Chlorine Production by Gamma Radiolysis of Acid Chloride Solutions and Its Effect on Analytical Procedures Hisashi Kubota Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. The gamma radiolysis of acid chloride solutions generates free chlorine above critical acid and chloride concentrations. The deleterious effect of this chlorine generation can be greater to certain analytical procedures than the ordinary radiolytic effect. The amount of chlorine produced increases with increasing acid and chloride concentrations. Thus, it is sometimes expedient to work at lower chloride and/or acid concentrations to minimize the effects of chlorine formation at the sacrifice of some other item. The experimental conditions used in this study were selected to simulate the conditions that are normally encountered in processing and analytical work. Mechanisms are proposed to explain the radiolysis.
THE RADIOLYSIS in solutions containing radioactivity or exposed to ionizing radiation is a problem of concern to all types of chemical considerations. In our program to assess the effects of radiation on analytical procedures, one of our avenues of approach has been to study the effects on principal anionic systems from which, hopefully, we can predict the effect on specific systems. The first of these anionic media studied is the chloride, and the results of the study are reported here. The chloride system is an important medium for many nuclear applications, particularly in the separation and processing of many actinide and lanthanide elements ( I ) . These metals are usually amphoteric, so the medium has to be kept acidic to prevent hydrolysis and retain the cations in a form amenable to separation schemes. This study is concerned with the behavior of chloride solutions, in particular acid chloride solutions, when exposed to gamma radiation. The investigation showed that free chlorine is produced by this radiolysis when the medium is acid but not when neutral or basic. The factors that determine the rate of production were determined. The deleterious effect of this radiolysis on analytical procedures was demonstrated on the spectrophotometric determination of zirconium with arsenazo 111. Baybarz ( 2 ) reported that measurable amounts of chlorine were found in the gases released by acidic 10M LiCl solutions containing sufficient curium to radiate several watts of power per gallon. It was also reported that there was a steady depletion in acidity, and frequent replenishment of acid was necessary to prevent the precipitation of the more easily hydrolyzable ions. Lukens et a / . (3) subjected very dilute hydrohalic acids and ammonium halide solutions to megarad levels of simultaneous gamma and neutron irradiation and found some W I in the vapor phase above the solutions, It is assumed that this isotope was present as free chlorine. Thus, it was demonstrated that the escape of free chlorine as a result of radiolytic and recoil processes can be a serious source of error in the determination of chlorine in aqueous solutions (1) R. D. Baybarz, B. S. Weaver, and H. B. Kinser, Nucl. Sci.
Eng., 17, 457 (1963). (2) R. D. Baybarz, J. Inorg. Nucl. Chem., 27, 725-30 (1965). (3) H. R. Lukens, F. M. Graber, and D. M. Settle, Trans. Am. Nucl. Soc., 9, 88 (1966).
by activation analysis. One of the objects of this study was to see whether high levels of gamma irradiation would bring about similar effects, and the results indicate that the radiolytic effects brought about by alpha and gamma radiolysis are similar at least in a qualitative manner in this system. EXPERIMENTAL
Reagent grade chemicals were used without further purification. Ordinary distilled water was used to make up the solutions and dilutions. Actual working conditions were approximated wherever possible, and the rigorous purifications normally adopted in radiation chemistry were not followed. Only the water used to make up the standard ferrous sulfate dosimetry solution was prepared according to radiation chemistry standards. A 3000-curie cobalt-60 source was used for irradiations. All irradiations were carried out at room temperature in stoppered glass containers. The solutions were air saturated for all practical purposes. Dose rates were determined with a 0.002M ferrous sulfate solution in 0.8N sulfuric acid, The increase in absorption of the 305-mp Fe(II1) peak following irradiation for a given time was used to calculate the dose rate from the following equation (4).
