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T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
The action of zinc oxide as an activator for thiocarbanilide and a retarder for aniline have been the subject of much recent discussion. It now appears that the first mechanism of its action with carbo-sulfhydryl accelerators is to form a zinc salt or mercaptide of the general formula R-S-ZnS-R, which in some manner is able to activate the sulfur and hand it on to the rubber. By preparing these mercaptides before compounding the zinc may be used in much lower quantities and pure gum cements may be made to cure a t ordinary temperatures.
8UMMARY 1-The vulcanization of rubber by ammonium hydrosulfide has been explained by oxidation and the liberation of free sulfur from the disulfide, The loss of S, from polysulfides has previously been proposed as the mechrtnism for vulcanization. 2-Meta-diamines are differentiated from para-diamines by their sulfur reactions. m-Tolylene-diamine forms stable disulfide-polysulfides to which its curing power is attributed. &-Sodium phenolates form disulfide-polysulfides similar to the meta-diamines and aniline. $--Aldehyde ammonia is very rapid in its reaction with
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sulfur a t or below curing temperatures, ”and forms ammonium polysulfide during vulcanization. 5-p-Nitrosophenol is believed to function only &s a secondary accelerator, acting in this manner similarly to litharge. 6-Zinc oxide or zinc mercaptides have been found neceseary in all rubber cements which cure a t room temperature. 7--Weak bases, such as aniline, will vulcanize a zinc oxidecarbon disulfide cement a t room temperature, just as piperidine or dimethylamine. 8-In cements containing amine and carbon disulfide, zinc oxide is dissolved and the cement may take on the appearance of a pure gum cement. 9-A mixture of aniline and thiocarbanilide will dissolve zinc oxide at ordinary temperatures. 10-A mixture of aniline and thiocarbanilide will vulcanize a cement containing zinc oxide a t room temperature, while either alone will not. 11-Zinc thiophenol and zinc ethylxanthate are given as two accelerators which are free from nitrogen or alkali and which function either in heat cures or in curing pure gum cements at room temperature. Zinc mercapto-bensothiazole acts similarly. 12-The ultimate mechanism of vulcanization by mercaptides and sulfur has not been discussed.
The Preparation and Instability of Tricalcium Arsenate’ By J. H.Reedy and I. L. Haag DEPARTMENT on CHFMISTRY, UNIVERSITY OA ILLINOIS, URBANA, ILLINOIS
Of late attention has been sharply directed to the instability of tricalcium arsenate, which is extensively used as an insecticide for certain crops. During storage this product undergoes some change in composition which results in a considerable increase in the amount of water-soluble arsenate. The latter substance is considered responsible for the “burning” of the foliage of the plants to which it is applied. At the time of preparation, according to the manufacturer’s analysis, the soluble arsenate content is well within the limit (0.75 per cent AszOs), but upon reaching the consumer the value may he considerably in excess of this. Contact with moisture and carbon dioxide of the air was thought at first to be the cause of this deterioration, but the use of airtight containers has not been sufficient to prevent it. Furthermore, manufacturers have not been able to control their processes so as to give products of uniform composition. The material from one run may be thoroughly satisfactory, and that from the next may have to he rejected on account of excess of water-soluble arsenate.
EXPERIMENTAL
g. of water at 25” as: CaHAs04, 0.3308 g.; Ca3(As04)2,0.014 g. These pure salts were used as controh in the study of the products obtained by methods available for large-scale production. INDUSTRIAL PREPARATION METHODS-while Several methods suggest themselves for the manufacture of tricalcium arsenate, only one has been found in practice to give a satisfactory product a t a sufficiently low cost. This involves the action of arsenic acid on a paste of slaked lime?
