was taken purposely near t h e upper portion of t h e curve t o obtain a rather pessimistic view of the precision. The average value obtained is 21.6 p.p.m. The estimated standard deviation is 1.2 p.p.m. or 2.4 y of arsenic because a 2gram sample was taken. Accuracy. A sample of selenium was analyzed by t h e gravimetric procedure a n d found t o contain 0.66% arsenic. The quantities of this sample t o contain t h e amount of arsenic indicated in Table I1 were obtained by aliquoting and analyzed by the colorimetric procedure. These results were also analyzed statistically by the general procedure described above (7, 11). Assuming the straight line y=a+bz
where y is the amount of arsenic found, z is the amount taken, and a and b are the straight line constants, the best estimates of the constants are
with thc 95% confidence intervals
< 0.18 90 < b < 1.11
-0 26 < a 0
The standard deviation for a single determination is 0.08 mg. (80 y) which probably reflects aliquoting errors. This supports the recommendation to use volumetric or gravimetric methods for larger amounts of arsenic. The data were fitted to a quadratic regression line, but no significant improvement in fit was noted. Hence, within the precision the results are consistent and accurate. ACKNOWLEDGMENT
The author thanks Douglas H. Shaffer and Robert Hooke of Westinghouse Research Laboratories for aiding in the statistical analyses presented herein. LITERATURE CITED
(1) Bennett, C. A , , Franklin, N. L., a = -0.04and b = 1.01
“Statistical Analysis in Chemistry
and the Chemical Industry,” p. 232, Wiley, New York, 1954. (2) Green, M., Kafalas, J. A,, Lowen, J., Massachusetts Institute of Technology, Tech. Rept. 43 (;\larch 1954). (3) Gullstrom, D. K., Mellon, M. G., ANAL.CHEM.25, 1809 (1953). (4) Hillebrand, W.F., Lundell, G. E. F., Bright. H. A,. Hoffman. J. I.. “Applied Inorganic Anal&,” p: 259, Wiley, Kew York, 1953. (5) Knudson, H. W., Juday, C., Meloche, V. \IT., IND.ESG. CHEY., ANAL. ED.12,270 (1940). (6) Luke, C. C., Campbell, N. E., ANAL. CHEM.25. 1588 (1953). (7) Mood, A . $I., “Introduction to the Theory of Statistics,” Chap. 11, 13, RIcGraw-Hill, Sew York, 1950. (8) Rodden, C. J., J . Research Xatl. BUT. Standards 24, 7 (1940). (9) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” p. 137, Interscience, Sew York, 1944. (10) Schemer, J. A,, J . Research Natl. Bur. Standards 21, 95 (1938). (11) Youden, IT.J., ‘,‘StatisticalMethods for Chemsts, Chap. 5, Wiley, New York, 1951. RECEIVEDfor review August 21, 1957. Accepted February 13, 1958. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1957.
Determination of Titanium in Presence of Niobium by Differential Spectrophotometry RODRIGO A. GUEDES d e CARVALHO laboratorios KUDI, Rua Miguel Bombarda, 199, Porto, Portugal ,Titanium can b e determined in mixed tantalite-columbite-ilmenite ores without prior separation of niobium. A reliable correction for niobium is made without determining the amount actually present. Application of the differential spectrophotometric method increases precision.
T
in ilmenites is usually determined by redox volumetric procedures. Some of these methods, especially those which do not require previous separation of iron, are rapid and widely used. However, niobium, the most important interference in the oxidimetric determination of titanium ( I ) , is present in almost all ilmenites and its complete separation from titanium is difficult and time-consuming. Hillebrand ( 1 ) states that the oxidimetric method for titanium cannot be used in the presence of niobium; amalgams or metals reduce the latter in acid medium, and when it is subsequently oxidized, high results for titanium are obtained. The difficulties are even greater when dealing with impure ilmenites from the dressing of ITANIUM
1 124
ANALYTICAL CHEMISTRY
tantalite-columbite ores, because these contain substantial amounts of niobium. T h e r e f o r e , a spectrophotometric method was developed which is based on the titanium-hydrogen peroxide complex. The interference of niobium in this method is small, and a correction can be applied after determining only the approximate amount of niobium in the sample. The use of two standards instead of one, a technique recently proposed by Reilley and Crawford ( 5 ) ,was adopted. As the concentration of titanium is usually high (from 30 t o 60%), this modification improves the accuracy. Iron was separated by Schoeller’s tannin method (6) prior to color development. According to the author’s experience, this method is the most rapid. PROCEDURE
Apparatus.
