A N E W VOLUMETRIC OXIDIZING AGENT PHILENA YOUNG Wells College, Aurora, New York
The properties of ceric sulfate as a wolumetric oxidizing agent are contrasted with those of oxidizing agents i n general use, such as potassium perrnanganate and potassium bichromate. Methods of preparing and standardizing solutirms of ceric sulfate are described and the many uses already developed for this n m oxidizing agent are listed.
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I
N THE past six years a considerable number-of
articles describing the results of research on ceric sulfate as a volumetric oxidizing agent have appeared. This research has been sufficiently successful to warrant a r6snm6 of parts of it for the readers of THISJOURNAL, for in any present-day discussion of oxidizing agents available for quantitative work ceric sulfate must receive consideration. If an oxidizing agent is to be of extensive value in analytical work, it must meet a number of requirements, among the more important of which are the following: (1) it must compare favorably in its properties with oxidizing agents already in use; 52) it must be available in some convenient form so that solutions of i t can be prepared readily; (3) it must be either a primary standard or a substance which can be standardized easily, and preferably against a primary reduction standard; (4) it must serve as its own indicator when the equivalence-point is reached, or suitable indicators for use with i t must be available; (5) it must react stoichiometrically with a considerable number of reducing agents. These points will be discussed in the order mentioned.
I Potassium permanganate has served for a long period as probably the most useful and generally available oxidizing agent for volumetric work. It has the advantages of being a very strong oxidizing agent in solution, of acting as its own indicator, and of reacting quantitatively in direct titration with many reducing agents. Among its disadvantages have been the fact
that i t is not a primary standard so that standard solutions cannot be prepared directly, the limited stability of its solutions unless they are prepared with special care and protected from the action of light and organic materials, the d i c u l t y of using i t in solutions containing more than a very small amount of hydrochloric acid, and the number of reduction products possible. For example, in the persulfate-arsenite method for manganese the reduction of the permanganate is not a clear-cut process. In solutions containing hydrofluoric acid trivalent manganese is formed. Potassium bichromate is not a sufficiently strong oxidizing agent to be of such general use as permanganate. It is, however, a primary standard, its solutions are stable over a long period of time, and they may be used to titrate reducing agent$ in hydrochloric acid solution. Potassium bichromate does not serve as its own indicator. Ceric sulfate solutious containing sulfuric acid are approximately equal in oxidizing power to permanganate solutions. Kunz (I) has shondq that the molal reduction potential of ceric and cerous sulfate in 1 molal sulfuric acid is -1.44 volts, a value very similar to that of -1.48 volts derived for the mold potential of Mn++, MnOa8H+ (2). The oxidizing element is alone in the positive ion and only one reduction product is possible: Ce4+ e = Ce3+. In the case of permanganate f i e oxidizing element is in a complex negative ion; a number of reduction products are possible and it is not always easy to limit the reduction products to one definite form of manganese. Ceric sulfate is hydrolyzed in aqueous solution but is stable in a sulfuric acid solution. It has been shown that 0.1 N solutions of this oxidizing agent, which are 0.5 M in sulfuric acid, can be kept without special precautions for a number of years with no change in normality, that similar solutions can be boiled under a reflux condenser for a period of five hours without change in their ceric ion content (3), and that solutions as dilute as 0.01 N, if 0.5 M in sulfuric acid, are stable for months (4). Thus a solution of ceric sulfate far
+
+
surpasses a permanganate solution in stability. Ceric sulfate may be used to titrate reducing agents in hot solutions containing high concentrations of hydrochloric acid. Under such ~ n d i t i o n spermauganate is useless, because of its reaction with the acid to form chlorine. At the present time there is no ceric salt on the market with the requisite properties of a primary staudard. Obviously, such a salt would have the advantage of a much higher equivalent weight than that of potassium permangauate or of potassium bichromate. It will be shown later that ceric sulfate solutions may be standardized readily in a number of ways, and that while such solutions do not act as their own indicators, internal oxidation-reduction indicators which have proved very satisfactory are available.
I1 Until recently ceric sulfate solutions for volumetric work have been prepared from ceric oxide (5, 6). A pure oxide is not required because the other rareearth elements usually preseut in the impure product have but one valence and therefore do not act as oxidizing agents. The solid oxide when heated with concentrated sulfuric acid is converted directly into the solid sulfate. CeOl
+ 2 H 9 0 , = Ce(SO& + 2H20.