3 Ca(OH)2 f 2 HsAs04 + Cas(AsO4)a f 3 H20 (1)
In the present work, a high-grade lime was slaked with three or four times its weight of boiling water, and a solution of arsenic acid was slowly added, with constant stirring, until the mixture was only faintly alkaline to phenolphthalein. This process involves approximately equal parts of lime and arsenic acid (estimated as As206),in the molecular proportion of 4 CaO: As206. The final product was assumed to be the tricalcium arsenate with an excess of calcium hydroxide. The reaction between calcium chloride and trisodium arsenate gave a product that was unsatisfactory on account of the high water-soluble arsenate content. A batch of the material was made as follows: A solution of disodium arsenate was converted into the trisodium salt by adding a molecular amount of sodium hydroxide, and this wm added to an excess of calcium chloride solution;
PREPARATION OF SALTS FOR comRoLs-Three well-defined arsenattes of calcium2 are known: the normal or tricalcium arsenate, Ca3(As0&; the secondary or calcium hydrogen arsenate, CaHAsOc; and the primary or monocalcium arsenate, Ca(HzAsO&. The last of these is decidedly unstable, and its presence in the commercial product is unlikely. Samples of the normal and secondary s d t R were prepared by The by-product of Reaction 1 is water only, while the the methods of Robinson.8 For the former, calcium chloride and disodium arsenate were allowed to react in slightly acid mother liquor of Reaction 2 contains sodium chloride and solution. The latter was made by the action of arsenic acid the excess of calcium chloride. These were removed from solution on an excess of saturated calcium hydroxide solution. the precipitate by washing. However, the washings contained The precipitates were washed, dried, and analyzed, indicating soluble arsenates, and the residue still remained high in the respective formulas, CaHAsOJhO and Ca3(AsO4)2.31120. soluble arsenates. The part played by these soluble salts Robinson determined the solubility of these salts in 100 in increasing the soluble arsenate content will be discxlssed later. Received June 13, 1921. Abegg, “Handbuch der Anorganischen Chemie,” 111 (1907), 641. a Oregon Agricultural Experiment Station, Bulletin 131 (1918). 1 I)
I
Cf. Haywood and Smith, U S. Department of Agricultrre. B&&n
760 (1918).
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1%was concluded, therefore, that the lime paste-arsenic acid method is the most suitable process for preparing a satisfactory grade of tricalcium arsenate. Further study has convinced the authors, on the other hand, that the whole technique of the preparation of tricalcium arsenate by the lime method is very important and most difficult to bring under control. The difficulties of the manufacturer in obtaining yields of low arsenate solubility have persistently appeared in this work. Hence attention was particularly directed toward the study of various influences that might affect the nature of the product. TEMPXRATURE EFFECTS-Without doubt, One very important influence is temperature, and here the results are somewhat conflicting with those of others, especially Hrtywood and Smith.l Four samples of tricalcium arsenate were prepared by adding arsenic acid solution to calcium hydroxide solution (not the paste) until the mixture was only slightly alkaline. (Table I .) In the first two cases the calcium hydroxide was TAIILE I-EFFECT OF TEMPERATURE ON WATER-SOLUBLE AszOa SAMPI,E
1 2 3 4 5 6
Temperature of Ca(0H)a
c.
80 80
23 23 80 23
Total As201 Per cent 42.88 41.92 40.78 41.32 43.70 43.70
Water-Soluble AS206
Per cent
1.53 2.43 8.41 9.25 0.28 1.62
hot and the precipitate was washed with hot water: in the third and fourth cases the operations were a t room temperature. The lime paste was used in the preparation of Samples 5 and 6. While the results for corresponding temperatures do not check very well, they indicate that, under the conditions employed, high temperatures favor low soluble arsenate values. It might be added that the experience of the manufacturers has led them to adopt the mixing of the solutions hot as the best plant practice. It is likely that the explanation of this temperature effect is that a t low temperatures the speed of reaction is slow, so that the calcium hydrogen arsenate (probably formed as an intermediate product) is never quantitatively changed over into tricalcium arsenate, but is more or less occluded in the excess of the latter. ARSENIC ACID SoLUTIoN-The purity of the arsenic acid solution is of prime importance. The presence of nitric acid always resulted in high solubility, notwithstanding the fact that there was an ample excess of lime to effect its neutralization. Consequently i t was found necessary t o evaporate the arsenic acid solution, which was formed by the action of nitric acid on arsenic trioxide, on a steam bath as long as acid fumes were evolved. LIME PASTE-Previous writers have pointed out that the lime used should be of high purity, and that it should be slaked tu form a smooth paste. The presence of a considerable amount of calcium carbonate is objectionable in that it will not react with calcium hydrogen amenate, and may therefore be responsible for the presence of this soluble material. MIxINa-Thorough mixing is also essential. The arsenic acid should be added ~lowly,so slowly that the mixture should at all times be alkaline. This is necessary to prevent the formation of acid arsenates, which seem to revert to the normal salt only slowly. Whether or not in the precipitation from hot solutions the stirring should be stdpped just as soon as the acid solution has been added is hard to say. This point was investigated in the present work, and rcsults seem to indicate that cooling without agitation gives thc best product. The highest purity was obtained in runs that, were allowed to stand over night before filtering. Precipitation from hot solutions with vigorous stirring always resulted in a very fine precipitate, which upon drying 1h
