V O L U M E 2 5 , N O . 6, J U N E 1 9 5 3
947
1. The use of spark excitation under the conditions outlined. This permits the simultaneous determination of the four major elements, as well a s the common minor elements, in plant material with an accuracy which compares favorably %-ithchemical methods. The author has seen no evidence indicating that such complete coverage can be satisfactorily attained with one exposure, by either direct current arc or flame techniques. 2 . The use of integrated emulsion curves representing different wave length portions of the spectrum. A reference line in one portion may be used in a valid intensity relationship throughout the density ranges with an analysis line in a portion where gamma characteristics are different. This simplifies photometry procedure. 3. The use of sets of correlated analysis scales in the manner shown for dealing directly with fluctuating conditions. The use of analysis scales is, of course, not new but the particular construction and application described may be of interest.
Table 11.
Reproducibility on Ten Analyses of Collaborative -4lfalfa Sample No. 3 (1953)
Element,
Yo
K
Ca Mg
P
Fe A1 Xa Mn. p.p.rn. B, p.p.rn.
Mean 2.23 1.42 0.225 0.263 0.041 0.053 0.037 30 24
Standard Deviation
0,099 0.068 0.0051 0.0058 0,0012 0.0014 0.0026 1.1 2.8
Coefficient of Variation 4.4 4.8 2.3 2.2 2.9 2.6 7.0 3.7 11.6
The precision and comparative accuracy of this method, as used routinely in this laboratory for the past 7 years, have been evaluated by statistical analysis of several sets of analytical data One such study involved analysis for major elements of some 240 eamples of alfalfa by both spectrographic and chemical methods in these laboratories. I t was shown that the two procedures were approximately equal in precision for potassium, calcium, and phosphorus in this material, with coefficients of variation for the wries not exceeding 3.0 in any case. The coefficient of variation for magnesium was 3.8 for the spectrographic and 8.8 for the chemical method. Details of the statistical treatment of these data will subsequently be presented for publication. Other comparisons of this method within these laboratories and with outside laboratories using chemical, spectrographic, and flame techniques have been made. Partial data from a study
(6) made in 1951 by the writer are shown in Table I. This tabulation shows the ranges of means obtained by different laboratories on collaborative sample S o . 3. Laboratory differences for this sample are typical of those shown for three additional samples used in the study. The means are grouped and averaged for the methods used. The means shown for this method for potassium, calcium, magnesium, phosphorus, iron, and copper are in satisfactory agreement with those obtained by the best respective procedures involved. The wide ranges of means shown for aluminum, sodium, manganese, and boron leave the actual sample content of these elements in complete doubt. 9 portion of this same sample was used in determining the current reproducibility of this method, shown in Table 11. This evaluation refers to instrumentation only, as the 20 exposures were made on aliquots of the same sample solution. Ten sample exposures and duplicates of the low and medium standards were placed on each of two films. Sample ratios from the different films were paired according to their order on the films, to constitute ten analyses under method conditions. The four ratios for each of the two standards were averaged to obtain the respective reference values for each element. The copper results on these films were erratic, owing to contamination of the particular batch of electrodes, and were not used. Previous evaluations for this determination have shown an average coefficient of variation of 6.5. The means shown in Table I1 (1953) are interesting in relation to the means obtained by this method in 1951 (Table I). The over-all indication from the collective studies is that this method is capable of a precision and potential accuracy quite in line with the best of the procedures that were used. LITERATURE CITED
Applied Research Laboratories, Glendale, Calif., “Instructions for Use of Calculating Board.” Farmer, V. C., Spectrochim. Acta, 4, 224 (1950). Feldman, C., ANAL.CHEM.. 21, 1041 (1949). Fred, M.,Nachtrieb, S . H, and Tomkins, F. S.,J . Opt. SOC. Amer., 37,279 (1947). Mathis, W. T , J . Assoc. Ofic. Agr. Chemists, 31, 562 (1948). Ibid., 35, 406 (1952). ,Milbourn, M.,and Hartley, H. G. R., Spectrochina. A d a , 3, 320-6 (1948).
Rossa, J. T., J . O p t . Soc. Amer., 40, 804 (1950). RECEIYED for review December 20, 1951.
Accepted March 25, 1933.
