by two different laboratories using the sodium thiosu1f:ite titration method. The estimated standard deviation between pairs of the three assays on the same s:rni;!r,s indicate that the x-ray method r: 11 be w r y nearly as accurate and reliable :LS the wet chemical method. That is, the agreement between instrumental and wet results is almost equivalent to that between two laboratories using the same titration method. hlthough the present system requires about 1 hour for completion of a single sample, with a possibility of completing 70 to 80 per 8-hour shift, it is reasonable to feel that a process control method, with slightly less accuracy, could be developed which would give reliable results within 20 to 30 minutes after sampling. As with any analytical method, the roasting technique has limitations. It has been tested only on a very narrow
range of copper concentration and on only one complex ore type. Further studies are required to determine its practicality on a wider variety of sulfide ores. Considerable care is required in reproducing rOasting temperature and time to obtain consistent results. ACKNOWLEDGMENT
The authors thank S. R. Zimmerley for his encouragement of this project and the Kennecott Copper Corp. for permission to publish the results. LITERATURE CITED
.,
(1) Adler. Isadore. Axelrod. J. M.. ANAL. CHEM.27, 1002 11955). ‘ (2) Andermann, George, Kemp, J. W., Ibzd., 30, 1306 (1958). (3) Brownlee, K. A., “Industrial Experimentation,” p. 51 f, 86 f , Chemical
Publishing Co., New York, N. Y., 1953. Quebec Departr
(4) Claisse, Fernand,
ment of Mines Project Rept. KO. 327 (1956). ( 5 ) Cullen, T. J., XSAL. CHEX 32, 517 (1960). (6) Davies, 0. L., “Design and Analysis of Industrial Experiments,” p. 106f, Hafner Publishing Co., New York, N. Y., 1956. (7) Kolthoff, I. M., Belcher, R., Stegner, V. A., hlatsuyama, G., “Volumetric Analysis,” Vol. 3, p. 349, Interscience, New York, N. Y., 1957. (8) Liebhafsky, H. A., Pfeiffer, H. G., Winslow, E. H., Zemany, P. D., “X-ray Absorption and Emission in Analytical Chemistry,” p. 283f, Wiley, New York, N. Y.. 1960.
(9, Lithe,-F. W., Botsford, J. I., Heller, H. A., U. S. Dept. Interior, Bur. Mines Rept. Invest. No.5378;ANAL.CHEM.31, 809 (1959). (10) Mortimore, D. M., Romans, P. A , Tews, J. L., Norelco Reptr. 1, 107 (1954). RECEIVED for review February 27, 1961. Accepted May I, 1961. 12th Annual Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy Pittsburgh, Pa., 1961.
Spectrophotometric Determination of Titanium in Silicate Rocks TYSON RlGG and HEINRICH A. WAGENBAUER Geology Division, Research Council of Alberta, Edmonton, Alberta, Canada b The colorimetric determination of titanium with disodium 1,2-dihydroxybenzene-3,5-disulfonate has been found to be particularly useful in the analysis of silicate rocks. The use of thioglycolic acid to prevent interference from iron offers considerable advantages over the dithionite method of Yoe and Armstrong. The method is based on measurements of the absorbance of the yellow titanium complex at a wave length of 380 rnp in a solution buffered at pH 3.8.
T
HE use of disodium 1, 2-dihydroxybenzene-3,5-disulfonate (Tiron) for the co!orinietric determination of titanium was first suggested by Yoe and Jones (6),who used the reagent for the determination of iron. Yoe and Armstrong (6) later applied the reagent to the simultaneous determination of both metals. The present study has been directed towards achieving optimum performance in determining the titanium content of silicate rocks. The method offers greatly improved sensitivity and reliability over the more usual peroxide method.
APPARATUS AND MATERIALS
All absorbance measurements were made with a Beckman spectrophotometer Model €3, using cylindrical absorp-
tion tubes (Type 12617) 95 X 25 mm. These tubes are not generally recommended for accurate work, but high precision can be achieved by using only two of these tubes, Le., one tube for the reagent blank and the other tube for all the calibration and unknown solutions. The tubes were marked so that they could be located accurately in the same position each time they were inserted into the tube holder. The size of these tubes facilitates handling and saves considerable time during the routine analysis of a large number of samples. The spectrophotometer was supplied with current from a Stabiline voltage regulator made by the Superior Electric
co.
