1884
ANALYTICAL CHEMISTRY
The sample in the form of a fine powder or filings (200 to 300 mg.) in a small platinum crucible (5 mm. in diameter), is placed on the polyethylene spoon, A , which is then introduced into a flask, and the whole is submerged in a water thermostat as shown in Figure 1. Pure hydrogen from a tank is flushed through the a paratus, in order to prevent the oxidation of titanium(II1). ?phe sample thrown into the acid by overturning spoon A dissolves, if it is fine enough and low in oxygen content, in -3147 acid in about 15 minutes; if it is high in oxygen, about 1 hour and 60 ml. of -6N acid are required. After completion of Reaction 2, the end point of which is shown by arrows on the curves of Figure 1, hydrogen evolution continues a t a very slow rate due to a secondary reaction ( 7 ) : TiFa
+ HF
4
TiF,
+ 0.5H~
end of Reaction 2 (see inset of Figure 1). One oxygen determination can be finished in about 1 hour, if the necessary inexpensive equipment is available. Limitations. Only pure titanium oxygen alloys which will completely dissolve in the acid are amenable to such analysis (Table I) with a precision within 1%; the maximum oxygen content that can be determined is just below 33.470 by weight; and oxygen cannot be accurately determined in mechanical mixtures of titanium and oxides higher than Tilos by the method described (the errors will be larger than l % ) ,because Equation 3 is derived on the assumption that Ti2O3is the highest oxide present. But free titanium can be determined in the mixtures.
(4)
which should not be taken into account.
ACKNOWLEDGMENT
RESULTS
The authors are grateful to the Wright Air Development Center for support of this work and for permission to publish the results obtained.
Ten titanium-oxygen alloys were analyzed for free titanium and for their oxygen content (Equation 3 ) , by the method described, and for their total titanium content by a method very similar to that of Rahm ( 4 ) . The alloys mTere prepared in a vacuum furnace by heating dry titanium powder (+65 mesh, 99.7% purity), supplied by the Belmont Smelting and Refining Works, Inc., with the calculated amount of titanium dioxide (99.9% pure) of the Fisher Scientific Co. ( 2 ) . The results summarized in Table I were obtained by the use of Equation 1 for free titanium and by Equation 3 for the oxygen content of the alloys. The results agree within 1% with results obtained by the method of Rahm (4). Alloys 1 to 8, containing up to 13% by weight of oxygen, give better results than those with an oxygen content up to 33.47,. This is understandable, because with samples of higher oxygen content it is difficult to estimate the
LITERATURE CITED (1) Codell, >I., Norwita, G., ANAL.CHEM.27, 312 (1955). (2) Ehrlich, P., 2.anorg. u . allgem. Chem. 247, 53 (1941). (3) Kirschenbaum, A . D., Mossman, R. A., Grosae, A. V., Trans. Am. SOC.Xetals 46, 525 (1954). (4) Rahm, J. d.,ABLL. C H E i r . 24, 1832 (1952). (5) Straumanis, 11. E., Ballass, J. I.,2.anorg. u. allgem. Chem. 278, 33 (1955). (6) Straumanis, 11. E., Chen, P. C., J . Electrochem. SOC.98, 234 (1951).
(7) Straumanis, I f . E., Cheng, C. H., Schlechten, A. W., Ibid., 103, 439 (1956).
(8) Walter, D. I., ANAL.CHEM.22, 297 (1950). RECEIVED for review January 3, 1956. Accepted August 9, 1956. Based on a portion of work carried out for the Wright Air Development Center under Contract No. AI33(616)-75.
Determination of Titanium in Titanium Ores and Metal by Automatic Derivative Spectrophotometric Titration HOWARD V. MALMSTADT and CHARLES B. ROBERTS Chemistry Department, University
o f ///inois,
Urbana,
111.
To provide a relatively rapid and accurate method for determination of titanium in titanium ores, metal, and titanium dioxide, a procedure was developed utilizing the automatic derivative spectrophotometric end point detection system and electrolytic generation of the titrant at constant current. This provides a completely automatic titration method, which saves technician’s time. After dissolution of the sample an aliquot of the solution is passed through a cadmium reductor and collected in a solution containing a known quantity of ferric iron. The titanium(II1) reduces an equivalent amount of ferric iron and the excess ferric iron is automatically titrated with electrolytically generated titanous ion. The total elapsed time for the determination of titanium in rutile or ilmenite is about 40 minutes, including fluxing and dissolution of the sample. The precision and accuracy are within about 0.1%.
