Determination of Cerium and Chromium in Cerium-Chromiu m-U ranium Mixtures CHARLES V. BANKS and JEROME W. O’LAUGHLIN Institute for Atomic Research and Department o f Chemistry, lowa State College, Amos, lowa
A procedure is described for the determination of cerium and chromium in cerium-uranium-chromium alloys. Chromium is determined (on an aliquot of the sample) titrimetrically after it has been preferentially oxidized to chromium(\-I) with fuming perchloric acid. Cerium is not oxidized by fuming perchloric acid under the conditions used. Cerium and chromium are determined on another aliquot of the sample by titrating with iron(I1) after complete oxidation of cerium and chromium with peroxydisulfuricacid in the presence of silver(1). ..i reason is suggested for the low results often obtained for chromium following the perchloric acid oxidation, and conditions are described for the quantitati\ e oxidation of chromium with perchloric acid.
C
E R I U M and chromium are commonly determined titrimetrically by first oxidizing them to cerium( IV) and chromium(VI), then titrating either directly or indirectly Tvit,h a standard reducing agent. The oxidation methods usually employed, such as a sodium peroxide fusion or oxidation in acid solution with sodium bismuthate or ammonium peroxydisulfate in the presence of silver nitrate (S),result in the oxidation of both cerium and chromium. Perchloric acid has been used to oxidize chromium, but does not oxidize cerium unless mixed lyith sulfuric ok phosphoric acid (12). It, would seem possible then to use the peroxydisulfate-silver nitrate oxidation to determine the total amount of cerium and chromium, and the perchloric acid oxidation to determine the chromium alone. Cerium could be determined by difference. The original T-on Knorre ( 1 5 ) pproxydisulfate method for the oxidation of cerium r a s not, completely satisfactory, but since the discovery of the catalytic effect of silver(1) by Barbieri ( I ) ,it has given excellent results ( 2 9 ) . Chromium is also quantitatively oxidized by ammonium peroxydisulfate and silver( I ) under the same conditions (9). Willard (16) first reported the oxidation of chromium rrith perchloric acid, and Lichtin ( 6 )and James ( 5 ) later recommended perchloric acid oxidation in the determination of chromium. Killard and Gibson ( 1 7 ) studied this oxidation, observing that manganese v a s not oxidized unless phosphoric acid was present. These workers reported that decomposition of the perchloric acid gave chlorine and oxygen 4 HClO4 --+ 2C12
+ 7 0 2 + 2H2O
rather than oxides of chlorine as reported by Lichtin (6),and that low results vere obtained for chromium if the oxidized chromium solut,ion stood for a n-hile before the chroniiuni x a s titrated. Lundell, Hoffman, and Bright (‘7) in a study of perchloric acid oxidation of chromium reported that results m-ere invariably low for chromium, but x i t h special precautions the amount of chromium oxidized could be increased. Smith ( l b ) attributed the apparently incomplete oxidation of chromium to the simultaneous oxidizing and reducing properties of hot concentrated perchloric acid: 4HClOa --+ 2C12 2HC104 +Clz
+ 702 + 2H20 + 30: + HzOz
