FR2909179A1 - Measuring the total complexing power of a solution comprising chemical complexing species, by adding cocations, extracting the mixture of cobalt cations in the complexed form and chemical complexing species, measuring the free cobalt ions - Google Patents

Abstract

Measuring the total complexing power of a solution of volume (V) comprising chemical complexing species, comprises adding a quantity (Q1) of cocations to a solution, waiting for duration to obtain a mixture comprising with their equilibrium concentrations of cobalt cations in the free and complexed forms, extraction of the mixture of cobalt cations in the complexed form and chemical complexing species, measuring the quantity (Q2) of such cobalt cations in free-form within the mixture, calculating the overall complexing power by dividing the difference between the (Q1) and (Q2) by volume. Measuring the total complexing power of a solution of volume (V) comprising several chemical complexing species, comprises addition of a quantity (Q1) of cocations (Co2>+>) to a solution, where (Q1) is excess compared to the sum of the quantities of the chemical complexing species, waiting for a sufficient time to obtain a mixture comprising with their equilibrium concentrations of cobalt cations (Co2>+>) in the free and complexed forms, extraction of the mixture of cobalt cations in the complexed form and chemical complexing species, measuring the quantity (Q2) of such cobalt cations in free-form within the mixture, calculating the overall complexing power by dividing the difference between the (Q1) and (Q2) by the volume.

Description

1 The present invention relates to the field of analytical methods

of a solution and relates in particular to a method for measuring the complexing power of a solution comprising several complexing chemical species.

In practice, it is essential to know the complexing power of a chemical species. This makes it possible, for example, to determine its ability to complex a cation and thus predict some of the harmful effects that this complexing chemical species may have on its environment. Such effects are for example the following: when it associates with a complexing chemical species to form a water-soluble complex, a cation (for example a heavy metal such as cobalt) migrates more easily in the environment and will thus pollute this last. This phenomenon, the amplitude of which is proportional to the complexing power of the complexing chemical species, is observed in particular in the leachate generated by percolation of rainwater through a discharge of household and / or industrial refuse. In addition, in the field of management of nuclear waste packages, this phenomenon can be accentuated by an increase over time in the complexing power of the chemical species contained in the leachate generated by these packages. This is due in particular to the fact that radiolysis and / or conditions favorable to chemical degradation destroy certain chemical species, in particular certain complexing chemical species, initially present in the leachate and produce new complexing chemical species, possibly with greater complexing power. . Knowing also that this modification of the composition of the leachate is most often accompanied by the release of flammable gases such as hydrogen, it becomes essential to be able to determine the evolution of the complexing power of a leachate generated by the waste. nuclear. - Molecular biology, in particular Nuclear Magnetic Resonance Imaging (MRI), uses contrast agents consisting of metal cation complexes. However, these complexes have an impact on the organism which is proportional to the complexing power of the complexing chemical species used. Many methods exist to date for determining the complexing power of a chemical species. They most often consist in measuring the equilibrium constant of a complexation reaction (hereinafter called complexation constant) in which, for example, a complexing chemical species associates with a cation.

These methods are based on the addition of a cation in the solution comprising the complexing chemical species, then the calculation of the complexation constant by measuring at equilibrium the concentrations in solution of the complex formed, of the cation remaining in the form free and of the complexing chemical species which has remained in free form. However, the solutions whose complexing power is sought to be determined most often include not one but several complexing chemical species. It is therefore difficult and tedious to evaluate the complexing power of such a mixture according to the abovementioned methods because these require for each complexing chemical species to measure its concentration and, if its complexing power is not known, by besides isolating this species to evaluate its complexation constant. This repetition of the operations for each complexing chemical species furthermore has the drawback of being a major source of errors.

Furthermore, due to the physicochemical constraints specific to these methods, they are most often carried out in a medium which has a composition different from the solution initially comprising the 5 complexing chemical species. They are therefore not carried out under the specific conditions of this solution (salinity, pH, solvent, etc.), which has the effect of modifying the complexation equilibria and therefore biasing the measurements taken.

