Spectrophotometric Determination of Iridium with Leuco-Crystal Violet GILBERT
H. AYRES
and WILLIAM T. BOLLETER'
Department of Chemistry, The University of Texas, Austin, rex.
F A very sensitive and precise spectrophotometric determination of iridium is based upon the oxidation of leucocrystal violet to its colored form by iridium(lV) in acetate-buffered solution in the pH range 3.5 to 4.7. The optimum concentration range is 0.5 to 4 p.p.m. of iridium for measurements a t 590 mp and 1.00-cm. optical path. The specific absorptivity is 0.25 per p.p.m.-cm., corresponding to a molar absorptivity for iridium of 4.8 X lo4. The color develops immediately at room temperature and is stable for a t least 12 hours. Reliable results require preparation of the iridium(lV) solution b y fuming down with nitric, perchloric, and phosphoric acids. Among the platinum elements, the method is highly selective for iridium. Gold(ll1) interferes seriously by also oxidizing the dye base; a simple method for complete removal of gold is given.
T
method for the spectrophotometric determination of iridium published to date is that of Westland and Beamish (8),in which a red color is developed with p-nitrosodiniethylaniline. The method lacks specificity for iridium among the platinum elements; it is not directly applicable in the presence of nonvolatile acids or their salts; and the sensitivity is not always reproducible, requiring simultaneous preparation of standards. Ayres and Quick ( 2 ) determined iridium on the basis of a purple color produced by heating iridium(1T') solutions with mixed perchloric, phosphoric, and nitric acids; by slight modification the method can be applied to solutions containing up to 40% (volume) of sulfuric acid (1). Among the platinum and iron group elements, only palladium interfered when present to the extent of about 80% of the iridium content. AIaynes and llIcBryde (6) found the precision of the mixed acid method unsatisfactory, and determined iridium by the red color produced by heating iridium sulfate solutions 15-ith cerium(1T') sulfate for 10 to 12 hours a t 70" C. Consistent results in both of the latter methods depend upon careful attention to temperature HE MOST SESSTITE
Present address, llonsanto Chemical Co., Texas City, Tes. 72
ANALYTICAL CHEMISTRY
and duration of heating; the rather long heating period is inconvenient, and the methods are not as sensitive as desired. MacNevin and Kriege ( 5 ) determined iridium as its chelate complex with disodium (ethylenedinitri1o)tetraacetate (EDTA) in alkaline solution; development of maximum absorbance (at 313 mp) required 24 hours a t room temperature, but only about 10 minutes a t 85" to 90" C. The method has about the same sensitivity as the methods of Ayres and Quick and lllaynes and hIcBryde, and is subject to interference from the other platinum elements and from nitric acid. In the method reported here, iridium(1V) is determined by reaction with leuco-crystal violet t o produce its colored (ouidized) form, the absorbance of which is measured at 590 mp. The sensitivity is about three times as great as that of the method of Westland and Beamish. Other platinum elements do not interfere unless present in amounts 5 to 50 times greater than the amount of iridium; the only serious interference is from gold(III), and it can be removed by a simple method. The present method was the outgron t h of preliminary work based upon a sensitive spot test proposed by Tschugapff ( 7 ) ,in which leuco-malachite green in concentrated acetic acid solution was oxidized by iridiuni(1V) to the green colored form, which was extracted into chloroform. I n preliminary work, extraction FTas not used; the absorbance of the green' aqueous solution vias measured at 620 mp. Colorless solutions of the leuco-base reagent in acetic acid gradually became green within a few days; solutions of the reagent in 0.1 to 1-TI phosphoric, sulfuric, perchloric, or hydrochloric acid remained colorless for much longer periods. Maximum color development m s attained in acetate-buffered solutions in the p H range 3.5 to 4.2; precipitation of the dye base occurred at higher pH. At room temperature full color development required more than 24 hours; heating for periods up to 30 minutes accelerated the color development, but the final absorbance vias less than that produced a t room temperature. Only solutions of iridium(1T) in perchloric acid gave good reproducibility, and aged iridium solutions produced green solutions of lower ab-
