971
V O L U M E 2 5 , NO. 6, J U N E 1 9 5 3 2. The contents of the receiver were again distilled into boric acid or standard acid solution. The Eastern Regional Research Laboratory, United States Department of Agriculture, as well as a number of other laboratories, repeated the above experiment and obtained comparable errors. C . L. Ogg, Eastern Regional Research Laboratory, concluded that the errors were caused by alkali carried over during the first distillation. Shedd (6) also reported trouble with copper sulfate and shomd that the error could be reduced by precipitating the copper with polysulfide. Several simple experiments proved that as little as 0.01 gram of copper sulfate in the distillation flask greatly increased the volume of hydrogen generated by the zinc during a normal distillation. That sodium hydroxide caused the high titrations was proved by receiving the ammonia in distilled water, boiling to dryness, and titrating the residue. Solutions of the residue were also compared with respect to sodium content using a flame photometer. As even the most efficient type of scrubber connecting bulbs permitted large errors in some cases, certain precautions should be taken when copper sulfate is employed as a catalyst. A suitable eubstitute for zinc should be used to prevent bumping during distillation, or the minimum weight of zinc should be used, with a thiosulfate or sulfide precipitation of the copper. By replacing potassium sulfate with dipotassium phosphate, it n as possible to obtain complete nitrogen recovery from nicotinic acid in only 6 minutes’ total digestion time. However, comparisons of the two salts in the digestion of feeds and feeding concentrates brought out practical objections to the use of the phosphate. The flasks were etched and low results were occasionally obtained. The nitrogen losses probably were caused by the temperature, which frequently exceeded 400” C. 4 s the digestion in the proposed method is extremely rapid, some concern was felt over the possibility of nitrogen losses caused by accidental continuation of the afterboil. Evidence that a considerable safety factor exists is furnished by the following study, carried out on samples of tankage digested by both gas and electric heat following the heating conditions recommended by
the procedure. The nitrogen recovery was still 100% after a total digestion time of 60 minutes. However, after 90- and 120-minute digestion times the nitrogen recovery fell to 99 and 98%, respectively. In laboratories that do not possess regular Kjeldahl digestion apparatus the digestion may be conducted in narrow-necked 500ml. Erlenmeyer flasks placed in a fume cupboard. Each flask is fitted with a 3-inch funnel which returns condensed acid and prevents mechanical loss. The heating schedule of the proposed procedure is followed approximately. Somewhat greater care is used to avoid overheating during the early stages of the reaction, The proposed method has already replaced conventional Kjeldah1 procedures in several large feed analysis laboratories. ACKNOWLEDGMENT
Very helpful criticisms and suggestions were received from C. 0. Willits and C. L. Ogg, Eastern Regional Research Laboratory, U. S. Department of Agriculture, and from H. -4.Davis, University of New Hampshire. The author also wifihes to thank Ralph Gibson and Donald Sinclair of Canada Packers’ research division for their assistance in testing the method on a variety of materials. LITERATURE CITED
(1) Assoc. of Offic. Agr. Chemists, “Methods of Analysis,” p. 2.24.
1950. (2) Gerritz, H. W., and St. John, J. L., IND.ENG.CHEM.,ANAL.ED., 7, 380 (1935). (3) Lake, G. R., McCutchan, Philip, Van Meter, Robin, and Keel, J. C., ANAL.CHEM.,23, 1634 (1951). (4) Lauro, M. F., IND. ENG.CHEM.,ANAL.ED.,3, 401 (1931). (5) Ogg, C. L., Brand, R. W., and Willits, C. O., J . Assoc. Ofic.A g t . Chemists, 31, 663 (1948). (6) Shedd, 0. M., Ibid., 10,507 (1927). (7) Stubblefield, F. M., and DeTurk, E. E., IND.ERG. CHEM.. ANAL.ED., 12, 396 (1940). (8) White, L. M., and Long, M. C., ANAL.CHEM.,23, 363 (1951). (9) Willits, C. O., Coe, hl. R., and Ogg, C. L., J . Assoc. O f i c . Agr. Chemists, 32, 118 (1949). RECEIVED for review .4prll 7,
1952. Bccepted December 10,1952.
Ultraviolet Spectrophotometric Determination of Cerium GEORGE TELEP’ A N D D. F. BOLTZ Wayne University, Detroit 1 , Mich.