tometer,
Unicam
S.P. 500.
spectropho-
Reagents, Fused potassium sulfate (potassium pyrosulfate) . Tannic acid. Ammonia; density, 0.920.
bi-
Sulfuric acid; density, 1.84. Hydrochloric acid; density, 1.19. Hydrogen peroxide, 30%. Ammonium oxalate, saturated solution. Ammonium chloride, 2% and saturated solutions. Preparation of Standards. Fuse 0.1250 gram of titanium dioxide,
accurately iveighed, in a silica crucible with 6 grams of potassium pyrosulfate. Warm the cake with 25 ml. of concentrated sulfuric acid until i t is dissolved. Cool, then dilute t o volume with distilled water in a 250-ml. volumetric flask. Each milliliter of this solution contains 0.5 mg. of titanium dioxide. Prepare another solution a t the same time, but without titanium dioxide. This is solution SO. Prepare other standards from the titanium solution by taking appropriate aliquots, adding 0.5 ml. of hydrogen peroxide, and diluting to 100 ml. Prepare standards containing from 20 t o 30 mg. of titanium dioxide per 100 ml. in 2.5-mg. increments. These standards are stable for as long as 2 months. Method. Use a 0.2500-gram sample if t h e ore contains 40 t o 60% titanium dioxide, and a 0.5000-gram sample if it contains less t h a n 4oyO.
o.060
r
Values will be obtained which may be expressed as:
o.0so
Cell 1 Cell 2 Cell 3 Cell 4
0.040 0.030 0.020
O.Of0
10
20
40
&'O
mg, h!&
SO
0
70
80
os/ 100 Mf!.
Figure 1. Absorbance curves for titanium-niobium solutions 0
60
niobium
and
Niobium alone Niobium 25 mg. of titanium dioxide per 100 ml.
+
Fuse the sample with 6 grams of potassium pyrosulfate in a silica crucible. Treat the cake with 75 ml. of saturated ammonium oxalate solution, warming t o dissolve it. Filter into a 600-ml. beaker and add dilute ammonia (1 to 3) to the filtrate until a faint cloudiness persists. Then add 6 drops of 1 to 1 hydrochloric acid. Add the same volume of saturated ammonium chloride solution and heat to boiling. Precipitate the titanium, tantalum, and niobium by adding, with vigorous stirring, an amount of concentrated, aqueous tannic acid solution equivalent to 3 grams of tannic acid. This solution should be freshly prepared. Boil the precipitate for 1minute, then filter through a medium-porosity filter (12.5 em.), using vacuum if possible. Set the filtrate aside and return the precipitate to the beaker with a jet of hot, 2y0 ammonium chloride, using 150 ml. in all. Refilter through the same filter and reserve the precipitate. To recover any titanium that may have passed into the first filtrate, boil the filtrate and add 1gram of tannic acid in solution. Add ammonia dropmise until the mauve color characteristic of the ferric-tannin complex appears. Filter the precipitate, wash, and ignite it in a silica crdcible. If iron has been retained by the precipitate, the oxide will be reddish brown instead of white. I n this case, the oxides must be fused again with pyrosulfate and the precipitation repeated. If only traces of iron oxide are observed, reprecipitation is unnecessary. Ignite the first precipitate obtained (which is usually free of iron) in the same crucible. This contains mixed oxides of titanium, tantalum, and niobium, as well as impurities, such as silica, tin dioxide, and the like. Fuse these oxides with 6 grams of potassium pyrosulfate, then treat the melt with 25 ml. of concentrated sulfuric acid, warming it to bring it into solution. Cool and make up to 250 ml.; this is solution S,. Place 25 ml. of this solution in a 50ml. flask, add 0.5 ml. of 3oY0 hydrogen
peroxide, and dilute to volume with solution So. Make an approximate determination by filling three spectrophotometer cells with the two extreme standards. The three cells now contain solution S L , the lowest standard; solution So, the unknown; and solution S H ,the highest standard. Regulate the spectrophotometer to give zero absorbance a t 425 mp with SL. This can be done by trial and error, using the three variables-dark current, slit width, and sensitivity. The procedure followed by the author was to fix the dark current knob in the maximum (extreme clockuTise) position, and then, by regulating the other two knobs, arrive a t zero absorbance by successive trials. As soon as the absorbance is regulated to zero for S L , determine the absorbance of X U and S H . A straight-line relationship with a slope of 11.25 was found to exist between absorbance and concentration of the standards. Therefore, the following relationship can be obtained: Cu = CL
+ 11.25 A o
where Co = concentration of unknown in mg. of titanium dioxide per 100 ml. CL = concentration of lower standard in mg. of titanium dioxide per 100 ml. A" = absorbance of unknown
Use this approximate concentration to select the two standards to bracket the unknown. It must be kept in mind that the cells are all different, and that these differences are greater in the differential technique because larger slits are used. Therefore, as soon as the approximate concentrations are determined and the standards are selected, determine the absorption of each cell under measurement conditions in the following manner. Fill four cells with solution S L and determine the absorbance of three of them with respect to the fourth.
