February, 1931
INDUSTRIAL AXD ENGINEERING CHEMISTRY
to 104-107" C. and there is no longer a separate layer of germanium tetrachloride. At this point another flask is substituted for the receiver and the distillate of constantboiling hydrochloric acid is collected until i t no longer gives a test for germanium with hydrogen sulfide. From 200 to 500 cc. of acid are collected over a period of 2 to 4 hours. Distillation is now stopped. The residue in the still contains a small amount of suspended matter which when investigated was found to be a mixture of 80 to 100 grams of sulfur and 15 to 20 grams of silicon dioxide. The silica was found to carry less than 20 mg. of germanium, and consequently was discarded, it being unprofitable to carry out the difficult separation of such a small quantity. The germanium tetrachloride collected in the first Erlenmeyer flask is shaken with sufficient crushed ice (about 1 kg.) completely to hydrolyze it to white hydrated germanium dioxide, and is allowed to stand for a few hours in order to facilitate the subsequent filtration. It is then collected on a filter, using suction, washed with a small amount of cold water, dried, and ignited. The germanium dioxide thus obtained is spectroscopically pure and represents 85 to 90 per cent of the total germanium in the ore. The filtrate and washings are combined with the second part of the distillate, consisting mainly of constant-boiling hydrochloric acid, made 6 normal with respect to this acid, and the germanium is precipitated by hydrogen sulfide as white germanium disulfide. This precipitate represents 5 to 10 per cent of the total element in the ore, bringing the total yield of germanium up to 95 per cent. When running continuously, this precipitation is not carried out until a large quantity of liquid has been collected, and the precipitated sulfide is then converted to the dioxide by the method of Dennis and Papish (3). If desirable, the dioxide thus obtained may be reduced to
207
the metal by the very satisfactory method described by Tressler and Dennis (11). Reasons for Inclusion of Stage Two
Although Stage Two may be omitted and the entire distillate of Stage One be hydrolyzed and distilled as described in Stage Three, it was found expedient to include it for the following reasons: (1) Most of the arsenic is discarded with the second fraction, thereby making the production of pure germanium dioxide in Stage Three relatively simple. If a large amount of arsenic is present in the material charged into the still a t Stage Three, the distillate obtained will contain appreciable amounts of it. This will necessitate a t least one redistillation in the presence of water and chlorine completely to separate the germanium from the arsenic. (2) Most of the sulfur chloride is discarded with the second fraction. The presence of its hydrolytic products (especially sulfur dioxide) in Stage Three is undesirable. In the first place, the presence of an uncondensable gas decreases the efficiency of condensation of the vapors. In the second place, this gas would have to be bubbled through the water in the gas-washing bottles to hydrolyze any germanium tetrachloride carried by it mechanically, and this would retard the rate of distillation. (3) The volume of material to be handled in Stage Three is reduced t o one-third. (4) It permits a relatively simple concentration of the gallium.
Literature Cited Dede and Russ, Ber., 61B, 2461 (1928). Dennis and Johnson,J. A m . Chem. Soc., 45, 1380 (1923). Dennis and Papish, Ibid., IS, 2131 (1921). Keil, Z. anorg. Chem., 152, 101 (1926). Kriesel, Chem-Ztn., 48, 961 (1924). Lunt, S. African J . Sci., 20, 157 (1923). Pufahl, Metall E n , 19,324 (1922). Pugh, J. Chem. Soc., 1929, 11, 2540. Schneiderhbhn, Mefall Ers, 17, 364 (1920). Thomas and Pugh, J . Chem. SOC.,125, 816 (1924). Tressler and Dennis, J. Phys. Chem.,81, 1429 (1927).
The Composition of Commercial Calcium Arsenate' C. M. S m i t h a n d C. W. Murray INSECTICIDE DIVISION,BUREAU OF CHEMISTRY AND SOILS, WASHINGTON, D. C.