Colorimetric Determination of Iron with Nitroso R Salt Restudy of the Method JOHK A. DEAN AND J. HAROLD LADY Cniversity of Tennessee, Knoxeille, Tenn.
V
AN KLOOSTER ( 5 )was the first to observe that nitroso R salt gave a bright green color reaction with very dilute solutions of ferrous iron. Little was done toward applying this reagent as a colorimetric method for iron because many of the other elements usually present in alloys also form highly colored complexes with nitroso R salt. Sideris ( 4 ) was aware of these interferences in his investigation of the microdetermination of iron in plant materials. Griffing and Mellon ( 3 ) extensively investigated the formation of the ferrous complex and the use of the complex as a colorimetric method. They concluded that the reagent was more suitable for color measurements of iron than either 1,lO-phenanthroline or 2,2‘-bipyridine; however aluminum precipitated in
the procedure of Griffing and Mellon, and cobalt, copper, nickel cerium, chromium, and uranium interfered because of the formation of colored complexes of their own or because of the color of their aquoions. These interferences severely limited the utility of the method. In the present investigation all these interferences appear to be eliminated by the use of hydrazine as a reducing agent and of a short heating period to destroy all diverse. ion complexes and any excess dye. Compared with other existing colorimetric methods for iron, the modification introduced renders nitroso R salt a more selective reagent for ferrous iron. The method has been successfully applied to a variety of copper-, nickel-, and aluminumbase alloys without any prior separations. The entire proredure
ANALYTICAL CHEMISTRY
948
This work was undertaken to obtain a more direct and specific eolorimctric method for iron. The reagent, nitroso R salt, reacts with ferrous iron in an acetate buffer to produce a green colored complex which has a maximum absorbancy at 720 mp. When hydrazine is used as the reducing agent, and the temperature is maintained at 75' to 80' C., most other interfering color complexes and any excess reagent are completely destroyed without affecting the color intensity of the iron complex. The procedure is applicable to the direct determination of as little as 0.0470 of iron in copper-, nickel-, and aluminum-base alloys without prior separation of the iron or chemical removal of any of the usual constituents of the sample. The method is rapid and requires only 20 minutes after sample dissolution. The elimination of the color of excess reagent permits the application of the method visually or with simple instruments.
requires approximately 20 minutes time following dissolution of the sample. APPARATUS
-4Klett-Summerson photoelectric colorimeter, glass cell model, employing both 2- and 4-em. cells and a 690-mfi broad band filter, was used for all colorimetric measurements. A Beckman Model H-2 p H meter was used for adjustment of PH. REAGENTS
A standard solution of iron, 1.00 ml. = 0.400 mg., was prepared by dissolving 0.400 gram of iron wire of known purity in 20 ml. of 1 to 1hydrochloric acid and diluting to 1 liter with distilled water. A weaker standard solution of iron, 1.00 ml. = 0.00400 mg., was prepared by pipetting out 10.0 ml. of the above solution, adding 10 ml. of 11 J4 hydrochloric acid, and diluting to 1 liter. Hydrazine dihydrochloride, 10% solution, was prepared t)>dissolving 100 grams of the C.P. salt in 1 liter of distilled water. Nitroso R salt, 1% aqueous solution, was prepared hy dissolving 5.0 grams of purified salt in distilled water and diluting to 500 nil. Commercial grade reagent was purified by dissolving in a minimum amount of hot water, cooling, and filtering thc crystals which formed, and finally washing with a 1 to 1 alcohol-water mixture. These steps were repeated until bright golden ycllom crystals were obt.ained and the sahrated mother liquor no longer had any- greenish tint due to the iron complex. After the final crystallization and filtration, the cryst'als were washed with absolute alcohol and air dried. h n alternative chromatographic purification procedure has been described ( 2 ) . Ammonium acetate or sodium acetate solut,ion, 2 JI,n-:w prepried by dissolving 154 or 164 grams, respectively, of the C.P. salt in distilled wat,er and diluting to 1 liter. EXPERIMENTAL