This acid mixture is then stirred with water and the acidic solution of ceric sulfate formed must as a rule be filtered unless pure ceric oxide has been used. It can be seen that this method of preparing solutions of ceric sulfate is not convenient. About a year ago ceric ammonium sulfate of a high degree of purity was put on the market* and now solutions are prepared very quickly (7)by dissolving the correct amount of this salt in cold, dilute sulfuric acid containing enough of the acid to make the final solution after dilution to the proper volume about 0.5 M in acid. Such solutions are ready a t once for standardization, while it is necessary to allow a permanganate solution to stand for several days and then to siphon ir from any manganese dioxide before standardization.
If arsenous oxide is the primary staudard the titratiou with ceric sulfate may be made (a) in a sulfuric acid solution with osmium tetroxide as catalyst and ophenauthrolme ferrous complex as indicator (8), (b) in a hydrochloric acid solution with iodine chloride as catalyst and cblorofoxm as indicator ( 9 ) , (c) in a w m hydrochloric acid solution containing iodine chloride as catalyst and o-phenanthroline ferrous complex as indicator (4), or (d) potentiometrically in a hot hydrochloric acid solution or a t room temperature in a hydrochloric acid solution containing iodine chloride as catalyst (10). Ass08
+ 6NaOH = 2Na&Ol + 3Hn0
Individual samples of arsenous oxide may be used or aliquot portions of a staudard solution. If electrolytic iron is the primary standard, the iron may be dissolved in sulfuric acid under an atmosphere of carbon dioxide, and the ferrous sulfate titrated a t room temperature with ceric sulfate either (a) with opheuauthroline ferrous complex ( l l ) , dipbenylamine sulfonic acid (7), diphenylamine or diphenylbenzidine (12), or methyl red, erio glaucine, or erio green (13),as an oxidation-reduction indicator, or (b) potentiometrically (5). 2FeS04+ 2Ce(SO&
+ (HISOJ = Fe(SO& + Cel(SO&
If the iron is dissolved in hydrochloric acid and the stannous chloride reduction method is used, the endpoint in the subsequent titration of the ferrous chloride with ceric sulfate may be determined with any of the oxidation-reduction indicators mentioned except o-phenanthroline ferrous complex, or . potentiometrically. Electrolytic iron is not available in every laboratory, but if i t is used the stannous chloride reduction method is to be preferred, because of the possibility of not having all the iron in the ferrous form before the titration if the sample is merely treated in an inert atmosphere with sulfuric acid. All the methods mentioned above in which sodium oxalate and arsenous oxide are used as well as the potentiometric titration of ferrous chloride after a I11 stannous chloride reduction and the titratiou of ferrous sulfate in a sulfuric acid solution with o-phenanthroline Any one of three primary reducing agents may be used to standardize solutions of ceric sulfate: sodium ferrous complex as internal indicator have been tested oxalate, arsenous oxide, or electrolytic iron. With by weight titrations and have shown a high degree of sodium oxalate a number of procedures are possible: precision. Thus there are a number of potentiometric the oxalate may be titrated with the ceric sulfate (a) in and indicator methods for the standardization of ceric a warm hydrochloric acid solution containing iodine sulfate solutions. In contrast, one substance, sodium chloride as catalyst and o-phenanthroline ferrous com- oxalate, has been specified as the most satisfactory plex as internal oxidation-reduction indicator (4), (b) primary standard for permangauate solutions, and the potentiometrically in a hot hydrochloric, sulfuric, or experimental conditions to be followed must be closely perchloric acid solution (5, 6), or (c) potentiometrically adhered to in order to obtain accurate results. a t room temperature in a hydrochloric acid solution IV coutaining iodine chloride as catalyst ( 5 ) . Solutions of cerous salts are colorless, while those of Na2C2O. 2Ce(SO& (&SO,) = Ce9(SOSs NaqSO4 2C01. ceric salts vary from yellow to orange-yellow in color. Although one or two drops of 0.1 N ceric sulfate produce a noticeable effect in 250 cc. of a colorless solution, in * By 0.T. C O ~ E L TJackson. , Michigan.