I
N THE manufacture of the very important agricultural insecticide known as calcium arsenate, it is necessary to use approximately 4 mols of calcium oxide to 1 of arsenic oxide to obtain a product that is insoluble enough to be safe for application to growing foliage. Naturally the question of the state of combination of the calcium and arsenic oxides soon arose, and in the absence of any definite information on the subject it was tacitly assumed that the finished product contained tricalcium arsenate together with calcium hydroxide and calcium carbonate equivalent to the extra mol of calcium oxide. This assumption has persisted to this day. The total arsenic in commercial calcium arsenate is always calculated to C S . ~ ( A S Oin~ )order ~ to obtain the percentage of active ingredient, the figure so obtained being in the neighborhood of 70 per cent. However, it has been known for many years that this point of view is probably incorrect, and that, as a matter of fact, the product probably contains a considerable proportion of a basic arsenate. Tartar, Wood, and Hiner (4) reviewed the evidence for this in an article published in 1924 and then presented an account of their own experiments designed to 1 Received October 9, 1930. Presented as a part of the Insecticide Symposium before the Division of Agricultural and Food Chemistry at the 79th Meeting of the American Chemical Society, Atlanta, Ga , April 7 t o 11,
1930.
substantiate it. By the hydrolysis of tricalcium arsenate and of calcium ammonium arsenate with boiling water they produced an end product of fairly fixed composition to which they assigned the formula 3Ca3(As04)z.Ca(OH)2,exactly similar to that of the well-known basic arsenate of lead. These authors were convinced that they were dealing with a definite compound, and not with a mixture of two materials giving a minimum solubility. However, Clifford and Cameron (1) have argued that neither this basic arsenate nor tricalcium arsenate itself exists, but that solid solutions are formed just as they are in the system Ca0-P205-Hz0 which had been previously studied by Cameron and others. One of the present writers undertook several years ago to study the basic region of the system CaO-AszO~-H~Oby means of phase-rule considerations, but obtained such erratic results that the work was never published. He considered that the difficulties arose from a very slow attainment of equilibrium and an attack of the glass flasks by the alkaline solutions during the long standing (as much as 9 months). Some slight evidence was obtained, however, of a compound with a molecular quotient CaO/AszOs of &that is, one considerably more basic than the compound reported by Tartar, Wood, and Hiner. No attempt seems to have been made to prove directly the presence of basic arsenate in commercial calcium arsenate.
INDUSTRIAL AND E,VGINEERING CHEMISTRY
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I t is customary in the analysis of this insecticide to determine the total CaO and total As205. That more than this is necessary to interpret the true composition is self-evident. An estimation of the proportion of calcium carbonate present is possible by a determination of the carbon dioxide content, but even then there remains some calcium oxide to be accounted for. With this point in mind Smith and Hendricks ( 3 ) developed a method for the estimation of free calcium hydroxide in calcium arsenate. This method is based on the reaction of the free calcium hydroxide with an alcoholic solution of benzoic acid, which medium does not attack either calcium carbonate or tricalcium arsenate, and it was shown that quite accurate results could be obtained on known mixtures of tricalcium arsenate and calcium hydroxide. Analyses
The present authors have applied this method to sixteen brands of recently manufactured commercial calcium arsenate that are being studied in relation to foliage injury in the hope that there might be a correlation between injury and degree of basicity. Complete detailed analyses were made, but all the results are not given here because they are too voluminous. I n general they differ in no marked degree from those quoted by McDonnell, Smith, and Coad ( 2 ) for a group of products manufactured about 1921, showing that, as far as gross composition goes, commercial calcium arsenate has become a well-standardized product. The average values obtained in the analyses of these materials are shown in Table I. It is seen that the total AszOsranges only from 40.3 to 44.4 per cent, with an average of 42.4 per cent. If, as is frequently the case, this were the only determination made (aside from water-soluble arsenic), a false idea of the similarity of various brands might be obtained. The total CaO has a somewhat wider range, and very marked differences are seen in the figures for calcium carbonate and calcium hydroxide, the former ranging between 1.2 and 11.7 per cent with an average of 6.4 per cent, the latter from 1.5 to 12.7 per cent with an average of 6.6 per cent. A close inspection of the individual figures for calcium carbonate and calcium hydroxide (not given here) will show that they are somewhat complementary t o each other, which merely reflects the fact that one is formed at the expense of the other when carbon dioxide is absorbed from the air during storage. T a h l e I-Average
Analyses of Sixteen Commercial Calcium Arsenates MAXIMUX MINIMUM AVERAGE
Moisture Insoluble in HC1 Loss on ignition Total CaO CaC03 Ca (OH)z CaO as arsenate R203 Me0 P b-0 Total AszOc AszOs
sn,
c1
- - I
NzOs
Water-soluble Ass0
70
%
%
3.22 0.97 12.72 47.70 11.73 12.66 40.59 1.14 1.14 2.49 44.39 0.52 0.78 0.39 1.81 2.37
0.74 0.27 6.94 40.42 1.21 1.52 31.49 0.22 0 13 None 40.32 Trace Trace Trace Trace 0.06
1.94 0.57 9.3 43.91 6.37 6.63 35,32 0.39 0.52 42142 0.18
...
