Chromous Sulfate as Reducing Agent in Volumetric Determination of Iron WILLIAMM. THORNTON, JR.,AND JOSEPH F. SADUSK, JR.,The Johns Hopkins University, Baltimore, Md.
C
HROMOUS salts, which
0.067 N instead of 0.1 N , as Chromous sulfate solutions which are suitable are undoubtedly v e r y origin a 11y intended, this does for analytical work may be conveniently prepared powerful reducing agents, not necessarily indicate that the from polassium dichromate (and doubtless f r o m have come into prominence for r e d u c t ion was incomplete, for other chromium compounds) by means of the use in deoxidation methods of the chromous salt probably sufJones reductor. analysis within the last few years. fered some oxidation from exAlthough this property of biposure to air during the transfer Chromous sulfate in sulfuric acid not stronger valent chromium was known a t to the storage bottle. than 0.18 N , if stored properly, can be kept for a a comparatively e a r l y d a t e , TITRATION APPARATUS. As period of 2 months without undergoing a n apprechromous chloride having been s o l u t i o n s of chromium comciable change in titer; and it is not necessary to employed in gas analysis ( I @ , it pounds (especially the chromic remove the dissolved zinc. was not until r e c e n t l y t h a t salts) are highly colored, it beBuehrer and Schupp ( 2 ) demoncame practically necessary to deIron in cold solution can be readily determined strated the feasibility of utilizing termine the end point electroby potentiometric titration with chromous sulfate, it in certain volumetric procmetrically in the present work. provided a small quantity of potassium thioesses. Since then, methods deA 250-cc. wide-mouthed Erlencyanate be added. It is believed that the results pending upon reduction by a meyer flask served as the reaction so obtained are very near to the true values. chromous comDound have been vessel. The Roberts titration brought out for the estimation outfit (Y), whose potentiometer of various elements either alone or in admixture with one is manufactured by the Leeds and Northrup Company, was or more others ( I S ) . used for following the relative electromotive force of the sohThis present contribution consists in preparing a solution of tion as the experiment proceeded. The electrodes, however, chromous sulfate of the requisite purity and stability by a were somewhat modified, particularly in that the calomel halfwell-known convenient procedure, devising a reliable ap- cell was made larger and the vertical part of the glass tube paratus for storing and using the solution so obtained, and covering the platinum electrode, down which the carbon dioxstudying the reduction of a ferric salt at such temperatures as ide passed, was lengthened so that it dipped well into the usually prevail in a working room. solution. The carbon dioxide thus served the twofold purpose of stirring the liquid and protecting it against atmospheric CHROMOUS SULFATESOLUTION USEID oxidation. Finally, the flask was closed with a four-holed rubPREPARATION. A number of investigators (2, 10, I d , I S ) ber stopper through which the buret tip, the two electrodes, have prepared chromous salts, utilizing in all a variety of and the outlet tube passed. processes but, as a rule, the resulting product did not show a PERMANENCE OF STANDARD SOLUTION.An approximately 0.1 N solution of ferric ammonium sulfate was prepared by satisfactory degree of stability. Since others (11) have demonstrated that chromium can be dissolving 48.2 grams of the crystalline salt [NH4Fe(S04)a. completely reduced to the bivalent condition by means of 12H20] in water, adding 50 cc. of concentrated sulfuric acid, the Jones reductor, certainly, when operating on the analytical and diluting to 1000 cc., the function of the acid being to scale, a slightly modified form of this familiar apparatus was prevent the formation of basic sulfate of iron. Any ferrous adopted for obtaining a solution of chromous sulfate. In salt that might have been present was oxidized with the least short, 5 liters of a solution of potassium dichromate, containing possible amount of 0.1 N potassium permanganate. The solu75 grams of the salt, which had been purified by two recrystal- tion was then boiled to decompose the small excess of perlizations of the c. P. material, and 125 cc. of sulfuric acid (sp. manganate, cooled, and allowed to stand for 2 days, when i t gr., 1.84) were passed through a column of amalgamated zinc was stored in a glass-stoppered bottle and kept in the dark. To 20 cc. of the ferric alum solution 12 cc. of sulfuric acid (40 cm. long and 1.9 cm. wide) at