Colorimetric Determination of Iridium with p-Nitrosodimethylaniline A.
D. WESTLAND and
F. E. BEAMISH
Department o f Chemistry, University o f Toronto, Toronto, Canada
Even though iridium has extensive cornniercial uses, few methods of determination are available and these are not sensitive. It was found that p-nitrosodimethylaniline produced an intense color with small quantities of iridium. In heated alcohol-water solution buffered to pH 7.2 to 7.3, a cherry-red colored complex is formed which absorbs over a broad band of wave length. The method fails in the presence of nonvolatile acids or salts, but in such cases iridium may be obtained as the chloro salt after hydrolytic precipitation using nickel as a collector. The nickel is removed on a cation exchange resin. In samples from which the other platinum metals have been removed, precise results can be obtained for iridium in the range 1.5 to 10 p.p.m.
1.2
1.0
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0.4
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0.0
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N L T two acceptable colorimetric methods for iridium other than direct measurement of the absorbance of hexachloroiridate have been developed and published to dat,e. Ayres .and Quick (1) used a mixed acid reaction, and Maynes and McRryde ( 4 ) used ceric sulfate. Both involve fuming n i t h acid, the final color depending on the care with which thie operat,ion is carried out.. These methods are not particularly sensitive. Several platinum metals can be determined with p-nitrosodimethylaniline. Although the met,hod presented for the determination of iridium using this reagent enjoys no specificity among the platinum group, it is sensitive and precise. Various procedure8 esist, for the separation from iridium of micro amounts of all the platinum metals except rhodium, and a procedure for t,he separation of traces of this metal is currently being investig:ited in this laboratory.
650
600
WAVE LENGTH
w Figure 1. Variation of absorbance with wave length A . 5.8 p.p.m. of iridium B . 2 . 4 p . p . m . of iridium C . Blank plu. reagent D . B-C
APPARATUS AND SOLUTIONS
Optical Instruments. A Klett-Summerson photoelectric colorimeter using matched tubes was used for most of the work. A Beckman Model DK spectrophotometer was used to examine the variation of absorbance with wave length. Standard Iridium Solution. A solut,ion of sodium iridium chloride prepared in 0.05N hydrochloric acid was standardized gravimetrically using 2-mercaptobenzothiazole according to the method of Barefoot, McDonnell, and Beamish ( 2 ) . The mean result of four determinations was 0.143 mg. per ml. The stock solution was diluted to one tenth of this concentration with water, and such solutions were stable for a t least a 2-week period. Color Reagent. -4solution of p-nitrosodimet,hylaniline (Eastman Kodak Co.) was prepared by dissolving 150 mg. in 100 ml. of 95% ethyl alcohol and filtering. The solutions tended to deposit a dark powder if kept more than a week or two. Alcoholic solutions n-ere required in order to form the color. Buffer Solutions. .Inalytical grade disodium hydrogen phosphate aiid potassium dihydrogen phosphate were dissolved together in n.at,er in varying amounts to prepare buffers 1 . O M in phosphate, the pH of these being determined with a Beckman Model G pII meter. The buffer used in the final procedure contained 10 grams of disodium h drogen phosphate and 4.1 grams of pot,assium dih)-drogen phospzate in 100 ml. of aqueous solution. COLOR REACTION
When an iridium chloride solution is heated with an alcoholic solution of p-nitrosodimethylaniline the solution develops a red color superimposed upon the yellow color of the reagent. It may be noted that p-nitrosodiphenylaniline and p-nitrosodiethylaniline did not produce an intense color with iridium chloride solutions. A plot of the absorbance of the complex and of the blank reagent, compared with water appears in Figure 1. It can be seen from the plot of the difference of these that there is a broad band
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Figure 2.