...
... ...
Little need be said concerning the minor impurities, most of which are to be expected from the nature of the raw materials used. The presence of lead in a few samples is probably due to the use of the same machinery for the manufacture of both lead arsenate and calcium arsenate. The high content of nitrogen pentoxide in three of the samples is rather surprising, but the presence of this impurity is probably explained by the failure to remove from the arsenic acid all the nitric acid used in its preparation. Some of these con-
Vol. 23, No. 2
stituents were not determined in all the samples, which explains the absence of an average value in the table. Molecular Quotients
The principd interest for our present consideration attaches to the values given in Table I1 for “molecular quotient.” These figures were obtained as follows: It was assumed that the smaller impurities could be neglected, which is partly justified by the fact that the acidic and basic ones tend to offset each other; the percentages of CaO equivalent to both the calcium carbonate and the calcium hydroxide present were then subtracted from the total CaO, the remainders converted to mols, and these figures divided by the mols of Asp06 to which the percentages of that component are equivalent. A quotient of 3.0 thus obtained represents tricalcium arsenate, one below 3.0 an acid compound, and one above 3.0 a basic component. It will be seen that all but one of the figures are higher than 3.0; in other words, fifteen samples show evidence of containing some basic compound. Table 11-Basicity of Commercial Calcium Arsenate MOLECULAR QUOTIENT MOLECULAR QUOTIENT SAMPLE CaO/AsrOr SAMPLE CaO/AsrOr S 4.1 3.3 9 3.7 3.4 10 3.4 3.4 11 3.7 3.6 12 3.6 3.3 13 3.2 3.3 14 3.1 3.2 15 3.0 16 3.4 Maximum.. . . . . . . , , , . , , , . , , , . 4 . 1 Minimum. . . . , . . . , , , . , , , , , , , 3 . 0 Average.. . . . . . . . . . . . . . . . . . . . . 3 . 4
It is quite impossible t o determine from these figures just what basic compound is present. Several of the figures approximate closely the value 3.33, which would be characteristic of the compound 3Caa(A~04)2.Ca(OH)~ reported by Tartar, Wood, and Hiner, but since nearly half are appreciably higher the evidence for its existence is not very conclusive. Since no value is less than 3.0 and only one slightly above 4.0, perhaps the best guess is that these samples consist of mixtures of 3Ca0.As205and 4Ca0.Asz05. The latter, as said before, was also suggested by some of the senior author’s earlier work. It is thus demonstrated that the arsenic in commercial calcium arsenate practically never exists solely as tricalcium arsenate, but that a considerable portion, and perhaps in some cases all of it, is present in the form of a basic arsenate of undetermined composition. It is quite possible that the nature of the basic compound might be more fully revealed by similar work on laboratory samples made from pure constituents under controlled conditions. Summary and Conclusions
Sixteen brands of recently manufactured calcium arsenate were analyzed in detail, the results tabulated, and the average composition of this important agricultural insecticide thus revealed. The presence of a basic arsenate of undetermined composition is definitely established. The average product contains 80 to 85 per cent of what is probably a mixture of tricalcium arsenate and this basic arsenate, together with about 6.5 per cent each of calcium hydroxide and calcium carbonate and small amounts of incidental impurities. The individual products may, however, differ rather widely from these average figures. Literature Cited (1) (2) (3) (4)
Clifford and Cameron, IND. END.CHEM.,21, 69 (1929). McDonnell, Smith, and Coad, U. S. Dept. Agr., Bull. 1116 (1922). Smith and Hendricks, IND.ENG.CHEM.,16, 950 (1924). Tartar, Wood, and Hiner, J. A m . Ckcm. Soc., 46, 809 (1924).