The titanium reagent consisted of a 5% solution of disodium 1,a-dihydroxybenzene-3,5-disulfonate (Eastman Kodak) in water, and was discarded a t the first sign of any yellow coloration. The thioglycolic acid (Fisher Scientific Co.) was used as a 20% (vol.) solution in water. A standard solution of titanium was prepared from National Bureauof Standards titanium dioxide (sample 154a) : 0.5 gram of the dioxide (dried a t 105’ C.) was heated with 10 grams of ammonium sulfate and 25 ml. of concentrated sulfuric acid until completely dissolved. The solution was cooled and diluted to 1 liter with IN sulfuric acid. Twenty-milliliter aliquots of this solution were further diluted, usually to 1 liter, with 0.1N sulfuric acid to provide a more convenient cc xentration (10 pg.
of TiOn per ml.) for colorimetric purposes. Acetic acid/sodium acetate buffer solutions were used to control the pH of the solutions during color development. The buffer finally chosen for routine use consisted of a mixture of 1 liter of 1.OM sodium acetate solution and 390 ml. of glacial acetic acid. The final p H of the colorimetric solutions using this buffer should be 3.8. The p H of solutions was checked with a Beckman Zeromatic pH meter using a glass/calomel electrode system. The solutions used in studying the interference effects of a number of ions were made from analytical grade salts dissolved in 0.1M hydrochloric acid. The concentrations of the various ions were chosen so that in all cases the volume of solution required would not exceed about 10 ml. EXPERIMENTAL
The absorption spectrum of the yellow titanium complex formed with disodium 1,2-dihydrosybenzene-3,5disulfonate was investigated and had a maximum a t a wave length of 380 mp, as reported by Yoe and Armstrong. Thioglycolic acid does not absorb a t this wave length. At p H 3.8, Beer’s law was strictly obeyed a t 380 mp for solutions containing up to a t least 200 pg. of Ti02 in a final volume of 50 ml. The absorbance a t this concentration was usually VOL. 33, NO. 10, SEPTEMBER 1961
1347
1
5
0
7
-
-
-
To determine titanium, a 5-ml.aliquot of the above solution is transferred to a 50-ml. Volumetric flask followed, in order, by 25 ml. of buffer solution, 5 ml. of hydroxybenzene sulfouate reagent, and 2 ml. of 20% thioglycolic acid. The contents of the flask are mixed by swirling, diluted to the mark, mixed again, and allowed to stand for a t least 1 hour before measurement of the absorbance a t a wave length of 380 mp. The absorbance of a series of solutions should be measured in the same order as they are treated with the reagent so as to minimize the effect of the verj slight further increase with time that continues after 1 hour's standing (Figure 1). In this laboratory the solutions are usually allowed to stand overnight, absorbance measurements being made the following day, but in all cases within 24 hours. The titanium content is determined by reference to the standard curve which is prepared by pipetting aliquots (0 to 10 ml.) of diluted standard solution into 50-ml. volumetric flasks and processing them in the same way, and a t the same time, as the batch of unknown samples. The plot of absorbance us. titanium content should be a straight line, not necessarily passing exactly through the origin because of slight differences between the tube used for the blank and that used for the standards and unknowns.
1 7
9 5 iI-
I
I t
I
I
1
-
. 15
5 IC Sfonding time ( h o u r s )
~
20
-J
25
Figure 1. Variation of absorbance with time after addition of reagent Upper curve, 1 60 pg. T i 0 2 lower curve, 80 pg. T i 0 2 in 5 0 ml.
in the range 1.60 to 1.70, depending on the particular cell or reagent batch in use a t the time. The interference effects of a large number of elements of interest in rock analysis were investigated in solutions containing 100 pg. of Ti02 in 50 ml. Tolerance limits for the various elements have been calculated on the basis of the amount required to cause deviations of *2 pg. a t this level. Further checks on interference effects were made with solutions containing 25 pg. of Ti02 in 50 ml. Iron, which would otherwise interfere very strongly by forming a deep blue complex with the hydroxybenzene sulfonate, is rendered harmless by the thioglycolic acid. Figure 1 shows the variation of absorbance with time after addition of the reagents and final dilution. An appreciable increase occurs during the first hour, but after this the increase is very slow. Unlike the complex of titanium with peroxide (f), the absorptivity of the complex with 1,2-dihydroxyben~ene-3~5disulfonate is not significantly dependent on temperature.
