ANALYTICAL CHEMISTRY
1540 Table 111. Record of Reagents Tested at the National Bureau of Standards Period 1928-30 1931-35 1936-40 1941-45 1946-50 Total
Number Tested 236 492 50 1 619 507 2355
Class A , % 65
72 80 87 79
Class B, % 9 10 10 6
9
Class C ,
70
26 18 10 7 12
quality but occasionally may require their relaxation. In keeping with the principle that a reagent standard must represent the closest approach to ideal purity that is commercially feasible, vigilance toward changes in available quality must be maintained. The standardization of methods of test has made it possible to do this readily. For the ACS committee this aspect of the standardizing process has been aided by the joint representation on the conimittee of producers and users. The phrase “available quality” is subject to a somewhat variable interpretation, of course, and the price that will be paid most often determines in the long run the quality that can be made available. The finechemical industry in the United States today is capable of reaching levels of quality unthought-of in 1915, and will meet almost any quality demand if the economics of the situation permit. The qualification “if the economics of the situation permit” is a n important one. There will be instances when chemicals conforming to currently accepted standards of quality are not sufficiently pure for special purposes, and the purer grade cannot be produced in limited amounts at a price the ueer is willing to pay. In such instances the chemist, now as in the past, will be obliged to undertake the special preparation or further purification of the needed material in his own laboratory. The question may be raised, has the development of betterdefined standards for reagent chemicals obviated the need for the analyst t o assure himself that the reagents he uses are sufficiently pure? Table 111, which shows the experience of the Xational Bureau of Standards in testing reagents since 1928, gives a partial
answer to this question. The chemicals tested are divided into three classes. In Class A are those that were found to conform fully to the purchase specifications, usually ACS. In Class B are those that failed in respect to a single requirement and were only slightly substandard in that respect Purchases represented by Class B were not rejected. Class C represents purchases that were rejected for failure to meet two or more requirements or were substantially substandard in a single respect. Results of tests made in the period 1928-30 have been previously reported (4). I t R i l l be noted that there was a continuing improvement in the quality of chemicals purchased by the bureau during the period 1931-45, with some recession during 1943-50. Bccause of the procurement policy of the Federal Government, which favors free competition among all suppliers, regardless of known technical competence, it is possible that the results given in the table are not representative of the quality avajlable to discrimating purchasers. The classification used in the table, which follows for convenience that used in the earlier survey, does not disclose differences in quality within Class or Class C. It seems likely that Class C currently includes fewer chemicals that are excessively bad than were formerly encountered. It is not unlikely that Class A contains a larger proportion than formerly of reagents of superior quality. The improvement of test methods probably has tended toward a more rigorous interpretation of requirements, so that even though requirements may be no more rigid than formerly, as numerically expressed, they may be more exacting as defined by the procedures. However, even the most optimistic attitude does not justify a blind confidence in labels or failure to run blank determinations when high accuracy is at stake. LITERATURE CITED
(1) Buc, H. E., J . I n d . Eng. Chem., 11, 1140 (1919). (2) Hillebrand, W.F., Ibid.,9, 170 (1917). (3) Spencer, G. C., Ibid., 15, 1281 (1923).
(4) Wichers, E., Isaacs, A , , and Schoonover, I. C., IND.ESG.CHEM., ANAL.ED.,3, 227 (1931).
RECEIVED August 31, 1951.
4th Annual Summer Symposium-Standards
Requirements for Primary Redox Standards V. A. STENGER T h e Dow Chemical Co., Midland, Mich.
I
T WAS a distinguished company that gathered in Berlin during the summer of 1903 for the Fifth International Congress of
Applied Chemistry. Sorensen was there, and Schiff, Lunge, Seliwanoff, and de Koninck. The United States was represented by F. W. Clarke, the eminent geochemist, and A. A. Noyes, subsequently known, among other reasons, for his part in developing the Noyes and Bray system of qualitative analysis. Among the 200 or more names on the official roster of the congress, many others are still familiar to analytical chemists after nearly 50 years. The session of June 6 was devoted to “Standards and Standardization’, and was presided over by W. Fresenius, with the assistance of P. Klason of Stockholm and H. Landolt of Berlin. In this setting, Wagner of Leipzig proposed that for each kind of volumetric reagent a series of pure compounds should be designated as standard substances. Thus it would be possible to check a solution of a given reagent against several standards. For permanganate solution he suggested potassium oxalate, sodium oxalate, potassium tetroxalate, and various other metal osalates (29) as suitable standards.