Finally, the result obtained would nevertheless not account for the complexing power of the solution comprising the complexing chemical species because these can interact with each other and / or compete with respect to the complexation of the same cation according to a law. complex 15 difficult or even impossible to model. One of the aims of the invention is therefore to achieve a method which is at the same time simple to implement, reproducible, sensitive and which gives a representative measurement of the complexing power of a solution comprising several complexing chemical species. The object of the invention relates to a method for measuring the overall complexing power (PCg) of a solution of volume V comprising several complexing chemical species, the method comprising the following successive steps: a) adding to the solution a quantity ( No) of Col + cobalt cations, the amount being in excess of the sum of the amounts of the complexing chemical species; b) wait a sufficient time in order to obtain a mixture comprising at their equilibrium concentrations these cobalt Co` + cations in free form and cobalt Co`t cations in complexed form; c) extracting from the mixture the cobalt Col + cations in complexed form and the complexing chemical species; 2909179 4 d) measuring the quantity of cobalt Col + cations in the mixture in free form (Nfree); e) calculate the overall complexing power according to the formula PCg = (Np - Nfree) / V.

In order to account for the complexing power of all the complexing chemical species (mineral and / or organic) within the solution studied, the inventors have introduced a new parameter which is the overall complexing power (denoted PCg). The PCg is calculated by dividing the equilibrium amount of the complexed cobalt Col + cation (No - Nfree) by the volume of this solution. It is expressed in moles or grams of cobalt cation Col + complexed per liter of solution. Advantageously, the PCg makes it possible to be completely free from the multiplicity of measurements which would have been necessary to evaluate the complexing power of the whole of the solution with the methods existing to date. The amount No of cobalt Col + cation in excess of the sum of the amounts of the complexing chemical species is an amount equal to or greater than that at which the PCg is constant. However, alternatively, this amount can be arbitrarily set at x times the sum of the amounts of the complexing chemical species (with x greater than 1, x preferably being between 5 to 10). It is considered that the PCg is constant when the value of the PCg does not vary by more than 10% between a first experiment in which the excess amount No is five times greater than the sum of the amounts of the complexing chemical species (x = 5) and a second experiment in which No is ten times greater than the same sum of the amounts of the complexing chemical species (x = 10).

2909179 5 It is however preferable not to introduce too large a quantity No, namely so that the difference (No - Nfree) is so small that it approaches the uncertainty of the measurement within the mixture of the quantity cobalt Col + cations in free form (Nfree). By way of example, the sum of the amounts of the complexing chemical species can be determined beforehand by measuring the TOC (Total Organic Carbon) of the solution when the species are exclusively or mainly organic compounds, or by another known method of those skilled in the art such as ion exchange chromatography when these species comprise both organic compounds and mineral species.

The time sufficient in step (b) to reach the complexation equilibrium is that from which the concentration of the cobalt cation Col + in free form or in complexed form is stable in the mixture. It is also the time from which, according to the above definition, the 20 PCg is constant. In general, one can consider that a time of 30 minutes is sufficient to reach equilibrium this complexation. One of the essential points of the invention is the choice of a specific cation, the cobalt Co2 + cation, to determine the PCg using the method of the invention. This choice has many advantages: the complexation constants of cobalt Col + with many complexing chemical species are high and the complexes formed are stable, in particular when these species are organic compounds. Thus, the measurement of PCg is not only sensitive and reproducible, but it also integrates the complexing power of a large number or even all of the complexing chemical species present in the solution studied. By way of example, the measurement of PCg makes it possible to representatively integrate the complexing power of chemical species such as acetate, tartrate, oxalate, citrate, NTA (Nitrilo Triacetic Acid), CDTA (Cyclohexane Diamine Tetraacetic Acid), EDTA (Ethylene Diamine Tetra Acetic Acid), DTPA (Diethylene Triamine Penta Acetic Acid), TTHA (Triethylene Tetramine Hexa Acetic Acid). It should be noted that even if certain complexation constants of the cobalt Co` '+ cation with the complexing chemical species of mineral nature are low, these species, if they have a sufficient concentration in the solution (for example they represent more than 80 % of the complexing chemical species for a solution comprising for example more than 10 moles / liter of complexing chemical species), can nevertheless contribute to a significant strengthening of the overall complexing power (PCg) of the solution, strengthening which is measured by the method of the invention. The process of the invention therefore allows a particularly representative measurement of the complexing power resulting from the conjugation within the solution studied of the complexing power of each complexing chemical species. It should be noted that the PCg value is of course dependent on the physicochemical conditions known to those skilled in the art to influence a complexation reaction and therefore on its equilibrium, such conditions being for example the temperature or the pH. . - at room temperature, the solubility products of the various hydroxides of the Col + cation are such that, for example, for usual concentrations of free cobalt Col + cation of the order of 10-2 M, this cation only precipitates from with a pH = 7.5. The method of the invention therefore allows the measurement of PCg for solutions whose pH is in a wide range. For high pHs (beyond about 9.5), it is however optionally possible to elute the solution beforehand through a proton exchange resin in order to adjust its pH to around 7. 10 - the cobalt cation Col + is particularly representative of the cations found in a nuclear waste package, such as the cobalt isotope 60Co2 +. By extraction step (c), the process of the invention has the advantage that only (or in a majority amount) the cobalt Col + cation which has remained in free form remains in the mixture obtained at the end of the process. 'step (b). this ensures that no further interaction with these complexing chemical species (interaction which would lead to a modification of the complexation equilibrium) is possible within this mixture. The quantity of cobalt Col + cation which remained in free form measured in step (d) is therefore representative of what it was once equilibrium was reached at the end of step (b). It should be noted that the method of the invention also makes it possible to measure the PCg of a solid. It suffices first to cryobrush and then leach this solid. The PCg is then measured on the leachate generated. It is expressed for example in mg of cobalt cation Col + complexed per liter of leachate and converted (after division by the concentration in the leachate of the solid expressed for example in grams of sample per liter of leachate liquid) in mg of cobalt cation Col + complexed per gram of massive sample.

