Radiolysis of Dilute Aqueous Thiourea Solutions

Radiolysis of Dilute Aqueous Thiourea SolutionsAuthor(s): A. Charlesby, P. M. Kopp and J. F. ReadSource: Proceedings of the Royal Society of London. Series A, Mathematical and PhysicalSciences, Vol. 292, No. 1428 (May 3, 1966), pp. 122-133Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/2415621 .

Accessed: 10/06/2014 02:40

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

.

The Royal Society is collaborating with JSTOR to digitize, preserve and extend access to Proceedings of theRoyal Society of London. Series A, Mathematical and Physical Sciences.

http://www.jstor.org

This content downloaded from 195.78.109.194 on Tue, 10 Jun 2014 02:40:49 AMAll use subject to JSTOR Terms and Conditions

http://www.jstor.org/action/showPublisher?publisherCode=rsl

http://www.jstor.org/stable/2415621?origin=JSTOR-pdf

http://www.jstor.org/page/info/about/policies/terms.jsp


Radiolysis of dilute aqueous thiourea solutions

BY A. CHARLESBY, P. M. KoPP* AND J. F. READ

Physics Department, The Royal Military College of Science, Shrivenham, Swindon, Wilts

(Communicated by Sir Willis Jackson, F.R.S.-Received 16 August 1965)

Thiourea, when present at low concentration, can act as a radiation protector to macro- molecules (biological or polymeric) in aqueous solution. Previous work has shown that much of this protection occurs against the indirect effect. Thiourea can also protect against the direct effect, though to a more limited extent.

In this paper the reactions of dilute aqueous thiourea solutions have been followed at a range of y radiation doses and dose rates, and in the absence and presence of oxygen. The rapid destruction of thiourea confirms that the effect is primarily indirect, and almost independent of concentration, and dose rate over the range 10 to 500 mg/l. The breakdown products of thiourea can react further. ln the presence of oxygen the loss of thiourea is more rapid, and different products are formed.

INTRODUCTION

Thiourea has been shown to be very efficient in protecting many chemical and biological model systems from the damaging effects of ionizing radiation. Its use in protecting biologically active compounds in aqueous solution was studied by Dale, Davies & Meredith (I949), Dale (I950) and Dale & Davies (I952), but owing to the complexity of their systems no clear-cut mechanisms could be formulated. It has been shown by Charlesby & Kopp (i 962, I 966) and Kopp & Charlesby (i 963) that in simple model systems such as aqueous polymer solutions, thiourea, even at very low concentrations, is very efficient in offering protection against the indirect effect of radiation (i.e. the radiolytic products of water) whereas it can give only a limited amount of protection against the direct action of radiation on the macromolecule. This dual type of protection behaviour arises from (1) the thiol- thione equilibrium structure of thiourea existing in aqueous solution, which per- mits a chain reaction with the H- and OH radicals (and possibly hydrated electrons) produced by the radiation of the water, thus eliminating these before they are able to attack the macromolecule; and (2) direct reaction of thiourea or its breakdown products with radicals on the macromolecule.

Several papers have been published on the fate of the thiourea when irradiated in aqueous solution (Dale I956; Dale & Davies I956, I957, I959) but only at high concentrations (0.8 and 8 %) and under different irradiation conditions to those used in our earlier protection studies. It is believed that the behaviour of thiourea may be representative of a number of other protecting agents of biological impor- tance (Charlesby & Kopp i962, I964; Kopp & Charlesby I963), and hence warrants more detailed studies at low concentrations comparable with those used in our model systems. The present work is therefore concerned with the radiolysis of very low concentrations (mainly at 50 mg/l.) of thiourea in aqueous solution, where the

* Now at Medizinisch-Chemisches Institut, Berne University

[ 122 ]



Radiolysis of dilute aqueous thiourea solutions 123

reaction is almost completely confined to the indirect effect, and involves the interaction of thiourea with the radiolytic products of water. The influence of oxygen in the water has also been studied.

EXPERIMENTAL

Solutions The thiourea was of Analar quality (B.D.H.) and was treated with activated

charcoal and twice recrystallized from triply glass distilled water. Samples irradiated in vacuum were thoroughly degassed three times on a high

vacuum line (< 10-5 mmHg). The effect of oxygen on the radiolysis was investigated by passing a fine continuous stream of pure oxygen through the sample during irradiation.