Smith (Id)reported that the first reaction \vas by far the more important, but that the second occurred simultaneously and the hydrogen peroxide formed by the react,ion reduced part of the chromium. Hoffman and Lundell(4) reported that chromium is volatilized as chroniyl chloride, even if no chloride is present in the solution initially, and that the loss of chromyl chloride is probably the chief cause of the low results found for chromium after a perchloric acid oxidation. Schuldiner and Clardy (11) and Ewing and Banks ( 2 ) published similar procedures; the chromium was oxidized in a flask equipped with a distillation head and any chromyl chloride volatilized \vas returned to the original solution before titration. They claimed quantitative recovery of chromium using this procedure. Roger (IO) reported that, in experiments with the perchloric acid method, chromium was not lost as chromyl chloride and the results for chromium were not influenced by the formation of oxygenated compounds. Roger (10) claimed the method is rapid and gives reproducible results. Chemists of the U. S. Steel Corp. ( 1 4 ) described a procedure for the oxidation of chromium with perchloric acid, which they claimed 77-ould give reproducible results. They made no provision for the loss of chromyl chloride, but cooled the solution immediately after oxidation as recommended by Smith and added silver nitrate to precipitate any chloride in the solution. Recently Lynn and Mason (8) published a procedure for the oxidation of small amounts of chromium with a mixed perchloric acidsulfuric acid-silver nitrat,e reagent. I n this investigation Conditions \\-ere found with which small amounts of chromium could be quantitatively oxidized with hot concentrated perchloric acid in the presence of uranium and cerium. Cerium \vas not oxidized. The loss of chromium as chroniyl chloride mas negligible \Then the recommended procedure \vas followed. The l o x results often found for chroniium after a perchloric acid oxidation may be diie to reduction of chromium by the chloride ion. REAGENTS
Ferroin Indicator, Fe(ClzH&2)3S041 0.025M. The ferroin indicator solution v a s prepared by dissolving 3.714 grams of 1,lO-phenanthroline 1-hydrate (G. Frederick Smith reagent grade) and 1.737 grams of reagent grade iron(I1) sulfate 7hydrate in water and diluting to 250 ml. A 0.OOlM solution was prepared by dilut,ing a 10-ml. aliquot to 250 ml. Iron(I1) Sulfate, O.lOLV. An iron(I1) sulfate solution was prepared by dissolving 39.5 grams of reagent grade iron(I1) ammonium sulfate 7-hydrate and 10 ml. of concentrated reagent grade sulfuric acid in v-ater and diluting to 1 liter. The solut’ion was standardized each day against standard ceric sulfate. Potassium Dichromate Solution. A solution \vas prepared bl? dissolving 18.9132 grams of primary standard grade potassium dichromate (National Bureau of St,andards, sample 136) in a small amount of nitric acid, transferring to a tared 2-liter volumet,ric flask, and diluting to approximately 2 liters. The weight of the solution was 2028.29 grams and it contained 3.2977 mg. of chromium per gram of solution. Cerium(1V) Sulfate Solution, 0.1N. A stock solution of cerium(1V) sulfate was prepared by dissolving 518.26 grams of reagent grade ammonium hexanitratocerate(1V) in 560 ml. of concentrated sulfuric acid and slowly diluting with vigorous stirring t,o 10 liters. The solution was standardized against electrolytic iron wire and arsenious oxide (hIallinckrodt primary standard) and found to be 0.1090S. Cerium(1V) Nitrate Solution. -4 solution was prepared by dissolving 3T.834 grams of reagent grade ammonium hexanitrato1338
V O L U M E 28, N O , 8, A U G U S T 1 9 5 6 Table I.
Oxidation of Chromium with Perchloric Acid Cr Idded,
Procedure Used Willard and Gibson ( 1 ; ) Ewing a n d Banks ( 2 )
Proposed procedure
Table 11. U Added, 31g. 94.0 5.0 94.0
0.0 5 0 0 0 0 0
94 0
1339
Alg.
8 8 7 8 8 16 38 70
Cr Found, 3Ig.
115 260 6iO ROO 280 96 66 76
7 56
7.23 7.43 8.19 8.29 16.94 38.64 30 80
Error, 3Ig. -0 56 -1 03 -0.24 -0.41 +o 01 -0 02 -0 02 t o 04
Effect of Cerium and Uranium on Perchloric .4cid Oxidation of Chromium Ce Added, 3lg. 5 .0 9-1 0 1 0 0 0 80.0 0 0 0.0 40 0
Cr ddded, 31g. 1 50 1 64 -1 72 8 28 12 23 16 96 3 8 66 83 07
Cr Found, 3Ig. 1 49 1 64 4 31 8 29 12.27 16 94 38.64 82 97
Erior, xg.
