 We have previously discussed about the actinides, their production, some of the actinides their discovery and also their electronic configuration. Now, as I have already mentioned before, the actinides are the relevance in the nuclear fuel cycle. So, we need to understand the chemistry of actinides particularly those which are relevant for the nuclear fuel cycle. So, in this lecture, I will be discussing about the chemistry of uranium to americium, that means uranium, leptinium, plutonium and americium. These four elements mainly, they are great relevance in the nuclear fuel cycle. Other details will be covered in separate lectures. This lecture is just a broad chemistry of uranium to americium. Now, when we are talking about the chemistry of actinides, we also should know about their complexation, that is the interaction of actinide ions with the ligands which are mostly levis based ion. The interaction with actinides are mainly electrostatic, that is ion ion or ion dipole. So, all the actinide metal ions are ions and we have the ligands which can be neutral ligand, in such case we have an ion dipole interaction and if it is an ionic ligand, then it is an ion interaction. Now, this interaction energy is a measure of the stability constant and it is proportional to the charge on the metal ion and also inversely proportional to the distance between the ions. I have shown some schematic here in which the different actinide ions are represented by either plus or plus plus, that is multiple charge metal ions and the ligand for simplicity I have taken just minus for all cases. Now, I have taken four cases where the metal ions size, I have also tried to show the size of the metal ion, the first case the size of the metal ion is bigger than that of the second one where the size is smaller and in case of the third one the ligand size is bigger than that of the metal ion and also in the fourth case we have the doubly charged metal ion which has a bigger size than that of the donor group of the ligand. As you can see, the radius between the metal ion and the ligating ion is varying depending on the size of the metal ion. So, now as I have mentioned the stability of the metal complexes will depend not only on the charge but also the distance between the ions. Now, actinides as we know that they are from actinium to Lorentzium, now this size of the actinides when we take a particular ionic species say plus 3 oxidation state then the size of the actinides will be changing. This phenomena is called the actinide contraction that is the ionic radii decreases with increasing the charge of the actinide ions. Now, this is similar to the lanthanide contraction and it is as a great significance in the complexation that means the heavier actinide will form a stronger complex compared to the lighter actinide. So, this is what will be discussed in subsequent part of this lecture. So, actinide also the ionic species as I will be discussing there are different type of species. Now, considering that the actinides have the oxidation state say from plus 3 to plus 6, these are the very commonly available oxidation state of the actinides then you can see that if it is a plus 6 oxidation state will form a definitely stronger complex than the plus 5 oxidation state which will form inter a stronger complex with the plus 4 oxidation state which will form a stronger complexation than the plus 3 oxidation state. The actinides plus 6 oxidation state and actinide in the plus 5 oxidation state they are extremely reactive in water and they do not exist at the plus 6 oxidation state the actinide plus 6, but they exist as the actiniline that is anO2 2 plus similarly an5 plus ion also in water medium it is metabolized and it is forming the species like anO2 plus. So, these are the ionic species for the plus 6 oxidation state that is anO2 2 plus and the plus 5 oxidation state that is anO2 plus. As will be discussed in subsequent lectures the charge and the plus 6 oxidation state this actiniline is little more than 3 plus and that of the plus 5 oxidation state actiniline that is anO2 plus is little higher than plus 2 that means the observed reactivity while forming the complexes for the actinine ion in the plus 6 oxidation state will be little lower than that of the plus 4 actinidine and it will be somewhat higher than that of the plus 3 oxidation state. Similarly, the actiniline in the plus 5 oxidation state that is anO2 plus will have somewhat higher reactivity compared to the plus 2 oxidation state and somewhat lower reactivity than the plus 3 oxidation state. So, the complexation activity of the actinide will be discussed in a subsequent lecture. So, before that we will be discussing the general chemistry of the actinides. The right-hand figure gives a comparison of the plus 3 plus 4 as well as plus 5 oxidation states of the actinides and it shows the ionic radii in picometer which is the atomic number of the actinides and as can be seen this ionic radii is continuously decreasing for the plus 3 oxidation state and plus 4 oxidation state also for plus 5 oxidation state also we see again some relatively less tip decrease but nevertheless there is decrease in the ionic side, ionic