b e between wide limits of cobalt concentration. Variation of p H had a marked effect on the color intensity of the system. A large excess of the reagent, leading to higher acidity, was found to lower the depth of color in itself but even if a higher pH was maintained, a large excess of the reagent was not tolerated. The optimum pH was between 4.8 to 5.25 (see Figure 2) and in the range of determinable cobalt (0.25 to 10 fig. of Co per ml.) 2 ml. of 1.OM thioglycolic acid was safely tolerated. The procedure was free of interference from most common anions in large concentrations. Most cations too were found to have no color reaction with the reagent, but iron, nickel, copper, manganese, chromium, vanadium, uranium, molybdenum, and tungsten, the usual interfering ions in colorimetric procedures for cobalt, caused serious positive or negative interference in the present procedure. All of these could, however, be easily eliminated by dithizone extraction of cobalt in basic citrate medium. The present method gave easily reproducible and accurate results in actual determinations of cobalt in solutions of known concentration. In its application to the determination of cobalt content of NBS samples of alloys and steels, the data obtained compare favorably with the specified certificate values. The thioglycolic acid procedure for
the determination of cobalt may, therefore, be claimed to be a simple, accurate, and dependable method of wide applicability for the determination of cobalt content of pure compounds, alloys, and steels. The technique evolved should ala0 find ready application to the determination of cobalt in soils, biological materials (such as animal tissue), ores, and other cobaltbearing samples. ACKNOWLEDGMENT
Sincere thanks of the authors are due to S. S. Joshi for keen interest and to R. H. Sahasrabudhe for permitting the use of the instruments. The award of a senior scholarship to V.D.A. by the Ministry of Scientific Research and Cultural Affairs is also acknowledged. LITERATURE CITED
(1) Allport, N. L., “Colorimetric Analysis,” p. 55,Chapman and Hall, London, 1947. (2) Bersin, T.,E. anal. Chem. 85, 428 (1931). (3) Buscarons, F.,Artigas, J., Anales real. soc. espafi. fh.y quim. (Madrid) 48B, 140 (1952); C.A. 46, 10044 (1952). Ibid., 49B, 375 (1953): C.A. 48, 2524 (1964). ’ (4)Chirnside, R. C., J . SOC.Glass Tech. 22 41-T(1938). (5)khirnside, R. C., Pritchard, C. F., Ibid., 23,26-T(1939). ( 6 ) Escudero-Tineo, R. E., Anales fac. farm. y bioquim., Univ. nucl. mayor
pect ro photo metric Determina ti o n ,I 0-phenanthroli
Sun Marcos ( G m ) 3, 489 (1952); C.A. 48,5735 (1954).
c.,
(7) Gupta H. M. L.,
SOgmi, No ANAL.d m ~31,9>8 . (1959). 18) Hart. 6. S.. Dtseert. Ab&. 13, 651 . ,(1953): (9) Koenig, R. A.,Johnson, C . R., J . Bid. C h . 142,233 (1942). (10)Ley, H.,Z. Ekktrochem. Soc. 10, 954 (1904). (11) Lyons, E.,J. A*. Chem. Soc. 49, 1916 (1927). (12) Marston, H. R., Dewe D. W., Azlstralian J . Exvtl. Biol. A d d . Sei. 18, 343 (1940). (13) Mayr, C.,Gebauer, A,, 2.anal. Chem. 113,189 (1938). (14’1Mevers. C. N.. J. Lab. Clin. Med. ‘ 6,359“192p ‘ (15) Mic aelis, L., Schubert, M. P., J . A m .Chem. Boc. 52,4418 (1930). (16) Misra, R. N., GuhwSircar, 5. S., J . Indian Chem. SOC.32,127 (1955). (17)Rosenheim, A., Davidson, J., 2. anorg. Chem. 41,231 (1904). (18) Sandell, E.B., “Colormetric Determination of Traces of Metals,” p. 543, Int,er!rRaience. __- ..-.. New York. 1959. (19)Ibid., p. i70. (20) Schubert,M. P., J. Am. Chem. Soc. 54,4077 (1932). (21) Snell, F. D.,Snell, C. T.! “Colorimetric Methods of Analysis,’ p. 319, Van Nostrand, New York, 1954. (22) Ubeda, F. B., Capitan, F. Analee real. SOC. espaii. fis. y quim. (Madrid) 46B, 453 (1950); C.A. 45, 5073 (1951). (23) Vogel, A. I., “A Text Book of Quantitative Inorganic Analysis,” p. 461,Longmans London J953. (24) Wenger, .PI E.,.&merman, c., Corbaz, A,, Mikrochzm. Acta 2, 314 (1938). (25) West, P. W.,Duff, M. A,, Anal. Chim. Acta 15,271(1956). RECEIVZD for review November 30, 1960. Accepted June 5, 1961.