+
+
+
+
+eve
practical work ceric salts can rarely be used as selfindicators. A number of internal oxidation-reduction indicators for use in titrations with ceric salts have already been mentioned. Of these o-phenanthrolineferrous complex appears to be the most valuable because of its unusually high oxidation potential, its reversible character, and its intense color change (11). In reduced form this indicator is red and in oxidized form a very pale blue. It is stable in acid solution during a titration at room temperature, and sufficiently stable for titrations in similar solutions at 50°C. (4). Diphenylamine sulfonic acid is a more satisfactory indicator in the titration of ferrous ion than either diphenylamine or diphenylbenzidine. This is due partly to its greater solubility, its sharper color change, and its slightly higher oxidation potential. Its oxidation potential is not sufficientlyhigh, however, to make unnecessary the presence in the solution of some substance such as phosphoric acid to remove the ferric ion as rapidly as it is formed, since this ion has a tendency to oxidize the indicator. Ferric ion has no oxidizing action on ophenanthroline ferrous complex and many important uses for this indicator are certain to be f o u n d a s in differentialoxidations in which diphenylamine sulfonic acid would be of little value. When the k s t articles on ceric sulfate as a volumetric oxidizing agent were published, neither of these indicators had been described and the potentiometric method was ordinarily used to determine the endpoint in a titration. This method, though accurate, requires apparatus which is not available to every experimenter. Now that these indicators are on the market* many new procedures with ceric sulfate are possible.
for determining the ferrous ion. The oxidizing agent may be permanganate as in the Zi~mnermatm-Reinhardt method. This titration requires an experienced operator, for the endpoint is neither sharp nor permanent. If bichromate is the oxidizing agent, the endpoint may be determined with an external indicator, potassium ferricyanidea tedious method now becoming obsolete, or with an internal oxidation-reduction indicator, such as diphenylamine sulfonic acid. Some experimenters have difficulty in detecting the first appearance of the purple color of the oxidized indicator in the solution, which is an intense green in color due to the chromic ion. If ceric sulfate is the oxidizing agent an internal indicator such as diphenylamine sulfonic acid may be used and a very sharp endpoint obtained, because the solution is practically colorless throughout the titratiou and changes to a deep purple color as the first drop of ceric sulfate in excess converts the indicator into its oxidized form. Trivalent arsenic causes no diiculty in the titration of ferrous iron with ceric sulfate, but it does interfere in titrations of ferrous iron with some other oxidizing agents. Soule (14) has found that ceric sulfate can be used to advantage in titrating the ferrous iron obtained from the decomposition of certain silicate rocks with hydrofluoric acid in pyrex containers, because reducing agents (possibly arsenic) derived from the glass do not affect this particular oxidizing agent. In the author's classes, students have been given at times a choice among these three oxidizing agents for titrating the ferrous iron in an ore after a stannous chloride reduction. Rarely have repetitions been required when ceric sulfate was used, but trouble has been experienced by some in seeing the endpoint with bichromate and diphenylamine sulfonic acid or from v high results with the Zimmermann-Reinhardt method. The determination of iron in an iron ore is one of the The author would suggest that readers who have not most valuable uses to which ceric sulfate has been put. used ceric sulfate in the laboratory experiment with it If the reduction of the ferric salt is carried out in a by analyzing an iron ore for iron, as only in such a way sulfuric acid solution with a Jones reductor, aluminum, can one become acquainted with a new chemical and cadmium, or zinc wire, the ferrous sulf+te is usually form an estimate of its worth. Detailed procedures titrated with permanganate and a very sharp endpoint for standardizing a ceric sulfate solution and for is obtained. S i c e iron ores must be decomposed determining the iron in an iron ore are given by Willard with hydrochloric acid and the latter removed by and Furman (7). Other materials for which analytical procedures an evaporation with sulfuric acid before any of the reducing agents mentioned above are used, the method involving the use of ceric sulfate have been developed of titrating the ferrous ion with permanganate in a follow: Calcium may be precipitated as the oxalate, the sulfuric acid medium is time-consuming. The much more widely used and more rapid procedure is the precipitate filtered, washed, and dissolved in hot, dilute direct reduction of the fenic ion in the hydrochloric sulfuric or hydrochloric acid and the oxalic acid titrated acid solution by the stannous chloride method. In either potentiometrically or with o-phenanthroline the solution thus obtained, and containing ferrous ferrous complex as indicator (4, 5 ) . Arsenic has already been mentioned under methods chloride, hydrochloric acid, and precipitated mercurous chloride, a number of indicator procedures are possible of standardizing ceric sulfate solutions (4, 8,9, 10, 15). Antimony chloride may be titrated potentiometrically a t room temperature in the presence of iodine mono* a-Phenanthroline ferrous complex may he obtained from 0 . T. Coffelt, Jackson. Michigan, or froin the G . Frederick chloride as catalyst (10); a t 50°C. in a hydrochloric Smith Chemical Co., Columbus, Ohio; and diphenylamine sul- acid solution with o-phenanthroline ferrous complex fonic acid in the form of its barium salt from the G. Frederick Smith Chemical Co. or from the Eastman Kodak Co.. Rochester. as indicator (4); and potentiometrically a t room temperature in the presence of small amounts of arsenic if New York.
