NOVEMBER 15, 1936
AIC‘ALYTICAL EDITION
(23) . , Lum. J. H.. and Curtis. H. A.. IND.ENG.CHEM..Anal. Ed... 7., 327-33 (1935). (24) Pieters’ H‘ J’s Koopmans’ H” and Hovers’ J’ T’’ 13, 82-6 (1934). Pitts(25) Porter, H. C., proc, Srd Intern, conf. Bituminous burgh, Vol. I, 613-30 (1931).
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449
(26) Porter. H. C.. IND. E X G .CHEM.. 27. 962-6 (1935). (27) Thiessen, R . , a n d Sprunk, G. C.’, Fuel, 13, 116-25 (1934). RECEIWDAugust 3, 1936. Presented before the Division of Gas and Fuel Chemistry a t the 9Znd Meeting of the American Chemical Society, Pittsburgh, Pa., September 7 to 11, 1936. Published by permission of the Director, U. S. Bureau of Mines. (Not subject to copyright.)
Hexanitrato Ammonium Cerate as a Proposed Reference Standard in Oxidimetrv J G. FREDERICK SMITH, V. R . SCLLIVAN.
4YD
GERALD FRANK, University of Illinois, Urbana, Ill.
C
ERIC sulfate has been generally accepted as a volumetric oxidation reagent comparing favorably with potassium permanganate in versatility, accuracy, and convenience. Many of its characteristics make it a preferred reagent by comparison with permanganate. Kew developments in high-potential, reversible, internal indicators having suitable color transitions, when used in connection with ceric sulfate oxidations, have eliminated the unfavorable comparison with self-indicating permanganate reactions. The commercial availability of the double salt, ceric ammonium sulfate, Ce(S04)2,2(NH4)~S04.2Hz0, which is easily soluble in dilute sulfuric acid, has eliminated the unfavorable necessity of preparing ceric sulfate solutions from relatively impure samples of ceric oxide. The notable stability of ceric sulfate solutions, during storage under ordinary conditions and in the case of hot solutions, is a favorable circumstance*in comparison to permanganate. It would appear that the most appropriate advance in the study of new developments in ceric salt oxidimetry still to be made is that of providing a salt suitable for use as a standard of reference. This problem a t first thought might be considered almost impossible of solution. The known difficulty associated with the separation of cerium from its associated rare earth elements, praseodymium, neodymium, and lanthanum as well as thorium, has been too often experienced. Attempts have been made to utilize the double salt, ceric ammonium sulfate, as the basis of a product suitable as a standard of reference. The influence of varying concentrations of sulfuric acid and ammonium sulfate upon solutions of ceric sulfate in the attempt to prepare a double sulfate of cerium with ammonium of definite composition greatly complicates the problem. The ceric oxide or oxalate suitable in such a synthesis would have to be of high purity and therefore inaccessible. A double salt is inherently objectionable for obvious reasons of variable composition. The double ceric ammonium sulfate also is hydrated and its equivalent weight so high that a reference standard based upon its use would, if one succeeded in its preparation, be prohibitive in cost.
Advantages in Use of Complex Nitrato Cerate 1. Hexanitrato ammonium cerate is a complex salt as distinguished from a double salt (with possible variations in combining proportions) and is of high equivalent weight (548.258). The salt is not noticeably hygroscopic under ordinary atmospheric conditions. The secondary ionization of the Ce(N03)6-- ion to form ceric and nitrate ions is not pronounced but is ample for the purpose of the oxidation of divalent iron (and probably other reducing agents) as well as for the oxidation of suitable indicators and for potentiometric end-point phenomena. The product is commercially available a t reasonable cost.
2 . A product 99.6 per cent pure can be easily prepared by one crystallization starting with a low-grade (40 to 50 per cent) thorium-free mixture of ceric oxide containing lanthanum, praseodymium, and neodymium. A second crystallization of hexanitrato ammonium cerate from concentrated nitric acid in the presence of excess ammonium nitrate results in an 80 per cent yield of product of practically perfect purity. 3. The newly proposed standard of reference is easily soluble in dilute sulfuric acid, forming a solution which is stable upon storage under ordinary conditions and is perfectly stable upon digestion a t 100” C. The crystalline salt is stable a t 110” C. and is easily freed from excess nitric acid and ammonium nitrate in contact with which it is prepared. Hexanitrato ammonium cerate is soluble in water (without hydrolysis) as well as in sulfuric, nitric, perchloric, and hydrochloric acids. Pure salts of ceric sulfate and cerous chloride are easily prepared from it by digestion with sulfuric and hydrochloric acids in excess.