T
H E quantitative determination of titanium in ores and metal is an important analysis and several methods have been proposed. Papucci (4) reported good accuracy and a determination time of about 80 minutes using pertitanic acid in a differential colorimetric method. Thompson ( 5 ) reduced titanium(1V) to titanium(II1) with a-Jones reductor, after prereduc-
tion with zinc, and titrated with a standard solution of ferric iron. A rapid and accurate titration procedure for the determination of iron in titanium materials ( 2 ) utilizes automatic derivative end point detection and electrolytic generation of the titrant a t constant current (1, 2 ) . The basic titration procedure can be readily adapted for the quantitative determination of titanium. The method consists of putting titanium material into solution by fluxing or acid dissolution, passing an aliquot of the solution through a cadmium reductor, and collecting the titaniumiII1) in a known quantity of ferric iron. The excess ferric iron is then automatically titrated using the derivative Ppectrophotometric titrator (1) and electrolytic generation of titanium(II1) titrant. The method is precise, accurate, and relatively rapid. The total elapsed time for the determination of titanium in rutile or ilmenite is about 40 minutes, including fluxing and dissolution of the sample. Less than one half of the total elapsed time requires technician’s time. The rest is used in the automatic titration and dissolving fluxed sample. Therefore, with a systematic arrangement it should be possible for one technician to run several ore samples simultaneously with little extra expenditure of time. The precision and accuracy are within about 0.1%. Another aliquot of the sample can be run by the automatic titration procedure for the determination of iron ( 2 ) with a relative precision within about 0.1c/o.
1885
V O L U M E 28, NO. 12, D E C E M B E R 1 9 5 6 Table I. Determination of Titanium in Pure Titanium Metal by Automatic Spectrophotometric Titration Aliquot 1
2
3
Ti Taken, hfg.
Ti Found,
81.5% 81,52 81,52 81.52 81.52
81,42 81.58 81.54 81.44 81.47 81.33
81.52
81.40
81.52
4 5 6
AY.
Mg.
Error, XI& Ti
Relative Error,
-0.11 + O , 06 + o , 02 -0.08
-0.13
c
+0.07 02 -0.10 -0.06 -0.23
+o
-0.05 -0.15 0.08
0.10
Table 11. Automatic Derivative Spectrophotometric Titration of 10-Ml. Aliquots of Commercial Titanium Sponge Each Containing 100.03 Xlg. of Saniple Aliquot 1 2 3 4
Ti Found, 3Ig. 99.36 99.35 99.33 99.15
rlr. Coefficient of variation = 0 . 10yc
Titanium, c-,C
55.33 95.32 $9.30 99.12 59 27
APPARATUS
Cadmium Reductor. Titanium(1V) is reduced to titaniuni(II1) by means of a cadmium reductor. This is more efficient than an amalgamated zinc reductor, the evolution of hydrogen gas is less, and it reduces only a small amount of chromium when it is present. KOblank correction is necessary. The flow rate through the reductor is sufficiently fast without the use of vacuum. The reductor column has a diameter of 1.9 cm. and is 27 em. long. A glass wool plug is placed in the bottom of the column, which is then filled to a height of 24 em. Kith cadmium shavings, made by turning a solid cadmium bar on a lathe and collecting the turnings. They should be made as small as ossible, and any large pieces should be cut up with a shears. &he cadmium turnings are attacked by sulfuric acid during the reduction process, and a small amount of turnings should be added about once a n-eek to maintain the proper height in the column. When not in use, the reductor should be filled with distilled water. Instrumentation. The spectrophotometric circuit and electrolysis apparatus are used in exactly the way described for the determination of iron(&). REAGENTS
Standard ferric solution. For each determination approximately 1 gram of titanium(1V) must be in the 150 ml. of solution to give a current efficiency of lOOyo. As the amount of titanium in the sample aliquot is considerably less than this, the standard ferric solution is made up to contain this amount of titanium for each 50 ml. of ferric solution. A stock solution of ferric chloride is made by accurately n-eighing approximately 25 grams of Mallinckrodt reagent grade iron wire and dissolving in a solution of 167 ml. of concentrated hj.drochloric acid and 300 ml. of water; after the iron is dissolved, he solution is diluted to 1 liter with distilled water. This quantity serves for 200 to 400 titanium determinations, depending