1209
V O L U M E 20, N O . 1 2 , D E C E M B E R 1 9 4 8 Table I . Determination of Iron and Titanium Melt
_-1
% Iron
% Titanium 3
2
2
1 0.030
-
3
0.054 0.055 0.030 0.029 0 05i 0.060 0.032 0.032 0.028 0.086 0.082 0 084 0.022 0.023 0.022 0.058 0.029 0.028 0.055 0.058 0.028 0.158 0.152 0,160 0,022 0,024 0,024 0.174 0.180 0.180 0.025 0,020 0.024 I n cases similar t o melts 73 and 74 where t h e iron differs in order of magiiitude f r o m the titanium, separate aliquots must be taken for the iron deterxiination. 63 66 il 72 73a 74’
0 053
0.060
8
Table 1 1 .
.icruracy of Determinations % Titanium
‘;C I r o n
reagent are present, as confirmed experimentally. The ultraviolet sensitive phototube is still used here along with the stray radiant energy filter. Calculate results using calibration curves, such as those of Figure 1, which are based on standards. The results given in Table I were obtained using: the above procedure. Good accuracy was obhined, as is indicated by Table 11. According to Yoe and Armstrong, solutions containing as much as 4 p.p.ni. of titanium and 10 p.p.m. of iron can he determined spectrophotometrically on the same sample. Above these concentrations the inteiisity of the color does not increase proportionately 13-ith the concentration and thus does not. conform to Beer’s lay.
-
LITERATURE CITED
~
Other laboratoriea U e l t ti6 H-1400 .A.S.T.>I.
0.05Y 0.036
0.053
So. of analystr
Tiron pro?. 0.0%
0.033 0.047
Other laboratones 0.027 0.033 0.032
So. of analysts
Tiron
5
0.027 0,031 0.030
3
proc.
(2j (3)
tile., Then observe the per cent transmittance of the titanium complex at 399 nip. Below about 390 m,u sodium dithionite absorbs light, although the absorption is slight a t longer wave lengths, and may be disregarded if very small quantities of the
Joseph, “Reagent Chemicals and Standards.” second printing, p. 6, New York, D. Van Nostrand Co., 1943. Snell, F. D., and Snell, C. T.. “Colorimetric Methods of rltialysis,” fourth printing, Vol. I, p. 363, New York, D. Van No>trand Co., 1943. Treadwell, F. P., and Hall, IT. T., “Analytical Chemistry,” 9th ed., Yo]. I, p. 548, New York, John Wiley & Sons, 1937. Yoe, J. H., and Armstrong, A. R., AXAL.CHEM.,19, 100 (1947). Yoe, J. H., and Jones, L. A., IKD. EX. CHEM., ANAL.ED.,16, 111
(1) Ilo,siii.
(4) (5)
(1944). RECEIVEDApril 30, 1948.
Colorimetric Determination of Rhenium A . D. 3IELiYEN
AND
K. B. WHETSEL, Cniversity of Tennessee, I- for qualitarive work, and with further refirir~tiientit might have quantitative :tpplicat ion. Hurd and Hiskey (81,in a method for determiniiig rhenium ill pyrolusite, used a combination ether extraction and steam tlidllation from sulfuric acid. .Ilthough this method is satisfactory for pyrolusite, which contains little or no molybdenum, i t is not entirely satisfactory for the determination of rhenium i i i niolybdenites because some molybdenum is carried over either inechanically or by distillation. FIoffinan and Lundell ( 5 ) used differential reduction with niercury to qeparate rhenium and mol)-bdenum. Under proper I
cunditions, only niolybdenum ia rtduced and this may he extracted with ether after treatment vith a thiocyanate. This method is satisfactory, except that it is long and detailed. These authors also separated rhenium from molybdenum by distilling the rhenium from a solution cont,aining hydrogen bromide, phosphoric acid, and perchloric acid (6). Hiskey and Meloche (4) used a modified steam distillation from sulfuric acid to effect separation. These authors report that any color formed by the molybdenum carried over during distillation will fade upon standing 20 to 00 minute,. APPARATUS AND REAGENTS
Spectrophotometer. .Z Coleman Electric Company Model 11 photoelectric spectrophotometer was used in this work. The 1.00-cm. rectangular cuvettes which accompanied the instrument were used. .\I1 readings of per cent transmittance were obtained by the direct reading met,hod. Volumetric Flasks. The volumetric flasks were calibrated to contain the desired volume a t 27” C. Pipets. The pipets were calibrated to deliver the desired volume a t 27” C. The same pipet was used throughout the work to measure the standard potassium perrhenate solution. Potassium Perrhenate. The potassium perrhenate was prepared in this laboratory and found to be 100.07, pure when a,nalyzed by the method of Willard and Smith (IO). -A standard solution containing 1.00 mg. of rhenium in 1.00 ml. was prepared by placing 0.3884 gram of potassium perrhenate in a 250-ml. calibrated volumetric flask, followed by enough distilled water to fill to the mark. The salt was dried at 110” C. and cooled in a desiccator before weighing.