vv
a dilute solution of cerium(II1) is treated with a concentrated solution of potassium carbonate, a white precipitate is formed which dissolves in an excess of the reagent. The resulting cerous complex is presumably oxidized by the oxygen in the air to give a yellow ceric complex. Plank ( 4 ) found that a turbidity developed if hydrogen peroxide was used to facilitate the oxidation. Therefore, Plank preferred to oxidize with a stream of pure oxygen for 10 to 15 minutes. Although his work showed the applicability of this colored complex to the colorimetric determination of cerium, the method has found little use. This study mas undertaken to determine the ultraviolet absorption spectrum of this cerium complex and to develop a spectrophotometric method for determining small amounts of cerium. HEN
APPARATUS AND SOLUTIONS
A Beckman Model DU spectrophotometer with 1.00-em. silica cells was used for all the absorbancy measurements. A hydrogen discharge tube was used for measurements taken from 220 to 400 mfi and a tungsten filament lamp for the region from 400 to 500 m/r. A glass electrode was used for all pH measurements. A standard cerium(II1) solution was prepared by dissolving Present address, E. I. du P o n t de Nemours & Co., Inc., Dacron Division, Kmston, K. C. 1
0.3130 gram of the hydrated cerous perchlorate in redistilled water and diluting to 1 liter. This solution was standardized using the gravimetric procedure of Brinton and James (I), according to which the cerium was initially precipitated as the iodate, converted to the oxalate, and then ignited to the oxide in a platinum crucible. One milliliter of this solution contained 0.086 mg. of cerium(II1). The hydrogen peroxide used was 3% and 28% analytical reagent grade. The potassium carbonate (100 grams/100 ml.) solution was prepared from the reagent grade salt dissolved in redistilled water. FUNDAMENTAL REACTION
9 dilute solution of cerium(III), when made basic with a concentrated solution of potassium carbonate and treated with hydrogen peroxide, gives an intense yellow complex. This complex exhibits maximum absorbancy in the ultraviolet region of the spectrum. The formula of the yellow complex is unknown, and an investigation is in progress to determine the exact constitution of this complex. EXPERIMENTAL
Cerium Concentration. The absorption spectra for several concentrations of cerium were determined under two sets of conditions with the position of the absorbancy maximum being
972 dependent upon the carbonate concentration and the pH. Conformity to Beer’s law in a concentration range 0 to 30 p.p.m. was found at either 304 mp or 320 mp. The characteristic absorbancy maximum at 304 mp is shown in Figure 1 when the recommended carbonate concentration is used and the solution is adjusted to a pH 10.1 to 10.5. The molar absorbancy index a t 304 mp is 5.38 X lo3and at 320 mp is 4.38 X 103. The optimum concentration is 3 to 30 p.p.m. of cerium, using 1-em. absorption cells. Carbonate Concentration. In order to deter-
mine the effect of carbonate concentration, 8.6 p.p.m. of cerium were used with 1 ml. of 3y0 hydrogen peroxide, and variable amounts of the potassium carbonate solution were utilized a t all times. The final pH was above 11 in all these m e a s u r e m e n t s . Each solution was compared against a reagent blank. Table I shows the effect of carbonate concentration upon t h e wave l e n g t h of m a x i m u m absorbancy. The effect of the carbonate concentration is small for 5 to 30 ml. of WAVE LENGTH, rnp the concentrated soluFigure 1. Absorption Spectra tion per 50 ml. but is of Cerium Complex more pronounced as the P.p.m. of cerium: 1 = 25.8; 2 = 17.2; 3 = 8.6; and 4 = 4.3 masimum amount of carbonate is approached. -_ An actual shift of the wave length of absorbancy maximum to shorter wave