Spectrophotometric Determination of Niobium in Niobium-Bearing Steels JAMES
L. KASSNER, ASDRUBAL GARCIA-PORRATA', end
E. L. GROVE
School o f Chemistry, M e t a l l u r g y , a n d Ceramics, University of A l a b a m a , University, A l a .
8-Quinolinol (oxime), analytical reagent. Hydrochloric (37%) and nitric (70%) acids, 1 to 1 ratio, reagent grades. Hvdrochloric acid wash solution.. 2%. Xiobium metal and oxide, C.P. Oxine solution, 1 f O.Ol%, in chloroform. Perchloric acid (ilo/O),reagent grade. Potassium bisulfate, analytical reagent. Sulfuric acid (97%), reagent grade. Sulfurous acid (freshly . prepared). - . Tantalum metal. Beckman p H meter, Model H-2. Beckman soectroohotometer. Model DU: 1.000-cm. matched cells; slit, o.io mm:
A simple procedure for the colorimetric determination of niobium in niobium-bearing steels has been developed which involves only two separations: a perchloric acid hydrolytic precipitation and a chloroform extraction of the metal derivative of 8-quinolinol. The great bulk of the steel components is removed as soluble perchlorates in the first separation. Elements that may contaminate the niobic acid precipitate do not interfere because they do not react with 8-quinolinol, their oxinates (quinolinates) are insoluble in chloroform, or they are not extracted by chloroform from the ammoniacal citrate solution. The method makes use of the yellow color of chloroform solutions of niobium oxinate which shows maximum absorbance at 383 mp. The method has been applied successfully to a niobiumbearing steel standard and to a series of composite steels. It involves a considerable saving in time and fewer mechanical operations without a sacrifice in accuracy.
T
H E hitherto little known element, niobium, is no longer a laboratory curiosity but is a n important element of metallurgy because of its role in preventing intergranular corrosion in stainless steels used a t high temperatures. Niobium is usually found along with tantalum, and their accurate separation by conventional wet methods is one of the most difficult of analytical problems. Schoeller (9) thoroughly studied the chemistry of these elements and developed the basic techniques for their separation and analysis. Modifications developed by Slavin (10) reduced the time for an analysis from about 15 to 5 days. A number of new methods have recently been developed for the analytical separation of and the spectrophotometric determination of this element. Hiskey and coworkers ( 2 )proposed the removal of interfering elements by the distillation of chlorinated products. Geld and Carroll (6) discussed a colorimetric method based on the yellow color of perniobic acid in concentrated sulfuric acid solution. Telep and Boltz ( 1 1 ) modified the method by making use of the absorbance of perniobic acid in the ultraviolet region. The yellow color of the niobium thiocyanate complex TYas also employed in a spectrophotometric method described by Freund and Levitt ( 5 ) . Hiskey (8) also has developed a simultaneous spectrophotometric method for the determination of tantalum and titanium using the peroxy complexes in concentrated sulfuric acid solution. I n this paper is described a procedure involving the spectrophotometric measurement of a chloroform solution of niobium oxinate, nThich has been extracted from an ammoniacal citrate solution. By this procedure four to six determinations can be performed in a day. The precision and accuracy of the method are good. REAGEYTS AND EQUIPMENT
Ammonium hydrovide, 1 =!= O.Ol-V, prepared from reagent grade. Chloroform, reagent grade. Citric acid solution, 25 f 0.05 grams per 100 ml., prepared from reagent grade. 1
Present address, University of Puerto Rico, Rio Piedras, Puerto Rico.