Procedure for Rock Analysis. Four tenths gram of dried ground (80- to 100-mesh) sample is weighed into a platinum crucible and 15 ml. of concentrated hydrofluoric acid and 5 ml. of concentrated sulfuric acid are added. The mixture in the covered crucible is digested overnight on a water bath. The lid is then removed and any splashed material washed back into the crucible with water. One milliliter of concentrated nitric acid is added and the crucible heated on a hot plate to evaporate the fluoride. Heating is continued until strong sulfuric acid fumes are evolved. The mixture is then cooled and diluted with 100 to 150 ml. of water in a Vycor beaker and evapcrated to about 50 ml. If a precipitate remains a t this stage it is removed by filtration on a small filter paper, collecting all the filtrate and washings in a 100ml. volumetric flask. 1348
ANALYTICAL CHEMISTRY
DISCUSSION
The procedure described above offers a number of advantages over the method originally developed by Yoe and Armstrong,and later used by Shapiro and Brannock ( I ) ,especially for samples containing iron. These authors used sodium dithionite to eliminate the interference due to ferric iron. The sef absorption of this substance, however, makes it necessary to measure the absorption of the titanium complex a t a wave length of not less than 410 mp rather than a t 380 mp where the titanium absorption is a maximum. Since the absorption varies strongly with wave length a t 410 mp a loss in precision could be expected in addition to the somewhat reduced sensitivity. A more serious limitation of the dithionite method is the tendency to develop turbidity, due to sulfur precipitat,ion, even a t pH 4.7 (3, 6). At the lower pH recommended in the present work, dithionite caused turbidity very rapidly. Samples treated with thioglycolic acid remain perfectly clear for many days. There is some discrepancy between the present work and that of Yoe and Armstrong regarding the pH of maximum titanium absorption. The present work indicates that maximum absorption occurs at pH 4.05 (Figure 2) whereas Yoe and Armstrong state that the absorption is nearly independent of pH in the range 4.3 to 9.6 and that maximum color intensity does not develop below p H 4.3. This difference ap-
t O.5Od-' 2.0
'
'
'
'
3.0
'
'
'
'
'
2
4.0
0
PH
Figure 2. Variation of absorbance with pH for solutions containing 80 pg. Ti02 in 50 ml.
A 0
pH of buffer added (measured) pH of fino1 solution
pears to be due to the thioglycolic acid. A pH 3.8 is believed to offer the best combination of high absorption, good buffer capacity, and minimum variation of absorption with pH. According to Nichols (3) a further advantage of keeping the pH below 4.0 is the avoidance of metatitanic acid precipitation. The relative freedom from interferences renders the present method particularly suitable for the determination of titanium in rocks. The effects of the more important elements were studied in considerable detail and those which interfere are shown in Table I where the tolerances are given in terms of the composition of the original rock sample (0.4 gram), assuming the subsequent treatment to be as described above. The average amount of these elements present in earth's crust material (2) is shown for comparison. All the interfering elements produced positive deviations, although if calcium ion is added directly to the colorimetric solution a decrease in color intensity is observed, The method will, however, tolerate gross amounts of calcium (up to 14 mg. of CaO in the case of 100 pg. of TiOz), and in any case the sulfuric acid digestion step precipitates calcium sulfate if large amounts are present. The acid digestion also eliminates several other ions that might otherwise interfere, e.g., F-, Ba+2, Pb+2, and Srf2. None of the following components of silicate rocks interferes with the titanium determination even in amounts corresponding to 100% of the sample (0.4 gram) : SiOl, AlzOa, FeO/FezOs, CaO, MgO, NazO, KzO, P20, MnO, S, Clz, Fz, BaO, ZrOz, and SrO.
