture until the desired color is obtained, and then adding a solution of aluminum sulfate followed by enough sodium hydroxide to neutralize the solution; the aluminum is thus precipitated as aluminum hydroxide on the particles. FORMULA. The following standard procedure was then set up which should give satisfactory colors: Dissolve 33.1 grams (0.1 mole) of lead nitrate in a liter of water. Add with constant stirring a solution containing 11.5 grams (0.0385 mole) of Na&rzOr.2Hz0, 1.7 grams (0.012 mole) of sodium sulfate, 2.8 grams (0,011 mole) of Na?Mo04.2H20,and about 2.8 grams (0.06 mole) of sodium hydroxide made up t o 1 liter. The pH after mixing will be in the range from 1.5 to 2.5. Stir for about 30 minutes to allow for development of the color and then add a solution of 2.3 grams (0.067 mole) of aluminum sulfate in 20 cc. of water, followed by a solution of sodium hydroxide to raise the pH to the neutral point (requires about 0.56 gram of sodium hydroxide). Stir for a few minutes, wash twice by decantation, filter, and dry at 100” C. The resultant product, as predicted from preliminary results, was a deep red in color, even redder than the commercial standard. However, it did not have as great tinctorial strength and was therefore not so valuable. Under the circumstances it seemed that the empirical work had failed to give all the information needed; therefore it was necessary to consider the problem on a purely theoretical basis.
fiable. When a solution of lead nitrate is mixed with a solution containing chromate, molybdate, and sulfate, three reactions may, and probably do, take place if sufficient lead is present :
+ +
+ +
Pb(NO& NazMoOl+.2NaN03 PbMoO4 (tetragonal) (4) 2Pb(NO3)1 Na~CrzOv H20 + 2NaN03 2HN08 2PbCrO4 (rhombic). (5) ., Pb(N0s)i Nd04 + 2NaNOa PbSO4 (rhombic or monoclinic) (6)
+
+ +
+
I n the presence of an excess of lead nitrate (which seems to be necessary for the production of the molybdenum orange color) each of these reactions goes practically to completion. Ordinarily we consider lead chromate, lead molybdate, and lead sulfate virtually insoluble. If this is true, as soon as these salts are precipitated, they will be removed from the region of activity and can no longer be affected by reactions in solution except to act as nuclei for other compounds. However, we know that some sort of a reaction does take place and that this goes on faster when the pH is lowered, either to produce the red “molybdenum orange” or the bulky crystals of monoclinic lead chromate. We also know that lead sulfate, lead chromate, and lead molybdate are all soluble to a considerable degree in strong nitric acid; therefore, in the hope that this might give a clue, their solubility in dilute acids was checked. The data in Table I1 were compiled from Seidell (17). These solubilities, calculated to grams per liter, are given in Table 111, together with the values from actual measure-
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302
ments of the pH of nitric acid of these concentrations in the laboratory with a pH meter. Tables I1 and I11 show that these salts, ordinarily considered insoluble, are appreciably soluble even in dilute acids. From these data and experimental observations, a reasonable explanation has been deduced for the changes observed in the preparation of molybdenum orange.
