amounts of the insoluble ruthenium trichloride were not successful. Platinum wa8 successfully chlorinated and recovered in chlorinations under the conditions found optimum for ruthenium. LITERATURE CITED
(1) Barefoot, R. R., Beamish, F. E., ANAL.&EM. 24, 840 (1952). (2) Deville, H. S., Debray, H., Morin, H., Chem. Zentr. 1874, 609.
(3) Fraser, J. G., thesis, University of Toronto, 1951. (4) Gilchrist, R., BUT.Standards J . Research 6, 421 (1931).
(9) Westland, A. D., Beamish, F. E., ANAL.CHEM.26, 739 (1954). (10) Westland, 8. D., Beamish, F. E., Mikrochim. Acta, in press. (11) Westland, A. D., Beamish, F. E., un-
(6) Hill, M. A., Beamish, F. E., ANAL. CHEM.22, 590 (1950). (7) Hill, M. A., Beamish, F. E., J . Am. Chem. SOC.72, 4855 (1950).
(12) Wichers, E., Schlecht, W. G., Gordon, C. L., J . Research Natl. Bur. Standards 33, 363 (1944).
(5) Gordon, C. L., Schlecht, W. G., Wichers, E., Ibid., 33, 457 (1931).
(8) Sandell, E. B., “Colorimetric Determination of Traces of Metals, 2nd ed., p. 494, Interscience,New York, 1950.
published research.
RECEIVED for review March 11, 1957. Accepted October 22, 1957. Work suported by the award of a fellowship by the ational Research Council (Canada).
R
Certain Rare Earths in Purified Thorium and Uranium Preparations Chemical Isolation and Spectrographic Determination CYRUS FELDMAN and JANUS Y. ELLENBURG Oak Ridge National laboratory, Oak Ridge, Tenn.
b Microgram quantities of rare earths can b e quantitatively separated from thorium nitrate dodecahydrate or uranyl nitrate hexahydrate b y dissolving the salt in an ether-nitric acid mixture, stirring with cellulose, and filtering. The rare earths are retained on the cellulose, from which they can b e removed with aqueous nitric acid (pH 2). The rare earth solution is purified by thenoyltrifluoroacetone extraction. An aliquot of this purified solution is treated with palladium internal standard and zinc buffer, and evaporated on the ends of graphite rods. These electrodes are sparked in a 20% argon-80% oxygen atmosphere, using a 5.5 radio-frequency ampere high voltage discharge having 20 to 23 breaks per half cycle. The presence of zinc prevents changes in sample composition from shifting or rotating analytical curves. The effectiveness of the procedure has been demonstrated for cerium, dysprosium, europium, gadolinium, lanthanum, neodymium, and samarium. It should b e effective for the other rare earths as well. Yttrium accompanies the rare earths through the entire procedure, but scandium will be lost if thenoyltrifluoroacetone is used to remove the last traces of thorium.
S
europium, gadolinium, and dysprosium have very large cross sections for the capture of thermal neutrons; other rare earths do not (7). In order to formulate realistic purity specifications for nuclear fuels, it has thus become necessary to develop methods for determining 0.1 to 100 p.p.m. of individual rare earths in p u d e d thorium AMARIUM,
418
ANALYTICAL CHEMISTRY
and uranium preparations. Emission spectrochemical methods are among the most accurate and reliable for this purpose. To perform this determination spectrographically, it is necessary first to isolate and concentrate these impurities. The present procedure was first developed for use with purified bhorium materials (total rare earth content 2 will produce a colloidal form of beaker. Cover the pulp with an 11.1 thorium hydroxide which redissolves volume % nitric acid-ether mixture. extremely slon-ly, even though the final Decant the mixture immediately before pH of the solution may be 1.0. This using the pulp. PREPARATION OF SAMPLE SOLUTION. colloid cannot be extracted by the subsequent purification procedure.) I n the present procedure, the sample Extract the residual thorium from the must be present initially as thorium solution by agitating with an equal nitrate dodecahydrate. If the sample volume of 0.5-lf thenoyltrifluoroaceis received as thorium oxide, weigh tone in xylene or kerosine. Discard out a representative sample which the organic phase. This extraction recontains approximately 20 grams of moves the thorium, iron, zirconium, thorium and transfer it to a 300-ml. and scandium and can be completed in platinum dish. Add 200 ml. of 1 to 1 5 minutes a t 60" C. Backwash the nitric acid and then 3 drops of -5 aqueous layer with xylene or kerosine to volume yo hydrofluoric acid. Stir the remove any dissolved thenoyltrifluorosuspension. Place the