6
races o
n
Application to Vanadium, Chromium, Titanium, Niobium, Tantalum, Uranium, Tun sten Metals, Alloys, and Compounds A. R. GAHLER, R. M. HAMNER, and
R. C.
SHUBERT
Research and Development Analytical laboratory, Technology Department, Union Carbide Metals Co., Division of Union Carbide Corp., Niagara Falls, N. Y.
b A rapid, accurate, spectrophotometric method for the determination of iron is described. It is particularly applicable for the determination of iron in high-purity vanadium, chromium, titanium, niobium, tantalum, and tungsten metals; aluminum-vanadium alloy; titanium-aluminum-vanadium alloy; uranium monocarbide; and compounds of these metals. The method is applicable over a wide concentration range-0.0001 to about 2%. Accuracy and precision data are pre-
sented for iron at various concentration levels in different materials. A determination can be made within ‘ / z hour after dissolution of the sample; therefore, it can be applied to control analyses.
accurate, sensitive, and rapid method has been needed for the determination of iron in the range of 0,0001 to 20/, in high-purity metals, alloys, oxides, carbides, metal halides, N
and other inorganic materials. The difficulty experienced in obtaining accurate results with existing procedures, particularly for iron in high-purity vanadium metal and alloys containing Vanadium, indicated that an improved method was needed. A method of wide applicability wets sought because of the general interest in small amounts of iron in various materials. A preliminary survey of published methods indicated that the spectrophotometric method for the determinaVOL 33, NO. 13, OECEMBER 1941
*
1937
tion of iron with bathophenanthroline (4,7 - diphenyl - 1,lO - phenanthroline) as proposed by Smith, McCurdy, and Diehl (6, 11) could be modified and advantageously applied to many products. The reagent is unique in that the iron complex is extractable into a n immiscible organic liquid containing an alcohol. Thus, interferences from colored ions such as chromium and vanadium are eliminated by the liquidliquid extraction procedure. Likewise, error in the absorbance measurement caused by the presence of a colloid or precipitate as a result of hydrolysis of metals such as tantalum or titanium during the color development process is eliminated. The reagent forms a very stable colored complex which is more sensitive than most of the common colorimetric systems for iron. The formation of the iron(I1) tris(bathophenanthro1ine) complex is fairly specific under the conditions recommended in the literature. This paper points out the versatility and applicability of this reagent for the determination of iron, particularly in the presence of colored ions such as vanadium and chromium, and presents the results of a study of the accuracy and precision of the method as recommended in this paper. The success of the method depends largely upon the reductant used. Sodium hydrosulfite, as proposed by Crawley and Aspinal (a), reduces iron in the presence of large amounts of complexing reagents such as citrate or tartrate which are required to keep metals in solution at the pH for extraction. Since this reductant has somewhat different properties than hydroxylamine (6, 11) which is usually used but fails to reduce iron completely in the presence of citrate or tartrate under the conditions of the extraction, a study of the effect of a number of diverse ions not formerly reported in the literature was necessary. EXPERIMENTAL