PROPOSED PROCEDURE
Weigh a 200-mg. sample (for steels containing between 0.3 and 3% niobium, otherwise change accordingly), place in a 600-ml. beaker, and dissolve with 50 ml. of a 1 to 1 mixture of 37% hydrochloric and 70% nitric acids Carry along a reagent blank treated exactly like the sample in this and succeeding operations. Add 35 ml. of i l % perchloric acid and heat until the acid begins to boil. Cover the beaker and heat so that the perchloric acid boils under reflux for about 30 minutes (6). Cool, dissolve n4th 50 ml. of distilled water, add 50 ml. of a freshly prepared saturated sulfurous acid solution and a small quantity of paper pulp, and dilute to about 300 ml. with distilled water Stir and boil gently for about 10 minutes, then digest for 2 hours. Cool, filter through No. 40 Khatman filter paper, wash well with 2% hydrochloric acid solution, and ignite the precipitate. Fuse the oxides nith 2.5 grams of potassium bisulfate in platinum crucibles a t as low a temperature as possible in order to avoid spattering, then cool and moisten with 5 drops of concentrated 97% sulfuric acid. Repeat the fusion and after cooling, dissolve the melt in about 40 ml. of a hot c i t i c acid solution containing 10 =k 0.02 grams of the acid, and dilute to 250 ml. with distilled water in a volumetric flask. Transfer a 15-ml. aliquot to a 125-ml. separatory funnel and add 15 ml. of a recently standardized 1 f 0.0LV ammonium hydroxide solution Mix thoroughly and then add 10 ml. of a 1 f 0 01% solution of ovine in chloroform. Shake uniformly for 3.0 minutes, draw off the chloroform layer, and read the absorbance a t 385 mp. With the use of a previously calibrated working curve, calculate the percentage of niobium in the sample. DEVELOPMENT OF PROCEDURE
Steel Composition. Siobium is one of approximately 25 elements nhich may be present in steels. These elements may be classified into four general groups: elements common t o all steels such as iron, carbon, phosphorus, and sulfur; common alloying and scavenging elements such as chromium, tungsten, molybdenum, cobalt, vanadium, aluminum, and titanium; elements used in special steels such as niobium, tantalum, and zirconium; and elements generally occurring as impurities such as tin, antimony, and arsenic. Perchloric Acid Digestion and Separation. Because of the large number of elements and the rather low selectivity of oxine, a prior separation of most of the elements in steel is necessary. The perchloric acid hydrolysis M-as selected because most of its salts are highly soluble and because of its strong oxidizing properties. The soluble perchlorates may be separated from the elements silicon, niobium, tantalum, tungsten, tin, and antimony, which form insoluble acids. Siobium and tantalum are precipitated quantitatively as the earth arids if the digestion is assisted with sulfurous acid (1). The insoluble acids may be contaminated with molybdenum,
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493
V O L U M E 27, NO. 4, A P R I L 1 9 5 5 vanadium, mangariese, boron, and bismuth, which map be coprecipitated to a small extent if they are present in large amounts or if the precipitates are voluminous. Titanium and zirconium phosphate may contaminate the acid precipitate if appreciable amounts are present, but are dissolved in a prolonged digestion with perchloric acid. A more nearly pure precipitate can be obtained by keeping the acid concentration quite high, thus preventing the formation of basic salts (12). Of the approximately 25 elements found in steels only six elements-namely, niobium, tantalum, tungsten, silicon, tin, and antimony-are precipitated by the perchloric acid hydrolysis. Molybdenum, manganese, vanadium, and boron may coprecipitate under special conditions. Titanium and zirconium map be prepent, as their phosphates are very slon-ly soluble in perchloric acid. 8-Quinolinol. The reactions of numerous organic complexing agents with niobium and tantalum solutions were examined. Of these 8-quinolinol appeared to be the most useful in aqueous tartaric and citric acid solutions. Niobium produced a yellow color, while tantalum produced no color. The color in the citric acid solution was much more intense than that in the tartaric acid solution. The action of 8-quinolinol with the interfering elements that may remain from the acid hydrolysis is summarized in Table I. hIanv of the data were obtained from Feigl(4); hoa-ever, the solutditics were also verified in this work. Boron, phosphorus, and silicon do not react. The reaction of tin and antimony are doubtful, since the extract does not show any absorbance. Molybtfenuni and tungsten form insoluble oxinates in chloroform. This is important, because tungsten is completely precipitated hv the perchloric acid digestion and molybdenum, lyhich may he coprecipitated, is often found in relatively large quantities. Tantalum and vanadium salts also are not extracted. Although the titanium oxinate is extracted and has high absorbance values at 385 mp, it did not introduce an error in composite steel samples (Table 11),and thus it must have been eliminated as a soluble perchlorate in the initial separation. Titanium probably exists chiefl\r as the carbide ( 3 ) and appears to go into solution quite rendilv.
Table 1 .
Table 11. Analyses of Composite Samples Prepared from NBS Samples" Niobium, % Error, I n sample Found % Impurity Ratio Si 5 : 1 0.39 -2.5 0.40 Si 5 : 1 0.46 +2.2 0.45 >lo 1 : l 0.57 -1.7 0.58 Ti 1 : l . Mn 3 : l 0.31 0.0 0.31 4 P 1 : 1 , hIn 9 : l 0.116 4-0.37 0.115 5 V 1 : 2 , Si 0 : l 0.103 -1.9 0.105 6 hIo 1:1, hIn G : l 0.103 * -1.9 0.105 7 w 1:l 0.72 0.0 0.72 8 Zr 1 : l 0.72 -1.4 9 0.73 T a 1:l 0.72 -2.7 10 0.74 V 1 : 1 , RIn 3:1 0.196 -1.0 0.198 11 Sn 1:l 0.75 +1.4 12 0.74 Sb 1 : l 0.73 -1.4 13 0.74 B 1 :l 0.G6 -1.5 14 0.67 Ti 1 : 1 , Si 2 : l 0.243 -2.0 15 0.248 W 5 : l , hln 10:1 0.102 0.0 16 0.102 Zr 1 : l 0.73 +1.4 17 0.72 T a 1 :l 0 . 7 3 1 . 4 18 0.74 Tantalum, zirconium, antimony, and boron added t o S B S samples.
