862
INDUSTRIAL AND ENGINEERING CHEMISTRY
Procedure SELECTION AKD PREPARATION OF SAMPLE. Procure a representative portion of the material to be analyzed, reduce to a fine powder, and dry in a desiccator. Proper care should be exercised, since trinitrobenzene can be detonated. MEASUREMENT. Weigh samples of 50 to 100 mg., dissolve in 95 per cent ethanol, and dilute to 50 ml. MEASUREMENT OF DESIRED CONSTITUENT. Withdraw an aliquot containing 0.1 to 1 mg. of trinitrobenzene, add 0.5 ml. of 10 per cent sodium hydroxide solution, and dilute to 50 ml. with ethanol. Measure or com are the color by any of the usual means within 10 minutes. A bge-green filter such as Corning No. 396 is recommended for photometric measurement, Beer’s law is valid for measurements a t 502 mw.
Summary A rapid colorimetric method for the determination of trinitrobenzene is described. The optimum range for measurements with a l-cm’ transmission is from Oaol to containing mg’ per 50 ml‘ Of By as high as 75 per cent trinitrobenzene can be accommodated
Vol. 14, No. 11
satisfactorily. With proper modification, the method should be applicable to samples such as air containing only small traces. Results were not affected by the presence of dinitrobenzene in an amount 100 times that of the trinitrobenzene.
Literature Cited (1) (2) (3) (4) (5)
Bost and Nicholson, IND.ENG.CHEM.,ANAL.ED.,7, 190 (1935). Brice, Reu. Sci. Instruments, 8 , 279 (1937). Drummond, J . SOC.Chem. Id.,41,338 (1922). Hammick, Andrew, and Hampson, J . Chem. SOC.,1932,171. Kay, Can. J . Research, 19B,86 (1941).
Lansing, IND.ENQ.CHBM.,ANAL.ED.,7, 184 (1935). (7) Mellon, “Methods of Quantitative Chemical Analysis”, p. 378, New York, Macmillan Co., 1937. (8) Rudolph, Z.anal. Chem., 60, 239 (1921). (9) Snell and Snell, “Colorimetric Methods of Analysis”, Vol. 11, p. 431, New York, D. Van Nostrand Co., 1937. (6)
ABBTRACTED from a thesis presented b y M. L. Moss to the Graduate School of Purdue University in partial fulfillment of the requirements for the degree ?f doctor of philosophy, May, 1942.
Colorimetric Determination of Iron with 272‘-Bipyridy 1 and with 272,‘2“-Terpyridyl M. L. MOSS WITH M. G. MELLON, Purdue University, Lafayette, Ind.
S
IKCE the work of Blau (1) on complex salts of 1,lOphenanthroline and 2,2’-bipyridyl, various analytical applications of these bases have been found aside from their use as colorimetric reagents for iron (4, 12, 17, 18, 19, 24). Inasmuch as 2,2’-bipyridyl, 2,2‘,2”-terpyridyl, and 1,lophenanthroline contain the same iron-specific, cyclic N-CC--N grouping, a basis for comparing the merits of these three compounds as iron reagents is desirable. (Since reactions between iron and possible pyridyl or phenanthroline isomers of 2,2’-bipyridyl, 2,2’,2”-terpyridyl, and 1,lOphenanthroline are not known, the shorter names, bipyridyl, terpyridyl, and phenanthroline, are used in this paper.) The determination of iron with 1,lO-phenanthroline has already been studied (5). Various workers have investigated the bipyridyl method, although not with particular attention to the effect of diverse ions (3, 6-11, 13, 20-23). Cooper (2) used terpyridyl in a study of the distribution of iron in sea water and in marine plankton, and reported that it is capable of detecting 1 mg. of iron in a cubic meter of water or 1 part per billion, a remarkable sensitivity. For low iron concentrations, it was preferred over bipyridyl. Morgan and Burstall obtained terpyridyl (2,6-di-2’pyridylpyridine) along with bipyridyl in the dehydrogenation of pyridine (15, 16).