-
Dose rate (radsig min) =
-
Do) X 6.023 X 1023 X 102 1 X G X d X IO3 X t X e X F
(0s
Do
absorbance of unirradiated dosimetry solution
D, = absorbance of irradiated dosimetry solution F = 6.24 X lo3 (eV/rad) I = cell length (cm) G = 15.8/100 eV = density of solution (giml) = molar absorptivity (2240) = time in minutes
d e t
The dose rates were of the order of 20,000 rads per minute with the particular irradiation geometry employed. Chlorine was determined spectrophotometrically with o-tolidine reagent on a Cary Model 14 spectrophotometer using the 436-mp peak. RESULTS
Identification and Analysis of Chlorine. Mass spectrometric and gas chromatographic analyses of the gaseous products from acid chloride solutions subjected to massive doses (lo8 rads) showed the presence of chlorine, hydrogen, and oxygen. The latter two are normal radiolytic products of water, and chlorine is the product unique to this system. A means to identify any chlorine that could or would be formed at lower dose levels was desired together with a procedure for quantitative analytical determinations. When a small amount of chlorine gas is dissolved in hydrochloric acid, the solution picks up a sharp absorption at 228 mp presumably because of the formation of a chlorine-chlo(4) G. S. Hochandel, Radiation Chemistry Lecture Series, ORINS,
3-26-57. VOL. 40, NO. 2, FEBRUARY 1968
271
0.e
c
li
2
4
6
8
IO
%-I
Figure 2. Effect of [Cl] on G(CIo) in the gamma induced radiolysis of chloride S U ~ U ~ ~ O I I S MHCl
Figure 1. Chlorine production by the gamma induced radiolysis of HCI solutions ride complex (5). Irradiated hydrochloric acid also shows a similar absorption whose intensity increases with increasing dose. The position of this peak shifts slightly with changing acid or chloride concentration, and the dose-absorption relation is not linear in the dose range 103-105 rads. Thus, this spectrophotometric method is useful in verifying the formation of free chlorine but is not suited for the quantitative determination of chlorine in the desired working range. The addition of a bromide or iodide to irradiated hydrochloric acid releases free bromine or iodine whose presence is readily shown spectrophotometrically. This is another evidence for the formation of a strong oxidant that is more powerful than bromine. The o-tolidine procedure (6) which is used in water analysis for the determination of residual chlorine was found to have the desired sensitivity and linearity, On the other hand, it is not specific for chlorine, and many strong oxidants will produce the same color. A radiolytic product of water with sufficient stability to constitute a possible interference is hydrogen peroxide. Tests showed that 3 hydrogen peroxide produces a light yellow color with o-tolidine only after several minutes of contact, but concentrations of the order produced by doses up to lo6rads in water brought out no color, Furthermore, chloride is a good scavenger for the hydroxyl radical which is the precursor of hydrogen peroxide (7); consequently, any hydrogen peroxide formation in high chloride medium should be smaller than in water, Another drawback of the o-tolidine procedure is the fading of the color with time. This was of no consequence here because the absorbances were determined very soon after the addition of the color reagent. The calibration for free chlorine was made with chlorine water of appropriate dilution. The chlorine water was standardized iodometrically. Chlorine Formation as a Function of Hydrochloric Acid Concentration. Hydrochloric acid solutions were prepared in concentrations from 1.2 to 12M in 1.2M increments. Five-milliliter aliquots contained in stoppered 10-ml volumetric flasks were irradiated from 1 to 5 minutes in the cobalt ( 5 ) G. Zimmerman and F. C. Strong, J. Am. Chem. SOC.,79,2063
(1 957). (6) D. F. Boltz, ed., “Colorimetric Determination of Nonmetals,” Interscience, New York, 1958, p 162. (7) T. J. Sworski, Radiation Res., 2,26 (1955).