the rate of about 40 cc. per minute, the deoxidized liquid being caught in a 6-liter flask (1 to 1) and 50 cc. of water were added. The resulting soluthrough which a stream of carbon dioxide was maintained tion was heated to the boiling point, the flask was set in place during the entire operation. This solution was thoroughly under the buret, and the system was swept with carbon dioxide mixed by continuing the flow of the carbon dioxide, and when for 10 minutes. Throughout the entire series of operations a cool, it was charged into the stock bottle of the storage ap- small flame was kept under the flask to maintain the test paratus previously employed by Thornton and Wood (9) for solution at a temperature of 80' to 100" C. When all but about 2 cc. of the required amount of chromous sulfate had titanous sulfate. It can be calculated from the reaction that the chromous been introduced, no further addition was made until a constant sulfate solution was not over 0.18 N with respect to sulfuric reading on the potentiometer had been obtained, after which acid. According to certain authorities ( I ) the acidity of the the standard reagent was added slowly till the end point was solution plays an important role in the decomposition of actually reached. In this way it was found that the solution did not undergo chromous salts, hydrogen being liberated in sufficiently acid media; hence it is reasonable to suppose that the gratifying an appreciable change of titer during a period of 2 months. PURITYOF SOLUTION.Although the use of an iron-free degree of stability exhibited by the solution was due in some measure, a t least, to its low concentration of free sulfuric acid. chromous salt is not necessary in the present case, contaminaAlthough when actually tested, the solution was found to be tion by this element would doubtless interfere in certain other 240
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 15, 1932
analytical processes (4). Accordingly, it was thought desirable to have a t hand a convenient and reliable method for detecting and, if possible, determining the amount of this well-nigh ubiquitous substance. Kolthoff and TomiEek (5) have estimated small quantities OF iron in titanous chloride solutions by titrating potassium dichromate electrometrically with the titanous salt, and by this method they were able to determine as little as 0.25 per cent of iron (reckoned on the content of trivalent titanium).
30
40 LL:
5
5
50
4 bb
A
I
I
C-----O.Jcc------J VOLUME, cc.
FIGURE1. TITRATION CURVES OF CHROMOUS SULFATE AGAINST POTASSIUM DICHROMATE
If iron is present in sufficient amount, two breaks in the titration curve can be observed: the first appears immediately after the complete reduction of the dichromate, and the second just after the deoxidation of the ferric salt (formed from the ferrous salt originally present in the titanous chloride solution at the expense of the dichromate). Consequently, the volume of standard reagent used between the first jump and the second corresponds to the iron content of the solution. An exactly analogous procedure was followed in order to test the solution of chromous sulfate for iron. A 0.1 N solution of potassium dichromate was prepared by dissolving the requisite amount of the salt (purified as described above) in water and diluting to the proper volume. A measured quantity of this solution was acidified with 12 cc. of sulfuric acid (1 to 1) and 90 sc. of water were added to it. An accurately known amount of iron was then intentionally introduced, and the composite solution was titrated in the same way as the ferric alum except that the reduction was carried out more slowly. TABLEI. TITRATION OF CHROMOUS SULFATE AQAINST POTASSIUM DICHROMATE No.
XzCrz01 TAKEN CC.
1
a
CrS04 REQUIRED"
.
Cc 53 53 7.5 7.5 7.5 7.5 7.5 7.5 7.5
35 35 2 3 5 5 5 5 5 5 5 Total volume; approximate only.
Fe TAKEN Gram
None None 0.00007 0.00012 0.00013 0.00020 0.00020 0.00041 0.00042
Fe FOUND Gram
None None None None
0.00015 0,00022 0.00022 0.00037 0.00030
The results are given in Table I, and some of the titration curves are plotted in Figure 1, the numbers of the curves corresponding to the numbers of the experiments. I n experi-
241
ments 1 and 2, however, enough dichromate (35 cc.) was taken to react with 53 cc. of chromous sulfate, but no iron was added. On actually titrating, none was found. From these findings it is logical to conclude that the chromous sulfate solution, which had been prepared with the aid of c. P. zinc, was virtually free from iron, or that it contained, a t a maximum estimate, less than 0.00015 gram per 50 cc. of solution.
