METHOD FOR DETERMINING THE ACIDITY OF AN ACIDIC AQUEOUS SOLUTION

Abstract

Methods for determining the acidity of an acidic aqueous solution. These methods make it possible to measure the total acidity of an aqueous solution comprising a strong acid or a mixture of strong acids and, if this solution contains one or more hydrolysable cations, to also measure its free acidity.

Claims

1. A method for determining a total acidity of an aqueous solution A1 comprising a strong acid or a mixture of strong acids, the method comprising at least the following steps: a) providing an aqueous solution A5 by mixing: a volume V1 of the solution A1, a volume V2 of an aqueous solution A2 of pH denoted pHA2, comprising a compound selected from a weak acid, a weak base, a salt of a weak acid or a salt of a weak base, and a volume V4 of a solution A4 comprising a pH-sensitive dye having an acid form and a base form, and a transition range between a first pH and a second pH, the second pH being higher than the first pH but lower than pHA2; b) determining a UV-visible absorbance spectrum of the dye present in solution A5 by using the solution A2 as a measuring blank; c) determining a concentration of at least one of the acid and base forms of the dye in the solution A5 from the absorbance spectrum obtained in step b); d) determining a pH, denoted pHA5, of the solution A5 from the concentration determined in step c); and e) determining the total acidity of the solution A1 from the pHA5 determined in step d); and wherein pHA2 is such that a mixture of the volumes V1 and V2 has a pH which is in the transition range of the dye.

2. The method of claim 1, wherein step b) comprises: acquiring a light intensity spectrum of the solution A2 in a UV-visible range, whereby light intensity values are obtained for the solution A2; acquiring a light intensity spectrum of the solution A5 in the UV-visible range, whereby light intensity values are obtained for the solution A5; and calculating a decimal logarithm of a ratio between the light intensity values obtained for the solution A5 and the light intensity values obtained for the solution A2.

3. The method of claim 2, wherein step c) comprises applying the Beer-Lambert law.

4. The method of claim 1, wherein step d) comprises applying equation (1): $\begin{array}{cc}{10}^{-\mathrm{pHA}5}=\frac{K_{\mathrm{HC}}\times \left[\mathrm{HC}\right]}{\left[C^{-}\right]}& \left(1\right)\end{array}$ wherein: K.sub.HC is a dissociation constant of the acid form of the dye in the solution A5; [HC] is the concentration of the acid form of the dye in the solution A5 in mol/L; and [C.sup.−] is the concentration of the base form of the dye in the solution A5 in mol/L.

5. The method of claim 1, wherein step d) comprises applying equation (2): $\begin{array}{cc}\mathrm{pHA}5=-\log \{K_{\mathrm{HC}}\left(\frac{1}{R}-1\right))& \left(2\right)\end{array}$ wherein: K.sub.HC is a dissociation constant of the acid form of the dye in the solution A5; and R is a ratio between the concentration of the base form of the dye in the solution A5 and a concentration of the dye in the solution A5.

6. The method of claim 1, wherein the total acidity of the solution A1, expressed in a molar concentration of H.sup.+ protons, denoted [H.sup.+], is determined by equation (3): $\begin{array}{cc}\left[H^{+}\right]=\frac{{10}^{-\mathrm{pHA}5}\times \left(V1+V2\right)-{10}^{-\mathrm{pHA}2}\times V2+Y}{V1}& \left(3\right)\end{array}$ wherein: V1 and V2 are expressed in L; and Y is determined by equation (4): $\begin{array}{cc}Y=\frac{{10}^{-\mathrm{pHA}5}\times \left[A^{-}\right]\times V2-K_{\mathrm{HA}}\times \left[\mathrm{HA}\right]\times V2}{K_{\mathrm{HA}}+{10}^{-\mathrm{pHA}5}}& \left(4\right)\end{array}$ wherein: if the solution A2 comprises a weak acid or a salt of a weak acid, then: K.sub.HA is an acid dissociation constant of the weak acid in the solution A5; [A.sup.−] is a concentration of a conjugate base of the weak acid in the solution A2 in mol/L; [HA] is a concentration of the weak acid in the solution A2 in mol/L; if the solution A2 comprises a weak base or a salt of a weak base, then: K.sub.HA is an acid dissociation constant of a conjugate acid of the weak base in the solution A5; [A.sup.−] is a concentration of the weak base in the solution A5 in mol/L; [HA] is a concentration of the conjugate acid of the weak base in the solution A5 in mol/L; V2 is expressed in L.

