Spectrophotometric Determination of Tungsten in Iron and Steel C. L. LUKE Bell Telephone Laboratories, Inc., Murray Hill, N . 1.
b A new semimicromethod is described for determining 0.005 to 5% of tungsten in ferrous alloys. Most of the iron is removed by extraction with methyl isobutyl ketone', and the remainder plus other interfering metals is removed from hyd rochloric-hydrofluoric acid solution by a two-stage extraction with a chloroform solution of cupferron. Tungsten is then determined spectrophotometrically by the thiocyanate method.
V
or grav.metric methods for the deteririinatiori of tungsten in iron and steel are time-consuming. Moreover, such method: are not suitable for the determination of less than about 0.1% of tungsten. For these reasons it has seemed desirable to develop a spectrophotometric method for this analysis. Kakita and Goto ( I ) have recently published such a method, in which tungsten is isols,ted by solvent extraction separations and then determined by the spectrophotometric thiocyanate method. This method was tested, but since difficulty was experienced in obtaining adequate removal of molybdenum it was concluded that a better method for the isolation of the tungsten was needed. In previous work it :?as been shown that Nb, Mo> Fe, and the other metals that normally interfere in the thiocyanate method for W can be removed by a cupferron-chloroform solvent extraction from HC1-HIT solution (4). I t seemed probable that, in the analysis of small samples of steel, this separation would isolate the V ' V a d q u a t e l y for the thiocyanate determination and that the same would be true, in the analysis of larger samples (> 10 rng.), provided that most of the Fe it removed by a preliminary solvent extraction from 6N HC1 by methyl isobutll ketone. This has proved to be true and as a result it has been possible to develop an excellent rapid method for the determination of 0.005 to 5yGof W' in all types of iron and steel. OLUMETRIC
PROCEDURE
Preparation of Cali'bration Graph. Transfer aliquots of standard W solution ( 3 ) equivalent to 0, 25, 50, 7 5 , and 100 pg. of W to 50-ml. beakers.
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Add 1 ml. of 10% S a O H solution (w./v.) and evaporate to about 0.25 ml. Cool, and add 4 nil. of SnClz solution (50 granis of SnC12 2 H z 0 dissolved in and diluted to 100 nil. with HC1). Swirl to neutralize all of the NaOH and allow to stand for 20 minutes. Wash the solution, and precipitate into a 75-ml. separatory funnel, and dilute to 16 ml. Swirl, and add 4 nil. of NH4CKS solution (40 grams of NH4CNS dissolved in and diluted to 100 ml. with water). Stopper the funnel and invert to mix. Add 10.0 ml. of methyl isobutyl ketone and shake vigorously for 30 seconds. Allow to settle and then drain off and discard the aqueous layer ~ J I U Sabout 0.5 ml. of the ketone layer. Pour all but about 0.5 ml. of the latter into a 50ml. conical flask containing about 2 grams of anhydrous S a 2 S 0 4 . Swirl to dehydrate the solution, fill a 1-cm. absorption cell, and immediately measure spectrophotornetrically at 410 mp, using pure methyl isobutyl ketone as the reference liquid Analysis of Steels Containing Less Than 0.1% W. Dissolve 100 mg. of the milled sample in a covered 100-rnl. beaker in a mixture of 3 ml. of freshly mixed HC1-HN03 (4 1) plus a drop Add of HF, by heating gently. about 0.5 ml. of formic acid and heat gently to destroy the "03. If necessary, add a few more drops of the formic acid. When foaming ceases, remove the cover and heat to expel all brown fumes. A4void excessive loss of acid during the above operations. Add 1 gram of tartaric acid and heat to dissolve. Cool, transfer to a 75-ml. separatory funnel with the aid of HC1 (1 1) from a wash bottle, and dilute