ANALYTICAL CHEMISTRY
1756 Table 111. Separation of Cerium from Individual Rare Earths (300 ml. of 0.5 M nitric acid) Earth Oxide CeOz CeOn Found, S o . of Present, Taken, Error, Mg. hlg. hlg. Pptions. hlg. 1 22.2 22.2 0 30 YiOa 1 26.6 27.0 +0.4 30 YzOa 22.2 22.3 +o. 1 2 30 Yzo3 1 -0.1 5,6 5,5 24 LazOa 1 28.3 30.1 +1.8 48 LarOa 2 23.8 23.6 $0.2 60 LazOa 2 22.9 $0.7 23.6 24 LazOa 2 25.1 +0.2 25.3 24 PrrOa 1 4.0 +o. 1 3.9 12 PrzOs 2 27.1 26.1 +l.O 24 PrzOa 2 25.5 25.6 -0.1 41 SdiOa 31.7 +0.2 2 31.9 82 NdzOa 1 48.5 45.9 +2.6 41 SdzOa 1 59.7 59.7 0 41 NdzOaa 22.3 22.3 0 2 30 SmiOa 2 25 3 25.4 -0.1 30 SmzOa 2 20.8 20.7 +o. 1 15 GdzOs 25.3 2 24.9 $0.4 30 GdiOa 25.3 25.3 2 30 GdiOa 21.2 2 -0.1 21.3 30 ErzOa 20.7 2 -0.1 20.8 30 ErzOa 0 500 ml. vol ume.
Oxidizing Agent Persulfate Bromate Bromate Persulfate Persulfate Persulfate Bromate Persulfate Persulf ate Bromate Persulfate Persulfate Bromate Bromate Persulfate Bromate Persulfate Persulfate Bromate Persulfate Bromate
error in the persulfate method if two precipitations are made. If the volume is increased to 500 ml., considerably larger amounts can be tolerated. Interfering Elements. The following ions gave no precipitate under the conditions recommended : magnesium, aluminum, cobaltous, nickelous, chromium, and molybdate. The following gave precipitates immediately: strontium, barium, bismuth, stannic, lead, silver, mercurous, mercuric, titanyl, uranyl, and thorium, and on standing manganese, ferric, zinc, cadmium, calcium, cupric, gallium, and indium precipitated at room temperature. Ceric oxide obtained in this way can, of course, be determined titrimetrically by the use of a standard reducing agent.
Table IV. Separation of Cerium from a Mixture of Yttrium, Lanthanum, Praseodymium, Neodymium, Samarium, Gadolinium, and Erbium (300 ml. of 0.5 IM nitric acid, two precipitations) Each Oxide, hlg. 5
CeOn Taken,
15
30 15
SEPARATION OF CERIUM FROM OTHER RARE EARTH ELEMENTS
Known amounts of cerium were added to known amounts of rare earth salts and the procedure was carried out in the usual manner. The results of the separation from individual rare earths are shown in Table 111. The separation of cerium from a mixture of seven rare earths is shown in Table IV. The total weight of the oxides is seven times the weight shown in the first column. I t is obvious that in general the persulfate method gives a better separation than bromate and that in general the presence of up to 30 mg. of any rare earth oxide introduces no appreciable
3Ig. 22.9 32.5 26.8 22.1
CeOz Found, 3Ig. 22.9 32.4 26.9 22.6
Error, Mg. 0 -0.1 f O .1 +0.5
oxidizine . .:
Agent Persulfate Persulfate Persulfate Bromate
LITERATURE CITED
(1)
Brinton, P. H. R.I. P., and James, C., J. Am. Chem. Soe., 41,
1080
(1919). (2)
Chernikhov, Tu. A,, and Uspenskaya, T. A., Zavodskaya Lab., 2, 276 (1940).
(3) Willard, H. H., . h a t . CHEM.,22, 1372 (1950). RECEIVED for review hIay 8, 1953. riccepted July 22, 1953. From a thesis submitted by Sylvia T'sai Yu t o the Graduate School of the University of Michigan in partial fulfillment of the requirements for the degree of doctor of philosophy.
Estimation of Water Content of Small Amounts of Proteinaceous Material HARRY SOBEL Division of Laboratories, Cedars of Lebanon Hospital, Los dngeles, calif. often necessary to estimate the quantity of moisture presIterials ent in 5- to 50-mg. samples of lyophilized proteinaceous masuch as pituitary hormones. The most widely used chemT IS
ical methods for the determination of water employ the Karl Fischer reagent, but existing methods, in general, are not applicable to small samples. A convenient colorimetric procedure has been developed which employs the Fischer reagent.
Table I. Analytical Data for Sample of Lyophilized Pituitary Gonadotropic Hormone Preparation Mg. Water in standard Protein Calculated water content
0
0.200 0.300 15 0.26
Instrument Reading (Absorbance) 1.50
0.83 0.23 0.48
METHOD
The reaction is carried out in a dry box or under ordinary conditions on a day with low humidity. All glassware is separately wrapped in brown wrapping paper and dried a t 110OC. for 2 hours. Pre aration of Reagent. Diluted Fischer reagent is prepared by ad8ing the concentrated reagent t o anhydrous methanol until a yellow color develops. The quantity of Fischer reagent to be added is arrived a t empirically according t o the following scheme:
Ten milliliters of the diluted solution is put into a standard Coleman cuvette and 0.02 ml. of anhydrous methanol is added, using a Sahli hemoglobin pipet with a rubber tube and a mouthpiece. The methanol is not blown in with the breath, but rather by compression of the rubber tube. The pipet is then used as a stirring rod. To a similar tube 1 drop of water is added to produce d e coloration; this is used for the blank setting of the Coleman Junior spectrophotometer. Measurement is made a t 500 mp. The contents of the first tube should have an absorbance of 2.0 if approximately 0.4 mg. of water is expected to be present, 1.5 for 0.3 mg.,and 1.0 for 0.2 mg. or less. The diluted reagent is useful for several days if kept in a dry atmosphere. Preparation of Standard Curve. Solutions of water in absolute methanol are prepared so that 0.100, 0.200, and 0.300 mg. are present in 0.02 ml. of solution. If greater accuracy is desired, intermediary standards should be prepared. Each standard is introduced into the diluted Fischer reagent and readings are made as indicated above. Procedure. The ca is removed from the bottle containing the lyophilized materia! 10 ml. of the diluted reagent is added, the bottleisstoppered, andits contents are mxedbyswirling, care being taken that the solution does not come in contact with the stopper. The bottle is quickly centrifuged, the supernatant solution is transferred to a colorimeter tube, and a reading is taken, using a m-ater-decolorized solution as a blank. The nonaqueous reactants may be determined on a heatrdried sample. In Table I analytical data are shown. The complete deterniination takes lees than 10 minutes. RECEIVED for review 3Iari.h 13, 1953. Accepted June 30, 1953.
