567
V O L U M E 25, NO. 4, A P R I L 1 9 5 3 Table IV. Element Fe V Sn Ti Nb Hf Zr
M0 Ta
Interference Studies Distribution Coe5cient Ca. 0 1.2 2.4 9.1 10.5 11 18.4 Ca. 20 22
Results of the analysis of the synthetic samples are given in Table 111. INTERFERENCE STUDIES
In the procedure developed, interference may occur during the Reparation steps or during the final aluminum measurement. .4ny metallic ions not forming anionic complexes will pass through the column and are potential interferences, depending upon their behavior with cupferron and the reagent used for the final determination of aluminum. When 8-quinolinol (15) or aluminon ( 7 ) is used, the interference picture is well known, as is the behavior of many metals with cupferron (I). The interference studies 1% ere confined, therefore, to those elements forming stable fluoride complexes and likely to be exchanged on the resin to an indeterminate extent. .is with zirconium, distribution coefficients were determined by equilibrating with Dowex-1 a 0.06 M hydrochloric acid and 0.8 M hydrofluoric acid solution containing the interfering metals. From the data tabulated in Table IV the relative behavior of various metals may be compared. The most serious interference is iron(III), which passes through the column and thus accompanies aluminum. A subsequent cupferron separation is therefore necessary. To a lesser extent tin(1V) and vanadium(V) would also come through and require further separation. Otherwise the metals tested would be exchanged on the column.
Sulfate was studied as a possible source of interference, because potassium bisulfate is csften used to fuse zirconium containing samples not soluble in acid. Two or 3 grams of the sulfate did not affect the determination of milligram quantities of aluminurn. The effect of the sulfate on the analysis of microgram quantities of aluminum was not determined. ACKNOWLEDGMENT
The authors wish to thank the Northwest Electrodevelopment Laboratory, U.S. Bureau of Mines, for its generous cooperation in supplying certain facilities and materials that aided in this research. LITERATURE CITED
(1) Furman, X. H., Mason, W.B., and Pekola, J. S.,h i L . CHEM., 21,1325-30 (1949). (2) Geist, H. H., and Chandlee, G. C., IND.ESG. CHEM.,As.iL. ED., 9,169-70 (1937). (3) Hahn, R. B., ANAL.CHEM.,21, 1579-80 (1949). (4) Kolthoff, I. AI., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” 3rd ed., p. 321, Kew York, Blacmillan Co., 1952. (5) Kraus, K. A,, and Moore, G. E., J . A m . Chem. Soc., 73, 9-13 (1951). . , (6) Kumins, C. 1., ANAL.CHEY.,19,376-7 (1947). (7) Luke, C. L., I b i d . , 24, 1122-6 (1952). ( 8 ) Lundell, G. E. F., and Knodes, H. B., J . Ani. Chem. SOC.,42. 1439-48 (1920). (9) RZicrochemical Specialties Co., Berkeley. Calif., “Ion Ex-
change Resins.- Laboratory Procedures.” (10) Osborn, G. H., Analpst, 73, 381-4 (1948). (11) Purushottam, A , , and Rao, R. S.V., I b i d . , 75, 684-6 (1950). (12) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., p. 175, Sew York, Interscience Publishers, 1950. (13) Killard, H. H., and Diehl, H., “ildvanced Quantitative .Inalysis,” pp. 74-81, Sew York, D. Tan Nostrand Co., 1943.
RECEIVED for review October 23, 1952. Accepted December 29, 1952. Presented in part a t the Pacific S o r t h n e s t Regional Meeting, AMERICAN CHEMICAL SOCIETY,Corvallis, Ore., J u n e 1952. Published with the approval of the Oregon State College Monographs Committee. Research Paper No. 218, Department of Chemistry, School of Science.
Spectrophotometric Investigation of Reaction of Titanium with Chromotropic Acid WARREN W. B R A N D T AND ALVIN E. P R E I S E R
Department of Chemistry, Purdue University, West Lafayette, Znd.
