ously under computer control. Such computer control removes timing and transfer errors of a human experimenter, yet does not take more than a small fraction of the running time of the computer. The computer can control the peripheral hardware without being,totally dedicated to it. Hardware to perform chemical experiments automatically is presently feasible. Although much of the equipment of ELLA is not standard, it can be built. By improving present instruments and building good transfer systems, the analytical chemist can build automated equipment to improve the speed and reliability with which he can perform his work. Hardware improvements are needed, however, to realize the full potential of automated systems. Well defined chemical decisions can be made by an on-line computer in a research environment. The decisions made by ELLA and similar systems appear relatively simple in nature but they are sophisticated enough to produce acceptable chemical data. The quality of the decision-making can be improved and new decisions can be implemented based upon what has been learned. The use of computer decisionmaking can greatly increase the speed at which knowledgeable decisions of how to proceed during an experiment can be made and can remove human error and bias from the decisions. In order to make decisions on-line by computer it is necessary to define the decisions carefully so that the computer can be programmed to make them. This, too, promises to add to the knowledge of chemical systems by forcing chemists to weigh variables more carefully and to define processes in greater detail. Computer decision-making may help to standardize research approaches and make laboratory procedures and equipment more universal in nature.
The ability of the laboratory oriented computer to d o sophisticated data analysis is established. A system which, both on-line and off-line, produces the type of analysis that the chemist needs to conduct informative experiments has been constructed. Powerful routines manipulate data into forms for visual presentation. The basis for an even more extended data analysis system exists. Moreover, the laboratory machine is able to run a special purpose time-sharing system with data analysis operating in the foreground and hardware oriented routines running in the background. The further applications of computerized research to analytical chemistry seem unlimited. Any set of decisions that can be completely defined can be programmed and, therefore, can be made automatically and without the bias of the human observer. The development of such systems will allow the chemist more time to devote to the theoretical aspects of chemistry, since the laboratory work necessary for hlm to draw his conclusion will be performed rapidly and thoroughly by automated systems. ACKNOWLEDGMENT
The authors wish to thank the Du Pont Company for the use of a modified Spectronic 20 and the digital pipet. They also wish to acknowledge Arletta E. Sherry for her technical assistance and Walter J. Blaedel for his counsel.
RECEIVED for review December 30, 1970. Accepted February 22,1971. This work was supported by the National Institutes of Health, Grant No. G M 10978, and the Graduate Fellowship Program of the National Science Foundation.
Determination of Titanium by Controlled-Potential Coulometry L. P. Rigdon and J. E. Harrar Chemistry Department, Lawrence Radiation Laboratory, University of California, Livermore, Calif. 94550
A procedure has been developed for the determination of titanium by means of controlled-potential coulometry. The method is based on the reduction of Ti(IV) to Ti(lll) at -0.20 V VI. SCE in 9M H2S04. Background corrections are low, and samples containing 0.1-10 mg Ti per ml can be analyzed with an accuracy and precision of 0.1-0.2%. The Ti(lV)-Ti(lll) couple is reversible, thus by reoxidizing the Ti(lll) to Ti(IV) at +0.22 V, the quantity of each oxidation state can be determined and certain interferences can be avoided. Principal interferences are As(lll), Bi(lll), Cu(ll), Mo(VI), Se(VI), and Te(lV). Five mg of Ti can be determined accurately in the presence of >5 mg Nb, >4 mg Zr, 5 mg V, 0.2 mg W, 2 mg Fe, or 0.5 mg CI-. The method has been applied to the analysis of Ti-W alloys of high tungsten content, after separation of the titanium by hydrolytic precipitation. ALTHOUGH EXISTING DATA on the electrochemical behavior of titanium suggest that this element could probably be determined by controlled-potential coulometry, no such methods have been reported. This technique would afford an accurate and precise alternative to the titrimetric and gravimetric methods presently used for the analysis of titanium metal, its alloys, and compounds ( I , 2). (1) E. R. Scheffer in “Treatise on Analytical Chemistry,” Part 11, Vol. 5 , I. M. Kolthoff and P. J. Elving, Ed., Interscience, New York, N. Y.,1961, pp 1-60. (2) R. Z . Bachman and C. V. Banks in “Progress in Nuclear Energy. Analytical Chemistry,” Vol. 3, Part 4,C. E. Crouthamel, Ed., Pergamon Press, New York, N. Y ., 1963, pp 95-104.
