320
ANALYTICAL CHEMISTRY
containing all reagents except the sulfite are obtained, when the directions given are adhered to closely. Larger amounts of more dilute solutions do not yield satisfactory results. It is suggested that in using the test both negative and positive control samples be run simultaneously with the unknowns. By negative control is meant a solution containing roughly the same amounts of thiosulfate, nitrite, etc., as the unknown, and by positive control is meant the same solution as the negative except that an amount of sulfite slightly above the threshold has been addedfor example, about 1 to 2 mg. of sulfite. By this means the distinction between solutions which contain sulfite and those which do not will be readily apparent. If the hydrochloric acid is added before the selenious acid to a solution containing thiosulfate but not sulfite, sulfite will be formed very rapidly from the thiosulfate and a false positive test will, therefore, be obtained. In the test as modified by the addition of sulfamic acid, the latter reagent effectively prevents the interference from nitrite. Without this reagent both positive and negative controls yield ungatisfactory results, the negative ones often rapidly yielding an orange color and the positive ones not showing appreciably more red color development than the negative. The qualitative tests for sulfite were developed to meet the specific need for such tests in connection with the analytical work on thiosulfate, nitrite, and sulfide solutions. They were based on the knowledge that: sel’enious acid reacts with sulfite to give a red color (selenium) and this reaction has been used in
the quantitative determination of sulfite (11) and that sulfamic acid is a reagent which reacts with nitrite (9). ACKNOWLEDGMENT
The author is grateful to E. St. Clair Ganta, Analytical Chemistry Branch head, for encouragement and suggestions in support of this work. This paper is published with the permission of F. W. Brown, technical director of the U. S. Naval Ordnance Test Station. LITERATURE CITED
(1) Bennett, H., and Harwood, H. F., Analyst, 60, 677 (1935). (2) Brasted, R. C., ANAL.CHEY.,24, 1111 (1952).
(3) Cool, R. D., and Coe, J. H., IND.ENG.CHEM.,SXAL. ED.,5 , 112 (1933). (4) Furman, N. H., ed., "Scott's Standard Methods of Analysis,” 5th ed., New York, D. Van Kostrand Co., 1939. (5) Griffin, R. C., ed., “Technical Methods of Analysis,” 2nd ed., New York, McGraw-Hill Book Co., 1927. (6) Khmelnitzkaya, I., and Verkhovskaya, A , Anilinokrasochnaya Prom., 4, 27 (1934). (7) Knowles, G., and Lowden, G. F., Analyst, 78, 159 (1953). (8) Merrow, R. T., Cristol, S. H., and Van Dolah, R. W., J . A m , Chem. SOC.,in press. (9) Pierson, R. H., AXAL.CHEM.,25, 1939 (1953). (10) Vinogrodov, A. V., and Dubova, A. O., Zavodskaya Lab., 11, 282 (1945). (11) Y o s t , D. bl., and Russell, H., “Systematic Inorganic Chemistry,” pp. 66, 332, S e w York, Prentice-Hall, Inc., 1946. RECEIVEDfor review January 31, 19.53. Accepted October 19, 1953. Presented in part before the Division of Analytical Chemistry a t the 123rd Meeting of the AMERICAN CHEMICAL SOCIETY, Los Angeles, Calif.
Accuracy of Determination of Hydrogen Peroxide by Cerate Oxidimetry EVERETT C. HURDIS and HENDRIK ROMEYN, JR. General Laboratories, United States Rubber Co., Parraic, N. J.
The accuracy of the cerate-hydrogen peroxide titration has been studied by comparison with a new absolute (gravimetric) method of hydrogen peroxide analysis. Results indicated a favorable comparison between the two methods. Additional information on the accuracy of the cerate-hydrogen peroxide titration was obtained by comparison between cerate and permanganate titrations. No significant difference in results by the cerate and permanganate methods was found. This further substantiates the accuracy of the cerate
T
HE determination of hydrogen peroxide concentration by
cerate titration has certain advantages of convenience over the usual permanganate titration method. However, although a number of authors (1, 2. 4, 6, 11) have reported successful use of the hydrogen peroxide-cerate titration, their work has not included any direct determination of the accuracy of the titration by comparison with an absolute method df analysis. The resulting doubt as to the complete reliability of the cerate method has tended to hinder its adoption for general use. It has, therefore, been the primary object of this work to investigate the accuracy of the cerate method by comparison with the results of an absolute method of hydrogen peroxide analysis. Huckaba and Keyes (7) have reported a careful study of the accuracy of the permanganate titration method for hydrogen peroxide determination. The work of these authors indicated a favorable comparison between the permanganate method and an absolute (gasometric) method. It, therefore, seemed desirable to include, aa a further object of the research reported here, an intercomparison between the permanganate and the cerate hydro-
titration, as previous investigators have reported good agreement between permanganate titrations of hydrogen peroxide and an absolute (gasometric) method of analysis. Intercomparison between hydrogen peroxide titrations by several different standard cerate solutions indicated that the accuracy was not affected by changing cerate concentration, by substituting sulfatocerate for nitratocerate, or by using reagent grade rather than primary reference standard grade cerate as oxidant.