T
HE color formed by the reaction of quadrivalent titanium with chromotropic acid (4, 5-dihydroxy-2, 7-naphthalene disulfonic acid) was first observed by Geissow in 1902 ( 5 ) . The early applications of the reaction as a qualitative and quantitative method for the determination of titanium set the pattern for its subsequent usage. Hall and Smith ( 7 )and Lenher and Crawford ( 9 ) worked R-ith dilute sulfuric acid medium while Levy (10) used concentrated acid. Later investigations (3,4,8,18,19) have followed these two lines of approach, and individual investigators i n general list no specific reason for utilizing the particular solvent they choose. Endredy and Brugger ( 2 ) made the first investigation which concerned itself with the understanding of the reaction between quadrivalent titanium and chromotropic acid. They determined formulas and stability constants for the complexes formed by means of equilibrium measurements. Their results indicated the existence of a 1 t o 1 complex in concentrated sulfuric acid and a 2 to 1 chromotropic acid-titanium complex in 2% sulfuric acid. They proposed a correlation b e k e e n the two specie8 present in
the different media. In their work they demonstrated the application of the color reaction to the quantitative determination of titanium using a Pulfrich photometer. The present investigation was concerned with furthering the study of the nature of the reactions of quadrivalent titanium with chromotropic acid. It included the possibility of the extension of the quantitative scheme of Endredy and Brugger ( 2 ) in concentrated sulfuric acid to more complex mixtures of ions than previously reported. A spectrophotometric investigation of the variables functioning in the dilute acid solution was carried out to determine the relative advantage of the two methods. Detailed studies and applications of the dilute acid system have been published since the completion of this work (If, 16). APPARATUS AND REAGENTS
All spectrophotometric curves were obtained with a General Electric automatic recording spectrophotometer having a 10-mp band width. The Beckman Model B spectrophotometer was used for measuring absorbance a t a particular wave length. One-
ANALYTICAL CHEMISTRY
568
The investigation was initiated to study the nature of the reaction of titanium and chromotropic acid in dilute and in concentrated sulfuric acids and to determine the extent of interference of diverse ions upon each system. The dilute acid system was shown to be extremely sensitive to pH and to reagent concentration in the pH range 4.0 to 6.0. The system was show-n to contain a mixture of species. The interference of several common ions was demonstrated. The concentrated acid system required far less attention to experimental conditions, and fewer
centimeter glass cells 11-ere used in every case. X Beckman Model H-2 p H meter was used for all p H measurements. The chromotropic acid was Eastman blue label which had been purified by dissolution in water, filtration, and subsequent precipitation with a saturated sodium chloride solution. The airdried product was slightly gray and yielded a concentrated sulfuric acid solution which \\as almost colorless. The spectrophotometric blank from freshly prepared solutions was negligible when distilled water was used as the reference component. However, upon standing, particularly in light, the concentrated sulfuric acid solution of chromotropic acid darkens considerably, and solutions which are a few days old introduce a small error in the spectrophotometric curve. This was corrected by utilizing reference solutions containing equal amounts of chromotropic acid The darkening of aqueous stock solutions of chromotropic acid was considerably retarded by the presence of sodium bisulfite and by the use of a blackened glass container for the reagent. It was noted in the latter phases of the investigation that solutions of chromotropic acid in 72% perchloric acid darken much more
ions interfered. A reaction of the colored complex in concentrated sulfuric acid with water was postulated as the reason for the destruction of the color upon dilution, and evidence was presented in support of this assumption. The results indicate the advantages of the method in concentrated sulfuric acid for the determination of titanium in the presence of common interfering ions and suggest other applications of the reagent in analysis. A new explanation of the interference of water indicates a new method of studying some inorganic reactions.
the molar absorptivity at that wave length. The wave length of maximum absorbance shou s a regular change, but the straight line is displaced for different concentrations (Figure 1). At the same time the molar absorptivity goes through a maximum a t pH 4.5 (Figure 2). .kt a higher ratio of chromotropic acid to titanium-e.g., 10 to 1-the same linear relationship is observed between the wave length of maximum absorbance and pH (Figure 1 ) . In this case, however, one straight line suffices for all the concentrations studied. The correlation between pH and molar absorptivity is different, in that no maximum in the curve is observed (Figure 2). In each case the highest molar absorptivity was used regardless of wave length. A further study of the effect of the ratio of the reactants was carried out a t a constant pH of 4 5. An increasing ratio of chro-
SlOR-ly.