Irradiation The samples were subjected to y radiation from a cobalt 60 source of 7000 curies

giving a maximum intensity of 3 Mrad/h. The irradiations were carried out at room temperature with the samples positioned vertically at various positions round the source, according to the dose rate required. Care was taken that the samples received a homogeneous dose. Dosimetry measurements were carried out with a Baldwin Farmer substandard dosemeter and with ferrous sulphate (Fricke) GFe3? = 15 5.

Ultraviolet spectra and pH measurements The u.v. spectra and pH of the solutions were measured immediately after open-

ing the irradiated vacuum tubes. The oxygenated samples were made up to the initial volume with triply glass distilled water before carrying out the measurements.

The u.v. spectra were recorded on a Perkin Elmer model 137 recording spectrometer and the pH measured with a Pye pH meter.

Paper chromatography

Owing to the low concentrations (50 mg/I.) of thiourea employed it was neces- sary to irradiate large volumes (4 x 250 ml.) of solution at doses of 1, 2 and 4 Mrad, all at 50 krad/h. After irradiation these were freeze dried and redissolved in 4 ml. of distilled water to give concentrations suitable for paper chromatographic analysis. Any insoluble matter was filtered off from the solution on a G. 4 glass sintered crucible, dried to constant weight and reweighed.

The clear solutions were loaded on Whatman no. 3mm chromatographic paper by making 25 applications of 0002 ml. with an Agla micrometer syringe. The paper was then subjected to descending chromatography using a butanol-ethanol- water (4:1:1) solvent. The unknown and reference compounds were run simul- taneously on the same paper. Substances on the chromatograms were located with FCNP spray and ammoniacal silver nitrate as described by Milks & Janes (I956).

Chemical analyses of irradiated solutions

One litre of solution containing 500 mg/l. of thiourea was irradiated in 250 ml. batches at 0-1 Mrad/h for doses of 12-5, 20 and 40 Mrad, and then freeze dried.



124 A. Charlesby, P. M. Kopp and J. F. Read

The solids were redissolved in 50 ml. of distilled water and the insoluble residues were filtered off on G. 4 glass sintered crucibles. The remaining solution was again freeze dried. The soluble and insoluble residues were weighed and subjected to elemental analysis and quantitative ammonia and sulphate determinations.

Gas analysis

Gases produced during radiolysis were identified in an A.E.I. MS 10 mass spectrometer. The amount of carbon dioxide was estimated by a volumetric method, employing a back titration. (The amount of sodium hydroxide neutralized by the carbon dioxide was determined by back titrating with hydrochloric acid using phenolphthalein as indicator.)

RESULTS

Destruction of thiourea: variation with accumulated dose and dose rate

Aqueous thiourea solutions show a characteristic double absorption peak in the ultraviolet at A = 198 nm and A = 236 nm which decreases on irradiation (figure 1). Since Beer's law is obeyed it is assumed that the decrease in optical density of the irradiated samples corresponds to a loss of thiourea. Measurement of the peak at A = 198 nm could not be relied on as this falls near the lower end of the spectrometer range. The background absorptions due to radiolytic products are uncertain, and have therefore been ignored. This renders the measurements at high degrees of destruction, and at low initial concentration, less accurate.

The radiolysis of thiourea was investigated spectrophotometrically at three selected concentrations (10, 50 and 500 mg/l.) and at dose rates of 10, 100 and 1000 krad/h in vacuo, and at 50 mg/l. at a dose rate of 1000 krad/h in the presence of oxygen. The decrease in thiourea concentration with dose (at an initial value of 50 mg/l.) is shown in figure 2 and the corresponding G values for all concentrations and dose rates are listed in table 1. It is seen that, in the absence of oxygen the initial G value for thiourea destruction varies only slightly with initial thiourea concentration and dose rate over a wide range; further irradiation causes a decrease in these G values, ascribed to competitive reactions of the breakdown products.