gen peroxide titrations. An additional, although indirect, check on the accuracy of the cerate titration would thus be obtained. In addition to the above objects, it was decided to investigate the consistency of the cerate-hydrogen peroxide determinations by studying the comparative accuracy of titrations by several different cerate solutions. The variations to be investigated included changes in standard solution concentration, changes in cerate ion-e.g., use of sulfatocerate rather than nitratocerate solution-and change in the purity of the cerate salt used. For the purpose of investigating the accuracy of the ceratehydrogen peroxide titration, a new absolute method for the analysis of aqueous hydrogen peroxide solutions wya_q devised. This method (here termed the indirect absolute method) resembles the gasometric method of Huckaba and Keyes ( 7 ) in that it depends on decomposing the hydrogen peroxide according to the equation catalyst 0 2 2H202 -2H2O
+
However, while the gaeometric method requires an apparatus
321
V O L U M E 26, NO. 2, F E B R U A R Y 1 9 5 4 for collecting the evolved oxygen and measuring its volume, in the indirect absolute method the oxygen is allowed to escape and the water formed is retained in the apparatus. The weight of oxygen evolved is then determined simply by subtracting the weight of residual water from the origins1 sample weight. For the analysis of concentrated hydrogen peroxide solutions, where the weight of the evolved oxygen is considerable in comparison t o the sample weight, the indirect absolute method should be capable of excellent precision. Since the method does not depend on the purity of any reference standard substance, and since there are no known side reactions in the hydrogen peroxide decomposition, there is little doubt that the method is truly an absolute one. Consideration indicated that, as compared with the gasometric method, the indirect absolute method would have advantages of simplicity in construction and operation of apparatus. The indirect absolute method was therefore chosen for use in this work. As specifically applied here, the method consisted of completely decomposing a hydrogen peroxide sample by repeatedly distilling in contact with platinum from one glass container to another. The gases from the decomposition were passed through a suitable absorption train in order to remove vapors, while allowing escape of the oxygen formed. A search of the available literature has failed to disclose any instance where this method has been utilized, or even proposed, for the analysis of hydrogen peroxide solutions. A method of hydrogen peroxide analysis described by Thoms (IO) utilized the principle of determining the oxygen evolved by the loss in weight of the sample. However, this method was not an absolute one, as it involved reacting the hydrogen peroxide with manganese dioxide in acid solution, rather than decomposing it catalytically. The indirect absolute method, like the gasometric method, is highly time-consuming compared t o such other methods as titration or refractive index determination. It is therefore proposed for use only as a means of calibrating more rapid and convenient analytical methods. INDIRECT ABSOLUTE RlETHOD
The apparatus shown in Figure 1 was assembled for the determination of hydrogen peroxide by the indirect absolute method. The essential parts of the assembly were a source of nitrogen, five 6-inch Schwartz-type absorption tubes, a decomposition tube of double U form, and a beaker of water used as a bubble counter. The borosilicate glass decomposition tube was made by joining two 6-inch Schwartz-type absorption tubes, removing the stopcock plugs, and sealing off the two inner stopcock plug openings. The two remaining stopcock plug openings were closed by tightly fitting rubber stoppers, which did not project far enough into the tubes to close the side tube openings. The decomposition tube was connected in series with the absorption tubes in the order shown in Figure 1, using glass-to-glass contact inside short lengths of rubber tubing for the connections. The absorption tubes contained anhydrous magnesium perchlorate. The function of tube A was to dry the stream of nitrogen used. Absorption tubes B, C, and D were to remove water from the gases coming out of the decomposition tube and also to decompose any hydrogen peroxide carried over in droplets from the decomposition tube. Tube B was connected to the nitrogen intake side of the decomposition tube to catch any water vapor carried out of this side of the decomposition tube by diffusion or by momentary backing up of gas caused by vigorous hydrogen peroxide decomposition. Tube E was included in the system to protect tube D from weight gain caused by water vapor diffusing back from the bubble counter at times when the nitrogen stream through the apparatus was stopped. Determinations of the hydrogen peroxide content of aqueous hydrogen peroxide solutions vere carried out in this apparatus by the following procedure. The decomposition tube was cleaned, rinsed with distilled water, dried a t 110’ C., and allowed to come to equilibrium with atmospheric moisture. hfter absorption tubes B , C, and D had been weighed, the decomposition tube was weighed filled with nitrogen and with both top and side arm openings closed by
stoppers. The catalyst samples to be added later were weighed with the decomposition tube, so that a correction for their weight need not be applied to the final tube weight. All weighings were made against similar glass tubes used as counterpoises. The analytical weights used had been calibrated by the Richards substitution method. About 1 gram of pure 85 to 90% hydrogen peroxide solution was then introduced into one side of the decomposition tube and about 2 grams of the hydrogen peroxide solution were placed in the other side. The decomposition tube was then reweighed to determine the sample weight. About 1 gram of water waa then added to the side of the decomposition tube containing 1 gram of hydrogen peroxide solution, the purpose of the added water being to moderate the rate of decomposition of the hydrogen peroxide. The exact weight of water added was determined by weighing the decomposition tube and contents again. The decomposition tube was then connected to the absorption assembly as shown in Figure 1, with the side of the decomposition tube containing the diluted hydrogen peroxide attached to absorption tube B. The liquids in both sides of the decomposition tube were then frozen by the use of solid carbon dioxideacetone baths. The absorption tube stopcocks were opened to bring the system to atmospheric pressure. The stopper of the left-hand side of the decomposition tube (labeled L in Figure 1) was then opened momentarily and the platinum decomposition catalyst was dropped in. The catalyst consisted of a piece of porous platinum weighing about 50 mg., and, so that it would not cause immediate decomposition of the hydrogen eroxide, it was held in a 1.5-cm. length of 6-mm. borosilicate g i s s tube sealed a t one end.