Table 1. Tolerance Limits for Interfering Substances in Analysis of Rock Containing 0.50% TiOz4
Interfering Allowable Average yo Substance yo in Rock in Earth’s Crust 0.029 CrlOi 0.84 0.13 0.022 VzOa 0.06 0.013 CUO 0.010 W08 0.19 0.002 MoOa 0.01 US08 >0.60 >O ,0005 a This assumes a permissible error of iO.OlyoTiOt. Table
II. Precision of Absorbance Measurements
Absorbance
TiOz in Mean of 5 Sample, Measure- Mean Grams ments Deviation 0.0011 10 0.0874 0.0038 80 0.6718 0.0042 160 1.3312
Accuracy Limit Accepted for Rocks Analyzed According to Method Recommended ( i o .Ol% TiOzin rock)
0.017 0.017 0.017
With samples containing an amount of TiOz equivalent to 0.125% in a rock, the tolerance limits are the same as shown in Table I except for VzOs and CuO which must be decreased by a factor of about 1.5. In all cases the observed interference appeared to be proportional to the amount of interfer-
ing substance present, thus if the permissible error is taken to be =t0.02% Ti02 the above tolerances can be doubled. Interferences from CuO, VpOa, and MOO3 were approximately additive. I t can be seen from Table I that a very unusual enrichment of the interfering substances would have to occur before a serious error could be caused in a titanium determination. The analytical procedure detailed above is suitable for rocks containing up to 1.0% Ti02. If more than this is present either a smaller sample weight or smaller aliquot should be taken so that there is never more than 200 pg. of Ti02 in the 50-ml. flask. If this is done any interfering substances present will have an even smaller effect than when the standard procedure is used. If it is necessary to use larger samples or aliquots to determine accurately a very small content of TiOz, great care should be taken to keep the concentration of interfering elements below those corresponding to the standard case of a 5-ml. aliquot taken from a 0.4-gram sample in 100 ml. of solution. In difficult cases a preliminary separation may have to be made or compensating amounts of known interfering substances added to the reagent blank and standards. The precision and accuracy of the method is very good, as can be seen from the results shown in Tables I1 and 111. Yoe and Armstrong investigated the discrepancy between their determinations and the figures quoted by the National Bureau of Standards (Table 111). They found that a silica
Table 111. Comparison of Results from Various Sources for Analysis of National Bureau of Standards Argiilaceous Limestone Sample 1A
Ti02 Found Yoe and Present N. B. S. Armstrong Work“ 0.16 0.192 0,189 0 . I90 0.190 0.185 0.187 a Three separate rock samples.
separation, as recommended on the certificate analysis resulted in the loss of about 10% of the titanium present in the rock. The‘ discrepancy is exaggerated by the fact that the certificate value was based on a spread of figures ranging from 0.11 to 0.25’%, the higest figure being excluded from the mean. LITERATURE CITED
(1) Hillebrand, W. F., Lundell, G. E. F:, “Applied Inorganic Analyses,” 2nd ed., Wiley, New York, 1953. (2) M;son, B., “Principles of Geochemistry, Wiley, New York, 1952. (3) Nichols, P. N. R., Analyst 85, 452 (1960). (4) Shapiro, L., Brannock W. W., U. S. Geol. Survey Bull. ho. 1036-C, iv, 19 (1956). (5P.Yoe, J. H., Armstrong, A. R., ANAL. CHEM.19,100 (1947). (6) Yoe, J. H., Jones, A. L., IND.ENG. CHEM.,ANAL.ED. 16,111 (1944). RECEIVED for review February 2, 1961. Accepted June 13, 1961. Research Council of Alberta Contribution No. 153.
Determination of Small Oxygen Deficiencies in Oxide Monocrystals HERBERT B. SACHSE and GORDON L. NICHOLS Keystone Carbon Co., St. Marys, Pa.
bA
direct method to determine small stoichiometric oxygen deficiencies of the order of 1 milliatom/mole in oxide monocrystals is described. The oxide is dissolved under vacuum either directly in H&Or in the presence of Fe+3 ions, or, if impossible [as in the case of rutile monocrystals (TiOz)] by a vacuum melting process in a mixed carbonate melt and subsequently also Fez(SO& solution. in H&Oa
+
T
HE DETERMINATION of small stoichiometric deviations was recently reported for oxide monocrystals with an oxygen excess, stressing their important, influence on the electrical
properties of semiconductors (4). Similar effects exist for oxygen-deficient oxides. Classic examples of the effect of oxygen deficiencies on electrical conductivity are Ti02 and ZnO. Between Ti02 and Ti01.995 the resistivity a t room temperature decreases by a factor 10’0. Simultaneously, the activation energy drops from 1.65 to 0.27 e.v. (6). Monocrystals of TiOz (rutile) make this transition a t 600” C. in pure hydrogen within minutes. By stoichiometric deficiency undetectable with normal chemical analysis, the dielectric loss can increase by a factor of 10 to 100, while simultaneously the apparent
dielectric constant increases from approximately 100 to more than 15,000 (7, 9). Further work with oxygen deficient TiOz monocrystals was done by Breckenridge and Hosler ( I ) and Cronemeyer ( 2 ) . When oxygen deficient synthetic crystals of pure ZnO with an initial resistivity of 0.3 to 0.7 ohm cm. and an activation energy of 0.01 e.v. are annealed for 30 hours a t 900” C. in oxygen, a final resistivity of 2.5 X lo6 ohm cm. and an activation energy of 0.04 e.v. is reached (S). This last figure is still far below the theoretical energy gap for intrinsic semiconduction in ZnO . VOI. 33, NO. 10, SEPTEMBER 1961
1349