TABLP~ 11. SOLUBILITIES OF LEADSALTS (17) Solvent,
“Os
PbMo01 c
0.1 M 0.5 M 1.0 M
0,0060 0.0665 0.2958
PbCrO4 Millimole/lOO -.c 0.0196 0.0548 0.1190
PbSO4 0.1405 0.3272 0.6667
OF PH ON SOLUBILITIES OF LEADSALTS TABLE111. EFFECT
“01
Conon. 0.1 M 0.5 M 1.0 M
Measured
PH
PbMoO4
1.00 0.50 0.35
-Grams/liter 0.020 0.244 1.086
PbCrOc
PbSO4
0.063 0.177 0.385
0.426 0.992 2.021
The precipitate originally formed consists of a mixture of lead chromate (yellow rhombic crystals), lead molybdate (white tetragonal crystals), and lead sulfate (white rhombic or monoclinic crystals), [According to the contention of Rassudova and Kasatchkina (IO),mixed crystals of lead sulfatelead chromate are formed at the instant of precipitation, and these may be present also.] Thus we have at least three crystalline phases. Whether these three phases have been formed simultaneously in a common vessel or in separate vessels, and then mixed is of no consequence as regards subsequent developments, according to example 4 of Lederle’s first patent ( 5 ) . During the stirring period following precipitation or mixing, according to the laws of chemical kinetics, a dynamic equilibrium is set up between the respective crystals, dissolved molecules, and ionization products of these compounds; molecules are constantly dissolved at the crystal faces and are replaced by other molecules from the solution. Equilibrium a t the crystal face can be presented by the expression: ‘
s
crystal e dissolved molecules P
(7)
The speed of the solution reaction, S, moving to the right to dissolve the crystal will depend upon the solubility of the crystal in question, and that of the precipitation reaction, P, causing the crystal to be built up will depend upon the number of molecules in solution capable of being deposited in the lattice of the crystal. Thus, the crystal will either dissolve or become larger, depending upon which of these tendencies is greatest. During this solution and deposition process, according to the laws of formation of mixed crystals, different compounds capable of existing in the same crystalline modification (isomorphous with each other) may build into the same crystal lattice, thereby forming a true mixed crystal. Since lead chromate is trimorphous with three possible forms (rhombic, monoclinic, and tetragonal), since lead sulfate is bimorphous (rhombic and monoclinic), and since lead molybdate is also bimorphous (tetragonal and monoclinic), the system may become rather complicated. We may have, for example, lead sulfate and lead chromate building into the same rhombic or monoclinic lattice, lead
VOL. 31, NO. 3
molybdate building into the monoclinic lead chromate lattice, or, what we are chiefly interested in, lead chromate building into the tetragonal lead molybdate lattice. Apparently this latter action produces the red color, since it is well known that tetragonal lead chromate is red. As pointed out above, the crystal which is least soluble will be the one to gain by this exchange of molecules since the speed of its solution action, 8, will not be so great as the speed of the precipitation action, P. At a pH of 1.0 or above (Table 111) lead molybdate is the least soluble of the three salts under consideration. Therefore, the tendency for lead chromate to build into the tetragonal lead molybdate lattice is greater than that of the molybdate to build into the monoclinic lead chromate lattice. Since lead molybdate cannot build into the rhombic lattice and since these crystals are composed entirely of lead chromate (or mixtures of chromate and sulfate), the tendency is for them to go into solution and for the resultant chromate molecules to build into the tetragonal lattice. The net result is disappearance of rhombic lead chromate and formation of tetragonal mixed crystals of lead molybdate-lead chromate. If, on the other hand, the pH is lowered to about 0.5 or lower where lead chromate is less soluble than lead molybdate (Table 111), the equilibrium balance is reversed. Although, under the conditions considered, lead chromate precipitates chiefly in the rhombic form, Wagner (14) pointed out that monoclinic lead chromate always exists in the precipitating vessel and is apparent on long standing. This would lead us to assume that the monoclinic form of lead chromate is the more stable, an assumption borne out by the literature and by experimental observations. It is possible that this form of lead chromate is actually less soluble than the rhombic and so makes its appearance on long standing by the mechanism already described. Since lead molybdate may exist in the monoclinic form, when its solubility is greater than that of lead chromate, monoclinic lead chromatelead molybdate is formed on the original lead chromate lattice by the same action, which promotes growth of tetragonal crystals a t higher pH. At a pH of 0.5 or lower, the rhombic transforms direct into the monoclinic form without passing through the red stage. At intermediate pH values when the molybdate is slightly less soluble than the chromate, a preliminary rapid formation of tetragonal crystals is followed by a slower formation of the monoclinic. It is