to 15 ml. with HCl (1 1). Add 10 ml. of methyl isobutyl ketone, shake vigorously for 30 seconds, and then drain the lower layer back to the 100-ml. beaker. Wash the ketone layer by shaking for 10 seconds with 3 or 4 ml. of HCl (1 1) and drain the lower layer to the beaker. Add 2 grains of silicon carbide and evaporate the acid solution to complete dryness. Add 5 ml. of "03 plus 3 nil. of HC104, cover, and digest on a low temperature hot plate to destroy organic matter. Finally, remove the cover and evaporate to dryness to expel all acid. Avoid excessive heat that would convert the salts to oxides. If the precipitate is red, indicating the presence of considerable amounts of chromic acid, cool, add 1 ml. of HCI, and evaporate to about 0.25 ml. Add 9), swirl, cover, and 2 ml. of H F (1 heat gently to dissolve d l salts. Cool,
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transfer to a 75-ml. separatory funnel, and dilute to 15 ml. Reserve for the cupferron-chloroform extraction. If, on the other hand, the precipitate does not contain appreciable amounts of chromic acid, add 1 ml. of 570 NaOH solution (w./v.), swirl to wet all salts, and then evaporate on a flame to about 0.25 mi. Add 2 ml. of HF (1 9) plus 2 ml. of HCl (1 9), cover, and heat gently to dissolve all salts. The solution should not be cloudy. Cool, transfer to a 75ml. separatory funnel, dilute to 15 ml., and reserve for the cupferron-chloroform extraction. Analysis of Steels Contaking 0.1 to 5% W. LOW-ALLOY STEELS. Dissolve 20 to 100 mg. of t h e sample containing 0.1 t o 1 mg. of W as directed above, b u t use 5 drops of HF. Destroy the "03, evaporate to 2 ml., add 20 ml. of HF (1 9), and heat nearly to boiling to ensure complete solution of the W03. Cool, transfer to a 100-ml. volumetric flask, dilute to the mark, and mix. Transfer 10.0 ml. of the solution to a 75-ml. separatory funnel, dilut,e to 15 ml., and reserve for the cuaferron-chloroform estraction. HIGH-ALLOY STF:E:LS-e.R., NBS 134a. Dissolve the samale in 3 ml. of HS03-HF (9 1) and then evaporate just to complete dryness. Add 3 ml. of HC1 plus 0.5 ml. of formic acid. Destroy the "03, evaporate the solution to 2 ml., add 20 ml. of H F (1 9), and continue as for low-alloy steels. CORROSION-RESISTING sTEhLS-e.g., SBS 123-a. Dissolve the sample as completely as possible in 3 ml. of HCI Add 3 drops of H?iO, plus 3 drops of HF and warm to oxidize the Fe. Destroy the "03, evaporate to 2 ml., add 20 ml. of HF (1 9))and then continue as for low-alloy steels. Cupferron-Chloroform Extraction. Add 5 ml. of a freshly prepared 4 7 , aqueous cupferron solution ( w . / v . ) , mix, add 20 ml. of CHCI3, shake vigorously for 30 seconds, and then allow t h e layers to separate. Drain off and discard the lower layer as completely as pos3ible. K a s h down the walls with 3 or 4 ml. of CHC13 from a wash bottle and drain off and discard. Bdd 1 ml. of HCI and 1 ml. of the cupferron solution, mix, and repeat the extraction with 10 ml. of CHC13. Finally, add 10 ml. of CHCI3, shake for 10 seconds, allow to settle, and drain off and discard the lower layer. Pour and wash the aqueous solution to a 100-ml. beaker and evaporate without cover to complete dryness on a 101%temperature hot plate. -4dd 1 ml. of 10% S a O H solution
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VOL. 36, NO. 7, JUNE 1964
1327
Table I.
Interference of Nb, Mo, and
V
Trans- %letal yo added, Transmittance mg. mittance 19 l = Mo5, 10 1 8 . 8 19 1 19.0 m 5 , o . 1 14.8 v b , 1 19.0 18.gb 17.6c Kb5, 1 3.0 19.O b ll.Ob V6,10 18.9 Mo5, 0 . 1 16.ga >foe, 1 19.0 19.1 15.8c 19. l b a Cupferron extraction omitted. Double cupferron extraction used. c Only 0.1 ml. of HC1 used in cupferron extraction. Metal added, mg. Kone
Table 11. Copper , added, mg.