Apparatus. All absorbance measurements were made in 1.000-cm. cells with a Beckman Model D U spectrophotometer. pH measurements were made with a Beckman Model G p H meter. Reagents. All chemicals used were reagent, but several were further purified to remove traces of iron. 4,7 - Diphenyl - 1,IO - phenanthroline (Bathophenanthroline) Solution, 0.1% (G. Frederick Smith Chemical Co.). Dissolve 0.25 gram of bathophenanthroline in 250 ml. of 95% ethanol (or methanol). The solution is stable. Sodium Hydrosulfite (Sodium Dithionite-Na&OJ Solution (10%) (Mallinckrodt). Prepare fresh immediately before use, Dissolve 10 grams of sodium hydrosulfite in 100 ml. of water and transfer to a separatory funnel. To remove traces of iron, add 10 ml. of bathophenanthroline solution and 15 1988
e
ANALYTICAL CHEMISTRY
ml. of chloroform, and extract the iron complex. Repeat the extraction until the chloroform layer is colorless. If this solution is not used within 30 minutes, discard and prepare a fresh solution. Sodium Citrate Solution (approximately 100 grams in 1 liter of solution). Dissolve 100 grams of sodium citrate in 1 liter of water and adjust the pH to 5 with citric acid. To remove the iron, transfer approximately 150 ml. of the sodium citrate solution to a separatory funnel, add 3 ml. of a cupferron solution (2 grams per 100 ml. of water), let stand several minutes, and extract the cupferrate with about 25 ml. of chloroform. Repeat the extraction with chloroform three to four times to remove all the iron cupferrate and the excess cupferron. FinaIly extract with 50 ml. of diisopropyl ether to eliminate all traces of cupferron in the aqueous solution. If all the cupferron has been removed, the solution will remain colorless for weeks. Discard the solution if it becomes yellow upon standing. Sodium Citrate Solution (approximately 300 grams in 1 liter of solution). Dissolve 300 grams of sodium citrate in a liter of water and remove the iron as described in the preceding paragraph. Standard Iron Stock Solution (1 ml. of solution contains 1 mg. of iron). Dissolve 0.1 gram of high-purity iron in 10 ml. of hydrochloric acid. Add 2 ml. of nitric acid and evaporate to near dryness. Add 5 ml. of HCl and dilute to 100 ml. in a volumetric flask. Dilute Standard Iron Solution (1 ml. of solution contains 0.01 mg. of iron). Transfer 5 ml. of standard stock solution to a 500-ml. volumetric flask. Add 25 ml. of hydrochloric acid and dilute to the mark with distilled water. PREPARATION OF CALIBRATION GRAPH
Place I-,2-, 5-, and 10-mI. aliquots of the dilute standard iron solution into separatory funnels. Carry a blank with the samples. Add 25 ml. of water, 20 ml. of the purified sodium citrate solution (100 gram/liter) to each funnel, and adjust the pH of the solution with ammonium hydroxide to between 4 and 8. Then add 20 ml. of the fresh preextracted sodium hydrosulfite solution. Mix between addition of reagents and allow to stand 15 minutes after the sodium hydrosulfite addition t o reduce the iron. Add 10 ml. of the bathophenanthroline reagent and then 15 mI. of chloroform. Shake for 30 seconds, allow the phases to separate, and draw off the chloroform layer into a 50-ml. volumetric flask previously rinsed with 95% ethanol. Add 10 ml. of chloroform to the separatory funnel and repeat the extraction. Dilute the combined chloroform extracts in the 50-ml. volumetric flask to the mark with 95% ethanol. Mix and read the absorbance of the solutions in a 1-em. cell a t 533 mp with the blank in the reference beam of the spectrophotometer. Plot absorbance ws. concentration.