Effect of pH and temperature
of absorption due to the complex extending over the whole wavelength range studied, with a maximum at 530 mp. The absorbance is dependent upon various factors such as temperature, pH, time of heating, and ionic strength. The first three factors are interrelated in a rather complex manner. A study of these relations mas carried out using sampleb containing 0.115 mg. of iridium in a 4-ml. volume to nhich \?-ere added 2 ml. of reagent and 2 ml. of buffer. The samples were contained in test tubes suspended in a bath of hot water, the temperature of which was controlled manually to 11' C. After heating for a given period the tubes were cooled in running u ater, the pH was measured, the samples were diluted to 25-ml volume with water, and the absorbance was determined. Temperature. Solutions buffered at various values of pH were heated for 40 minutes a t various temperatures. From the results plotted in Figure 2 it can be seen that the absorbance remained constant over a fairly broad range of pH only when the samples 1776
1777
V O L U M E 27, NO. 11, N O V E M B E R 1 9 5 5 were heated a t 70" C. At this temperature the absorbance nas nearly constant over the p H range 6.7 to 7.5. At higher valurs the iridium is probably hydrolyzed. At 80" C. a brown precipitate appeared. This was sometimes seen also at i o " C., but if the sample was made up to volume with 6N hydrochloric acid the precipitate dissolved. S o change in the absorption due to the complex was caused by this treatment, but the excess reagent color was bleached. This is fortunate, in that it reduced the considerable absorption of the blank. The appearance of a precipitate during heating mas not attended by any discrepanry in the results. The relative heights of the various curves are not significant, since the sensitivity frequently varied from one simultaneously prepared set of samples to the nest. On the basis of the increased proportion of hydrolysis of chloroiridate, one may account for the anomalous position of the 75" C. curve. Such factors as the temperature of the water used for dissolving the sample and the time of standing befoie color formation can govern the effect. -~ --
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Table 1. Effects of Quantity of Reagent and Volume on 4hsorhance Iridium Taken, 3.43 P.P.hI. Yolume, nil. .4bsorbanre
Iridium Taken, 5.7 P.P.M. Reagent, nil. Absorbance
7 8 9 10 11
0.301 0.301 0.299 0,290
0.286
Table 11. Precision and .Adherence to Beer's Law Final Concentration, P.P.M.
Absorhancc 0.141 0 279 0.424 0 570 0 714 1.000
0.0993 0.0973
0.0988 0,0995 0.09'% 0,0999
Time of Heating. Color developed more rapidly in samples of low pH than in those of high, so that heating t,imes at various values of pH were studied. The plot,s in Figure 3 show that color development was complete after 36 minutes' heating at i o " C. A heating period of 40 minutes was chosen. Choice of Buffer. A buffer yielding a solution having a pH of 7.2 to 7.3 was used in subsequent work, because at this pH the absorbance was less dependent on variations in temperature, heating time, and residual acidity in the sample. Quantity of Reagent Added. The absorbance of the solution depends upon the quantity of reagent added. As can be seen from Table I, the absorbance was greater for higher reagent conrentrations. More than 2 ml. of reagent in a volume of 10 ml. vas considered undesirable because of the high reagent color. Clorrespondingly, as can be seen from the table, the absorbancbe was somewhat dependent upon the volume of solution heated. Treatment of Acidic Samples. As iridium is usually handled in acidic solutions, a study directed a t finding the best met'hod of preparing such solutions for analysis was made. I t was found that if the solutions were neutralized or merely brought to pH 5 or 6 before adding the buffer, 1011- results were obtained. This is probably due to partial hydrolysis of the iridium in regions of high hydroxy! ion concentration occurring as the base is added. Slow addition with stirring overcame this trouble in part only. Because of the high temperature, homogeneous neutralization by boiling with urea resulted in the development of no color whatever, w-hen samples were evaporated t o less than 0.5-ml. volume, 2 ml. of reagent were added, and the solution was diluted to volume with a 1M phosphate buffer with pH of 7.8 acceptable results were obtained with stock solutions.
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Figure 3.
Effect of heating time A. E. C. D. E.