Sample 1 2 3
a
These were then diluted to 35 ml. wit,h water and estracted with 10 ml. of a 1%solution of oxine in chloroform. The effects that various volumes of the l.OLV ammonium and sodium hydroxide solutions have on the amount of niobium oxinate and tantalum oxinate extracted is shown in Figure 1. The absorbance was measured a t 385 mp. The solution is highly buffered a t about a pH of 9.4 when 12 to 18 ml. of 1.ON ammonium hydroxide are added to 15 ml. of a 4% citric acid solution of the sample. Under these conditions the niobium oxinate extract exhibits a well defined color n-hile the tantalum oxinate shows no absorbance and so does not interferc.
08-
0
' 1
NiOH
Action of Oxine on Possible Interfering Elements
Element Antiinony Boron Manganese h'lolvbdenui 11 Niobium Phosphorus Silicon Tantalum Tin Titanium Tungsten Vanadium Zirconium
Action Doubtful S o action Reacts Reacts Reacts S o reaction N o reaction Reacts Doubtful Reacts Reacts Reacts Reacts
Solubility of Oxinate in CHCh
. . ...
Extraction from Ammoniacal Citrate Solution, pH 9.4 h*ot extracted
Soluble Insoluble Soluble
Eu t r act'e'd S o t extracted Extracted
SoILbie
Kot exir'acted Not extracted Extracted Kot extracted Not extracted Extracted
...
SolLbie Insoluble Soluble Soluble
Thus, of the potential interfering elements only manganese and zirconium remain to be considered. Manganese may be present because of coprecipitation and zirconium possibly because of the slow attack by perchloric acid. The procedure calls for an extended digestion in order for the perchloric acid to act on any resistive compounds of zirconium t h a t may be present. The results obtained viith synthetic samples (Table 11)containing relatively high ratios of manganese and zirconium to niobium indicated no interference. Optimum Conditions for Extraction. To determine the proper conditions for the complexing reaction and extraction, 15-ml. aliquots of 4% citric acid solution containing 0.080 mg. of niohium and 0.100 mg. of tantalum were treated with different quantities of I A' sodium and ammonium hydroxide solutions.
0.0
0
2
Figure 1.
4
6
8 10 12 ML. LON BA5E
14
16
I8
Effect of bases on chloroform extraction of niobium and tantalum oxinates
Effect of Time and Light. Chloroform solutions of certain other 8-quinolinates have been reported to undergo slow photochemical decomposition (Y), especially when expcsed to bright light. A niobium sample, along with a blank, was evtracted under the described conditions and the persistence of the developed color was studied for a period of 30 hours. The samples were kept in the Beckman Corex cells and protected constantly from light. The effect of the elapsed time was found to be negligible (Table 111),as the difference between the absorbance value of the oxinate and the blank remained constant. However, samples exposed to strong sunlight decomposed rapidly. Precision. The precision of the method, based on six samples of niobium containing 0.040 mg. per sample, is shown in Table 117. The deviations of the readings are within the experimental error. Absorption Characteristics. The absorbance of niobium
ANALYTICAL CHEMISTRY
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oxinate shows a maxima at 385 mp, Figure 2. The strong absorption band below 360 mp is due to the unreacted oxine; therefore its concentration must be kept as low as possible. One 10-ml. portion of 1% solution was found to be sufficient to react with all the niobium present in the samples used in this work. Seven samples of niobium oxinate-chloroform solution extracted from the buffered citric acid solution a-ere studied. At the concentrations employed the interference of the unreacted oxine was shown to be negligible, Figure 2. Also there is no displacement of the band as the concentration of niobium is changed. The absorptivity a (the relation log Io/I = a d ) averaged 122 when the concentration, c, was expressed in grams of niobium per liter and the light path was 1 cm. The calculated molar absorptivity, e, averaged 1.13 X lo4. The absorbance data a t 385 mp adhere to Beer’s law. A graph of the data a t 385 mp from Figure 2 us. Concentration of niobium s h o m a straight line intercepting a t the origin. ThuP, such factors as ionization, dissociation, and association may be considered to be absent. Since these factors are negligible, they can have no effect upon the distribution constant betLveen the two liquid phases. In the procedure, accurate control of the volumes of the two immiscible liquids is possible and recommended. Also, the oxinate is highly soluble in the chloroform solution. Therefore, under these conditions only one extraction is necessary. Results obtained using three extractions were no better than those obtained with one extraction. I n terms of milligrams of niobium, the concentration range corresponds to about 0.02 to 0.08 mg. per 15-ml. sample or from about 1.5 to 6.0 p.p.m. The niobium concentrations were brought into this optimum range by the proper selection of sample size and/or aliquoting. ANALYSIS OF STEEL SAMPLES
The validity of the procedure was checked by the use of KBS niobium-bearing steel 123a, which contains tantalum and
Table 111. Effect of Time on Color Stability of Niobium Oxinate in Chloroform (Protected from Light)a Time, Hours 0 1
3 6
15 24 30 0
Total
Absorbance Blank
l l b oxinate
0.442 0.442 0.443 0.447 0,450 0.452 0.453
0.125 0.126 0.129 0.130 0.132 0.134 0.135
0.317 0.316 0.315 0.317 0.318 0.318 0.318
Sample contained 0.040 mg. of niobium per 15 ml. of solution.