!
\
A
8
ABSORPTION TUBES
‘WITROGEN
L
R
OECOYPOSlTlON TUBE
GAS S U P P L Y
c
D
ABSORPTION
TUBES
R U B B E R T U B E TO BUBBLE COUNTER
Figure 1. Apparatus for the Indirect Absolute Hydrogen Peroxide Analysis
A stream of nitrogen regulated to about two bubbles per second was then passed through the assembly and the hydrogen peroxide in the left hand side of the decompositiQn tube was melted by gentle warming. By tapping the tube, the platinum was brought into contact with the hydrogen peroxide, so that decomposition started. The decomposition was regulated by immersing the tube in hot or cold water as needed to ensure a rapid but not violent reaction. Most of the vapors from the decomposition were condensed in the right-hand side of the decomposition tube, as this side was kept immersed in solid carbon dioxide-acetone mixture; the small amount of vapors not condensed in the decomposition tube was absorbed by tubes C and D. After the decomposition had become slow, a gas flame was used to distill the remaining liquid over to side R of the decomposition tube. The contents of side R were then melted by gentle warming and mixed by tapping the tube, so that the water from the decomposition in side L diluted the undecomposed hydrogen peroxide in side R to about 50% concentration. The liquid in side R was then refrozen, the nitrogen stream was stopped, and the section consisting of tubes B and C and the decomposition tube was momentarily disconnected and turned around, so that the order of tubes was then A , C , decomposition tube, B, D, and E.
With both sides of the decomposition tube cooled by solid carbon dioxide-acetone, a 50-mg. piece of porous platinum in a short borosilicate glass tube was added to the side containing the frozen hydrogen peroxide solution. (Both sides of the decomposition tube thus contained platinum catalyst a t this point. ) -4fter again starting the nitrogen stream, the hydrogen peroxide sample was melted and was decomposed by distilling in contact with platinum, as previously. The steady stream of nitrogen used during decomposition was very useful by carrying vapors in the desired direction and by preventing condensation in the inlet tubes of the absorption tubes. Since the double distillation-decomposition described left a p proximately 2% of the original hydrogen peroxide content undecomposed, three more stages of decomposition were carried out, reversing section B-C again for each additional stage and distilling the remaining sample from one side of the decomposition tube to
ANALYTICAL CHEMISTRY
322 the other each time. The net result of this treatment was that one third of the hydrogen peroxide sample was distilled five times and two thirds of the sample was distilled four times in contact with platinum. Absorption tubes B, C, and D were then disconnected and weighed to determine water absorption. The decomposition tube was weighed a t intervals until it showed a weight change of no more than 0.1 mg. in 15 minutes. (Before weighing, each tube was opened momentarily to ensure that the gas inside was at atmospheric pressure.) After the decomposition tube attained constant weight, the completeness of decompo sition was checked by rinsing out the contents, acidifying with sulfuric acid, and adding one drop 0.2N sulfatocerate solution. There was not enough residual hydrogen peroxide left after any of the determinations to decolorize this amount of standard cerate solution. It was thought that operation of the apparatus might result in a mist containing hydrogen peroxide being carried into the absorption tubes, where the hydrogen peroxide might be decomposed only slowly. For this reason the absorption tubes were allowed to stand overnight before final weighing. Delayed decomposition of hydrogen peroxide in the absorption tubes occurred t o a very minor extent, if a t all, however, since the average total weight loss of tubes B , C, and D on standing overnight was only about 0.1 mg. The absorption tubes kept their desiccant efficiency for inany determinations, since most of the water was retained in the deromposition tube during each run. The total weight gain of the absorption tubes during each determination was only about 40 mg. Removal of water vapor from the gas stream was essentially complete in tubes B and C, the average weight gain of tube D per run being only 0.3 mg. The efficiency of the platinum catalyst was not noticeably altered by continued use. The catalyst was dried thoroughly after each determination by heating in a crucible over a gas flame. The weight of v,-ater remaining after hydrogen peroxide decomposition was computed by adding the total weight gain of the absorption tubes to the weight of water remaining in the decomposition tube (minus the weight of water added for dilution). Subtraction of total remaining water from the sample weight gave the weight of oxygen evolved in the decomposition. Per cent of hydrogen peroxide was then calculated by the formula: weight of oxygen evolved X 212.60 % HzOz weight of sample As a correction for the dissolved air in the sample, 0.1 mg. was subtracted from the calculated weight of oxygen evolved in each determination. This correction was scarcely significant, since its application changed the average result of the determinations by only 0.008%. A series of blank determinations was made using the exact procedure described, except that 2 to 3 grams of water were substituted for the hydrogen peroxide sample. Results indicated a standard deviation of 0 . i mg. of water, with no significant difference between water taken and water found. COMPARISON OF ABSOLUTE AND CERATE ANALYSES
For direct comparison with the results of the indirect absolute method, the hydrogen peroxide titration with standard nitratocerate solution was used. The standard solution (here termed cerate solution A ) contained about 0.38 equivalent of cerate per 1000 grams of solution (volume normality 0.46) and was prepared by dissolving the calculated quantity of ammonium hexanitmtocerate (ceric ammonium nitrate) in 2 S nitric acid. The ammonium hexanitratocerate used was a sample of primary reference standard quality (purity by titration 99.95% or higher). The cerate solution was standardized against sodium oxalate and arsenious oxide (National Bureau of Standards samples 40e and 83a). The sodium oxalate used showed no loss in weight on drping 1 hour a t 105" C.; it8 effective purity was taken to be 99.96'%,