40 minutes 36 minutes 32 minutes
28 minutes 24 minutes
In the authors' opinion, huM e l er, a better method of eliminating acid consists in evaporating to dryness solutions containing a little sodium chloride. Evaporations were carried out in 50-ml. beakers on a hot plate until the solution just barely covered the bottom of the beaker, a t which time the beaker was placed on a steam bath and allowed to go thoroughly dry. The absorbance of the solution was found to decrease with increasing ionir strength of the solution; hence the amount of salt present should be controlled. Samples containing 4.29 p.p.m. of iridium and 40, 80, and 120 mg. of sodium chloride gave absorbances of 0.428, 0.416, and 0.404, respectively. The amount of color developed in iridium solutions which had been treated by different procedures was unpredictable, unless some means were taken to standardize the form of dissolved constituent immediately prior to the determination. Low results were obtained with aliquots of stock solution which had been diluted to a 200-ml. volume and evaporated to dryness with salt. From this it appears that even under these conditions iridium is partially hydrolyzed. This trouble was overcome by making such solutions a t least 0.5Nwith hydrochloric acid before evaporating, then treating the residue with aqua regia and hydrochloric acid in the usual manner, and again evaporating to dryness. As the sensitivity was not alu ays reproducible under the conditions of the procedure, simultaneous standards were necessarily employed. An indication of precision and conformity to Beer's law is represented in Table 11. The same Klett tube \\as used to contain the samples and absorbances 4 ere obtained by dividing the Klett unit b> the factor 519 ( 3 ) . Recommended Procedure. .4n acid sample of iridium to which are added 2 ml. of 2% sodium chloride is evaporated t o dryness in a 500-ml. beaker on a hot plate and finally on a steam bath. The residue is treated with 4 nil. of aqua regia, and this is then evaporated to dryness. The crystals are moistened with concentrated hydrochloric acid and this is evaporated three times, being dried thoroughly to get rid of residual acid. A minimum of water is added to dissolve the salts, and the solution is rinsed into a 15 X 150 mm. test tube containing 2 ml. of buffer and 2 ml. of color reagent. The solution is diluted to &ml. volume with water, and thc test tube suspended in a 70" C. bath for 40 minutes. The test tubes are then cooled in running water and the solutions dilnted to 10-ml. volume with 6K hydrochloric acid. SEPARATION OF TRACES O F IRIDIUM FROM NONVOLATILE ACID
If the solution contains much sulfuric acid or fixed salt, it is not feasible to apply the determination directly. I t was found
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ANALYTICAL CHEMISTRY
that small quantities of iridium can be separated from such solutions by hydrolytic precipitation if a carrier precipitate is formed as well. Thus, if nickel is added t o the solution, quantitative removal of the hydrated iridium oxide can be effected. The nickel can then be removed by passing the redissolved precipitate through a column of Doweu 50 ion exchange resin. To rnsure that the iridium in the column process is entirely anionic, the precipitate must be heated with strong arid containing chloride. Tahle 111. Recovery of Iridium by Hydrolytic Precipitation Number 1
2
3 4 5 0 7 8 9 10
I
Iridium Taken,
Iridium Recovered,
Y
Y
%
58 116 580 1160 29 58 116 14.5 29 58
57.5 116 580 i- 8 1160 i- 6
99 100 100 100 100 99 93 99 91 99
29
57.5 108 14.3 26.5 57.5
Recovery,
Aliquots of iridium solution were fumed nith various quantities of sulfuric acid until the final volume was about 2 ml. or less. The solution was allowed to cool, diluted to 10 ml. with water, and 50 mg. of nickel in the form of a solution of its chloride and 5 nil. of 10% sodium bromate were added. The solution was nearly neutralized with sodium hydroxide and finally adjusted to pH 6.7 to 7.5 by means of dilute sodium bicarbonate and dilute hydrochloric acid. The nickel hydroxide turns black in this pH range. The solution was finally boiled gently for 0.5 hour and filterpd through a porous-bottom crucible of 5-ml. capacity.