5
b
Interference of Copper
HC1 added, ml.
Transmittance 19.1= 0 43 0" 1 27 0 0.1 l b 1 16 39.0 1 0.5b 30.7 1 0.3b 25.5 1 0.l b 19.2 Cu ferron extraction omitted. HC! present a t cupferron extraction.
Table 111. Interference of Various Metals Found in Iron and Steel c /C
Metal Trans3letal 92 added, mitadded, Transmg. tance mg. mittance S i 2 , 10 19 0 Ta5, 1 19 2 Co2, 10 18 8 Ti4, 1 18 9 Fe.3. 10 19 2 Zr4, 1 19 0 Cr", 10 18 7 Composite 19 On %nZ,1 19 4 19 0" &In2,1 18 9 Sample contained 10 mg. of Fe plus 0.1 mg. of all metals hsted in Tables I to 111 ewept Xb, and double cupferron extraction was made.
( w . / v . ) and shake, tilt, and turn the beaker so as to wet the salts on the bottom and sides. Heat on a flame to a volume of about 0.25 ml. Proceed as directed in Preparation of Calibration Graph. DISCUSSION
In developing a thiocyanate method for W in iron and steel it is necessary to conyider the possible interference of metals that are normally encountered. That of S b is SO severe that this metal must be completely removed before attempting the thiocyanate extraction of IT. As much a* 100 p g , of X b can be 1328
ANALYTICAL CHEMISTRY
Fe in spite of the complexing action of t,he HF. However. by complexing the W with t'artrate and making the extraction of t'he FeC13 with methyl isobutyl ketone a completely satisfactory separation is achieved. This separation is recommended in the proposed method. When analyzing low-alloy steel containing less than 0.1% of W,difficulty may be encountered in obtaining complete solution of appreciable amounts of precipitated tungstic acid- e.g., > 50 pg. of W--in HF. For this reason it is safer to treat with XaOH before adding the HF. When analyzing large samples containing appreciable amounts of Cr it is necessary to obtain this metal in the trivalent state to prevent oxidation of the cupferron and high blanks in the thiocyanate extraction. In the analysis of samples containing 0.1 to 5% of W,direct solution of the tungstic acid is usually feasible, since more HF can be used. Moreover, it is probable that in the case of high-alloy steels, solution in H F is easier because of the complexing action of other
quantitatively separated by means of the recommended double cupferronchloroform extraction from HC1-HF solution but further separation is required if larger amounts of Xb are present. The interference of 110 i b considerably less than that of Nb because the color of the extracted Mo-thiocyanate complex ii brown rather than yellow. -1s much as 10 nig. of 110 ab well as Fe and V can be adequately separated by a single cupferron-chloroform extraction. Part of the Co present in a sample will accompany the JY in the thiocyanate extraction but will cause no interference, since the Co-thiocyanate complex is blue. Surprisingly, Cu causes severe interference in the thiocyanate method for W. When more than traces of Cu are present at the time of the thiocyanate extraction, low results for W are obtained, due possibly to the formation of a Cu-W complex. Hence, it is necessary to keep the acidity as low as possible in the cupferron extraction so as to ensure complete removal of Cu. When the cupferron extraction is made from dilute acid solution, however, removal of Mo, V, Ti, and T a is incomplete. To remove these metals adequately it is necessary to follow the first extraction by a second from stronger acid solution. Surprisingly, no difficulty is encountered in the extraction of up to 1 mg. of T a as cupferrate. Perhaps the same would not be true when more than 1 mg. is present ( 2 ) . In the presence of appreciable amounts of Fe, difficulties are encountered in the control of the acidity a t the first cupferron extraction. Aforeover, the large amount of ammonium salts produced in the cupferron separation causes trouble in the subsequent thiocyanate extraction. For this reason it is necessary, when iron or steel samples larger than about 10 mg. are to be analyzed, to make a preliminary separation of the bulk of the Fe. Removal of the Fe by an ether extraction of FeCl3 from HCl-HF solution fails because some of the W accompanies the
Table V.