Color Reaction. The reagent solution itself is yellow. K i t h fiwous iron it, forms a bright green complex with a broad transniittancy minimum around 720 mp. The color of the complex is stable for a t least 24 hours, even in the absence of e x c ~ s s rengent. pH. I n agreement with Griffing and 11ellon ( 3 ) it was found that no complex formation took place below p H 3. Partial development occurred a t p H 4; between p H 5 and 7 the color development. is complete and suitable for quantitative work. By heiting the composited solutions the color development is hTstcned and there is no variation in color intensity abovc, p H ;2.3, as rrpx-ted by the above authors, until a p H of 8 or highy, is :rttained. The increased color intensity a t higher p H vzilues 15 attributed to the change in the color of the dye itself which turns t.,n green. ('olor development is most rapid between pH 6 and T : likewise thra destruction of the diverse ion complexes is most rapid :it this pH. However, appreciable amounts of copper, if pi,rsent, :ire reduced to th? metallic state which necessitate? working at ii lower pH value, around 5 . Reagent Concsntration. Gencmdly 1 nil. of lYc nitroso It salt qolution is requir ld for each milligram of sample. Additional amounts of dye solution are n c d e d for samplrs containing large amounts of cohalt. Approximately 3 ml. of t,he dye solution should be added for each milligram of cobalt, present as the cobalt, complex forms much more readily than any of the othw complexes, including ferrous iron, and t,his tics up thtx reagrliit 01)hrfore complete formation of the ftxrrous comp1c.s rail tained. 1 x 2
Reducing Agent. Hydrazine, a stronger reducing agent than hydroxylamine which had been used by all the previous inrrstigators, readily destroys the color complexes of all divers? ions upon heating the solution adjusted between p H 5 and i . The destruction of these color complexes may be due to a reduction t o a lower valence state, as in the case of copper and cobalt, or it may be due to the removal of the dye molecule from the compl~x t)y reaction with hydrazine. Any excess nitroso R salt is :tho reacted upon in such a manner as to yield a nonreactive molr,cule from the standpoint of complex formation. One tenth milliliter of 10Yc hydrazine dihydrochloride solution is sufficient to react completely with 2 ml. of the nitroso R reagent. The success of t'he method really depends on thc cific stability of the ferrous comple p H 6 to 7 and a t elev temperatures in the presence of ex hydrazine, a t which point the dissociation of the other compl is sufficiently complete to permit the reaction betTveen the dissociated dye molecule? :ind hydrazine to take place. At p H 5 the ferrous complex is less stable a t elevated temperatures in the presence of hydrazine, and the heating of the solution must be discontinued as soon as the h s t traces of excess dye are removed. Effect of Temperature and Time of Heating. I n thc ahst,rice of appreciable amounts of copper, the rolor of the ferrous coniples is developed prcfcrably in a solution buffered a t p H 6 to T , and a t a temperature just helon- the boiling point until the hydrazine has reacted with tht. diverse ion complexes and esc('ss dye. The completion of the reaction is noted by the disappearance of the brown color caused hy the diverse ion compleses and only the bright green color of t.he ferrous complex remains. The ferrous complex is very stahle and is not affected by a prolonged heating period of several minutes after the last traces of excess dye are removed. However, under these condition? nirtnllic copper, 1vht.n copper is present in the sample to the extent of 8 mg. or more, usually precipitates as a finely divided suqwnsion. This difficulty ciin be circumvented by working at 1" 5 in order to deci,ease thr reducing action of hjdi,azinr. AAtp€€ 5 though, the Feri~ouscwn-
Table I.
Effect of Diverse Ions
25,0 50 0
II.011 Found, Mg." 0,0400 0,0397
Element Added Cerium C hromiuin
1
.o
0.0397
Cobalt
1.0
0.0404
0 0 0 0
0.0400 0.0402 0.0400 0.0410
1 0
2 0 5.0
0,0395 0,0403 0.0410
Thorium
1. 0
0.0402
Uranium
1. o
0,0398
Vanadium b
1
.o
0,0407
Zinc
1.0 3.0
0.0400 0.0410
Copper
Sicke
1 5 10 20
a 0,0400 mg. of iron present in each case. b Solution was bright green while hot and lavender when c o d e d : ficulty eliminated by raising p H above 6.
dif-
949
V O L U M E 2 5 , N O . 6, J U N E 1 9 5 3 Table 11.