RECOMMENDED GENERAL PROCEDURE
The following general method was followed in the analysis. The dissolution procedures are described under the section Dissolution of Metals, Alloys, and Compounds for the Determination of Iron. Dilute the sample or aliquot containing from 0.005 to 0.12 mg. of iron to a volume of about 30 ml. with water, add iron-free sodium citrate solution, the amount varying with the metal content of the aliquot or sample, and adjust the pH to 4 to 6 with ammonium hydroxide. The p H adjustment may be made with p H paper, but with highly colored solutions such as vanadium or chromium, a p H meter is necessary. Transfer the sample to a separatory funnel, add 20 ml. of ironfree sodium hydrosulfite colution prepared immediately before use, mix thoroughly, and allow to stand for exactly 15 minutes. Add 10 ml. of bathophenanthroline reagent, then 15 ml. of chloroform, and shake for at least 30 seconds. Allow the two layers to separate and draw off the chloroform layer into a 50-ml. volumetric flask containing 2 to 3 ml. of ethanol. Repeat the extraction of the aqueous phase in the separatory funnel with 10 ml. of chloroform. Allow the layers to separate and draw off the chloroform solution into the 50-ml. flask. Dilute to volume with ethanol and mix. Measure the absorbance of the solution at 533 mp using in the reference cell a blank on the reagents which has been carried through the same treatment as the sample. Determine the iron concentration by reference to an absorbance us. concentration graph. DISSOLUTION OF METALS, ALLOYS, AND COMPOUNDS FOR DETERMINATION OF IRON
A brief description of the dissolution of several materials follows. In all cases, a blank on the reagents must be carried with the samples. When nonhomogeneity in a sample is known to occur, it is advifiable to take a larger sample and determine the iron on an aliquot of the solution. Specific sample sizes and volumes of reagent solutions are indicated for those samples where hydrolysis may occur. Vanadium Metal. Add 10 ml. of HC1 and 3 ml. of “03 for each ram of sample. Add 10 ml. of HzSO, Tl: 1) and evaporate to light fumes of sulfuric acid. Cool, add 10 ml. of water and 5 ml. of sulfurous acid, and again evaporate to light fumes of sulfuric acid. Cool, dilute with water, and determine the iron as described under the Procedure. Vanadium Pentoxide. Add 20 ml. of HC1 for each gram of sample. Warm to dissolve the sample, add 5 ml. of sulfurous acid, and evaporate to a low volume t o remove most of the HCl. Dilute with water and determine the iron as described in the Procedure. Ammonium Metavanadate. Add 30 ml. of HCl (1:2) for each gram of sample. Heat gently nith occasional
stirring until the sample is dissolved, then evaporate to a low volume to volatilize most of the acid. Add 5 ml. of sulfurous acid and heat to reduce the vanadium. Cool and determine the iron in the sample. Chromium Metal. Add 30 ml. of HCI (1:2) for each gram of chromium metal. Heat gently until the sample is dissolved and then evaporate to remove most of the HCl. Cool and proceed with the color-forming procedure. Niobium and Tantalum Metal. Dissolve 0.5 gram of metal in a platinum dish in 5 ml. of hydrofluoric acid and 10 to 15 drops of Hxo3 added dropwise. Evaporate to near dryness (do not bake), cool, add not more than 1 ml. of hydrofluoric acid and 5 ml. of water. Add 25 ml. of purified sodium citrate, transfer to a separatory funnel, and proceed with the reduction and colorforming process. A 1.0-gram sample can be used with proportionally larger volumes of reagents. Titanium Carbide. Dissolve 0.5 gram of sample in a platinum dish in 5 ml. of hydrofluoric acid and 20 to 25 drops of HN03, add 10 ml. of H2SO4, and fume until the solution becomes colorless, adding more HxC'O3, if necessary. Dilute to a 100-ml. volume, take an appropriate aliquot, and continue with the reduction as described under the Procedure. Titanium Metal, Titanium-Aluminum-Vanadium Alloy. Add 20 ml. of HC1 (1:3) and 10 to 15 drom of HF for each 0.5 gram of sample In a platinum dish. After dissolution, add 15 ml. of sodium citrate solution (300 grams per liter), 25 mi. of boric acid (40 grams per liter), dilute to volume, and take an appropriate aliquot, if necessary, and continue with the reduction as described under the Procedure. Alternately, the sample may be dissolved in dilute sulfuric acid. Tungsten Metal. The method of Crawley and Aspinal (3) consists of dissolution of the metal in a mixture of hydrofluoric and nitric acids, addition of citric acid, and evaporation to dryness on a water bath. The residue is dissolved in sodium hydroxide solution, citric acid added, the p H adjusted, and the colored system developed. Uranium Carbide (UC). Transfer, in a dry box flushed with argon, about 0.500 gram of the uranium carbide from the sample bottle to a weighing bottle of known weight. (Handling in the dry box is necessary only if the material is of very small particle size.) Obtain the weight of the weighing bottle and contents. Transfer the sample in the weighing bottle to a 250-ml. beaker. Cautiously add from 10 to 15 ml. of water and then HNOJ dropwise at intervals until a volume of from 10 to 15 ml. of HSOs has been added. Digest a t a low temperature until all action ceases. Add 10 ml. of H2SO4 (1 : 1) and evaporate to fumes of sulfuric acid to remove all traces of HNOs. Cool, add 25 ml. of water, transfer t o a 250-ml. volumetric flask, and dilute to volume. Transfer an aliquot t o a separatory funnel, add sodium citrate solution,
adjust the p H and proceed as described for development of the colored system. Uranium Oxide. Add 5 ml. of HNOa and 10 ml. of HsS04 (1:l) to each gram of sample. Evaporate to light fumes of sulfuric acid, cool, add 10 ml. of water, and 5 ml. of sulfurous acid. Boil and evaporate to light fumes of sulfuric acid. Cool and dilute with water.