as given by its certificate of analysis. The arsenious oxide sample had a weight loss of 0.03% when dried for 1 hour a t 105" C. Therefore in calculating the results of standardizations against arsenious oxide, a correction of 0.03% was subtracted from its stated purity factor of 99.99% The sodium oxalate and arsenious samples used were not hygroscopic and their moisture content did not change significantly during several months of use in the laboratory. As an aid t o accuracy, weight burets were used for all titrations reported here. The following procedures, based on methods described by Smith and Getz ( 9 ) , were used for the standardizations. Standardization against Sodium Oxalate. An accurately weighed sample of about 1.6 grams of sodium oxalate was transferred to a 600-ml. beaker and dissolved in a mixture of 80 ml. of water, 1 drop of 0.025M 5-nitro-I, 10-phenanthroline ferrous sulfate solution, and 20 ml. of 6iV perchloric acid. The phenanthroline indicator was added to the water before the acid in order to prevent the formation of a precipitate which redissolved only slowly. Also the solution was acidified by the use of perchloric acid, rather than nitric acid, because nitric acid ordinarily contains variable amounts of reducing substances. The oxalate solution was then titrated with 0 . 4 6 s nitratocerate solution to the first permanent disappearance of the pink color. .4 yellowbrown precipitate which formed during the titration invariably dissolved before the end point was reached and was not found to affect the accuracy of the result. The indicator blank for the determination (corresponding to 0.004 gram of standard solution) was determined by adding diluted cerate solution to the reagent solution used in the titration. Since no attempt was made to split drops in the titrations described here, an additional blank correction of one-half drop of standard solution (0.022 gram) was subtracted from the buret reading to compensate for the resulting average overstepping of the end point. Concentration of the solution in equivalents per 1000 grams of solution was calculated by the following formula:
C =
weight of sodium oxalate X purity of oxalate 0.067007 X weight of standard solution
Standardization against Arsenious Oxide. A sample of about 1.2 grams of arsenious oxide nTas accurately weighed on a small watch glass and the sample transferred to a 600-ml. beaker by placing the watch glass in the beaker. One gram of sodium hydroxide chips and 10 to 15 ml. of water were then added and the mixture was swirled gently until the arsenious oxide was all dissolved. Seventy milliliters of water, 1 drop of 0.025144 nitrophenanthroline ferrous sulfate solution, and 20 ml. of 6LV perchloric acid were then added. As catalyst, 2 drops of O.OL1.1" osmium tetroxide in 0.LV sulfuric acid was used [method of Gleu ( 5 ) ] . The solution was then titrated with standard cerate solution to the first permanent disappearance of the pink coloration. The blank correction \vas determined as in the standardization against sodium oxalate. Concentration in equivalents per 1000 grams of solution was calculated by the following formula:
C =
weight of arsenious oxide X purity of arsenious oxide 0.049455 X weight of standard solution
Titration of Hydrogen Peroxide by Standard Nitratocerate Solution. A sample of 85 to 90% hydrogen peroxide weighing about 0.45 gram (about 9 drops) was accurately weighed in a 3ml. microbeaker covered with pure aluminum foil. Microbeakers used for weighing hydrogen peroxide samples were reserved for this purpose only and were cleaned before each use by immersing for a t least an hour in 327, nitric acid, rinsing thoroughly with tap water followed by distilled water, and drying. To prevent dust contamination, each beaker was covered by pure aluminum foil during storage. After weighing the hydrogen peroxide sample, the foil cover was removed and the microbeaker was placed in a 600-ml. beaker. Eighty milliliters of water, 1 drop of 0.025M nitrophenanthroline ferrous complex indicator, and 20 ml. of 6N perchloric acid were then added, and the solution was titrated with standard nitratocerate solution to the disappearance of the pink coloration. The blank correction was determined as in the oxalate standardization. The weight per cent hydrogen peroxide was calculated by the following formula:
% Hz02
weight of standard solution X concentration X 1.7008 weight of sample
V O L U M E 26, NO. 2, F E B R U A R Y 1 9 5 4 (Concentration of the standard solution was expressed in equivalents per 1000 grams of solution). The concentrated aqueous hydrogen peroxide used for compariqon of absolute and titration methods was an extremely pure commercial material (8). Its purity was evidenced by a very low decomposition rate (0.2% loss of titer per month when stored in glass containers a t room temperatures near 25’ (3.). Because of the high quality standards maintained in the preparation of this material, it may be safely assumed that the total impurities present (other than dissolved air and oxygen) yere not over 12 parts per million and thus entirely negligible in the analyses reported here. In order that the results would not be biased by the slow decomposition of the hydrogen peroxide sample analyzed, the same number of titrations and absolute analyses were made, each titration being carried out on the same day as the corresponding absolute determination. Also. since the standard nitratocerate solution was s l o d y decreasing in titer on storage, the standard solution was standardized both directly before and directly after the series of determinations, the concentration of standard solution on a given day then being obtainable by interpolation. The titer of the standard nitratocerate solution used dropped by about 0.1 % over a 2-week interval.