sorbance than freshly prepared iridiuni solutions. Efforts to modify the procedure and/or conditions to give satisfactory precision n-ere not successful. By substituting leuco-crystal violet for leuco-malachite green, and preparing the iridium solution for analysis by a special method, a very sensitive method of high precision was developed. APPARATUS
Absorbance measurements were made with a Beckman Model DU spectrophotometer, operated a t high constant sensitivity, and matched 1.00-cm. Cores cells. The cell compartment was maintained a t 25" C. Xeasurements of p H mere made with a Beckman Model H-2 line-operated meter. The platinum elements or their compounds were checked for possible impurities with a 1.5-meter Applied Research Laboratories grating spectrograph. REAGENTS
Iridium metal powder and the other platinum metals or their compounds were obtained from A. D. Mackay, Inc. Spectrographic examination of the iridium showed only a trace of platinum, palladium, and rhodium, none of which interferes in the proposed method. Iridium Stock Solution. Iridium metal powder was attacked by fusion with sodium peroxide or mixtures of sodium peroxide and sodium hydroxide in a silver crucible a t dull red heat; the final solution was prepared essentially as described in earlier work ( 2 ) . The stock solutions contained 1.00 mg. of iridium per ml. Appropriate aliquots of the stock solution were used to prepare more dilute working solutions in the color development procedures. Color Reagent. 4,4',4"-Hexa methyltriaminotriphenylmethane (leucocrystal violet), obtained from the Eastman Kodak Co., was used as a 0.10% solution. Solutions in concentrated acetic acid (as used with leuco-malachite green in the Tschugaeff spot test) became blue on standing for a fen- days. Solutions in 1JI phosphoric, sulfuric, perchloric, or hydrochloric acid were much more stable against atmospheric oxidation. I n the final procedure, a 0.10% solution in 1M phosphoric acid was used; fresh solutions were prepared weekly. Buffer Solution. Sodium acetateacetic acid buffer solution l+as prepared by adding 700 ml. of glacial acetic acid t o 300 nil. of 6M sodiuni
hydroxide: the DH of the buffer solutcon was 3.9. * Test Solutions. Stock solutions containing 1.00 mg. of metal Der ml. were usedvin interfvrence tests: Platinum, palladium, and gold solutions were prepared by dissolving the metal in aqua regia, followed by repeated evaporation with hydrochloric acid, and final dilution to volume with water. Osmium solutioii was prepared by dissolving osmium tetroxide in 0.2M sodium hydroxide. Ruthenium, rhodium, iron, cobalt, and nickel solutions were prepared from their hydrated chlorides. All other chemicals were C.P. reagents, or reagents conforming to ACS specifications.
Table 1.
Iridium Concn., P.P.M. 0.20 0.40 1.00 2.00
3.00
4.00 5.00 6.00
PH
Crystal Violet Absorption Spectrum. Figure 1 is a plot of log absorbance against wave length for a crystal violet solution formed by oxidation with 2 p.p.m. of iridium(1V) in acetate buffer a t p H 3.9; maximum absorbance occurs a t 590 mp. I n the final standardized procedure the color developed immediately a t room temperature and was stable for a t least 12 hours. Table I shows that the system conforms to Beer’s law. Amount of Reagent. One milliliter of 0.10% leuco-crystal violet solution per 25-ml. final volume was adequate for full color development of the largest amount of iridium used (6 p.p.m.). S o difference in absorbance was produced by use of the same volume of a 1.0% solution, but 1 ml. of 0.01% solution was inadequate for full color development. Solutions of the reagent gave reproducible results for about
2.40 3.00 3.22 3.48
-
IRIDIUM -CRYSTAL VIOLET
SYSTEM
Absorbance at 590 Mp 0.052 0.102 0.252 0.507 0.756 1.02 1.26 1.50
Table 11.