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ANALYTICAL CHEMISTRY
H + = 1.2 M
source. The flasks were positioned to assure constant irradiation geometry. The reagent (2 ml of 0.1 % o-tolidine in 1.8M HC1) was added directly to this flask. The absorbance us. exposure time (or dose) plot was made for each acid concentration. A linear absorbance-dose plot was obtained at each concentration level. The average rate of chlorine preduction per unit dose (G value) (molecules of products formed per 100 eV absorbed dose) was calculated and plotted as a function of the acid concentration in Figure 1. An exponential type relation between the rate of chlorine formation and acid concentration was obtained. Chlorine Formation as a Function of Chloride Concentration at Constant Acidity. A series of solutions was prepared which was 1.2M in HCI and which contained increasing amounts of chloride added in the form of lithium chloride. The solutions were irradiated and analyzed for chlorine in the same manner as was described for the hydrochloric acid solutions. The yield-dose relation is plotted in Figure 2. Here again there is an exponential type increase in chlorine production, this time with increasing chloride. Critical Acid Concentration for Free Chlorine Formation. The radiolysis curve of hydrochloric acid solutions showed no chlorine formation below 1.4M acid, yet the 1.2M HC1 containing added chloride produced chlorine. The highly concentrated lithium chloride solution used to determine acid loss described later in this paper produced chlorine even when the acidity was of the order of 0.2M. Thus, there seems to be a critical acid concentration at any given chloride concentration below which the radiolysis to free chlorine does not occur. This critical acid value was determined at several levels of chloride concentration. The criterion for chlorine production was taken arbitrarily as that acid concentration at which a 106 rad dose gave a spectrophotometric otolidine absorbance of 0.05 or more. The results are plotted in Figure 3. It is seen that essentially any concentration of acid will bring about this radiolysis above 5Mchloride. Effect of Gamma Radiation on the Arsenam III-Zirconium Complex in HCI Medium. Arsenazo I11 forms sensitive colored complexes with many lanthanides and actinides. The complexes with the tetravalent metal ions of thorium, zirconium, and uranium(1V) are particularly sensitive, and molar absorptivities of the order of 105 or greater have been reported (8). These colors are developed in strong acid medium, and hydrochloric acid is preferred. (8) S. B. Savvin, Tulanra, 8,673 (1961).
Zr = 2,ug/ml .O!% ARSENAZO SOLUTION X = 665rnp
I
I
20,000
30,000
I
i0,ooo
Figure 3. Critical acid concentrations at various [Cl-] for radiolytic Cl0 production
m
RADS
Figure 4. Effect of gamma radiation on Zr-Arsenazo 111 in HCl medium
Zirconium as the pure metal or in alloy form is used frequently in nuclear technology particularly as container material for processing solutions. Thus, it is possible that zirconium may be corroded and enter the solution. This metal ion was used as a model ion to test the effect of gamma radiation on an analytical procedure in which an acid chloride medium was involved. The molar absorptivity of the zirconium-arsenazo I11 complex increases with increasing hydrochloric acid concentration. The absorbances of this complex at a given zirconium concentration at differing HCl levels are shown as the starting points at zero dose in Figure 4. It is seen that the absorbance in 9 M HCl is nearly double that at 6 M and about four times that at 3 M. The decrease in absorbance with dose is also shown in the same figure. This rate of decrease is seen to parallel the rate of chlorine formation which was shown to be a function of the HCl concentration. The greatest stability of the color in these three acid levels occurs in 3M HC1 where there is roughly 5 decrease in absorbance resulting from a dose of 30,000 rads compared to the completely unreliable results in the 9Mmedium at the same dose. Thus, it may be more expedient at times to work at lower HC1 concentrations with its lower sensitivity but greater radiation stability, DISCUSSION
The principal radiolytic effect to aqueous solutions is what the radiation does to the water itself, and direct effects to solutes are usually rather small. The radiation chemistry of water is a subject that has been studied very intensely, and there is a good understanding of most of the fundamental processes that are involved. When water is exposed to ionizing radiation, the net effect is the formation of free radicals like H and OH and in air saturated medium, HO:!radicals, along with molecular products like H2 and Hz02. The principal radiation effects then will be what these radicals and molecules from the radiolysis of water will do to solutes, and direct interaction between radiation and solutes in the ordinary concentrations used in analysis will be expected to be relatively minor. We usually speak of media for processing solutions in terms of the principal anionic component because it is usually the complex forming ability of the anion that is utilized for the purpose at hand. To be sure, the large concentration of anion must be balanced by some cation or cations. In a majority of cases these cations are the relatively stable (not subject to redox reactions) alkali or alkaline earth metals.