Table I. Comparison of Absolute and Cerate Titration Methods for Analysis of Hydrogen Peroxide Wt. % Hi02 B y absolute method 87.24 87.35 87.24 87 29
87 18
B y titration methoda 87.34 87.24 87.18 87 21 87 25
Ar. 87.27 87.24 a Figures in this column are the results of consecutive single titrations, each made on the same d a y a s the corresponding absolute determination.
The data obtained in a series of five absolute determinations and five titrations are listed in Table I. The results indicate a favorable comparison between the indirect absolute and the cerate titration methods. Statistical analysis of the data indicated that the 0.03% difference in the average by the tlyo methods is not significant. INTERCOMPARISON OF PER’MMAhG4NATE AKD CERATE HYDROGEN PEROXIDE TITRATIONS
On the basis of comparison with an absolute (gasometric) method, Huckaba and Keyes ( 7 ) have recommended procedures for potassium permanganate standardization and hydrogen peroxide titration which give excellent absolute accuracy. Therefore, an additional valuable check on the accuracy of the ceratehydrogen peroxide titration should be obtainable by comparison with the permanganate procedure of Huckaba and Keyes ( 7 ) . The literature lists only one comparative study of the cerate and permanganate hydrogen peroxide titrations ( e ) ,and this was not made using the recommended permanganate procedure of Huckaba and Iceyes ( 7 ) . I t was therefore considered advisable to make a direct intercomparison between the permanganate titration and titration by the cerate solution .4 used for titrations in comparison with the indirect absolute method of hydrogen peroxide analysis. Cerate solution A was standardized against sodium oxalate and arsenious oxide by the procedures described. To avoid error from possible decrease in titer of the standard solutions, both cerate .and permanganate solutions were restandardized directly before and after the comparison titrations and the average normality was used for calculating peroxide concentrations. Cerate
323 Table 11. Comparison of Permanganate and Cerate Titrations for Hydrogen Peroxide Analysis Xormality Before HzOl After H20, titrations titrations Standardization of permanganate= Against arsenious oxide Against sodium oxalate
0.1076 0.1074
0.1076 0.1073
Standardization of cerateb Against arsenious oxide Against sodium oxalate
0.3838 0,3838
0.3837 0.3837
Titrations of Hydrogen Peroxide Permanganate Cerate titration, titration,
%
nzol
86.16 86.24 86.41 86.29 86.18 a
b
70 HzOz 86 85 86 86 86
54
96 44 42 38
Ar. 86 26 86 35 Solution of potassium permanganate in water. Solution of ammonium hexanitratocerate in 2 5 nitric acid.
