ANALYTICAL EDITION
16
It is significant that tartaric acid gives more precise reacetic acid, and the senior author intends to follow
SUltS than
this investigation in the hope of explaining the interference of certain acids in this determination. LJTERATURE CITED (1) Fales, “Inorganic Quantitative Analysis,’’ p. 358, Century, 1925. (2) Gooch and Heath, 2. anorg. Chem., 55, 119 (1907).
VoI. 5 , No. I
(3) Haen, Ann., 91, 237 (1854). (4) Hillebrand and Lundell, “Applied Inorganic Analysis,” p. 198, Wiley, 1929. (5) M ~ z, anal. ~ Chem., ~ ~ 43 ,597 (1904), (6) Murray, “Standards and Tests for Reagent Chemicals,” Van ’ Nostrand, 1920. (7) Riimpler, J. prakt. Chem., 105, 193 (1868); Z. anal. C h m . 8 ,
465 (1868).
R ~ C E I Y EJune D 9, 1932.
Determination of Cadmium A Critical Study of the Evrard Method LORENC. HURDAND RICHARD W. EVANS,University of Wisconsin, Madison, Wis.
A
LTHOUGH cadmium is a fairly common constituent of many zinc ores and concentrates, there does not exist a rapid and specific method for its determination. In a recent communication Evrard (3) reported a method which appeared to be simple and accurate and which functioned in the presence of large amounts of zinc. Inasmuch as such a method is sorely needed, the writers thought it worth while to repeat the work of Evrard and investigate the possibilities of the method. Evrard’s determinations indicated that the cadmium iodide addition product of allyl iodourotropine was quantitatively insoluble and could be represented by the formula CdIz[ (CH2)6N&Hd]2. Inasmuch as the molecule contains but 11.44 per cent of cadmium and in light of its reputed insolubility in the presence of excess precipitant or in 95 per cent alcohol, the determination appeared to offer excellent possibilities. Zinc, according to the original author, had little effect upon the determination, for in the presence of large amounts of a zinc salt and in moderate concentrations of sulfuric acid a recovery of 99.7 per cent of added cadmium was obtained.
EXPERIMENTAL PROCEDURE PREPARATION OF MATERIALS.Allyl iodide, prepared according to the method of Datta ( I ) , was added to an equimolar solution of urotropine in chloroform. The melting point of the precipitated allyl iodourotropine checked with accepted values. A 10 per cent aqueous solution was used throughout the work, Cadmium sulfate was purified by subjecting c. P. material to the treatment described by Reilly (4). The recrystallized product, freed from iron and zinc, was dehydrated in vacuo, ignited in quartz, cooled, moistened with sulfuric acid, and again ignited. The anhydrous salt was dissolved in water to give solutions containing approximately 0.001 gram of cadmium per cubic centimeter. RESULTSOF ANALYSIS.Evrard reported little detailed information regarding the exact procedure followed in the analysis of his solutions. As nearly as the writers were able to determine, the precipitations were carried out by adding the cadmium solutions to the reagent. This order was followed in a large number of cases, but it was found that the characteristics of the determination were such that the order of addition was relatively unimportant. The precipitated complex was found to be a white flocculent compound which, in the absence of excess precipitant or other electrolyte, exhibited a marked tendency to pass into the colloidal state. In a series of preliminary analyses an attempt was made to simulate as nearly as possible the conditions recommended by Evrard. The precipitations were