upon t,he amount of titanium in the samples being analyzed. h tit,anium(IV) solution is prepared by dissolving 10 grams of Cramet high-purity titanium sponge in 250 ml. of water and 75 nil. of concentrated sulfuric acid, using a 500-nil. Erlenmeyer flask for dissolution. This is heated below the boiling point for *3 to 4 hours until the sponge is completely dissolved; the solution is cooled, and most of the titanous ion is oxidized by the addition, x i t h stirring, of 6 grams of potassium permanganate. The oxidation is then completed by adding from a pipet a saturated solution of potassium permanganate until a slight excess is indicated by the pink permanganate color. The excess permanganate is destroyed by adding approximately 0.5 gram of sodium azide, and the solution is boiled for 7 minutes to destroy excess azide. After cooling, the solution is poured into a 500-ml. volumetric flask and the required amount. of ferric chloride solution is added. Two strengths of iron solution were used in this work. One contained 1.25 grams of iron per 500 ml. of solution, and the other 0.62 gram of iron per 500 ml. of solution. The former is prepared by adding 50 ml. of the stock solution of ferric chloride to the titanium(1V) solution in the 500-ml. volumetric flask and diluting to volume; the latter is prepared in the same way, but 25 ml. of the stock ferric chloride solution are used.
For routine titanium determinations it is best to prepare a very large quantity of titanium-ferric chloride solution. The ferric chloride solution (containing 20 mg. of titanium per ml.) is then standardized by titrating a 50-ml. portion. It is pipetted into the s ectrophotometer cell, 16 to 17 ml. of concentrated sulfuric a c i i are added, and the solution is diluted with distilled water to a volume of about 150 nil. The solution is boiled for 5 minutes, cooled to about 78O, and titrated like any iron sample by the automatic dei.ivative spectrophotometric method (1, 9). I n this way the number of milligrams of ferric iron in 50 ml. of standard solution is determined and is used for all subsequent determinations of titanium. -411 other reagents used in the analysis are the same as those used for the determination of iron (1, 2). PROCEDURE FOR DETERMINATION OF TIT.4SIC31 IN TITANIUM METAL
Size and Dissolution of Sample. A 1-gram sample of titanium sponge was added to a solution containing 25 ml. of water, 7.5 ml. of concentrated sulfuric acid and 10 nil. of 4870 fluoboric acid, and heated gently until d:ssolut.ion \vas complete. This required about 15 minutes, after which the solution was cooled and made up to 100 ml. in a volumetric flask. Pretitration Treatment. A 10-ml. aliquot was pipetted into the cadmium reductor and the stopcock \vas opened until the solution passed below the top level of the cadmium. The sulfuric acid which drained through the reductor was returned to the top of the reductor to prevent any loss of titanium and to prevent air from coming in contact with the cadmium. The titanium aliquot was alloived to remain in contact with the cadmium for 2 to 3 minutes while 50 ml. of the standard ferric iron solution Jvere pipetted into the titration cell. The reductor was then lowered until its tip dipped into the iron solution contained in the 180-ml. titration cell, the stopcock was completely opened, and the solution was stirred as titanous ion entered. The column was washed with 251 sulfuric acid and the stopcock was closed when the titration cell contained about 150 nil. of solution. Six to 7 ml. of concentrated sulfuric acid were added to the solution in the t h a t i o n cell to bring to proper acid strength. The titration cell was removed after the tip of the reductor and glass stirring rod were washed from a wash bottle. The solution was boiled for 5 minutes and cooled to about 78" C., and the excess ferric iron was determined bg- t t e automatic derivative spectrophotometric method. Calculations. The per cent of titanium in the cr.inple was calculated as follows:
Weight of sample ( g r G where eq. Fe = equivalents of iron in 50 nil. of stanciard ferric chloride solution. EXPERIMENTAL RESULTS FOR THE DETER3IINATION OF TITANIUM IN TITANIUM METAL
Table I shows the percentage of titanium found in 99.95yo pure titanium (prepared by the iodide process), each determination being an aliquot of a 10-gram sample. The same procedure was used for commercial titanium sponge with results as s h o r n in Table 11. Each determination is an aliquot of a 10-gram sample. PROCEDURE FOR DETERMINATION OF TIT.iSICM IN TITAKIUM DIOXIDE