Figure 3 shows the fall in G(-thiourea) with dose; each determination was made by considering the fall in optical density over a small dose range (0.05, 0-5 and 5 Mrad for 10, 50 and 500 mg/l.). For oxygen-free samples the shapes of the curves (figure 3) are more nearly similar if comparisons are made, not at the same dose, but at the same dose divided by initial concentration. In other words the initial G values are approximately similar, but to produce an equivalent reduction, the required dose must be increased in proportion to the amount of thiourea initially present, i.e. competition between thiourea and its breakdown products is equally effective only if the concentrations of both are varied in the same proportion. For the same small dose the amount of breakdown products is almost independent of initial thiourea concentration (similar G value); hence the dose must be increased in proportion to the thiourea concentration to provide a similar degree of competition. Therefore, these results are to be expected if the radiation-induced changes occur by an indirect effect, and result in breakdown of thiourea into other products which can




OMrad 0.1

o*5

~1.o-

1.0 0.5

0 -

0U? - -1 -

300 250 200 wavelength (nm)

FIGURE 1. Change of the u.v. spectrum of an aqueous thiourea solution (50 mg/I.) with dose.

1,0 -

0 0

0-5

x~~~~~

0 1 2 3

dose (Mrad)

FIGURE 2. Disappearance of thiourea (50 mg/I.) with dose studied by u.v. spectroscopy. Dose: A, 10 krad/h, vacuum; x, 100 krad/h, vacuum; 0, 1000 krad/h, vacuum; *, 1000 krad/h, oxygen.




still scavenge the radiolytic products of water. From the curves in figure 3, we can estimate that doses of 0-15, 0 6 to 1-4 and 10 Mrad are needed to halve the initial G values for thiourea loss for initial thiourea concentrations of 10, 50 and 500 mg/l. According to figure 2 and table 1 the residual amount of thiourea after these doses is about 48, 42 to 54 and 50 o and this reacts with half the radiolytic

TABLE 1. G VALUES FOR THIOUREA DISAPPEARANCE UNDER DIFFERENT

CONDITIONS STUDIED BY U.V. SPECTROSCOPY initial

thiourea concentra- dose loss of loss of

tion gas rate dose thiourea thiourea G(_thiourea)

(mg/i.) phase (krad/h) (Mrad) (mg/i.) (Go) G(-thiourea)y cumulative

10 vacuum 1000 0-05 2-4 24 0-61 0-61 0-10 4-4 44 0-51 0-56 0-15 5G7 57 033 0-48 0-20 6-8 68 0-28 043 0-25 7-5 75 0-18 0-38

50 vacuum 10 0 5 20-5 41 0-52 0-52 1.0 31-6 63 0-25 0 40 1.5 41-1 82 0 24 0 35 2-0 454 91 011 029 2 5 46 9 94 0 04 0 24

50 vacuum 100 0 5 17 2 34 0 44 0 44 1-0 26-4 53 0.23 0 33 1.5 33 8 68 0-19 0-29 2-0 39-6 79 0.15 0-25 2 5 42 6 85 0 08 0-22

50 vacuum 1000 0 5 13 2 26 0 34 0 34 1.0 23 3 47 026 0 30 1.5 304 61 0-18 0-26 2-0 353 71 012 022

2-5 385 77 0-08 0-19

500 vacuum 1000 5 144 29 0 37 0 37 10 252 50 0-27 0.32 15 316 63 0 16 027 20 368 74 0-13 0.23 25 408 82 0 10 021

50 oxygen 1000 0-1 16 5 33 2 09 2 09 05 41-7 83 0 79 1-06 1-0 47-2 94 0-14 0-60 1-5 48.8 98 004 0-41

* G(-thiourea) Over short dose range 0.05 Mrad for 10 mg/I., 0 5 Mrad for 50 mg/I. and 5 Mrad for 500 mg/I.

products of water in competition with an approximately equivalent amount of breakdown products. Thus thiourea is approximately equally effective as a scavenger for the radiolytic products produced in the water.

In the presence of oxygen the fall in a values with increasing dose is far more rapid, about 0 35 Mrad being needed to halve the initial G value for (50 mg/l.) thiourea loss. According to figure 2 the residual amount of thiourea after this dose is only 24 %. Since this can react with half the radiolysis products of water in



Radiolysis of dilute aqueous thtourea solutions 127

competition with 76 % of the breakdown products of initial thiourea, thiourea is approximately three times more effective as a scavenger for the radiolysis products produced in water and oxygen.