Advantageously, the complexing chemical species consist in molar percentage of organic compounds for more than 50 preferably more than 70 still more preferably more than 90 In the case of a solution comprising, as complexing chemical species, both organic compounds and at the same time mineral species, it may be desirable to include in the measurement of the PCg only the complexing power of the organic compounds. In this case, the solution is previously purified of mineral species by means known to those skilled in the art, such as the selective precipitation of mineral anions or the elution of the solution on resins allowing the selective retention of these mineral species. . Thus, it is possible to remove the carbonate ions by eluting the solution beforehand through an On Guard H resin sold by the company DIONEX and then by bubbling helium or nitrogen into the eluate (on thus eliminates the carbonic acid formed by passing over the resin).

It is also possible to eliminate the chloride ions by elution from the solution through a resin of the On Guard Ag type marketed by: a company DIONEX while verifying by a test that this resin does not retain the complexing organic species of 'interest.

It is also possible to get rid of sulfate ions by eluting an aliquot of the sample through an On Guard Ba resin marketed by the company DIONEX (but here too, not the complexing organic species of interest.

30 Preferably, those negatively charged from the cobalt Col + cations in complexed form are extracted from among the complexing chemical species those negatively charged by bringing the mixture into contact with an anion exchange resin capable of retaining the cobalt Col + cations in complexed form. negatively charged and negatively charged complexing chemical species. An anion exchange resin has the property of retaining only the negatively charged chemical species. Only the complexes (namely the cobalt Co` + cation in complexed form) and the complexing chemical species negatively charged at the pH of the solution are therefore extracted by the anion exchange resin according to the invention. It should be noted that a complex of the cobalt cation Col + can be negatively charged because it is, for example, associated with several complexing chemical species (which is frequent in the case of cobalt, which is one of the cations which complexes most easily. ), for example at least two carbonates, at least two oxalates or at least three nitrates. The resin according to the invention therefore makes it possible, by virtue of their negative charges, to extract from the mixture both the complexing chemical species and the complexes, both being negatively charged.

The resin according to the invention has a good affinity for a number of negatively charged complexing chemical species and for the negatively charged complexes formed: these are therefore efficiently extracted from the mixture. On the other hand, it has no affinity for the cobalt cation Col +: on the one hand the cobalt cation Col + therefore remains in free form in the mixture, on the other hand the complexes formed with the cobalt cation Col + do not. are not dissociated. These perfectly matched affinities ensure that the resin according to the invention has little or no influence on the complexation equilibria. The measurement of the PCg according to the method of the invention therefore allows a measurement which is entirely representative of the PCg of the solution studied.

Even more preferably, the resin comprises at least one ionizable group which is an ammonium group. The presence of at least one ammonium group as an ionizable group within the anion exchange resins according to the invention has the advantage that these resins can be used over a wide pH range, typically with a pH of 0 to 14 and therefore most often allow the measurement of PCg under pH 10 conditions representative of the solution studied. Even more preferably, this ammonium group is a quaternary ammonium group. Advantageously, such a group does not include a dissociable proton, which guarantees its great structural stability and therefore the use of the resin over a wide pH range. Preferably, the Col + cobalt cations added in step (a) are each associated with at least one counteranion chosen from OH-, NO3-, a halide ion, preferably Cl-.