Kuhling of Charlottenburg differed with Wagner, preferring to have only a few substances designated as official standards (16). Under his plan, one primary standard would be provided for each analytical purpose. Sorensen adopted this viewpoint, and in a later paper ( $ 6 ) proposed two fundamental principles concerning primary standard substances: 1. The purity of a primary standard should be tested by welldefined qualitative procedures whose sensitivity may be determined for each material. Each new lot of the compound must be subjected to these tests. 2. If a compound of qualitatively established purity is proposed as a primary standard, it must be tested further by various methods suited to the nature of the material. This testing must be done as accurately as possible, but need be carried out only once.
Another set of rules was drawn up by McBride (21) to cover the requirements for a substance that might be used both as a primary reference standard and a regular working standard. He was concerned particularly with the use of sodium oxalate as a standard for permanganate.
V O L U M E 23, NO. 1 1 , N O V E M B E R 1 9 5 1 Primary standards for use in oxidation-reduction titrimetry are discussed with reference to the requirements outlined by SSrensen, McBride, and Kolthoff. Among the standard substances considered are sodium oxalate, arsenic trioxide, potassium iodide, potassium iodate, and potassium dichromate. Although the purity of a standard is of much importance and should be assured by testing or by
1. There must be reasonable case of preparation and accuiate cproducibility. 2. The purity must be determinable with sufficient accuracy, and the purified material must be stable under ordinary conditions of the laboratory. 3. The use of the material in regular work must demand neither complex apparatus nor difficult manipulations. 4. Such precision must be obtainable when the substance is used with ordinary care that one, or a t the most, a very few determinations suffice for fixing the value of a standard solution. 5. The accuracy obtained under ordinary conditions of standardization must be at least as great as that required in the use of the solution to be standardized. I
Kolthoff (11) proposed that three further qualifications for standard substances should be required in addition to those of Sorensen, and he expressed another idea that defines a permissible exception to Sorensen’s first rule: 1. A standard substance must be capable of maintaining its composition unchanged during storage. 2. A standard substance must not be so hygroscopic that it takes up water during weighing. 3. The volumetric reaction must exclude disturhing side reactions during the standardization. In addition, the titration error must be negligible or must be easy and exact to deteimine experimentally. 4. If a method has been devised by which a substance can be prepared entirely pure at any time, it is unnecessary to apply qualitative tests to material that has been prepared in careful accordance with the method. Kiihling’s paper before the Fifth International Congress listed standard substances for onl> thiee redo\ solutions: potassium tetroxalate for permanganate, potassium dichromate for thiosulfate, and arsenic trio\ide for iodine. Apparently the life of a volumetric analyst was fairly simple in those da? q In contrast, a laboratory well equipped for general analysis a t present is likely to employ solutions of permanganate, dichromate, periodate, bromate-bromide, hypochlorite, one or more salts of tptravslent cerium, iodine, thiosulfate, arsenite, trivalent titanium, diand trivalent iron, and possibly chromous, ferro- and ferricyanide, or chlorite, not to mention the Karl Fischer reagent and other nonaqueous solutions. The busy modern industrial chemist would like to add one more rule for standard substanc they should always be conirnercially available in a eon pure state. Fortunately, this qualification is met in several cases, thanks t o the efforts of the National Bureau of Standards and certain manufacturers of laboratory chemicals. Let us now consider some of the standard substances that are commonly used in redox titrimetry, to see whether the foregoing rules adequately cover the requirements. SODIUM OXALATE
For a number of yeais sodium oxalate was considered an ideal standard substance for permanganate. Its preparation for use in the standardization of acids had been studied thoroughly by Sorensen and others, and adequate tests for its purity had been developed. Material of better than 99.95y0 purity was, and still is, available from the S:itional Bureau of Standards. The procedure generally used ill standardizing permanganate consisted in weighing out the oxalate, diluting with water, adding sulfuric
1541
a prescribed preparation method, the stoichiometry of its reaction during standardization may affect the accuracy of the result even more. In selecting a standard substance for a particular purpose, therefore, one should consider the entire system: standard-titrant-titration medium. Studies of reaction kinetics and induced reactions can contribute to a more advantageous use of redox standards.