solution d lost titer during storage a t an average rate of 0,2y0 per month during an 8-month period. The standard permanganate solution used was a 0.1N solution of analytical reagent grade potassium permanganate in water. Sfter being made up, the solution was allowed to stand several weeks and was then filtered through the sintered-glass plate of a filter funnel. iis thus prepared, the solution a a s of excellent storage stability, showing an average decrease in titer of only 0.1% per month during 8 months’ storage. The solution was standardized against sodium oxalate and against arsenious oxide, using the procedures given by Fowler and Bright ( S ) , except that in the standardization against arsenious oxide the end point was determined by the permanganate color instead of potentiometrically. Standardizations against arsenious oxide gave values for the normality which averaged 0.1 to 0.2% higher than those obtained in standardizations against sodium oyalate, possibly owing to failure to use potentiometric indication in the arsenious oxide standardization. A similar effect was noted in standardixing various cerate solutions, where normalities against arsenious oxide have averaged 0.04 to 0.1% higher than those against sodium oualate. A series of ten titrations was made in which a sample of pure concentrated hydrogen peroxide was titrated alternately by nitratocerate solution, using the procedure described, and with potassium permanganate solution, using the procedure recommended by Huckaba and Keyes ( 7 ) . Titration and standardization data obt%ined are listed in Table 11. Calculations of per cent hydrogen peroxide by the two titration methods were based on the average normality from an equal number of sodium oxalate and arsenious oxide standardizations. Seither standard solution decreased in titer significantly during the period in which the titrations were carried out. The data obtained are considered to demonstrate good agreement between cerate and permanganate titrations, since statistical analj-sis indicated that the 0.1% difference in per cent hydrogen peroxide by the two methods was not significant. The comparison also indirectly demonstrates essential agreement between the gasometiic method of Huckaba and Iieyes ( 7 ) and the indirect absolute method as used in this study. According to the results shown in Table 11, both the permanganate and cerate hydrogen peroxide titrations are excellent with respect to absolute accuracy. However, the cerate method may be preferable in many instances because certain precautions necessary for accurate permanganate titrations are unnecessary with cerate titrations. Thus, in cerate titrations, ordinary distilled water serves adequately without any boiling to remove organic matter and there is also no need to regulate the speed of
324
ANALYTICAL CHEMISTRY
titration. The following additional advantages of convenience of nitratocerate titrations should be considered in choosing a method for hydrogen peroxide analysis: Cerate end points are not subject to fading; heating is not required for the standardization against sodium oxalate; cerate solutions are light-colored enough so that the bottom of the meniscus can easily be read; and cerate solutions do not form any precipitate on burets, flasks, etc.they may be filtered through ordinary filter paper and the unused cerate standard solutions may be returned to the storage container without harmful effect. A nitratocerate standard solution loses titer on storage slightly faster than permanganate solution stored with proper precautions. However, this disadvantage is compensated for by the fact that the cerate decomposition is not autocatalytic, as in the case of permanganate solution, but proceeds a t a steady rate which gradually decreases with increasing age of the solution. HYDROGEN PEROXIDE TITRATIONS BY VARIOUS CERATE SOLUTIONS
The literature on the cerate-hydrogen peroxide titration has not heretofore included any data on the effects of varying cerate solution concentration, of substituting sulfatocerate for nitratocerate, or of using reagent grade rather than reference standard quality cerate. Accordingly, the influence of these changes in the standard cerate solution on the accuracy of the cerate-hydrogen peroxide titration was investigated by making an intercomparison between titrations by four standard cerate solutions. These solutions comprised, in addition to the cerate solution A used in the preceding work, a 0.1N nitratocerate solution, a 0.2N sulfatocerate solution. and a 0.1N nitratocerate solution prepared from reagent grade rather than primary reference standard grade ammonium hexanitratocerate. The compositions of these solutions are given in Table 111.
Table 111. Composition of Standard Cerate Solutions Solution A
B C D
~
0xi dan t Ref. std. amm. hexanitratocerata Ref. std. amm. hexanitratocerate Ref. std. amm. hexanitratocerate Reagent amm. hexanitratocerste
Acid Z'V HNOa 2N "03 2N HdOd 2 N "03
Oxidant Canon., Equiv./1000 G . Soln. 0.38 0 1
0 2 0 1
~~~
Table IV.
Intercomparison of Various Cerate Solutions for Hydrogen Peroxide Titration
I
HlOs Samplea
I1
I11
H10z %, by titration with Cekste solution A 87.58 85.21 ... Cerate solution B 87.59 Cerate solution C ... Si:& 8i:81 Cerate solution D ... ... 81.76 0 Each tabulated figure is the average of four hydrogen peroxide titrations.
The 0.1N nitratocerate solutions were standardized against sodium oxalate and arsenious oxide by the same procedures used for standardizing cerate solution A. However, the weights of standard substances used were reduced proportionately to the decrease in cerate concentration, so that about 60 grams of standard cerate solution were used for each titration. For the hydrogen peroxide titrations with 0.1N cerate solutions, a sample weight containing about 0.2 gram of hydrogen peroxide was used, EO that each titration consumed about 100 grams of cerate solution. Otherwise, the procedure was identical with that described for titrations with 0.46N cerate solution. Comparison between the average values for hydrogen peroxide concentration obtained by titrations with cerate solutions A and B (tabulated in Table IV) indicates excellent agreement between determinations by 0.46 and 0.1N nitratocerate solutions.
