Photometric Determination of Columbium, Tungsten, and Tantalum in Stainless Steels LUTHER IKENBERRY, Research Laboratories, Armco Steel Corp., Middletown, Ohio, AND J. LEO MARTLN AND W. J. BOYER, Rustless Division, Armco Steel Corp., Baltimore, M d . The increased use of ferrocolumbium containing appreciable amounts of tantalum (5 to 25%) has made it necessary to determine the tantalum as well as the columbium content of Type 347 stainless steel. These steels often contain varying amounts of tungsten which inadvertently gets into the melt with the scrap. Columbium. tungsten, and tantalum are readily separated as a group by the conventional hydrolysis scheme. Photometric procedures have been developed for determining these three elements after the group separation. ' The columbium and tungsten are determined by additive absorbancies utilizing the color reaction with hydroquinone, and the tantalum is determined by utilizing the color reaction with pyrogallic acid. The method was found to be satisfactory when applied to the mixed oxides of columbium, tungsten, and tantalum after a preliminary group separation. This scheme can also be applied to the mixed oxides obtained from other materials such as ores and slags.
I
T HAS been customary to determine the combined columbium
and tantalum contents of Type 347 stainless steel by the conventional hydrolysis technique and to report the combined weight of oxides as per cent columbium. In most instances this practice was satisfactory because the tantalum and tungsten contents of Type 347 stainless steel were low. However, because of the recent use of ferrocolumbium containing appreciable amounts of tantalum (5 to 25%) and the use of scrap containing variable amounts of tungsten, it has become necessary to know how much of each of these elements is present. A photometric scheme of analysis appeared to offer the best means of avoiding the customary separation of these elements, which are lengthy and tedious, and therefore subject to inherent errors. Columbium and tungsten form colored complexes with hydroquinone in concentrated sulfuric acid. Bogatski (I), Heyne ( d ) , and Johnson ( 3 ) have used this colored complex for the quantitative determination of columbium and/or tungsten. The hydroquinone complexes of columbium and tungsten might be determined simultaneously by additive absorbancies, providing their maxima occur a t sufficiently separated wave lengths to apply this technique. Tantalum forms a colored complex with pyrogallic acid in an acidified ammonium oxalate solution. Thanheiser ( 5 ) used this colored complex for the quantitative determination of tantalum. APPARATUS
The Kromatrol photometer was employed in most of this work; the test tube supplied with the instrument was used as the absorption cell. The cylindrical absorption cell has a light path of approximately 1 cm. Kromatrol filter 3, with maximum transmittance a t 460 mp, and filter 5, with maximum transmittance a t 525 mg, were used for the determination of columbium and tungsten. Kromatrol filter 2, with maximum transmittance a t 430 mg, was used for determining the tantalum. The photometer should be adjusted to zero absorbancy, with air a t each wave length, in order to obtain absorbancy values for the reagent blanks. These blank values will serve to detect the beginning of decomposition of the columbium and tantalum reagents. The calculations that are involved in additive absorbancy
photometry are much more conveniently made by converting all per cent transmittance values to an absorbanc value by the relation, absorbancy = 1000 (2 - log T ) . T&s relation eliminates all decimals, thus simplifying the calculations. A Beckman DU spectrophotometer was used to prepare the absorbancy curves of the columbium, tungsten, tantalum, and titanium complexes. REAGENTS