probable that in this range both types of crystals are built up, the monoclinic proceeding more slowly than the tetragonal. When all of the rhombic crystals originally formed are exhausted, the monoclinic crystals continue t o build up a t the expense of the tetragonal form which now is composed chiefly of chromate. The result is a loss of color of the crystalline mass and, on continued stirring, an almost complete transformation to the monoclinic. The important problem in the production of molybdenum orange seems to be that of stabilizing tetragonal lead chromate and preventing formation of the monoclinic modification. This problem is not unlike that of stabilizing light chrome yellow (rhombic lead chromate) in many respects since both are concerned with prevention of the formation of monoclinic crystals. A short survey of the work which has been done in the light chrome yellow field is given in the following paragraphs. Jablczynski (2) discussed the effect of lead sulfate on preventing the “reddening” of chromes. He assumes that the color change is due to the formation of the basic chromate rather than to change in crystal structure but states that any soluble lead salt as well as the sulfate is capable of preventing the change. Milbauer and Kohn (9) made an intensive study of the system :
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INDUSTRIAL AND ENGINEERING CHEMISTRY
They concluded that for stability of chrome yellow, a slight excess of lead acetate or lead nitrate must be used in order to keep some lead sulfate in the system; for lead sulfate disappears in the presence of excess potassium chromate since reaction 8 has a greater tendency to move to the right. They also concluded that the sulfochromate theory tracing the stability of chrome yellow to lead sulfochromate is improbable since they found no evidence for the existence of such a double salt. Wagner (IS) stated that rhombic lead chromate crystals with a sheath of lead sulfate are much more stable. Rassudova and Kasatchkina (10) did considerable work on mixed chromates, chiefly with the monoclinic form, and came to the conclusion that lead sulfate-lead chromate crystals are formed simultaneously at the moment of precipitation. Such crystals, they claimed, are identical with pure monoclinic lead chromate, as shown by the Debye diagrams. BrusilovskiI and Tsarev ( 1 ) discussed the preparation of light-fast highly dispersed chromates. They listed, from Wagner and Reidel (16),conditions affecting the color of lead chromates as follows: “variations in acidity of medium, temperature, concentration of initial solutions, conditions of mixing, etc.” They claimed to have been able to stabilize rhombic lead chromate effectively by the use of highly surface-active compounds which form films on the surface of the particles and thus prevent further action at the surface, with consequent transition to the monoclinic form, They found that the stabilizing effect of these compounds was much more effective at pH 6.8 than at 3.8, which is in accord with the writer’s observation that rhombic lead chromate is more stable in neutral solutions than in acid. They tried the following compounds : petroleum sulfonic acids, oxidized paraffin (hydroxy acids), soaps obtained by saponifying solid highmolecular-weight animal fats and vegetable oils (such as linseed oil), and protective colloids such as gelatin and sodium alginate. Best results were obtained with petroleum sulfonic oils and their mixtures with soaps. Sapgir, Rassudova, and Kvitner (11) were able to prepare stable rhombic lead chromate without the use of sulfate under carefully controlled conditions as follows: precipitation from neutral solutions, room temperature or lower, excess of lead acetate, wash with dilute lead acetate or distilled water, drying in vacuum at room temperature (about 25’ C.), and use of only small batches (up to 40 grams). They concluded that in chrome yellows of the formula mPbCr04.nPbS04, rhombic crystals could be obtained at any value of the m : n ratio, whereas for the monoclinic form the ratio must be 1. The preceding literature survey shows that most authors consider the presence of lead sulfate or other lead salts necessary to prevent the transition to the monoclinic form. No one seems to know just how lead salts prevent this transition; many conflicting theories have been advanced. Since we are interested in maintaining lead chromate in the rhombic form only so far as this promotes formation of the tetragonal modification, such methods as the use of surface-active materials or the formation of sheath crystals are not of much value during the period of tetragonal crystal formation. It is possible that these sheath crystals may be advantageously ‘employed to prevent further transformation, once the red color is formed. This probably is the reason for the use of numerous addition materials by the color makers in their attempts to stabilize molybdenum orange. Review of the articles cited might be of value in suggesting better applications in this field. As already mentioned, little work was done along this line in this laboratory, since it seemed preferable to stabilize the products as far as possible by control of
