Cerate Oxidimetry Preparation and Stability of Solutions G. FREDERICK SMITH AND C. A. GETZ University of Illinois, Urbana, Ill.
A
Ceric oxide, CeOz, anhydrous ceric sulfate, Ce(S04),, and the so-called double sulfate, Ce(S04)2.2(NHa)2S0a.2H20, originally served as materials for the preparation of solutions for use in analvsis. The use of the complex salt, hexanitrato ammonium cerale, (NH4)2Ce(K03)6, was proposed by Smith, Sullivan, and Frank (6). The corresponding potassium salt, K2Ce(NO3)8, may be similarly employed. The electrolytic oxidation of cerous sulfate, nitrate, and perchlorate t o form the coniplex acids H2CeH2Ce(N03)6,and H2Ce(C10& by an improved procedure not requiring the use of a diaphragm cell was described by Smith, Frank, and Kott (3). The potential relationships of the couple Ce+-++ + e = Ce+*+ in hydrochloric, sulfuric, nitric, and perchloric acids were studied by Smith and Get2 (4), who developed a cerate-ion concept. Improved procedures in the standardization of the nitrato and perchlorato cerate anion, using sodium oxalate and arsenious oxide as reference standards, were described by Smith and Getz ( 5 ) , who used nitro-ferroin for the first time as indicator in this work.
D I S T I S C T advantage in using tetravalent cerium in volumetric analysis, rather than permanganate, with which it compares favorably in the number of practical applications, is the fact that its sulfate solutions are completely stable during long storage and are stable a t the boiling temperature for considerable periods of time. Moreover, hydrochloric acid in moderate concentrations is not a complicating factor, as in the case of permanganate. The fact that permanganate serves as its own indicator while the tetravalent cerium does not is no disadvantage, since the high potential indicator, ferroin, has come into wide use. Cerium in volumetric analysis has one disadvantage as compared to permanganate. The oxidation potential of the e = Ce+++ in molar sulfuric acid is 1.44 system Ce-++volts, whereas that of the couple bIn045e 8H+ = Mn++ 4H20 is 1.52 volts. The former system in hydrochloric acid medium has a potential of 1.28 volts. This fact, which places such reactions on a par only with dichromate reactions as to oxidation potential, is not sufficiently recognized. The use of tetravalent cerium in nitric and perchloric acid solution provides the potentials 1.61 and 1.70 volts, respectively, for the system Ce++++ e = Ce+++. At these potentials such solutions extend the field of possible analytical applications. Standard solutions of tetravalent cerium would be expected t o undergo change as a result of the slow formation of oxygen, owing to the reaction of the couple, O2 4e 4H+ = H20,the potential of which is approximately 1.2 volts. It is the purpose of the present study to determine the magnitude of the last-mentioned effect.
+
+ +
+
Preparation and Materials Hexanitrato ammonium cerate, (NH4)2Ce(.?r03)6,was prepared as previously described ( 5 ) . Salts of two different types were employed: a pure grade, 98 to 100 per cent, and an impure grade, 90 t o 95 per cent. Hexanitrato potassium cerate was similarly prepared, using potassium nitrate in place of ammonium nitrate. Solutions of perchlorato ceric acid, H2Ce(C10,)6,in perchloric acid were prepared by electrolytic oxidation of pure cerous perchlorate ( 2 ) . The cerous perchlorate was completely oxidized before use. Solutions of pure cerous nitrate in nitric acid were completely oxidized electrolytically to nitrato ceric acid, H2Ce(N03)8,by the method of Hengst,enberger ( 2 ) .
+
+
+
Preparation and Standardization of Solutions Solutions were all standardized using Bureau of Standards sodium oxalate and arsenious oxide, as well as ferrous sulfate that had been standardized indirectly with sodium oxalate and sulfato cerate solution, using ferroin as indicator in all cases. The met'hods have been described in detail by Smith and Getz ( 5 ) . The solutions tested for stability in storage are described in Table I.
Previous Studies The most recent summary of published data in the use of tetravalent cerium in volumetric analysis has been given by Furman ( I ) , who presents a general review of the subject and an extensive bibliography.
Yariation i n Solution Normality with Time of Storage
TABLE I. COMPOSITION A X D ST4XDiRDIZ iTION Y4LOES O F NITRATO AXD PERCHLORAT0 C E R 4 T E SOLUTIONS (Solutions numbered in the order oi their stabilitj i Solution S O .