made in the cold, and all washings were made in the prescribed manner with a dilute solution of the reagent followed by small portions of 95 per cent ethyl alcohol. An average recovery of 96.4 per cent of the added cadmium was obtained. Although a ninefold excess of precipitant was used in all of the preliminary experiments, it was thought that a further increase might result in a more nearly quantitative recovery. Accordingly a study was made of the effect of excess reagent. It was found that, when an amount of allyl iodourotropine representing a fourteen fold excess was employed, the apparent recovery reached 116.1 per cent. Decreasing the amount of reagent resulted in decreased recoveries. From a study of the data obtained during the execution of some hundred analyses, it became apparent that erratic recoveries of added cadmium resulted from two principal causes. Incomplete removal of reagent or salt adsorbed by the precipitate gave rise to a positive error, whereas the actual solubility of the precipitate contributed to significant negative errors. I n such cases it is only by a fortuitous balance of conditions that results approaching theoretical are obtained. Inasmuch as the precipitate is somewhat less soluble in alcohol than in water, it appeared worth while to attempt to carry out the determination in alcoholic solution. Accordingly numerous analyses were made on solutions, the ethyl alcohol content of which varied from 45 to 67 per cent by volume. Cadmium recoveries ranging from 32.5 to 129.4 per cent were obtained. Prolonged washing with ethyl alcohol dissolved a major portion of the precipitate. Although solutions of zinc salts do not yield precipitates when added to allyl iodourotropine solutions, the presence of zinc has a marked effect upon the cadmium determination. Apparent recoveries of as high as 132 per cent of added cadmium were obtained when the determination was carried out in the presence of small amounts of zinc. The obvious inference is that zinc is co-precipitated or otherwise carried out of solution when the cadmium complex is precipitated. Sulfuric acid in concentrations of 0.01 to 0.05 M had little effect on the solubility of the precipitate. Only when the concentration exceeded 0.06 M , was the relative percentage of cadmium recovered greatly decreased. A series of solubility determinations carried out a t 25” * 0.5” C. indicated that the complex was soluble to the extent of 0.0014 gram per cc. of water. The solubility in ethyl alcohol was somewhat less, an average of 0.00062 gram per cc. being obtained. The use of allyl iodourotropine as a qualitative reagent was also investigated. It was found that a number of metallic ions formed insoluble complex derivatives when treated with the reagent. This is in harmony with the results of
January 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
DelBpine (a), who found that the base was precipitated by HgC12, HgBrz, HgI2, KBiI4, and other salts. Following are the approximate molar concentrations a t which precipitation of various metallic derivatives of allyl iodourotropine take place. SALT SbCla Bi(NOda FeCla PbClr
MOLARCONCN. 5 x 10-4 5 x 10-3 7 x 102 . 4 x 10-7
SALT Hg(N0a)r SnCL CUClZ AsC1a
MOLAR CONCN. 4 x 10-4 6 . 7 X 10-8
x 1x
5
1010-
Pentavalent arsenic and antimony do not yield precipitates when treated with the reagent. Likewise molar solutions (or in some cases saturated) of LiC1, NaCl, KCl, BaC12, CaC12, SrClz, Mg(N03)2, ZnSO4, A1 (NO&, La(N03)3, Ce(KO&, Ce(N03)*, ZrOC12, Th(NO&, SnC14, MnC12, KReO?,
17
CrC13, NiCI2, and FeSOc do not form insoluble complex allyl iodourotropine derivatives. The use of allyl iodourotropine as a precipitant for cadmium is, therefore, not a reliable method because of solubility and adsorption errors. The interference of other metals has been indicated. LITERATURE CITED (1) D a t t a , R. L., J.Am. Chem. Soc., 36, 1006 (1914). (2) DelBpine, Bull. soc. chim., [3] 17, 293 (1897). (3) Evrard, Ann. chirn. anal. chim. a p p l . , 11, 322 (1929). (4) Reilly, J., “Physico-chemical Methods,” p. 607, Van Nostrand, 1926. RECEIVED March 17, 1932. L. C. Hurd‘s present addrees is Marienstr. 33, Hannover, Germany. This paper is from the senior theais of R. W.Evans, University of Wisconsin, 1932.
Determination of Fluorine in Cryolite F. J. FRERE,Pennsylvania Salt Manufacturing Company, Philadelphia, Pa.