It has been shown from photochemical studies that the net reaction H+
+ OH + C1-
=
C1"
+ HzO
(1)
is favored thermodynamically while the reaction in neutral or basic solution
OH
+ C1-
=
C1"
+ OH-
is not favored (9). The results of this study are consistent with this concept of the formation of free chlorine when acid chloride medium is exposed to gamma radiation. Reaction 1 given above requires the depletion of acid in an amount equivalent to the free chlorine that is formed. This has been shown to be the case qualitatively for alpha radiolysis by Baybarz (2). The determination of the very small decrease in acid molarity in the presence of a very large amount of acid is a difficult assignment; therefore, this determination was attempted on a 0.2M HCl-11M LiCl solution in a sealed container at large dose. The G value for chlorine formation determined for a short period in what is essentially an open system will not be the same over long periods in a closed system. Consequently, the chlorine production was determined separately with thiosulfate titration, and the free chlorine titer was compared with the loss in acidity at the same dose. The acid titration was made after the free chlorine had been reduced with thiosulfate. A typical analysis gave 0.0330 meq of chlorine formed by the irradiation of 3 ml of solution with a dose of 2 X IO7 rads. The corresponding decrease in acidity was 0.0340 meq. Thus, the gamma radiolysis of acid chloride solutions results in the consumption of hydrogen ion just as reported for the alpha radiolysis. This acid depletion also is in line with the radiolytic reaction that was assumed. A recent Soviet publication (IO) describes the radiolysis of purified aqueous hydrochloric acid solutions with 60-keV x-rays. Irradiation of nitrogen or hydrogen saturated solution gave chlorine as the sole radiolytic product. When the irradiated solution was air or oxygen saturated, chlorine dioxide and hydrogen peroxide were found in addition to the chlorine. The G value for chlorine formation in 6N acid was given as 1.2 in oxygen free, 2.7 in air saturated, and 4.4 in oxygen saturated media. The absorbance of the Cla- ion (9) H. Taube and W. C. Bray, J . Am. Chem. Soc., 62,3357 (1940). (10) L. T. Bugaenko and V. M. Moralev, Russ. J. Inorg. Chem., 11, 464 (1966). VOL 40, NO, 2, FEBRUARY 1968
273
was the analytical technique used. The values reported by these workers are higher than those found in this study. Figure 3 outlines the acid and chloride concentrations which mark the division between chlorine and no chlorine production by radiolysis. There is a demarcation at 1.40 HCI concentration which represents the lower chloride limit. A higher acidity at lower chloride was not attempted because the addition of another acid besides hydrochloric would add a new factor to be resolved. As the chloride content increases, the critical acid concentration decreases. There is a fair amount of hydrogen peroxide produced in low chloride solutions which decreases with increasing chloride. This amount is of the order of the chlorine produced but far below that required to color o-tolidine and reacts rapidly with any chlorine that is produced as long as the acidity is low. The reactivity of peroxide with chlorine diminishes with increasing acid concentration. There is more peroxide formed at low chloride than at high chloride. Thus, a higher acidity will be required at low chloride concentration to decrease the reactivity between peroxide and chlorine and bring the net chlorine production to a level close to that at high chloride concentration and low peroxide formation. This explanation is essentially what was proposed by the Soviet workers, (IO) and a similar suggestion has been made for the radiolysis of aqueous bromide system
As stated before, this investigation was aimed primarily at the extent of radiolytic damage that can occur to analytical systems. Thus, all chemicals used were reagent grade with no further purification, and ordinary distilled water was used. No quantitative explanations with the radical and molecular yields in aqueous systems are attempted. The radiolytic measurements were made on solutions prepared from different batches of reagents at various times. There was variation between measurements made at different occasions, but the overall trends remained the same. The values usually ranged within 30% of the average. Thus, it is believed the data of Figures 1 and 2 describe rather closely what radiolytic effects can be expected in ordinary acid chloride media. Very little difference was found between lithium and sodium as the cation in these acid-salt systems, and it was concluded that these cations do not play significant roles. Similar behavior is expected of other alkali and alkaline earth cations. Lithium chloride was the choice here because its solubility makes possible a wider total chloride range. RECEIVED for review October 11, 1967. Accepted November 17, 1967. Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corporation. (11) A. Rafi and H. C. Sutton, Trans. Faraday Soc., 61,877 (1965).