dish on a steam acetone. Discard the organic phase. bath and cover it with a tightly fitting Increase the pH of the aqueous phase watch glass. Allow the mixture to to 4.5. Agitate this as above n-ith a digest until dissolution of the sample is fresh portion of thenoyltrifluoroacetone. complete (approximately 90 minutes for The rare earths are extracted into the 20 grams of thorium oxide). Do not organic phase. Retain this, but disstir it during the first hour; stir it card the aqueous phase, which contains occasionally after that, if desired. the impurities other than rare earths. Replace the original watch glass by a Agitate the organic phase for 5 minribbed watch glass, and evaporate the utes with approximately 15 ml. of 1-U solution to dryness on the steam bath. hydrochloric acid a t 60" C. Backwash The residue is thorium nitrate dodecathe aqueous layer with xylene or kerohydrate. sine as above to remove residual theIf the sample is received as metallic noyltrifluoroacetone. The aqueous thorium, dissolve 20 grams as above in phase now contains rare earths only 1 to 1 nitric acid containing hydro(including yttrium). fluoric acid and evaporate the solution Procedure for Purified Uranium to dryness on a steam bath. The resiCompounds. Prepare the sample in due is thorium nitrate dodecahydrate. the form of uranyl nitrate hexahyIf the sample is received as a thorium drate. Condition cellulose pulp and compound other than thorium nitrate isolate rare earths as in the above dodecahydrate, or as some hydrate of procedure, but use a 4.76 volume % thorium nitrate other than the dodecanitric acid-ether solution ( 5 volumes hydrate, convert it to this compound of nitric acid plus 100 volumes of by appropriate chemistry, ending with ether) in place of the 11.1 volume % evaporation of a 1 to 1 nitric acid solunitric acid-ether solution throughout. tion of the sample on a steam bath. Dissolve the thorium nitrate dodecaEvaluation of Procedure. The hydrate in a minimum quantity of effectiveness of the cellulose procedure concentrated nitric acid (-45 ml. for a in recovering microgram quantities of 20-gram sample). Add this solution slowly and with rapid stirring to rare earths from gram quantities of VOL. 30, NO. 3, MARCH 1958
419
Table 1.
Recovery of Rare Earths by Cellulose-Ether-Nitric Acid Procedure
Mixture 1. E U * ~ * - ( 20 g. Th 2. Eu16*--4 1 mg. Ce+4 20 g. Th
+ + + Eul62-4 + 1 mg. La + 20 g. Th 1 mg. Gd + 1 mg. Dy + 2 mg. Sm + 8 g.
3. 4.
Th
Method of Measurement Tracer Tracer Grav. (CeOz) Tracer Grav. (LazOi) -Spectrographic
% Recovery 99.2 99.7 97.7 99.1
98.4
Gd 98.7
Dy 99.3
Sm 99.3 Table II.
Effect of Changes in Composition of Rare Earth Mixture on Individual Rare Earth-Palladium Intensity Ratios
Value of Intensity Ratio for Indicated Mixture Composition Original mixture 0 . 6 y Sm, Original Original Original 0.2 y Dy, Original 0.2 y Gd, mixture mixture mixture mixture 0.5yPd +2yCe +8yCe +0.5yLa +2yLa
0
Intensity Ratio Measured Gd 3422.47 0.958 Pd 3421.24 Dy 3531.71 1.03 Pd 3421.24 Sm 3568.26 o.535 Pd 3421.24 Rare earth line not detectable.
0.621
0.717
0.748
a
0.619
0.673
0.780
a
0.417
0.303
0.482
a
thorium was tested by tracer, gravimetric, and spectrographic methods (Table I). Tracer experiments were carried out by treating the nitric acid solution of thorium nitrate with Eu1S2-' (y-emitting) tracer and following the above procedure. Colorimetric tests with thoron showed that the final rare earth solution contained less than 2 y of thorium per ml. after the separation. The EulSz-' activity in this solution was measured with a y-ray scintillation spectrometer. I n some cases, milligram quantities of rare earths were added to the thorium nitrate solution along with the tracer. In these cases, recoveries were measured by both tracer and gravimetric methods. The slightly low recoveries obtained in the gravimetric cases were probably due to incomplete collection of the small precipitates. I n the spectrographic tests, the mixture described in Table I (mixture 4) was put through the entire separation procedure, and the rare earths were determined by the spectrographic technique. The results given in Table I show that recovery of these rare earths was essentially complete. The data of Kember (9) and Bronaugh and Suttle (3) indicate that recovery would be equally good for the remaining rare earths. SPECTROGRAPHIC DETERMINATION OF RARE EARTHS
Development of Procedure. TIVITY.
