INDUSTRIAL AND ENGINEERIYG CHEMISTRY
912
tions in the crystal. For until these dimensions are known,
it is not possible to distinguish between the presence of simple planes and glide planes of reflection or between simple axes and screw axes of symmetry, and it is thus difficult if not impossible to arrive a t the correct space group. Richer fiber diagrams of this form of gutta-percha or the production of specimens in m hich a higher order of orientation is achieved will aid in this determination. But as has been often pointed out (6, 11), in the absence of suitable macroscopic crystals it nil1 be necessary to employ all available chemical and physical evidence in order to arrive at a convincing solution of this phase of the crystal structure. S o attempt at a complete treatment of the alpha diagram of gutta-percha is possible on the basis of the evidence obtained 50 far. I t is difficult to explain the discrepancies that apparently exist in the identity period calculated for this diagram. Rather than attribute a very long identity period to the gutta-percha molecule [Clark (5) reported finding such spacings in rubber and cellulose], it seems more reasonable to postulate the existence of three forinsoof the hydrocarbon with fiber periods & i i ,9.00, and 8.70 A. or multiples of these figures. The latter tTvo are represented by the points of Table IT’. Speculations of this nature, howel-er, are of little value until further data are secured. One result of this work has been to show that the crystal structure of gutta-percha is considerably more complicated than generally has been realized. Only by continued investigation by means of x-rays and electron rays can the true solution of the crystal structure of this and other high-polymeric substances ultimately be solved. It is hoped that the experiments summarized here will serve a4 a qtep in the direction to this solution.
VOL. 28, SO. 8
Acknowledgment The author wishes to express his thanks to A. R. Keriip for suggesting this work and to F. S. Malm for supplying t,he samples.
Literature Cited (1) Bruni, G., and K a t t a , G., S t t i accad. Lincei, 19, 206 (1934); tr. in Rubber Chem. Tech., 7, 603 (1934). ( 2 ) Clark, G. L., ISD.ENQ.CHEM.,18, 1131 (1926). (3) Clark, G. L., and Corrigan, K. E., Radiology, 15, 117 (1930). (4) Clark, G. L., Warren, W. J., and Smith, W. H., Science, 79, 433 (1934).
( 5 ) Hauser, E. A., and Rosbaud, P., Kautschuk, 3, 1; (1927); Z. Elektrochem., 33, 511 (1927); Hauser, E. A , Hunemorder, M., and Rosbaud, P., Kautschuk, 3, 228 (1927). (6) Hauser, E. A , , and Susich, G. von, Ibid., 7, 120, 125, 146 (1931). (7) Hopff, H . , and Susich, G. von, Ibid., 6 , 234 (1930); Rev. ggn. caoutchoirc, 7, 23 (1930). ( 8 ) Kata, J. R . , “Die Rontgenspektrographie als Untersuchungsmethode,” Berlin, Urban und Schwarzenberg, 1934. (9) Kata, J. R . , Cummi-Ztg., 41, 2035, 2091 (1927). (10) Kirchhof, F., Kautschuk, 5, l i 5 (1929). (11) Mark, H., “Physik und Chemie der Cellulose,” p. 136, Berlin, Juliiis Springer, 1932. (121 hleyer, K. H., and Mark, H., “Der Aufbaii der hochpoiynieren organischen Naturstoffe,” 1930. ( 1 3 ) Ott, E., ~~\htu,.wl‘ssenschu~~ften, 14, 320 (19%). (14) Park, C . R., IND.ESG. CHEX, 17, 162 (1926). (15) Sauter, E . , 2. Krist., 84, 453 (1933). (16) Stillwell, C. IT,, and Clark, G. L . . IXD.ENG.C‘HEK, 23, 7‘06 (1931). RECEIVED Alar 15, 1936. Presented before the Dirision of Rubber Chemistry at the 91st Meeting of the American Cheinical Society, Kansas City, Mo.. .ipril 13 t o 17, 1936.
Selenium Compounds in Soils K.T. WILLIAMS A N D H. G.BYERS Soil Chemistry and Physics Research Division, Bureau of Chemistry and Soils,Washington, D. C.
