The adsorption of thorium has been determined on freshly prepared, coagulated sols of silver halides and silver thiocyanate. The amount of adsorbed thorium has been measured radiometrically by addition of thorium-234. It was found that the adsorption of thorium was much stronger on positively charged sols; on the negatively charged sols the adsorption was much less pronounced . The acidity also influences the adsorption, so that it is higher at greates pH values (measured up to pH = 4.5). The adsorption is nearly equal on coagulated sols of silver chloride, silver bromide and silver iodide; on silver thiocyanate is considerably less

K. F. Schulz

M. J. Herak

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C R 0 A T I C A C H E M I C A A C T A 30 (1958) 1 ~7 CCA-126 541.183.24:546.841 :546.571.2 Adsorption of Thorium on Silver Halides and Silver Thiocyanate So ls K. F. ·Schulz and M. J. Herak Institute for M edical Research, Yugoslav Academy of Sciences and /\.rts, an d Institute »Ruder Boskovic« Zagreb, Croatia, Yugoslavia Received May 23, 1958 The adsorption of thorium has been determined on freshly prepared, coagulated sols of silver halides and silver thiocyanate. The amount of adsorbed thorium has been measured radio-metrically by addition of thorium-234. It was found that the adsorption of thorium was much stronger on positively charged sols; on the negatively charged sols the adsorption was much less pronounced. The acidity also influences the adsorption, so that it is higher at greates pH values (measured up to pH = 4.5). The adsorption is nearly equal on coagulated sols of silver chloride, silver bromide and silver iodide ; on silver thiocyanate is considerably less. The presence of sulphate ion prevents the adsorption of thorium on negatively charged sols, and there is no effect on positive sols. The adsorption equilibrium was established in less than 5 minutes. When the coagulation of lyphobic sols is carried out with neutral electro-lyte solutions of highly charged counterions - especially with trivalent and tetravalent ions - usually the phenomenon of so called »irregular series« will be observed; instead of one coagulation region we have two regions. At high and low concentrations of electrolyte it comes to the coagulation, medial values on the contrary stabilize the sol. This phenomenon can be formally explained: if coagulation ions are strongly charged it is necessary to add only a litle amount of electrolyte to coagulate the sol. With slightly higher concentrations there will be no coagulation and the charge of the sol will be reversed. Only considerably higher concentrations of the electrolyte can coagulate again. And now the coagulating ion is no more the highly charged counter-ion but usuall:y the lower charged accompanying ion. The mechanism of the reversal of the charge of the lyophobic sols is not yet quite explained. Generally it is considered that the products of hydrolysis of these highly charged counter-ions are responsible for the phenomenon of the reversal of the charge1• This might prove the fact that the change of charge of sols will occur in neutral and alkaline solutions but not in strongly acid solutions. On the other hand it is certain that the mechanism of the reversal of the charge with multivalent ions differs from that with great monovalent ions, as are the inorganic complex ions and great organic ions. The thorium ion, as highly charged ion, is very convenient for studying the mechanism of the reversal of charge of negative sols and it was often used for this purpose. In the first place we were interested in the adsorption of thorium on coagulated sols of 128 K. F. SCHULZ AND M. J. HERAK silver halides and silver thiocyanate hoping that by varying the conditions of the investigation some data could be obtained which will help to explain the action of thorium ions on these sols. The quantity of adsorbed thorium can be measured easily by adding the solutions radioactive thorium (Th-234). EXPERIMENTAL The systems investigated were prepared in