(a) (b) (c) c)M (d)

08 -

0-6

0*2-

0 001 0 2 0 1 2 3 0 10 20 30 0 1 2

dose (Mrad)

FIGURE 3. Variation of G(-thiourea) with dose. (a) 10 mg/I. thiourea in vacuum at 1000 krad/h; (b) 50 mg/l. thiourea in vacuum: A, 10 krad/h; x, 100 krad/h; 0, 1000 krad/h. (c) 500 mg/I. thiourea in vacuum at 1000 krad/h; (d) 50 mg,'l. thiourea in oxygen at 1000 krad/h.

60

0

x~~~~~~ x~~~~~~~~~~

4-

x

2L I I I 0 0-02 0 04 0 06 0 08

rlc (r = dose (Mrad); c = concentration (mg/I.))

FIGURE 4. Variation of pH of thiourea solutions with dose. *, 10 mg/l. thiourea in vacuum at 1000 krad/h; 0, 50 mg/I. thiourea in vacuum at 1000 krad/h; x, 500 mg/I. thiourea in vacuum at 1000 krad/h; OI, 50 mg/l. thiourea in oxygen at 1000 krad/h.

Variation of pH wtith dose

The variations of pH with dose for the different series are shown in figure 4. It can be seen that for the 10 mg/I. solution the change in pH is only slight. In the case of the vacuum irradiated 50 and 500 mg/I. thiourea solutions the pH is seen




to decrease to a minimum at 1-5 and 15 Mrad, respectively. This initial fall to- gether with other evidence can be ascribed to the destruction of thiourea to give an unknown intermediate of an acidic nature. The dose at which this minimum in pH occurs corresponds to the point where over half the initial thiourea has been destroyed. The subsequent rise in pH can be taken to be due to the more rapid destruction of this intermediate product. In the case of the oxygen irradiated 50 mg/l. solution no further rise in pH occurs once this minimum has been reached. This finding agrees with the difference in the rate of thiourea destruction. These results are in good agreement with those from the u.v. studies and with the paper chromatographic results.

Identification of radioly8is products and elemental analysis

Ultraviolet spectra

A comparison of the u.v. spectra of the irradiated thiourea solutions and some commercially obtainable related compounds indicated that no useful information regarding the nature of the products formed could be obtained by this technique.

Paper chromatography: evacuated samples

The results are indicated in table 2. The majority of the thiourea in the 50 mg/l. solution has disappeared by 1P5 AMrad with a simultaneous increase in intensity of the spots identifiable as cyanamide and guanyl thiourea up to 1 Mrad. At a dose of 2 Mrad these spots in turn have disappeared, indicating that these are the first decomposition products. Owing to the large thiourea spot dicyanamide could not be positively identified (Rf values 0 45 and 0 49, respectively). The streak (Rf value 0 to 0 25) which is present at doses above 1 Mrad has been tentatively ascribed to guanidine sulphate. Although by itself it gives an orange streak, in the presence of other possible compounds it gives a blue green streak identical to that in all the irradiated samples. Ammonium thiocyanate, thiourea dioxide and urea were not detected in any of the irradiated samples.

Paper chromatographic analyses were also carried out for the more concentrated solutions (500 mg/l.) irradiated at correspondingly (10 x ) larger doses. The results agreed with those presented above for the more dilute solutions, indicating that no changes in the reactions and products are brought about by simultaneous increases in concentration and dose.

Oxygenated samples

The paper chromatograms of the oxygenated samples were different from those presented above and only two of the seven spots, thiourea dioxide and guanyl thiourea at 0 5 Mrad could be identified. At 0 5 Mrad the thiourea had almost completely disappeared and at 1 Mrad both the identified products were absent from the chromatograms.