radius. Now, coming to the different ionic species of the actinides as I have mentioned in the previous slide. The lower oxidation states such as plus 3 plus 4 and plus 2. So, there is relatively less hydrolysis in the bottom medium and the actinide ions exist in those charged states. You can see for example, mericium 3 plus, plutonium 4 plus and nobelium 2 plus. So, these are existing these plus 3 plus 4 and plus 2 oxidation charged species in the actinide medium. On the other hand, the higher oxidation states show the formation of the oxo species as shown below. The MO 2 plus and MO 2 2 plus exist as symmetrical nearly linear structure where the MO bond order is around 2 for the bond length of around 1.7 between the metal ion and the oxygen and then due to complexation MO bond order is lower that is if it is around 1.25 for the bond length is also elongated and we have 1.95 angstrom at the MO bond length. This ionic species like N3 plus gets oxidized to N4 plus. Similarly, MO 2 plus gets oxidized to MO 2 plus and these couples are reversible and are fast reactions are possible. On the other hand, the N4 plus to N4 2 plus that is the tetravalent actinide to the pentavalent actinide couple is slow and it is not reversible. This is because of the bond formation between the metal and the oxygen and this is a slow process and that is why this is not reversible couple. For protactinium 5 though one can predict species like PaO2 plus similar to that of neptunium pentavalent or plutonium pentavalent species but the reported species is PaOOH2 plus. So, this is the actual species which is existing and then reported in the aqueous medium. Plutonium in the plus 7 oxidation state is not stable in the normal acidic aqueous medium and is stable only in the alkaline medium the species being MO5 3 minus. All these actinide ions they have different colors if taken in appreciable concentrations and I have given a table here where you can see the color of the actinides. For example, uranium in the plus 3 state it is red color, uranium in the plus 4 states is the green color, uranium in plus 5 state is colorless and uranium in plus 6 state is the well known yellow color which we see most of the uranium compounds yellow color. Neptunium plus 3 state is a blue purple plus 4 state it is yellow green plus 5 state that is MO2 plus that is neptunium O2 plus this is a green color NPO2 2 plus this species neptunium 6 oxidation state it is pink red and the anionic species of neptunium 7 that is NPO5 3 minus in alkaline medium is dark green color. Same also for plutonium in the plus 7 oxidation state that also we get the anionic species PUO5 3 minus and also we get a dark green color similar to the neptunium plus 7 oxidation state. But generally we do not deal with the alkaline medium in the nuclear fuel cycle that is why there are only academic interest for the neptunium and plutonium plus 7. On the other hand the other oxidation states are of great relevance for example, for plutonium mostly plutonium 3 plus that is color of blue to violet color plutonium 4 plus which has a tan orange to brown color plutonium 5 that is PUO2 plus it has a reddish purple color and plutonium 6 that is PUO2 2 plus it has a yellow orange color. So these are the ionic species of plutonium and americium as mostly americium 3 plus as the ionic species that is having a color of pink or yellow color depending on the medium americium 4 plus can also be obtained it has a red brown color americium 5 plus can be done by electrochemical oxidation of americium it has a yellow color americium 6 that is AMO2 2 plus as a rum color in the active solution. Curium exist in plus 3 oxidation state and it has a pale green color bercelium plus 3 oxidation state in the green color. Bercelium is also existing in plus 4 oxidation state and it has a local californium as a green color. I am covering only up to this because other actinides are produced at a very low concentration so we will not be dealing with it. I summarize now the chemistry of these early actinides that chemistry of iranium as mentioned in the previous slide iranium exist in oxidation state plus 3 to plus 6 but the plus 6 oxidation state is most common that is UO2 2 plus ionic species being the most stable and we are dealing with now chemistry labs iranium in the plus 6 oxidation state. Iranium 3 plus it is a very strong reducing agent because it is not at all stable and in view of its soft nature it can form organometallic compounds that play with ligands like cyclopentadonyl and this UCP whole thrice this type of complexes are reported for iranium. Iranium 4 plus is obtained by reducing iranium 6 oxidation state that is iranilion by passing this yellow iranilion solution through a zinc amalgam column and it produces the dark green iranium 4 as it is shown in this. Now this iranium 4 also forms the cyclopentadonyl complex and the structure is given here where you have the four cyclopentadonyl ligand coordinating to the iranium plus 4 species and this can be reduced to give this iranium CP3 that is the cyclopentadonyl complex of the iranium plus 3 oxidation state. These organometallic compounds are very very unstable in air and you