Analysis of Bureau of Standards Samples by Modified Nitroso R Salt Method Certified Fe Value,
Sample Sheet brass 37d 71 Cu, 27 Zn Manganese bronze 62b 58 Cu. 38 Zn
%
Copper-Base Alloys 0.076 0.82
Phosphor bronze 63 78 Cu, 10 Zn Phosphor bronze 63b 78 c u
0.27
Ounce metal 124b 84 Cu, 5 Zn Sickel silver 157 72 Cu, 18 Xi, 0 . 1 Co bilicon bronze 158 91 Cu, 2 Zn .4luminum brass 164 64 Cu, 22 Zn
0.26
0.47
0.053 1.48 2.52
Value Found
%
0.077. 0.067, 0.77, 0.78. 0.80, 0.27, 0.26, 0.44, 0.47, 0.46, 0.24, 0.28, 0.065, 0,052, 1.49, 1.52, 2.52, 2.56,
0.073 0.076 0.81 0.77 0.82 0.27, 0 . 2 6 0.26 0.44 0.47 0.46 0.20 0.26 0.058 0.065, 0.044 1.44 1.60 2.54 2.60
Sickel-Base Alloys llonel 162 0 34 0 38, 0 . 3 6 66 Xi, 29 Cu, 0 . 5 Co 0 38, 0 37 Sickel-base casting 161 15.0 15.1, 15.2, 15.1 64 Xi, 17 Cr, 0 5 Co Aluminum-Base Alloys Aluoiinurn alloy 85a 0,208 0.21, 0.22 94 AI, 2 c u , 2 Mg 0.206, 0,212, 0.206, 0.214, 0.209, 0.208 Aluniinum alloy 87 0.46 0.50, 0 . 4 9 89 4 1 , 2 Zn .4luminum alloy 86c 0.90 0.92, 0.94 91 d l , 6 Si, 8 Cu
plr\ is less stable to high development temperatures and to proIringrd heating periods after the excess dye has been removed. truction of the diverse ion complexes also takes place owlp. h-evertheless, amounts of copper as large as :in he successfully handled a t pH 5 if the t'eniperature during the hrnting period is maintained between i5' and 80' C., and tht, heating is discontinued immediately after the last trace of 1)rownish color due to the diverse ion complexes disappears. t-sual heating time required under these conditions is 10 minutes. Tho solutions must be cooled immediately to room temperature, and the transmittancy be measured without delay. Longel, heating or heating at, higher temperatures usually results in p a ~ t i a ldecomposition of the iron complex and risk of eventual further reduction of copper to the metallic state. At pH value Io\ver than 5 , there is less risk of copper precipitating but the ferinus complex is partially decomposed even upon immediate cooling, probably because a much longer time is required to dwtro). the diverse ion complexes. As a precaution it is suggested th:it the solution be ed for 10 minutes a t i 5 O to 80" C. l f thv rolor of the dive1 In complexes disappeam, the method is :ii)l)licat>le. DISCUSSIOS
Thp concentration rangcl of t m t accuracy for tbe colorimetric, niethod, as recently illustrated hy Aiyres(I), extends from about 1 G to 60 micrograms of iron per 50-nil. volume when using 2-cm. cuvrttea. With the coloiinietc~used, the minimum detectable R mount \vim found to he 0.1 niicarograni of iron per 5O-nil. volume. Tlic concentration range \voulc! lie from 8 t o 30 micrograms of ii,on per 50 ml. for -I-(.ni. cuvettw, corresponding to a minimum oi 0.04% iron for copper-haw :iIlo>-s based on the maximum amount of copper which can hc hnndled by the procedure, Ai smd1 blank, equivalent to appt,osini:ttely 1.00 microgrnni of iron, ~ v : i r found to be pracatic.ally con3t:int utilws unusually large :iinount$ of buffer solution were used. Large amounts of the niairi ronstit~i~iit-. of' wpper-, nickel-, :t~id aluminum-base alloys ma>-be preyent without ofiering m y interfei,c~nce. Table I summarizes the effects of othcr metal ion. upon the modified nitroso R salt nir,thod. Only the major ions pirsc:nt iri c'opper-, nickel-, and aluniinum-base alloys, anti fou~idI)?Griffing and blellon (3) to interfere seriously, ~ v c r eiixtrstcti. The noninterference of mppcr, vobalt, nickel, zinc-, chromium, and uim~iuniis notable. In many environments the method 1 1 0 ~ heromcs tilmost specific for iron. The method i8 rapid. requires