TITRATION OF IRON Titrations were next conducted, first, with the solution of ferric salt hot, and then with it cold, the procedure being otherwise the same as that described above. On comparing the results obtained in hot and in cold solution (Table 11),it can be seen that there is, on the average, a difference in the volume of chromous sulfate consumed amounting to 0.29 per cent, a quantity too large to be disregarded in accurate quantitative work. Although this error could doubtless be compensated for by standardizing under the same conditions that prevail in an actual analysis, the objection would still remain that the reduction of iron by chromous sulfate in cold solution proceeds slowly in the vicinity of the end point. It is well known that energetic reducing agents are oxidized, more or less rapidly, by the dissolved atmospheric oxygen in water, and this phenomenon might account for the greater volume of chromous sulfate that was used up when titrating a cold solution of ferric alum. Moreover, in those analyses in which the ferrous iron is oxidized by the careful addition of potassium permanganate prior to the reduction titration, the small excess of permanganate being destroyed by boiling, some of the dissolved oxygen is, in all probability, removed. Accordingly, titrations were performed on solutions of ferric ammonium sulfate that had been first boiled and then cooled down to room temperature by means of an ice bath while a current of carbon dioxide was passing through the liquid. The results, as shown in Table 11,are slightly lower, the mean discrepancy being 0.10 per cent, than those previously obtained with cold solutions that had not been subjected to the preliminary treatment, yet noticeably higher than those obtained with hot solutions. TABLE11. TITRATION OF CHROMOUS SULFATE AGAINST FERRIC ALUM
(20 cc. of ferric alum titrated in each experiment)
No.
CrSOi (Fe SOLN.
HOT) Cc
1 2 3 4 rlv.
.
CrSO4 Fe SOLN. cLD)
cc
.
CrSOa (Fe SOLN. BOILEDAND COOLED)
cc.
30.62 30.62 30.62 30.64
30.72 30.70 30.74 30.70
30.07 30.68 30.68 30.71
30.63
30.72
30.69
Thornton and Wood (9), when verifying the end point obtained by the use of potassium thiocyanate as indicator in the reduction of iron with titanous sulfate, found that the thiocyanate seemed to behave as a catalyst, thus expediting the reaction. Therefore, it was decided to ascertain whether or not the above-mentioned thiocyanate has a like effect in the titration of iron with chromous sulfate. For this purpose, 20 cc. of the ferric alum solution were diluted with 40 cc. of water, acidified with 12 cc. of sulfuric acid (1 t o l), and treated with 10 cc. of potassiuwhhiocyanate (10 per cent solution). After sweeping with carbon dioxide for 10 minutes, the titration was conducted a t the customary rate until the intense reddish color due to ferric thiocyanate had faded to an appreciable extent, whereupon the reducing agent was added slowly up to the point of complete reaction. Throughout the entire series of experiments, wherein 10 cc. of the thiocyanate solution were used, the values seem to decrease with increase of temperature (Table 111). Thornton
ANALYTICAL EDITION
242
Vol. 4, No. 2
and Chapman (8) obtained similar indications when titrating with titanous chloride. Furthermore, a number of workers ( 7 ) have observed the bleaching of ferric thiocyanate in conhection with the colorimetric determination of iron.