-0.01 0.00 -0.01 0.01 0.04 -0.02 -0.02 -0.10
This may be accomplished more easily if the solution is contained in a beaker. For the oxidation the solution is transferred t o a 500-nil. Erlenmeyer flask, 10 ml. of i27, perchloric acid are added, and the solution is placed on the hot plate and heated until the perchloric acid begins t o fnme. A refluxing still head is placed on the neck of the Erlenmeyer flask and the solution is fumed 3 t o 5 minutes after the conversion of chromium(III)> green, to chromium(VI), red, is complete. The solution is removed from the hot plate, immediately placcrl under a hot water t a p t,o start the cooling, and cooled under the cold water tap. [If Vycor glassware is used, the hot flasks may be immediately plunged into an ice-water mixture for even more rapid cooling ( I S ) . ] The cool sample is diluted as quickly as possible to 100 t o 150 ml. and 2 to 3 grams of sodium bicarbonate are added. The solution is again placed on the hot plate and boiled for 10 t o 15 minutes until moist starch-iodide paper held over the flask no longer gives a chlorine test. The samples are cooled and 5 ml. of concentrated sulfuric acid are added. The chromium(V1) is determined by adding a known excess of standard iron(I1) sulfate and 1 ml. of 0.001M ferroin indicator. The excess iron(I1) sulfate is back-titrated with itandard reriuni (IT‘) sulfate. DETERMINATION OF CERIUZI
.ln aliquot of the uranium-chromium-cerium sample is pipettcd cerate(1V) (G. Frederick Smith Chemical Co.) to 2 liters with 1IM nitric acid. This solution was compared with the standard cerium(1V) sulfate solution and found t o contain 4.257 mg. of cerium per ml. Perchloric Acid, reagent glade, 70 t o 7270 HClO,. Uranium(V1) Nitrate Solution, UO,(SO,),. A stock solution 11as prepared by dissolving 10.0069 grams of primary standard grade uranium oxide, U308, in concentrated nitric acid and diluting t o approximately 2 liters in a tared flask. The weight of the solution was found to be 2000.38 grams. The uranium solution was compared with the standard cerium( IV) sulfate and was found to contain 4.242 mg. of uranium per gram of solution. This agrees well with the calculated value of 4.242 mg. of uranium per gram of solution. Sodium bicarbonate, reagent grade. Sulfuric acid, reagent grade, sperific gravity 1.84, 987, H,SO,. S i t r i c acid, reagent grade, specific gravity 1.42, 705, HSOs. EXPERIM EYTA L
Samples of the standard rhromium solution were weighed into 250-ml. Erlenmeyer flasks using weight burets. The chromium(V1) was reduced by adding 1 t o 2 drops of 30yc hydrogen peroxide. The excess peroxide was destroyed by boiling a few minutes. The chromium samples were oxidized using the procedures recommended by Willard and Gibson ( I Y ) , Ewing and Banks ( d ) , and the proposed procedure. I n all cases the chromium( V I ) present after oxidation mas determined by adding a known excess of iron(I1) sulfate and back-titrating with standard cerium(1V) sulfate t o the ferroin end point. The results obtained are tabulated in Table I. The effects of cerium and uranium on the perchloric acid oxidation of chromium were studied by adding known amounts of standard cerium and uranium solutions t o the chromium samples before oxidation. The cerium was added as cerium(IV), but was reduced to a cerium(II1) by the hydrogen peroxide used to reduce the chromium(V1). The results obtained are given in Table 11. The total cerium and chromium in the presence of uranium was found by oxidizing cerium and chromium to cerium( IT’) and chromium(V1) with potassium peroxydisulfate and silver. This procedure has been used in this laboratory for some time for the determination of cerium and chromium in uranium with excellent results. T h e amount of cerium v-as calculated for two known samples by correcting the experimentally determined total for the amount of chromium added (Table 111). DETER%lINATIOY OF CHROMIURI
-4sample of the uranium-chromium-cerium alloy is weighed into a 400-ml. beaker and dissolved with dilute hydrochloiic or nitric acid. K h e n dissolution is complete, the solution is transferred to a volumetric flask and diluted to volume. .In aliquot containing 2 to 50 mg. of chromium is taken for analysis. If the sample was dissolved in hydrochloric acid, the sample is taken down several times with nitric t o remove the hydrochloric acid.
into a 600-ml. beaker and 5 to 10 ml. of concentrated sulfuric acid are added. T h e solution is evaporated until fumes of sulfur trioxide are evolved to remove all of the hydrochloric acid. The solution is cooled and diluted t o 250 ml. with water. .Ibont 0.05 gram of silver nitrate and 2 grams of potassium peroxydisulfate are added. The solution is boiled vigorously for about 20 minutes and the solution is removed from the hot plate and cooled. A known excess of standard iron(I1) sulfate and 1 ml. of 0.001X ferroin indicator are added. The excess iron(I1) sulfate is back-titrated with standard cerium(1V) sulfate. The amount of iron(I1) sulfate used is corrected for the amount required to titrate the chromium. T h e amount of chromium present is found by the perchloric acid oxidation procedure. The cerium is then Calculated by difference. DISCUSSION
The results obtained 13)- the reroniniended procedure indicate that chromium can be quantitatively oxidized by perchloric acid. The necessary conditions, as found by the authors and reported by others ( I d , 14,I 8 ) , seem to be the rapid cooling of the sample after oxidation, and the immediate dilntion of the sample n-heii cooled sufficiently.