All other chemicals used in the course of the investigation were reagent grade quality. The solutions in dilute sulfuric acid were prepared by mixing a solution of titanium dioxide in concentrated sulfuric acid with the appropriate amounts of an aqueous solution of chromotropic acid and then adjusting the solution t o the proper pH value. I n some cases aqueous solutions of titanium potassium oxalate were used. Buffers Meere prepared according t o the directions of Clark and Lubs (1). The Concentrated acid solutions were prepared by mixing both reactants, which had been dissolved in concentrated sulfuric acid, and diluting t o volume with concentrated acid. The solutions containing a definite percentage of water were prepared by mixing the concentrated sulfuric acid solutions of the reagents and adding t o this a cooled mixture of the necessary water and enough conceintrated acid t o make the volume almost the desired total, The flasks were then filled t o the mark with less than 1 ml. of concentrated acid. The stock solution of titanium dioxide in concentrated sulfuric acid was standardized gravimetrically by weighing as the dioxide.
I
I
I
-2 Y
ui
m
COLOR REACTION IN DILUTE SULFURIC ACID
A sequence of colors is observed when quadrivalent titanium and chromotropic acid are mixed a t regularly increasing pH values. The mixture is yellow from p H 1 to about p H 2.5, where it becomes red. If the p H is increased t o 4.0, the solution is orange and it becomes increasingly yellow from p H 5.0 t o 14. I n the region of pH 7.0 to 12.0 the yellow color changes to orange within a few minutes and to red in a few hours. The red fades back to an orange over a period of 2 t o 3 days. No evidence of precipitation was observed. Above p H 12.0 the yellow solution changes to orange within 3 to 4 hours and precipitation is noted after 12 hours. In view of the work of Endredy and Brugger (2) a t very low p H values and the instability of the solutions above pH 7.0, particular attention was given t o the p H range 4.0 to 6.0. The effects of pH, concentration, and the ratio of the reactants were investigated in order to determine their influence upon the spectrophotometric characteristics of the system. At a low ratio of chromotropic acid to titanium-i.e., 3 to 1and in the concentration region of 10-4 M titanium, the p H of the system affects both the wave length of maximum absorbance and
a
x a I U 0
I
c
(3
6 w
>
I
I
I
I
4.0
5.0
6.0
PH
Figure 1. Effect of Variation of Reagent Ratio on Relationship between Wave Length of Maximum Absorption and pH
V O L U M E 2 5 , NO. 4, A P R I L 1 9 5 3
569
motropic acid to titanium causes a decrease in the wave length of maximum absorbance and an increase in the molar absorptivity until a ratio of approximately 50 to 1 is reached (Figure 3).
(molar absorptivity = 16,000) of this color reaction in dilute acid is heavily countered by the many variables affecting the system, the necessity for rigid pH control, and the number of elements that interfere even a t very low concentrations. COLOR REACTION IN COKCEKTRATED SULFURIC ACID
The product of the reaction of quadrivalent titanium with chromotropic acid in concentrated sulfuric acid is completely different from the one observed in dilute acid. The color formed is a purple, very similar to that of the familiar permanganate ion. I
I
I
I
I
0, I x 10-3)
-. -'... ...'..