Sample 155 ( W 0.5, Cr 0.5) 155 ( W 0 5, Cr 0 5) 153 (Mo8, Cr4, T2, C08) 153 (Mo8,Cr4,V2,Co8) 134-a (Mo8,Cr4,Vl) 134-a (Mo8,Cr4,5'1) 123-a (Cr.Ki.Sb)
Table IV.
NBS sample 170 19-f 101-c
126-b 19-f
101-c 126-b 19-f
Analysis of Composite Samples of Steel
x t . of IT' in aliquot sample analyzed analyzed, Added, Found, mg. Pg. rg. 100 10 0 11 0 in n 100 -i n _ ~i 25 0 25 00 100 50 0 52 0 100 100 75 0 76 0 100 75 0 73 OQ 100 100.0 95 0 100 100 0 97 0 I
101-c 10 io o io i 19-f 10 25 0 24 5 10 25 0 26 0 170 126-h 10 50 0 50 0 101-c 10 50 0 50 0 73 0 170 10 75 0 19-f 10 100 0 98.0b 126-b 10 100 0 97 0 a CrB reduced before cupferron extraction. 1 mg. Cu added with W.
Analysis of NBS Steel Samples
W't. of sample analyzed, mg. 10 10
5 5 5 5 10 10 50 50
10
10 50 \50
w,% Present 0.517 0.517 1.58 1.58 2.00 2.00 0.11 0.11 0.11 0.11 0.18 0.18 0.18 0.18
Found 0 52 0 52 1 59 1 58 2 2 0 0 0 0
00 00
12 11 11 11 0 18 0 18 0 17 0 17
refractory metals with the W. If solution difficulties are encountered in spite of precautions taken to avoid loss of too much HF during the dissolution steps it may be advisable, in the analysis of samples containing 0.1 to 5% of W,to modify the procedure as follows:
W was then extracted as the thiocyanate
After the destruction of the "OS, pour, and wash the solution to a platinum dish. Add 2 ml. of HF, heat to dissolve the WO3, cool, add 50 ml. of water, pour, and wash back into the beaker and thence into the 100-ml. volumetric flask. Then proceed as directed.
T o evaluate the proposed method for the analysis of steels, 0.1-gram samples of various XBS steels plus varioui ahquots of standard ITT solution (1 mg. or 0.1 mg. of W per ml.) were transferred to 100-ml. beakerb and the composite samples were then analyzed for W by the proposed method. I n some instances the analysis was made on the whole sample after removal of the bulk of the Fe by extraction with ketone (analysis of Iow-JY steels); in other instances, on an aliquot of the sample solution (analysis of low-alloy steels containing 0.1 to 5Y0 IT). Correction was made for any W found in the steel itself.
RESULTS
T o investigate the removal of interference of various met hls in the determination of JT by the proposed method, 1 ml. of standard W solution (0.1 mg. of If' per ml.) plus 2 rrl. of HF(1 9) plus 1 ml. of HC1 plus 1-ml. aliquots of standard chloride or fluoride solutions of various metals was added, in the order indicated, to a 75-ml. separatory funnel. The solution was diluted to 15 ml. and, unless otherwise stated, a single cupferron separation was made. The solution was evaporated and the
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and measured spectrophotometrically. The results obtained are shown in Tables I to 111. The ketone extract of the JY was slightly brown in the analysis of the samples containing 10 mg. of N o .
Of those steel samples tested, the only one found to contain It' was KBS sample 101-e. This sample n a s found to contain 0.0274 W. The results obtained are shonn in Table IV. Several W-bearing XBS samples of
steel were analyzed by the proposed method. Samples 158, 153, and 134-a were dissolved in HSOa-HF (9 1) and samples 123-a and 123-b were dishdved in HCI. The, latter samples were analyzed on aliquots containing 10 mg. of sample and also on 50-mg. portions of the samples after removal of most of the Fe by ketone extraction. I n the latter instances the samples were dissolved, oxidized, and treated with formic acid as directed in the procedure for corrosion-resisting steels and then treated as directed in the procedure for steels containing less than 0.174 W. Chromic acid was reduced before the cupferron extraction.