the usual way2 by rruxmg 5 ml. of AgN03 solution with 5 ml. of a solution which contained the halogen component and NaN03 or Na2S04. The solution of Th(N03) 4 were added to the solution of pre-dpitating component in excess which contain€d also radioactive Th-234 isolated in a very simple way from U02(N03}2 as described by Dyrssena. To be able to separate the solid phase from the solution it was necessary to work in the coagulating region, and for this reason we added the necessary amount ·Of NaN03 and Na2S04 respectively. The precipitate was separated from the solution by centrifuging it and the activities were measured in the aliquot volume of the solutions. The counting was performed by a G.-M. counter (made in Institute »Ruder Boskovic«). A strong linear function between the concentration of thorium and .counted numebr of impulses was obtained. The activity of the systems was always determined against the standard solutions. These standard solutions were made without adding one of the precipitating components (silver nitrate or potassium or sodium halide) . When silver was in excess we did not add the halide component and when the halide was in excess silver nitrate was not added. The acctivities were measured half an hour after preparing the systems and after 24 hours. Generally there was no diff€rence ip. activities between these two readings. RESULTS On Fig. 1 the results are shown with the systems: KI-AgN03-Th(N03).-NaN03 and KI-AgN03- Th(N03)4- K 2S04 • The results were in a way unex-pected. The adsorption of thorium (final concentration in solution was 100 f---~~'.::::::...._---+---~K~J-,, 1~,10,.,..,(7Ag..,-N"'0,-,, 4"°".10"'·-0"'1,"'1 ......-, ----1 ~ AgNOjl•ld'~CKJ · 1.10·L4 162N 1 o No NQ1. 1•10-'N 6 No1SQ.. 3.irlO-'N \ Th(NO)~ hlO--sN 0 ~so f-----~1----<~---+-----+-----1 Q'. 0 "' 0 " ~o . o f-----~1-------+-"""'""-a~.l).- -~---1 · 1 LOG.CONC Ag NO,INI -l -1 LOG CONC KJI NI Fig. 1. Adsorption of thorium on freshly coagulated sols of silver iodide in presence of 0.01 N NaN0 3 and 0.03 N Na,so4 respectively. The concentration of Ag! was 0.001 M and the concentration of Th in the systems· was 1 X 10-s N. 1 X 10-6 N) was stronger on positively charged sols than on negatively charged · sols. The adsorption increased with excess of Ag+ ions, so that at an excess of ·0.039 M nearly 1000/o of the added amount of thorium was adsorbed. At nega-tively charged sols, where we may expect a greater adsorption, it was signi-ficantly lower. There was also a great influence of the accompanying ion on the negative sols. In the presence of sulphate ions there were no adsorption at all. On the positive side there was no difference in adsorption. ADSORPTION OF THORIUM 129 Because of the great influence of acidity on the precipitation of silver halides in presence of thorium4 we also measured the adsorption at different pH. The results are shown on Fig. 2. The higher the pH the greater the 100 0 50 w CD a:: 0 vi 0 <{ 0 ' I . 3 1-() .AgNO, 4xl0- N KJ lx10 N Th(NOJ)4 lxl0.5 Ni NaNOJ 2x10.'N f() 0 AgN01 hll·' N KJ 4x!0.1 N () Th(''""''".'"·"""'•~.,··:; 6.AgNO 1 xlO -~ N KJ MO N J s -2 Th (N03)4 lxlO - N' Na~ .. 3x10 N ---t-7 I y / () I 0 ~-- ~.0. ~I 6 I 0 2 pH Fig. 2. Influence of pH on the adsorption of thorium on freshly coagulated sols of silver iodide in presence of 0.01 N NaNOa and 0.03 N Na2SO• respectively. The concentration of AgI · was 0.001 M and the excess of Ag+ or I- was 0.039 N. 100 '------- Th(N03)4 :lxlQ-sN , NaN03. 2x1o"'N; I o NH4CNS lxlO ~:N () NaQ 1x10 N AgN~ ·MO-"N I • KJ :1x10-3N <---.. D.. NaBr ·1x10-3N ' --c. . IJ':---....