solution was enough to cause the potential of the test solution to fall from the value corresponding to trivalent iron to the very low value corresponding to bivalent chromium (Figure 2). As an additional means of testing the va1idit.y of the proposed method, determinations of iron in a sample of ferrous TABLE 111. TITRATION OF CHROMOUS SULFATE AGAINST FERRIC ammonium sulfate were made. ALUM The iron value of the ferric alum solution was found to be (20 cc. of ferric alum titrated in each experiment) 0.005706 gram per cubic centimeter by reducing two 25-cc. CrSOr CrSO4 CrSOd CrSOa portions in the reductor according to the instructions of ROOM^ 7 K C N S (10% s o h ) addedNo. TBMP. 1 cc. 5 00. 10 0 0 . 20 0 0 . Lundell and Knowles (6, 12) and then titrating the ferrous c. cc. cc. CC. cc. salt with potassium permanganate, which in turn had been 1 19.6 30.69b ... 30.72 .. .... standardized against the certified sodium oxalate of the 2 19.7 30.67b ... 30,70 3 20.0 ... 30.67 Bureau of Standards (Standard Sample No. 40). Taking the 4 20.0 ... ... 30.68 ... 5 20.5 ... ... 30.72 ... mean of the results obtained in experiments 1 and 2 of R Table 111,and correcting the volumes to the standard tempera... ... 30.67 ... 2!4*!U . I 7 ... ... ... so * zo ture, 20" C., and for the graduation errors of the measuring 8 21.0 6U.Il ... 9 2!*? 30.67 30:jO 3O.?Cj 30:?3 instruments, we find that' 30.69 cc. of chromous sulfate were 10 21.0 ... ... 80.116 ... equivalent to 20.01 cc. of ferric alum. Therefore, 1 cc. of the 11 22.0 ... . . . 30.62 ... chromous sulfate solution represented 0.0037203 gram of iron. 12 22.2 ... ... 30.54 ... 22.8 30.51 Judging from the experience of Thornton and Wood (9) 14 l3 26.7 3b:jO 3b:55 30.37 3Q:i7 it would seem that specimens of ferrous ammonium sulfate a Temperature of chromous sulfate solution. b Solution boiled and aooled to room temperature before adding thiotaken a t random do not always contain the theoretical oyanate. percentage of iron. Consequently, the c. P. salt was recrystallized in such a way as to procure a fine-grained, homogeneous Experiments were made with varying quantities of potassium thiocyanate present, and the results show that the material (9), and the iron content of the finished product was amount of chromous sulfate required is influenced not only by carefully determined by the reductor method. It was found the temperature but also by the concentration of the former to contain 14.30 per cent of iron instead of 14.24, as demanded reagent, there being a noteworthy diminution in the volume by theory. An accurately weighed portion of the Mohr's salt was disof the chromium solution accompanying an increment in the solved in 40 cc. of water, 12 cc. of sulfuric acid (1 to 1) were added, and the ferrous iron was oxidized with 0.1 N potassium permanganate, care being taken not to overstep the end point. The solution was then boiled for 10 minutes to decompose the small excess of permanganate inevitably introduced, and the cooled liquid was titrated with chromous KSCN sulfate in the usual manner, 1 cc. of potassium thiocyanate (10 per cent solution) being added for its accelerating effect upon the reduction of the ferric salt. The potassium permanganate was tested for iron, but none was found. . I .
40tC"0"L:
TABLEIv. TITRATION O F CHROMOUS SULFATE FERROUS AMMONIUM SULFATE
4
No. 1 2
3
MOHR'SSALT CrSOi
Fe (BY Fe TAKEN REQUIRDD REDUCTOR) FOUND Grams
Cc.