Apparatus and Solutions Transmittancy measurements were made with a General Electric recording spectrophotometer adjusted for a spectral band width of 10 mw. A glass electrode pH-meter was used (14). A standard ferric nitrate solution prepared from iron wire of known purity contained 0.01 mg. of iron per ml. and sufficient nitric acid to prevent hydrolysis. Measured quantities of this solution were reduced with 10 per cent solutions of hydroxylamine hydrochloride or sulfate to obtain known concentrations of ferrous iron. Buffering was accomplished with a 20 per cent solution of ammonium acetate and pH adjustments were made with 6 N solutions of hydrochloric acid and ammonium hydroxide. Ten per cent sulfuric acid and 20 per cent phosphoric acid were prepared from the c. P. reagents. One-tenth per cent solutions of bipyridyl and terpyridyl were
used as the color-forming reagents. Bipyridyl dissolves in water, although not readily, while terpyridyl is not sufficiently soluble to permit preparation of a 0.1 per cent solution. Cooper used a 1 per cent solution of the base in 0.2 9 hydrochloric acid. This is a satisfactory procedure. since a wide latitude in pH is permissible in carrying out the color reaction. Not knowing this in advance, it seemed preferable to use the hydrochloride in order to avoid an excess of acid. The hydrochloride is easily prepared by passing dry hydrogen chloride into an ether solution of the base or by evaporating a small volume of hydrochloric acid containing the desired amount. Since bipyridyl and orthophenanthroline monohydrate do not dissolve readily in cold water, it would be advantageous to use these bases as hydrochlorides also. Cobalt nitrate hexahydrate was weighed directly to prepare a solution containing 2.5 mg. of cobalt per ml. Electrolysis of aliquots showed this to be a valid procedure. Standard solutions of the anions studied were prepared from the alkali metal salts in most cases. Sitrates, chlorides, and sulfates were used for the cation solutions. Each contained 10 mg. of the ion in question per ml. of solution.
COLORREACTION.The reddish-purple color formed by bipyridyl with ferrous ion, exhibiting its maximum absorption at about 522 mp, has been attributed to a complex ion containing 3 molecules of bipyridyl in which the 6 coordination positions of iron are occupied by nitrogen atoms. Similarly, terpyridyl gives a complex ferrous ion. Unlike bipyridyl and phenanthroline, however, only 2 molecules of the base are required, terpyridyl containing 3 nitrogen atoms. The terpyridyl complex resembles permanganate in hue and its absorption maximum occurs a t about 552 mp. For solutions containing 0.1 mg. of iron, 4 ml. of bipyridyl or 1 ml. of terpyridyl solution develops the maximum color and an excess produces no effect. EFFECT OF pH. Variations in p H between 3 and 9 do not influence the intensity or hue of the color produced with bipyridyl. Ignatieff (9) reports 3.5 to 8.5 as the optimum range. If the p H is less than 2.5, development of the color proceeds very slowly. If less than 2 or greater than 9.5, fading begins immediately and complete color development is not attained. With terpyridyl, p H variations between 3 and 10 do not produce any measurable effect on the intensity or hue
,
ANALYTICAL EDITION
November 15, 1942
O
I00
410
I
440
460
I
I80
100
I
I10
540
I
160
110
I
600
620
1
640
660
680
ID0
Navelength, i n m+
of the color. The workable range for phenanthroline is 3 to 9. Kide tolerances such as these are not always permissible with colorimetric methods. EFFECTOF IRON CONCENTRATION. Beer's law is valid for both reagents a t the wave lengths of maximum absorption. The range of iron concentrations amenable to measurement using a 1-cm. transmission cell is indicated in Figures 1 and 2. All solutions contained an excess of hydroxylamine and reagent. Visual comparison in Nessler tubes is best confined between 0.05 and 2 p. p. m. The molecular extinction coefficient for bipyridyl-ferrous ion a t 522 m p is 8650 and for terpyridylferrous ion a t 552 m p is 11,500. The latter value represents only a very small improvement in sensitivity over 11,100 for phenanthroline. A solution containing 0.001 p. p. m. of iron developed a slight color with terpyridyl, the transmittancy a t 550 mp being 98.5 per cent for a 5-cm. thickness of solution. REDUCING