~ ~ I : so '--- - ----w OJ a: 0 vi 0 4 o~o o~~~~L~-.] -2 LOG CONC AgN03 (N) -3 -2 .lDG CONC KJ; et; Br: CNS 0 (N) Fig. 3. Determination of the adsorption of thorium on coagulated sols of AgCl, AgBr, AgI, and AgSCN respectively. The concentration of added NaNOa was 0,2 N . The amount of solid phase was 0.001 M and the concentration of thorium in the sy stems 1 X 10- ' N. adsorption on positive sols, and at pH = 4 the adsorption was nearly 1000/o with respect to the amount of thorium. On the negatively charged sols we found the adsorption of thorium only at pH higher than 3. In the presence of sulphate 130 K. F. SCHULZ AND M. J. HERAK ion there was no adsorption, regardless of the pH. The measurements were carried out at the pH values lower than 4.5; at the higher there is a significant precipitation of thorium because of hydrolysis (see e. g.5). We varied the pH by addition of nitric acid and measured it on a Beckman pH-meter with a glass electrode. I I 10~5 100 -------------0 0 0 . I ~·-· "" % /I ~-.r:; -;! >-0 ~ w CD ;; so 10-6 er. 0 Ul 0 0 I ~ w '---0-~ CD ~ a: AgN~ : Gx10-1 N 0 (/) KJ : 1x10·1 N u 0 : 2x10-' N z 4: NaN OJ 0 I u 10-7 ci B I 9 10-~ -s 10 10-6 LOG CONC Th(N01'., (M) Fig. 4. Dependence of the adsorption of thorium on its concentration in solution. The concentration of Ag! was 0.001 M with an excess of Ag+ 0.039 N . ThlN~ 14 ·--- 1 · ll..s N 100 ..__...__NaNO, ,--- 2·1l-' N 0 HNO, -- 1 ·ll-4 N p-Ag---2 ~ §So -l---i----o~:__----+-·-----+---t 0 < I I p-H " <t>------A----- -A- ----A------- -------,. I ll-2 LOG. CONC AgJ<MJ 3,5 Fig. 5. Dependence of the adsorption of thorium on the amount of solid phase. The Ag! sols were prepared with an excess of Ag+ 0.01 N in· presence of 0.2 N of NaN03• The concentration of thorium in the systems was 1 X 10-s N. The comparison of the adsorption of thorium for different coagulated silver halide sols and silver thiocyanate sols was also interesting, as can be seen on Fig. 3. When the halide or thiocyanate component was in excess there was no adsorption under experimental conditions. When silver was added in excess the adsorption of thorium was nearly equal at AgCl and AgBr sols; the ADSORPTION OF THORIUM 131 adsorption curve for Ag! sols differs slightly from the curves for AgCl and AgBr sols. The adsorption on AgSCN sols is lower and begins at a greater amount of Ag+ ions, i.e. at higher positively charged sols. The dependency of the adsorption on the concentration of thorium is given on Fig. 4. The sol was positive and its concentration was 0.001 M with an excess of 0.039 M AgN03 and was coagulated with NaN03 (final concentration 0.2 N). As we expected the amount of adsorbed thorium increases with its concentration. The adsorption is constant at nearly 1X 10-5 N of Th and amounts to s x 10-s moles/I. when the quantity of the solid phase is 0.001 moles. At the concentrations of thorium larger than 0.0001 M the adsorbed should be determined on the precipitate and not in the solution. The concentration of thorium in the solution is in excess and therefore very little changed by adsorption. The precipitate should be completely freed from the solution. In this separation the change of adsorption equilibrium might take place. 50 -§.. ~ .r; 25 Cl w m C(. 0 I/) Cl < I I I I I I I I I I I I 0 0 " //'',,.. I I I I I I 0 J_Q AgNOi :Gx10"1 N ; Th(N01),. : 2x10"' N 2 LOG TIME IN MINUTES 0----c> KJ :lxlO·J N NaNOJ: 2x10· 1 N ) Fig. 6. Ad~.orption of thorium as a function of time. AgI was precipitated with an excess of Ag+ 0.039 N and in the presence of 0.2 N NaNOa. The concentration of thorium in the systems was 2 X rn-• N. When the quantity of the solid phase at a constant excess of Ag+ (pAg = 2) and a constant pH of 3.75 was varried the quantity of the adsorbed thorium increased and became steady at 0.022 M Ag!. In alll our systems the adsorption equilibrium was established in rather short time, after 5 minutes or even earlier (Fig. 6). The exact time we could not determine since we . needed 5 minutes to prepare the systems for measurement. DISCUSSION As we mentioned before tqe results were in a way surprising. We expected the adsorption of the positively charged thorium ion or its complex ion to be much stronger when sols are negatively charged, but it was not so. This phe-nomenon is now difficult to explain. On the positively charged sols the ad-sorption is the greater the more positively charged the sols are, i. e. the greater is the excess of the stabilizing ion. 132 K. F. SCHULZ AND M. J. HERAK At