7. A method for determining a free acidity of an aqueous solution A1 comprising a strong acid or a mixture of strong acids and one or more hydrolysable cations, comprising at least the following steps: a) providing an aqueous solution A3 by mixing a volume V1 of the solution A1 and a volume V2 of an aqueous solution A2 of pH denoted pHA2, comprising a compound selected from a weak acid, a weak base, a salt of a weak acid or a salt of a weak base, and an agent complexing the hydrolysable cation(s) at a concentration C2; b) determining a UV-visible absorbance spectrum of the hydrolysable cation(s) present in the solution A3 by using the solution A2 as a measuring blank; c) determining a concentration C3 of the hydrolysable cation(s) in the solution A3 from the absorbance spectrum obtained in step b); d) providing an aqueous solution A5 by mixing the solution A3 and a volume V4 of a solution A4 comprising a pH-sensitive dye having an acid form and a base form, and a transition range between a first pH and a second pH, the second pH being higher than the first pH but lower than pHA2; e) determining a UV-visible absorbance spectrum of the dye present in the solution A5 by using the solution A2 or the solution A3 as a measuring blank; f) determining a concentration of at least one of the acid and base forms of the dye in the solution A5 from the absorbance spectrum obtained in step e); g) determining a pH, denoted pHA5, of the solution A5 from the concentration determined in step f); then h) determining the free acidity of the solution A1 from the concentration C3 determined in step c) and from the pHA5 determined in step e); and wherein: pHA2 and the volumes V1 and V2 are such that the solution A3 has a pH, denoted pHA3, which is within the dye transition range; the concentration C2 and the volumes V1 and V2 are such that the complexing agent is in excess with respect to the hydrolysable cation(s) in the solution A3.

8. The method of claim 7, wherein the compound present in the aqueous solution A2 is also the complexing agent.

9. The method of claim 7, wherein step b) comprises: acquiring a light intensity spectrum of the solution A2 in a UV-visible range, whereby light intensity values are obtained for the solution A2; acquiring a light intensity spectrum of the solution A3 in the UV-visible range, whereby light intensity values are obtained for the solution A3; and calculating a decimal logarithm of a ratio between the light intensity values obtained for the solution A3 and the light intensity values obtained for the solution A2.

10. The method of claim 9, wherein step c) comprises applying the Beer-Lambert law.

11. The method of claim 9, wherein step e) comprises: acquiring a light intensity spectrum of the solution A5 in a UV-visible range, whereby light intensity values are obtained for the solution A5; and calculating a decimal logarithm of a ratio between the light intensity values obtained for the solution A5 and the light intensity values obtained for the solution A3.

12. The method of claim 11, wherein step f) comprises applying the Beer-Lambert law.

13. The method of claim 7, wherein step g) comprises applying equation (1): $\begin{array}{cc}{10}^{-\mathrm{pHA}5}=\frac{K_{\mathrm{HC}}\times \left[\mathrm{HC}\right]}{\left[C^{-}\right]}& \left(1\right)\end{array}$ wherein: K.sub.HC is a dissociation constant of the acid form of the dye in the solution A5; [HC] is a concentration of the acid form of the dye in the solution A5 in mol/L; and [C.sup.−] is a concentration of the base form of the dye in the solution A5 in mol/L.

14. The method of claim 7, wherein step g) comprises applying equation (2): $\begin{array}{cc}\mathrm{pHA}5=-\log \{K_{\mathrm{HC}}\left(\frac{1}{R}-1\right))& \left(2\right)\end{array}$ wherein: K.sub.HC is a dissociation constant of the acid form of the dye in the solution A5; and R is a ratio between a concentration of the base form of the dye in the solution A5 and a concentration of the dye in the solution A5.

15. The method of claim 7, wherein the free acidity of solution A1, expressed in molar concentration of H.sup.+ protons, denoted [H.sup.+], is determined by equation (3): $\begin{array}{cc}\left[H^{+}\right]=\frac{{10}^{-\mathrm{pHA}5}\times \left(V1+V2\right)-{10}^{-\mathrm{pHA}2}\times V2+Y}{V1}& \left(3\right)\end{array}$ wherein: V1 and V2 are expressed in L; and Y is determined by equation (7): $\begin{array}{cc}Y=\frac{10^{-p\mathrm{HA}5}\times \left(\left[A^{-}\right]\times V2-\frac{n}{m}C3\times V1\right)-K_{HA}\times \left[HA\right]\times V_1}{K_{HA}+10^{-p\mathrm{HA}5}}& \left(7\right)\end{array}$ wherein: m is a stoichiometric coefficient of the hydrolysable cation(s) in a complexing reaction of the hydrolysable cation(s) by the complexing agent; n is a stoichiometric coefficient of the complexing agent in the complexing reaction of the hydrolysable cation(s) by the complexing agent; if the solution A2 comprises a weak acid or a salt of a weak acid, then: K.sub.HA is an acid dissociation constant of the weak acid in the solution A5; [A.sup.−] is a concentration of a conjugate base of the weak acid in the solution A2 in mol/L; [HA] is the concentration of the weak acid in the solution A2 in mol/L; if the solution A2 comprises a weak base or a salt of a weak base, then: K.sub.HA is an acid dissociation constant of a conjugate acid of the weak base in the solution A5; [A.sup.−] is a concentration of the weak base in the solution A5 in mol/L; [HA] is a concentration of the conjugate acid of the weak base in the solution A5 in mol/L; V1 and V2 are expressed in L and C3 is expressed in mol/L.