Advantageously, the presence of these counter-anions in the mixture confers one or more negative charges on the few complexes formed which would be free of charge, such as for example the complexes between the cobalt cation Col + and a single oxalate molecule or a single oxalate molecule. single molecule of tartrate, each of these two complexing molecules comprising at most two negative charges. Such complexes are therefore more easily extracted from the mixture. Furthermore, such counter-anions have no or very little affinity in solution for the cobalt cation Col + with which they will therefore not form a parasitic complex which would increase the value of the PCg. This can be verified by a preliminary test in which the PCg value of a solution containing only the cobalt 2909179 11 Col + cations and these counter-anions (measurement of the blank) will have been measured. If the measured PCg is non-zero due to a certain affinity between these counter-anions and the cobalt cation c02- F, subsequent measurements of the PCg of solutions comprising these counter-anions 5 will be corrected by adding the quantity of the cobalt Col + ions. thus complexed to the amount Nfree. Preferably, the amount Nfree is measured in step (d) using a spectrometer chosen from an atomic emission spectrometer with inductively coupled plasma (ICP / AES), an atomic absorption spectrometer or an atomic absorption spectrometer. Inductively Coupled Plasma Ionization Mass Spectrometer (ICP / MS). The use of such a spectrometer allows the measurement of the amount of the cobalt Col + cation in free form (Nfree) with high selectivity (which may be important if the solution studied contains other cations) and high sensitivity. Such selectivity is for example provided by the choice of the measurement of the quantity N free at several emission wavelengths of cobalt for atomic emission spectrometry with inductively coupled plasma (ICP / AES) or the same measurement. for several isotopes of cobalt for an inductively coupled plasma ionization mass spectrometer (ICP / MS). Preferably, the pH of the solution or of the mixture is adjusted before step (b) to a value between pH ′ = 7 and pH2 = hydroxide pH + 0.5 to 2 pH units, the hydroxide pH being the pH from which the cobalt Col + cation in free form remaining in the mixture precipitates as the hydroxide.

This has the advantage, that at such basic pHs, a greater number of complexing functions of negative charge (s) (such as for example an acid function -CO2-) are available in a certain number of dies. complexing chemical species, in particular in organic compounds, for example polycarboxylic compounds (such as DTPA, TTHA). Consequently, the increase in the number of negative charges of these complexing chemical species and / or of the 5 complexes formed will promote their extraction from the mixture and therefore the quality of the measurement carried out. Moreover, under these conditions where complexation is favored, the measurement of the PCg then makes it possible to evaluate the complexation potential which may still be available at pHs at which the cobalt Col + cation in free form precipitates. Other objects, characteristics and advantages of the invention will emerge more clearly on reading the description which follows, given by way of illustration and without limitation. 1 - Measurement at different pH of the PCg of a solution comprising a mixture of organic compounds. - In a 25 ml volumetric flask, a solution comprising a mixture of organic compounds known for their complexing properties was produced by introducing 20 in the form of sodium salts 10-6 moles of each of the organic compounds (EDTA, citrate, oxalate, acetate) in 10 ml of Milli-Q water (R = 18.2 M12.cm, TOC <50 ppb). The pH of the solution was 6. It was adjusted to pH 7 using NaOH solution.

Preferably, the pH is adjusted by adding to the solution or to the mixture of at least one mineral acid (for example HCl) or of at least one mineral base (for example NaOH), such an acid or such a base not exhibiting no significant complexing properties compared to the cobalt Co` + cation. This avoids the adjustment of the pH by adding a buffer solution, the constituents of which most often have complexing properties which would distort the measurement of the PCg. Alternatively, the pH is neutralized by eluting the solution or the mixture on a proton exchange resin. Such a resin is, for example, an On Guard H resin marketed by the company DIONEX and advantageously makes it possible to avoid the introduction of any anion which could have a complexing power and which would bias the measurement of PCg. Then, an excess quantity No = 25.10-6 moles of a solution of CoC12 (0.5 M) was added. As the pH dropped when the cobalt was added in acidic solution, it was readjusted to a pH value = 7. The addition of Milli-Q water made it possible to make up the volume Vo of the mixture to 25 ml. Complexation equilibria are established almost instantly; a wait of 30 minutes was nonetheless respected in order to ensure that the complexation equilibrium was properly established. After this time, a 10 ml aliquot of the mixture was taken and then eluted on an On Guard A resin cartridge marketed by the company DIONEX (reference P / N 042102) and comprising as ionizable group an associated -NR3 + group. to the exchangeable counterion HCO3. The first two milliliters of the eluate were not stored, the next six milliliters were collected, then refrigerated at 4 ° C. while awaiting analysis.