lengths is observed. Although the absorbancy maximum a t 320 mp is satisfactory, the formation of a slight white precipitate within 1 hour after complexation was encountered. It should be pointed out that an absorbancy maximum has been found at 320 mp for cerium (IT)in acidic solutions (2, 3). The formation of the precipitate was more rapid with concentrations larger than 10 p.p.m. of cerium. Inasmuch as this precipitate was formed upon standing in a strongly basic solution, it seemed desirable to establish whether there was a pH range in which maximum stability could he attained. Effect of pH. In adjusting the pH, 25 ml. of the concentrated potassium carbonate solution were added and then partially neutralized with an acid. The use of a sulfuric acid solution is not recommended due to the low solubility of the potassium sulfate which is formed. d 6 N hydrochloric acid solution was used, since the solubility of potassium chloride is much larger. In order to study the effect of pH on the complev formation, the following procedure was used. A definite amount of the standard cerium solution (8.6 p.p.m.) was added by means of a microburet to a 50-ml. volumetric flask. Exactly 25 ml. of the potassium carbonate solution were added to the flask from a buret. Variable amounts of the 6 N hydrochloric acid were added from a microburet, very slowly to prevent excessive frothing. The contents of the flask were diluted t o the mark, and after mixing thoroughly, 1 ml. of 3% hydrogen peroxide was added from a microburet. The complexation is immediate, and absorbancy measurements were taken a t 304 mp. The pH measurements were made after the absorbancy measurements. A reagent blank was used in the reference cell.
Table I1 shows the effect of pH on the absorption spectrum. These data indicate that the optimum p H range is 10.1 to 10.5. Therefore] it was decided to utilize a mixture of 25 ml. of potas-
ANALYTICAL CHEMISTRY Table I.
Effect of Potassium Carbonate Concentration
Concn. oi KtCOs, Grams/SO 111.
Wave Length of Absorbancy Maximum,
Absorbancy a t Maximum 0.275 0.280 0.282 0.281 0.273 0.269 0.250 0.244
hfI.’ 320 320 320 320 320 320 278 278
5
10 1 ;7
20 25 30 40 Max. amt.
Table 11. Effect of pH PH 11.32 10 93 10.71 10.59 10.42 10.35 10.11
Wave Length of Maximum Absorbancy,
Absorbancy a t 304 M p 0.2fio 0.290 0.325 0.334 0.344 0.344 0.345
M p
320 320 304 304 304 304 304
sium carbonate solution and 10 ml. of 6 3- hydrochloric acid solution in a final volume of 50 ml. to give a pH of 10.35. Stability. The instability of the complex was quite pronounced for solutions exceeding 10 p.p.m. of cerium when using 25 ml. of potassium carbonate solution in a final volume of 50 ml. However, a t a pH of 10.35, the solutions were stable for 24 hours with no evidence of either a precipitate or a turbidity. Solutions containing as high as 30 p.p.m. of cerium did not give a precipitate when the pH was properly adjusted. Hydrogen Peroxide Concentration. The effect of various concentrations of hydrogen peroxide was studied using 8.6 p.p.m. of cerium. I t was found that with 0.5 to 5.0 ml. of 3y0 hydrogen peroxide, maximum complexation was obtained, the absorbancy being measured at 304 mp. Because the commercial 370hydrogen peroxide gives a slight yellow color in basic solutions due to the presence of acetanilide, a preservative, a 2870 solution of hydrogen peroxide was also used. It was found that with 0.1 to 0.5 ml. of the %yosolution there was slight effect upon the maximum absorbancy values. The 3Y0 solution is favored because there is less bubble formation in the absorption cells.
Table 111. Interfering Diverse Ions Ion
ildded As
Amount Added. P.P.M. 2 13 450 10 10 37 10 23 10 500 500 5fi 100 100
‘70
Error 17.4 0.0 11.0 68.6 23.5 71.8 13.9 0.0 15.1 19.7 16.3 57.0 42.4 13.9
Permissible Amount.