Elemental analyses of the irradiated evacuated samples

The elemental composition of the soluble and insoluble fractions of the samples irradiated in vacuum are shown in table 3. It is of interest to note the decrease with




TABLE 2. PAPER CHROMATOGRAPHIC DATA

(a) Reference compounds ammoniacal silver nitrate

before after 10 min compound Rf Rt FCNP spray heating at 100 ?C

guanidine sulphate 0-0-32 0-0 70 orange streak light brown light brown thiourea dioxide 0-17 0 37 blue-purple magenta dark magenta urea 0-37 0-81 red light brown yellow brown guanyl thiourea 0 37 0-81 blue-green grey brown grey brown ammonium thiocyanate 0-42 0 94 blue white thiourea 0 45 100 purple brown grey brown dicyanamide 0-49 1-09 magenta - white cyanamide 0 70 156 magenta yellow yellow sulphur 0 0 grey grey

(b) Irradiated thiourea sol8ution (50 mg/I.; doses of 1, 2 and 4 Mrad at 1 Mrad/h)

ammoniacal silver nitrate dose A

(Mrad) before after 10 min spot Rf Rt 1, 2, 4 FCNP spray heating at 100 ?C

origin (sulphur) 0 0 v v v - grey brown grey brown streak (guanidine- 0--0 25 0-0 57 v V v blue green light brown light brown

sulphate) A 0413 0 30 V\ v - white white grey B 0-19 043 V - - pink - -

C 0-22 0 50 V V V yellow - white green

D 0-27 0-61 V v' - pink white E(guanyl thiourea) 0 37 0-84 V - - blue green grey brown grey brown

F 0 39 0-89 - - V green dark brown dark brown G(thiourea) 0 44 100 V/ - - purple brown grey brown

H 0-47 1-07 - - V yellow pink grey pink orange

I(cyanamide) 0-69 1P57 V - - magenta yellow yellow

TABLE 3. ELEMENTAL ANALYSES OF IRRADIATED THIOUREA

SOLUTIONS (500 mg/I.) AT 1 Mrad/h IN V.AcUo

elements present (%)

dose C H N S 0 (Mrad) difference

(a) soluble fractions

0 16-6 5-6 36-4 389 25 12*5 13*0 5.7 32*4 32-1 16-8 20 10*3 5*8 30-6 26*6 26-8 40 6*5 6*1 25*9 19*3 42*2

(b) insoluble residues

12*5 74.5 20 78-1 40 2*5 0*4 0.5 90.0

9 VOL. 292. A. A.




increasing dose of sulphur in the soluble fraction accompanied by a simultaneous increase in oxygen. In the oxygenated solutions no weighable residues remained.

The soluble fractions were found to contain sulphate and ammonium. Sulphate was determined gravimetrically by precipitation with barium chloride (Vogel) and ammonium was analysed colourimetrically with Nessler's reagent. The results are shown in table 4.

From the analytical results it can be seen that the insoluble residues consisted mainly of sulphur.

The results obtained with the mass spectrometer indicated appreciable quanti- ties of hydrogen and carbon dioxide and traces of nitrogen and oxygen. It was found that for a 500 mg/l. thiourea solution a dose of 40 Mrad produced a 20 % conversion of the carbon to carbon dioxide with a C value of 0 03.

TABLE 4. G VALIUES OF SOME PRODUCTS FORMED IN THE RADIOLYSIS OF AQUEOUS

THIOUREA (500 mg/l. = 5*68 mM) SOLUTIONS AT 1 Mrad/h IN VACUO

molar ratio: conen. product

dose of solution product initial analysis for (Mrad) (mg/i.) (mM) thiourea G value

sulphate 12-5 87*0 0.91 0.138 0 07 20 158*8 1*66 0*252 0.08 40 314*2 3.27 0*497 0 09

ammonium salt 12-5 20*7 1l15 0.174 0 09 20 46*5 2*58 0*392 0*12 40 101*6 5.65 0*858 014

sulphur 12-5 14.2 0 44 0-067 0 03 20 20*6 0O64 0.097 0*03 40 70-2 2*19 0*332 0 05