AUGUST,1934
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469
the concentration of hydrochloric acid present is h-i ~ h(1.5). Vanudyl ion may be titrated in hot hydrochloric, sulfuric, or perchloric acid solution (16,17). It can be titrated in the presence of chromic and ferric salts and of tungstic acid (17); also, ferrous and vanadyl salts can be titrated successively (16). All endpoints have been determined potentiornetrically.
U(S0Sr f 2Ce(SOSr f 2H2O = UOlSO4
+ Cen(S0,)r + 2HzSO4.
\ - - , ~
Iodide ion may be titrated potentiometrically in a sulfuric acid solution (23) : or potentiornetrically in the presence of a cyanide (23) : KI
+ 2Ce(SO& f KCN + (HISO,)
= ICN
+ Cez(SOSr + K&O+
Ferrocyanide ion may be titrated potentiometrically Chromium may be oxidized (a) with excess of standard at room temperature in a hydrochloric or sulfuric acid ceric salt, the excess being titrated differentially in the solution or with a ceric sulfate solution containing a presence of chromic acid with standard sodium nitrite or sodium oxalate; (b) with excess of ceric sulfate, small amount of femc iron as indicator (24); also with after which nitrite is added in slight excess to destroy o-phenanthroline ferrous complex as internal indicator the ceric sulfate, followed by urea to remove all nitrite, (4). and then the chromic acid is titrated with standard ferrous sulfate; (6) with excess of ceric sulfate, the excess being removed with sodium azide and the Hydrogen Peroxide may be titrated potentiometrically chromic acid titrated with standard ferrous sulfate. in dilute hydrochloric, sulfuric, nitric, or acetic acid Iron and large amounts of manganese do not interfere; solution (25) or with o-phenanthroline ferrous complex if vanadium is present it is determined with the chro- as indicator (4). An indirect determination of lead mium (18). An indicator method can be used in (b), peroxide based upon treatment of the peroxide in a and also in (6) if the solution is boiled for 10 minutes nitric add solution with an excess of standard hydrogen after the addition of the sodium azide in order to remove peroxide and determination of the excess by titration all hydrazoic acid, and then cooled to room tem- with ceric sulfate has been proposed (25). perature. Equations for the oxidation: Hydrazmc Acid may be oxidized in a practically neutral solution by an excess of standard ceric sulfate. Cn(SO& GCe(SO& f 8Hn0 = 2HCr0, 3Ce2(SOJa f 6HxS04 The excess of oxidizing agent has been determined by (a) 2Ce(SOSr f NaNO* HIO = Cez(SOSr NaNOx addition of potassium iodide in excess and titration f HdO, of the liberated iodine with standard thiosulfate soluor Ce(SO& NanCnO~ (HzSOS-already given tion (26). Obviously, the excess of ceric snlfate could be determined by other methods.
+ +
+
+ +
+
(c)
2Ce(SO&
+ 2NaNs + ( H 9 0 d
= CedSOdr f 3N2
+ Na.SO4
H2Cr0, f FeS0.f HlSO4already given
Thallium may be titrated potentiometrically in a hydrochloric acid solution (19). Fe+++, CU++,Bi+++, Cd++, Pb++, Sn4+, Hg++, Zn++, &Ox--, TeOs--, As04---, Sb5+, and Cr+++do not interfere.
"
TlCl
+ 2HCI + 2Ce(S01). = TlCls + Cs(S0dr 4- H 8 0 , .
Tellurous Acid may be oxidized in a hot sulfuric acid solution by an excess of standard ceric sulfate in the presence of chromic sulfate as catalyst. Selenons acid and cupric salts do not interfere. The exFss of ceric sulfate may be titrated potentiometrically with standard ferrous sulfate (27). H2TeOa
+ 2Ce(SO& + HzO = HaTeOd + Cel(SOJa + HSO,.