For constant-current coulometry, Ti(II1) can be electrogenerated from Ti(IV) with 100% current efficiency at platinum electrodes in strong mineral acid media (3). However, preliminary investigations showed that these conditions are not directly applicable to coulometry at controlled potential. The concentrations of titanium in the electrolyzed solutions are some one-hundred times lower in controlled-potential coulometry, and the background current from hydrogen evolution at the platinum electrode is relatively large at the applied potentials needed for quantitative Ti(1V) reduction. A mercury working electrode, on the other hand, appears to be ideally suited to the determination of titanium. Polarographic literature (4) indicates that the electrochemistry of the Ti(IVkTi(II1) system is well defined in several supporting electrolytes. Of these, strong sulfuric acid (5-8) was chosen for development of the controlled-potential coulometric procedure. This electrolyte is compatible with most (3) J. J. Lingane, “Electroanalytical Chemistry,” 2nd ed., Interscience, New York, N. Y., 1958, pp 588-598. (4) “Handbook of Analytical Chemistry,” L. Meites, Ed., McGraw-Hill, New York, N. Y., 1963, Sec. 5 , pp 55-97. (5) J. J. Lingane and J. H. Kennedy, Anal. Chim. Acta, 15, 294
(1956). (6) D. K. Banerjee, C. C. Budke, and F. D. Miller, ANAL.CHEM., 31,1836 (1959). (7) G. M. Habashy,Z. Anorg. Allg. Chem.,306,312 (1960). (8) G. M. Habashy, Collect. Czech. Chem. Commun., 25, 3166 (1960). ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
747
titanium dissolution media ( I , 6 ) and it forms stable solutions with most metals commonly associated with titanium. EXPERIMENTAL Apparatus. The details of the construction and operation of the mercury-pool electrolysis cell (9) and the controlledpotential coulometer system ( I O ) have been reported previously. The integrator was calibrated electrically ( l o ) , and its readout voltages were measured with a Non-Linear Systems Model 484A digital voltmeter. The reference electrode used was a Fisher Scientific Co. Cat. No. 13-639-210 SCE; its potential was checked occasionally against a laboratory prepared SCE. All volumetric glassware was certified or calibrated. Micropipets were used to take aliquots of the solutions for coulometric analysis. Reagents. All chemicals were of reagent grade or equivhlent purity. Standard solutions of titanium were prepared from Materials Research Corporation Marz-grade, zone-refined titanium wire and from National Bureau of Standards SRM No. 154a TiOz. The titanium wire had a nominal purity of 99.97%; examination of the material by sparksource mass spectrography, vacuum fusion analysis, atomic absorption spectrophotometry, combustion analysis, and emission spectrography showed that its total impurity level did not exceed 0.040%. The principal contaminant was carbon. Prior to analysis and use as a primary standard, the titanium wire was etched with either H2S04 or dilute “ 0 3 -
HF. In tests of the method for the determination of titanium in titanium-tungsten alloys, synthetic mixtures were prepared from Marz-grade tungsten wire and titanium of a lower quality from a different source. The titanium assayed 99.8 %, and this was taken into account in the recovery data. Sample Dissolution and Pretreatment Procedure. If the sample is dissolved by fusion and with potassium pyrosulfate ( I ) , dissolve the melt by heating carefully with 5-10% H2S04. Cool and transfer the solution to a volumetric flask; rinse and dilute to volume with 5-10 % HzS04. If the sample is dissolved by heating with (NH4)2S04 and HzS04( I , l l ) ,pour the solution with stirring into cold water in a volumetric flask; rinse the dissolution flask and dilute to volume with 5 HzS04. GENERAL ALLOYSAMPLES.Add 30-40 ml of cool 9-10M H2S04and 3-5 drops of concentrated H N 0 3 to the sample containing 0.1 to 1.0 gram of Ti in a 250-ml beaker. If H F is used, add 5-10 m l 4 8 z H F to the mixture in a Teflon beaker. If the dissolution appears to cease and the blue color of Ti(II1) persists, heat carefully and add a few more drops of “Os. When the solution has been evaporated to fumes of SO3,but before sample dissolution is complete, cool the solution and add 10-15 ml of 5 % H2S04while stirring; then continue the dissolution. When dissolution is complete, slowly evaporate the solution to copious fumes of SO3 to remove the last traces of nitrogen oxides and fluoride. Do not evaporate to salt formation. Cool the solution, add 15 ml of 5 % HzS04, and cool again. Transfer the solution to a volumetric flask, rinse the beaker and dilute to volume with 5 H2S04. Ti-W ALLOYS. Weigh sufficient sample to contain 100-400 mg Ti into a 250-ml Teflon beaker. Add 20 ml water and 5-10 ml 48% HF. After the initial reaction has subsided, add H N 0 3 dropwise and warm until the sample is completely dissolved. Dilute the solution to 200 ml with cold water, then precipitate the Ti by adding 50 % NaOH until the pH of the solution reaches 12. Filter the mixture through Whatman No. 42 paper and wash the precipitate several times