Table IV. Sample 1
2 3 4 5 6
Precision of Chloroform Extraction of Niobium Oxinate Siobium, Rlg. Present Found 0,040 0,040 0.040 0.040 0.040 0.040
Average
0.039 0,039 0.035 0.039 0.038 0.039 0,039
Absolute Error, M g . -0.001 -0.001 -0.002 -0.001 -0.002 -0.001 -0.001
Table V. ;inalysis of Kiobium-Bearing Steel 123a Sample
Klobium, 70 Certified Found 0.75 0.75 0.75 0.75 0.75 0.75 0 75
Average
0.74 0.74 0.73 0.75 0.76 0.74 0.75 0 74
Absolute Error, So -0.01 -0.01 -0.02 0.00
tO.O1 -0.01 0.00 0 01
Figure 2. 1. 0.120 mg. of 2. 0.096 mg. of 3. 0.084 me. of 4. 0.072 m i . of
Absorption spectra of niobium oxinate Present per 15 ml. of solution niobium 5. 0.060 mg. of niobium niobium 6. 0.048 mg. of niobium niobium 7. 0.024 me. of niobium niobium 8. 200 mg.-of oxine
titanium in small quantities The results are summarized in Table V. The average error of the seven samples is 1.3% based upon the amount of niobium present As steel 123a contained only tantalum and titanium in small quantities, composite samples were prepared with steel 123a and other NBS samples to provide samples with relatively high ratios of the potentially interfering elements to the amount of niobium present. The results of these analyses are summarized in Table I1 including the ratio of the impurities. The elements that may occur as major constituents, such as silicon, manganese, and tungsten, were tested in different ratios. Less important elements were examined only once. The errors of the determinations are small, although the ratios of the interfering elements to niobium are large The procedure effectively removes the interfering elements although they are present in relatively large quantities. LITERATURE CITED Am. Soc. Testing Materials, Philadelphia, P a . , “ A S T M Methods of Chemical Analysis of M e t a l s , ” pp. 79, 58, 1946. Atkinson, R. H., Steigman, J., a n d Hiskey, C. F., ANAL.CHEM., 24, 477 (1952). Bain, E. C., “Functions of Alloying E l e m e n t s , ” p . 64, American Society for Metals, Cleveland, 1943. Feigl, F r i t z , “Chemistry of Specific, Selective a n d Sensitive Reactions,” Academic Press, Kew York, 1949. F r e u n d , H., a n d L e v i t t , A. E., A N A L .C m x , 23, 1813 (1951). Geld, I., a n d Carroll, J., Ibzd., 21, 1098 (1949). Moeller, Therald, and Cohen, A. J., I b i d . , 22, 656 (19.50). Palilla, F. C., Adler, N., a n d Hiskey, C . F., Ibid., 25, 926 (1963). Schoeller, W. R., “ d n a l y t i c a l Chemistry of T a n t a l u m a n d Niobium,” C h a p m a n & Hall, London, 1937. Slavin, bI., P i n t o , C. &I., and P i n t o , RI., “ 4 T a n t a l i t a d o Nordeste,” Ninisterio d a Agricultura, D e p a r t m e n t o Nacional d a Produca Mineral, Rio de Janeiro, Brasil, Bol. 21 (1946). Telep, G., a n d Bolte, D . F., ANAL.CHEM.,24, 163 (1952). Willard, H. H., a n d F u r m a n , N. H., “Elementary Q u a n t i t a t i v e Analysis,” Van ii-ostrand, S e w York, 1949. RECEIVED for review May 15, 1954. Accepted January 3, 1955. Submitted by Asdrubal Garcia-Porrata in partial fulfillment for the degree of doctor of philosophy. Presented before the Southeastern Regional Meeting of the AMERICAN CHEMICAL SOCIETY, Auburn, Ala., October 1952.