Stannous Chloride Solution. Dissolve 30 grams of stannous chloride in 100 ml. of hydrochloric acid (1 4). by- heating- in a covered beaker. Ammonium Oxalate Solution. Transfer 80 grams of ammonium oxalate to a 2000-ml. beaker, add approxigately 1000ml. of water, and heat until dissolved. Transfer the hot solution to a 2000-ml. volumetric flask and dilute nearly to the mark with cold water. Cool to room temperature, dilute to the mark, and mix. Filter before use. Hydroquinone-Sulfuric Acid Solution. Dissolve 6.0 grams of hvdroquinone in 100 ml. of sulfuric acid by use of B mechanical sther.- Prepare the amount needed fresh"each day. Pyrogallic Acid Solution. Transfer 50 grams of pyrogallic acid to a 300-ml. glass-stoppered Erlenmeyer flask and add 100 ml. of water. Stopper the flask and swirl occasionally until completely dissolved. Thir usually requires about 1 hour. This solution is unstable and gradually becomes colored, but it may be used for 24 hours if a blank is obtained with each group of determinations. Standard Tantalum Solution. Transfer 0.272 gram of potassium tantalic fluoride (K2TaF7) to a platinum crucible, add 15 drops of sulfuric acid (1 l),and evaporate on a sand bath until all excess sulfuric acid has been volatilized. Add approximatel 6 grams of sodium bisulfate and heat a t a temperature just higg enough to dissolve the oxides and obtain a clear melt. Allow the crucible and its contents to cool to nearly room temperature. Add 5 drops of sulfuric acid and again heat to a temperature just high enough to obtain a perfectly clear melt, but no longer, lest an excessive amount of sulfuric acid be volatilized. Cool the crucible and its contents to room temperature, transfer to a 250ml. beaker, and add 125 ml. of ammonium oxalate solution. Cover the beaker and digest on a low temperature hot plate until the fusion has been dissolved. Filter the solution through a KO.40 JYhatman paper containing a small amount of ashless paper pulp. Wash the paper with a small amount of water and then dilute the filtrate to 500 ml. with ammonium oxalate solution. Mix thoroughly. This solution contains approximately 0.00025 gram of tantalum per mi. It may be standaEdized as follon~s: Transfer 50-mI. aliquots to two 400-ml. beakers, add 30 ml. of sulfuric acid (1 l ) , and dilute to 200 ml. with water. Cool the solution to below 15' C. and precipitate the tantalum with 30 ml. of an aqueous solution of cupferron (6%). Add a small amount of ashless paper pulp, filter, and wash the paper and precipitate well with cold water. Ignite to constant weight in B platinum crucible.
+
+
+
wt' Of 50
0'819
=
grams of Taperml.
Standard Columbium Stock Solution. Transfrr 1.OOO gram of potassium columbate (4K30.3Cb~Ob.16H~O) and 8 grams of sodium bisulfate to a platinum crucible. Heat a t a temperature just high enough to dissolve the oxides and obtain a clear melt. Allow the crucible and its contents to cool to room temperature, add 5 drops of sulfuric acid, and again heat to a temperature just high enough to obtain a perfectly clear melt, but no longer, lest an excessive amount of sulfuric acid be volatilized. Cool the crucible and its contents to room temperature, transfer to a 250ml. beaker, and add 125 ml. of ammonium oxalate solution. Cover the beaker and digest on a low temperature hot plate until the fusion has been dissolved. Filter the solution through a No. 40 Whatman paper containing a small amount of ashless paper pulp. Wash the paper with a small amount of water, dilute thefiltrate to 500 ml. with ammonium oxalate solution, and mix
V O L U M E 2 5 , NO. 9, S E P T E M B E R 1 9 5 3
1341
thoroughly. This solution contains approximately 0.0008 gram of columbium per ml. It may be standardized as follows: Transfer 25-ml. aliquots to two 400-ml. beakers, add 30 ml. of sulfuric acid (1 l), and dilute to 200 ml. with water. Cool the solution to below 15" C. and precipitate the columbium with 30 ml. of an aqueous solution of cupferron (6%). ildd a small amount of ashless paper pulp, filter, and wash the paper and precipitate with cold water. Ignite to constant weight in a platinum crucible.
+
Wt'
Of
Cbz06 25
0'699
=
grams of Cb per ml,
Standard Columbium Working Solution. Transfer exactly 25 ml. of the above stock solution to a 200-nil. volumetric flask and dilute to the mark with ammonium oxalate solution. Standard Tungsten Solution. Dissolve 0.0897 gram of reagent grade sodium tungstate ( Na2W04.2H20) in water and dilute to 500 ml. This solution contains 0.0001 gram of tungsten per ml.