1210
ANALYTICAL CHEMISTRY
1
I
1
I
9.0
I
The molybdenum color was developed by adding 3.0 ml. of the g 1.8 molybdenum solution (1 mg. of molybdenum per ml.) to approxi8 80 mately 87 ml. of distilled water, 1.6 followed by 2 ml. of potassium thioZ cyanate, 2 ml. of stannous chloride, 6o 1.4 and enough distilled water to make k? < 100 ml. in the order given. From 13 40 this curve it is evident that there 9 1.2 is no wave length in the visible spectrum a t which the molybdenum PO 1.0 0 0.5 1.0 1.5 2.0 complex does not interfere with RHENIUM, MG. IN 100 ML. transmittance measurements on the I I I 1 Figure 2. Transmittance-Concentrarhenium complex. Consequently, 300 400 500 600 700 800 tion Curves for Rhenium Thiocyanate if rhenium is to be determined in WAVE LENGTH, Ml.r Complex the presence of molybdenum, either Figure 1. Spectral Transmittance Curves 1. Rhenium solution taken through final the molybdenum complex must be procedure 1. Molybdenum thiocyanate complex (3 mg. of prevented from forming, or the 2. Rhenium eolution plus 2 ml. of potassium Mo i n 100 m l . ) thiocyanate plus 2 ml. of stannous chloride 2. Rhenium thiocyanate complex (3 mg. of Re i n molybdenum must be removed be100 ml.) fore the color-developing reagents are added. Molybdenum Solution. An aqueous solution containing 1 mg. When per cent transmittance was measured over a period of of molybdenum per ml. of solution was prepared from Baker’s time on a solution containing 1 mg. of molybdenum in 100 ml., Analyzed molybdic acid (85% MOO?). values of over 90% transmittance were never obtained. S o Potassium Thiocyanate. A 20% solution of potassium thiohigher values were obtained either by changing the concentracyanate waa prepared by dissolving 200 grams of Baker’s Analyzed potassium thiocyanate in 800 ml. of water. tions of the color-developing reagents or by adding hydrochloric Stannous Chloride. A 20yosolution of stannous chloride was acid before the development of the color. Attempts to prevent prepared by dissolving 476 grams of Meqck stannous chloride the formation of the molybdenum complex by the addition of dihydrate (SnC12.2H20) in 400 ml. of distilled water and 935 ml. tartaric, citric, or oxalic acid were unsuccessful. of hydrochloric acid (d = 1.19). The solution was filtered into a glass-stoppered bottle, 10 grams of metallic tin were added, and In a qualitative test for rhenium, Anismov (1) used a 10% the bottle was wrapped in brown paper. solution of hydroxylamine hydrochloride to cause the color due a-Benzoinoxime. A 2.5% solution was prepared by dissolving t o the molybdenum complex to fade without affecting the rhenium 2 grams of Eastman a-benzoinoxime in 100 ml. of absolute ethyl complex. Experiments were carried out in which the color was alcohol. Lead Acetate. A 4y0 solution of lead acetate was prepared developed in various concentrations of hydroxylamine to deterby dissolving 4.6 grams of Baker’s Analyzed lead acetate trimine whether this reagent might be of value in this method. hydrate [Pb(C2H302)2.3H20] in 95 ml. of a 0.5% acetic acid Figure 3, a plot of per cent transmittance against concentration solution. of hydroxylamine hydrochloride shows that the amount of fading Hydroxylamine Hydrochloride. The solutions of hydroxylamine hydrochloride were prepared by dissolving Eastman that takes place is a function of the amount of hydroxylamine hydroxylamine hydrochloride in distilled water. hydrochloride present up ;to a concentration of about 10%. Above this concentration there is no increased fading, and the STUDY OF RHENIUM THIOCYANATE COMPLEX highest value of transmittance obtained was 98%. A blank Before any work was done on solutions containing both containing all reagents except molybdenum was used as the refrhenium and molybdenum, the rhenium thiocyanate complex was erence. studied. Curve 2 in Figure 1 is the spectral transmittance curve for a sample containing 3 mg. of rhenium in 100 ml. of solution. The color was developed using the reagent concentrations recommended by Hiskey and Meloche (4). This curve shows that maximum adsorption takes place at 400 mp, and it was a t this wave length that all further work was done. When per cent transmittance a t 400 mp was measured over a period of time, about 90 minutes were required to obtain a constant value. The time required to obtain a constant value of per cent transmittance a t higher wave lengths R as considerably longer. However, on adding 2 ml. of potassium thiocyanate and 2 ml. of stannous chloride in the order given to 90 ml. of the 90 rhenium solution and diluting to 100 ml., a constant value of 10 15 0 5 PO per cent transmittance a t 400 n i l \?as obtained after 60 minutes. % HYDROXYLAMINE HYDROCHLORIDE When per cent transmittance a t 400 mp was measured on a Figure 3. Transmittance-Concentraseries of samples containing varying amounts of rhenium and tion Curves log T plotted against concentration, it was found that Beer’s Solutions contain 