Gram
Gram
0.8602 0.5034 1.0509
33.05 19.31 40.36
0.1230 0.0720 0.1503
0.1230 0.0718 0.15016
AGAINST
DIFF. #ram
0.0000 -0.0002 -0.00015
HOT COLD
(1) Berthelot, Compt. rend., 127, 24 (1898); Chem., 26, 193 (1898); Doring, J . prakt. Chem., (2) 66, 65,
JKSGN
25
30
LITERATURE CITED
35
~
VOLUME, C C
FIGURE2. TITRATION CURVESOF CHROMOUS SULFATE AGAINST FERRIC ALUM concentration of the thiocyanate a t temperatures ranging above 22" C. When only 1 cc. of a 10 per cent solution of potassium thiocyanate was added, however, the result was not affected by a considerable rise of temperature; yet the reaction appeared to proceed just as rapidly as in the presence of much larger quantities of the salt. Besides, the change in potential was greater than that observed in either cold or hot solution in the absence of the accelerator. I n fact, one drop (approximately 0.03 cc.) of the 0.067 N chromous sulfate
(1902). (2) Buehrer and Schupp, IND.ENQ.CHEM.,18, 121 (1926). (3) Hostetter and Roberts, J. Am. Chem. Soc., 41, 1337 (1919); Roberts, Ibid., 41, 1358 (1919). (4) Knecht and Hibbert, "New Reduction Methods in Volumetric Analysis," 2nd ed., p. 3, Longmans, 1925. (5) Kolthoff and TomiEek, Rec. trav. chim., 43, 775 (1924). (6) Lundell and Knowles, J. Am. Chem. SOC.,45, 2620 (1923). (7) Stokes and Cain, Bur. Standards, Bull. 3, 115, 157 (1907); Mellor, "A Treatise on Quantitative Inorganic Analysis," p. 200, Griffin, 1913. (8) Thornton and Chapman, J. Am. Chem. Soc., 43, 91 (1921). See also English, J. IND. ENG.CHEM.,12, 994 (1920). (9) Thornton and Wood, Ibid., 19, 150 (1927). (10) Traube and Goodson, Ber., 49, 1679 (1916); Traube, Burmeister, and Stahn, 2. anorg. aZ2gem. Chem., 147, 50 (1925). (11) Van Brunt, J . Am. Chem. SOC., 36, 1426 (1914); Lundell, Hoffman, and Bright, IND.ENQ. CHEM.,15, 1067 (1923); Lundell nnd Knowles, Ibid., 16, 723 (1924). (12) Von der Pfordten, Ann., 228, 112 (1885); Jannasch and Meyer, Ibid., 233,375 (1886) ; Anderson and Riffe, J. IND. ENG.CHEW, 8, 24 (1916).
April 15, 1932
IN DU STR IA L A N D EN G INE ER IN G C HE M ISTRY
(13) Ziritl and Rienacker, 2. anorg. allgem. Chem., 161, 374, 385 (1927); Brintzinger and Oschats, Ibid., 165, 221 (1927); Brintzinger and Rodis, Ibid.,166, 53 (1927) and 2. Elektroehem., 34, 246 (1928); Zintl and Zaimis, 2. angew. Chem., 40, 1286 (1927) and 41,543 (1928); Zintl, Rienacker, and Schlof-
243
fer, 2. anorg. allgem. Chem., 168,97 (1927) ; Zintl and Schloffer 2. angew. Chem., 41, 956 (1928); Brintzinger and Schieferdecker, 2. anal. Chem., 76, 277 (1929) and 78, 110 (1929). RECEIVED June 9, 1931.
Microdetermination of Protein in Cereal Products REXJ. ROBINSON AND J. A. SHELLENBERGER, University of Washington, Seattle, Wash. ICROMETHODS of analysis were originally developed for use when only a small sample of the material could be obtained. More recently they have proved so accurate, rapid, and economical that they are now used in many determinations even when there is an abundance of sample available. This paper deals with an adaptation of the microdetermination of nitrogen for usage in the cereal industries, just as Bermann (3) applied micromethods to the determination of nitrogen in the fermentation industry.