acid, heating to around 80” C., and titrating while hot. Many chemists carried out their first permanganate titrations in this way, while studying quantitative analysis, and it was a poor student who could not obtain “accurate” results. Unfortunately for the self-confidence of those chemists, two men working a t the Bureau of Standards proved in 1935 (6) that accurate results cannot be expected by the classical titration procedure. These two men, Robert M. Fowler and Harry Bright, found permanganate titers from 0.2 to 0.4% too high by the old method, compared with results obtained in standardizations against very pure iron. Their findings were later confirmed by Kolthoff, Laitinen, and Lingane (IS),who compared the oxalate titration with the potentiometric titration of iodide. Folvler and Bright postulated that the error was caused mainly by decomposition of oxalic acid during heating of the acid solution, and they proposed to avoid this by carrying out most of the reaction a t room temperature. By dissolving the oxalate in cool, dilute acid, adding most but not all of the permanganate, and then heating, they were able to complete the titration with very reproducible results. Kolthoff, Laitinen, and Lingane demonstrated the average error of the new procedure to be +0.07 & 0.03%’o,as compared with +0.2 & 0.1% for the old. The point to he noted for the purpose of this paper is that although sodium oxalate has the good inherent properties that should be required of a standaid substance, it did not in actuality become a suitable standard for permanganate until the conditions under which it should be used were determined experimentally. In other words, even though the standard was a pure, reproducible substance, the results obtained were unreliable until the stoichiometry of its use had been established. Even today we do not knom- enough about the stoichiometry of the periliariganate-oxalate reaction to regard sodium oxalate as the best standard for permanganate. In spite of numerous kinetic studies (1, 6, 19, 20) the complicated reaction mechanism is not a t all clear. Part of the difficulty lies in the nature of manganese with its intermediate valences. When several valence states are possible, a reaction can follow several paths, and if one of the paths involves a slow step, one of the reagents may not be consumed completely a t the end point. Consider a reaction mechanism somewhat modified from that proposed by Launer and Tost ( 2 0 ):
+
+ + + + +
+
2hlnO43Mn(II) 16Hf+5hln(IV) 8Hz0(rapid) Mn( IV) hfn( 11) e 2Mn( 111)(rapid, reversible) Mn(IV) CtO4--+Mn(III) COZ Cop-(measurable) lln(1V) COZ--.COZ Mn(II1) (rapid) Mn(II1) 3CzO4-- F? [Mn(III)(Cz04)3]---(reversible) hln(II1) CZO~--+ Mn(I1) COL COZ- (measurable) hln(II1) COP-+ Mn(I1) CO~(fair1yrapid) COZ- 0 2 ~ O Z C O (rapid, Zreversible) OzCOz- Mn(I1) 2 H + + Mn(II1) COz HZO2 2Mn(III) H 2 O z - + Mn(I1) 2H+ Oz ( 0 z C 0 ~ - 2Cz04-4H++4COZ +2Hz0 COZ-)
+ + + + + +
+
+
+
+
+
+
+
+
+
+ +
+
+
(1) (2) (3)
(4) (5) (6) (7) (8)
(9) (10) (11)
This scheme is mentioned, not because it is necessarily correct in all details, but because it illustrates a number of possible cases. Permanganate does not react instantaneously with oxalate in
ANALYTICAL CHEMISTRY
1542
acid solution, but after some manganous ions have Iormed or been added, the reaction proceeds rapidly (Reaction 1). If much manganous ion is present, Reaction 2 occurs and thenceforward most of the steps involve complexing or oxidation by trivalent manganese (Reactions 5, 6, 7 ) . If, on the other hand, no manganous salt is added, tetravalent manganese is likely to be present in larger amounts and Reaction 3 can take place. This produces some sort of a free radical, such as the free “formyl” ion shown, which can take up oxygen to form a peroxy-type radical (Reaction 8) and may then lead to the kind of trouble called “induced oxidation.” In Reactions 9 and 10 the oxidation is normal, the peroxy radical bringing about a one-electron reduction and regenerating oxygen. But if there should be a reaction like 11 (hypothetical in this case), the effect would be to consume atmospheric oxygen in the oxidation of oxalate. Meanwhile, the free formyl radical would be regenerated, so that the induced oxidation might be continued through a number of cycles. Such phenomena are probable when ferrous iron is part of the system, and Kolthoff has recently been investigating the behavior of ferrous iron with peroxides (14, 15). He suggests also that the peroxy radical, OZCO2-, may disproportionate into 0 2 and COZ. Further study of this point is needed. Bnother source of error is illustrated by Reaction 5 . A reaction like this, if only s l o d y reversible, could by binding oxalate cause too low a consumption of permanganate and thus apparently give the same effect as an induced oxidation. Yet there would be a difference, for in the case of complex formation the end point would not be permanent. Although the Fowler and Bright method of titration gives results that are excellent for all practical purposes, there is little doubt that small variations are caused by side reactions such as those mentioned. Possibly if we knew enough about the kinetics involved, we could add suitable catalysts or inhibitors and secure even better results. From the standpoint of the possible valence changes, one might expect a ceric solution to be better suited than permanganate for standardization against oxalate. That is certainly true in one sense; indeed, the great increase in the use of ceric reagents during the past few years can be credited at least in part to their simple, one-electron, oxidation-reduction step. This diminishes the probability of interfering side reactions