T
HE determination of fluorine in minerals containing appreciable amounts of this element has always been accomplished by difficult and tedious procedures which, a t their best, have left much to be desired. Many compounds have been proposed for the gravimetric determination of fluorine, the most common of which are: calcium fluoride (S), lanthanum fluoride (IO), thorium fluoride ( l a ) ,and lead chlorofluoride (16). While good results have been reported in some cases, preliminary trials proved none of these methods applicable to complex compounds of high fluorine content such as cryolite, except that in which the fluorine was precipitated as lead chlorofluoride. The volumetric methods of Hempel and Scheffler (6), Wagner and Ross (16)’ and Penfield (11), which depend upon the evolution of the fluorine as silicon tetrafluoride, are impractical, as it is not possible to recover all the fluorine by such distillations. According to Reynolds, Ross, and Jacob ( l a ) , 92 per cent is about all that can be obtained by the best available procedures. Shuey (14) has made an investigation of the recovery of fluorine from sodium fluoride and states that the average of the results obtained would suggest the possible use of a factor in placing the recovery of fluorine on a 100 per cent basis. His results, for the most part, are rather inconsistent and it is felt that the use of a factor would be unreliable. Kurtenacker and Jurenka (8) have proposed the use of cerous nitrate as a reagent for the direct titration of fluorine using methyl red as an indicator. More recently Batchelder and Meloche ( I ) have outlined a procedure involving the use of ampho magenta as an adsorption indicator for the direct titration of fluorine by means of cerous nitrate. Data have been given showing a comparison of results obtained by this method and those obtained by the method of Kurtenacker and Jurenka. According to Batchelder and Meloche, methyl red gave slightly better results in the case of smaller quantities of fluoride, and in a later publication (9) they have given further details concerning this reagent. It has been the writer’s experience that methyl red is the more satisfactory of the two indicators. Kurtenacker and Jurenka reported difficulty in obtaining results in agreement with the theoretical values based upon the cerium content of the solution added. Differences amounting to about 4 per cent have been observed by them. This has been the writer’s experience in nearly all cases. Batchelder and Meloche reported no such discrepancy.
DETERMINATION OF FLUORINE USINGYTTRIUM NITRATE The writer has found that yttrium nitrate may be used as a satisfactory reagent for the direct titration of fluorine using methyl red as an indicator. Quantities of sodium fluoride ranging from a few tenths of a milligram to 0.3 gram were titrated with equally good success. I n pure solutions the results were in close agreement with the theoretical values based upon the yttrium content of the solution added. However, titrations could not be made a t 80” C. as in the case of cerous nitrate. It was found that potassium chloride and nitrate produced errors varying from -3.3 per cent at 1 gram per 100 ml. to -6.2 per cent at 4 grams per 100 ml. with practically no increase up to 8 grams per 100 ml. The corresponding sodium salts produced an error varying from -1.2 per cent to -3.8 per cent under the same conditions. Mixtures of potassium and sodium salts gave errors varying from -2.3 per cent at 0.5 gram per 100 ml. to - 6.2 per cent a t 4 grams per 100 ml. Sulfates caused an over-titration in concentrations greater than 0.1 gram per 100 ml. MATERIALSUSED. The sodium fluoride used as a primary standard was furnished through the courtesy of V. W. Meloche, of the University of Wisconsin. Several sodium determinations, as well as tests for impurities, showed the material to be of excellent quality. Natural cryolite obtained from Greenland was used for these experiments. The material was very carefully selected by hand and gave the following analysis: THEORETICAL
Na AliOa
FOUND
%
%
32.86 24.26
32.85 24.25
White reagent-grade cerou8 nitrate was used. The solution was standardized according to the method of Metzger (9) and showed a purity of 99.0 per cent as compared to the cerium determined gravimetrically. Reagent-grade yttrium nitrate was used, which tests showed to be of a good quality. This reagent may be readily obtained in pure form.
PROPOSED METHOD After a thorough investigation of the methods already described, it was decided that the use of yttrium nitrate for the direct titration of fluorine in cryolite offered the best possibility.