The material used for this determination 1%-asTational Bureau of Standards sample 154, which has a titanium dioxide assay of 98.7YG. The dissolution n-as made in a manner similar to that recommended by the National Bureau of Standards ( 6 ) . Size and Dissolution of Sample. A I-gram sample was placed in a 250-ml. Erlenmeyer flask Jvith 10 grams of ammonium sulfate and 25 ml. of concentrated sulfuric acid. A short-stemmed glass funnel was placed i n the neck of the flask and the contents were heated without boiling until all of the sample was dissolved. The solution was poured into 50 ml. of water with stirring, cooled, and made up to 100 ml. with 131 sulfuric acid. Pretitration Treatment and Titration. The same procedure was used as for the determination of titanium in titanium metal. EXPERIMENTAL RESULTS FOR DETERMINATION OF TITANIUM IN TITANIUM DIOXIDE
The results of this analysis are shown in Table 111. Tn-o different samples were used with determinations of aliquots from each.
ANALYTICAL CHEMISTRY
1886 Table 111. Determination of Titanium Dioxide in National Bureau of Standards Sample 154 (98.7% TiOp) by Automatic Derivative Spectrophotometric Titration Sample NO.
Aliquot h-0.
Ti02 Found,
Deviation,
98.62 98.69 98.63 98.66 98,70
0 04 0.03 0.03 0.00 0.04
98.66
0.03
7%
Av.
%
Table IV. Determination of Titanium Dioxide in a Rutile Sample Aliquot T102, CC 1 2 3 4
88.21 88.46 88,47 88.46
Average Ti02 in rutile = 88.40% Coefficient of variation = 0.13%
PROCEDURE FOR DETERMINATION OF TITANIUM IS RUTILE AND ILMENITE
Rutile and ilmenite are not readily dissolved by the ammonium sulfate-sulfuric acid procedure as used for titanium dioxide. A fluxing procedure must be used, and in order to test its accuracy a sample of National Bureau of Standards sample 154 titanium dioxide was fluxed in a way similar to that used for the ores.
A 1-gram sample was placed in the bottom of a large platinum dish and covered with a mixture of 30 grams of potassium acid sulfate and 2 grams of Sa2B40,. 10H20. The mixture was heated over a hleker burner until the titanium dioxide had completely fluxed, It was cooled, i 5 ml. of 131 sulfuric acid were added, and the contents of the dish were stirred, while being heated gently, until all solid material had dissolved. This required 10 to 15 minutes. The solution was then poured into a dry 100-ml. volumetric flask, the platinum dish was rinsed with 1 M sulfuric acid, and the solution was made up to volume by adding 1J1 sulfuric acid. Several aliquots gave about the same precision and accuracy as the results from the ammonium sulfate-sulfuric acid procedure shown in Table 111. The procedure n-as then applied to rutile. Size and Dissolution of Sample. The rutile was ground in a mortar to the fineness of flour. h 1-gram sample was accurately weighed and placed in the bottom of a large platinum dish, towhich a mixture of 30 grams of potassium acid sulfate and 2 grams of ;?;a2R4O1.10H20were added. The dish was placed in an electric oven at approximately 1000" C. and fluxed for 4 minutes. The fluxed material \vas cooled, 150 ml. of 111 sulfuric acid were added, and the mistuie was stirred while being heated gently until all solid material was dissolved. This required about 15 minutes. Pretitration Treatment. The solution ITas cooled and made up to 200 ml. in a volumetric flask n-ith 1.11 sulfuric acid. A 20-ml. aliquot x a s placed in the cadmium reductor and the same procedure was followed as for the deter niination of titanium in titanium metal. EXPERIMENTAL RESULTS FOR TITASIUJI IN RUTILE AND I L3I EN ITE
The results for several aliquots of a rutile sample are shown in Table IV. The same procedure n-as applied to a sample of ilmenite to give a titanium dioxide content of 64.50%. Too few determinat,ions were made to test its precision, but it appeared to be comparable to that, of rutile. PROCEDURE FOR DETER3IINATION OF IRON IN RUTILE AND ILhIENITE
The same solution of either rutile or ilmenite which was used to determine titanium may be used to determine the amount' of iron in the ore. High-purity titanium was dissolved by the same method as previously described for the standard iron solution, and an iron blank was made by the automatic derivative spectro-
photometric method to determine the weight of iron in 50 ml. of solution.