I
S T H E various publications of thi, division, in which the presence of selenium in soils and vegetation is discussed and which have recently been summarized (1, Z), only incidental attention has been directed to the particular chemical forms in which the element is found. This question is important, since there appears to be no definite relation between the quantities of selenium present in a soil and that absorbed by a given species of vegetation ( 2 ) . Considerable work has been directed by Franke and other. (6, 7 ) and by Horn, Selson, and Jones (S) to the selenium compounds present in grain, but no definite compound has yet been identified.
Pyritic Selenium Work previously reported (8) showed that selenium is apparently always present in varying quantities in iron pyrites. I t always appears that selenium is always present in the shales of the upper Cretaceous period and particularly in the lower portion of the Pierre and the upper portion of the Siobrara formations ( 2 ) . In these formations pyritic nodules are often found. Two of them haye been analyzed. One is from the Pierre shale in Boyd County, Sebr., and contains 205 part.s per million of selenium. The other is from Lane County, Kans., and contains 80 p. p. in.
Since the soils of the semi-arid areas have undergone relatively little weathering during the soil-forming processes and are very iminat.ure, it may be inferred that one of the forms in which selenium may be present in the soils is as pyrit’es carrying selenium as a part’ial substitute for sulfur. Such occurrence is relatively unimportant, except for its bearing upon the problem of the sources of selenium, since in pyrited selenium is certainly not available as a plant food or poison. This statement, while true in general, may not apply to cases where the sulfur compound is marcasite which weathers rapidly and may release selenium in soluble form (8).
Selenium as Selenite In the course of examination of hundreds of samples of shale, of svhich only a small portion has been reported, analyses of separate limonitic concretions or limonitic pieudomorphs frequently showed a selenium content higher than the ininiediately adjacent strata, For example, an iron oxide concentration in the Siobrara shale had a selenium content of 48 p. p. in.lvhereas the adjacent shale had only 3 p. p. m. It was therefore a,..sunied that the high results represented residual selenium from m-eathered pyrites. This assumption, however, does not establish the chemical form in which it remains either in the concretions or in the soil itself.
AUGUST, 1936
INDUSTRIAL AND ENGINEERING CHEZIISTKY
The solution of the problem is not a n easy one since the quantity of selenium present in soils is extremely small. Indeed, the maximum quantity of selenium found as a result of the examination of several thousand soil samples is 82 p. p. m. or 0.008 per cent. The normal quantity found in soils which are considered “seleniferous” is much less, and in most cases is only from 1 to 6 p. p. m. or 0,0001 to 0.0006 per cent. Approximately a hundred samples of soil were extracted with water. The procedure was to add to 60 grams of the soil 600 cc. of water, shake overnight in a mechanical shaker, and filter off 500 cc. of the liquid by aid of a Pasteur-Chamberland filter. The selenium content of the water extract from the samples used ranged from less than 0.1 to 38 p. p. m. In about 80 per cent of the extracts no selenium in excess of 0.1 p. p. in. could be detected when subjected to the usual process of examination. One of these, a sample of raw shaly soil found in Gove County, Kans., was selected for careful examination. It contained 22 p. p. m. of selenium. This sample was orange in color and contained 5 i per cent calcium carbonate, 4.4 per cent iron oxide, and but 0.2 per cent organic matter. It also contained 0.6 per cent of water-soluble