While filter 3 does not appear to be the ideal wave length for maximum columbium absorption and minimum tungsten absorption, actual tests indicated that it was more satisfactory than filters with transmittance maxima a t shorter wave lengths. When the data obtained on pure solutions (Table I ) are plotted as shown in Figure 2, it is found that the colors of the hydroquinone complex of columbium and tungsten obey Beer's law a t the wave lengths obtained with the two filters chosen. By measuring the combined absorbancies of the hydroquinone complexes of columbium and tungsten iyith filters 3 and 5, it should be possible to calculate the percentage of columbium and tungsten present by means of simultaneous equations.
900
/
800 Table I.
Absorbancy of Columbium and Tungsten at 460 and 525 RIP
Columbium, Mg.
0.33 0.66
(Kromatrol photometer) Filter 3, 460 hIp Filter 5, 525 M p Absorbancy Slopea Absorbancy SlopeQ 280 848 74 224 561 852
0.99
Bv.
850 860 853
o 600 0 0
220
148 218
7 00
220 22 1
X
500 -
5Z 400 4
a
Slope =
d) D!
absorbancy X 1000, mg. C b or W
0 Q 0
2 2004
Q
150-
0
63 4
loo50 -
I
--’ I
0.OZG. Cb I
I
I
1.8
1.6
\.4
3
4
5
4.3
3,3
2.2
0
I
2
I
P H
ml
H3POh
Figure 4. Effect of pH on Absorbancy of Pyrogallic Acid Complexes of Tantalum and Columbium Beckman DV .pectmphotometer
V O L U M E 2 5 , NO. 9, S E P T E M B E R 1 9 5 3
1343
]\-hen solution is complete. add 20 nil. of perchloric arid and evaporate until the perchloric acid vapors condense on the watch glass and all the chromium has been converted to chromic acid. Cool, and add 125 ml. of water, 50 nil. of a saturated sulfurous acid solution, 5 ml. of hydrochloric arid, and a small amount of ashless paper pulp. Boil the solution approximately 5 minutes t,o hydrolyze the columbium, tungsten, and tantalum. Digest a t 63' for a t least 30 minutes or until the supernatant liquid is perfectly clear. Filter through a S o . 40 JYhatman paper containing a small amount of ashless paper pulp. Scrub the beaker thoroughly with a policeman and then wash with dilute hydrochloric acid (2 98). Wash the paper and precipitate ten times with dilute hydrochloric acid (2 98). Transfer the paper and precipitate to a 30-nil. platinum crucible, char the paper, and then ignite a t a dull red lieat (not over i 5 0 ° (2.). Tre:it the mised osides with 1 ml. l), and approxiof perchloric acid, 10 ml. of sulfuric acid (1 mately 1 ml. of hydrofluoric acid (48%). Evaporate the contents of the crucible to copious fumes of sulfuric acid and continue heating until the volume of acid is reduced to approximately 3 ml. Cool and transfer the contents of the crucible to a 400-ml. beaker by means of 150 ml. of dilute hydrochloric acid (2 98). Scrub the crucible thoroughly with a policeman and wash any addition:tl residue into the beaker. Add 50 ml. of a saturated sulfurous acid solution and a sinall amount of ashless paper pulp. Boil the solution approximately 5 minutes to hydrolyze the columbium, tungsten, and tantalum. Digest a t 65' C. for at least 30 minutes or until the supernatant liquid is perfectly clear. Filter through a S o . 40 \Vhat,man paper containing a small amount of ashless paper pulp. Scrub the beaker thoroughl!with a policeman and then ash Kith dilute hydrochloric acid (2 98). K a s h the paper arid precipitate ten times with dilute 98). Transfer the paper and precipitate hydrorhloric acid ( 2 to a :(O-ml. platinum crucible, char the paper, and ignite a t a dull red heat (not, over 750" C.). To the mised osides in the platinum crucible, add approximately 5 grams of sodium bisulfate and heat. a t a temperature j w t high enough to dissolve the oxides and obtain a clear melt. AIloa. thrl crucible and its contents to cool to nearly room ternpewture. Add 5 drops of sulfuric acid and again heat to a temperature just high enough to obtain a perfectly clear melt. Do not heat longer than is necessary to obtain a clear, glassy melt, lest an exressive amount of sulfuric acid be