303
solution conditions. The almost universal practice of using sulfate in stabilizing light chrome yellows probably accounts for the ready acceptance of the prevalent opinion that molybdenum orange cannot be prepared without the use of sulfate. A number of points become much more easily explained in the light of the foregoing theory of transformation. The reason for the enormous effect of differences in p H on reaction rate and properties of the finished product is clear. If the solution after precipitation is neutral (pH 7.0), very little transformation takes place since lead chromate, molybdate, and sulfate are very insoluble in such a solution. Consequently, the dynamics of solution equilibrium represented by Equation 7 are not very great, and there is little tendency to change from the forms originally precipitated. If the pH is lowered slightly (34),the solubility is increased and rhombic lead chromate is dissolved to build up the tetragonal form, resulting in a change of color to the red. If the p H is lowered still more (1.5-2.5), the solubility is further increased so that the transformation takes place more rapidly and completely to give a redder color. If, on the other hand, the pH is raised very much above the neutral point (up to 9.0), monoclinic lead chromate is formed directly, possibly as a result of the decomposition of some of the lead salts to form plumbates; this leaves an excess of chromate which is apparent in solution a t extremely high pH values, An investigation of basic lead chromates was not carried out since such colors were believed to be outside the range of true molybdenum oranges and to be more nearly allied to the chrome oranges. The theory also explains the effect of increased temperature and concentration in speeding up the transformation from yellow to red. An increase in temperature accelerates the movement of molecules and thus increases the rates of reactions taking place in the crystal-solution equilibria; the whole process is thus materially hurried. An increase in concentration increases the ratio of solid particles to liquid after precipitation so that solution of one type and deposition in the other form is facilitated, since the distance between particles is not so great. Substantiation of the theory to some extent is found in the fact, as previously pointed out, that the transformation from yellow to red takes place whether the crystals are coprecipitated or precipitated in separate vessels and then mixed, as well as by the following observation: When a solution of 88 mole per cent sodium chromate and 12 mole per cent sodium molybdate, together with enough sodium hydroxide to give a final pH of 2.2, was added to a slight excess of lead nitrate and the mixture was allowed to settle immediately, the precipitate was distinctly two phase. One of these phases was an orange red, which settled rapidly; the other was a light yellow, which remained in suspension longer, apparently as a result of smaller particle size. On further stirring, all of the yellow portion of the precipitate disappeared while the redness of the remainder increased. Since in this case only molybdate and chromate were present, the only possible explanation for such a phenomenon seems t o be the formation of two types of crystals (rhombic lead chromate, yellow; and tetragonal lead molybdate-lead chromate, red) which later formed a single type.
Application of Theory to Increasing Tinctorial Strength As stated previously, the preliminary colors were not nearly so strong as the commercial standard. According to theory, if more nuclei of lead molybdate could be produced for the chromate to build upon, the net result would be smaller particles with a consequent increase in strength. According to von Weimarn’s principle (16),the degree of
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304
dispersion, or fineness of particle size, depends upon solubility, supersaturation, and viscosity of the medium as shown by the expression : d=-
Q
-%K
S
where d = degree of dispersion Q = quantity of material in solution at the instant of mixing S = normal solubility of the compound formed v = viscosity of the medium K = a constant, depending upon the nature of the system Since we wish to increase d as much as possible for lead molybdate, and v and K remain fixed, we must either decrease the solubility or increase the supersaturation of lead molybdate at the instant of mixing, As pointed out before, the more nearly neutral the solution is, the lower will be the solubility of this salt. (Supersaturation can be increased by use of more concentrated solutions and more rapid mixing, but this was not feasible here since the work was already being done almost a t the limits of these variables.) If the mixture of lead salts is precipitated in neutral or faintly acid solution rather than at pH 1.5 to 2.5, as was formerly done, the strength should be greatly increased. By actual trial, precipitating a t pH 4.5 and then lowering the pH to 2.8 with nitric acid gave as good as, or better strength than that of the commercial standard.