PROCEDURE Using a microchemical balance, from 10 to 20 mg. of sample are weighed into a micro-Kjeldahl digestion tube. A small crystal of copper sulfate and 2 ml. of concentrated nitrogenfree sulfuric acid are then added. The sample is thoroughly mixed with the acid and then heated until the organic matter is charred. This usually requires about 4 minutes. A microKjeldahl digestion rack is very convenient for use in this operation, since several samples may be digested a t the same time. The digestion mixture is allowed to cool, after which 1 gram of potassium persulfate is added ( 7 ) . The mixture must be cooled to at least 100’ C., otherwise the potassium persulfate decomposes a t the surface of the hot liquid, producing an inefficient oxidation. The mixture is again heated gently until complete decomposition of the potassium persulfate has taken place. A perfectly clear sample is usually obtained after heating approximately 1 minute, and the time for the entire digestion is usually less than 10 minutes. A distillation equipment similar in principle to that of Kemmerer and Ballett (4) was used for this investigation and gave accurate results, although it was not so suitable for routine work as their more elaborate apparatus. The digested sample is washed into the distillation flask and sufficient 40 per cent carbonate-free sodium hydroxide solution added to neutralize the excess sulfuric acid. The ammonia is expelled by steam distillation, assisted by the heat from a microburner beneath the distillation flask. About 25 ml. of the distillate are collected in a known volume of N/70 sulfuric acid. The time required for distillation is from 5 to 8 minutes. The back titration is made with N/70 carbonate-free sodium hydroxide solution, using methyl red as the indicator. Satisfactory results were obtained without first boiling the acid distillate as recommended by Pregl(6), or without cooling to 15” C. as specified by Allen and Davisson (1). PRECAUTIONS Distilled water should be freed from dissolved ammonia and carbon dioxide, or otherwise large errors would be incurred. With certain waters a satisfactory purification may only be accomplished by redistilling from an acid permanganate solution, However, it has been the authors’ experience, using distilled water from a source originally containing very small quantities of impurities, that the freeing from ammonia and carbon dioxide was accomplished most easily by vigorous
boiling for several minutes. A metal container is required for this operation, since hot water extracts appreciable quantities of alkali from glass vessels, as shown by Walther (8). Distilled water containing alkali yields high nitrogen results, as it is used to dilute the acid in the ammonia-distillate receiver. Reg1 recommended that a fused quartz tube be used in the condenser to eliminate the possibility of the steam and hot water extracting alkali from the glass. In this investigation a Pyrex tube was used and very satisfactory results were obtained, indicating that the time of contact was insufficient to incur error. Blank determinations should be made from time to time and appropriate corrections applied if necessary.
EXPERIMENTAL RESULTS To demonstrate the accuracy and practicability of the method, pure acetanilide was analyzed. The percentage of nitrogen found was 10.32, the theoretical value being 10.37. Results equally as satisfactory were obtained when using certain cereal products, as shown in Table I, the micro results being compared with those found by the Official Gunning Method ( 2 ) . TABLE I. COMPARISON OF MICRO-AND MACROMETHODS SANPLE MATERIAL MOISTURE
1 2 3 4 5 6
7 8
Flour
Flour Flour Flour Wheat
Wheat Wheat Corn
--PROTEIN ( N X 5.7)BY:Macro Micro Diff.
%
%
%
%
12.5 14.0 13.4 13.2 8.0 8.5 12.6 10.6
11.74 10.66 14.84 10.88 11.87 16.20 12.69 7.81
11.73 10.41 14.41 10.74 12.07 16.36 12.71 7.68
-0.01 -0.24 -0.43 -0.14 4-0.20 +0.16 +0.02 -0.13
The wheat and corn samples were ground on a Wiley laboratory mill (9) so that the entire sample passed through a screen of 0.5-mm. mesh. Samples 1, 2, and 5 were collaborative samples of the Association of Pacific Northwest Cereal Chemists, and consequently the macro results reported for these samples represent the averages of the protein as determined by a t least seventeen different laboratories. I n the other cases the micro results are compared with those obtained by the authors using the Official Gunning Method. On a routine basis an analysis can be made in about 35 minutes, The rapidity of this method for determining protein should be of great value to the cereal industries, inasmuch as it provides a means for rapidly checking their products. LITERATURE CITED Allen and Davisson, J. Bid. Chom., 40, 183 (1919). Assoc. Official Agr. Chem., Methods, p. 8 (1925). Bermann, Mikrochemie,2, 169 (1924). Kemmerer and Hallett, IND.ENQ.CHEY., 19, 1295 (1927). Parnas and Wagner, Biochem. Z., 125,253 (1921). Pregl, “Quantitative Organic Microanalysis,” 2nd ed., translated by Fyleman, p. 101, Blakiston, 1924. (7) Scott and Meyers, J. Am. Chem. Soc., 91,304 (1925). (8) Walther, J. prakt. Chem., 91,332 (1915). (9) Wiley, IND.ENQ.CHBM.,17,304 (1925).
(1) (2) (3) (4) (5) (6)
RECEIVBID October 19, 1931.
