1778
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 to 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 to 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 to the effluent, and this was evaporated to 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 22, 1403 (1950). (1) Ayres, G. H., and Quick, Q., ASAL. CHEIII.. ( 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
V O L U M E 2 1 , NO. 11, N O V E M B E R 1 9 5 5
1779
I
exceeds 90% under the conditions used in this method. I t is seen from Table I that a i t h 10 ml. of solvent, a straight-line relationship can be assumed with an accuracy comparable with most routine absorptiometric methods. The same is found rrith 20 ml. of solvent, but in using 5 ml. (Table 11) serious errors occur in the lower range. If a calibration curve is drawn, however, these can be largely eliminated. The partition coefficient for chlorine dioxide is much less than for chlorine (below 75%) and a calibration curve must be constructed (see Figure 1). The precision of the two methods is then comparable. -4s the molar absorbance coefficient for chlorine dioxide is some ten times that of chlorine, the method can be made much more sensitive on a molarity basis, although on a normality basis the difference is less marked.
~
Table I.
of this solution is measured without delay against water using a spectrophotomet,er wit.h light of wave length 3270 A. The cell containing the chlorine solution should not be exposed to the light in the spectrophotometer for longer than about 30 seconds. A blank determination is made on 5.0 ml. of solution of the same composition as the sample solution, but which contains no chlorine. The preparation of such a solution is not difficult, but depends on t'he exart nature of the samples being analyzed. The carbon tetrachloride used in this method is of reagent quality and freed from any material oxidizable by chlorine by shaking the solvent with about 2 t,o 3 ml. of saturated chlorine water per liter of carbon tetrachloride. The solution should then have an absorbance of 0.05 or less. If it is greater, nitrogen is passed through the liquid until an absorbance of 0.02 or less results. The solvent containing the chlorine must not be exposed to direct sunlight or to bright lighting from mercury fluorescent discharge tubes; however, moderate diffuse lighting is satisfactory, especially if the separating funnel is painted black on the exterior. This method is directly suitable for amounts of chlorine between l and 12 mg., or 3 and 30 X 10-$.TI chlorine in 5 ml. of sample, and a linear relation is found between concentration and absorbance. The range can be increased by using a slightly larger funnel and 20 ml. of carbon tetrachloride when exactly one half of the absorbance results. For concentrations of chlorine below about 10-2A14(3.5 mg. of chlorine in 5 ml.) 5.0 ml. of carbon tetrachloride may be used, but it is then better to use a calibration curve for the lower concentrations as a straight-line relation is not strictly obeyed. The limit of sure detection using 5 ml. of carbon tetrachloride lies around 15 . p.p.m. of chlorine in . the aqueous solution. Chlorine Dioxide. The method is, in essentials, the same as that used for chlorine except that, for convenience of absorbance measurement, the relative volumes of carbon tetrachloride and sample solution are altered. For amounts of chlorine dioxide up to 1.5 mg., 1.00 ml. of sample solution is used together with 5.0 ml. of 5 M hydrochloric acid and 15 ml. of carbon tetrachloride. If the chlorine dioxide is present between 1.5 and 5 mg., 1.00 ml. of sample solution is again used, but 50 ml. of carbon tetrachloride. The absorbance is measured in a 1-cm. cell using light of wave length 3550 A4. For both quantities of carbon tetrachloride a calibration curve must be constructed as the concentrationabsorbance relationship iq not linear. DISCUSSION
T o suppress hydrolysis of chlorine solutions to hypochlorite the pH should be below 1, but for safety and to ensure a fairly uniform electrolyte concentration, irrespective of the composition of the sample solution, the extraction is made from solution 2.5M with respect to hydrochloric acid. The use of a similar molarity of sulfuric acid appears to be satisfactory, but this point has not been tested extensively. The hydrolysis of chlorine dioxide in acid solutions is not dependent upon acidity unless the pH exceeds 2, but for uniformity a procedure has been used similar to that employed for chlorine. The partition coefficient for chlorine into carbon tetrachloride
Extraction of Chlorine Using 10 311. of Carbon Tetrachloride
Mean factor, assuming linear relationship. 1Iolarity = absorbance X 2.34 x 10-2 LIolarity Calculated x 10-20i 5Iolarity % Sample Absorbance x 10-2 Error 0.535 1.095 1.77 1.97 2.67 2.8G
0.235 0.470 0 750
0.865
1,120 1.215
0,530 1.10 1.76 2.02 2.62 2.83
+3 +0.5 -0.5 +2.3 -2