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gave a very friable, fluffy powder. Such a condition is necessary to insure a sufficient suspensibility to make the product serviceable as a spray material. EXPOSURE TO AIR-Tricalcium arsenate deteriorates to a marked degree when exposed to air for R considerable length of time. Tables I1 and I11 ahow the effects of laboratory air and moist carhsn dioxide, respectively. Confessedly, the latter is not comparable to ordinary air, but it does show in TABLE 11-EFRECTOF EXPOSURE TO AIR -Water-Soluble Initial Value Per c e n t 0.35 0.07
SAMPLE
C I
O F LABOR.4TORY
-
AFzOa After 4 Wks ' Exposure Per cent 3.69 0.52
an exaggerated degree the change that takes place undw atmospheric influence, The apparent capriciousness of the TABLE 111-EFFECTOF Gain in Weight Per c e n t 0.015
SAMPLE C
G
C A R B O N DIOXIDE -Water-Soluble AszOsInitial Value After 21 Hrs.' Exposure Per cent Per cent
0.35 1.62 0.07
0.021 0.032
I
9.50 12.70 3.32
results may be due to the various degrees of impurity of the samples used. For example, Sample I may have had a higher content of free calcium hydroxide, which protected it to a large degree. However, the figures suggest that other influences than the removal of the free base are involved. InlPvRrTrEs-~able Iv shows the effects of certain common contaminating substances, as compared with distilled water. IMPURITIES IN WATER of Water-Soluble AsrO-. Tap 0.02 Per cent 0.02 Per cent Water NaCl FeSO4 Per cent Per cent Per cent
TABLE IV-EFFECT -------Amount
SAMPLE
Distilled Water Per cent
I
0.07 0.35 1.62
C
G
OF
2.31 7.42
1.27 3.55 5.26
..
8.25 18 00 25.00
The tap water contained sodium, magnesium, and iron in the form of bicarbonates and sulfates, anions that are believed to be very active in the decomposition of tricalcium arsenate. The action of the nitric acid in the arsenic acid in causing high solubility, as mentioned above, falls under this head. I n this case, it is the calcium nitrate that effects the decomposition of the material. An excess of calcium hydroxide, on the contrary, has been shown by Robinson' to stabilize the product, reducing the amount of water-soluble arsenate to practically zero. The various influences affecting the amount of soluble arsenate, as brought out in this work, are summarized in Table V. The property of suspensibility is relative, and was estimated by the volume of the precipitate and its slowness in settling. It will be noticed that the samples reported are, for the most part, high-grade products. TABLE V-SUMMARY Tem-
O F THE INFLUENCES
perature
of Reac-
tion SAMPLE C. A 80 B 80 80 C D 80 G 23 I 80 80 J
AFFECTINQ THE
TRICALCIUM ARSENATE
Acid STIRRING Added Thorough Slowly Thorough Slowly Thorough Slowly Little Fast Thorough Slowly Thorough Slowly Thorough Slowly
Stood
Water-
Hrs. 0.5 0.5 0.5
Per cent 0.23
before Soluble Filtering AszOs
0.5 0.5 12 12
0.16
0.35 1.04 1.62 0.07 0.09
QUALITY OF
THE
SUSPENSIBILITY Excellent Excellent Fair Poor Poor Good Good
NATUREOF THE DECOMPOSITION The reaction which occurs during the deterioration of tricalcium arsenate is evidently hydrolysis, and is represented by the following equation: Caa(AsOc)2 -J- 2 H2O c* 2 CaHAsOa Ca(OH), Evidence to substantiate this assertion cannot be found in analytical data, since, until either calcium hydroxide or calcium hydrogen arsenate begins to precipitate, the com-
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position of the dissolved substance will be identical with presence of soluble salts, a behavior exactly analogous to the that of the solid. Very conclusive evidence, however, was action of salts in catalyzing the hydrolysis of esters. brought out by the following experiment: Some tricalcium This explanation, of course, is a t variance with the asarsenate of the best purity was placed in a glass-stoppered sumption that tricalcium arsenate, since it is the most insolbottle, covered with distilled water containing a few drops uble, is therefore the most stable of the calciuq arsenates. of phenolphthalein, and shaken occasionally for several days. Insolubility is not an infallible index of stability, as might be A considerable alkalinity developed. To meet the objection shown by citing a number of cases in which insoluble substances that the alkalinity might be due to calcium hydroxide present are hydrolytically converted into more soluble ones. Magin the arsenate, this water was drawn off, and analysis nesium ammonium phosphate is a single example. From showed that it contained a corresponding amount of soluble this point of view, tricalcium arsenate in contact with mpisture arsenate. The water over a slightly impure sample (contain- is a metastable substance, and its transformation into the ing a trace of sodium chloride) reddened more rapidly than more stable secondary arsenate may be easily effected by many substances. in the case of the pure substance. This concept of a reversible hydrolysis also explains SUMMARY the other facts in the behavior of tricalcium arsenate. The 1-The most favorable conditions for making a stable stabilizing effect of the excess of lime is due to the repression of the hydrolysis. On the other hand, decomposition will be form of tricalcium arsenate that will have a low soluble arfavored by introducing anything that will combine with senate content and be otherwise suitable for use as an inthe calcium hydroxide formed, such as acidic substances secticide on plants are: (a) high temperatures, ( b ) purity or materials that will convert it into a more insoluble com- of materials, (c) excess of lime, (d) thorough mixing. 2-The water used in preparing sprays should be as pure pound. This is illustrated in the action of the tap water and of the ferrous sulfate (Table IV). The effect. of the sodium as possible. 3-The decomposition of tricalcium arsenate is due to chloride and calcium nitrate is probably of a different nature. The easiest and most direct explanation of such action is tfhat hydrolysis, which seems to be catalyzed by many substances the decomposition of tricalcium arsenate is catalyzed by the that may be present as impurities.