volatilized. Allow the crucible and its contents to cool to room temperature. .Idd 20 ml. of ammonium oxalate solution to the crucible and heat the contents :it n Ion- temperat,ureuntil the fusion is completely dissolved. Alloi\- the crucible and its contents to cool to room temperature : m t l transfer the solution to a 100-ml. volunietric flask. Dilute t o the mark with amnioniuni oxalate solution, mix thorouglil!., : i n t i i'cwrve as Sohition A. To determine the t:iiitnluni, transfer a 50.0-ml. aliquot (1.0 grani) of Solution to a 10O-ml. volumetric flask. Add 10 nil. of tlilutc i)hosphoric acid (1 3) and dilute to approximately GO nil. with :iiinnoniuni omlate solution. Khile swirling the flask, add 20 1111. of freshly prepared pyrogallic acid solution, dilute to the ni:irI. with ammonium oxalate solution, and mix thoroughly. .kl,jiist the temperature of the solution to 30' & 1.0" C. and allon10 miiiutes for maximum color development. Measure t'he abs o r h n i y with filter 2 (masimum transmittance a t 430 mp). The ab~oi.lnncy for the pyrogallic acid complex of tantalum must tie c-oii,ecatedby the absorbancy obtained on a blank representing all the rwgents used beginning ivith the fusion operation. Further rimwtion must be made for the slight effect due to the columbium :ind tungsten that are nlso present. The numerical value of these corrertioris can hc obtained from the data shown in T:i\)le 111. TI) (letermine the columbium and tungsten, transfer an aliquat of exactly 5.0 nil. (0.1 gram) of Solution d to a 125-mi. 1,ileiin)eyer flask. ridd 10 ml. of sulfuric acid, 1 ml. of dilute ~ ~ 1 i o s ~ ) I i oacid r i c ( 1 3). and 10 nil. of nitric acid. Evaporate the s ~ i l u t i o i i to fumes of sulfuric arid. fume for approximately 5 i1iiiiutt.s; and allow the flask and contents to cool to room tenip~i~:iture.Add 1 drop of stannous chloride solut niet1i:rtely and thoroughly. Add approximatelj droquinone solution a n d again mis thoroughly solution to a dry 100-1111. volumetric flask, nash Hnsk three tinies xith approximately 10-ml. portions of the hydroquinone solution, and tmnsfer each washing to the 100-nil. f1:isk. Dilute to the ni:irk with hvdi,oyuinone solution a,nd mix tlioroughly. .Idjust the temperature of the solution to 30" =t I " C . :ind allox- 10 minute.q for m:t\;iiiium color development. Hctore making the absorbancy nicasurements of the (loloreti solutions adjust the Kromatrol photometer to zero absorbancy with air-Le., without an absorption cell or tube in the holder. Then add at least 10 ml. of the colored solution to a clean. dry nl>sorption tube and me:rsui'c' the :ik)soih:iii(,j- with both filters 3
+
+
+
+
+
and 5. These absorbancy readings must be corrected a t both wave lengths by the absorbancy of a blank prepared by transferring 10 ml. of sulfuric acid to a 100-ml. volumetric flask. Add 3 drops of phosphoric arid (85%) and 1 drop of stannous chloride solution. Dilute to the mark with the same 6% hydroquinone solution and mix thoroughly. Determine the percentage of columbium and tungsten present in the sample of steel from these corrected absorbancy values using previously established calibration curves and formulas. Calibration Curves. Determine the calibration curves for the hydroquinone complex of columbium using filters 3 and 5 as follows: Add 0, 3, 6, and 9 ml. of the standard columbium working solution to each of four 125-ml. Erlenmeyer flasks. Also add to each flask 10 ml. of sulfuric acid and 1 ml. of dilute phosphoric acid (1 3). Evaporate the solutions to fumes of sulfuric acid and continue as directed under standard procedure. Correct all absorbancy values for the blank and determine the slope for each filter by dividing the corrected absorbancy by the n-eight of columbium present. Make a similar calibration for the hydroquinone complex of tungsten with filters 3 and 5 . Repeat these calibrations, preferably on different days, and determine the average slope values.