= 0 absorbance = a2 absorbance unit = a3 absorbance unit = ar absorbance unit
where 0 5 a2 5 a3 5 a4. Fill cells 2 and 4 with standard solutions S L and SH, respectively. Now regulate the spectrophotometer to give infinite absorbance with solution S H and [0 - (a, - a 2 ) ] absorbance with solution S L . Fill cells 1 and 3 with the unknown solution and determine the absorbance. Correct the observed absorbance by subtracting a2 from the absorbance reading for cell 1 and (a3 - az) from the reading for cell 3. The true concentration of. the unknown solution may now be obtalned by interpolation between the two known standards. Finally, to see if niobium is present in the sample, measure 25 ml. of solution S , into a 50-ml. volumetric flask, add 20 ml. of hydrogen peroxide and 2 ml. of concentrated sulfuric acid, and dilute to volume with solution SO. Measure the absorbance difference between this and the former solution (solution Su) and calculate the niobium interference from Figure 3. After subtracting the titanium dioxide contribution of the niobium, determine the concentration of titanium dioxide present in the original ore from the following equation:
9% . . TiOz =
corr. concn. (mg./100 ml.) x 5 initial weight of ore, mg.
x
100
DISCUSSION
Differential Spectrophotometry. The two standards chosen for each determination must be close t o one another to increase the accuracy of the method. For ores containing between 40 and 60% titanium dioxide, the following limits were chosen: Ti02 in Ore,
% 40 t,o 45
45 to 50 50 t o 55 55 to 60
Standards, Mg. TiOn/ 100 M1. LoQTer Higher 20.0 22.5 25.0 27.5
22.5 25.0 27.5 30.0
-
T17hen this technique is used, there is no need of a calibration curve. HOB-ever, a previous approximate determination is necessary in order to choose the two standards to bracket the solution being measured. Interference of Iron. Iron interferes because the color it imparts t o solutions causes high results. Neal (3) has studied the influence and concluded that it increases with the amount of titanium present. When a standard containing 15 mg. of titanium dioxide per 100 ml. is used, the presence of 1% VOL. 30, NO. 6, JUNE 1958
1125
0.080
\.
0.875
0.070
0.750
0.060
0.615
0.0.50
0.500
0.040
0.375
0.030
b
$
\“
2
ZO
/O
“8. Ns,
f00
mi.