Maximum absorption of the iron(I1)
tris(bathophenanthro1ine) (6) occurs near 533 mp. Since the reagent does not exhibit absorption at the wave length of the absorbance measurement, the volume of the reagent added to the blank and sample does not need to be accurately measured. Dilution of the chloroform extract with ethanol is advantageous because i t clears any turbidity in the organic phase cawed by small droplets of water. Varying the ethanol (95%)-chloroform ratio from 1 : 2 to 1 : 6 has no effect on the colored system. It is recommended that about 10 ml. of ethanol (9501,) be added t o the CHCls solution (total volume of 60 ml.), The red colored system measured at 533 mp follows Beer's law from 0.01 to 0.12 mg. of iron(I1) in 50 ml. of chloroform-ethanol solvent using 1-cm. cells (absorbances 0.08 and 3.0, respectively). Reductant. Although several authors successfully used hydroxylamine hydrochloride as reductant for iron, low recoveries result when either tartrate or citrate is present. The determination of iron in the presence of many metals requires some complexant to avoid precipitation of the metal during p H adjustment. Craw-
COLOR REACTION
Several solvents have been used for extraction of the iron complex. Those described in the literature are isobutyl, amyl, isoamyl, or hexyl alcohol, and nitrobenzene (1-3, 6, 8-11). Crawley and Aspinal (3)recommended the chloroform-ethanol extraction system proposed for copper with neocuproine (7). The chloroform-ethanol extraction system was selected because the organic liquid is heavier than the aqueous and can be readily separated from the aqueous phase. Iron (0.1 mg.) is completely extracted within about 30 seconds into 15 ml. of the chloroformalcohol solvent. The aqueous phase is usually washed with an additional 10 ml. of chloroform. The colored system in the chloroform-ethanol solution is stable for a t least 1 week. Methanol may be substituted for ethanol,
Table
l.
Effect of Diverse Ions on Extraction and Spectrophotometric Determination of Iron with Bathophenanthroline
Diver8e Elernent
Added
T\"~!~os
F'
1 g.
Cr Nb Ta
Cr metal Nb metal
u
cu
Ta met,al -. .~.. . ~ . ~ in 0.45 g. of UsOS in 2.0 g. of UaOe
cu co
Cu metal Cu metal Co metal
h10 Ni
Ammonium molybdate Ni metal
F F
HF
HF
Concentration of Diverse Element (mg.) 436 1000
500 375 380 1700 0.5
5.0 1 .o 1.o
540 1.o
2.0 5.0
10.0 1 ml. HF
2 ml. HF
F
1 ml. HF
F
2 ml. HF 5 ml. HF
HaBOa Citrate Nitrate HNQa
2.0 g. sodium citrate 5.0 g. sodium citrate 0 2 g. NaNOs 0.5 g. NaN03 I
"0s
1000 1000
1290 3220 146 225 2 ml. HNOa
Fe Added (mg.) 0.050 0,050 0.050
0.050
0.050 0,050
none 0.050 0.050 0.050 0.050 0.050 0.050 0.050
0,050
0.050 0.050 0.050
0,050 0.050
Fea ReRecovered covery (mg.) (%) 0,049 98 0.050 102 0.050 0.050 0,050
0.052 3 . 9 p.p.m. 0.050 0.054 0.035 0,049 0,050 0.050 0.050
0.044 0.026
100 100 100 104
98
100 108b
75
98C 100 100
100 88 52 100