and enables one to use a ceric reagent with more confidence than would be justified for a compound like permanganate. Even so, there can be difficulties in the standardization of ceric solution against oxalate. Some of the earlier methods required heating an acidified oxalate solution and thus presumably involved the same decomposition that has already been mentioned. This error can be avoided by the use of iodine chloride both as a catalyst and as an indicator, which permits titration at room temperature ( S I , 32). Probably, however, the error is not great if ferroin is used as indicator and iodine chloride as catalyst, for a temperature of only 50” C. is sufficient in this case. Osmium tetroxide can also be used as a catalyst. Another possible error resides in the fact that midway in the titration the solution contains both cerous ions and oxalate ions in appreciable concentrations. If the solution is not sufficiently acid, or if the titration is delayed, cerous oxalate may precipitate. Such an occurrence would produce essentially the same effect as formation of a manganic oxalate complex in a permanganate titration ARSENIC TRIOXIDE
Although arsenic trioxide has long been known as a reliable standard for iodometry, its use as a standard substance for permanganate has been neglected until relatively recently. The difficulty lay in the slow reaction and the intermediate manganic compounds formed. The catalytic effect of iodine compounds, discovered by C. Lang ( l 7 ) , overcame the difficulty, but it was not until studies by R. Lang ( I S ) , Kolthoff, Laitinen, and Lingane ( I S ) , and Bright (3) had been published that arsenic trioxide was demonstrated to be one of the best standards for
permanganate. bletzler, Myers, and Swift ( 2 3 ) tested the most suitable conditions for the permanganate-arsenite reaction and recommended that it be carried out in 0.6 N hydrochloric acid, with iodine chloride as catalyst and ferroin as indicator. With their procedure they obtained an average deviation of 0.01% and a maximum deviation of 0.03%. Here is a case in which a substance, long thought to be unsatisfactory for a particular purpose, turned out to be one of the most reliable standards available for that purpose after the proper conditions had been determined. Coincidentally, the same substance (arsenic trioxide) went through a similar course as a standard for ceric solutions. In the original work of Willard and Young (Sf ), in which iodine chloride served as a catalyst during standardization, titers about 0.3% Ion, were obtained. Swift and Gregory ( 2 7 ) later were able to secure excellent results with the method by increasing the hydrochloric acid concentration to 4 M. Subsequently Willard and Young (34) demonstrated that the chloride concentration is the critical factor, and that a lower acidity is permissible if the chloride concentration is sufficient. When the required conditions are met, arsenic trioxide is an excellent standard for solutions of tetravalent cerium. POTASSIUM IODIDE
Although potassium iodide can be prepared in very pure state and meets the requirements for stability during storage and weighing, it was not considered as a volumetric standard for redox purposes until Kolthoff, Laitinen, and Lingane ( I S ) determined the stoichiometric exactness of its reaction with permanganate. The color of iodine interferes unless the titration is carried out potentiometrically or in the presence of cyanide, as in Lang’s method. By titrating to the formation of iodine cyanide, Kolthoff and coworkers found the corresponding permanganate normality to be in error by only +0.04 f.0.03%. It would be desirable to know xhether results of comparable accuracy can be obtained in the standardization of ceric reagents against potassium iodide, in the presence of cyanide. POTASSIUM IODATE AND BROMATE
Potassium iodate is an excellent standard which can be used in a variety of ways. By reducing it with pure sulfur dioxide in acid solution and boiling out the excess sulfurous acid, one can prepare a known iodide solution for use in standardizing silver nitrate or permanganate. hlore often the iodate is used directly for standardizing acids or thiosulfate. The stoichiometry of such use seems to be beyond question, a t least when any simultaneous air oxidation of iodide is avoided. Only two objections to potassium iodate as a primary substance appear to be possible. The first is its low equivalent weight as an oxidizing agent. This is offset by the stability of its solution, which permits working with aliquot portions. The other is the lack of sensitive chemical tests for certain impurities, particularly for rubidium. [Milstead (24) reported that commercial samples of C.P. potassium iodate liberate too much iodine and that on recrystallization from water the results become worse. He suspected the presence of sodium iodate as an impurity. Further investigation would be desirable.] Rubidium iodate is less soluble than potassium iodate and if present might tend to be enriched in the solid phase during recrystallization. Fortunately, the manufacturers of analytical reagent potassium iodate have eliminated this difficulty in some way; spectrographic analyses recently made at the author’s request by T. M. Hess and E. L. McLaughlin of the Dow Spectroscopy Laboratory have shown less than 0.001% rubidium in any sample tested. Current products from three manufacturers \yere represented and one *ample came from analytical reagent material that had been recrystallized by the writer a number of years ago. The quantities of rubidium found were well below those likely to be present in crude potassium salts. N o significant amount of sodium or cesium was present.