carbon dioxide 40 106*3 2.42 0*367 0 03

DISCUSSION

The destruction of thiourea in evacuated dilute aqueous solutions occurs with an initial C value between 0 34 and 0-61. These values are little affected by a 50- fold range in concentration and a 100-fold range in intensity. Uncertainties arise from unknown background absorptions which become large at high doses and low initial thiourea concentrations. The G values quoted here for destruction of thiourea in oxygen free solution are considerably smaller than were observed in the protective effect of thiourea in aqueous solutions of polymer. Thus in 5 % polyvinylpyrrolidone with 50 mg/l. of thiourea the G value for crosslinking falls from 1-8 to 0-61. The difference cannot be accounted for by the destruction of thiourea since this occurs with a C value of 0*34 without competition from the polymer. The suggestion we previously put forward involves some degree of non- destructive protection by thiourea. That this C value does not decrease at very low thiourea concentrations indicates that even at 10 mg/l. it is fully effective at capturing at least one of the radiolytic products of water.

The nature of this scavengeable entity from water is not known as yet, but further work is proceeding using pulse radiolysis. It is significant that in vacuum




the oxygen content of soluble breakdown products of thiourea is greatly increased indicating that OH radicals play a major role in this reaction. The hydrogen concentration remains approximately constant, so the H- atoms are mainly involved in abstraction and addition reactions. In the presence of excess oxygen the destruction of thiourea is more rapid than in vacuum (as previously found by Dale & Davies I957). The G value is 0-8 at 0 5 Mrad as compared with a G value of 0 34 in vacuo. The latter value is close to that found for initiation of ionic reactions in radiation induced polymerization and may therefore be associated with electrons ejected during ionization. Preliminatry experiments with pulse radiolysis have shown that in the presence of thiourea the concentration of solvated electrons is considerably reduced.

In investigating the chemical reactions involved we must consider two stages; in the first the main reaction is the destruction of thiourea. This occurs up to a dose (Mrad) of about 002 c where c is the concentration (mg/l.). Subsequently the breakdown products are themselves destroyed by the inidirect effect of radiation to give secondary products. Ultraviolet investigations gave no positive identification of the radiolysis products of thiourea. Paper chromatographic analysis of the evacuated samples revealed the presence of cyanamide, guanyl thiourea, dicyanamide, sulphur, and possibly guanidine sulphate up to doses of 1 5 Mrad, so these may be considered as possible first breakdown products. At higher doses all these products except sulphur, in turn disappear to give a possible five components which could not be identified.

The quantitative elemental analysis of the residues indicates that sulphur is precipitated in increasing amounts with dose. The G values for sulphur precipitation in our case are very small, of the order of 0 04 indicating that sulphur precipitation does not appear to be a main process, but possibly occurs as a side reaction, i.e. from the breakdown of an intermediate such as formamidine disulphide which is only stable in strong acid conditions (Storch I890; Werner I94I; Dale & Davies 1957). This disulphide would be expected to be formed during the radiolysis of evacuated aqueous thiourea solutions. Gravimetric analysis indicates that sulphate formation is a more important reaction as opposed to sulphur precipitation. Further, since sulphur irradiated in water in vacuum yields no sulphate, the two processes must proceed by different routes.

A sharp decrease in pH results with increasing dose, which after 15 Mrad slowly increases. This suggests that a large proportion of the initial radiation products are oxidized to sulphur derivatives of an acidic nature which become further oxidized to sulphate as the radiation dose increases. No ammonia or hydrogen sulphide is evolved during irradiation. However, ammonium salts are present in the soluble residue. The G value for ammonium salt production (0.09 to 0.14) seems to indicate that deammination is an important process as has been shown for many other amino compounds (Swallow i960). The conversion of the carbon to carbon dioxide proceeds with a G value of 0 03.

In the presence of oxygen, the destruction of thiourea is accompanied by a rapid decrease in pH. Paper chromatographic analysis indicates that different products are formed from those discussed above. Of these only two compounds were identifiable




at 0.5 Mrad, namely, thiourea dioxide and guanyl thiourea. The former compound is not present when thiourea is irradiated in vacuo. It is notable that thiourea dioxide can be readily prepared by the action of hydrogen peroxide on thiourea under suitable conditions (Kitamura 1939). The G value for hydrogen peroxide formation in oxygenated water is 0*67 and it is reasonable that the thiourea dioxide is produced by the following oxidation reaction:

H2N H2N 0

C=S+21H202 -+- C-S +2H20

H2N H2N 0

Thiourea dioxide can exist in two tautomeric forms

HN OH 112N 0

C-s \ * - (c==s

H2N 0 H2N 0

and its acidity could account for the more rapid decrease in pH in the oxygenated samples.