420
SENSI-
Preliminary tests showed ANALYTICAL CHEMISTRY
that the greatest absolute sensitivity was obtained by sparking nitrate residues on graphite electrodes in an argon-oxygen atmosphere. As the sample waa concentrated on the surface of the electrodes, i t was felt that the energy of the spark source would be utilized more efficiently if this energy were delivered in the form of many small bursts per half cycle, rather than a few large bursts. An air-interrupted spark source, operated at 20 to 23 breaks per half cycle (5.5 radiofrequency amperes) gave excellent precision, and sensitivity which was ten times greater than that afforded by the same quantity of energy per half cycle delivered in the form of a single burst. INTERELEMENT EFFECT. I n many cases, microgram quantities of one rare earth were found to change the position or slope of the analytical curves of another. Table I1 shows typical changes in rare earth-palladium intensity ratios caused by adding cerium and lanthanum to a given rare earth mixture. It is obvious that quantitative analysis of unknown mixtures of microgram quantities of rare earths is impractical by this method, unless some way is found to make individual intensity ratios-e.g., Gd 3422.47/Pd 3421.24, etc-insensitive to changes in the composition of the mixture. Trial of a number of buffer materials showed that the desired effect was best obtained by adding 30 y of zinc as the nitrate to sample and standard rare earth residues. When this was done, analytical curves were independent of the composition of the standard mix-
ture and recoveries were essentially quantitative (Table V). Even the presence of 2 y of lanthanum, which completely suppressed all rare earth lines in the absence of zinc, had little apparent effect when zinc was present. PRECISIOK. Good precision in the use of spark sources of high power rating (> '/a kv.-amp.) depends on accurately reproducing the curve of current us. time from one discharge to the next. In air-interrupted sources this means primarily that the breakdown voltage of the auxiliary gap must be closely reproducible. This breakdown voltage depends principally on the distance between the auxiliary electrodes, but is also affected by the shape, smoothness, and cleanliness of their surfaces and the resistivity of the ambient atmosphere. Consequently, it cannot be accurately and reliably predetermined merely by reproducing a given auxiliary gap width, regardless of how accurately this is done. This is especially true for small gaps. To achieve the desired reproducibility, the usual auxiliary gap-setting procedure was altered to cause this gap to break down at a predetermined voltage, rather than at a predetermined gap width. The new procedure n-as aa follows: A. Auxiliary gap opened to 10 mm.
or wider.
B. Autotransformer in primary circuit set at zero.
C. Source turned on. D. Autotransformer adjusted to give a predetermined voltage across its variable arm. E. Auxiliary gap narrowed until one breakdown occurred on each half cycle. F. Autotransformer adjusted to give the desired number of breaks per half cycle.