have to take special precursor to exert them. As I mentioned iranium plus 6 oxidation state that is Uo2 2 plus is the most stable oxidation state of iranium and in this case you basically you are having actually iranium with two axial oxygens and there is a equatorial plane which is available for the coordination by the ligand atoms. So, complexation of iranium ion in solution will be discussed in a subsequent lecture. I have shown here the spectra of the iranial ion see generally you get this type of pattern where you have the different bands are there and at appreciable concentration of iranium this can be used for estimation of iranium but if the low concentration of iranium is there then we have to go for different methods by spectrophotometry what we can do is we can use a ring agent and then we can measure optical density to find out the concentration of iranium ion. Iranial ion gets hydrolyzed in aqueous medium the pH is around 4 to 6 that region also gets hydrolyzed and at high pH values it forms dimeric or polymeric space. I have shown this species and diagram of iranium you can see here and up to pH 4 it is iranial ion is stable and at increasing pH values you have the other species the Uo2 OH plus Uo2 OH 2 is a neutral species and also you have subsequently you have this dimeric and polymeric species at higher pH now coming to the chemistry of neptunium. Neptunium has the electronic configuration as I mentioned is IVF4 6D1 and 7S2 so as per this if all electron valence electrons are removed then you can have neptunium 7 plus 7 species which is an anionic species I have just discussed in the previous previous slide and such species are present in alkaline medium on the other hand you can also have neptunium in the plus 4 plus 3 that will be less stable and plus 5 and plus 6 oxidized. As I have already mentioned neptunium is the first trans-iranium element discovered by the reaction of 238 iranium the neutron capture giving iranium 239 which undergoes beta decay to produce 239 neptunium this is how we get this neptunium 239 in the reactor and this is used in the laboratory studies a very good quality tracer is obtained other important isotopes are neptunium 237 237 neptunium particularly is very important because it has a very long half-life now in the radioactive waste management this neptunium 237 has to be necessarily separated otherwise the waste has to be kept under the geological repository and the surveillance period will be very very long that I will be discussing in a separate lecture this neptunium 239 as I have mentioned it has a application that it decays to give neptunium 239 which has lot of relevance because it is a precise isotope of neptunium. Now in the nuclear reactor we can get 237 by these two reactions mentioned here 235 iranium we can get as well as from 238 iranium we can get by capture of past neutrons for the 238 iranium and also by capture of thermal neutron we can get it for 235 iranium and we get the 237 iranium for the 235 iranium which undergoes beta decay to give 237 neptunium similarly for the 238 iranium capture of past neutron gives 239 iranium and it emits two neutron to become 237 iranium which undergoes beta decay to give the 237 neptunium also the decay product of 241 americium by alpha decay can give 237 neptunium so these are the sources of 237 neptunium and the separation of this neptunium as I mentioned it is required for this radioactive waste management purposes and while doing this this valency state of neptunium has to be adjusted so that this separation can be done by solenoid extraction or ionic system of methods. Neptunium can be converted to the plus per valency by ferrous sulphamide. Neptunium can be oxidized to the plus 6 oxidation state by dichromate and this neptunium plus 4 and plus 6 these two ionic species are possible to extract using 30 percent tbp in a non-polar like odorless kerosene or dodecan and I have shown this figure here the right side where you see the neptunium 4 how it is extracted as a function of the nitric acid concentration and neptunium 6 extraction also is given but lower nitric acid concentration neptunium 6 of extraction is higher and at higher nitric acid concentration neptunium 4 extraction is higher so neptunium 4 and 6 can be selectively separated from nitric acid solution by 30 percent tbp and tuning the nitric acid concentration on the contrary if you see the neptunium 5 extraction it is much much less because neptunium 5 ionic potential is much lower and also it has charged little higher than that of plus 2 that is why the interaction with tbp is less. Neptunium 5 it is a very important for carrying out lot of studies in the environment the environmental chemistry of neptunium is mostly focused on the neptunium 5 that is npo2 plus which is very very mobile in the environmental conditions and lot of studies in the laboratory can be carried out using npo2 plus which is prepared by using sodium nitrite or even by Fe3. Now this neptunium tbp whatever I have shown the extraction it forms this extraction equilibrium is operative that means 1 neptunium 4 plus ion with 4 nitrate ions and 2 tbp this is neptunium 4 is in the aqueous phase the nitrate