.v
x
10-4
0.6
'4
I
5t
-1.6
3:1 C T A : T i I - - - )
A
4.0
6.0
6.0
04
PH
Figure 2. Effect of Variation of Reagent Ratio on Relationship between 3Iolar Absorptivity and pH
0 2
I
1:9
I
3:7
I
I:I
I
r 3
I
9.1
RATIO C T A : T i
Figure 4. .Study of Continuous Variations in Concentrated Sulfuric Acid
e0:1
40:l
RATIO
so:1
CTA,Ti
Figure 3. Effect of Variation of Reagent Ratio on Wave Length of Maximum Absorption and Molar Absorptivity at Constant pH
Despite the variable system involved, it was possible to verify Beer's law a t 450 mp for a pH of 4.5 in the concentration range of 5 X 10-6 to 6 X 10-6M titanium. If the solution were protected from direct sunlight, no change in absorbance was noted over a 24-hour period. Utilizing the conformance to Beer's law, a continuous variations study (20) was carried out. The results indicated a serious intermingling of species a t this pH. The graphs indicated that the main species involved were the 2 to 1 and 3 to I, since the maxima on the graph fell in this region. More influence of the 1 to 1 complex would probably be encountered a t lower pH values and different wave lengths. I n order to determine further the utility of the system for the quantitative determination of titanium, 22 anions and 12 common metal ions were tested for interference a t 460 mp (Table 1). I t is concluded that the advantage of the extreme sensitivity
The reaction is typical of titanium and phenolic compounds in general, and many possible reagents have been surveyed in this respect (6, 9, 12-14). Although the reaction has been used primarily for qualitative testing (3,4 , 9 , 18, 1 9 ) , particularly directly on the metal surface, Endredy and Brugger (2) demonstrated the practicability of its quantitative application using a Pulfrich photometer. These authors also determined the ratio of titanium to chromotropic acid to be 1 t o 1, by means of equilibrium measurements. This ratio was verified in the present work bj- the method of continuous variations ( 2 0 )a t 535 mp (Figure 4). However, despite the l to l ratio, an excess of reagent is necessary for
Table I.
Interferences in Dilute Sulfuric Acid"
Interfering Ionsb Cr(II1) Mn(I1) Fe (111) V(V) Zr(1V) U(V1) MO(V1)
Concn. Causing Appreciable Error, M 1 x 10-5 1 x 10-6 1 x 10-5 1 x 10-5 1 x 10-5 1 x 10-4 1 x 10-4
Ions S o t Interfering in 0.1 M Concentrationb
Some of these interferences were also noted b y Orenston, Parker, and Hatchard ( 1 1 1 , whose study a t p H 6.0 appeared following completion of this work. b Titanium concn. of 8 X 10-6 M .
570
ANALYTICAL CHEMISTRY
reproducibility. This is shown in Figure 5, as only a t ratios of chromotropic acid to titanium greater than 6 to 1does the absorbance remain constant a t the 535 mp maximum. (Molar absorptivity = 4000.) The possibility of extending the application of quantitative determination in concentrated sulfuric acid using this complex formation was investigated by studying the reactions of a number of cations which might be encountered in the determination of titanium. Table I1 summarizes the errors due to the primary interfering ions. The extreme sensitivity of some of these colors, and the separation of the maxima of the columbium and tantalum colors, suggest several further applications which are currently being investigated.
I '
I
I
1
I
I
15
L'
I
I
I
I
WAVELENGTH (mp) Figure 6. Spectra of Titanium-Chromotropic Acid System in Different Concentrations of Sulfuric Acid
Numbers refer to per cent sulfuric acid 65
00
I5
70
85
80
SO
S5
PER CENT HgS04
Figure 5.
Effect of Variation of Reagent Ratio on Absorbance in Strong Sulfuric Acid Solution
The interference due to vanadium is readily removed by the addition of ferrous ion. The addition of ferrous ion to the vanadium results in a colorless solution. This reaction is being investigated further. The products of the iron-vanadium reaction do not interfere with the titanium-chromotropic acid color. The interference due to chromium(V1) is large, but it is rendered insignificant by reduction to chromium(111). The concentrated acid system thus offers considerable advantage over the dilute acid with respect to interferences and the number of variables requiring careful control.