Table 111. Determination of Cerium b>-Difference after Peroxydisulfate-Silver Oxidation of Cerium-Chromium Total Cr .Idded,
U Added,
31g.
AIg.
Ce Added, .\I&
Ce Found, hlg.
Error, hlg.
5 02 1 7 ,22
5.0 94.0
239.9 195.4
239,P 195.1
0.0 0.3
The authors believe the lo^ results often obtained for chromium iollowing a perchloric acid oxidation are caused by reduction of chromium(TT) by chloride ion in the concentrated perchloric acid medium. It can be readily demonstrated t,hat chloride ion xi11 reduce chroniium(V1) in concentrated perchloric acid. Furthermore, the presence of the chloride ion in chroniiuni samples after a perchloric acid oxidation has been observed by the authors and others (8, 14). The hydrogen peroside mechanism postulated by Smith (12) does n o t seem to explain the slov reduction of chromium(V1) in concentrated perchloric acid medium after the sample R i cooled. Hydrogen peroxide should not form in the cool sample. and, if any formed during the osidation of the chromium, it ~ ~ o n l t l be expected to react with the chromium(T’1) immediately. In the procedure rrconimended, the sample is immediiitely cooled and diluted after oxidation. Sodium bicarbonate is added, n-hich reduces the ucidity and helps sn-eep any chloriiir out of the solution bj- liberation of large quantities of carboil dioxide. This tends to reduce the amount of chloride fornit4
1340
ANALYTICAL CHEMISTRY
through disproportionation of chlorine and, by reducing the hydrogen ion concentration, prevents oxidation of any chloride already formed by shifting the chromium(V1) to chromium(II1) reduction potential t o a more negative value than the chlorine to chloride ion reduction potential. APPLICATIONS
The procedure described vas found to be generally useful for the determination of chromium and cerium in uranium-chromiumcerium alloys, giving equally good results for both uranium-rich and cerium-rich alloys. -4s cerium is determined by difference, it is desirable that the cerium-chromium ratio b r large. LITERATURE CITED
Barbieri, G . .I.A,t t i accad. Lincei 25, I, 37 (1916). Ewing, R. E., Banks, C. V., ANAL.CHEM.20,233 (1948). Hillebrand, IT. F.,Lundell, G. E., Bright, H. A,, Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., p. 527, Wiley, h-ew York, 19.53. (4) Hoffman. J. I., Lundell, G. E., J . Research il’atl. Bur. Standards 22,465 (1938).
(5) James, L. H., Chemist-A?dysl 19, S o . 5, 14 (1930).
Lichtin, J. J., ISD.EXG.CHEM.,A N ~ LED., . 2, 126-7 (1930). (7) Lundell, G. E., Hoffman, J. I., Bright, H. A., “Chemical Analysis of Iron and Steel,” pp. 298-300, Wiley, Kew York, 1931. (8) Lynn, S., Mason, D. AI., ANAL.CHEM.24, 1855 (19.52). (9) Rodden, C. J., “Analytical Chemistry of the Manhattan Project,” 1st ed., pp. 445-6, 1IcGraw-Hill, Sew York, 1950. (10) Roger. L., Fonderie 1951, 2359-62. (11) Schuldiner, S.,Clardy, F. B., ISD. ESG. CHEX., ANAL.ED., 18, 728-9 (1946). (12) Smith, G. F., I b i d . , 6,229 (1934). (13) Smith, G. F., private communication. (14) U. S. Steel Corp. chemists, “Sampling and Analysis of Carbon and Alloy Steels,” p. 141, Reinhold, New York, 1938. (15) 1-on Iinorre, G., Z . angew. Chem. 11, 717 (1897). (16) Willard, H. H., Cake, W.E., J . I n d . Eng. Chens. 11, 480 (1919). (17) Willard, H. H., Gibson, R. C.,I m . ESG. C H E x , ANAL.ED. 3, 88 (1931). (18) Willard, H. H., Young, P.. I b i d . , 6, 48 (1934). (19) Willard, H. H., Young, P., J , Am. Chem. SOC.50, 1379 (1928). (6)
RECEIVED f o r review March 7, 1956. Accepted M a y 17, 1956. Contribution 447. Work performed in the Ames Laboratory of the U. S. Atomio Energy Commission.
DeterminatioR of Traces of Gallium and Indium in Germanium and Germanium Dioxide C.