Modified Procedure for Molybdenum Orange Since theoretical considerations had disclosed a method of overcoming the low strength of the product, the procedure for the preparation of molybdenum orange was modified as follows: 1 was made up of 34.7 grams of lead nitrate (0.1 mole +Solution 5 per cent excess) dissolved in a liter of water. The excess
lead nitrate is used t o be sure that no excess of chromate will be left in solution. Solution 2 consisted of 11.5 grams (0.0385 mole) of NazCrzOl.2Hz0, 1.7 grams (0.012 mole of sodium sulfate, 2.8 grams (0.011 mole) of Na2MoO4.2Hz , and 3.2 grams (0.08 mole) of sodium hydroxide, dissolved in enough water to make a liter. The excess sodium hydroxide is added so that the pH of the solution after preci itation will be about 4.5. The amount used is approximately &at required to convert the sodium dichromate to monochromate according t o Equation 3. The pH of solution 1 will be about 4.8 to 5.0 if crystallized lead nitrate is used. If the solution is prepared from metallic lead and nitric acid or from litharge and acid, the pH of the solution may be adjusted t o this value by the addition of caustic or account may be taken of this by addition of excess caustic t o solution 2. The adjustment of the lead nitrate solution is recommended since by this method the pH at no time during precipitation will be lower than a minimum precipitation value of about 3.0. Solution 2 is run into 1; addition in this manner maintains the necessary excess of lead salt. Rapid mixing is advocated on the basis of von Weimarn's principle, since supersaturation is increased in this way so that the strength should be greater. Solutions should be at room temperature or lower. When precipitation is complete, the pH of the mixture is lowered by the addition of nitric acid to 2.5 to 3.0, which permits the transformation to take place comparatively rapidly (from 15 to 30 minutes). Lower pH gives a greater degree of transformation and a redder color, but the shade is not so readily controlled as when the pH is about 3.0. Also, at lower pH the time factor becomes increasingly important, the secondary transformation to the monoclinic form being rather rapid below pH 2.5 and not interferingnearly so much at pH 3.0. As a matter of fact, at pH 3.0, the stirring time may be varied from 30 minutes to 3 or 4 hours without a great deal of change; the outcome is greater ease in plant manipulation and more reproducible results. When the desired color is reached, as in the previous formula, a solution of 2.3 grams of aluminum sulfate in 20 cc. of water is added, followed by a solution of sodium hydroxide to raise the H of the mixture and effect the formation of a film of aluminumtydroxide on the particles. Just enough sodium hydroxide is added to raise
(I
VOL. 31. NO. 3
the pH to the neutral point (about 0.56 gram is usually required). Care must be taken not to exceed a pH of about 8.5, since an alkaline solution will cause transformation to the monoclinic with resultant loss in color. A final pH of 6.0 t o 8.0 is recommended, since it is found that the precipitate is flocculent and settles very slowly in the presence of aluminum hydroxide below pH 5.5. The precipitate may be washed as desired, filtered, and dried at a temperature as high as 100' C. without appreciable loss in color, although lower drying temperatures are recommended on the basis of the work of Sapgir, Rassudova, and Kvitner (11).
Substitution of Other Salts for Lead Sulfate There is, no doubt, some advantage in the presence of lead sulfate for maintaining lead chromate in the rhombic form, as is evident from its wide application in light yellow chromes; and these experiments show that an excess of some lead salt definitely increases stability of molybdenum orange. However, it does not seem necessary to limit this to the sulfate. Jablczynski (2) pointed out that any soluble salt of lead will prevent change to the darker form, and Sapgir, Rassudova, and Kvitner (11) showed that lead sulfate is not necessary to the stability of rhombic lead chromate if an excess of lead acetate is maintained. No x-ray examination of molybdenum orange crystals was made here; consequently the statement of Wagner, Haug, and Zipfel (14) that a true mixed crystal of lead chromate, lead sulfate, and lead molybdate is obtained cannot be contested. (They even go so far as to call it a compound and give a characteristic formula of 5PbCrO4.3PbMoO~.lOPbSO~.) But their statement cannot be accepted entirely. By definition a mixed crystal is a single crystalline phase formed by replacement of isomorphous (capable of existing in the same form) compounds in indefinite proportions within the crystal lattice. Therefore, for two compounds to form true mixed crystals, they must be capable of existing in the same crystalline modification. Lead sulfate seems to be dimorphous, since it is able to exist in either the rhombic or monoclinic form. No reference has been found to a possible existence in the tetragonal form, which would be necessary in order for it to enter into the tetragonal lead molybdate-lead chromate lattice. It is possible that lead sulfate is trimorphous or that the definition of mixed crystals is in error, but in view of the facts the inclusion of lead sulfate as a part of the tetragonal lattice of molybdenum orange is open to question. It may be joined to the crystal or merely mechanically mixed as lead sulfate or as lead sulfate-chromate mixed crystals. Therefore, it should be possible to substitute another slightly soluble salt of lead for the sulfate. By means of the procedure given previously and substitution for the 1.7 grams of sodium sulfate of its equivalent of the salt in question, colors were prepared from the following salts: Grams