0.050
0.043 0.041 0.052 0.051
86
82 104
0.050 0.050 0.050 0.050 0,050
102 0.051 102 Results erratic 0.049 98
0.050 0.050
104
0.050 0.050
0.050 0 050 0.050 0.060
100
0.050
0,049 0,048 0.052 0.052 0,050
98 96
104 100
0.050 0.050
100
2 ml. HsP04 100 f ml. HCIOI 0.049 98 Amount of Fe present in original material corrected in calculations. * Sample had to be filtered because of fine precipitate formed upon reduction with NazSzQ4. Additional reagent (total of 20 ml.) added instead of usual 10-ml. volume. HsPOr HCIO,
VOL. 33, NO. 13, DECEMBER 1961
e
1939
ley's suggestion (3) to use sodium hydrosulfite as reductant ovwcomes the difficulty experienced with hydroxylamine hydrochloride. Since sodium hydrosulfite solution is instable, i t must be prepared and purified immediately before use. Low results for iron are experienced when a solution of the reductant is used which has been standing for more than one-half hour. Stannous chloride was avoided as a reductant for several reasons. This reagent must be added before citrate (1) for complete reduction of the iron. Thus, hydrolysis is likely to occur when iron is determined in easily hydrolyzed metals such as tantalum and niobium. Additional citrate would also be re-
Table II. Effect of Manganese on Extraction of Iron with Bathophenanthroline Concentration iron present, 0.05 mg. Manganese Iron Concentration Recovered bg.1 ( %) 10 100 15 88 20 78 25 78 50 58 100 36 150 100 200 100 250 100 a 30 ml. of reagent (0.1%) used in the extraction instead of the usual 10-ml. volume.
Comparison of Bathophenanthroline Method with Other Methods for Iron in Various Metals and Alloys '% Iron % Iron Sample (Other Remarks Pertaining to Values Identi(BathophenNo. of Methods) Obtained by Other Methods ficat'ion Detns. anthroline) Vanadium ... 2 0.029,0.024 v-1 ... 2 0.090 ,o ,089 v-2 V-2 Preliminary separation of iron with 0.24 0.24 . 1 a sodium hydroxide separation 0.26 0.25 v-4 1 with La as carrier for Samples 0.25 0.27 v-5 1 V-3 to V-6 0.50 2 0.50, 0.52 V-6 Iron determined titrimetrically 1.17 1.16 v-7 1 Vanadium Oxide Preliminary ammonium hydroxide 0.018 2 0,019, 0.019 vo-1 separation with Al as carrier, H2Sseparation to remove remaining V and colorimetric Fe with thioglycolic acid 0.023, 0.024 0.031 2 vo-2 0,008 1 0.012 vo-3 V-A1 alloy
Table 111.
(40%
v)
TT-AI-~
2
0.38, 0.39
0.39
V-A1-2 V-.41-3 V-A1-4 NHdVOa NV-1 Ti sponge
1 1 1
0.19 0.32 0.42
0 .43
2
0,0024, 0.0024 0,0017
Ti-1
1
0.0066
0.0073
1
Average of 3 determinations. Preliminary HzSNaOH separations before color development with thioglycolic acid
0.19
0.29
Cr metal Cr-2
1
0.13 0.091
0.11 0.092
Cr-3 Gr-4 Iodide-Cr Battelle
2
0.046, 0.047
1
0.056
0.044 0.058
1
10 p.p.m.
10 p.p.m. Analysis by Battelle Memorial Inst. spectrographic method
2
3.8, 3.9p.p.m.
4 p.p.m.
1
0.96
0.96
2
0.030, 0.031
Cr-1
Uranium oxide Uranium carbide Ta metal Ta-1
1940
0
ANALYTICAL CHEMISTRY
...