EXPERIMENTAL
070 060
Standard Series Calibration Data
Absorbance 0.450 0.497 0.501 0.507
Absorptivity Per P.P.M.-Cm. 0.26 0.255 0,252 0.254 0,252 0.255 0.252 0.250
Effect of pH
pH
3.77 4.01 4.62 4.70 Above 4.7
Absorbance
0.507 0.507
0.507 0.507 Turbid
1 week after preparation, after which the measurements were less reliable, especially for small amounts of iridium. Effect of pH and Buffers. I n preliminary tests with sodium acetateacetic acid buffers it was found t h a t deviations in absorbance were associated with the origin of the sodium acetate used; buffers prepared from acetic acid and sodium hydroxide gave consistent results. Use of 10 ml. of buffer solution per 25-ml. final volume gave excellent precision. The p H of the solutions was varied by adding small amounts of 6M sodium hydroxide or 6M phosphoric acid. Phosphoric acid could be added either before or after the acetate buffer without affecting the results. K i t h sodium hydroxide, reproducible results mere obtained only TT hen the buffer TYas added before adding sodium hydroxide; a t pH greater than 4.7 the leucobase was precipitated. Stable colored solutions were produced in the pH range 3.5 to 4.7; the data in Table I1 are for solutions containing 2 p.p.m. of iridium. I n the standardized procedure given later, the p H of the final solutions was in the range 3.6 to 3.9, depending upon the size of the aliquot of the acidic iridium solution taken. Temperature. The temperature of the cell compartment of the spectrophotometer was varied from 15’ t o 39’ C.; a solution developed from 2.0 p.p.m. of iridium decreased in absorbance from 0.513 to 0.497; the change in absorbance was linear with temperature, and was reversible. Variations in Iridium Solution. “Working solutions” prepared by merely diluting the stock iridium solu-
tion containing nitric and hydrochloric acids did not give satisfactory precision, and the amount of leucocrystal violet oxidized !vas less from a n aged iridium solution than from one freshly diluted from the stock solution, Westland and Beamish (8) also observed that the amount of color developed by p-nitrosodimethylaniline depended upon the previous treatment of the iridium solution, and $vas unpredictable unless a standardized procedure of pretreatment of the iridium solutions was followed. Much of the work of the present investigation was concerned with finding conditions for the preparation of an iridium solution that would give reliable results. Chloride-free solutions of iridium were prepared by boiling down aliquots of the stock solution with sulfuric, phosphoric. acetic, or perchloric acid. Only the perchloric acid solution gave fair precision in the reaction with leuco-crystal violet, and then only in freshly prepared solutions. Maximum color development required 15 minutes. Addition of phosphoric acid or of diamnionium hydrogen phosphate in the color-development procedure improved the precision of the absorbance measurements. Iridium solutions prepared by boiling down with a mixture of perchloric, nitric, and phosphoric acids gave color developed solutions with leuco-crystal violet of better reproducibility than had been obtained previously; substitution of sulfuric acid for the phosphoric acid in the mixture resulted in poor absorbance precision. Preparation of Stable Iridium S o h tion. The following procedure gave a dependable iridium solution for color development. To an aliquot of the iiidium stock solution, 10 ml. of 70 to 727, perchloric acid and 10 ml. of concentrated nitric acid were added, along with sufficient 85% phosphoric acid to make the solution about 1M in phosphoric acid when finally brought t o known volume. The mixture was heated in an Erlenmeyer flask fitted with a fume eradicator. During the heating operation the amber solution turned to purple, then slowly faded to a pale yellow color; a t this point fumes of perchloric acid appeared. Heating was continued until volatilization of perchloric acid was complete, as indicated by cessation of vigorous boiling of the solution. After cooling, the solution \\-as diluted considerably, boiled gently for 30 minutes, then finally diluted to known volume. The boiling after dilution Tvas necessary in order to get maximum and reproducible absorbance of the crystal violet. Aliquots of this working solution were used for developing the color with leuco-crystal violet by the standardized procedure given below. Working solutions containing up t o about 10 p.p.m. of iridium gave reproVOL. 29, NO. 1, JANUARY 1957
73
ducible results for 1 or 2 days after preparation; solutions containing 25 p.p.m. or more gave reproducible results for several weeks. Variations in phosphoric acid from 0.5 to 3M in the working solution had no effect on the absorbance of the final colored solutions. Working solutions which were further diluted before use in the color reaction gave less final color of crystal violet than solutions of the same iridium concentration prepared as described above. Standardized Procedure. An aliquot of a n iridium solution, liM in phosphoric acid, prepared as above, mas transferred t o a 25-m1. volumetric flask; 1 ml. of 0.1070 solution of leucocrystal violet in 1M phosphoric acid and 10 ml. of acetate buffer were added, and the solution was diluted to volume. The color developed immediately, and was stable for more than 12 hours. Absorbance was measured against a reagent blank. Reproducibility. The precision was evaluated by measuring the absorbance of 48 solutions consisting of triplicate aliquots from each of 16 different working solutions prepared by the fuming procedure described earlier. The working solutions were of three different concentrations-10, 25, and 50 p.p.m. of iridium-and the final solution for measurement contained 2 p.p.m. of iridium. No results were discarded. All results were in the absorbance range 0.495 t o 0.514; the average was 0.503, with a mean deviation of 0.003. Optimum Range. The optimum concentration range for measurement at 1.00-cm. optical path is about 0.5 to 4 p.p.m. of iridium. For a difference of 0.2% transmittance, the relative analysis error in this concentration range is about 0.67,.
Table 111.