4.57 3.68
3.40 1.40 3.58 1.18
3.57 3.94 1.37 3.61
Formula NazBdOr. lOHzO NaBOs. 4Hz0 NazSiOs .9Ha0 NaCl NaI NaCN NazFe(CN)aNO . 2 H ~ 0 KaFe(CN)6 ("4) &Or. Hs0 NaBrOa
Name Borax Sodium perborate Sodium metasilicate Sodium chloride Sodium iodide Sodium cyanide Sodium nitroprusside Potassium ferricyanide Ammonium carbonate Sodium bromate
I n these preparations no sulfate was used; 0.6 gram of sodium aluminate was substituted for the 2.3 grams of aluminum sulfate added after the color had formed. The borate, perborate, silicate, cyanide, nitroprusside, ferricyanide, chloride, and carbonate gave approximately the same results as the sulfate; the color developed in the
MARCH, 1939
INDIJSTRIAL AND ENGINEERING CBEiVIISTRY
same manner, with some variations due to the differences in characteristics of the corresponding lead salts. Disodium phosphate, sodium metaphosphate, sodium tartrate, sodium stannate, hydrofluoric acid, sodium iodate, and sodium vanadate were also tried, but colors prepared in the presence of these salts either failed to change or did not become as red as the standard adopted. Scratch-outs were made from those colors which did give equivalent results, together with ZC-to-1 reductions of these colom with zinc white. An interesting sample of dserence due to characteristics of the lead sulfate is shown by the iodide, also included, which was quite brown by comparison. It is interesting to note that the color made with ferricyanide was as strong, more brilliant,, and much faster to light than any of the colors produced with sulfate. Of all the salts used as addition agents, the pigment from bromate showed the most vivid and strongest color. There are, no doubt, many other salts which could be substituted for sulfate but these are sufficient to show that sulfates are not necessary to the production of the red orange molybdenum colors.
h a d ChromateLead M o l y b d a t e without Addition Materials Tbe possibility of producing molybdenum orange from lead chmmate and lead niolybdate alone was investigated. It had been previously demonstrated by the work of Schultze (la) and of Jaeger and Germs (4). A trial run was made in which the procedure previously given was foUowed except that the sulfate was omitted. The transformation from yellow to orange-red took place in much the same way as it did in the presence of sulfate, and the resultant color was just as strong and brilliant. The greatest difference was that the color was not quite so rrrl. Thus, although the sulfate or many other slightly soluble salts of lead are not necessary to the forniabion of t,he color,
INKMIXER (Note buttery co&ttenoy
at this stase.)
305
they do help to intensify it,, either by sti ng the equilibrium in the direction of tetragonal crystal formation to stabilize it or by influencing light absorption by their presence in the mixture. This trial run served to show also that production of molybdenum orange without sulfate or any other slightly soluble lead salt required more careful control than when the salt was present. By the preparation of red mixed crystals of lead chromatemolybdate, the problem had heen reduced t o a two-component system (neglecting the water and excess of lead nitrate) which could be investigated much more easily from the standpoint of composition than could a three-component system. RATIO OF MOLYBDATE TO CHBOMATE.Schultze (18) stated that he obtained tet.ragona1 crystals of lead chromate and lead molybdate when he used as much as 42 per cent of l e d chromate and that crystals containing above 58 per cent of lead molybdate were dark red. This would seem to indicate that more molybdate than chromate should he used to obtain the red color. However, preliminary work showed that, although this might work out in the cases of fusion, it was not true when the ma,terials were precipitated from aqueous solutions. As shown in patent literature on molybdenum orange, a t least 4 to 5 per cent of lead molybdate (1 to 1.25 per cent molybdenum) must be present in the finished pigment to obtain the desired orange or rod color. With this figure as the minimum, a series of colors was prepared containing, respectively, 5, 10, 20, and 30 mole per cent of lead molybdate (corresponding approximately to 1.25, 2.5, and 7.5 per cent molybdenum) in the finished product. Scratehauts of the color rubbed out in No. 00 Litho varnish (2 grams of color to 0.5 gram of oil), as well as 20-to-1 reductions of these colors with zinc white, showed that the region of greatest redness of color lies at about 10 mole per cent of lead molybdate. A closer determination of the ratio is shown by seratchouts from samples prepared with mole percentages of lead molybdate ranging from 8 to 16 as a check on the previous determination. The optimum amount of lead molybdate seems to be about 10 to 12 mole per cent of the finished pigment (corresponding t o approximately 2.5-3 per cent niolybdenum). The previously given analyses of two commercial colors showed that they contained 12 per cent lead molybdate, amounting to about. 