x
x
(9) J. E. Harrar, U.S. Atomic Energy Comm. Rep., UCRL-50335, Livermore, Calif., November 1967. (10) J. E. Harrar and E. Behrin, ANAL.CHEM., 39,1230 (1967). (11) Provisional Certificate of Analysis, TiOl SRM No. 154a: National Bureau of Standards,Washington, D. C . 748
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
with warm, dilute NaOH to remove the fluoride and nitrate. Place the stem of the filter funnel in a 50-100 ml volumetric flask and dissolve the precipitate into it with warm 10% HzS04. Wash the filter paper several times and dilute to volume with the 10 % HzS04. Coulometric Analysis Procedure. Place the supporting electrolyte, 9 M HzS04, in the two salt bridge tubes. If an inert-gas scrubber solution is used, it should contain approximately 9 M HzS04; dilute scrubber solutions will saturate the gas stream with water, which in turn will be removed by the hygroscopic supporting electrolyte in the cell. Introduce mercury into the cell until its surface is level with the upper surface of the disk stirrer. Turn on the stirrer and add sufficient Hi304 electrolyte so that the final solution with sample will be 5-7 ml 6-9M HzS04. Pipet an aliquot of sample solution containing 0.1-10 mg Ti into the supporting electrolyte. Deoxygenate the solution for 7 min. Preelectrolyze the solution at +0.22 V E S . SCE until the current reaches 5-25 PA. Zero the integrator, then reduce the Ti(1V) at -0.20 V and measure the readout voltage when the current has decreased to 10-25 PA. The initial current at -0.20 V and the time for a complete electrolysis will be approximately 70 mA for 5 mg Ti(1V) and 15 min, respectively. If required, after zeroing the integrator, reoxidize the Ti(II1) by electrolysis again at +0.22 V us. SCE, and measure the readout when the current has decreased to 10-25 PA. Valence state analysis of mixtures of Ti(II1) and Ti(1V) may be carried out by determination of Ti(1V) at -0.20 V (without preelectrolysis) followed by measurement of the total amount of titanium at +0.22 V. Determine the background correction by electrolyzing the supporting electrolyte alone in the same manner and for the same lengths of time as required for the titanium sample. With preelectrolysis at +0.22 V cs. SCE, the background correction at -0.20 V typically should not exceed the equivalent of 0.015 mg Ti for a 15-min electrolysis. RESULTS AND DISCUSSION Sample Dissolution and Pretreatment. Most of the dissolution procedures developed for titanium compounds and alloys are based on sulfate media (1, 6, 11, 12), and thus are ideal for coulometric analysis in sulfuric acid supporting electrolyte. Sulfuric and nitric acid together, and with hydrofluoric acid, dissolve titanium metal and alloys such as NBS SRM No. 173 (6A1-4V), 174(4A1-4Mn), and 176 (5A1-2.5Sn) very readily. Some nitrate and halides can be tolerated in the coulometric determination; however, it is best to remove these completely by fuming the sample solution with sulfuric acid. It was found that considerable care must be exercised in the solution preparation to avoid the hydrolytic precipitation of Ti(1V). The /3 form of Ti02 is precipitated when solutions low in sulfuric acid are heated, and this material is difficult to dissolve (12). Heat from the dilution of the concentrated acid solutions is sometimes sufficient to cause this precipitation. Because the possibility of TiOz precipitation is lessened, the faster dissolutions using hydrofluoric acid are recommended for titanium and its alloys. At room temperature, 1 M H2S04solutions containing 10 mg Ti(1V) per ml were stable indefinitely with respect to hydrolytic precipitation and electrochemical properties. General Characteristics of the Ti(1V)-Ti(II1) System. Polarographic investigations of the Ti(1V)-Ti(II1) system in sulfuric acid (5-8) have shown that the couple behaves reversibly when the supporting electrolyte concentration exceeds approximately 5 molar. Below this concentration, (12) J. J. Lingane, “Analytical Chemistry of Selected Metallic Elements,” Reinhold, New York, N. Y., 1966, pp 108-9.