~~~~
30
50
10
PER CENT H z S O l
EFFECT O F WATER ON CONCENTRATED SULFURIC ACID REACTION
Levy (IO) and others ( 6 , 9) have noted that the addition of water to the colored product of the titanium-chromotropic acid
Table 11. Interfering Ion
Interferences in Concentrated Sulfuric Acid Molar Ratio of Ion/Ti 5:l 5:1 5:l
50:l 1:l
a
+loo
Color of Complex tilth CT.4 Red Yellow Red
+zoo
d r &e
Rel. To Error +IO0
+10 -5
Ions Not Interfering in 0.1 M Concentrationa Mn(I1) Fe(II1) Fe(11) Cu(I1) Yi(I1) Cd(I1) Z ~ ( I I ) , ' A I ( I I I ) ,U W I ) , & o ~ - - , ' i r ( W c r,('I I i j Titanium ooncn. of 5 X 10-4 M .
a
Figure 7. Comparison between Complex Formation and Water Activity in Sulfuric Acid Solutions
reaction causes the color to fade. Because this phenomenon is unusual for colored complexes of analytical significance, a careful study of the effect was undertaken. Figure 6 shows the effect of increasing concentrations of water upon the absorptiometric curves obtained. It is noted that there is little change up to approximately 30% water. Further addition results in a marked decrease in absorbance, until a colorless system is obtained a t approximately 80% water. Further decrease in the sulfuric acid concentration of the solution does not promote color formation until the point where the colors characteristic of the dilute acid system begin to form. The decrease in absorbance is accompanied by a considerable shift in the wave length of maximum absorption. Increase in the ratio of chromotropic acid to titanium de-
V O L U M E 25, NO. 4, A P R I L 1 9 5 3
571
creases the variations in absorbance a t high concentrations of sulfuric acid, such that quantitative application is possible betneen 7 5 and 95% sulfuric acid at 6 t o l or greater ratios. This range adds considerably t o the convenience of the system. This effect a-as illustrated in Figure 5. I n an attempt t o explain the effect of water on the complexation, several other types of experiments were carried out. The generality of the reaction vias checked with several other phenolic-type compounds, and Table I11demonstratesthe similarity in behavior of the other reagents tested. This emphasizes the possibility that Rater is itself involved, and that it is not a change in the organic molecule which is causing the decreased absorbance. Endredy and Brugger ( 2 ) suggest a quinoid structure as being responailile for the anomalous color in concentrated acid.
Table 111.
Effect of Water on Titanium Complexes
Complexing Reagent C‘hromotropic acid P3 rocatechol P)rogallol I-Saphthol 2-Saphthol Phloroglucinol
Color in Concd. H ~ S O I Deep purple Deep orange Red-orange Lavender Red Orange
Color in Dil. HzSO4 Colorless or yellow Colorless Faint orange Yellow Colorless Yellow
The effect of other solvents xas investigated, and it was found that the same purple color can be prepared in 72% perchloric acid and in concentrated hydrochloric acid (37’35). Qualitative tests showed that after the purple color had been destroyed by the addition of water it could be restored by the addition of acetic anhydride or more concentrated sulfuric acid. Dilution of the color in concentrated sulfuric acid with glacial acetic acid or concentrated hydrochloric acid did not destroy the color. Dilution with 72% perchloric acid led t o a new phenomenon, discussed be]OW.