It is not necessary to know the exact value of the breakdown potential, if this value is uniquely determined by the procedure used. When the auxiliary gap is too wide to permit a breakdown, a given setting of the autotransformer always corresponds to a given voltage across the auxiliary gap. The use of a precise voltmeter in step D should thus enable the operator to reproduce a given breakdown potential despite variations in line voltage, atmospheric humidity, and auxiliary electrode surface conditions. An experimental check showed that this technique gave better precision of intensity ratios than when breakdown potentials were regulated by resetting the auxiliary gap separation to a given distance. Two series of exposures were made in which the auxiliary gap was thrown out of adjustment and reset before each exposure (Table 111). In Series A, this gap was reset to a given separation by means of the shadow-image projector provided with the power
source. I n Series B, the above procedure was used. Procedure. It is assumed that the samples to be analyzed by this spectrographic procedure contain no non-rare earth metals except yttrium and scandium. The original matrix in which the rare earths occur is, of course, immaterial, if the rare earth group can be quantitatively isolated. SPECIALAPPARBTUS. Gas chamber and V-block rider for supporting chamber independently on the optical bench, same as in Figure 4 of (4), except that this chamber has two holes 3/16 inch in diameter. In the present case, the body of the chamber may be borosilicate glass. Graphite electrodes, Grade AKGSP, Sational Carbon Co., Cleveland, Ohio, 1 3 / r x 3,'/16 inch diameter, ends polished flat. Diameter reduced to 0.111 A= 0.001 inch for a distance of inch from the sparking end. STOCK SOLUTIONS.Stock solution of palladium, 10.00 f 0.01 mg. of palladium per ml. Prepare by dissolving the metal (Johnson-Matthey Specpure palladium sponge S o . J M 940 or equivalent) in aqua regia; convert to and store as a solution of the chloride in dilute hydrochloric acid. Stock solution of zinc nitrate, approximately 10 mg. of zinc per ml. Prepare stock solutions of individual rare earths in dilute nitric acid containing 10.00 mg. of metal per ml. PREPARATION OF STANDARD SOLUTIONS. Prepare all standard solutions other than the above-mentioned stock solutions immediately before use. Prepare standard mixed solutions containing the various rare earth nitrates of interest for the sample a t appropriate concentration levels. The concentration of any one rare earth may be from 2.5 to 250 y per ml., if the total weight of rare earth metal deposited on one pair of electrodes does not exceed 10 y. Before making each standard solution up to volume, treat it with 75 pl. of the 10 mg. per ml. zinc solution for every milliliter of final volume, and 100 pl. of a freshly prepared 100 y per ml. palladium solution for every milliliter of final volume. Adjust the acidity of the standard solutions with nitric acid to 0.1N. PREPAR.4TION O F SAVPLE SOLUTION. The double thenoyltrifluoroacetone purification yields the rare earths in the form of hydrochloric acid solution. Evaporate this to dryness and convert to the nitrate by a few repeated treatments with small volumes of nitric acid. Take up in a small volume of O.1N nitric acid; add 75 pl. of the 10 y per ml. zinc solution for each milliliter of . freshly final volume and 100 ~ 1 of prepared 100 y per ml. palladium solution for each milliliter of final volume, and dilute to volume with water. PREPARATION OF ELECTRODES.Dip the necked part of a pair of electrodes into 2% solution of Plicene (Central Scientific Co., Chicago, Ill.) in benzene and allow t o dry. Repeat the process.
Table 111. Effect of Auxiliary G a p Adjustment Procedure on Precision of Intensity Ratios
Line Pair Dy3531.71 Y 3600.73 Gd 3422.47 Y 3600.73 Sm 3568.26 Y 3600.73 Dy3531.71 Y 3710 29
Relative Average Deviation of Intensity Ratio, yo Series B. Series A. Gap Gap adjusted to separation breakdown reset at predetervisually to mined predetermined secondary distance voltage 7,3 6,
8,3 5,2
0.95 2.7 1.4 1.2
Table IV. Analytical Lines and Useful Ranges for Spectrographic Determination of Rare Earths
Rare-Earth Element Ce Dv
E; Gd La La
Nd
Nd Sm Y SCb Pd (int. std.)
Spectral Line,
A. 3801.53 3531.71 4129.74 3422 47 3265 675 3249 35 3951 15 4061.09 3568.26 3600.73" 3642.79 3242.70 3421.24 3553.08
Useful Range ( y of R.E. on Electrodes) in Presence of 30 r o f Zn 0.2-2.0 0.03-0.25 0.03-0.25 0 03-0 25 0 2-1 0 0 5-2 0 0 5-2 0 0.2-2.0 0.2-2.0 0.005-0.04 0.005-0.04
(Int. std.)
Not most sensitive line of this element. Will not be present if rare earths have been purified by TTA procedure. a b
Impregnate the electrodes immediately before they are used (the waterproof coating deteriorates during storage). Transfer the electrodes to a graphite heating block kept a t 50" f 2" C. Onto the tip of each electrode, carefully pipet 20 pl. of the test or standard solution. If some other volume is delivered to the electrodes, change the zinc and palladium content of the solution in such a way that the final residue contains 30 y of zinc and 0.4 y of palladium (total for both electrodes). Warm the electrodes until they are dry (approximately 10 minutes). Allow the electrodes to remain in the drying block or store them in a desiccator until used. ELECTRODE ARRANGEMENT AND OPTICAL SYSTEM. Adjust the optical system so that light from the center 50% of a 2-mm. analytical gap is photographed [See (-0.Figure 31. . Adjuit thk speitrograph so that the 3100- to 4500-A. region of the spectrum will be photographed a t a reciprocal linear dispersion of 2.5 A. or less per mm.
