Effect of Gamma Radiation on Aqueous Solutions of (Et hylenedinit riIo)te traace tic Acid H. E. ZITTEL Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge,:Tenn.
b
The effects of gamma radiation on aqueous solutions of EDTA, used as a complexing agent, were studied at various pH's, EDTA concentrations, and radiation levels. These factors all affect the shape of the curve for the amperometric titration used to measure EDTA concentration; however, none significantly affects the sensitivity of EDTA to irradiation. Under all conditions studied, EDTA was degraded by gamma radiation to the extent of -1.4 X mmoles per milliliter of s o b tion irradiated per roentgen. The infrared absorption spectrum of an irradiated solution of EDTA was determined. The radiolysis products were not identified with certainty. Xylenol orange, eriochrome black T, and alizarin red S, which are commonly used in EDTA titrations, are fairly insensitive to gamma radiation insofar as visual end point detection is concerned.
E
THYLENEDINITRILO)TETRAACETIC ACID (EDTA) is an extremely
useful titrant for the determination of many elements (10). A visual indicator is normally used to determine the equivalence point. However, under conditions where use of a visual indicator is not feasible, another method for detecting the equivalence point can be used (4, 7). The extent to which gamma radiation affects such methods has not been determined before. Radiation chemistry has become an important field in the last several decades. Although much work has been done to ascertain the effects of various types of radiation on numerous systems ( I ) , little effort has been made to measure the effects of radiation on analytical methods. With the advent of nuclear power sources, the need to do analytical work under conditions of high radiation has increased greatly. Those experienced in analyzing highly radioactive samples have observed that some analytical methods are affected deleteriously by the radiation. The work reported in this paper is the result of a quantitative study of both the effects of gamma radiation on aqueous solutions of EDTA and the significance of such effects in several methods for determining the equivalence point in EDTA titrations.
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ANALYTICAL CHEMISTRY
EXPERIMENTAL
Reagents. STANDARD SOLUTION of E D T A , -0.02M. Dissolve 7.4 grams of disodium (ethylenedinitri1o)tetraacetic dihydrate (Eastman organic chemical No. 6354) in water and dilute the solution to 1 liter. Standardize the solution by any of the established procedures (10). More dilute standard solutions may be prepared from this solution. FERROUS AMMONIUM SULFATE SOLUTION, 0.0251. Dissolve 0.8 gram of Fe(N\'Ka)2(S04)2.6H20in 100 ml. of 0.01-If HClO,. ST.4NDARD SOLUTION OF VANADYL SULFATE, -0.05M. Dissolve 10 grams of V0SO4.2Hz0 in water and dilute the solution to 1 liter. Standardize the final solution by titration against a standard solution of Zn+2 (see Procedures below). All other solutions required were prepared from ACS grade chemicals and, when necessary, were standardized by conventional methods. Apparatus. Polarograph, O R K L Model Q-1160 (6). For all the amperometric titrations, this instrument Tvas used both as a source of constant potential and as a current recorder. Electrodes. Indicator 1: platinum foil, 1-sq. cm. area. Reference 1: saturated calomel electrode (S.C.E.) with agar-KC1 salt bridge. Syringe-buret Titrant Delivery ApDaratus. This amaratus is described elsewhere (4). Radiation Source. An %-curie Co60 gamma source was used to make all the irradiations. This source was cahbrated with a Fricke dosimeter (9 i . Spectrophotometer, Cnry Model 14. Procedure. TITRATION OF EDl'*I WITH Vo+2. Transfer to a 50-nd. beaker a test aliquot of the sample that has been irradiated a t the desired level. Add 20 ml. of 1120 and 2 ml. of 3-11 CH3COOKa solution. Adjust the p l i to 4 with 0.1M HC104. Apply a potential of +0.6 volt us. S.C.E. to the platinum electrode. Titrate with the standard solution of T'OEOa, - . 0 5 X . Record the current and the corresponding volume of titrant during the titration; rapid increase in current indicatcq that excess titrant has been added. Locate the equivalence point by extrailolating the two segments of the titmtion graph to their point of intersection. In the same manner, titrate a control sample that has the same original composition as the irradiated sample but that has not been irradiated. Take