Figure 2. Effect of peroxide concentration absorbance of niobium pentoxide solutions X
0.5 mi. of hydrogen peroxide 0 40 ml. of hydrogen peroxide
iron increases the amount of titanium found by O.OOi%, The interference of iron \vas in\-estigated under the conditions outlined above by comparing the color of irontitanium-peroxide solutions with that of titanium-peroxide solutions. When 20 mg. of titanium dioxide per 100 ml. was used with varying amounts of iron, the following results were obtained: Iron, Mg./100 bI1. 0.3 1.5 3.1
Error Due to Iron (Mg. Ti02/100 311.) 0
+o.oz
+0.07
When iron is separated from titanium by the tannin method, a trace of iron is retained by the titanium precipitate. However, numerous experiments showed that never more than 2 mg. of iron was retained. As a 1 to 1 dilution is made prior to colorimetry, this is less than 1 mg. of iron per 100 nil. in the final sdution. Thus, the interference from iron is so small that it is usually not considered. Interference of Niobium. The absorbance of titanium-free solutions containing varying amounts of niobium is shown in Figure 1. The solutions contained 2.4% potassium pyrosulfate, 10.0% (v./v.) sulfuric acid, and 0.5% (v./v.) hydrogen peroxide. Similar determinations were made with solutions containing 25.0 mg. of titanium dioxide per 100 ml. Figure 1 shows that the presence of titanium does not modify the interference of niobium. Separation of the two elements is not advisable. Only the chromatographic 1 126
0
ANALYTICAL CHEMISTRY
0.020
0.125
0.010
40
30
0s /
a250
on
Figure 3. Effect of peroxide concentration on absorbance of titanium-niobium solutions All solutions contain 2 5 mg. of titanium peroxide per 100 ml. X 0.5 mi. of hydrogen peroxide 0 40 mi. of hydrogen peroxide
method ( 2 ) is reliable, but it is timeconsuming. Therefore, a rapid method was developed to estimate the amount of niobium present. According to Palilla, Adler, and Hiskey (4), the color due to the perositie complex of niobium increases with sulfuric acid Concentration [attaining a maximum a t 95% (w./w.) sulfuric acid] and with hydrogen peroxide concentration (attaining a maximum when the peroxide-niobium ratio is 20 in concentrated sulfuric acid). The color of the peroxide complex of titanium decreases when the sulfuric acid increases but it is independent of the hydrogen peroside concentration if the peroxide-titanium ratio is greater than 1. These facts are the basis of the method for estimating the amount of niobium present. The estimate is based on the absorbance difference between a sulfuric acid solution of titanium and niobium which contains a small amount of hydrogen peroxide, and another solution, the same in all respects except that it contains a higher concentration of hydrogen peroxide. This method was developed after carrying out two series of experiments, one with niobium solutions and another with niobiumtitanium solutions. Sulfuric acid solutions containing varying amounts of niobium were prepared by fusing exact weights of pure niobium pentoxide with potassium pyrosulfate in a silica crucible. The melts were dissolved in sulfuric acid and diluted to an exact volume with water. The concentrations of pyrosulfate and sulfuric acid in the final solution were the same as prescribed above-2.4 and 1070, respectively.
Two equal aliquots of each concentration were measured into two 100ml. volumetric flasks. To the first was added 0.5 ml. of 30% hydrogen peroxide, and to the second were added 40 ml. of 30% peroxide and 4 ml. of concentrated sulfuric acid (to maintain the same final acidity). The absorbances were determined, using a similar solution without niobium for comparison. The results are shown in Figure 2. Similar experiments were carried out with solutions containing 25 mg. of titanium dioxide per 100 ml., comparing them with a solution containing the same aniount of titanium but no niobium. These results are shown in Figure 3, Although the absolute absorbance values in Figures 2 and 3 are not the same, the differences between the values increase in the same ratio as the niobium concentration. Thus, there is a straight-line relationship between the niobium content and the difference in absorbances which is not influenced by the amount of titanium present. Calculated from the data shown in
Table
I. Results Showing Precision
and Accuracy Titanium Dioxide. -. Gram Difference, Taken Found Gram 0.1251 +0.0001 0,1250 0.1251 +o. 0001 0.1250 0 0.1250 0.1250 -0.0002 0.1248 0.1250 +o. 0001 0.1058 0.1057 0 0.1161 0.1161 $0.0003 0.1262 0.1259 0.0003 0.1366 0.1363
+
Figure 3, the following conclusions can be reached:
Table II.
A difference of 0.0018 absorbance unit between the upper and lower curves is equivalent to 1 mg. of niobium pentoxide per 100 ml. Under the conditions of the method, each 1 mg. of niobium pentoxide is equivalent to 0.001 absorbance unit (lower curve), Each 0.001 absorbance unit is equivalent to 0.0125 nig. of titanium dioxide per 100 mi.
Ti02
RESULTS
A n-eighed amount of titanium dioxide was carried through the procedure and the titanium was determined in four equal aliquots of the final solution. Various weights of titanium dioxide were also carried through the procedure. The results of these experiments, which illustrate the precision and accuracy of the method, are shown in Table I. Experiments were also performed with titanium dioxide to which iron or iron and niobium had been added. In the latter case the correction for niobium was applied. These results are shown in Table 11. Titanium dioxide was determined in a n ilmenite free of niobium (Table 111). The titanium dioxide content had previously been determined volumetrically and confirmed gravimetrically as 48.5%. With concentrated ilmenites-that is, those containing 40 to 60% titanium dioxide-the sample weight must be 0.2500 gram. I n such n case, if the
Present, Gram Fez03 Nb&
0.1250 0.1250 0.1250 0 1250 0 1250
0.1250 0.1250 0.1000 0 1000 0 1000 Table Ill.