A 50-ml. aliquot of the rutile solution, or a 25-m1. aliquot of the ilmenite solution, was added to 50 ml. of the high-purity titanium solution, proper acidity being obtained by the addition of concentrated sulfuric acid; and the determination for total iron n-as made as previously described. The iron blank was subtracted from this to give the amount of iron in the aliquot of ore solution and consequently in the sample. The rutile sample by this method contained 5 . i 3 % iron and the ilmenite sample 21.38% iron with a relative precision m-ithin about 0.170. By this method both titanium and iron content for the ores can be determined from the same solution and nith the same titration technique, within a total time of 1 hour or less. DISCUSSION
\Then titanium sponge is dissolved in either sulfuric acid or a fluoboric-sulfuric acid mixture, approximately 95% of the titanium is in solution as titanium(II1). The remaining 5 % is titanium(IV), which is reduced when the solution is passed through the cadmium reductor. I n order to test the efficiency of a single pass through the reductor, a sponge solution was completely oxidized to titanium(1V) before passing through the reductor. The results for per cent titanium were the same as those obtained when only 5% of the titanium was in the +4 oxidation state. The higher acid strengths of the pipetted aliquots cause considerable error unless the pipets are calibrated for the solution which is being used, and because the solutions have a higher viscosity than water, drainage time should be a t least 1 minute. By determining the density of a representative solution and weighing the amount delivered by each pipet, the volume delivered can be accurately determined. This value can then be used for all solutions of corresponding acid strength. While the automatic spectrophotometric method is very convenient, because it requires no operator time after the titration has started, the end point may also be observed visually by noting the complete disappearance of methylene blue color, a t which time the relays stopping electrolysis and the clock are manually terminated, This is of particular use with the ores, as titanium content is usually fairly closely known before the determination; thus the amount of ferric iron can be regulated so that the excess will be small and consequently the electrolysis time will be of the order of a minute or tv-0. For laboratories which do not have a spectrophotometer and associated equipment, this could provide a simple method for determining titanium in ores. For samples containing large amounts of iron it has the disadvantage of iequiring considerable operator time for the titration: automatic spectrophotometric detection makes this time available for the preparation of other samples. INTERFEREKCES
Elements commonly found in titanium ores and metal such as iron, magnesium, manganese, and chromium (less than 1%) do not interfere with the procedure. If titanium alloys are to be analyzed for titanium, elements such as mol-bdenum, tin, copper, or vanadium must be removed if they are present in significant quantities. Separation may be effected by using the mercury cathode (5) or by precipitating the sulfides of these metals (3). LITER4TU-RE CITED (1) lIaImstadt, H. V.,Roberts. C. B., ;Isa~.CHEN.28, 1408 (1956). (2) Ibid., p., 1412. f3) Xorwita G.., Codell. 31.. Ibid..25., 1438 .(1953). ~ -~ ~ ~ (4) Papucci, R. A . Ibid:, 27, 1175 (1955). (5) Thompson, J. AI., Ibid., 24, 1632 (1952). (6) U. S. Dept. Commerce, S a t l . Bur. Standards Provisional Certificate of ;inalysis of Standard Sample 154 Titanium Dioxide. \-,
~
~
~~~
~
~
~I
RECEIYED for rei iew J ~ l y16, 1950. Accepted August 30, 19.56. Work supported in part bv a grant-in-aid from Cramet, Inc., Chattanooga, Tenn.