V O L U M E 2 3 N O . 11, N O V E M B E R 1 9 5 1 Potassium bromate can be prepared in as pure form as the iodate (12, 28). Aside from the somewhat IoTver molecular weight, it too would be a suitable standard for thiosulfate, were it not for the rather sluggish reaction with iodide. The reaction rate is accelerated by increase in the acid concentration, or even more markedly by the addition of a little sodium molybdate as a catalyst. This has been known for at least 30 years (IO),but it still seems to be ignored by many chemists. If the bromateiodide reaction is not allowed to reach completion before the solution is titrated, the remaining bromate oxidizes part of the thiosulfate to sulfate rather than to tetrathionate. POTASSIUM DICHROMATE
The purity of various commercial products was tested and reported by Willard and Young in 1935 ( 3 0 ) and by McClellan in 1949 ( 2 2 ) . None of these investigators found any product to assay less than 99.90% or more than 100.04% after drying; in fact, the average deviation from 100.00% was only lt0.025. Furthermore, the Bureau of Standards offers primary standard potassium dichromate of 100.00% purity by t,itration. There is no doubt that material of sufficient purity for practically any purpose can be obtained without much difficulty. The Association of Official Agricultural Chemists has accepted dichromate as a standard substance for t,hiosulfate (8) and has suggested it also for the standardization of titanous chloride (2). RIany analysts prepare standard dichromate solutions directly from the salt for use in iron determinations. Potassium dichromate is excellent both as a primary standard and as a working standard. Nevertheless some care is required in the standardization of thiosulfate against dichromate. During the early part of the century numerous workers reported that the reaction of chromic acid with iodide is not stoichiometric and that an induced oxidation may occur. It remained for Bruhns (4)and Kolthoff (9) to find the proper conditions of acidity, which must be neither too high nor too Ion-. Kolthoff recommended using a solution about 1 11- in hydrochloric acid and mixing thoroughly before titration of the liberated iodine. Hahn ( 7 ) noted that the reaction reaches completion slon-ly because of complex formation. The more recent AOAC directions ( 8 ) employ an aridity of only about 0.2 N , but specify that the dichromate-acid-iodide mixture should be allowed to stand for 10 minutes in the dark before titration ( 2 5 ) . Thiosulfate solutions so standardized were found to give results deviating by ltO.2%, in analyses of a standard iodate solution by various analysts. It would he desirable to have a thorough comparison of dichromate and iodate as standards for thiosulfate under various conditions, including both the presence and absence of oxygen. DISCUSSION
Having examined some of the better knoTvn standard substances in the light of the previously formulated rules, let us now re-examine the rules in the light of what me have learned about the standards. Usually it has not been difficult to obtain compounds in a state of sufficient purity. On the other hand, purity alone did not assure that the result of a redox standardization would be accurate. I t was equally important, perhaps even more so, to ascertain by stoichiometric studies that the standard substance n-ould undergo a quantitative reaction 15-ith the active component of the titrating solution. In several cases it was necessary to standardize the conditions for titration in order to tiring about a quantitative reaction. Therefore, in selecting a standard substance for a particular purpose, one should consider the entire system: standard-titrant-titration medium. This point is implied in RlcBride’s rules 4 and 5 , though without emphasis on the nature of the processes in question. I t is also touched upon by Sorensen’s second rule. The latter would cover the case completely if it were enlarged by one phrase: “If a compound of qualitatively established purity is proposed