This latter correction is applied to determine the contribution of niobium to the absorbance reading for titanium. Even though this method does not measure the amount of niobium accurately, it gives a sntisfactory correction.
Accuracy in Presence of Iron or Iron and Niobium
... 0.0050 0 0250 0 0500
Niobium TiOnFound, Correction, Gram Gram (Corr.)
Found, Gram TiOz Nb206 0.1248 0.1249 0.1250 0 1251 0.1257
... 0.0025 0 0240 0 0490
0.1250 0 1248 0 1251
0 0000 0 0003 0 0006
Determination of Titanium Dioxide in Ilmenite O r e
Ore Taken, Gram
Added, Gram
0.2500 0,2500 0 2500
0.01oo 0.0500
Nbn05
niobium pentoxide content of the ore is 5% (which is abnormally high), there is 0.0125 gram of niobium pentoside in the sample. This amount of niobium would increase the amount of titanium dioxide found by only 0.00015 gram, which is equivalent to about 0.06% of the amount actually present. Thus, with concentrated ilmenites there is no need to consider the interference of niobium because it is so small that the error it contributes is within the limits of accuracy of the method, If the niobium pentoxide content of the ore is greater than 5%, the correction must be applied. These cases are frequent in mixed ilmenites-columbites, impure columbotantalites, and the like. Some of the ores encountered in this laboratory have 30 to 50% niobium pentoxide and only 1 to 5y0 titanium dioxide. When the titanium dioxide content is betn-een 20 and 40%, 0.5 gram of the
+-
TiOJ Found, yo SnectroVolumetric p hdtometric 48.47 48.53 48.49
48.52 49.68 59.08
ore should be taken. For ores containing less than 20% the initial weight should be 0.75 gram. However, if the ore is a tantalite-columbite, it is not advisable to take more than 0.5 gram because of the voluminous precipitate obtained in the tannin precipitation. LITERATURE CITED
Hillebrand, K.F., Lundell, G. E. F., “Applied Inorganic Analysis,” 2nd ed., pp. 584-51 FT’iley, Kew York, 119531.
Me&er, R. A., \Tells, R. A., Analyst 79, 339 (1954).
Neal, W.T. L., Ibid., 7 9 , 4 0 3 (1954). Palilla, F. C., Adler, X., Ilislrey, C. F., ANAL.CHEM.25, 936 (1953). Reillev. C. N.. Crawford. C. hl.. Ibid.; 27, 716 ’(1955). Schoeller, W. R., Powell, A. R., “Analysis of Minerals and Ores of the Rarer Elements,” 3rd ed., p. 126, Griffin, London, 1956. RECEIVEDfor review December 1956. Accepted February 10, 1958.
11,
Fixation of Atmospheric Carbonyl Compounds by Sodium Bisulfite K. W. WILSON Department o f Engineering, University o f California, los Angeles ,The bisulfite method of Goldman and Yagoda for atmospheric formaldehyde has been evaluated for other aldehydes and ketones a t concentraA moditions from 0.3 to 0.5 p.p.m. fication is described which results in increased accuracy and precision.
THE
wlfite method of Goldman and bib Yagoda (S), in which excess bisulfite is removed with iodine and the
24, Calif.
aldehyde bisulfite compound is titrated with standard iodine after decomposition with a n acetate-carbonate buffer, has been widely used to estimate atmospheric aldehydes (2, 5, 7‘). Goldman and Yagoda demonstrated t h a t formaldehyde a t concentrations of 7 p.p.m. (by volume) and higher could be trapped from air in 1% sodium bisulfite with 957, or higher efficiency when air flows as high as 28 liters per minute were
used. Other investigators used the method to determine mixtures of various aldehydes in the atmosphere at concentrations as low as 0.1 p.p.m. Occasional comments about the probable trapping efficiency of bisulfite under these conditions (1, 6) indicate that higher aldehydes are not trapped quantitatively; however, formaldehyde is the only carbonyl compound which has been studied in controlled experiments. VOL. 30, NO. 6, JUNE 1958
1127