Preparation of Mannose from Ivory-Nut Shavings' By Paul M.Horton BATONROVQE,LOUISIANA AUDUBON SUQAR SCHOOL,LOUISIA+ STATE UNIVERSITY,
For a number of years it has been customary in the chemical laboratory of the Louisiana State University to assign to advanced students in the Audubon Sugar School the preparation of various rare sugars in the pure form. These preparations as a rule were not very satisfactory, either as to yield or product, nor wag it always easy to tell where the trouble lay. During the past few years the rare sugars, however, have been attaining considerable technical importance. The following description of the preparation of mannose presents our experience in an attempt toward its simplification. The procedure described has been tried out by six or eight students of average technique in organic chemistry with uniformly satisfactory results, both as to yield and quality. This procedure is presented not as offering anything essentially new, but in order that any one attempting to prepare this rather costly sugar may be assured of results. Though mannose occurs in many substances, the usual source is ivory-nut shavings, a by-product in the manufacture of buttons. A review of the literature will indicate that the source of the mannose produced from this source is largely the reserve mannocellulose. Whether the nuts actually contain fructo-mannan or not is immaterial, but it is certain that gums and other extractives are present, complicating the procedure by the steps needed for their removal. By the older processes the final sirup was always difficult to crystallize, owing to the various impurities passing through. For this reason the mannose was usually first separated as the hydrozone. Bourquelot and Herissey2 probably first described the method of hydrolyzing mannocellulose by means of cold 75 per cent sulfuric acid, but C. S. Hudson3 has recently much improved the procedure by crystallizing the sugar direct from 1 Presented before the Section of Sugar Chemistry and Technology a t the Blst Meeting of the American Chemical Society, Rochester, N. Y., April 26 t o 29, 1921. * Comfit. r e n d , 133 (19011, 302. 8 J . A m . Cirem. Soc., 39 (1917). 670.
glacial acetic acid, thus avoiding the use of the expensive reagent, phenylhydrazine acetate. The method finally adopted in our laboratory is as follows: Dissolve 20 g. of commercial sodium hydroxide in 2000 cc. of tap water and heat to boiling, preferably in a porcelsbin dish. Stir 175 g. of 20-mesh ivory-nut powder or shavings into the boiling solution and remove at once from the source of heat. Allow the mixture to stand for 1 hr. with frequent stirring. Filter off the deep brown extract, and wash the residue until the runnings are clear and neutral. This filtration can be carried out effectively in an 8-in. Buchner funnel with suction, using heavy toweling as the filtering medium. Theresidue is sucked dry and finally transferred to a tray and dried at room temperature or in an electric oven at not over 60" C . Mix 130 g. of the absolutely dry residue with 130 g. of cold 75 per cent sulfuric acid (1000 g. of 1.84 acid plus 400 cc. water for stock). If the residue has been well dried there will be very little evolution of heat, and the absence of irritating vapors is especially noticeable as contracted with the results obtained when using the unextracted shavings. The heavy, brownish, dough-like mass may be allowed to stand indefinitely without much decomposit,ion, although 6 hrs. are sufficient for the reaction. Dissolve the mass in water to obtain a final volume of about 1.5 liters. Heat t o boiling and allow to simmer for at least 6 to 8 hrs., keeping the volume constant by adding water from time to time to replace evaporatiqn. A reflux may be used. At the end of this period there will still remain a brownish residue in the solution, consisting for the most part of the outer skin of the nuts, which will weigh not over 10 g. when dry. Without filtering, allow the solution to cool to room temperature. Prepare a creamy solution of calcium hydroxide by slaking 50 g. of freshly burned lime in about 300 cc. of warm water. When cold add to the mannose solution with violent stirring and agitation in order that no, portion