In the presence of oxygen sulphur precipitation was very small (probably less than 1 % of the amount formed under similar radiation conditions in the absence of oxygen), insufficient for chemical micro-analysis. Dale & Davies (I957) observed large G values for sulphur precipitation in the presence of oxygen; they stressed however the large dose rate dependence of GCS), the values being drastically reduced at higher dose rates.

A qualitative test confirmed the presence of sulphate, however, the extremely small samples were insufficient for a quantitative estimation. The remaining spots at 1, 2 and 4 Mrad were not identified.

The analysis is far from complete due to the limitations of microchemical analysis techniques for such dilute solutions. In the absence of oxygen and for a dose of 40Mrad (500 mg/l. thiourea) the following picture represents our present knowledge:

H2N 500/% sulphur as (S04)2- 2

\ / 80% of 43% nlitrogen as NH+ C- C=S total

H/1 sulphur prec dsulphur H2N/ 30%0/ sulphurprecipitateJ CO2 (20% of total carbon)

The remaining percentages of nitrogen, sulphur and carbon are probably taken up by the unidentified products.

CONCLUSION

The destruction of dilute aqueous solutions (10, 50 and 500 mg/l.) of thiourea irradiated in vacuum proceeds with an initial G value between 0'34 and 0-61. These G values are only slightly affected by a 50-fold concentration range, and a 100-fold range in intensity, indicating that even at very low concentrations thiourea is very effective in scavenging at least one of the radiolytic products in




water. This results in the destruction of thiourea to give breakdown products which can compete with thiourea as a scavenger. Some identification of these breakdown products has been given. The result is a reduction in pH of the solution.

Further irradiation in turn destroys some of these breakdown products, pro- ducing a rise in pH. The dose at which the reactions of the breakdown products first predominate over thiourea depends on its concentration, and corresponds to about 002c Mrad (c in mg/I.).

In the presence of oxygen, thiourea is destroyed with a higher efficiency G(-thiourea)

08, and different products are formed. It has previously been deduced that in macromolecular solutions much of the

protective ability of thiourea is due to a thiol-thione equilibrium, whereby H atoms may be temporarily gained or lost to repair polymer radicals. The much higher G values for this protective reaction, as compared with C(thiourea) derived here confirms this view.

REFERENCES

Charlesby, A. & Kopp, P. M. i962 Int. J. Radiat. Biol. 5, 521. Charlesby, A. & Kopp, P. M. I966 (In the press.) Dale, W. M. 1950 Conference on the Biological Hazards of Atomic Energy. New York: Wiley

and Sons (1952). Dale, W. M. 1956 Nature, Lond. 177, 531. Dale, W. M. & Davies, J. V. I952 Disc. Faraday Soc. 12, 293. Dale, W. M. & Davies, J. V. 1956 Progress in Radiobiology, p. 119. Edinburgh: Oliver and

Boyd. Dale, W. M. & Davies, J. V. 1957 Radiat. Res. 7, 35. Dale, W. M. & Davies, J. V. I959 Int. J. Radiat. Biol. 1, 189. Dale, W. M., Davies, J. V. & Meredith, W. J. I1949 Brit. J. Cancer, 3, 31. Kitamura, R. 1939 J. Pharm. Soc. Japan 59, 33. Kopp, P. M. & Charlesby, A. I963 Int. J. Radiat. Biol. 7, 174. Milks, J. E. & Janes, R. H. I956 Analyt. Chem. 28, 846. Storch, L. I890 Monatsh, Chem. 11, 452. Swallow, A. J. I960 Radiation chemistry of organic compounds. Oxford: Pergamon Press. Vogel, A. I. I96I Textbook of quantitative inorganic analysis. London: Longmans. Werner, A. E. A. I94I Sci. Proc. R. Dublin Soc. 22, 387.

9-2



Documents

Radiolysis of Dilute Aqueous Thiourea Solutions