(14.
Measurement of Total Organoaluminum-Reactable Impurities in Hydrocarbons by Continuous Flow Thermometric Analysis T. R. Crompton and Brian Cope Carrington Plastics Laboratory, 'Shell' Research, Ltd., Urmston, Manchester, England
An instrument is described for the continuous or semicontinuous measurement in hydrocarbons of total triethylaluminum-reactable impurities such as water, dissolved oxygen, and low molecular weight alcohols. The sensitivity of the instrument is such that it produces a measurable response for concentrations as low as 1 ppm of these substances in the sample. Although a specific application of the instrument is described, the principle of continuous flow thermometric analysis could be used in other problems connected with the on-line analysis of process streams.
FREQUENTLY, in chemical processes or in end-use applications, critical limits exist regarding the concentration in hydrocarbons of dissolved impurities such as water, oxygen, and low molecular weight alcohols. A thermal analysis procedure involving reaction of the hydrocarbon with an organoaluminum compound was considered a feasible proposition for the determination of these impurities at the required level. Triethylaluminum was chosen as the reagent because of its solubility in the hydrocarbon and because of the sensitivity afforded by its high heat of reaction with water (approximately 100 kcal/mole). In some experiments, with a thermometric titration apparatus using a thermistor end-point detector of the type described by Everson (1) and Everson and Ramirez (2) a sample of a CS (1) W. L. Everson, ANAL. CHEM., 36, 854 (1964). (2) W. L. Everson and Evelyn M. Ramirez, Ibid.,37,806 (1965).
274
ANALYTICAL CHEMISTRY
hydrocarbon was titrated with an 8% v/v solution of triethylaluminum in the hydrocarbon. The titration was carried out under dry, oxygen-free nitrogen to eliminate thermal interference due to reaction of the reagent with atmospheric oxygen and moisture. This procedure revealed that the hydrocarbon contained impurities which liberate heat on reaction with triethylaluminum. A continuous method of analysis based on this principle had obvious advantages, such as high sensitivity and the fact that all substances present in the hydrocarbon which destroy triethylaluminum, would evolve heat upon reaction and provide an estimate of the concentration of total impurities present in the hydrocarbon without any knowledge of their chemical nature. Priestley et al. (3, 4 ) have described an apparatus for the continuous flow enthalpimetric analysis of aqueous streams. In this apparatus, the sample stream from a small reservoir and a stoichiometric excess of reagent stream are pumped at controlled, constant-flow rates into a reaction cell where thorough mixing occurs. This method is not truly continuous but does handle a flowing 25-ml sample. Priestley mentions the possibility of applying this technique to continuous process control. Three thermistors are used in conjunction with a suitable bridge circuit to monitor the difference between the temperature of the contents of the reaction cell and the average (3) P. T. Priestley, W. S. Sebborn, and R. F. W. Selman, Analyst. 90, 589 (1965). (4) P. T . Priestley, J . Sci. Znstr., 42, 35 (1965).