the system becomes increasingly irreversible and more complicated, because of the hydrolysis of the titanium and sluggish equilibrium among the titanium(1V) species (5, 7, 8). It was shown by Lingane and Kennedy (5) and Habashy (8) that reversibility results from the formation of a Ti(1V)-bisulfate species, but the exact nature of this complex ion is not known. The dc polarographic half-wave potentials continue to shift anodically with increasing sulfuric acid concentration to a value of f0.17 V us. SCE in 15MHzS04 (5). These general characteristics are also evident in controlledpotential electrolyses at the mercury pool, as shown in the form of coulograms in Figure 1. The reduction of Ti(1V) to Ti(II1) is still quantitative at appropriate applied potentials in the lower concentrations of sulfuric acid; however, the rates of electrolysis are inconveniently slow. Complete electrolysis in 3M HzS04 at -0.35 V us. SCE requires 40 min, compared with less than 15 min in 9 M H2S04 at -0.20 V. Thus, the higher concentrations of sulfuric acid are desirable for coulometry, and at the recommended control potential of -0.20 V, the final concentration of sulfuric acid in the electrolysis cell should be at least 6 molar. N o evidence was found for the further reduction of Ti(II1) to Ti(I1). Anodic coulograms in exact symmetry to those of Figure 1 are obtained for the oxidation of Ti(II1). The formal potentials (including the saturated KCl liquid junction potentials) found for the Ti(1V)-Ti(II1) couple are: 9 M HzS04, +0.034 V us. SCE; 6 M , -0.015 V; and 3M, -0.125 V. For the 9 M HzS04supporting electrolyte, the formal potential did not vary appreciably with the concentration of Ti(1V) in the range used for coulometry. For the determination of titanium in the presence of certain interferences, the solution may be pre-electrolyzed at +0.22 V without reduction of the Ti(1V). This applied potential is also optimum in 9M H&04 for the quantitative oxidation of Ti(II1) to Ti(1V). An interesting feature of the electrolysis of Ti(1V) and Ti(1II) is that, in strong sulfuric acid, Ti(1V) and Ti(II1) associate in a mixed valence complex (13, 14). During reduction the solution is observed to change from colorless, to dark-reddish-violet (with the maximum intensity occurring when the TiOV) is approximately 50 % reduced), and finally to the pale violet of Ti(Il1). Goroshchenko and Godneva (14) examined the visible absorption spectra of solutions of Ti(1V) and Ti(II1) in 2-18M H2S04 and concluded that the mixed valence complex contained Ti(1V) and Ti(II1) in the ratio of 1 :l. The maximum absorptivity of the mixed valence complex occurred in 7-9MHzS04. The formation of this mixed valence complex has not been considered in previous electrochemical investigations of the titanium system in strong sulfuric acid (5-8, 1 3 , chiefly because its existence is obscured by the reversibility of the electrode reaction. Using 25-100 Hz square wave polarography, Tanaka and coworkers (15) found a value of 3.99 X cmjsec for the apparent standard rate constant in 8M H2S04. The current-time behavior of the coulometric electrolysis and direct measurements of the bulk electrolytic rate constant [by means of the predictive coulometry system (16)]indicate (13) M. B. Robin and P. Day in “Advances in Inorganic Chemistry and Radiochemistry,” Vol. 10, H. J. Emeleus and A. G. Sharpe, Ed., Academic Press, New York, N. Y . , 1967, pp 247422. (14) Ya. G. Goroshchenko and M. M. Godneva, Russ. J. Inorg. Ckem., 6,744 (1961). (15) K. Tanaka, K. Morinaga, and K. Nakano, Nippon Kukagu Zusshi, 89, 1060(1968); Chem. Abstr., 70,63477b (1969). (16) F. B. Stephens, F. Jakob, L. P. Rigdon, and J. E. Harrar, 42,764 (1970). ANAL.CHEM.,
0.3
I
I
I
0.2
0.1
0
I
-0.1
E
VI.