+
0
400 4
X
t
I
+
a\
+
+
I
0
1.0
I
I
2.0 30 mg Tan+a\urn
Figure 5 . Absorbancy of Pyrogallic Acid Complex of Tantalum at 430 mp Kromatrol photometer
For example, using the Kromatrol photometer in the Arnico Research Laboratory, the slope for 1.0 mg. of columbium is equivalent to an average absorbancy reading of 853 n-ith filter 3 and to 221 with filter 5. The elope for 1.0 mg. of tungsten is cquivalent to an average ahsoibancy reading of 502 with filters 3 and to 311 with filter 5. Equgtions. When the slopes have b w n accurately determined, rquations for the calculation of the percentages of columbium :i,nd tungsten may be readily p r e p a i d . The follon-iny exaniplr illustrates these calculations: If A is the total absorbancy a t t,he indicated wave lengths, and because the colored compounds of the two elements absorb independently of each other, the following simultaneous equations 111;iy be set up:
+ 502 X m g . of \V = A460 mp + 311 X mg. of if-=
853 X nig. of Cb 221 X mg. of Cb
(1)
By multiplying Equation 2 by the factor 1.614 and then s u b tracting Equation 2 from 1, we get Equntion 3. 85.7 X mg. of Cb :(Si
x
mg. of Cb
+ 502 X mg. of \Y = .-lr.,o
mp
+ 502 x mg. of \\- = l.(ill x rlj2jmG
496 X nig. of Cb
=
my
-
1.614 -45ihnip
(I
)
(2)
(XI
Then : d a n mp - 1.614 X A S ; mp J[g, of Cb = A_ _ ~ ____ 496 Jig. of Cb = 0.00202 . 1 I R O nip - 0.00323 A s ? : mp.
(4) Simi1:irI~~. by substituting for columbium and solving for tung.sten: M g . of \V = 0 . 0 0 5 3 iiiZa mp - 0.00143 dio mp (5) From the milligrams of columbium and tungsten so found, the percentage can be readily determined.
1344 '
ANALYTICAL CHEMISTRY
Table IV. Sample NBS 123a 10270
AIS1 Armco Std. 30
Reproducibility of Method for Columbium, Tungsten, and Tantalum % No. of Present 0.75 0.11 8.02 0.76 0.15 0.19 0.60 0.02 0.35
Element Cb W
0.82
Cb
0.12
Ta
.,.
Ta
Cb W Ta Cb W Ta
w
Values 8 8 7 5 5 6 9 9 9 12 12 12
Range 0.73-0.75 0.06-0.10 0.00-0.01 0.71-0.76 0.15-0.15 0.20-0.23 0.57-0.62 0 00-0.03 0:31-0.36
Av.
0.74 0.074 0.008 0.74 0.15 0.22 0.59 0.01 0.34
u
0,009 0.014
..
0.018 O:t)12 0,015 0:OlS
0.81-0.86 0 . 8 2 0,013 0.34-0.39 0 . 3 6 0.018 0.11-0.12 0 . 1 2 0.005
may be obtained by the pyrogallic acid method by using a larger initial sample. The accuracy of a method is difficult to determine, but the values obtained on standard samples indicate that the columbium and tantalum values are in reasonably good agreement with values obtained by other methods. The tungsten values appear to be slightly low (probably because of incomplete recovery in the hydrolysis separation) but should be satisfactory for most work where the tungsten content is less than 1% and where the total of the columbium and tantalum contents is more than the tungsten content. ACKNOWLEDGMENT
The authors wish to acknowledge the permission of the Armco Steel Corp. to publish this paper. iicknowledgment is also due the other members of the Research and Rustless Division Laboratories for technical assistance.