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the difference between the two titers as a measure of the amount of EDTA decomposed by the radiation. TITRATION WITH EDTA USINGF e f 2 as ENDPOIKT INDICAIWR. Transfer to a 50-ml. beaker a suitable aliquot of a standard solution of Th7-4 (or Zn+2). Add 2 ml. of 351 CH3COONa solution: then adjust the p H to -4 Tvith 0 LIT HC1O4. Dilute the resulting solution t o about 25 ml. with dibtilled water; deaerate it with nitrogen. .4dd 0.2 ml. of 0.0251 Fe(S€14)2(S04)2solution. Apply a potential of +0.4 volt us. S.C.E. to the platinum electlode. iitrate with the EDTA solution that hciy been irradiated a t the desired level. Record the current and the coiresponding volume of titrant. Locate the equivalence point aq described above. Titrate mother equal aliquot of thr standard solution of T h L 4(or Zn+*)with nonirradiated EDTA solution of the same original composition as the irradiated EDTA solution. Czlculate the titer of each EDTA solution. Take the difference between the two titrrs as a measure of the amount of EDTA degraded by the rr,diatinn. Infrared Study. A 1 X 10-*M aqueous solution of E D T A was subjected to 1 X 106 r. of gamma radintion. The irradiated solution was freeze dried and peIletiLed with K B r The infrared spectrum of the KBr pellet was obtained and compared with t h a t of K B r pellet t h a t contained unirradiated E D T A and t h a t was prepared in the same way. RESULTS AND DISCUSSION
During the light particle irradiation of an aqueous bolution the major chemical change occurs in the solute, whereas the water undergoes very little change (1). Since the Ivater is the major component and must therefore absorb most of the radiation energy, some means must exist whereby this energy may be transmitted to the solute. Free radicals (H and OH), as well as molecular fragments, result from the breakup of the water. All these in turn either react with solute molecules or recombine to form H20. The free radicals may also form Hz and H2O2when like pairs meet. The eiact molecular constitutions of the free radicals are not known, but it is know1 that one, OH, is a powerful oxidant. whereas the other, €1, is a strong reductant. Tlie effect of these free radicals on a solute depends on the
relative susceptibility of the solute to oyidation and reduction. When heavy particle radiation of a n aqueous solution occurs, the main products of the water decomposition are molecular Hz,02,and HzOZ. These normally have much less effect on the solutr for a given erergy input than do the free radicals fornied by light particle radiation ( 2 ) . Therefore in this study the effect of the Cow gamma radiation is considered as s o m w h a t of a “boundary” condition. ’ilhile it is acknowledged that, under actual “hot” analysis conditions, other types of radiation will he present, i t i: proposed that the effect of the gamnii radiation will be a maximum effect per roentgen for the EDT.1 system. The effect of gamma radiation on aqueous solutions of EDT.4 itself has not heen reported. Fricke, Hart, and Smith ( 3 ) h a r e studied the effect of x-radiation on a r,umber of organic compounds but none so complex as CDl‘h. Since the purpose of this nork nab t o measure the effects of gamma radiation on E D T A 2s it is used in analytical methods, no systematic study TI as made t o identify the degradation products prebent as a rcwlt of gamma irradiation. The radiation chemist often uses the term ‘.G value” to express a change that occurs in a given system as a result of irradiation. The G .;slue is the number of molecules of .material changed for each 100 e.v. of radiation. Since moqt analytical chemists are more familiar with the term “roentgen” than with G value, all results given in this work are in terms of millimoles of irradiated solute degraded per milliliter of solu-
a result of the irradiation, caused t h r increase in absorbance mas considered to be highly improbable because an irradiated blank was being used as a reference. The experimental results indicated t h a t the degradation of the E D T A caused by radiation could not be followed by means of absorption spectrophotometry. The results also indicate that direct spectrophotometric method. should not be used to measure EDTA concentrations under conditions of high radiation. Since the increase in absorbance way considered to be due to the formation of degradation products of EDTA\< an attempt was made to identify them. The infrared absorption spectrum of a 10-3Jf solution that had been irradiated was studied. KO definite statement can be made a? to the structures of the degradation fragments, but the infrared study showed that many changes occur in the E D T d molrcule as a result of the gamma irradiation. Since spectral means mere not accurate for following the degradation of EDTA cauqed by gamma radiation, two amperometr ic procedures u ere used for this purpose in the remainder of the study. One procedure was the direct aniperometric titration of T h + l n i t h E D T A by using Fe+2 as a n end point indicator ( 4 ) . The other consisted of adding EDTA in ewes$ to a standard solution of Th+4 and of aniperometrically back-titrating the excess E D T A 111th which also actas the end point indicator ( 5 ) .