AGENTS, Although the color reactions are with ferrous ion, ferric or total iron may be determined by using a suitable reducing agent. Cooper used a 10 per cent sodium sulfite solution with terpyridyl and allowed 24 hours for development of the color. For bipyridyl, the following reducing agents have been recommended : titanous chloride, hydroquinone, ascorbic acid, sodium dithionite (sodium hyposulfite, sodium hydrosulfite), sodium sulfite, sulfurous acid, and hydrazine sulfate. Hydroxylamine hydrochloride was found to be very effective with the phenanthroline method (5) and works equally well with bipyridyl and with terpyridyl. Its use with bipyridyl has been suggested but not studied (23). Good results were also obtained with titanous chloride, hydroquinone, sodium dithionite, and ascorbic acid. Hydrazine and sulfurous acid are not recommended. PERMANENCY OF STANDARDS. The solutions represented in Figure 1 were kept for one year in glass-stoppered Pyrex bottles exposed to daylight. Transmission curves run periodically showed no changes within the limits of visual accuracy in color matching, Bipyridyl-iron standards are valid, there-
863
fore, for a t least 1 year. Hill (8) reported that the colored system was unchanged after standing 3 years in a sealed tube. Terpyridyl-iron standards for visual work are reliable for a t least 3 months. McFarlane recommends cobalt nitrate hexahydrate as a permanent color standard with the bipyridyl method, especially for low iron concentrations (IS). From the standpoint of hue, the choice of this salt is most appropriate, as will be seen on comparison of the curves in Figure 5. A 2-cm. transmission cell was used. I n view of the stability of the iron-bipyridyl complex, the need for artificial color standards is questionable, a t least for concentrations exceeding 0.5 p. p. m. EFFECTOF DIVERSEIONS. I n measuring the effect of diverse ions on the bipyridyl method, a standard solution of the ion in question FYas added to 0.1 mg. of iron measured as ferric nitrate. One milliliter of 10 per cent hydroxylamine hydrochloride solution and 4 ml. of 0.1 per cent bipyridyl were then added and, after dilution to 50 ml., the apparent iron concentration was determined using 2-em. transmission cells with distilled mater in the blank cell. Two per cent deviation from 2 p. p. m. is negligible for ordinary work and this arbitrary tolerance is used as a basis for defining freedom from interference in the following discussion. A similar procedure was followed with terpyridyl and 0.5 ml. of reagent solution was used for 0.05 mg. of iron in 25 ml. Hydroxylamine hydrochloride was used as the reducing agent and, in all cases, the ion in question was added to the iron before reducing. This is essential if the over-all effect is desired, because some ions interfere by preventing or retarding the reduction. The color develops immediately with both bipyridyl and terpyridyl (unless ions such as phosphates are present) and was measured within 10 minutes. I n certain cases, the effect of extraneous ions depends on the procedure followed, pH, time of standing, presence of other ions, and the order in which the various reagents are
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 14, No. 11
There may be justification for using phosphoric acid in the analysis of samples containing orthophosphate or a few other interfering anions. The reason for the curious behavior of phosphoric acid in eliminating phosphate interference is not apparent. Conversion of the orthophosphate to pyrophosphate in solution seems out of the question. The effect cannot be explained on the basis of acidity. As shown in Figure 3, 500 p. p. m. of carbonate or orthophosphate can be accommodated satisfactorily. The solutions contained 0.5 ml. of 10 per cent sulfuric acid, 0.5 ml. of 20 per cent phosphoric acid, and 5 ml. of 20 per cent ammonium acetate in 50 ml. Sulfuric acid, and not phosphoric acid, is responsible for the action and the order in which they are added is significant. Phosphoric acid should be introduced before the bipyridyl. Interference by carbonate, formate, nitrite, oxalate, orthophosphate, pyrophosphate, and silicate was completely eliminated by the phosphoric acid procedure, although the method is not satisfactory with metals such as cadmium, cobalt, copper, mercury, or nickel.