Factors Indicating Complex Salt Composition It is not within the scope of this paper t o prove by physical chemical means the belief that hexanitrato ammonium cerate is a complex salt and not a double salt such as ceric ammonium sulfate. That (NH4)zCe(N03)6is ionized in solution to form ammonium ions and nitrato ceric ions, (KH4)&e(K0& F? 2NH4+ Ce(NO&--, is, however, clearly indicated. Thus, a solution of the pure acid-free salt in water is not hydrolyzed to form insoluble ceric salts as are the double salt ceric ammonium sulfate and other ceric salts. Solutions of the complex nitrato ammonium cerate in nitric acid are salted out by the addition of excess ammonium nitrate intro+, but are not similarly salted ducing the common ion (“4) out using excess nitric acid as a result of the addition of the common ion (?\TOs-). Clear solutions of the complex salt in water can be made which are more than 2.5 N , with color production about equal to 0.1 N ceric ammonium sulfate in normal sulfuric acid. A nitric acid solution of the nitrato cerate is noticeably slow in oxidizing reaction at the equivalence point when reduced by ferrous sulfate. The remarkably clean separation of the nitrato cerate from solutions containing equal concentrations of the other cerium group metals, except thorium, indicates a distinctly different composition since the other cerium group metals are known to form double salts as distinguished from complex salts. Lastly, the recrystallization of the nitrato cerate from concentrated nitric acid solutions by evaporation gives a product which tends towards a high ceric equivalency. This condition would be interpreted to indicate an impurity of H2Ce(XO3)6 in the salt (;?;H4)zCe(SO&. This tendency is eliminated by precipitation in the presence of excess ammonium nitrate.
+
INDUSTRIAL AND ENGINEERING CHEMISTRY
450
was then dissolved in the solution, which was cooled in ice water, centrifuged, and dried 1.5 hours a t 110" C. The purity of the product thus obtained was determined by dissolving samples in 100 ml. of 1 to 10 dilute sulfuric acid and titrating with standard ferrous sulfate using o-phenanthroline ferrous complex as indicator. The ferrous sulfate was standardized against standard ceric sulfate solution which had been standardized using Bureau of Standards sodium oxalate as reference, The results are shown in Table 11. From an examination of these data it is observed that in recrystallization from concentrated nitric acid in the presence of from 15 to 30 per cent excess of ammonium nitrate the composition of the resultant product is theoretical within ordinary analytical accuracy. A 10 per cent excess of ammonium nitrate is apparently too low probably because of the formation of H2Ce(N0& in small but appreciable amounts. Each of the five preparations was made from a different portion of the two preparations cited in Table I, the first two from the first sample and the last three from the second sample.
Preparation of Hexanitrato Ammonium Cerate A chocolate-brown ceric oxide containing approximately 46 per cent of cerium oxides was used as starting material. It contained approximately 40 per cent ceric oxide and was free from iron and manganese. This product dissolved completely in hot, concentrated nitric acid leaving a residue of foreign material, evidently oxide scale from metal trays used in its ignition during preparation. The concentrated nitric acid solution of the sample was diluted with an equal volume of water, the clear solution decanted from the insoluble foreign material, and the ceric-ion value determined. The theoretical quantity of ammonium nitrate was then added and the solution concentrated to cause the separation of the orangecolored crystals of nitrato cerate. The crystals thus obtained were separated from the mother liquor centrifugally and dried a t 100" C. until practically all excess nitric acid was removed. The results of a series of fractional crystallizations following this scheme are given in Table I. The necessity of using a thorium-free starting material is stressed by Cuttica and Tocchi (1). The two first crops of crystals were combined and 50- to 100-gram portions dissolved in boiling hot, concentrated nitric acid using 270 ml. of the concentrated acid for each 100 grams of salt taken. Ammonium nitrate in amounts between 10 and 30 per cent of the total weight of the salt recrystallized
Stability of Solutions in Nitric and Sulfuric Acids Approximately 0.1 N solutions of hexanitrato ammonium cerate in 0.5 N to 2 N nitric and sulfuric acids were digested under a reflux condenser a t 100" C. for various periods of
TABLEI. PREPARATION OF HIGH-GRADE HEXANITRATO AMMONIUM CERATE FROM LOW-GRADE THORIUM-FREE OXIDES Impure (46%) Oxides Taken Grams
1968 3280
Hexanitrato Ammonium Cerate Obtained Total .yield Total 1st Crop 2nd Crop 3rd Crop Obtamed Theoretical RePurity Purity Purity Purity Yield covery Grams % Grams % Grams % Grams Av. % Gams % 1567 99.40 475 98.82 390 90.1 2432 97.7 2540 93.5 2435 99.43 ..