These effects also indicate the importance of water as the reagent and not just a diluent in the observed effect. I n this con-
I
I C X E l
1.0
a I 5 MIN
2 3
25
95 285
5 6
9
“
-
910
1350 1565 2070
7 8
As
LITERATURE CITED
I
4
CONC. H2S04
-
0.6
0.2
450
500
550
600
WAVELENGTH (rng)
Figure 8. Variation of Spectrum of TitaniumChromotmpic Acid System with Time In mixture of 40%
by volume of concentrated acid in concentrated sulfuric acid
nection the per cent of color formed at different concentrations of sulfuric acid (using the amount formed in concentrated sulfuric acid as 100% formation) was plotted along with the activity of water at these same concentrations ( 1 6 , 1 7 ) . The resulting comparison in Figure 7 adds further indication that the destruction of the color is due t o a reaction of the purple complex or the constituents of the complex with water. Such a situation could ivell be fulfilled if it is assumed that the purple complex is formed by the reaction of a simple quadrivalent titanium ion with the chromotropic acid. The addition of water t o such a system would favor the formation of a hydrated titanium ion which is no longer capable of coordination with the organic molecule until the acidity is decreased t o the normal dilute acid range. The decrease in intensity would then represent the equilibrium between the coordination of the organic molecule and water with the simple quadrivalent titanium ion. If such a postulation is correct, this type of system offers an excellent means for studying the reaction of water with many ions which tend t o form very stable hydrated species. At the same time such systems provide new series of color reactions for these ions. The previously mentioned dilution of the complex in concentrated sulfuric acid with 72% perchloric acid presents a peculiar phenomenon for which no explanation is advanced. This procedure is not recommended for laboratory practice unless adequate precautions are observed. However, the authors observed no difficulties with the solutions even after several days’ storage. The titanium-chromotropic acid color is identical in either concentrated sulfuric acid or 72% perchloric acid, but mixtures of the two acids cause the color t o shift rapidly to a red of twice the intensity, which then fades t o a B-eak yellow. The phenomenon is illustrated in Figure 8. The isosbestic point at 520 mp aould indicate that the shift t o the red is a change within the species, which is then followed by a decomposition or fading of the entire system t o the yellow end product. As the purple color is stahle in either solvent alone, it would appear that the combination had affected the activity of water or introduced some other speciese.g., an oxidant-which either is complexed by the titanium or reacts to alter the organic portion of the existing complex.
perchloric
(1) Clark, W.If., and Lubs, H . 9., J . Biol. Chem., 25, 479 (1916). (2) Endredy, .4.,and Brugger, F., 2. anorg. allgem. Chem., 249, 263 (1942). (3) Evans, B., and Higgs, D., Analyst, 70, 75 (1935). (4) Feigl, F., “Quantitative -4nalysis by Spot Tests,” 3rd ed., p. 161, New Fork, Elsevier Publishing Co., 1946. ( 5 ) Geissow, Munich dissertation, 1902. (6) Hall, R., and Smith, E. F., J . Am. Chem. SOC.,27, 1369 (1905). (7) Ibid., p. 1391. (8) Kusnetsov, V.,J . Gen. Chem. (U.S.S.R.), 14, 903 (1444). (9) Lenher, V., and Crawford, TT’. G., J. Am. Chem. SOC.,35, 138 (1913). (10) Levy, L., Compt. rend., 103, 1075, 1195 (1886). (11) Orenston, T. C. J., Parker, C. -4., and Hatchard, C. G., A n a l . C h i m . Acta, 6 , 7 (1952). (12) Roseman, R., and Barac, G., Compt. rend. SOC. b i d , 140, 657 (1946). (13) Rosenheim, 4.,Raibmann, B., and Schendell, G., 2. anorg. allgem. Chem., 196, 160 (1931). (14) Rosenheim, rl., and Sorge, O., Ber., 53, 932 (1920). (15) Rosottee, R., and Landon, E., A n a l . Chim. Acta, 6, 149 (1952). (16) Shankman, S.,and Gordon, A. R., J . ,471~.Chem. SOC.,61, 2370 (1939). (17) Stokes, R. H., Ibid., 67, 1686 11945). (18) Tananaev, K.,and Panchenko, G., Z . anorg. allgem. Chem., 150, 163 (1926). (19) Thanheiser, G., and Waterkamp, l l . , Arch. Eisenhiittenw., 15, 129 (1941). (20) Vosburgh, W., and Cooper, G., J . rlm. Chem. SOC.,63, 437 (1941). RECEIVED for review October 9 , 1952. Accepted January 8, 1953. Presented before the Division of Analytical Chemistry at the 121st hleeting of the AMERICAN CHEMICAL SOCIETY, Buffalo, N. Y.