tion irradiated per roentgen of radiation. Effect of Gamma Radiation on Aqueous Solutions of EDTA. Since aqueous solutions of EDT.1 absorb ultraviolet light of approximately 200-m,~wavelength ( 8 ) ,it was believed that the degradation of EDTA by gamma radiation could be measured by absorption spectrophotometry of 200 mp. hccordingly, portions from a n aqueous solution 1.2 X 10-3X in EDT-4 were irradiated at various levels of ganima radiation, and the spectra from 190 to 220 mw \%-ereobtained. The absorbance in the region from 190 to 220 m,u increased rather than decreased as the radiation level was increased. I s the radiation dosage was increased u p to -lo6 r., the absorbance increased; thereafter a slight decrease in absorbance occurred. Since the absorbance values were obtained a t one eytreine end of the usable ultrariolet range of the instrument, it could not be ascertained whether the observed increase in absorbance na.; an apparent one due to a shift in the peak or nas real. The experiment was repeated, and esentially the same results were obtained. An attempt nas then made to titrate the EDTA solution, which had been esposed to lo6 r., v i t h a standard solution of Th+l. The irradiated solution did not contain EDTA which indicated t h a t the observed increase in absorbance was not a function of the E D T A concentration. The possihility that H202,formed as 0
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0.33
1 0.66
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350
I
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400
450
Figure 1 . Amperornetric titration curves of irradiated solutions of EDTA having different initial pH values Test conditions EDTA ,4 X 1 O-4M as irradiated, 3-ml. aliquot titrated; pH, as noted (adjusted with 0.1M N a O H or HClO4); titrant, 6 X 10-4M VOSOd; Gamma dosage, 7.2 X 103 r. Solution PH A (control) 7
650
Figure 2. Spectra showing effect of gamma radiation on alizarin red S when used as indicator for EDTA Solution composition: 10 ml. 1.2 X 10% EDTA 3 ml. N H & z H ~ O Z - H C ~ H ~buffer OZ 0.5 ml. 1 % alizarin red S Hz0 to total volume of 50 ml. A, B, C, 13,series has Th+4 added to just cause color change Reference, the above solution less the indicator Aliquot Gamma dosage, r. A, A‘ (control) None
B
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C D E
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600
WAVELENGTH, m p
2.33
JCLUME OF T I T R A N T , mi
L 500 550
B, B’
7.0 x 104
1 .4 X l o 6 2.4 x 105
VOL. 35, NO. 10, SEPTEMBER 1 9 6 3
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Table 1. Effect of pH on Sensitivity of EDTA to Gamma Radiation Procedure: see Procedures section Test solution: [EDTA], 1 X 10-3JI DH. as indicated iadiusted nith 0.1JI
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h d i a t i o n damage, yc = titer of cwntrol - titer of saniple x 100. _____ - ____ titer of control Table II. Effect of EDTA Concentration on Sensitivity of EDTA to Gamma Radiation
Procedure, see Procedures section pH, S for all test solutions Titrant. 0.02.11 VOSOd Hadiation tlarnage
factor,"
l)ns:rge,
[ll. The enliaiicernrnt is r dosage-: aiid loiv EIlT.1 concentrations and it is ahirnt after a dosage of 10: r. The high current might have been due to the presenw of H20Z formed during irradiation. Hoivever, addition of H202 to control solution:: re,sulted in a decrease in the current after the end point, in contrast ivith the behavior shown in Figure 1. d possible es1,lanation is t'liat dcgradation producbs of E D T l might still br able to chelate with YO+*. Such comlileses would have equilibrium constants different from those of the EDTA complex. The fact that the phenomenon seems to be ahsent for E:DT1 iolutions exposed to a higher dosage tends to bear out this po.4ljle explanation since any such active dcgradation products would be degraded to a still greater degree by more radiation. The effect of the original pH of the EDTA solution irradiated on the titer required R i shown in Tahle 1. From the curves in Figure 1: an exact tleterinination of t h r cnd point is difficult. Howevc~r.within the limits of such mea+
Effect of Gamma Radiation on Some Organic Indicators Commonly Used with EDTA I'rocwhire, see Procedures section, [EDTA], 1 X l0FJJ/
Table 111.