TTith a pair of trial electrodes in the excitation chamber, deliver to the chamber of 20% argon-80yo oxygen mixture at a total flow rate low enough to avoid displacing the spark. Maintain a positive pressure of approximately 5 em. of water with respect to the atmosphere. Flush the chamber with this mixture for 45 seconds before each exposure. ADJUSTMEKTOF POWERSOURCE. Inspect the tungsten auxiliary gap electrodes. The sparking surfaces should be flat and free from coatings, pits, or protuberances. The edge of the sparking surface should be rounded to a radius of curvature of 0.5 to 1.0 nim . Adjust the power source as above, using the following parameters: C = 60 ph. (including 20 0.005 pf.; L ph. residual); primary resistance 13 Cl -Le., ballast only; secondary resistance zero. Voltage across variable arm of autotransformer with auxiliary gap wide open, 55.0 volts. Breaks per half cycle, 23. Radio-frequency amperage, 5.5. STANDARD AND SAMPLEEXPOSURES. Expose for 45 seconds. No preburn. REDUCTION OF DATA. Perform photographic processing by a standardized procedure. Methods preferred by the authors for calibrating film and measuring relative intensities have been described (1, Sections 9, 12, and 14). Using the lines shown in Table IV, plot log (IRE. line/IPd line) against log y R.E. on electrode. The lower figure in the Useful Range column for each spectral line is the concentration level below which the standard deviation of the R.E./Pd intensity ratio begins to exceed &lo% because of low line-to-background ratio. This figure is hereinafter referred to as the quantitative sensitivity limit. The absolute limits of detection are two- to threefold lower than the values listed. When all of the rare earths from a 20gram sample of thorium are concentrated into 1 ml., and 40 pl. of this solution is used for an exposure, the quantitative sensitivity limit in terms of parts per million-i.e., micrograms of rare earth per gram of thorium-ranges from 0.006 p.p.m. for yttrium to 0.25 p.p.m. for cerium, neodymium, and samarium. Separate experiments have shown that it is practical to evaporate 1 ml. of solution on one pair of electrodes, if necessary t o achieve maximum sensitivity; recoveries of 100 =t 3.6% were obtained in this manner. When this procedure is applied to the purified rare earth fraction from a 20-gram sample of thorium, the quantitative sensitivity limits range from 0.00025 p.p.m. for yttrium to 0.01 p.p.m. for cerium, neodymium, and samarium.
-
Evaluation of Procedure.
REPRO-
GIVEN INTENSITY RATIO. Eight rare earth-palladium ratios were measured in each of nine exposures, and the relative standard
DUCIBILITY
OF
VOL. 30, NO. 3, MARCH 1958
421
REFERENCES
Table V.
Recovery of Individual Rare Earths in Mixtures, Using
Mixture No. 1
Recovered, Element Gd Dv
Taken, 0.200 0.400
0,500 1.00 0.0200 2.00
Ce Iid
Y
Zn 2
0.0200
Not measured
(as suppressant) 0.080
DY Sm Ce Sd Y La
0.240 1 50 2 00 0 I0200
0 0 0 1 2 0
0.080
0.80 (as suppressant)
The experiments whose results are described in Table V were performed primarily to check the effectiveness of zinc in eliminating the above-described interelement effect. These solutions were
100
(1) -4m. SOC.Testing Materials, Phil-
adelphia, Pa., Photographic Photometry (SM 2-2) in “hfethods for Emission Spectrochemical Analysis,” 1953; Tentative Method E 116-56T, 1957. (2) Blackmore, R. H., Bearse, A. E., Calkins, G. D., Battelle Memorial Institute, Columbus, Ohio, Rept. BMI-261. (3) Bronaugh, H. J., Suttle, J. F., U. S.
075
083
94 103
245 50
08
102 100
104
0190
95
S o t measured
30 0
RECOVERYOF IXDI~IDUAL RARE EARTHSIN RARE EBRTH RIIXTURES.