matter, mostly calcium sulfate, with traces of chlorides and bicarbonates Extraction of this material with water, as described, brought into solution only 0.2 p. p. m. of selenium, or approximately 1 per cent of that present. When treated with a quantity of hydrochloric acid, just insufficient to dissolve the limestone present, no increased removal of selenium was effected. The residual material was freed from chloride by repeated washing and then refluxed for 5 hours with a 10 per cent solution of potassium sulfite. The filtered extract contained 0.4 p. p. m. of selenium or about 2 per cent of the original content. This treatment may be considered as effecting removal of any selenium present in the elemental form ( 3 ) . I t should also remove any “exchangeable anions” by double decomposition. The result obtained seem- to indicate t h a t the free element is essentially absent and also t h a t the compound present is exceedingly insoluble in n-ater. This latter indication is emphasized by the fact t h a t refluxing the sample with 10 per cent sodium sulfate and then with 10 per cent disodium phosphate likewise removed practically no selenium. On the other hand, refluxing the residue with 6 N wlfuric acid effected the solution of all but 0.7 p. p. m. of the selenium, or approximately 95 per cent of that originally present. The sulfuric acid treatment was sufficiently drastic to dissolve all iron oxide present and leave a grayish white residue. It therefore seems probable that the selenium in this material is associated with iron oxide in a very insoluble form. T o determine whether soils containing iron oxide render selenium insoluble, solutions containing I .2 mg. of selenium as sodium selenite in 600 cc. were shaken overnight with 60 granis of Cecil sandy clay loam (11 per cent iron oxide) and Xipe clay (70.6 per cent iron oxide). I n neither case was a quantity of selenium in excess of 0.01 mg. found in 500 cc. of the filtered liquid. On the contrary, when the same experiment was repeated, using 1 mg. of selenium as sodium selenate, 0.6 mg. of selenium was recovered from 500 cc. of the filtered liquid. Ferric hydroxide gave similar results under like treatment. Further evidence of the same relation is indicated b y the fact t h a t the selenium in the 6 i Y sulfuric acid extract of the soil sample is completely reduced to elementary selenium by sulfur dioxide. This reaction does not occur with selenates. I n order to determine the form of ferric selenite probably . a present in the soils, an attempt wa9 made to synthesize compound having the required properties by reaction between ferric chloride and selenites. This reaction has been studied by Berzelius and many others, and the inqoluble material
913
has been shown to be of variable composition according to the conditions of precipitation but approximates the composition of the neutral salt Fe?(Se03)s(6). This salt is, however, not sufficiently insoluble in water to account for the remarkable persistence of selenium in ferruginoui humid soils (unpublished data). Ferric chloride and sodium selenite were mixed in varying ratios and differing, but always great, dilutions, In all ca.es a precipitate iy fornied though, in the more dilute mixtures, recourse was had to salting out with sodiuin chloride in order to effect coagulation. The precipitated material was then analyzed and the iron-selenium ratios were determined. The concentration. employed and the resulting ratios are given in Table I.
_ _ _ _ ~ -
I
I.
P R O D W T b O F REICTIOX BETWEET DILUTE SOLUTIONS OF SODITVSELEXITE IUD OF SODIVX SELEVITEWITH FERRIC
TABLE
CHLORIDE”
Elements in Reagents Fe as Seas Expt. 1-0. FeCli Sa2SeOa 1 2 3 4 5 R
k9 10
8.0 8.0 8.0 8.0 8.0
u n
16.0 32.0 16 0 16 0
8 n 8.0 16 0 24 0 5 0
i n
5.9 5.9 15 06 24.00
Elements in Ppts. de
.\toniic Ratios
Fe 6 6 8 8 6
9 9 0
0 6
6 fi 10 6
5 5 6 6
5 ,‘3 6
i
:!