P.P.M. 0 13 90 0 0 0 0 23 0 10 10
0 0 5
Effect of Diverse Ions. The effect of diverse ions was studied using 8.6 p.p.m. of cerium and a mixture of 25 ml. of potassium carbonate and 10 ml. 6 N hydrochloric acid. A reagent blank, containing 1.0 ml. of 370 hydrogen peroxide and the same carbonate-hydrochloric acid mixture, is freshly prepared and used in the reference cell for all measurements. The absorbancy measurements were taken a t 304 mpin order to ascertain any deleterious effects of the various ions. A negligible error was obtained with 1000 p.p.m. of the acetate, arsenate, bromate, bromide, bisulfate, chloride, chlorate, citrate, iodate, lithium, stannate, tartrate, thiosulfate, tetraborate, and sodium. Ions interfering because they form a precipitate or a turbidity with the
V O L U M E 25, NO. 6, J U N E 1 9 5 3 carbonatehydrochloric acid mixture are aluminum, barium, calcium, cadmium, plumbous, manganous, silver, strontium, and zinc. Table I11 lists the other ions and their effect upon the absorbancy values in per cent error of the desired constituent when 8.6 p.p.m. of cerium are present. The permissible amount is estimated to give an error of less than 2.5’7c. RECOMMENDED GENERAL PROCEDURE
Procure a representative sample of the material and n-eigh, or measure by volume, a sample containing an amount of cerous ions such that the final solution contains not more than 0.10 mg. of cerium per milliliter of solution. Make this solution just acidic to litmus with dilute hydrochloric acid and dilute to a definite volume in a volumetric flask. Transfer a suitable aliquot of this prepared solution by means of a microburet to a 50-ml. volumetric flask. Add 25 ml. of the potassium carbonate solution from a buret and carefully add sufficient 6 X hydrochloric acid solution (approximately 10 ml.) to bring the pH of the resulting solution between 10.1 and 10.5. Dilute to the mark with redistilled water, add 1 ml. of 3% hydrogen peroxide, and mix thoroughly. Use a reagent blank in the reference cell and measure the absorbancy a t 304 mp in 1.000-cm. silica cells. DISCUSSION
This spectrophotometric study has shown that the cerium complex as formed n-ith hydrogen peroxide in a concentrated potassium carbonate solution has an absorbancy maximum in the ultraviolet region. The wave length of maximum absorbancy can be obtained a t 304 mp and a t 320 mp, depending upon the carbonate concentration and the final pH of the resulting solution. There is a marked increase in sensitivity when using the absorbancy maximum at 304 mp as compared to that a t 320 mp. The stability
973 of solutions containing more than 10 p.p.m. of cerium is increased when the p H of the resulting solution is maintained between 10.1 and 10.5 The main interferences are these ions which impart a yellow color to the solution, namely, uranyl, ferrocyanide, chromic, chromate, and vanadate. Ferric, tungstate, molybdate, nitrate, and titanic ions also interfere in the ultraviolet region. Some of these interferences probably could be minimized by utilizing a compensating blank solution. The procedure as developed is applicable to the determination of small amounts of cerium. An indication of the precision of the recommended procedure was ascertained from the results obtained on 8 samples, each containing 17.2 p.p.m. of cerium. These samples which were analyzed on 3 different days gave a mean absorbancy value of 0.669 with a standard deviation of 0.006, or 0.9%. The occasional formation of gas bubbles on the absorption cell windows will introduce an error unless the bubbles are removed prior to spectrophotometric measurement by gently tapping the absorption cells. The main advantage of this method over the acidic solution-persulfate oxidation procedure is that it eliminates the interferences due to nitrate ions produced by Oxidation of any ammonium ions and to the residual persulfate ions. LITERATURE CITED
(1) Brinton, P. D., and James, C., J . Am. Chem. SOC., 41, 1081 (1919). (2) Freedman, A. J., and Hume, D. K.,ANAL.CHEM.,22, 932 (1950). (3) Rledalia, A. I., and Byrns, B. J., Ibid.,23,453 (1951). (4) Plank, J., Z.anal. Chem., 116, 312-15 (1939). RECEIVED for review .4pril 14, 1952. Accepted J a n u a r y 31, 1953. Presented before the Division of Analytical Chemistry at the 1 2 1 s t Meeting of the AMERICAN CHEMICAL SOCIETY, Buffalo, S e w York.