Discussion Inasmuch as the same iron-specific grouping is present in bipyridyl, terpyridyl, and phenanthroline, the reactions of these compounds with iron are similar and there is little basis added, thereby limiting any general statement aa to the exact extent of interference. Five hundred p. p. m. of the following ions may be present without causing more than 2 per cent error in the bipyridyl method : aluminum, ammonium, barium, calcium, cerium, lead, lithium, magnesium, potassium, sodium, strontium, uranyl, acetate, arsenate, arsenite, benzoate, bromide, carbonate, chlorate, chloride, fluoride, formate, iodide, lactate, nitrate, nitrite, orthophosphate, oxalate, perchlorate, pyrophosphate, salicylate, silicate, sulfate, sulfite, tartrate, tetraborate, thiocyanate, and thiosulfate. Uranyl ion does not affect the color in the spectral region 510 to 700 mp. The slight change in hue below 510 can be excluded readily from the color measurement. Interference of benzoate carbonate, formate, nitrite, orthophosphate, oxalate, pyrophosphate, and silicate was eliminated by using the phosphoric acid procedure (21). I n the case of fluoride full color development required 45 minutes. Reduction of tetraborate was carried out on the steam bath. Antimony, bismuth, and tin precipitate. Formation of bipyridyl complexes of cobalt, nickel, beryllium, and certain other metals results in interference which cannot be removed satisfactorily by using an excess of reagent. The extent of interference for various ions is summarized in Table I. With terpyridyl, 500 p. p. m. of the following ions may be present: aluminum, ammonium, barium, calcium, lead, lithium, magnesium, potassium, sodium, strontium, uranyl, acetate, arsenite, bromide, chlorate, chloride, fluoride, iodide, lactate, nitrate, perchlorate, salicylate, sulfate, sulfite, tetraborate, thiocyanate, and thiosulfate. Certain ions including carbonate may be removed easily, Transmission curves for solutions containing typical interfering ions are shown in Figures 3 and 4. Two-centimeter cells were used. PHOSPHORIC ACIDPROCEDURE, I n the determination of ferrous iron in drugs with bipyridyl, Schulek and Floderer ($1) recommend addition of sulfuric and phosphoric acids before developing the color and use of ammonium acetate as a buffer. This treatment is reported to stabilize the ferrousferric ratio. The procedure gives satisfactory results, provided sufficient time is allowed for the color to develop (phosphates retard the color reaction). For most work this is an unnecessary complication, since the color does not develop fully until about 7 hours after adding the bipyridyl.
TABLE I. EFFECT OF DIVERSE IONS WITH BIPYRIDYL Ion
Added nu AgNO: Cd(N0a)r Be(N0dr Co(NO1)r Crr(S0S: Cu(N0:)r HgNO; Hg(N0i)r Mus04 Ni(N0dr Th (NO:) 4 Ti(S04h Zn(N0:)r Zr(N0:)c NaiBkOi HCOiH (NH4)zCnOk CsHsCOiNa KCN KzCriOi (NH4)rMoOa KNOr KVO: NaiWOc
Preaent
Error
Amount Permissible
P . p . m.
%
P . p . m.
25 50 50 50 50 6 10
11 0 0 5 7.5 0 5.5 0 2 5 0 5 7 0 2 0 50 0 0 0 3 2 2 3
5 50 50 20 15 5 5
10
75 20 100 37 10 50 100
100 6 100 10 100
10
50
60 10
10 75
10 100 20 5 50 100 100 0 100 10 100 7
50 50
7
November 15, 1942
ANALYTICAL EDITION
TABLE 11. EFFECT OF DIVERSEIONS WITH TERPYRIDYL [On
Added as
Present P. p . m. 150 50
100 500 5 5 10 10
200
5 300 10 50 500 250 500 10 50 5
Mood-NO1
5 5 50 50 100
-voa WOd - -
10 75 5
CN CrrO7 --(Cr)
-
HPOI--(Plod P?Oi-- - 810:
10 10
Error
% ’ 3 0 0
3
30
77
Amount Permissible P . p . m. 100 50 100
300 0 0
2 0 0 6 0 0 0 2 1 2.5 5 0 0
10 200 0 300 10 50 500 250 400 5 50 5
1s 6.5 0 0 2 46 44 4 6 21
0 0 50 50 100 0 0 5 25 0
10
on which t o recommend any one over the others, aside from such considerations as cost and availability. (Terpyridyl is not listed by the chemical supply houses.) Advantages of terpyridyl in sensitivity and freedom from interference by p H variations and presence of diverse ions are hardly significant enough to consider in ordinary work. There are several outstanding differences, unpredictable in nature, which should be enumerated. Terpyridyl gives a cobalt complex sufficiently colored to be useful in the determination of cobalt. Bipyridyl and phenanthroline give practically no color with cobalt but form highly colored copper and molybdenum compounds. Silver interferes seriously with the bipyridyl and phenanthroline methods for iron, although 100 p. p. m. may be present if terpyridyl is used. Cobalt and copper cause somewhat more interference with terpyridyl than with the other two reagents. The wave lengths of maximum absorption for phenanthroline, bipyridyl, and terpyridyl are 510, 522, and 552 mp, respectively, and the hues vary accordingly. Terpyridyl has a more distinct band than the others and its maximum lies in the spectral region most favorable for visual measurements.. A satisfactory method for the determination of cobalt with terpyridyl has been studied. 1,lO-Phenanthroline, on the other hand, may be used for determining copper, and its ferrous complex is a valuable oxidation-reduction indicator (24). Ferrous complexes of bipyridyl and terpyridyl are not sufficiently stable in hot acid t o compare favorably with the “ferroin” indicators. The versatility of these cyclic bases containing the N-C-C--N grouping seems to justify further investigation as new compounds become available.