2 6 iR.9
:J (I i :3 0
14.1
:3 3
pH of
of Ppts. Fe Se
Filtrate
2:1.13 9:1.09
.3,12 2.99
2 : l 17 2 : l 18 ’:0 94 2:1 22 2:O 61 2:0.38 2:0 31 2:0.33
3.40 :3.27 3.20 3 10 2.80 2.63 2.95 2.95
The quantities of fesric chloride indicated were made up to 450 cc. in water and the selenium salts added a t a dilution of 50 cc. b Selenium as sodium selenate. 0
The data shoTv that, when extremely dilute solutions of selenites react with ferric chloride, a very insoluble precipitat’e is formed and approximates the composition of basic ferric selenite, Fe2(0H)$eo3, though the relation of iron to selenium is not constant. Corresponding reactions take place between selenates and ferric chloride, but’ the product is by no means so insoluble in water. Experiment 8 in the table is of special interest since i t indicates incomplete precipitation of selenium when the reaction mixture is sufficiently acid. This is quite in accord with observations previously reported on a profile developing under acid conditions ( 8 ) . The general conclusion from available data seems warranted that soil may contain selenium in the form of exceedingly insoluble basic ferric selenites,
Selenates B s already mentioned, about 20 per cent of the soils examined for water-soluble selenium gave appreciable quantities. TT7hen this quantity exceeds 0.1 p. p. m. of the soil, i t may be assumed that some forin other than basic ferric selenite is present. Treatment of the aqueous extracts with barium chloride is, in general, effective for essentially complete removal of selenium from solution. I n all soils examined in this manner, the aqueous solution contained much soluble sulfate, chiefly calcium sulfate, with or without accompanying chlorides. These solutions, when made 5 to 6 normal with sulfuric acid, gave no precipitate of selenium with sulfur dioxide, but, when made strongly acid with hydrobromic acid, the selenium was completely precipitated. Synthetic mixtures of selenates and sulfates behave similarly in all respects. Selenite and sulfate mixtures are also essentially freed from selenium by treatment with barium chloride, but in 6 N sulfuric acid the selenium is precipitated by treatment with sulfur dioxide. These observations indicate that soils containing watersoluble selenium may have i t present in the form of selenates, and in the samples examined a,s calcium selenate. Also, the soils examined which showed the presence of water-
soluble selenium were gray or dark gray, and isere tlrerefore not highly ferruginous, a t least with free iron oxide. These consideratious indicate that the presence of selenates in humid soils is very unlikely.
Organic Selenium Exainination of a soil from Fall River County, S. I)&. (B17742), which contained 40 p. p. in. of selenium, gave an aqueous extract which contained 28 p. p. in. of selenium. The extract, after treatment with barium chloride and filtration, still contained approximately 25 per cent of the selenium originally present in the soil. The soil was extremely saline and darker in color than normal for the area in which it W R S fuiind. It seems probable that, since the selenium in dried green vegetation is largely water-soluble, organic matter may contain water-soluble and easily removable organic selenium compounds. The nature of these coinpoundx i.i a i yet. unknswn. The
problem presents great difficulties hut is being investigated at the present time. The results are not yet ready for pohlication.
Literature Cited Ryera, H. G.. U. S. Dilpt. Agr., Tech. Rdl. 482 (1935) Ibid.,in press. Chcraskova, E., and Voiabontc. I.., IND. EN^. Carnu., Annl. Ed., 7,407-8 ( 1 ~ 3 5 ) . Franke.X.\h’.,J.Niilrition.S,60Y~13 (1934). Horn, M. J., Nelson. E. M..and Jones, D. W.. Cereal Chem., 13, 126-39 (1936). ~llellor,3. W.. “Treatise on Inorganic arid Theoretical Chemiatry:’ voi. BP. sawn, N ~ WY d . L O ~ ~ Z ~~r e~ cDn& ~CO.. ,
x,
1930. Painter. e. P.. and Frsnke. X. W.. J . Bid. Chem... 111.. 043-51 (1935). Williams, K. T.. and Byere. H. G., INU. Exe. C ~ m r . .4nnl. . Ed.. 6 . 296-7 (1934).
K a c r r v ~ oSlay 4, 1938
No. 68 in the Berolzheimer series ,>f Alchemical and Historical Reproduelions is from the hrush of the famous
Dutch painter, Jan Eiavicksz Steen, mually designated &s Jan Steen. He was imrn in Leyden in 1626, studied drawing under de ‘tiot in Haarlem. and psinting in Utrecht under Xicolas Knupfer, the Gcrman historical painter. Later he worked under Adrian van Ostade in Haarlcm. During an active career Steen produced over five hundred paintings, and died at Leyden in 1679. The original of this reproduction was painted in 1688, is 106 by 82 centimeters in siEe, and is in the Staedelschea Kunst Institut in Frankfort-on-the-Main. I t is also known as “The Alchemist” and as “An Alohemist Melting the Jewelry of His Wife.” This pninting, formerly in the possevsion of the Earl of Crawford, is the second of Steeds Forks we have reproduccd, the first, No. 29, appeared in our May, 1933, issue, page 562. The general idea portrayed by Steen in both of these paintings was undoubtedly copied from Teniors’ eighth painting in this series, No. 33, reproduced in September, 1933, page 1041.
DEGOUDZOEKER By Jan Steen
