Volumetric D etlermination of Tungsten A Study of the Method Proposed by Dotreppe M. LESLIEHOLT,University of Wisconsin, Madison, Wis.
A
LTHOUGH it has been quite generally accepted that a satisfactory volumetric determination of tungsten based on a reduction and subsequent oxidation is impracticable (g), the method proposed by Dotreppe (1) appears simple and very accurate. The present author, having need for a rapid determination of tungsten, thought it worth while to repeat the work and investigate its possibilities. The method of Dotreppe involves reduction of an alkali tungstate by zinc and concentrated hydrochloric acid in an atmosphere of carbon dioxide, oxidation with ferric alum, titration of the resulting ferrous iron by permanganate, and calculation based on an apparent reduction of WOS to WaOr. The results reported show that 99.6 per cent of the tungsten, the average of the nine values included, was recovered. Table I presents results obtained when the method cited above was followed as exactly as possible. TABLE I. RESULTS USINQTHE DOTREPPE METHOD SAMPLE.
VARIATION IN NUMBER TUNQ~TSN OF TRIAL@ REICOV~RED
AVB~RAQS R~COVBRY
%
%
86 t o 94 74 to 94 79 to 98
86 84 93
tried. A sharp end point was obtained by reduction of the tungstate in the presence of a small amount of phosphoric acid and titration of the ferrous iron with potassium dichromate, using diphenylamine sulfonic acid indicator prepared and used as suggested by Sarver and Kolthoff (3). Phosphoric acid prevents the precipitation of tungstic acid, but still has no influence on the reduction or on the reduced compound (1). Table I1 presents results obtained by using this modified method. TABLE 11. RESULTSUSINGTHE DICHROMATE METHOD SAMPLS KzWO4+ KC1 KaWO4 NanWOc
NUMBER TRIAL^
OF
12 6 12
VARIATION IN TUNQBTEN AVSRAQB RB~COVEIRB~DRB~COVIORT
%
%
71 to 97 88 to 98 76 to 90
88 93 80
These again were very poor, even though the end point was quite clearly defined. CONCLUSION This proposed volumetric determination of tungsten has been found unsatisfactory mainly because of the apparent impossibility of obtaining consistently identical reductions of WOS.
MODIFIED PROCEDURE These poor results were apparently due to difficulties in the reduction of the tungstate and to a lack of sharpness in the permanganate end point. The end point was obscured by a flocculent precipitate of pale yellow hydrated tungstic acid which appeared when the solution waa diluted before being titrated. Conditions of reduction were kept as exactly alike as possible, although variations in time, temperature, concentration of the acid, and amount of zinc were
LITERATURE CITED (1)
Dotreppe, G., Chimie
& Industrie, Special No. 173-8 (March,
1931).
Hildebrand and Lundell, “Applied Inorganic Analysis,”
p. 551, John Wilev & Sons. N. Y . .1929. (3) Sarver, L. A., and Kolthoff; I. M., J . Am. Chem. SOC.53, 2902,
(2)
2906 (1931). RECEIVEID September 16, 1934.
A Simple Laboratory Apparatus for Vacuum Distillation ALBERTW. STOUT AND H. A. SCHUETTE, University of Wisconsin, Madison, Wis.
A
LL the current types of receivers for collecting the successive fractions of a condensate from a vacuum distillation without interrupting the operation apparently trace their origins to one of two p r e c u r s o r s : One is t h e T h o r n e “ t r i a n g l e ” (6),the other is e i t h e r t h e Gorboff-Kessler v a c u u m desiccator (4) with r e v o l v i n g table, or its modification, t h e Bruhl apparatus (2), which appears to have been accepted to the exclusion of its predeFIGURE 1 cessor. It in turn seems to have inspired the less cumbersome, compact Bredt “star” device (1) or its antecedent, a receiver described by Gautier (3). The triangle types are not well suited for sharp
separations where quantitative analyses are involved. For this purpose the Briihl and star receivers are superior. The need, arising during the course of the fractionation of methyl esters of fatty acids in the analysis of oils, for a small receiver with tubes not too large to be weighed on an analytical balance, yet readily stoppered for storage, led to the construction of a device (Figure l), patterned after the von Wechmar (6) apparatus, from one 250-cc. and six 50-cc. Erlenmeyer flasks and a ground-glass joint. Because of its simplicity of construction and efficiency of operation-a 3-mm. vacuum can easily be maintained in the system and a recovery loss of less than 0.3 per cent is not unusual (Table I) -it is described here, in the hope that others may find it a useful tool in the analysis of fatty oils or in other fields. An outlet tube, throu h which the system is evacuated, is sealed into the side of t%e Erlenmeyer flask. Equally spaced around the eriphery of its base are sealed six 10 X 40 mm. outlets to whicg the receivers are subsequently attached by means of rubber stoppers. One is not necessarily limited to six o enings, and ground-glass joints may be substituted for rutber sto pers. The receivers are made by givin the neck of a 50-cc. Erinrneyer flask a 90’ bend. Into the ne& of the large flask a
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