Tolerance for Foreign Ions
Foreign Ion Palladium( 11) Platinum(IV) RhodiumiIII) Nickel( I1j Cobalt(I1) Gold( I11)
Tolerance, P.P.M. 100
50 10 200 30 0.01
Effect of Foreign Ions. I n testing for interference by foreign ions, aliquots of the iridium stock solution and of the ion under investigation Eere fumed in the mixed acids as described previously, and the color was then developed by the standardized procedure. Interference was taken as the largest amount of foreign ion that could be present, with 2.0 p.p.m. of iridium, and give a solution of trans74
ANALYTICAL CHEMISTRY
mittance not more than 0.4% (absolute) different from that produced by the iridium alone. This tolerance corresponds to about 0.005 absorbance difference for 2.0 p.p.m. of iridium, which is about in the middle of the optimum concentration range.
by also oxidizing the leuco-crystal violet. Extraction of gold(II1) from 10% hydrochloric acid solution into ethyl acetate, by the method of Lehner and Kao (C), is not a sufficiently complete removal. Reduction by iron(I1) or tin(I1) is not practical
HC104 (HNI&l)
002 0015
400
450 WAVE
500
550
600
LENGTH, inp
Figure 2. Spectral curves of 50 p.p.m. of iridium in solutions of different acids Final acid concentration approximately 1 M
The tolerances for various foreign ions are shown in Table 111. Osmium and ruthenium do not interfere because they are volatilized as their tetroxides during preparation of the stabilized iridium solution. The absorbance of solutions containing platinum increased slowly; the tolerance listed for platinum is based on measurement of absorbance within 1 hour of development of the color. When palladium was present in relatively large amounts-e.g., about 100 mg.-fuming down with the acids produced a brown precipitate which oxidized leuco-crystal violet, with dissolution of the precipitate which was presumed to be palladium(1T) oxide. Iron(III), in concentrations as low as 5 p.p.m., was precipitated a t the pH of the acetate buffered solution; removal of iron is preferably made prior to fuming down the acid mixture. Sulfate ion in moderate to large amounts interfered to some extent, but a concentration of about 0.01M in the final solution could be tolerated. Sulfate ion resulted in a decrease of absorbance, whereas all other interfering ions caused an increase. Removal of Gold. Gold(II1) in extremely minute amounts interfered
because of the necessity for removing excess reducing agent or its oxidation product. Reduction of gold(II1) in dilute hydrochloric acid by hydroquinone gave colloidal gold that could not be coagulated sufficiently for removal by filtration or centrifugation. Complete removal xas accomplished by amalgamation after reduction. Solutions containing the gold(II1) were made 1 t o 2M in hydrochloric acid; a two- t o threefold excess of hydroquinone was added, and the solution was heated for several minutes. After cooling to room temperature, a few milliliters of mercury were added and the mixture was shaken vigorously for about 1 minute, then filtered and mashed well with water. The quinone and excess hydroquinone were destroyed by adding 25 ml. of nitric acid and boiling down to about 25 ml., followed by the usual procedure for preparation of the solution for iridium analysis. As much as 200 mg. of gold was completely separated by this method. Although a detailed procedure was not worked out, it appears that a very sensitive method for the determination of gold could be based on the oxidation of leuco-crystal violet by gold(II1).
~~
I n the color reaction, gold(II1) was reduced to the elemental condition, by a three-electron change, and it produced an amount of the oxidized dye three times as great as that produced by a n iridium(1V) solution of the same molar concentration; i t is therefore concluded that in the redox reaction with leuco-crystal violet the reduction product is iridium(II1). During the investigation of methods for preparing the iridium working solution from the stock solution, separate samples of the latter were fumed down with different acids and acid mixtures, and the solutions were examined for possible direct spectrophotometric determination of iridium; spectral curves for the various solutions are shown in Figure 2 . Only the perchloric acid solutions were suitable for direct determination of iridium. Solutions containing perchloric acid were boiled vigorously to evaporate to dense white fumes. The 11-ine-red solutions had niasimuni absorption at 495 mp. The absorbance was not reproducible unless the samples, after dilution, were boiled for a t least 30 minutes to expel decomposition products of perchloric acid (chlorine and its oxides). llaximum absorption then occurred a t 510 mp, and absorbances were well reproducible.
Table IV. Absorbance of Iridium in Perchloric Acid Solution
Iridium Concn., P.P.11.
Absorhance at 510 1Ip
5.0
0.042
25 0
0.215 0 424
10.0 50 0 75 0 100 0
0.084 0 636 0 840
Absorptivity Per P.P.;\I.-Cni. 0.0084 0.0084 0.0086
0 0085 0.0085 0 0084
The solutions conformed to Beer's la^, as sho1r.n in Table IV, if freshly prepared samples )!-ere measured. Solutions containing up to about 25 p.p.m. of iridium were stable for 1 to 2 days; solutions containing 100 p.p.m. were stable for several weeks. Although the sensitivity is not high, perchloric acid solutions could be used for the direct determination of iridium. However, the nietliod is not recommended in the presence of palladium, gold, or platinum. Palladium is partially precipitated; gold is precipitated if present in amounts greater than 50 mg.; platinum is precipitated as platinum(1T') oxide, which is Yery difficult t o separate from the solution on account of very small particle size.