The 0.2N sulfatocerate solution used for intercomparison (cerate solution C) was prepared by dissolving the calculated quantity of reference standard grade ammonium hexanitratocerate in 2N sulfuric acid. This solution had excellent storage stability, no significant change in titer being detected over a storage period of 20 months. The sulfatocerate solution was standardized by the following procedures: Against Sodium Oxalate. An accurately weighed sample of about 0.9 gram of sodium oxalate was transferred to a 600-nil. beaker and dissolved in a mixture of 180 ml. of water and 20 ml. of 6 N sulfuric acid. (The 200-ml. total volume was used to avoid danger of exceeding the solubility product of cerous sulfate.) Then 90 to 95% of the required sulfatocerate solution was added from a buret. The solution was thereupon heated to 70" C. and the titration was continued to the first permanent yellow coloration. The amount of excess cerate present a t the end of the titration was estimated by comparing the color with that of blanks containing 0.050 and 0.025 g r a m of standard sulfatocerate solution in thP same volume of water acidified with sulfuric acid. Against Arsenious Oxide. An accurately weighed sample of about 0.6 gram of arsenious oxide was transferred to a 600ml. beaker and 1 gram of sodium hydroxide chips and 10 ml. of water were added. The mxture was swirled gently until the arsenious oxide was all dissolved and was then diluted by the addition of 170 ml. of water and 20 ml. of 6 N sulfuric acid. One drop (0.04 ml.) of 0.025-11 1,lO-phenanthroline ferrous sulfate indicator solution and 2 drops of 0.01.11 osmium tetroxide solution in 0.1N sulfuric acid were then added. The solution was then titrated with standard sulfatocerate solution to the first permanent disappearance of the pink coloration. The indicator blank for the titration was determined as in the standardization of nitratocerate solution against sodium oxalate. For the titration of hydrogen peroxide by sulfatocerate solution the use of 1,lO-phenanthroline ferrous sulfate indicator was tried. However, this indicator in the vicinity of the end point was oxidized by a transient excess of cerate solution and only returned slowly to its reduced state. The resulting fading of indicator color made the titration tedious and the end point noticeably less definite than in the titration by nitratocerate solution using nitrophenanthroline ferrous complex indicator. The use of a hydrochloric acid medium as recommended by Willard and Young ( 1 1 ) did not eliminate indicator fading, although it limited fading to a region closer to the end point. The titrations of hydrogen peroxide with sulfatocerate solution were therefore carried out using the cerate color for the indicator. The cerate color gives an end point of excellent sharpness, although careful observation is required to detect the faint yellow color from the first drop of cerate solution in excess. The following procedure was used for the titrations. A sample of 85 to 00% hydrogen peroxide solution weighing about 0.2 gram (about 4 drops) was accurately weighed in a 3ml. microbeaker covered with pure aluminum foil. After weighing, the foil was removed and the microbeaker placed in a 600ml. beaker. The sample was diluted by the addition of 180 ml. of water and 20 ml. of 6N sulfuric acid and was then titrated with sulfatocerate solution to the first permanent yellow coloration. The blank correction was determined by comparing the color a t the end point with standards containing 0.025 and 0.050 gram of sulfatocerate solution in water acidified with sulfuric acid. Data from the intercomparison of hydrogen peroxide titrations by 0.46N nitratocerate solution and 0.2.V sulfatocerate solutions (listed in Table IV) indicate excellent agreement between the two methods. Because the reagent grade of aninionium hexanitratocerate (purity by titration about 95%) is much cheaper than the reference standard grade, it was considered to be of interest to investigate the accuracy of titrations using reagent grade ammonium hexanitratocerate. For this purpose, a sample of hydrogen peroxide was titrated both by 0.1N reagent grade and 0.1N primary reference standard grade ammonium hexanitratocerate solutions. Data obtained are tabulated in Table IV. The agreement be-
V O L U M E 2 6 , NO. 2, F E B R U A R Y 1 9 5 4
325 Furman, N. H., and Wallace, J. H., J . Am. Chem. SOC.,51, 1449
tween the two methods was excellent, the averages of four titrations by each solution being found to differ by only 0.06%.
(1929).
Gleu, K., Z.anal. Chem., 95,305 (1933). Greenspan, F. P.,and MacKellar, D. G., -4s.4~.CHEM.,20, 1061
ACKNOWLEDGMENT
(1948).
Huckaba, C.E.,and Keyes, F. G., J . Am. Chem. SOC.,70, 1640
The authors wish to thank V. L. Burger of this laboratory for originally suggesting the investigation of the cerawhydrogen peroxide titration and for much helpful advice regarding the analytical techniques used in the work.
(19481. ~--,_ - .
Shanley, E S., and Greenspan, F. E'., Ind. Eng. Chem., 39, 1536 (1947).
Smith, G . F.. and Gets, C. A., IND.ENG.CAEM.,ANAL.ED., 10,304 (1938).
Thorns, H., Arch. Pharm., 225, 335 (1887). Willard, H. H.,and Young, P., J . A m . Chem. SOC.,55, 3260
LITERATURE CITED
(1) Atanasiu, J. A.,and Stefanescu, V., Ber. deut. ehem. Ges., 61, 1343 (1928). (2) Berry, A. J., Analyst, 58, 464 (1933). (3) Fowler, R.M.,and Bright, H. A., J . Research Natl. Bur. Standurds, 15, 492 (1935).
(1933). RECEIVED for reriew October 1, 1953. Accepted October 23, 1953. Work supported b y the U. S. Navy Bureau of Ships under Contract KObs 531667 Presented before the Division of Analytical Chemistry a t the 124th Meeting of the .kMERICAN CHEMICAL SOCIETY, Chicago, Ill.