1.0 mg. of molybdenum law is followed up t o a concentration of approximately 1.5 mg. of rhenium in 100 ml. of solution (curve 2, Figure 2 ) . At higher wave lengths the law is foilowed over a much wider concentration The transmittance of the rhenium thiocyanate complex in the range, but the precision is considerably lower. presence of hydroxylamine hydrochloride still follows Beer’s law. However, when the color is developed in solutions containSTUDY OF MOLYBDENUM THIOCYAIVATE COMPLEX ing both rhenium and molybdenum, a cdor is formed that exThe spectral transmittance curve for a solution containing hibits maximum absorption a t 475 mp, the transmittancy of 3 mg. of molybdenum in 100 ml. is shown by curve 1 in Figure 1. which does not follow Beer’s law. 100
W
22
f
f
”I-
d
-
h
V O L U M E 20, NO. 12, D E C E M B E R 1 9 4 8 In view of the unpromising nature of these results, it seemed clear that no satisfactory method for the removal of the interference due to molybdenum could be worked out along these lines and that molybdenum must be removed before the development of the color. SEPARATION OF RHENIUM AND MOLYBDENUM
In order to separate rhenium and molybdenum by a precipitation and filtration technique, a reagent was necessary that would quantitatively precipitate the molybdenum and leave the rhenium in a colorless solution. It was found that 8-hydroxyquinoline, a reagent often used to precipitate molybdenum, was not suitable for this procedure because of the excessive color left in the solution containing the rhenium. Lead acetate was found to precipitate the molybdenum and leave the rhenium in a colorless solution, but a small amount of rhenium was always lost during the procedure. Because approximately 4y0 of the rhenium was lost when the precipitation was carried out in aqueous solution, it seemed very unlikely that better results could be obtained in the presence of sulfuric acid where a large precipitate of lead sulfate would be encountered. Consequently, no further work was done on this procedure. Another standard reagent for precipitating molybdenum is a-benzoinoxime. This precipitation is carried out in a sulfuric acid solution, a fact which is particularly important, because samples obtained by the recommended steam distillation will have sulfuric acid present. Preliminary tests showed that the molybdenum was completely precipitated and the rhenium left in a colorless solution. After a series of modifications of the standard method for determining molybdenum (2,9) and standardization of certain variables, an effective separation of the elements and a method for determining the rhenium became possible with the use of this reagent. PROCEDURE
Twenty-five milliliters of a 4 to 7 N sulfuric acid solution which contains the rhenium and molybdenum are cooled to about 5’ C., and 2 ml. of 2.570 alcoholic a-benzoinoxime solution are added slowly with stirring. The solution is allowed to stand in the ice bath for 10 to 15 minutes with occasional stirring. The solution is filtered while cold and washed with a cold, freshly prepared solution made by adding 30 ml. of the a-benzoinoxime solution and 10 ml. of concentrated sulfuric acid to enough water to make 1 liter. The filtrate is collected in a 100-ml. volumetric flask and enough of the wash solution used to make the volume of the filtrate 95 ml. The filtrate is allowed to stand 1.5 hours for the excess reagent to crystallize out of solution. Then 2 ml. of 20% potassium thiocyanate, 2 ml. of 20% stannous chloride, and enough Tvater to fill to the mark are added in the order given. Bfter standing 10 to 15 minutes, the colored solution is filtered and the first 20 to 25 ml. are discarded. This procedure serves t o saturate the filter paper with the colored complex. The remainder of the solution is filtered and used for the analysis. The filtrate is allowed to stand 30 to 40 minutes, out of direct sunlight, before the per cent transmittance is measured. .4 blank containing all the components except rhenium is carried through the procedure and used as the reference sample in the spectrophotometer. DISCUSSION
The original 25 ml. of solution should be a t least 4 S nith respect to sulfuric acid; othcrwise, the intensity of the color is proportional to the acid concentration. In the concentration range of 4 to 7 S the intenqity of the color is essentially constant. It is desirable that no excess reagent crystallize out of solution during or after the development of the color. That rhenium found is as much as 50Vc low when this procedure is not followed, indicates that the rhenium complex is adsorbed during the crystallization process. One and one-half hours are sufficient for the filtrate to stand, but no harm is done if the time of standing is somewhat longer. The volume of the filtrate is made 90 to 95 ml. with the wash solution to prevent any appreciable crystallization of the reagent upon the dilution that follows the development of the color.