.. ...
..
..
...
..
...
OF PURE HEXANITRATO AMMONIUM CERATE TABLE 11. PREPARATION FROM
0.03959 N (NHdoCe(N0s)a FeSO4 No. Taken Required Grams MI. 1.0000 46.20 1 2 1.0000 46.15 0.08085 N FeSOc 2.0000 45.08 a 2.0000 45.12 4 6 2.0000 45.12
99.5
% STOCK
(NHi)pCe(NOs)s Found
Purity Found
NHtNOs, Excess
Error
Grams
%
%
%
1.0028 1.0016
100.28 100.16
4-0.28 +0.16
10 20
1.9984 2.0000 2.0000
99.92 100.00 100.00
-0.08
30 15 20
10.00
j=o.oo
OF HEXANITRATO AMMONIUMCERATE IN HOTNITRIC TABLE 111. STABILITY AND SULFURIC ACIDS
T y e of Solution of (Nd)*Ce(NOa)cUsed
Acid Conoen- Normality tration durbefore ing Heating Heating
Time of Normality Heatin at aftef Heat100' ing Min.
Hexanitrato Ammonium Cerate Solutions in 0.5 N and 1.0 N 0.09621 0.5 Approximately 0.1 N in 0.5 N H2S04 0.09621 1.0 0.09621 1.0 0.09621 1.5 0.09621 1.5 0.09621 2.0 0.09621 2.0 0.09602 1.0 Approximately 0.1 N in 0.5 N H:S04 0.09552 1.0 Approximately 0.1 N in 1.0 N HaSOa 0.09552 1.5 0.09552 2.0 0.09552 1.0 Hexanitrato Ammonium Cerate Solutions 1.0 N and 2.0 N Annroximatelv 0.1 N i n 1.0 N "0s" 2.0 0.09107 3.0 0.09107 4 0 0.09107 Approximately 0.1 N i n 2.C . NHNOta 0.09236 ... 0.09236 0.09236 ~~
0
___ ...
8.
Sulfuric Acid 0.09618 60 0.09618 30 0.09620 60 0.09624 30 0.09622 60 0.09618 30 0.09622 60 0.09602 180 0.09560 60 0.09553 60 0.09569 60 0.09551 180 Nitric Acid 45 0.09043 45 0,08942 45 0.08882 . . a
...
. . I
48 hours' storage at room temperature between each standardization.
VOL. 8, NO. 6
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...
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time and were then cooled and the- ceric-ion value was determined by titration with standard ferrous sulfate. The results are shown in Table 111. A sample of 25.00 ml. of the solutions was used in each case and standardized before and after heating for the specified periods. From an examination of Table I11 it is observed that approximately 0.1 N solutions of the nitrato cerate in 0.5 N to 2 N sulfuric acid media are stable after digestion a t 100" C. over liberal time intervals. Solutions of the nitrato cerate in 1 N to 2 N nitric acid are stable in the cold but not a t 100" C. It will require a more extended time interval to prove the stability of these solutions stored under ordinary conditions over long periods of time. Reasoning by analogy with the stability of ceric ammonium sulfate solutions which have been shown to be stable at the boiling point for several hours and which have likewise been found permanently stable under ordinary storage conditions, the solutions of the nitrato cerate should prove likewise to be permanently stable.
Influence of Nitrate Ion in Determination of Reducing Agents
It is beyond the scope of this paper to test the use of solutions of the nitrato cerate in 0.5 N to 1.0 N sulfuric acid for all the applications to which ceric sulfate solutions in the same acid have been employed. The nitrate ion has no influence upon the determination of ferrous iron, the most commonly determined element. It is not expected to interfere in most other cases. The value of the proposed new standard need not be in the least minimized by such considerations. From a solution of a known weight of hexanitrato ammonium cerate in excess sulfuric acid, a solution of ceric ammonium sulfate is easily obtained free from nitric acid by the simple process of digestion. All question of the possible undesirable presence of the nitrate ion is thus eliminated.
NOVEMBER 15, 1936
ANALYTICAL EDITION
Summary Conditions have been established for the preparation in pure form of hexanitrato ammonium cerate, (NH4)&e(NO&, starting with a low-grade thorium-free mixture of 40 to 50 per cent ceric and cerous oxides containing 50 to 60 per cent of mixed oxides of praseodymium, neodymium, and lanthanum. The properties of this proposed new standard of reference in ceric oxidimetry are discussed and the indications pointing to its complex nature as distinguished from the double salt type of ceric salt, such as ceric ammonium sulfate, are pointed out.