105
Xot measured
(as buffer)
deviation was calculated for each intensity ratio. These relative standard deviations ranged from 0.0 t o 2.8%; their mean was 1.3%.
Recovery, % 96 95 100 99
Kot measured
30.0 (as buffer)
Gd
Zn
0.192 0.190 0.400 0.495 1.05
0.200
Sr;l
La
Y
30-7Zinc Buffer
not put through the chemical procedure, but were mounted directly on electrodes. Table V s h o w that the inclusion of 30 y of zinc in standard and sample residues overcomes the serious matrix effects exhibited by cerium and lanthanum in Table 11. Two years of application of this procedure have shown that when the latter elements are absent, spectrographic recoveries range from 98 to 102%.
Atomic Energy Commission Rept. LA-1561 (1953). (4) Feldman, C., Ellenburg, J. Y.. ANAL.CHEY.27, 1714 (1955). (5) Gordon, L., Firsching, F. H., Shaver, K. H., I b i d . , 28, 1476 (1956). (6) Hettel, H. J., master’s thesis, Ioiva State College, Ames, Iowa, 1956. (7) Hughes, D. J., Harvey, J. A., Brookhaven National Laboratory Rept. BNL-325 (1955). (8) Hyde, E. K., Tolmach, J., U. S. -%tomicEnergy Commission Rept. ANL-4248.
(9) Kember, N. F., United Kingdom Department of Atomic Energy (Ministry of Supply) Rept. CRLAE-23; reissued by Technical Information Division, U. S. Atomic Energy Commission, Oak Ridge, Tenn. (10) Lerner, M. TI’., Petretic, G. J., ANAL.CHEM.28, 227 (1956). RECEIVEDfor review April 7.0, 1957. A4cceptedNovember 21, 1957.
Microdetermination of Zirconium in Sulfuric Acid Solutions with Pyrocatechol Violet J. P. YOUNG, J. R. FRENCH, and J. C. WHITE Oak Ridge National laboratory, Oak Ridge, Tenn. ,The blue color of the zirconiumpyrocatechol violet complex has been utilized in the development of a very sensitive method for the colorimetric determination of zirconium in sulfuric acid solutions. The molar absorbance index of this complex is 32,600 a t a wave length of 650 mp. Its absorbance conforms to Beer’s law up to a concentration of zirconium of 2 y per ml. Trace amounts of zirconium can b e determined in the presence of thorium, uranium, lanthanum, cerium, iron, nickel, and chromium. Fluoride, citrate, oxalate, and tartrate ions interfere by decolorizing the complex. The coefficient of variation for the determination of zirconium by this method is 2%.
A
s
THE importance of zirconium grows in the fields of chemistry and metallurgy, a method is needed for
422
ANALYTICAL CHEMISTRY
the colorimetric determination of trace amounts of zirconium in solutions that contain sulfuric acid. One of the more common reagents for spectrophotometric determination of zirconium is alizarinsulfonic acid. The molar absorbance index of the zirconiumalizarin red lake is approximately 7000 (6); sulfate ion decreases its absorbance (13). Another reagent for the determination of zirconium, p-dimethylaminoazophenylarsonic acid, is also adversely affected by sulfate (20). The spectrophotometric determination of zirconium with quercetin (7) is very sensitive, but in the presence of sulfuric acid, the color reaction is grossly hindered; furthermore, thorium and iron cations interfere seriously. Horton (9) developed a method for the determination of zirconium with 2-(2-hydroxy-3,6disulfo-1-naphthylazo) benzenearsonic acid (Thoron). The molar absorbance
index in this system is approximately 3200. Sulfate ion causes moderate interference. A fairly sensitive determination of zirconium (1) makes use of 2-(p-sulfophenylazo-)-l,8-dihydroxy-3,5-naphthalenedisulfonic acid (SPADNS); thorium likewise forms a complex with SPADNS. No mention is made of the effect of sulfate ion; however, it is recommended that the sample be present in a hydrochloric acid solution. Chloranilic acid has been studied rather extensively for the determination of zirconium by Thamer and Voigt (19), Frost-Jones and Yardley (5), Menis ( 1 4 , and Bricker and Waterbury (2). Pyrocatechol violet, 3,3’4‘-trihydroxyfuchsone-2”-sulfonic acid, forms colored complexes with many cations. Suk and Malat (16) have reviewed the properties of this dye and described some of its applications in