SCE
I
I
I
I
-0.2
-0.3
-0.4
-0.5
- volts
-0.6
Figure 1. Coulograms for the reduction of Ti(IV) to Ti(II1) in
H2S04 4.70 mg Ti(IV) in 5.5 ml solution
Table I. Analyses of Standard Solutions of Titanium by Controlled-Potential Coulometry (7-9M HISOl supporting electrolyte; reduction at -0.20V us. SCE and Ti metal standards except as noted; N = 6 for mean value) Titanium, mg Found, Re1 std dev, Re1 error, Taken mean value 9.741 9.744 0.06 +0.03 9.696 9.687a 0.04 -0.09 4.868 4.869 0.14 +o. 02 4.083b 4.083 0.10 0.00 4. 990b 4.989 0.16 -0.02 4. 990b 4.973” 0.08 -0.3 0.9741 0.9727 0.09 -0.14 0.4868 0.4858 0.04 -0.2 0.09741 0.0965 0.3 -0.9 By oxidation at +0.22 V us. SCE. NBS SRM 154a Tion.
z
z
a reversible reaction with no kinetic complications. It should be fruitful to examine this reaction in more detail by means of higher frequency ac polarography and high speed cyclic voltammetry. Accuracy and Precision. Table I shows the accuracy and precision obtained in the analysis of standard titanium solutions. The amounts of titanium taken were calculated from the assumption that the metal was 100.00% pure. The TiOz standards were corrected for 0.06z moisture, determined as recommended by the NBS (11). The high purity and ease of solution preparation of the zone-refined titanium metal recommend it as a primary standard, especially since the NBS Ti02 is no longer available. The small negative bias in the results obtained by reoxidation of the Ti(II1) is statistically significant; it was found, by means of H~OZ colorimetric tests, to be due to the diffusion of titanium species through the porous Vycor tube into the counter electrode compartment. National Bureau of Standards SRM No. 173, 174, and 176 were also analyzed by the coulometric technique. No titanium certification is given for these samples ; however, the results were within 0.5z of the titanium content calculated from the listed minor constituent values, and no interference effects were observed. Interferences. A number of potential interferences were examined and the results of these tests are summarized in ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
749
Table 11. Tolerance of Diverse Substances in the Determination of Titanium by Controlled-Potential Coulometry
Table 111. Determination of Titanium in Titanium-Tungsten Binary Alloys by Precipitation Separation and Coulometry
(5 mg Ti, 9 M HISO, supporting electrolyte)
z
Substance
Amount to cause 0.3 re1 error, mg Preelectrolysis'at Prereduction at +0.22 V vs. SCE, -0.20 V US. SCE, then reduction at then oxidation at -0.20 v +0.22 v
Ti, Sample Synthetic mixtures
>1.0
As(II1) Bi(II1) BlCd(I1) Ce(1V) c1c104-
Cr(V1) Cu(I1) Fe(II1) Ge(1V) HgW HdIU In(II1) Mn(VI1) Mo(V1) Nb(V)
NOzNosPb(I1) Po43WV) Se(V1) Sn(1V)
Te(1V) TU) UWI) V(V)
b c
d
0.01
0.01
0.05 0. 5a
0.05
z
Error,
z
1.697 0.00 2.085 0.00 13.13 0.00 20.08 -0.04 24.75 -0.11 42.21 -0.03 2.37, 2.45, 2.48 17.35, 17.40
2.0 0.55
1.0 0.10
2.5
0.05
0.05
2.0 0.01
0.6
>1.0 5.0
0.02 1.0 0.05 >5.0
>1.6 0.05
2.0b 0.05 1. o c
0.25
0.20
0.20
0.05 >5.0 0.05 >5.0
0.05
> 100
0.03 0.10 0.10
0.05 >5.0 5.0
Table 11. All anions were added as their sodium salts. Lead(I1) and Hg(I), which have a low solubility in the supporting electrolyte, were added as the acetates, and Bi(II1) was tested as the nitrate. All other cations were added as their sulfates. No reduction and no bias at the levels indicated in Table 11 were found for Cd(II), Nb(V), Pb(II), Sn(TV), Tl(I), and Zr (IV). From their known reduction potentials, Hf(IV), Taw), and Mn(I1) are not expected to interfere. Large amounts of phosphate decrease the rate of electrolysis but do not cause a bias. In the presence of chloride and bromide, Ti(1V) oxidizes the mercury of the electrode to insoluble Hg2CI2or HgzBr2. Both compounds are quantitatively reduced at -0.20 V us. SCE; thus reduction at only -0.20 V along with the remaining Ti(1V) avoids a bias in the determination. However, the presence of the insoluble material extends the time of the electrolysis, thus the amount that can be tolerated is somewhat arbitrary and depends on how carefully background corrections are made. For substances that cause the oxidation of mercury, the levels given in Table I1 refer to a 30-min electrolysis.