negatively charged sols the adsorption is less pronounced but it depends on the agents which influence the reversal of the charge: they are primarily the pH and the accompanying ion. The adsorption is greater at higher pH values; when the pH < 1 there is no adsorption either on the positively or the negatively charged sols. The adsorption reaches its maximum at pH approxi-mately equal to 4. This may indicate that the adsorption is connected with the formation of complex thorium ions. At still higher pH values the products of hydrolysis of thorium precipitate and we cannot measure the adsorption. The influence of pH is in accordance with the proposed mechanism for the reversal of the charge of highly charged ions. In strongly acid solutions positively charged complex products are not formed and the negative sols will not change the charge. In this pH region there is no adsorption. The influence of sulphate ions is very interesting. In presence of sulphate ions there is no adsorption of thorium on negative silver halide sols. On the positively charged sols there is no influence of the sulphate ion on the adsorp-tion. It is evident that the adsorption mechanism on positive sols differs from the mechanism of the negative sols. This influence of sulphate ion may be distinctly seen at the reversal of the charge on the negative silver ~alide sols with thorium. To accomplish the reversal of the charge in a sulphate medium we need much higher concentration of thorium, about 0.01 N . In the presence of nitrate ion the charge of the sol is changed! by addition of 8X 10-5 N of thorium4• This effect of the sulphate ion probably could be explained in this way that thorium builds sufficient stable complex Th(S0.) 2 ; the nitrate complexes are less stable6• In spite of the difference in the charge densities on the silver halide sols (which is increasing from AgCl to AgI), it is not especially pronounced at the adsorption of thorium. The exception is only the thiocyanate ion because the character of this ion differs from that of halide ions. The results show that the adsorption of thorium and related phenomena, as e. g. the reversal of the charge of sols, are complicated processes which are influenced by many factors. Some of these influences are not yet clear and therefore could not be explained. REFERENCES 1. H . R. Kruyt and S . A. Tro e 1 s t r a, KoHoid-Beih . 54 (1943) 262; J. Th. Ov er-b e e k in: CoHoid Science, edited by H- R. Kruyt, Amsterdam 1952, p . 314. 2. B. T e z a k, E. Mat i j e v i c and K. S c h u 1 z, J. Phys. & CoUoid Chem. 55 (1955) 1557. 3. D. D yr s s en, Svensk Kem. Tidskr. 62 (1950) 153. 4. K. F. S c h u 1 z, to be published. 5. Sirrkka Hie tan en, Acta Chem. Scand. 8 (1954) 1626. 6. E . L. Z e bro ski, H . W. A 1 t er and F. K. H e um a n n , J . Am. Chem. Soc. 73 (1951) 5646. . IZVOD Adsorpcija torija na solovima argentum halogenida i argentum tiocijanata K. F. Schulz i M. J. Herak Adsorpcija torija bila je odredivana kod svjeze priredenih, koaguliranih solova argentum halogenida i argentum tiocijanata. Kolicina adsorbiranog torija bila je mj er ena radiometrijski dodatkom Th-234. ADSORPTION OF THORIUM 133 Nadeno je, da je adsorpcija torija mnogo jaea na pozitivno nabijenim solovima; na negativno nabijenim solovima ona je znatno slabija. Na adsorpciju utjeee i aciditet, tako da je adsorpcija veca kod visih pH vrijednosti (mjereno do pH = 4.5). Adsorpcija je gotovo jednaka na koaguliranim solovima argentum klorida, argentum bromida i argentum jodida; znatno je slabije izrazena na solovima argentum tiocijanata. Nazocnost sulfatnog iona potpuno sprecava adsorpciju torija na negativno nabi-jenim solovima, a bez efekta je kod pozitivno nabijenih solova. Adsorpciona ravno-tefa uspostavlja se u vremenu kracem od 5 min. INSTITUT Z<\ MEDICINSKA ISTRAZIVANJA JUGOSLA VENSKE AKADEMIJE ZNANOSTI I UMJETNOSTI I INSTITUT »RUDER BOSKOVIC« ZAGREB Primljeno 23. svibnja 1958. 

Adsorption of Thorium on Silver Halides and Silver Thiocyanate Sols

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