The precipitate was washed with a few milliliters of 1% ammonium chloride solution. The crucible was returned to the original beaker, and the precipitate was dissolved in 8 ml. of aqua regia and then evaporated to about 2-ml. volume. Two milliliters of concentrated hydrochloric acid were added, and this was evaporated slowly t o 2-ml. volume. The solution was transferred to another vessel and the crucible leached on the steam bath with 5 ml. of slightly acidified water. This was added to the main bulk of the solution and the crucible was leached a second time. The solution was diluted to a 100-ml. volume and passed a t a rate of about 2 ml. per minute through a 10-em. deep bed of Dowex 50. The column was washed with 50 ml. of water, 10 ml. of concentrated hydrochloric acid, and 2 ml. of 2% sodium chloride solution were added t o the effluent, and this was evaporated t o dryness. The residue was treated with aqua regia and hydrochloric acid as described above and the determination was carried out. Results of samples subjected to this procedure appeared in Table 111. Nos. 1 to 4 contained no sulfuric acid; Nos. 5 to 7 were fumed with sulfuric acid to about 2-ml. volume, and Nos. 8 to 10 were fumed to less than 0.5 ml. in 50-ml. beakers so th'at a green oily film remained. ACKNOWLEDGMEY'I
This xork was supported hy a grant from the Sational Research Council (Canada). LITERATURE CITED (1) Ayres, G. H., and Quick, Q., ASAL. CHEIII.. 22, 1403 (1950). ( 2 ) Barefoot, R. R., McDonnell, W. J., and Bearnish, F. E., Ibzd., 23, 514 (1951). (3) Currah, J. E., Fischei, A , , AIcRryde, W. A. E., and Beamish, F. E., Ibid., 24, 1980 (1952). (4) Maynes, A. D., and McBryde, W. A . E , Analyst, 79, 230 (1954).
RECEIVED for reiiew February 23, 195.5. Accepted July 13, 19.5.5
Determination of Chlorine or Chlorine Dioxide in Dilute Aqueous Solutions Containing Oxidizimg Ions M. 1. SHERMAN and J. D. H. STRICKLAND British Columbia Research Council, Vancouver 8, B. C., Chlorine or chlorine dioxide may be determined in aqueous solutions in the presence of many oxidizing ions by extracting the chlorine or chlorine dioxide into a measured volume of carbon tetrachloride and measuring the absorbance of the extract with a spectrophotometer at 3270 A. for chlorine or 3530 A. for the dioxide. Extracts need not be filtered and photochemical decomposition is not serious if reasonable care is taken. The procedure is suitable for quantities of chlorine between 1 and 12 mg. in 5 ml. of sample, and for quantities of chlorine dioxide between 0.1 and 5 mg. in 1 ml. of sample.
D
URING the course of certain experiments in these labora-
tories it was necessary to determine chlorine and chlorine dioxide in aqueous solutions a t concentrations of a few hundredths molar. The problem is relatively simple by iodometry in the absence of interfering ions, but with substantial quantities of ferric iron, cupric copper, ceric cerium, dichromate, etc., the determination becomes complicated. A simple solution to this problem arises from the fact that both chlorine and chlorine dioxide can be extracted from aqueous solution into carbon tetrachloride, and there estimated directly by spectrophotometry. Losses by gas partition and photochemical decomposition are not serious and a very rapid technique which is suitable for routine analytical work can be used. The need for such an analysis may arise infrequently but may prove useful.
Canada
The great advantage of: the procedure is its wide applicability to almost any aqueous solution free from solid matter or extractable organic material. Under the conditions recommended no appreciable quantities of metals in a cationic or anionic form can be extracted, and interference is limited to the other halogens and, possibly, a relatively few nonionized inorganic substances. Direct tests in the presence of O.1N ferric iron, cupric copper, ceric cerium, antimony(V), and dichromate gave no difference in the absorbance of a standard chlorine extract. The method is not highly sensitive and cannot replace trace methods such as the otolidine procedure. I n carbon tetrachloride solution both chlorine and chlorine dioxide give well-defined absorption peaks a t 3270 A and 3550 A., respectively. The band for the dioxide reaches just into the visible region of the spectrum, giving a pale yellow solution, but the chlorine solutions are quite colorless in the dilutions used. The peaks are too broad and too close together for any useful differentiation between chlorine and chlorine dioxide to be possible spectrophotometrically. PROCEDURE
Chlorine. Exactly 10.0 ml. of carbon tetrachloride are measured from a buret into a 30-ml. separating funnel with a short stem drawn out to a narrow tip. Next 5.0 ml. of 5M hydrochloric acid are added, followed by 5.00 ml. of sample solution. The funnel is shaken vigorous1 for 1 to 2 minutes, and the layers are then allowed to separate &r exactly 1 minute. The lower layer of carbon tetrachloride is run into a 1-em. quartz absorption cell, filling it to the brim. Then the cell is stoppered. The absorbance