*
1.703 2.087 13.13 20.12 24.86 42.24
Ti found,
>5.0
2.0,5d VIV) 0.20 0.20 WVI) Zr(1V) >4.0 No preelectrolysis. This interference eliminated by sulfamic acid. Solubility limit. With prereduction at -0.20 V during deoxygenation.
750
Ti-W alloys
z
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
The strong oxidants, Ag(I), Ge(IV), Cr(VI), Fe(III), and Mn(VII), oxidize the mercury to Hg(1I). The platinum metals, plutonium, and Au(II1) would be expected to behave similarly. With milligram amounts of these species, the Hg(l1) formed is soluble, is reduced during the preelectrolysis at f0.22 V along with any remaining oxidant, and no bias is found. With larger amounts of oxidant, the solubility limit of HgS04 is reached, and the tolerance level is established, as discussed above for the halides, by the extended time of the electrolysis. Uranium (VI) and In(II1) are irreversibly reduced at -0.20 V, thus their interference can be avoided by reoxidation and measurement of the titanium at f0.22 V. Arsenic(III), Bi(III), Cu(II), Mo(VI), Sb(V), and W(V1) interfere by being reduced and oxidized at nearly the same applied potentials as titanium. Germanium(IV), Se(VI), and Te(1V) in the presence of Ti(IV) are reduced at -0.20 V to form an insoluble film on the electrode, causing a slow electrolysis and a positive bias. These elements are not reoxidized at f0.22 V, but the surface film prevents the rapid reoxidation of Ti(II1). Vanadiumw) slowly oxidizes the mercury electrode, and is reduced to V(1V); the V(1V) formed and any remaining V(V) are then partially reduced to V(II1) during the electrolysis at -0.20 V. However, this reduction to V(II1) is irreversible; thus the tolerance to vanadium is increased by the use of the reduction-oxidation procedure. When this two-step procedure is used to avoid interferences, the prereduction at -0.20 V can be conducted during the deoxygenation of the solution. In the case of V(V), this results in an increased tolerance level, because less of the V(V) is thereby allowed to react with the mercury electrode. Perchlorate ion apparently is catalytically reduced in the presence of titanium at -0.20 V, and a steady electrolysis current above the background level is reached. However, if sufficient time is allowed for complete reduction of the Ti(IV), the determination by reoxidation is accurate. Nitrate interferes both by oxidizing the mercury and by chemically oxidizing the Ti(II1) during the course of the reduction of Ti(IV). Thus a high, steady background current is also observed with nitrate, but low results are obtained on reoxidation. As shown in Table 11, 0.25 mg NO3- can be tolerated, but it is best to remove it during the sample pretreatment. Nitrite also attacks the electrode and should be removed either in the pretreatment or by reaction with sulfamic acid, before the solution contacts the mercury. Analysis of Titanium-TungstenAlloys. After a preliminary separation of the components, the coulometric procedure was applied to the determination of titanium in binary alloys con-
taining high W/Ti ratios. Hydrolytic precipitation of the titanium proved to be a simple and effective separation technique. Adjustment of the pH of the dissolution medium to 12 (in the presence of fluoride to keep the tungsten in the acid solution) quantitatively precipitated the titanium with insignificant coprecipitation of the tungsten. Results of the analyses of synthetic mixtures of titanium and tungsten and two alloy samples are given in Table 111. Several experiments indicated that, in this manner, titanium can also be quantitatively separated from molybdenum.
ACKNOWLEDGMENT Impurity analyses of the titanium metal were carried out by J. W. Fischer, R. Lim, J. R. Stevens, and E. G. Walter. RECEIVED for review December 14,1970. Accepted February 16, 1971. This work was performed under the auspices of the U. S. Atomic Energy Commission. Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U. S. Atomic Energy Commission to the exclusion of others that may be suitable.