Hydroquinone may be titrated at room temperature Thallons chloride may be titrated also a t 50°C. in a in a sulfuric or hydrochloric acid solution either pohydrochloric acid solution with o-phenanthrolineferrous tentiometrically or with diphenylamine or methyl red complex as internal indicator (4). Mercurous Mercury may be oxidized in a hot sulfuric as indicator (28). acid solntion by an excess of standard ceric sulfate, CIHLOH)~ 2Ce(SO& = CsH109 f Ce2(SO& H3SOa. and the excess titrated potentiometrically with standOrganic Acids such as tartaric, malonic, malic, ard ferrous sulfate (20). Mercuric ion in large amount glycolic, and citric acids can be oxidized in hot sulfuric does not interfere. acid solution by an excess of standard ceric sulfate and Hg,S04 + 2Ce(S04)% (HISO,) = 2HgSO' CedS03r. the excess titrated with standard ferrous sulfate. Uranium in the form of uranous sulfate may be ti- Formic, acetic, snccinic, fumaric, and maleic acids are trated potentiometrically in a hot sulfuric acid solution not oxidized by ceric sulfate in a hot sulfuric acid solu(21); also, the uranium in the triple acetate of sodium tion, while benzoic, phthalic, and salicylic acids are and zinc or magnesium in the determination of sodium oxidized to a variable extent under similar conditions (22). Uranous sulfate when titrated at 50°C. in a sul- (29). Thiosulfate solutions may be standardized against furic add solution containing o-phenanthroline ferrous standard ceric sulfate by treating a measured volume complex as indicator gives a very sharp endpoint (4).
+
+
+
+
of the ceric sulfate with excess of potassium iodide and titrating the iodine liberated with the thiosulfate (30). Since there are, however, excellent primary standards, such as iodate and bichromate, against which to standardize thiosulfate, this suggested use for ceric sulfate is not of great importance. This list of analytical methods, all of which depend upon the use of ceric sulfate, offers adequate proof of the value of this oxidizing agent. Ceric sulfate should be discussed along with the usual oxidizing agents in all
courses in elementary quantitative analysis and a procedure involving its use should be included, if possible, among the laboratory experiments assigned. Moreover, this reagent should be given a trial in any research problem which requires a strong oxidizing agent, and the attention of organic chemists might be called a t this time to the possibility of using ceric sulfate in certain organic oxidations in which in the past the usual reagents, such as permanganate or bichromate, have not produced the desired effect.
LITERATURE ?TED
KUNZ,J. Am. C h m . Soc., 53, 98 (1931). From the data of BROWNAND TEPE, ibid., 48, 1128. (1926), and of Towen, 2.physik. Chmn., 32, 568 (1900), as computed by G E ~ "International , critical tables." 1929, Vol. VI, p. 332. WILLARD AND YOUNG, I . Am. Chem. Sac., 51, 149 (1929). WILLARD AND YOUNG, ibid., 55, 3260 (1933). WILLARDAND YOUNG, $bid.. 50, 1322 (1928). F~MAN ibid., , 50,755 (1928). WILLARD, H. H. AND FURMAN, N. H., "Elementary quantitative analvsis." D. Van Nostrand Co., New York City. -7-
-.
GLEU,- . . .... ,.---, . SWE AND GREGORY, J. Am. Chcm. Sac., 52, 901 (1930). WILLARD AND YOUNG. ibid.. 50.1372 (1928). WALDEN, H e m . AND ~AhM~N,'ibid.; 55, 2649 (1933); and Dissertatmn of RAYPARKIN CHAPMAN, 1932, Columbia University. AND YOUNG, J. Am. Chem. Soc., 50,1334 (1928). WILLARD
FURMANAND WALLACE, I. Am. Chem. Soc., 52,2347 (1930). S~ULE ibid.. , 51,2117 (1929). FURMAN, ibid., 54,4235 1982) FURMAN, ibid., 50, 1675 ti928): WILLARD AND YOUNG, Ind. Eng. Chem., 20,972 (1928). WILLARD AND YOUNG. J . Am. Chem. Soc.. 51. 139 (1929). WILLARD AND YOUNG, ibid., 52, 36 (1930). ' WILLARD AND YOUNG,ibid., 52, 557 (1930). FURMAN AND SCHOONOYER, ibid., 53,2561 (1931). FURMAN. CALEY. AND SCAOONOVER, ibid., 54,1344 WILLARD AND YOUNG, ibid., 50,1368 (1928). F u ~ n u wAND EVANS.ibid., 51, 1128 (1929). FURMAN AND WALLACE, ibid., 51,1449 (1929). MARTIN, ibid., 49,2133 (1927). ' WILLARDAND YOUNG, ibid., 52, 553 (1930). FURMAN AND WALLACE, ibid., 52,1443 (1930). WILLARDAND YOUNG. ibid.. 52, 132 (1930). FURMAN AND WALLACB, ibid., si,lzsB (1931)