ions are in the also in the aqueous phase the tbp is in the organic phase and this this is neptunium NO3 4 times and 2 tbp this goes to the organic phase so that is how the extraction is done. Now this how it is detected because if it is converted to neptunium 4 plus then we can detect by spectrophotometry there are very characteristics peaks of neptunium 4 plus and this is at one peak is there at 704 nanometer as shown in this extreme left table and another peak is there at 961 nanometer so there are two peaks characteristic of neptunium 4 plus and then as shown that profile of this neptunium 4 extraction is shown as a function of nitric acid concentration and this extra liquid is spectrophotometry is done then you get the 704 nanometer peak is slightly shifted to 703 nanometer and this suggests that neptunium 4 is present in the organic phase. Neptunium 4 is relatively weaker acid as compared to both uranium 4 and plutonium 4 hence the hydrolysis chance of neptunium 4 plus ion in the actose medium is relatively lower and you can find neptunium 4 plus being stable even at point 1 molar nitric acid but if you go to lower nitric acid concentration it gets hydrolyzed for example at point 0 1 molar nitric acid concentration neptunium 4 plus gets hydrolyzed hydrolysis. On the other hand if you want to work with the plutonium 4 and uranium 4 ions that is p u 4 plus and u 4 plus ions then point 1 molar nitric acid also they get hydrolyzed so for safe first side we go for point 5 molar nitric acid to carry out the studies of uranium 4 plus and p u 4. Continuing the chemistry of neptunium as I have already mentioned neptunium the interest is in the neptunial ion that in the plus 5 oxidation state which is also the most stable oxidation state of neptunium and plus 5 and plus 6 oxidation states they are forming dihexocatans plus 7 oxidation state actually only in the basic medium and neptunium ions they are actually having undergoing disproportionation that is as I have shown here 2 neptunium 5 plus ions in the presence of strong acid conditions they disproportionate one of them becoming neptunium 4 plus ion and the second ion is becoming neptunium 6 plus ion and that is how this disproportionation reaction is taking place it has a equilibrium constant of minus 6.72 this is a slow kinetics of this disproportion reaction due to the neptunium O on formation and breaking both cases one case actually we are finding this one of the NP O2 plus ion is getting converted to NP O2 2 plus so this is not a slow process but the NP O2 plus ion converted to NP 4 plus is a slow process because that the neptunium oxygen bond breaking is called this also there are other disproportionation reactions like neptunium 4 and neptunium 6 they undergo simultaneous delboxylation reduction reaction as the reaction is given here and this is just the opposite of this equilibrium reaction I have given at the top which is a disproportion reaction of neptunium 5 and at low acidity that is at high pH neptunium 5 is dominant and we can carry out study without bothering much about its stability this is in short contrast to the iranium 5 and plutonium 5 species which readily disproportionate even at low acid solutions and neptunium hydrolysis strain is neptunium 4 gets easily hydrolyzed which is higher than that of neptunium 6 that is the neptunilion NP O2 2 plus which is again higher than that of NP 3 plus and which in turn is higher than the NP O2 plus that is neptunium 5 they follow the ionic potential trend of these ions this neptunium 3 it is very very unstable but it can be made stable at pH 4 to 5 under a very very special condition otherwise this gets quickly upside down to neptunium 4 in air neptunium 4 as I mentioned it gets hydrolyzed very easily ion species of the type NP H2O twice OH 3 plus these type of species are palm neptunium 5 this hydrolyzed above pH 7 to form species like NP O2 OH whole twice minus this type of species neptunium 6 this is stable only below pH 3 to 4 otherwise it gets hydrolyzed and neptunium 7 clean up now coming to plutonium it exists plus 3 to plus 7 oxidation states so there are 5 oxidation states all these oxidation states can be prepared in the laboratory plutonium 3 and plutonium 4 are more stable in acidic solution plutonium 5 is stable in near neutral solution but it disappears and it's ready so therefore very direct solutions are favored for plutonium 5 plutonium 6 is favored in acidic solution also plutonium 7 also is stable only in highly basic medium and strong oxidizing conditions plutonium 8 has been reported but not confirmed redox potentials plutonium potentials close to 1 volt for all these plutonium oxidation states this suggests that plutonium undergoes disproper solution as the reactions are given here and see that plutonium 4 plus 3 plutonium 4 plus ions undergo disproper solution giving 2 plutonium 3 plus ions and 1 plutonium 6 that is pu o2 2 plus ion and the log k is minus 2.08 similar also is the case for the plutonium 5 oxidation state that is pu o2 plus again 3 of those ions under highly acidic conditions they give rise to plutonium 3 plus and also plutonium 6 plutonium 5 also can undergo this proportionation to give 