REPRODUCIBILITY AND ACCURACY OF METHODS
h statistical study of the recommended procedure was made by applying it to a series of Type 347 stainless steel samples. The values obtained from this study are shown in Table IV. These values were obtained over a period of 2 months by one analyst. The average standard deviations (Iu) were 0.014% for columbium, 0.016% for tungsten, and 0.011% for tantalum. This indicates that the reproducibility of the method (2u) is &0.03% for columbium and for tungsten, and +0.02% for tantalum. Recent experiments indicate that with tantalum contents below O . l % , a reproduribilitv of better than 3=0.02%
LITERATURE CITED
(1) Bogatski, G., 2. anal. Chem., 114, 170 (1938). (2) Heyne, G., 2. angew. Chern., 44, 237 (1931). (3) Johnson, C. M., Iron A g e , 157, 66 (1946). (4) Knudson, H. W., Meloche, V. W., and Juday, Chancey, IND. ENG.CHEM.,ANAL. ED.,12, 715 (1940). (5) Thanheiser, G., Mitt. Kaiser-Wilhelm-Inst. Eisenfwsch. DQsseldorf, 22, 258 (1940). ( 6 ) Weissler, h.,IND.ENG.CHEM.,-4x.t~.ED.,17, 695 (1945). RECEIVED for review April 23, 1963. Accepted July 2, 1953. Presented before the Pittsburgh Conference on Analytical Chemiatry and .4pplied Spectroscopy, March 2, 1953.
Automatic Coulometric Titration with Photometric Detection of Equivalence EDW4RU K. WISE', PAUL W. GILLES, AND CHARLES A. REYNOLDS, J R . Department of Chemistry, University of Kansas, Lawrence, Kan.
To eliminate the troubles associated with anomaIous behavior of sensing electrodes and to reduce the number of electrodes in a titration vessel have been dual purposes of a successful attempt to develop apparatus and procedures for automatic photometric detections of equivalence in coulometric titrations. An all-electronic self-contained instrument, operating from the 115-volt alternating current supply, to produce constant current over a continuously variable range and to terminate the titration by photometric detection has been designed, constructed, tested, and operated. Coulometric titrations producing three types of color change at equivalence were investigated. In each titration the response of a photocell to the color change opened the generating and timing circuits. Samples of from 0.2 to 1.0 meq. w-ere titrated with average errors of from 0.06 to 0.15%.
T
HE detection of the equivalence point in coulometric titra-
tions has received considerable attention. Both potentiometric (2-4, 6) and amperometric (1, 6,7 , 8, 10, 13, 1 7 ) methods have been investigated, and a detailed study of electrometric processes has been published (16). With such methods, the problem of obtaining consistent electrode behavior has arisen, and has occasionally necessitated meticulous care in the handling of the electrodes. It has sometimes been necessary to store the electrodes under controlled conditions between titrations, and to pretreat the electrodes before a titration. In automatic coulo1 Present address, Department of Chemistry, University of Arizona, Tucson, Ariz.
metric determinations. electrical coupling between the generat ing circuit and the indicating circuit may prove troublesome. The present investigation was undertaken to eliminate the troubles associated with sensing electrodes, and also to reduce the number of electrodes in a titration vessel, by employing photometric detection of equivalence in coulometric titrations. This type of detection is limited to systems which give a suitable optical indication of equivalence, such as by the formation of a colored species, like iodine, following the attainment of equivalence, or to systems to which a chemical indicator may be added to produce a change in the absorbance (optical density) of the solution to provide the desired indication of equivalence. Other