no pi lor separations, is adaptable to routine determinations, and requires no expensive reagents or elaborate equipment. It is more sensitive than the popular 1,lO-phenanthroline and 2,2'bipyridine methods, and circumvents the interferences common t o both. Unlike the thiocyanate method, the color complex does not fade upon normal standing and the color complex obey. Beer's law over the entire useful range. Since excess dye is removed by the hydrazine treatment, the method becomes adaptable to visual colorimetric comparisons or with simple instruments although it is difficult to detect small changes in intensity for a green rolor. Table I1 summarizes the results obtained on Bureau of Standards copper-, nickel-, and aluminum-base alloys. I n most caQes the results obtained were within the prerision attainable bv normal colorimetric methods and in good agreement with the certificate values. It was noted, however, that in samples containing less than 0.1% of iron the results were generally high although it would be expected that amounts as Ion- as 0.04% could he accurately determined. The reason for this was not apparent. PROCEDURE
Calibration Curve. Suitable aliquot portions of thp w a k r r standard iron solution are taken, and to each are added 0 . j 1 1 . of 10% hydrazine solution t o ensure that the iron is completrxly in the ferrous state, then 10 ml. of 170nitroso R salt solution, followed by sufficient 2 h' ammonium acet,ate solution t,o adjust the pH between 6.0 and 7.0. The solution is placed in a watw bath maintained a t 95" C. for 5 to 10 minutes, or unt'il the yellow color of the excess reagent becomes indistinguishable from the bright green color of the iron complex. The solution is then cooled immediately t o room temperature, transferred to a 50-ml. volumetric flask, and diluted to the mark. The transmittancy is determined at a wave lengt,h of 690 to 750 mp. -4concentration range of 0 to 60 micrograms of iron per 50-ml. volume will cover the useful range of the Klett-Summerson colorimeter using a 2-cm. cuvette. For Copper- and Nickel-Base Alloys. The sample is dissolved in 1 to 3 hydrochloric acid by t,he addition of sufficient nitric acid just to cause dissolution. T o the entire sample or a suitnhle aliquot portion is added 0.5 ml. of 10% hjrdrazine solution, 10 nil. of 17 nitroso R salt solution, and sufficient 2 M solution or ammonium acetate solution to adjust the pH to 5.0, The solution is transferred to a SO-ml. volumetric flask and diluted to the mark. Then the flask is placed in a water bath maint,aincd txtween 75" to 80" C. until the brownish color or precipitate of the copper and nickel complexes disappears, leaving a bright green color. The flask is immediat,ely removed and cooled to room t,emperature and the transmittancy of the solution is measured without delay. For Aluminum-Base Alloys. The procedure for aluminumbase alloy? differs slight]!- from that desci,ibed for the copprrhase allo!-s because the large quantit>iesof aluminum present result in the formation of a gelatinous precipitate under the conditions normally employed for the development of the ferrous iron complex. The reagents are added in the usual manner and the color is developed as before except that it is done in a small heaker in order that the solution may be stirred continuously while hmting to ensure thorough mixing. .\fter the brownish color of the diverse ions disappea1.s the solution is cooled to room temperature and concentrated sulfuric acid is added dropnise and slovc-ly with constant stirring until the gelatinous precipitate redissolves and a clear green color remitins. The solution is then immediately transferred t o a 50-nil. volumetric flask, diluted to the mark, and the transmittancy is measured. Gsually the resultant pH is 3. S o rapid decomposition of the ferrous iron complex occurs a t room temperature unless the pH is lowered helo17 2.5. LITERATURE CITED (1) .lyres, G . €I., .%XAL. CHEM.,21, ti53 (1949). ( 2 ) D e a n , ,J. d..Ibid., 23, 1096-7 (1951). (3) Griffing. AI., and AIellon. 11.G., I h i d . , 19, 1014-16 ( 1 9 4 7 ) . ( 4 ) S i d e r i s , C. R., 1x1). E s o . CHEM..~ h - . < r , . ED.,14, 75ti ( 1 9 4 2 ) . ( 5 ) T a n K l o o s t e r , H. S..J . A m . CRem. S o c . , 43, 743 (1921).
KECEIVEE hugust 6. 1952. Accepted hIarch 28, 1953. Abstracted from a thesis suhinitted by James Harold Lady to the Graduate School of the University of Tennessee in partial fulfillnient of the requirements for the degree of master of science in chemistry, 1952. Presented at the Southeastern Regional Meeting. Auburn, .Ila.. 1932.