11 mole per cent of the pigment. EXCESSLEAD SALT. The experiments described so far have shown that an excess of Ipad salt was necessary to form and stabilize molybdenum orange. The next problem was concerned with how much excess was necessary mid what the effect of a large excess would he. A series of colors was prepared from the same t,ype of standard solutions of lead nitrate, sodium dichromate, and sodium molybdate described in the first part of this report; the lead nitrate solurion was as nearly equivalent to the chromate and molybdate solutions as possible, cubic centimeter for cubic centimeter. The molybdate and chromate solutions were mixed in a ratio of 1 to 9 so that the resultant product would he 10 mole per cent lead molybdate. Then 50-cc. portions of this mixture (measured with a pipet) were diluted to 500 cc. and added to measured amounts of lead nitrate solution, also diluted to 500 eo.; 50 cc. were used a t the beginning and the amount was gradually increased. After mixing, an excess of either lead or chromate was easily determined by allowing the precipitate to settle and adding a few drops of a supernatant
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INDUSTRIAL AND ENGINEERING CHEMISTRY
liquid to chromate and lead nitrate solutions. Formation of a precipitate in the lead nitrate indicated excess chromate; a precipitate in the chromate solution showed excess lead. The mixture of 50 cc. of lead nitrate and 50 cc. of chromatemolybdate solution gave a faint excess of chromate. I n this batch the color did not develop other than from the formation of monoclinic lead chromate. A 1 per cent excess of lead nitrate gave a much redder color but not so red as a 5 per cent excess; a 10 per cent excess gave still better color. Twentyfive and fifty per cent excesses of lead nitrate seemed to give better color during preparation, but this was not apparent when the dry color was rubbed out. PREPARATION OF MOLYBDENUM ORANGEWITHOUT SULFATE. The preparation of molybdenum orange without sulfate is not essentially different from the other procedure but requires more careful control to produce the desirable color: Solution 1 consists of 36.4 grams (10 per cent more than 0.1 mole) of lead nitrate dissolved in a liter of water. Solution 2 consists of 11.96 grams (0.045 mole) of NazCrzOr.2H20, 2.42 grams (0.01 mole) of NaaMoO4.2HZ0,and 4.0 grams (0.1 mole) of sodium hydroxide dissolved in a liter of water. Mix in the same manner as in the procedure using sulfate, adding solution 2 to solution 1 rapidly with constant stirring. The pH after mixing should be about 4.5. Lower to 2.5-3.0 by the cautious addition of nitric acid (about 0.5 cc.). Stir until the color has fully developed (about 30 minutes). Then add a solution of 0.6 gram of sodium aluminate in 15 cc. of water followed by enough sodium hydroxide to raise the pH to 6.0-8.0, taking care not to go above 8.5 (about 0.8 gram is required). The precipitate may be washed, although greater stability to drying is obtained when it is filtered directly, since washing removes the excess lead salt which acts as a stabilizer. Care should be taken that stirring is not continued too long after the color has developed since this has a tendency to promote the formation of monoclinic crystals which develop much more rapidly in the absence of slightly soluble lead salts other than chromate and molybdate.
Comparison of Colors Prepared with and without Sulfate Since comparison of reactions or results observed a t different times is difficult, molybdenum orange colors were prepared with and without sulfates at the same time under as nearly identical conditions as possible. The foregoing procedure was followed, merely substituting 1.70 grams of sodium sulfate for its equivalent of 1.79 grams of NazCrz07.2Hz0 in the case of the color containing sulfate. The color developed just as rapidly without the sulfate as with it, attaining a maximum in about 15 to 20 minutes after the pH of each solution was adjusted to 3.0. The color containing sulfate at this point was only faintly, if any, redder than the one without sulfate. On continued stirring for a total of an hour, however, the color without sulfate seemed to lose a little of its redness, whereas the sulfate color retained its maximum color. After addition of the sodium aluminate and sodium hydroxide, the precipitates were allowed to settle. The one containing sulfate settled more slowly, which indicated possible smaller particle size. After two washings by decantation, the difference in color between the two was more apparent, the sulfate color being considerably redder. When filtered, the precipitate containing no sulfate was much bulkier than the other; this result indicated that it had started to change to the monoclinic form. The colors were dried overnight a t 80-85” C. The one containing sulfate was perfectly stable to drying and remained a uniform orange-red; the one without sulfate lost some color and became yellow a t the edges. The yields were as follows: with sulfate, 34.7 grams; without sulfate, 35.6 grams. The results of this experiment showed definitely that a slightly soluble salt of lead promotes stability of molybdenum orange although it is not necessary to its formation. It has
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not been possible to produce quite so red a color when chromate and molybdate are used alone as when sulfate or some other slightly soluble lead salt is employed. However, by careful control of conditions, the best properties of sulfatecontaining colors have been approached.