1211
I t is not necessary to control the time between the development of the color and the second filtration, so long as 10 to 15 minutes elapse between the two operations. Although the intensity of the color is almost constant 1 hour after development, there is still some change. Therefore, the time between the development of the color and the measurement of transmittance should be kept reasonably constant. During this time the colored solution should not be allowed to stand in direct sunlight, as ultraviolet light causes a pronounced fading of the color. No difference was detected between samples that were allowed to stand in the room out of direct sunlight and those kept in total darkness. RESULTS
Following the above procedure, the transmittance was determined on a series of samples containing 0.10 to 0.80 mg. of rhenium in 100 ml. of solution, and a transmittance-concentration curve was prepared (curve 1, Figure 2). Samples containing higher concentrations of rhenium were not used, as previous results indicated that the transmittance of the rhenium complex, in the presence of sulfuric acid and a-benzoinoxime, does not follow Beer’s law if more than 0.90 mg. of rhenium is present in 100 ml. of solution. Table I shows the results obtained when nine samples containing 0.00 to 0.80 mg. of rhenium and 1 mg. of molybdenum were carried through the above procedure. Only two of the samples show an error of more than 2%, which is said to be the limit of precision of the instrument when the direct reading method is used. Although complete separation of 0.10 to 1.0 mg. of rhenium from 1 mg. of molybdenum was effected, it was impossible t o separate these quantities of rhenium completely from 3 mg. of molybdenum or more. Apparently when a large amount of molybdenum is present, a small amount of rhenium is occluded in the voluminous precipitate formed by molybdenum (VI) and a-benzoinoxime.
Table I. Sample
Determination of Rhenium in Presence of Molybdenum“ Rhenium Present
.Mg.
8 a
0.100 0.200 0.300 0.400 0.500 0.600 0.700 0 800
Rhenium Found M Q
.
0,100 0,191 0,292 0,400 0,500 0,603 0 697 0 791
Error
70 0.0 -4.5 -2 7 0.0 0.0 +0.5 -0 4 -1 1
1 mg of molybdenum In 100 ml of solution.
LITERATURE CITED
Anismov, B. S . , J . Applied Chem. (17.S . S . R.),17,658-9 (1944). (2) Furman, N. H., ed., “Scott’s Standard Methods of Chemical Analysis,” 5 ed., Vol. I, p. 591, New York, D. Van Nostrand
(1)
co.. IRRR. .. , ...
(3) Geilmann, W., Wrigge, F. W., and Weihke, F., Z . anorg. allgem. Chem., 208, 217-24 (1932). (4) Hiskey, C. F., and Meloche, 5’. W., IND. ESG.CHEM.,A s . 4 ~ED., . 12, 503-6 (1940). ( 5 ) Hoffman, J. I., and Lundell, G. E. F., J . Research Xatl. Bur. Standards, 23, 497-508 (1939). (6) Ibid., 22, 465 (1939). (7) Hurd, L. C., IIVD.ESG. CHEY.,ANAL.ED.,8, 11-15 (1936). (8) Hurd, L. C., and Hiskey, C. F., Ibid., 10, 623-6 (1938). (9) Knowles, H. B., J . Research Natl. Bur. Standards, 9, 1 (1932). (10) Willard, H. H., and Smith, G. M., ISD.EKG.CHEM.,AXAL.ED., 11, 305-6 (1939). RECEIVED December 19, 1947. Presented before the Division of dnalytical a n d Micro Chemistry a t the 112th Meeting of t h e A\IERICIN CHEMICIL SOCIETY,Kew York, N . Y. Contribution 51 f r o m t h e Department of Chemi s t r y . Cnirersity of Tennessee.