45 1
The stability of solutions of the nitrato cerate in 0.5 N to 2.0 N sulfuric acid a t 100" C. has been studied and perfect stability shown. The many desirable properties possessed by the proposed new standard, which make its use in the new role desirable, are outlined.
Literature Cited (1)
Cuttioa and Toochi, Gazz. chim. ibal., 54, 628 (1924).
RECEIWDJuly 17, 1936
Construction of Glass Helices for Packing Fractionating Columns A Rapid Mechanical Method W. W. STEWART, Ontario Research Foundation, Toronto, Canada
S
INGbE-turn glass helices have been used by several investigators (I,$, S,6)as packing material in laboratory fractionating columns. Up to the present time their use, particularly in large columns, has been limited to some extent by rather slow and tedious methods of construction. A method for making these helices by hand was first described by Wilson, Parker, and Laughlin (7). A more detailed account of the construction, breaking, and sorting of glass helices has been reported by Roper, Wright, Ruhoff, and Smith (4), who wound glass spirals from Zmm. Pyrex rod by hand. The fiber diameter was about 0.6 mm., and the outside diameter of the coil 4.4 mm. About 5 cc. of finished product were produced in 1.5 hours, 1 hour of which was required for breaking and sorting. Recently, a partially mechanical method of constructing glass spirals has been described by Young and Jasaitis (8). The distilling column which has been used in this laboratory has a packed section of 300 cc. It would take about 80 hours to prepare a sufficient number of helices to pack this column by using the technic of Roper and his co-workers, whereas it was accomplished in 15 hours using the method here outlined.
Winding the Spirals A mechanical device for winding glass spirals similar to those described by Roper was constructed from Meccano parts. The design of this machine was based on that of a device developed by Tapp (6) for winding spirals from quartz fibers. The machine was built to wind directly a glass spiral, with a fiber diameter of about 0.6 mm. and 11turns per centbe meter, from a Pyrex rod 2 mm. in diameter. The fiber for the spiral was drawn from this rod as the spiral was wound on a winding form. The mechanical device was constructed in two parts: (1) a unit which rotated the winding form a t a uniform rate and, at the same time, moved this form in a direction a t right angles to the plane of rotation at a uniform speed; (2) a similar unit which fed the rotated glass rod onto the winding form a t a uniform rate. The details of construction of these units are clearly shown in Figure 1. The power plant was a Bodine electric motor, ty e CR2, equipped with a 595 to 1 reduction gear. The gear (ko. 27a, Meccano part number) on the motor shaft revolved at 16 r. p. m.
and meshed with the gear wheel (No. 31) on the horizontal drive shaft, causing the latter to revolve at 28 r.p.m. The pinion (No. 26) on the drive shaft meshed with the centrate wheel (No. 28) which drove the worm gear (No. 32). The worm gear meshed with the inion (No. 25) on the shaft driving the chain sprocket (No. 96af The chain, which was fastened at one end, passed under an idler sprocket and then over the driving sprocket. The other end of the chain was fastened to a spring which applied a tension, thus reventing a slipping motion as the drive sprocket meshed with tge chain links and carried the carriage forward. The forward motion was at the rate of 1 mm. per revolution of the winding form which rotated at 28 r. p. m. The glass spirals were wound on a 3.18-mm. (0.125-inch) diameter steel drill rod, which was coupled to the main drive shaft, and passed through a bearing fixed on the end perforated flange plate of the carriage. The free end of the winding form assed through a bearing which was fastened to anothw pergrated flange plate (right of Figure 2), the latter being fixed to the base board. These two bearings served to steady and guide the form during the winding operation. The end of the winding rod was slotted. The glass spirals were wound from a 2-mm. Pyrex glass rod, fed through a gas-air flame onto the winding form at right angles to it and in the same horizontal plane. The feeding mechanism is shown in Figure 1, the glass rod being clamped in the second carriage in exactly the same position as that occupied by the winding form in the first. The relative positions of the glassrod feeding device and the carriage bearing the winding form are shown in Figure 2. The inner cone of the gas-air flame was about 4 cm. high, and the outer tip was placed under the glass rod approximately 0.5 cm. from the winding form. The tip of the inner cone of the
FIGURE1. GLASSHELIX-WINDING APPARATUS