1 plutonium 4 plus ion and 1 plutonium 6 plus ion and this has a slightly lower log k values for this equilibrium that is 4.20 this suggests that plutonium 4 plus plutonium 5 plus ions can be plutonium 5 that is pu o2 plus ions can be easily underwent disproper solution under the log k conditions the absorption factor of plutonium will be taken up in a separate lecture hydrolysis of plutonium plutonium 4 is greater than data plutonium 6 which is greater than data plutonium 3 and this follows the ionic potential trend and plutonium 3 is stable below pH 4 and it is obtained by reducing with hydroxylamine hydrochloride or hydrogen plutonium 4 is stable above 0.5 molar acid and plutonium 4 is produced by the nitrite ions usually by NaNO2 which is used which converts all the plutonium from plus 3 to plus from plus 3 to plus 4 state and also from plus 6 to plus 4 state so nitrate has a unique role of oxidizing plutonium 3 to plutonium 4 and also reducing plutonium 6 to plutonium 4 and plutonium 6 this production can be done below pH 3 by treatment with hot boiling HClO4 or by oxidation by silver oxide or ozone coming to americium we know this americium oxidation state mostly common is plus 3 oxidation state and this is based on this electrone configuration 5 F7 7 H2 1 would expect that americium should be stable in the plus 2 oxidation state as well because you have the 5 F7 electrone configuration 5 or americium 2 plus is not commonly existing in the aqua solutions and in very rare cases in solids only you can important isotopes of americium are 241 americium such a half life of 432 years and it is 5 meter it also has a characteristic 60 kV gamma ray americium 243 it has a half life of 7370 years used mostly for studies where this radiolysis possibilities are to be lesser than that of americium source of americium is already discussed so I will not mention it further americium 241 it is used as a neutron source along with beryllium this is called americium beryllium source the reaction is given here americium separation it is very very important so far the radioactive waste management is concerned it is not so important in the nuclear ql reprocessing the extraction equilibrium for americium extraction by tbp is given here where you have americium 3 plus in the aqua space and 3 nitrate again in the aqua space the 3 tbp in the organic phase going to americium nitrate thrice and 3 tbp this is the species in the organic phase under what condition americium can be extracted by tbp it can be done by using nitrate salts that means we can have sodium nitrate high concentration without having nitric acid and then under that condition we can extract americium 3 plus coming to the chemistry of americium any species americium is mostly present in the plus 3 oxidation state plus 2 oxidation state also is reported using some of the pulse radiolysis studies where americium 2 plus is produced but it is not very stable curium 2 plus and californium 2 plus are also formed in a similar manner americium 4 plus is again an unusual oxidation state of americium and it can be produced by electrolytic oxidation or a strong oxidizing agent like ammonium persulphate but requires complexation to stabilize americium can be oxidized to the plus 5 as well as plus 6 oxidation states where it gets species like am o 2 plus for the plus 5 oxidation state and am o 2 2 plus for the plus 6 oxidation state and this oxidation of americium to the plus 6 oxidation state is exploited for the suppression of americium from curium in a process called cisam process which will be discussed in a subsequent lecture americium 5 that is am o 2 plus it disproportionates in the same way as of other actinial ions like leptinium, iranium and clupanium in their pentavalent state in aqueous ligand they behave also americium 4 plus it also undergoes disproportionation I have mentioned here this americium 3 americium 4 plus plus 2 H2O giving 2 americium 3 plus and americium o 2 2 plus and as shown in the lower figure this disproportionation of americium from 6 molar SCO4 is given here where you find that the profile of americium 5 is slowly decreasing and after 2 hours you can see that it becomes almost one third less than one third that we started with and you have this growing profile of americium 6 as well as americium 3. Now, coming to the chemistry of americium in the trivalent state it is very similar to that of lanthanides we will find that this americium creates lot of problem in the radioactive waste management because of the similarity to the trivalent lanthanides. So, lanthanide actinide separation is one of the very important steps in the radioactive waste management also this americium chemistry is very similar to that of curium and so it was really very difficult to separate and that is how C bar faced lot of problem in the separation of the americium and curium and it was being discovered then absorption spectra of americium the strong absorption band they have shown here at 503 nanometer this characteristic of americium 3 plus is due to the transition to 7F025L6 transition which has epsilon value of 400 cytolysis and complexation of americium will be taken up in a subsequent lecture