1
2 3 4 5 6 7 8 9 10 11
12 13 14 15 16 17 1s
Starting Material
Type of Storaee
0.1008 0.05147 0.07082 0.08406 0,07020 0.06333 0.05642
Purified Commercial Purified Commercial Commercial Purified Purified
Dark Dark Dark Diffused daylight Diffused daylight Dark Dark
0.04274 0 . 0 0 7 143 0.01132 0.007351 0.09700 0.05439 0.01039 0.05624 0.009081 0.09540 0.04188
Commercial Purified Purified Purified Commercial Purified Purified Purified Commercial Purified Purified
Dark Dark Dark Diffused Dark Dark Diffused Dark Diffused Diffused Diffused
Standard Factor
2.0 N 3.0 A' 1.4 N 1.0 N 1.4 N 1.0 N 3.0 N
HClOi
HKOa "01
HClOi HNOi HSOs HCIOa
1.0 N HClOi 1 . 4 N HNOa
N N N N N N N 2.0 N 8.0 N 2.0 1.4 4.0 8.0 2.0 1.0 1.0
HClOi HNOa HClOa HClOa HClOi HClOi HClOi HClOi HClOi
Duration
Purity of Starting Material
Acid Concentration
339
of
Test Days 233 183 237 246 237 179 167
181 24 1
daylight
242 212 17 183 243
daylight daylight daylight
95 236 115
daylight
171
Data showing the change in normality Tyith time for ten solutions prepared as shown in Table I are given in Table 11. Some solutions studied are omitted to conserve space. The solutions omitted, all of which were acidified with perchloric acid from 1 to 8 N in strength, were increasingly less stablein the following order: 11, 12, 13, 14, 15, 16, 17, and 18.
Discussion of Results Of the seven most stable solutions five were stored in the dark, which indicates that decomposition is in part a photochemical reaction. The most stable solution consisted of purified liexanitrato ammonium cerate in 2 Ar perchloric acid. For change in normality of 1 part in 10,000, 10 days' storage time is required.
IXDUSTRIAL A S D ENGINEERING CHEMISTRY
340
TABLE11. Solution NO.
Time
Korrnality
~ A R I A T I O SI N SOLCTIOK I\;ORMALITY DURISG STORAGE F O R Total Normality Change in
Days 1
0 19
S5 134 235
4
5
0 30 76 183 0 18 87 134 237 0 20 52 94 143 246 0 18 87
0.10080 0.10050 0.09985 0.09961 0.09874 0.03147 0.05110 0.05103 0.05048 0.07082 0.07050 0.06974 0.06958 0.06850 0,08406 0,08363 0,08313 0.08230 0,08187 0.01958 0.07020 0,06996 0.06971
.....
0.00030 0.00095 0.00119 0.00206
:
0 00037 0.00044 0.00099
.....
0.00032 0.00106 0.00124 0.00232 .
.
I
.
.
0.00043 0 00093 0.00156 0.00219 0.00448
.....
0.00034 0.00049
Change
Av. perChange Day
%
% o:oi6
0:io 0.94 1.18
0,008
2.04
0.008
0.011
Solution So.
Time
5 (cont’dj
Days 134 237
6
o:j2 0.85 1.94
0: 024 0.011 0.011
7
0:i5
1.56 1.75 3.26
0 ,’ ii,j 0.01s 0.013 0.014
8
0: il 1.71 1.86 2.60 5.33
0:0i5
9
0.021 0,020 0.018 0,021
0:is 0.70
0:027 0.008
Hexanitrato ammonium cerate in 1.0 to 3.0 S perchloric or nitric acid made u p the ten most stable solutions 1%-ithout much preference for one or the other. Solutions of perchlorat0 ceric acid in perchloric acid were the least stableobviously a result of their higher potential as compared to solutions containing nitrate. This potential difference amounts to 0.2 volt or more in some cases. I n every case the stability increased with time. This is to be expected, since the potential of each solution falls as cerous ions accumulate. All of the first ten solutions had a very satisfactory stability. The tenth solution in the order of stability would require restandardization only every 4 days on the average before a difference of 1 part in 10,000 could be detected. Solutions of perchlorato ceric acid in 8 A- perchloric acid, the least stable solution of Table I, would require standardization each day when freshly prepared and once every 4 days when 4 months old to keep values within a maximum variation of 1 part in 10,000, if stored in diffused daylight. The oxidation potential of perchlorato ceric acid in 8 N perchloric acid, when oxidized to as great an extent as possible, is over 2.0 volts. This value rapidly falls as the cerous ion accumulates. Stability increased fourfold when 10 per cent of the perchlorato cerate ions were reduced to cerous ions. The study shows no advantage in the elimination of the last 5 to 10 per cent of the associated rare earth elements such as lanthanum, neodymium, and praseodymium. Four of the eight most stable solutions were prepared from commercial grades of materials, in which were present a few per cent of the cerous ion. I n most of these cases the solutions were stored in diffused daylight and their stability could be enhanced by storage in the dark.