Fe was separated from Cr by a cupferron extraction before colorimetric determination with 1,lOphenanthroline
Analysis by the National Bureau of Standards
quired to complex the tin to prevent precipitation a t the p B of the extraction. Effect of pH. Iron is completely extracted over the p H range of 3 to 9. However, the p H should be below 6 to eliminate the deleterious effect of small concentrations of copper (refer to section Effect of Diverse Ions). At a p H of 3.0, the sodium hydrosulfite reducing agent tends to decompose, forming a slight turbidity in the aqueous layer. Therefore, it is recommended that the pH of the solution be adjusted between 4 and 6 before addition of the reductant and subsequent extraction. EFFECT OF DIVERSE IONS
Under the conditions of the recommended procedure, which require the presence of citrate for complexing metal ions and sodium hydrosulfite for reduction of iron, several interferences have been noted in variance with the interferences listed in the procedure proposed by Smith, McCurdy, and Diehl (11) in which hydroxylamine was used as reductant. I n addition to the interference of large amounts of cobalt, copper, and nickel, manganese also interferes. Table I shows the effect of these and other elements. More than 10 mg. of manganese requires excess bathophenanthroline for extraction of the iron, With a 10-ml. volume of reagent (O.lcJO), 0.05 mg. of iron is completely recovered in the presence of up to 10 mg. of manganese. By increasing the amount of reagent added, the same amount of iron can be extracted from a t least 25 mg, of manganese (Table 11). Therefore, the manganese apparently reacts with the reagent and when manganese is known to be present, the amount of reagent must be increased, I n samples containing manganese, cobalt, or nickel as major components, a prior separation of the iron in an acid solution by a preliminary extraction with cupferron into chloroform is required. After evaporation of the chloroform, the iron cupferrate is destroyed with HNOs and HC10, (or H7S04)acids, and then the iron determined colorimetrically. Cobalt also reacts with bathophenanthroline; therefore, excess reagent is required to complex both the iron and cobalt. Iron (0.05 mg.) can be completely extracted in the presence of 1 mg. of cobalt if 20 ml. of 0.1% reagent is added instead of the usual 10 ml. (see Table I). Up to' 2.0 mg. of nickel may be present. Low recovery of iron results when 5 mg. of nickel is present, probably due to complexation of the nickel with the reagent. With lange conyentrations of nickel, the colored system has a yellowish hue. The effect of large concentrations
of vanadium, niobium, tantalum, chromiurn, titanium, and uranium wm studied by the standard addition technique. Known amounts of iron were added to solutions of these metals and the recovery was complete (see Table I). Small amounts of fluoride (up to 1 ml. of fluoride as HF) do not interfere. With 1.4 grams of boric acid present, 2 ml. of hydrofluoric acid cause no interference. Sulfate, chloride, perchlorate, nitrate, borate, and phosphate do not interfere (see Table I). Since perchlorates interfere seriously with other iron reagents, there is a definite advantage in using bathophenanthroline. Five grams of sodium citrate do not retard the extraction of iron. Attempts to overcome the deleterious effect of copper, nickel, cobalt, and manganese were not made, although the literature cites several examples using different reduction systems. For example, iron can be determined in copper by forming the bathophenanthroline complex, addition of cyanide to complex the copper, and extraction of the iron complex (4). Iron in bismuth is determined by reduction of the iron with tin(I1) chloride and complexation with bathophenanthroline followed by addition of disodium(ethy1enedinitrilo)tetraacetate and citrate to complex the bismuth ( I ) . Cupferron, if it is not completely removed during the sodium citrate solution purification process, interferes by coextracting with the iron(I1) bathophenanthroline complex. Reagent purification is necessary to keep the reagent blank to a minimum. DISCUSSION AND RESULTS
Vanadium interferes with the formation of the color of iron(I1) with 1,lOphenanthroline and several other reagents. Therefore, the determination of iron in vanadium metal and alloys with these reagents requires separation of the iron from the vanadium. Several methods of separation are applicable but all require appreciable time. The common practice of separation by precipitation of traces of iron with sodium hydroxide in the presence of peroxide is subject to considerable error unless a carrier is introduced. For example, the iron in vanadium metal samples V-3, V-5, and V-6 (Table 111) was determined colorimetrically with 1,lO-phenanthroline after a preliminary sodium hydroxide-peroxide separation of the iron from the vanadium. The results on these three samples with 10 mg. of lanthanum as carrier were 0.24 (V-3), Q.25 (V-5), and 0.50 (V-6) yo,and without carrier 0.09 (V-3), 0.06 ( T U ) , and 0.34 (V-6) %, respectively. This is typical of the erratic low recoveries obtained without a carrier. The addition of lanthanum improved the recovery of iron but the method is lengthy.
Table IV.
Precision and Accuracy Study of Spectrophotometric Method for Iron with Bathophenanthroline
Sample Vanadium Vanadium oxide
vo-4
vo-5 Ti-AI-V alloy NBS 178
% Iron By Other Methods
No. of Detns.
...
9
% Iron
Range
Std. Dev.
0.0010
0 * 0003
...