Summary and Conclusions A formula has been developed which gives satisfactory and reproducible results. A reasonably accurate explanation has been presented for the observed phenomena; the production of molybdenum orange is thus made a logically controlled procedure, and the foundation is laid for constructive experimentation rather than the trial-and:error method commonly employed in this field. Molybdenum oranges with the most desirable properties are produced under the following conditions: (a) in the presence of excess lead salts; ( b ) at room temperature or lower; ( c ) at low concentrations (0.1 molar); (d) precipitation in solutions such that the final pH is about 4.0-5.0; (e) development of color in solutions of pH 2.5-3.0; (f) formation of aluminum hydroxide film on the particles to decrease oil absorption and increase brilliancy, and (8) drying a t temperatures up to 100” C., preferably as low as practical. Lead sulfate, as a component of the pigment, can be substituted by the following slightly soluble salts of lead: borate, perborate, silicate, iodide, ferricyanide, cyanide, nitroprusside, carbonate, and chloride. The properties of the finished color depend somewhat on the salt in question, ferricyanide giving better results than any of the others. It is possible to produce tetragonal mixed crystals of lead chromate-lead molybdate without the use of any agent other than an excess of soluble lead salt and those employed in controlling pH. Such mixed crystals have all the desirable properties of colors containing sulfate or other slightly soluble lead salts, except that they are not quite so red and their stability to long stirring, washing, and drying is not quite so good. The ratio of molybdate to chromate should be about 1PbMo04.9PbCr04, and approximately 10 per cent excess soluble lead salt should be used in production of lead chromate-lead molybdate colors.
(1) (2)
Literature Cited BrusilovskiI, A. M., and Tsarev, B. A., Org. Chem. Ind. (U.S. S. R.), 3, 218-23 (1937). Jablczynski, K., Chem. Ind. (Ger.) Nuchr. Ausgube, 31, 731
(19081. (3) Jaeger and Germs, 2. unorg. allgem. Chem., 119, 145 (1921). (4) Ibid., 119, 169 (1921). (5) Lederle, Ekbert, German Patent 22 F. 1.62.30 (Aug. 14, 1930); British Patent 34,451 (Nov. 15, 1930); U. 8. Patent 1,926,447 (Sept. 12, 1933). (6) Lederle, E., U. S. Patent 2,030,009 (Feb. 4, 1936). (7) Ibid.. 2,063,254 (Dec. 8. 1936). (8) Linz, Arthur, Belgian Patent 327,260 (Feb. 27, 1937). (9) Milbauer and Kohn, 2. physik. Chem., 91,410 (1915). (10) Rassudova and Kasatchkina, Org. Chem. Ind. (U. S. S. R.), 3, 634 (1937). (Id) Sapgir, Rassudova, and Kvitner, Lakokrasochnuyu Ind., Za, 42, 56 (1932). (12) Schultze, Ann., 126,62 (1863). (13) Wagner, H . , Farben-Ztg., 42,83 (1937). (14) . . Wagner, Haug, and Zipfel, 2. unorg. allgem. Chem., 208, 249 (1932). (15) Wagner and Reidel, Furben-Ztg., 31, 1067 (1926). (16) Ware, “Chemistry of the Colloidal State,” 2nd ed., pp. 179-80, New York, John Wiley & Sons, 1936. (17) Willard and Cassner, “Solubilities of Inorganic and Organio Compounds,” private communication to Seidell, 1936.
RBCEIWDAugust 19, 1938.