Influence of Nitric and Perchloric Acids The use of tetravalent cerium in volumetric analysis in the absence of hydrochloric or sulfuric acids, xhich are ordinarily present, and in the presence of nitric or perchloric acids, improves conditions of analysis. The potentials attainable are from 0.16 to 0.42 volt higher. The determination of iron. arsenic, oxalate ( 6 ) , and many other frequently determined ions is better carried out using the solutions which have been described in this work. If a lower oxidation potential is in demand, the addition of the sulfate ion immediately converts the nitrato and perchlorato cerate ions to the sulfato cerate ion with an abrupt drop in potential (4). A greater lowering of the potential results from the addition of hydrochloric acid.
EXTESDED TIMEPERIODS
Kormality
0.06946 0.06903 0.05333 0.0528s 0,03264
Total Normality Change in
0.00074 0.001 17
.....
0 27 75 179
0.06084
0.00045 0.00069 0.00249
0 13 63 167
0.08642 0 05607 0 05554 0.05377
0.00035 0.00088 0.00265
0 2s 76 181 0 21 91 241
10
VOL. 12, NO. 6
0 22 92 242
0.04274
0.04223 0.04208 0.03938 0.007143 0.007077 0.006990 0.006755 0.01132 0.01119 0.01088 0.01071
.....
.....
0.00049 0.00066 0.00336
.....
0.000066 0.000283 0.000368
....,
0.00013 0.00044 0.00061
Change
Av. perChange Day
a
?A ._
1.05 1.69
0.00s
0.007
1.29 4.67
0:84
o:i31 0.017 0.026
0 : 62 1.36 4.72
o:oi1 0,025 0.028
i:i4 1.52 7.86
0:041 0.020 0.043
0:iz 3.54 5.42
0 : 644 0.039 0.022
i:i5
0:oiz 0.042 0.022
3.89 5.39
Using hexanitrato ammonium cerate as solute, but a small concentration of nitrate ions results upon acidification with perchloric acid. Consequently, the potential of such solutions is but slightly less than that of the perchlorato cerate ion in perchloric acid. Both the nitric and perchloric acid solutions of the corresponding cerate ions are suitable for oxidation of the oxalate ions (6) a t room temperature. Ferroin or nitroferroin may be used as indicator, preferably the latter. All solutions when heated showed marked decrease in stability. Because of the high potential of such solutions this is not important, since all reactions are rapidly completed a t ordinary temperatures.
Summary Eighteen solutions of the nitrato and perchlorato cerate ions in various concentrations of nitric and perchloric acid were prepared and their stability under various conditions and for extended time intervals was determined. A small but detectable photochemical decomposition was observed in the case of solutions stored in diffused daylight. Sitric and perchloric acid solutions of hexanitrato ammonium cerate between 1 and 3 molar in nitric and perchloric acids were found to show the highest stability. Perchloric acid solutions of perchlorato ceric acid required standardization a t frequent intervals. The stability of all solutions increased with time, showing the influence of the accumulation of cerous ions in lowering the oxidation potentials, 5 to 10 per cent increasing the stability two- and threefold. Five to 10 per cent of other rare earths of the cerium group present as impurities did not affect the stability adversely. A 0.1 N solution of commercial hexanitrato ammonium cerate in 1.0 A- perchloric acid was sufficiently stable in ordinary light not to require restandardization for a period of 20 days. Literature Cited (1) Brennecke, E., Furman, N. H., Stamm, H., Lang, R., and Fajans, K., tr. by Oesper, “Xerrer Methods of Volumetric
Chemical Analysis”. pp. 27-52, New York, D. Van Nostrand Co., 1938. (2) Hengstenberger, “Electrolytic Oxidation of Cerous Salts and Chlorination Experiments in Contact with Cerous Chloride”, Munich, Germany, J. Fuller, 1914. ( 3 ) Smith, Frank, and Kott. IND. ENG. CHEN.,Anal. Ed., 12, 268 (1940). (4) Smith and Gete. Ibid., 10, 191 (1938). ( 5 ) Ibid., 10, 304 (1938). (6) Smith, Sullivan, and Frank, Ibid., 8, 449 (1936).