1543 as a primary standard, it must be tested further by various methods suited to the nature of the material and to the purpose for which it is to be used., . .‘I Kolthoff’s third rule applies directly to the situation and deserves more attention than it has received. All the other rules have to do with the purity of the substance, or the convenience with which it i s prepared and used. Leaving out of consideration matters of convenience (which nil1 take care of themselves in acceptance or rejection of a compound by future generations of chemists) the requirements for primary redox standards may be summarized as follon-s:
A primary standard must be stable enough to permit accurate weighing and orJinary analytical manipulations. The purity of a primary standard must be assured: through well-defined qualitative tests of known sensitivity, or through preparation by a method that has been demonstrated to yield a pure product consistently, and storage under conditions in which the product is entirely stable. The suitability of a primary standard for a particular purpose must be demonstrated by stoichiometric studies. These studies must be carried out as accurately as possible, but need be made only once for each case. ACKNOWLEDGMENT
Thanks are extended to I. PIT. Kolthoff for a review of this paper and for permission to make use of some material that will appear in Volume I11 of “Volumetric Analysis.” LITERATURE CITED
(1) Barredo, J. hl. G., and Senent, S., i\Tnturwissenschu~ten,31, 550 (1943). (2) Breit, J. E., J . Assoc. O f i c . Agr. Chemists, 30, 504 (1947); 31, 573 (1948). (3) Bright, H. A,, IXD. ENG.C H E h I . , ~ A L ED., . 9, 577 (1937). (4) Bruhns, G., 2. nnorg. allgem. Chem., 49, 277 (1916); J . prukt. Chem., 93, 73,312 (1916); 95, 37 (1917). (5) Fessenden, R. IT., and Redmon, B. C., J . Am. Chcm. SOC.,57, 2246 (1935). ( 6 ) Fowler, It. XI., and Bright, H . A , , J . Research Nafl. B u r . Standards, 15, 493 (1936). (7) Hahn, F. L., J . Am. Chem. SOC.,57, 614 (1935). (8) Johnson, G. hi., J . Assoc. Ofic.Agr. Chemists, 25, 659 (1942); 28, 594 (1943); 31, 44 (1945). (9) Kolthoff, I. M., 2. anal. Chem., 59,401 (1920). (10) Ihid., 60, 345 (1921). (11) Kolthoff, I. hI., and Furman, N. H., “Volumetric Analysis,” 1st English ed., Val. 11, pp. 47, 48, New York, John TViley & Sons, 1929. (12) I b i d . , p. 367. (13) . . Kolthoff. I. hZ., Laitinen, H. A,. and Lingane. . J. J.. J . Am. Chem. Soc., 59, 429 (1937). (14) Kolthoff, I. h l . , and Medalia, A. I., I h i d . , 71, 3777, 3784 (1949). (15) Kolthoff.I. hl., Medalia, A. I., and Raaen, H. P., Ihid., 73, 1733 (1951). Fifth Intern. Congr. Applied Chem., I, 323 (16) Kuhling, O., PTOC. (1903). (17) Lang, C.. Chem.-Ztg. (Repertorium),29, 48 (1905). (18) Lang, R., Z . anorg. allgem. Chem., 152,203 (1926). (19) Launer, 13. F., J . Am. Chem. Soc., 54, 2597 (1932); 55, 865 (1933). (20) Launer.‘H. F.. and Yost, D. A I . , Ihid., 56, 2571 (1934). (21) hTcBride. R. S., Ibid., 34, 393 (1912). (22) McClellan, G., J . Assoc. O f i c . A g r . Chemists, 32, 557 (I94b). (23) Metsler, D. E., hfyers, R. J., and Swift, E. H., IND.ENG. CHEM.,ASAL.ED., 16, 625 (1944). (24) Milstead. K. L., d . Assoc. Ob’ic. Agr. Chemists, 22, 567 (1939). (25) Rue, S.O., IND.E x . CHEII.,AXAL.ED., 14,802 (1942). (26) Sorensen, S. P. L., 2. anul. Chem., 44, 141 (1905). (27) Swift, E. H., and Gregory, C. H., J . Am. Chem. Soc., 52, 901 (1930). (28) Van Dame, H. C., J . Assoc. O f i c . A g r . Chemists, 30, 502 (1947). (29) Wagner, J., Proc. F ~ f t hIntern. Congr. Applied Chem., I, 314 1903. (30) Willard, H. €I., and Young, P., IND.ENG.CHESI.,ASAL. ED., 7, 57 (1935). (31) TTillard, H. H., and Young, P., J . Am. Chem. S O C . ,50, 1322 ( 1928). (32) Ibid., 55, 3260 (1933). RECEIVED July 9, 1951