Determination of Formation Constants of Copper( II) Complexes of Ethane-1-hydroxy-1,l-diphosphonic Acid with a Solid State Cupric Ion-Selective Electrode Hiroko Wada and Quintus Fernando Department of Chemistry, University of Arizona, Tucson, Ariz. 85721 The acid dissociation constants of ethane-l-hydroxy1,l-diphosphonic acid, (H4Y), and the formation constants of the copper(l1) complexes, CuY2-, CuHY-, and CuH2Y, have been determined in aqueous solution at 25 OC at an ionic strength of 0.1. The values of these concentration constants that were obtained potentiometrically with a solid state cupric ion-selective electrode have been confirmed by potentiometric measurements with a glass electrode and by a spectrophotometric method. On the basis of the formation constant measurements, it has been proposed that the copper (11) complex has a six-membered chelate ring structure.
methods that complement each other: the free copper(I1) ions in solution could be determined potentiometrically with a solid state cupric ion-selective electrode, the hydrogen ions that are released upon complex formation could also be determined potentiometrically with a glass electrode, and, finally, the concentration of a copper(I1) complex that is formed in solution could be measured spectrophotometrically. This approach should yield reliable complex formation constants which could be used as a basis for deducing the kinds of metal complexes that might be expected in solution. EXPERIMENTAL
ETHANE-~-HYDROXY-~ ,1-DIPHOSPHONIC ACID, (EDPA), is a polydentate ligand which forms soluble complexes with most metal ions and selectively precipitates thorium(IV), scandium (111), and the lanthanides from acid solutions ( I ) . A method for the selective determination of thorium(1V) has been proposed, in which the formation of a soluble binuclear ternary complex of thorium(1V) has been postulated ( I ) . If the oxygen donors on one phosphonic acid group in the EDPA molecule participate in metal complex formation, mononuclear and binuclear complexes containing fourmembered metal chelate rings will be obtained. If the oxygen donors are from two different phosphonic acid groups in the same EDPA molecule, six-membered chelate rings will be formed. In addition to four- and six-membered rings that can be formed, a variety of neutral and protonated metal chelates may also be present in solution. To assess the usefulness of EDPA as an analytical reagent it is important to recognize the different types of complexes that are obtained when solution parameters such as acidity and ligand concentration are varied. A study of the metal complex equilibria involving copper(I1) and EDPA was undertaken therefore, as part of a systematic study of polydentate ligands of the same type. This particular system was chosen since it seemed feasible to determine the concentrations of various species in solution by at least three (1) R. Pribil and V. Veseley, Tulunta, 14, 591 (1967).
Synthesis of EDPA. Ethane-1-hydroxy-1,l-diphosphonic acid was prepared by the action of acetic anhydride on phosphorus acid (2). One mole of solid phosphorus acid and 1.1 moles of acetic anhydride were heated to 100 "C, maintained at this temperature for one hour, and the mixture steam distilled until the distillate was no longer acidic. The oily residue in the distilling flask was dissolved in water, and the pH of the resulting solution was adjusted to a value between 8 and 9 with sodium hydroxide. Ethyl alcohol was added to the solution until a white precipitate of the trisodium salt, EDPA.3Na, was formed. The crude salt was separated and purified by recrystallization from an ethanol-water mixture. The di- and trisodium salts of EDPA were found to be slightly hygroscopic and the free acid and the monosodium salt were extremely hygroscopic. An elemental analysis of the trisodium salt indicated that it contained about 8 % water, (Found: C , 8.11%; H, 2.50%; P, 21.13z; Calcd forEDPA.3Na.1.21H20; C, 8.11 H,2.56%; P,21.09%). After intensive drying the compound was titrated with a standard solution of (CH&NOH and was found to be 100% pure and free of water and acidic contaminants. The lH NMR spectrum of the compound in D20 exhibited the expected triplet that arises from the methyl protons that are split by two phosphorus atoms. Standard solutions of EDPA were prepared by dissolving EDPA in deionized water and standardizing the solution potentiometrically with standard (CH&NOH.
z;
( 2 ) B. T. Brooks, J. Amer. Chem. SOC.,34, 496 (1912). ANALYTICAL CHEMISTRY, VOL. 43,
NO. 6, MAY 1971
751