Table V.
Comparison of Methods for Iridium
Wave Length Specific of Maximum Optimum AbsorpAbsorbance, Range, tivity, Ref. M p P.P.M.0 P.P.M.-Cm.
Method Mixed acids Ceriuni(1V) p-Nitrosodimethvlaniline EDTA Leuco-crystal violet a
~
Treatment for Full Absorbance
(2)
564
10-80
0 013
(8) (')
505 530
15-100 1,510
1 hr. at lJO", 0.5 hr. at 150 0.0092 10-12 hr. at 70" 0.099'~ 40 min. at 70
(5)
313 590
8-60 0.54
0.017 0.25
10 min. a t 80-90' Instantaneous, room temperature
Approximate concentration range for absorbances from about 0.15 to 1.0. tubes; optical path not given in ( 8 ) . Molar absorptivity given in ( 5 ) ,3.2 x 105; range calculated as in
* Bbsorptivity per p,p.m. in Klett-Summerson e
5.
The mine-red component of the iridium perchloric acid solution was adsorbed on and eluted from a cation exchange resin (Dom-ex-50). I n a study of iridium solutions Dwyer and Gyarfas (3) reported an unstable red-violet form of iridium(1V) obtainable only by reduction of the brown iridium(V1) solutions; they suggested this unstable product was [IrO]++,which hydrolyzed to a stable blue-violet product, probably [IrO OH]+, and that this latter complex was also the substance produced by oxidation of iridium(II1) solutions. The purple component in phosphoric acid solution n-as adsorbed on and eluted from an anion exchange resin (IR-4-B), shon-ing iridium to be in the form of a n anionic complex. The different ionic forms of the iridium in perchloric acid and in phosphoric ncid are consistent with the known properties of these acids, the former being a poor complex former (if a t all), and the latter forming very stable anionic complexes with many metallic ions. Tests on the hydrochloric acid solutions of iridium showed them to contain a considerable amount of the element in oxidation state less than + 4 . This fact inay account for the poor precision in the crystal violet method, when hydrochloric acid solutions of iridium were used; the amount of dye base oxidized decreased with the age of the iridium solution used. Addition of chloride ion to the wine-red solutions of iridium in perchloric acid caused slow change to a blue color mhich, when the solution was heated, changed through blue-green, green, yellow-green, and finally to yellow. DISCUSSION
Table T' compares existing methods for the spectrophotometric determination of iridium. In addition to the high sensitivity and immediate color derelopment a t room temperature, the
crystal violet method has the advantage of no interference from other platinum elements unless they are present in large amounts relative to the iridium: interference from gold is easily removed. The principal disadvantage is the time required for fuming down the iridium solution with the acids and the additional boiling after dilution. in order to get an iridium solution that gives reliable results. ACKNOWLEDGMENT
The early part of this work was supported jointly by The University of Texas and the United States Atomic Energy Commission under the terms of Contract N o . AT-(40-1)-1037. Grateful acknodedgment also is made to the Texas Eastnian Co. for a fellowship awarded to Killiam T. Bolleter for thc academic year 1954-55, during T-J hich the work was completed. LITERATURE CITED
(1) Ayres, G. H., Maddin, C. If., AYAL. CHEM. 2 6 , 671 (1954). (2) Ayres, G H., Quick, Q., I b t d , 22, 1403 (1950). (3) Dwyer, F. P., Gyarfas, E. C., J . Proc. Roy. SOC. 8.W . 84, 123 (1950, (4) Lehner, V., Kao, C., J. Phus. Chem. 30. 126 11926).
W.' M., Kriege, 0. H . ANAL.CHERI.28, 16 (1956). (6) Mavnes, A. D., McBryde, W. -4.E , Analyst 79, 230 (1954j.' ( 7 ) Tschugaeff, L. A,, Ann. inst. plntz'ur 7. 205 (1929). (8) TF7e∧ A. 'D., Beamish, F. E., ANAL.CHEX 27, 1776 (1955). ( 5 ) M&evi< '
RECEIVEDfor review June 16, 1956. Accepted October 11, 1956. Division of Analytical Chemistry, 129th Meeting, ACS, Dallas, Tex., April 1956. Condensed from a dissertation submitted by William T. Bolleter to the faculty of the Graduate School of The University of Texas in partial fulfillment of the requirements for the degree of doctor of philosophy, May 1953 VOL. 29, NO. 1, JANUARY 1957
75