Use of Sulfuric Acid in the Detection And Estimation of Steroidal Sapogenins HENRY A. WALENS, ARTHUR TURNER, JR., and MONROE E. WALL Eastern Regional Research Laboratory, Philadelphia 18, Pa.
A rapid method for the identification and estimation of steroidal sapogenins was required. It was found that steroidal sapogenins dissolved in sulfuric acid have characteristic ultraviolet absorption spectra in the region 220 to 400 mp, which can be used for the detection and estimation of these substances. Optimum reaction conditions are the use of 0.1 to 5.0 mg. of sapogenin dissolved in 10 ml. of 94% sulfuric acid and warmed at 40" C. for 16 hours. Under these conditions, the determinations are reproducible and follow Beer's law. Constituents of binary mixtures can be spectrophotometrically determined, using appropriate equations based on previously determined absorptivities. More complex mixtures can be determined after preliminary separation by means of chromatography. A qualitative scheme is suggested for the identification of the 13 most commonly occurring steroidal sapogenins, using chromatographic behavior, spectra of the sulfuric acid chromogens, and melting points. The method can be applied only to purified sapogenins, not to crude mixtures.
D
URIK'G the course of a study on the partition coefficients
of steroidal sapogenins, the need for a method for determining relatively small quantities of these compounds became apparent The method finally adopted was based on the studies of Diaz, Zaffaroni, Rosenkranz, and Djerassi (1). They showed that steroidal sapogenins, on treatment with concentrated sulfuric acid, give characteristic chromogens. It was found necessary to make a thorough study of this reaction, and it has been observed that under suitable conditions the reaction can be used both for identification and for estimation of the various Sapogenins. ,4 qualitative scheme has thus been developed for the identification of the 13 most commonly occurring steroidal sapogenins, using chromatographic behavior, spectra of the sulfuric acid chromogens, and melting points. PROCEDURE
General. The sample, preferably 5.0 mg. (although quantities as low as 0.1 mg. can be used) is weighed into a 10-ml. volumetric
flask. Alternatively, a sample in this weight range is dissolved in chloroform, and transferred to a 10-ml. volumetric flask, and the solvent carefully evaporated to dryness. Sulfuric acid, 94% by volume, is added to the 10-ml. mark. The flask is then immersed for 16 hours in a water bath maintained a t 40' C. The flask and contents are then cooled to room temperature and the contents diluted, if necessary, to volume with 94% sulfuric acid. For the qualitative determination of the sapogenins, the ultraviolet spectrum is obtained over the range 220 to 600 m p using a Beckman DU spectrophotometer or, preferably, a Gary recordin spectrophotometer, with 1.0-cm. matched quartz cells, using 949f sulfuric acid as the blank. Four spectral curves, as determined by means of a Gary recording spectrophotometer, are shown in Figure 1. By comparison of the spectrum obtained with Figure 2 and Table I, most sapogenins can be easily identified. Confirmatory methods have been previously described ( 2 ) . Single-Component Samples. For estimation of the quantity of sapogenin present, the absorbance of the sample is measured a t 250 mg. The concentration of the solution can then be calculated. using the absorptivities listed in Table 11, and the amount of sapogenin present can be determined.
Table I. Wave Length Positions and Intensities of Absorption RZaxima of Sulfuric Acid Chromogens of Steroidal Sapogenins Sapogenin Chlororenin Diosgenin Desoxydiosgenin Gitogenin Hecogenin Desoxyhecogenin Hecogenone Kammogenin Xryptogenin Manogenin Markogenin Rockogenin Samogenin Sarsasapogenin Desoxysarsasapogenin Sarsasapogenone Smilagenin Desoxysmilagenin Smilazenone Tigoggnin Desoxytigogenin Yuccagenin Cholesterol
Absorption Maxima, hlp 270.330.416 271; 415; 514 271,312 272, 308 276,350,396 268,348,394 269,347 233,272,349 280.383 276,348,400,468 270,308 273,379 270,308 271,310 272,308 267,310 272,312 273.308 268.310 270; 312 274.296 240,268,405 308,416
Logarithm of molecular absorptivity.
Log ea a t Corresponding hfaxima 3 . 9 6 . 4 . 0 0 3 71 3 . 9 9 ; 4.06: 3 . 6 4 4.00,3.93 4.08, 4.11 4 . 0 6 . 4 . 1 0 ,4 . 2 4 3.97,3.98,3.61 3.95,3.89 4.11,4 02,3.89 3.77,4.06 4 .OO, 4 . 0 1 , 3 . 6 9 , 3 . 4 4 1.00,3.90 3,85,4.07 4.01,3.91 3.98,3.86 3.97,3.92 4.01.3.76 3.98.3.90 4 .OO, 3 . 9 4 4.06.3.86 3 . 9 4 ;3 . 8 8 3.99,4.03 4.11,4.09,3.83 3.95,3.58