L. LUKE
and M A R Y E. C A M P B E L L
Bell Telephone Laboratories, Murray Hill,
N. J,
In the photometric determinalion of 1 to 10 p.p.m. of gallium or indium in germanium and germanium dioxide germanium is removed by distillation as chloride. Gallium in the residue is isolated by ether extraction from 6.V hydrochloric acid, followed by oxinechloroform extraction from alkaline cyanide solution, and is determined by photometric measurement of the yellow oxine-chloroform extract. Indium in the residue from the germanium distillation is isolated by dithizone-chloroform extraction from alkaline cyanide solution, followed by oxine-chloroform extraction from aqueous solution at pH 3.5, and is determined by photometric measurement of the yellow oxine-chloroform extract.
vented by adding a small amount of lead nitrate to the solution just before the oxine extraction; the precipitated lead molybdate remains in the aqueous phase. Interference due to thallium can be eliminated by reducing this metal to the monovalent state. On the other hand, no method of preventing interference of iron was found. For this reason the oxine extraction a t pH 3.5 was abandoned in favor of extraction from alkaline cyanide solution ( 2 ) . S o n e of the metals that accompany the gallium in the ether extraction will accompany it in an oxine extraction from alkaline cyanide solution. Thus by using these two extractions in proper sequence the gallium can be readily isolated and determined. Reagents. DISTILLEDWATER. I n the development of the methods for gallium and indium ordinary distilled water was assed through a mixed cation-anion exchange resin mixture efore use. STANDARD GALLIUMSOLUTION ( 5 y of gallium per ml.). Transfer 0.0134 gram of gallium oxide, Gaz03, to a 125-m1. Vycor conical flask. Add 5 ml. of hydrochloric acid, cover, and warm gently to dissolve. Cool, transfer to a 200-ml. volumetric flask, 1 j, and mix. dilute to the mark with cool hydrochloric acid (1 Pipet 50.0 ml. of this solution to a 500-ml. volumetric flask, dilute to the mark with cool hydrochloric acid (1 I), and mix. ACID-WASHED ETHER. Transfer 200 ml. of anhydrous reagent grade ether to a 500-ml. separatory funnel. Add 30 ml. of hydro1) in which about 50 mg. of sodium sulfite has chloric acid (1 been dissolved. Shake gently, opening the stopcock frequently, until excess pressure ceases to build up. Then shake vigorously for 30 seconds. Drain off and discard the acid layer. Repeat the washing with two more 30-ml. portions of hydrochloric acid (1 1) plus sodium sulfite. vi-CRESOLPURPLEISDICATOR SOLUTION (O.l’%J Dissolve 0.1 gram of m-cresol purple in 10 ml. of water containing 1 pellet of sodium hydroxide, by warming gently. Cool and dilute to 100 ml. v i t h water. SoDrubi CYANIDESOLUTION.Transfer 20 grams of sodium cyanide to a 500-ml. polyethylene bottle, add 200 ml. of m-ater, and swirl to dissolve. Keep stoppered when not in use. OXIKESOLUTIOS. Transfer 2.000 grams of 8-quinolinol to a clean dry 200-ml. volumetric flask. Dilute to the mark with chloroform and mix. Prepare fresh just before use. Preparation of Calibration Curve. Transfer 0-, 1.0-, 3.0-, 5.0-, and 7.0-ml. portions of standard gallium solution (5 y of gallium
E
T
HE present investigation represents a continuation of the authors’ vork on the development of photometric methods of determining traces of Group I11 and Group V metals in semiconductor materials (3,4). The methods described provide for the determination of 1 to 10 p , p . m ~of gallium and indium in germanium and germanium dioxide. Suitable separations are included for the elimination of possible interference from any of 59 commonly encountered metals. The apparatus recommended previously (4j was used in the present investigation, except that a Beckman Model B spectrophotometer was used. DETERMINATION OF GALLIUM
Moeller and Cohen ( 5 ) have shown that small amounts of gallium can be determined by extraction with a chloroform solution of oxine (8-quinolinol), a t p H 3.5, followed by photometric measurement of the yellow extract. Because this method is by no means specific for gallium, a preliminary isolation of the gallium by ether extraction r i l l be required in most work. Of the metals that accompany gallium into the ether layer, only molybdenum, thallium, and iron interfere in the oxine extraction a t pH 3.5. The interference due to molybdenum can be pre-
+ +
+
+