Recommended Procedure SAMPLE. Procure a representative portion of the material to be analyzed and subject it to the necessary preparative treatment. Measurement. Weigh or measure by volume a quantity of sample containing 1 m . of iron or less. This is the maximum iron content advisable for measurements with a 1-cm. transmission cell. For visual work using Nessler tubes, color comparison for solutions containing more than 2 p. p. m. is impracticable. Treatment. Dissolve the sample by appropriate means. Any of the common mineral acids except phosphoric acid may be used. Remove any interfering ions present, bjl suitable methods, a:cording to the tolerances listed in Tables I and 11. With 2,2
-
86s
bipyridyl, interference by 50 mg. of benzoate, formate, nitrite, orthophosphate, oxalate, pyrophosphate, or silicate may be eliminated by adding 2 ml. of 10 per cent sulfuric acid, 2 ml. of 20 per cent phosphoric acid, and 10 ml. of 20 per cent ammonium acetate at this point. Maximum color development requires about 8 hours if phosphoric acid is present. DESIREDCONSTITUENT.Measurement. Transfer the solution to a 100-ml. volumetric flask and add 2 ml. of 10 per cent hydroxylamine hydrochloride solution. Adjust the pH to 3 to 9 (ammonium acetate, 6 N ammonium hydroxide, and 6 N hydrochloric acid are satisfactory), add 10 ml. of 0.1 per cent bipyridyl solution or 5 ml. of 0.1 per cent terpyridyl solution, dilute to the mark, and mix well. The color develops immediately and may be measured or compared by any of the usual methods. (Ironbipyridyl standards for visual comparison may be kept in glaasstoppered bottles for at least 1 year. Terpyridyl standards are valid for a t least 3 months.) For photometric measurement, a green filter with maximum transmission between 480 and 540 mp is recommended for bipyridyl. With terpyridyl a filter with maximum transmission in the vicinity of 550 is desirable.
Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24)
Blau, Monatsh., 19, 647 (1898). Cooper, Proc. Roy. SOC.(London), B118, 419 (1935). Feigl, Krumholz, and Hamburg, 2. anal. Chem., 90, 199 (1932). Ferran, Ann. cMm. a p p l h t a , 27, 479 (1937). Fortune and Mellon, IND.ENQ.CHBM.,ANAL.ED.,10, 60 (1938). Gerber, Claassen, and Boruff, Ibid., 14, 364 (1942). Gray and Stone, Ibid., 10, 415 (1938). Hill, Proc. Roy. SOC.(London), B107, 205 (1930). Ignatieff, J . Soc. Chem. Id., 56, 407 (1937). Jackson, IND.ENG.CHEM.,ANAL.ED., 10, 302 (1938). Koenig and Johnson, J. Biol. Chem., 143, 159 (1942). KomarowskiI and Poluektov, Mik*ochim.Acta, 1, 264 (1937). McFarlane, IND.ENG.CHEM.,ANAL.ED., 8, 124 (1936). Mellon, “Methods of Quantitative Chemical Analysis”, p. 413, New York, Macmillan Co., 1937. Morgan and Buratall, J. Chem. SOC.,1932, 20. Ibid., 1937, 1649. Nieuwenburg, van, and Blumendal, Mikrochaie, 18, 39 (1935). Parker and McFarlane, Can. J . Research, 18B, 405 (1940). Poluektov and Nazarenko, J. Applied C h a . (U.8,S. R.), 10, 2105 (1937). Scharrer, 2.Pflanzensmllhr., Dtingung Bodenk., 33A, 336 (1934). Sohulek and Floderer, Ber. ungar. pharm. Ges., 15, 210 (1939). Shorland and Wall, Biochem. J . , 30, 1047 (1936). Thiel, Heinrich, and van Hengel, Ber.,71B, 756 (1938). Walden, Hammett, and Chapman, J. A m . Chem. Soc., 55, 2649 (1933).
ABSTRACT~D from a thesis presented by M. L. Moas to the Graduate School of Purdue University in partial fulfillment of the requiremenk for the degree of doctor of philosophy, May, 1842.