1 Absorbance,
Indicator Eriorhronir black T
Radiaticm, r. ?;one (Control) 2 6 X lo4 4 b x 104 1 2 x 105
Alizarin red
P
Sone (Control) i 0 X IO4 1 4 x 105 2 4 x 105 Sone
in EDT.1 concentration or in radiation dosage. The author believes that the factor is d f i c i r n t l y accurate to permit a correction for radiation damage in analyses made under conditions of high gamma radiation. Tlie data indicate that a-: much gamma radiation at IO3 r. woultl have little effect on tlie accuracy of such EDT.1 titrations. However. if the EIl'LA\ is used for amperometric titrations. the effect of radiation on the shape of the c u r w should be recognized. Effect of Gamma Radiation on Some Organic E n d Point Indicators. To establish the cffcct of gamma rariiation on those EI>Th tit'rations iri \vhicli org aiiic end point. indicator. are used, the indicators sylenol orangr. alizarin r t d S! and eriochrome black T were studied. I n Figure 2 ishown the effect of gamma radiation on both forms of alizarin red S i i s d as indicator in a n EDTYi solution. If the dye is iisetl only as an end point indicator and not as the colorimetric reagent, as much gamnia radiation a- ' l o 5 r. can be tolerated without effecting the visual ease of end point tletrrmination. The data for all t'hree iridicators are given in Tahle 111. 111 each case. the d f w t of the radiation on both form> of each indicator w i ~ measured--e.g.. n-ith an excess of EDT.1 ~iresc'nt aiid with an excess of metal ion pre.-rnt. l+kc*el)tin the cabe of the rriochrome hlack T, no hignificant difference in sensitivity to gamma radi:ition hc.twen the two form. was noted In this ?a*?, an excess of cation apparently cause> an increase in sensitivity to radiation. The end point indicatorstudied are sufficiently re,sistant to radiation to tolerate a gamma radiation dose of 105 r. in such a titration. 1-nder such radiation. the eiid point will still be sharli and visible t o the naked eye. Effect of Gamma Radiation on Cationic End Point Indicators. Tlie two procrdurr+ used for following t,he E D 'I- -\ d rgra d a ti o n. tis iixntionr (1. are amprroinet~rictitrations. -1diort study \vas m:idt~of the effect of gamma radiation on the Fr-2 and T-O+2, which are esbentially ustd as elid point intlicators. In the case of 1 X 10-3-11 VO-2, a radiation dose of lo4 r. did not affevt the acwiracy of the titration. I n t h e ca3e of the FeL2 end Iioint indicator. n more comijlicated situation exists. .i~inclicatrd in tlie Procedures section. the Fr+' i- added in small
amounts to the solution heing titrated. I'lic sen.iti\ity of FeA2 to oxidation hy gamma radiation is high ( I ) . Since the method depends on the change in the oxidation potvntial of the Fe+* ~ 1 2it is changed to the Fe(FXW.t)2-2 cwmplex, the radiztion interferes by ~ m m a t u r e l yoxidizing tlie Fe L2. The Linil)eronietriccurrent, rather than being ~-mtially con,-taiLt before the end ;)oint, increaheq gradually, and no h r p hrenk is olltained at the end ,mint. It iq ther:fore recoinmended t liat in radiation field. of intendty
(6) Kelley, AI T , ilIiller, H H I Zbzd
greater than lo3 r., the VO+*procedure be used whenever possible.
,
24,1895 (1952) (7) Ringbom, A , Kilkman, B , ilcta Chem Scand 3, 22 (1949)
Y B , Bricker, C E , ANAL.CHEM.26, 19; (1954). ( 9 ) Weiss, J., Allen, A. 0.: Schm-arz, €1. A,, Proc. I n f e m . Conj. Peaceful C'srs At. Energy, Genera 14, 179 (19:5). (10) Kelcher, F. J., "The Analytical Cses of Ethglenediaminetetraacetic Acid," Van Sostrand, Princeton, S . J., 18) Sneetser.
LITERATURE CITED
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(1) Allen, A . O., "The Radiation Chemistry of FVater and Aqueous Solutions," F-an Sostrand, Princeton, K. J., 1961. ( 2 ) Dale, JV. M., Gray, L. I