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The results obtained are shown in Table V. LITERATURE CITED
(1) Kakita, Y., Goto, H., S a . R r p t . Res. Znst. 7'ohoku Cnzo. 15 ( A ) , N o I , 1 (1963 1 ( 2 j Luke, C. I,., AXAL.CHEM.2 8 , 1275 (1956). (3) Zbid., 33, 1365 (1961). ( 4 ) Zbid., p. 1964.
RECEIVED for review February 3, 1064. Accepted Ifarch 30, 1964.
A Combus tio n, T herma I -Conduc tivity An a Iyze r for Carbon in Steel L. L. LEWIS
and
M. J. NARDOZZI
Applied Research laboratory, United States Steel Corp., Monroeville, Po.
b For carbon determinations, steel samples are burned in an induction furnace in which carbon is converted to carbon dioxide in a sti,eam of oxygen. The gaseous mixture i:, swept from the furnace and through a molecular-sieve trap, which collects the carbon dioxide a t ambient temperature. After all of the carbon dioxide ha 5 been collected, the trap is placed in art oxygen carriergas stream of a gas chromatograph and heated to desortl the carbon dioxide, The desorbed carbon dioxide then passes through a thermal-conductivity detector that gives an electrical signal, which indicates the carbon content of the sample. The analysis is complete 30 seconds Zfter the end of the flush and combustion period, which requires 95 seconds. Data obtained on a group of different steels containing from 0.01 to 0.9%, carbon showed a relative standard deviation of 2.570, the maximum being 4.3%. The relative standard deviation a t the 0.001 level was 10%.
70
F
A N D MORE SENSITIVE control analyzers for carbon are required to keep pace with the new developments in steelmaking operations. These
ASTI:,K
analyzers must be rugged and simple for reliable routine use, and must be accurate and sensitive enough to analyze steels containing as little as 0.001% carbon. Chemical methods for the determination of carbon invariably involve combusting the sample in osygen t'o convert the carbon to carbon dioside (1, 4, '7). The carbon dioxide content can be determined in about 2 minutes with inexpensive and sturdy equipment. The lower sensitivity limit for rapid routine carbon determinations, however, is only about, 0.037, ( 2 ) . The measurable carbon range has been extended below 0.03y0 by absorbing the carbon dioside in a solution of barium or sodium hydroxide and relating the change in the electrical conductivit'y of the solution to the carbon content of the sample. The time required for one determination is relatively long, about 5 minutes, because the osygen flow rate must be limited to about 400 cc. per minute for complete absorption of the carbon dioxide in the solution (a!6). Recently developed carbon-control analyzers are based on the thermal conductivity of a mixture of carbon dioxide and oxygen ( 3 , 9). The carbon
dioxide-oxygen combustion mixtures usually contain some argon and nitrogen. The argon and nitrogen contents n-ill vary during measurements in accordance with the nitrogen content of the metal sample, the purity of the oxygen used for the combustion, and the completeness with which air is purged from the measuring a1)paratus before the determination. Argon and perhaps nitrogen may often be pres:rnt in quantities large enough to affect thermal-conductivity measurements and thus limit the precision and accuracy of carbon determinations, es1)rcially a t carbon concent'rations be lo^ 0.057,. When argon and nitrogen are not separated from the carbon dioxideoxygen mixture, the carbon detcrininations will be susceptible to the described accuracy limitatioiis. 1he trchniyue of using a molecular r 7
sieve to concentrate carhon dioxidc rapidly and q u a n t i t a t i v e l ~ for ~ detrrmining carbon or oxygen content in metals has been mentioned in the literature ( 11 ) . With this techriiq~e. argon and nitrogen do not interftw with Present address, I.B.31. Itesearch Center, Yorktonn Heights, S . Y VOL. 36, NO. 7, JUNE 1964
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