0.0090
5
0.0303
0.0015 0,0019
0,0008 0.0008
0.16
6b
0.153 0.149
0.008
0.014
0.006 0.003
0.009
7
Mean” % Iron 0.0179
60
Ti sponge NBS 351 0.022,d 5 0,0220 0.0013 Chromium cr-5 0.040 10 0,0390 0.0023 Cr-7 ... 9 0.0032 0.0012 Niobium 0 00220 0,0025 0.0002 7 Tantalum Ta-3 ... 7 0.0115 0.0005 0 Averages are reported to an additional decimal place to show range. b Sulfuric acid dissolution procedure. c HF-HC1 dissolution procedure. d Average of 4 deterrmnations with l,l0-phenanthroline. Average of 6 determinations with l,l0-phenanthroliie. ~
I n contrast, the procedure as described in this paper requires no separations other than the extraction of the colored iron bathophenanthroline complex into the immiscible organic solvent. The results of a study of the accuracy and precision of the method for iron in vanadium, titanium-aluminum-vanadium alloy, aluminum-vanadium alloy, vanadium oxide, ammonium vanadate, titanium, chromium, niobium, tantalum metal, and uranium oxide are shown in Tables 111and IV. Although not many standards are available for determining the accuracy of the method, agreement between values obtained by other methods is excellent. The agreement is good between values for National Bureau of Standards 173 Ti-Al-V alloy containing a reported 0.16yo iron ‘and results by the bathophenanthroline procedure. (Table IV shows the average to be 0.15%). A sample of uranium oxide analyzed by the bathophenanthroline procedure contained 3.8 and 3.9 p.p.m, of iron by two determinations. The National Bureau of Standards analysis is 4 p.p.m. A sample of iodide-chromium from Battelle Memorial Institute was analyzed for iron and a value of 10 p.p.m. was determined. This compared with a similar value of 10 p.p.m. determined by Battelle. Precision data of the method on various materials are shown in Tables III and IV. The values mere obtained by six chemists. A separate study of t w methods ~ of dissolution of Ti-AI-V alloy (NBS 173) was made. The average of six determinations using sulfuric acid done was 0.15%, and the average of six
0.0005 0,0007 0.0005 0.0001 0.0002
determinations using hydrofluoric and hydrochloric acids was also 0.150j0. The advantages of this method are its sensitivity, specificity, rapidity, and simplicity. At least four samples and a blank may be analyzed for iron in less than 45 minutes after dissolution of the samples. Consequently, the method has been used for control analyses. It has been demonstrated to be both accurate and precise for the determination of iron in vanadium, vanadium products, and a wide variety of other metals, alloys, and inorganic materials. ACKNOWLEDGMENT
The authors thank I. H. S. Fraser, Galen Porter, and J. W. Norwood for their support of this work and furnishing samples for testing the method. LITERATURE CITED
(1) Booth, E., Evett, T. W., AnaEyst 83, 80 (1958)~ (2) Collins, P., Diehl, H., ANAL.CHEM:. 31,
1692 (1959).
(3) Crawley, R. H. A., Aspinal, M. L., Anal. Chim. Acta 13, 376 (1955). (4) Diehl, H. Buchanan, E. B., Talanta 1 , 76 (1958j. ( 5 ) Diehl, H., Smith, 6. F., “The Iron
Reagents: Bathophenanthroline; 2,4,6Tripyridyl-2-triazine; Phenyl-2-Pyridyl Ketoxime,” G. Frederick Smith Chemical Co., Columbus, Ohio, 1960. (6) Diehl, H., Smith G . Frederick, “Quantitative Analysis,” p, 370, Wiley, New York, 1952. ( 7 ) Gahler, A. R., AXAL. @HEM. 26, 577 (1954).
(@-Peterson, R. E., Ibid., 25,1337 (1953). (9) Seven, M. J., Peterson, R.E.,ZMd., 30, 2016 (1958). (10) Sherwood, B. M., Chapman, F. W., Ibid., 27, 88 (1955). (11),Smith, 6. F., McCurdy, VV. H., Diehl, H., Analyst 77, 418 (1952). RECEIVEDfor review April 21, 1961. Accepted October 6, 1961. VOL. 33, NO. 13, DECEMBER 11961
* 194’1