The quantity of Col + cobalt cation in free form remaining in the aliquot was measured at ambient temperature (25 ° C.) using an ICP / AES OPTIMA 3000 spectrometer sold by the company PERKIN ELMER according to the following operating conditions: 30 - Plasma gas flow rate: 15 1 / min Auxiliary gas flow rate: 0.5 1 / min Nebulizer gas flow rate: 0.8 1 / min 2909179 14 - Cobalt detection wavelengths: 228.612 nm - 238.889 nm - 230.786 nm This measurement made it possible, taking into account the dilution factors, to obtain the free quantity N of cobalt cation 5 Col + in free form in the volume Vo of the mixture. The PCg was then calculated according to the following formula: PCg = (No - Nlibre) / V = (25.10-6 moles - 21.2.10-6 moles) / 25.10-3 liters = 0.152.10-3 moles / liter at pH = 7 10 - The process of the invention was then repeated twice according to the same operating protocol as that described above, except before completing the volume of the mixture to 25m1; the pH of the mixture was no longer brought to a value of 7 but to a value of 5 and 9.

15 The results obtained are as follows: -> PCg = 0.1168.10-3 moles / liter at pH = 5 -> PCg = 0.1904.10-3 moles / liter at pH = 9 - We therefore clearly see an increase in PCg when the pH is increased, in particular above the pH value at which the cation precipitates as the hydroxide. For a free N value = 10-3 M, this precipitation pH is theoretically 8.

Claims (10)

1) Method for measuring the overall complexing power (PCg) of a solution of volume V comprising several complexing chemical species, said method comprising the following successive steps: a) adding to said solution a quantity (No) of cobalt Col + cations, said amount being in excess of the sum of the amounts of said complexing chemical species; b) wait a sufficient time in order to obtain a mixture comprising at their equilibrium concentrations cobalt Col + cations in free form and cobalt Col + cations in complexed form; C) extracting from said mixture said cobalt Col + cations in complexed form and said complexing chemical species; d) measure within said mixture the quantity of said cobalt Co2 + cations in free form (Niibre) 20 e) calculate the overall complexing power according to the formula PCg = (No -Niibre) / V. 2) Process according to claim 1, characterized in that said complexing chemical species consist in molar percentage of organic compounds for more than 50%, preferably more than 70%, even more preferably more than 90%. 3) A method according to any one of the preceding claims characterized in that one extracts from said cobalt Co '+ cations in complexed form those negatively charged 2909179 16 and from said complexing chemical species those negatively charged by bringing said mixture into contact with an anion exchange resin capable of retaining said cobalt Col + cations in negatively charged complexed form and said negatively charged complexing chemical species. 4) Method according to claim 3 characterized in that said resin comprises at least one group: ionizable 10 which is an ammonium group. 5) Method according to claim 4 characterized in that said ammonium group is a quaternary ammonium group. 15 6) Method according to any one of the preceding claims characterized in that said cobalt Col + cations added in step (a) are each associated with at least one counter-anion chosen from OH-, NO3-, a halide ion, from Preferably Cl-. 7) Method according to any one of the preceding claims, characterized in that said Nlibre quantity is measured in step (d) using a spectrometer chosen from an atomic emission spectrometer with inductively coupled plasma ( ICP / AES), atomic absorption spectrometer or inductively coupled plasma ionization mass spectrometer (ICP / MS). 30 8) Method according to any one of the preceding claims characterized in that the pH of said solution or of said mixture is adjusted before step (b) to a value between pHl = 7 and pH = hydroxide pH + 0.5 to 2 pH units 2909179 17, said hydroxide pH being the pH from which said cobalt Col-1- cation in free form remaining in said mixture precipitates in the form of hydroxide. 5 9) Method according to claim 8 characterized in that the pH is adjusted by adding to said solution or to said mixture of at least one mineral acid or at least one mineral base. 10) Method according to claim 8 characterized in that the pH is adjusted by eluting said solution or said mixture on a proton exchange resin.