Determination of Ammonia in Hydrazine JOHN E. DEVRIES AND E. S T . CLAIR GANTZ ..lnalytical Chemistry Branch, U . S. lTaval Ordnance Test Station, China Lake, Calif, ( 2 , 7 , 8, 10) for determining ammonia in F hydrazinemethods were not satisfactory for the types of hydrazine XISTING
J
mixtures encountered in this laboratory. Interferences in these methods occurred when such materials as guanidines, and alkyl and aryl hydrazines were present. The Jamieson iodate method ( 1 ) is very suitable as a titrimetric method of analysis for hydrazine and for compounds containing hydrazino groups (3,6,6,8,9). Since these oxidations by iodate yield nitrogen it appeared feasible to determine ammonia in mixtures of hydrazino compounds by use of such oxidation as a preliminary step. The stoichiometry of the general reaction is represented in the form NaHsf
+
108-
+ HaO+ + C1- +Ni + IC1 + 4Hz0
A simple additional step was necessary for removal of the iodine monochloride and excess iodate. This was conveniently accomplished by adding crystals of sodium sulfite until the solution was colorless, followed by air bubbling for removal of excess sulfite. An ordinary Kjeldahl distillation of ammonia was used to complete the determination. The method thus developed is conveniently incorporated into the procedure for titrimetrically determining hydrazines. One sample serves first for the iodate hydrazine titration and then for the ammonia determination. REAGENTS AND APPARATUS
Standard solutions are required in the Kjeldahl titration, 0.1 N hydrochloric acid and 0.05 N sodium hydroxide solutions being recommended. Methyl red was used as indicator. Approximately 0.1 N potassium iodate solution, 10 N potassium hyroxide solution, and 0.2% aqueous amaranth indicator solutions are required. All other chemicals should be C.P. quality selected for low ammonia content.
The usual Kjeldahl apparatus is used. A gas bubbler containing dilute hydrochloric acid and glass tubing with fritted-glass tip are recommended for the air bubbling operation. PROCEDURE
Select a sample of such size that an aliquot will furnish a maximum of about 0.5 to 0.6 meq. of ammonia (approximately 10 mg. of “8). Pipet this quantity into a flask containing 25 ml. of 12 N hydrochloric acid plus enough water so that the final volume (acid plus water plus sample) will be 70 ml. Immediately add from a buret approximately 0.1 N potassium iodate solution until the color of the solution has progressed through a deep brown to a light yellow hue. Then add 2 drops of amaranth indicator and complete the titration to the disappearance of the pink color. Add sodium sulfite crystals until the solution becomes colorless. Bubble air which is free of ammonia through the solution for 15 minutes to remove excess sulfur dioxide. Transfer the solution to a Kjeldahl flask; rinse several times with distilled water; and add 50 ml. of 10 N potassium hydroxide. Complete the usual Kjeldahl distillation, receiving the distillate in 20 ml. of 0.1 N acid. Conclude the determination by titrating the standard acid plus distillate with 0.05 A- alkali, using methyl red indicator. The acidity must be carefully controlled for best results in the Jamieson procedure. If the titration requires more than 40 ml. of iodate solution as described above, an additional 5 ml. of hydrochloric acid should be added. The final acid normality should be 3 to 5 (48)to ensure quantitative reaction and a sharp end point. RESULTS
The results of the analyses of several synthetic samples are shown in Table I. The three series of samples were made using aliquots from a standard ammonia solution for which the titer had been determined both by direct titration and by the Kjel-