16. (canceled)

17. The method of claim 1, wherein the solution A2 comprises oxalic acid or a salt thereof.

18. (canceled)

19. The method of claim 1, wherein the dye is bromocresol green.

20. The method of claim 1, wherein the solution A1 is an aqueous solution of nitric acid.

21. The method of claim 7, wherein the solution A2 comprises oxalic acid or a salt thereof.

22. The method of claim 7, wherein the dye is bromocresol green.

23. The method of claim 7, wherein the solution A1 is an aqueous solution of nitric acid.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0105] FIG. 1 illustrates, by way of example, the light intensity spectrum (denoted I and expressed in counts) acquired by means of a CCD camera, over the wavelength range between 400 nm and 750 nm, for a solution A2 comprising 0.27 mol/L of sodium oxalate, with a pH of 5.7.

[0106] FIG. 2 illustrates, by way of example, the light intensity spectrum (denoted I and expressed in counts) acquired by means of a CCD camera, over the wavelength range between 400 nm and 750 nm, for a solution A3 obtained by mixing 1 000 μL of the solution A2, the spectrum of which is shown in FIG. 1, and a sample of 50 μL of an aqueous solution A1 comprising 2 mol/L of nitric acid and 0.56 mol/L of uranium(VI).

[0107] FIG. 3 illustrates, by way of example, the light intensity spectrum (denoted I and expressed in counts) acquired by means of a CCD camera, over the wavelength range between 400 nm and 750 nm, for a solution A5 obtained by mixing the solution A3, the light intensity spectrum of which is shown in FIGS. 2, and 150 μL of an aqueous solution A4 of bromocresol green at 0.02% by mass.

[0108] FIG. 4 illustrates, by way of example, the absorbance spectrum (denoted A and expressed in arbitrary units) of the uranyl cations present in the solution A3, this spectrum having been obtained by using the light intensity spectrum shown in FIG. 2 as a measurement and the light intensity spectrum shown in FIG. 1 as a measuring blank.

[0109] FIG. 5 illustrates, by way of example, the absorbance spectrum (denoted A and expressed in arbitrary units) of the bromocresol green present in the solution A5, this spectrum having been obtained by using the light intensity spectrum shown in FIG. 3 as a measurement and the light intensity spectrum shown in FIG. 2 as a measuring blank.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

[0110] The method of the invention has been validated by tests aimed at verifying whether this method makes it possible to find with an acceptable bias (ideally less than 5%) the free acidity, i.e. the acidity linked to the sole presence of nitric acid, of aqueous solutions comprising, in addition to nitric acid in a known concentration, uranium(VI) and/or plutonium(IV), also in a known concentration.

[0111] These tests were performed manually on the one hand and by automation on the other hand.

EXAMPLE I—MANUAL TESTS

[0112] Manual tests were carried out at ambient temperature (20-25° C.) in nuclear glove boxes using: [0113] as solution A2: an aqueous solution saturated with sodium oxalate, comprising 0.27 mol/L of Na.sub.2C.sub.2O.sub.4 where the pH has been adjusted to 5.7 by the addition of sulfuric acid; [0114] as A1 solutions to be analysed: a series of aqueous solutions comprising: [0115] nitric acid in a concentration ranging from 1.0 mol/L to 4.99 mol/L, and [0116] either uranium(VI) in a concentration of 33 g/L, 66 g/L or 133 g/L, or plutonium(IV) in a concentration of 20 g/L; [0117] as solution A4: an aqueous solution of bromocresol green at 0.02%; and [0118] as spectrophotometer: a SpectraPro SP500i UV-visible spectrophotometer (Roper Scientific) equipped with a cell with an optical path equal to 10 mm.

[0119] Each test was performed by using a 1 000 μL volume of solution A2, a 50 μL volume of solution A1 and a 150 μL volume of solution A4.

[0120] The acid dissociation constant K.sub.HA of oxalic acid and the dissociation constant K.sub.HC of the acid form of bromocresol green in test conditions were determined previously by experiment.

[0121] In test conditions, K.sub.HA is equal to 3.8 whereas K.sub.HC is equal to 4.55.

[0122] The tests were carried out according to the following operating protocol: [0123] 1. introducing the volume of solution A2 in the spectrophotometry cell and acquisition of the light intensity spectrum in the UV-visible range (here 400 nm- 750 nm); [0124] 2. adding into the spectrophotometry cell the volume of solution A1, mixing of this volume with solution A2 already present in this cell and acquisition of the light intensity spectrum on the same wavelengths as previously mentioned; [0125] 3. adding into the spectrophotometry cell the volume of solution A4 and mixture of this volume with the mixture solution A2/solution A1 already present in this cell and acquisition of the light intensity spectrum on the same wavelengths as previously mentioned; [0126] 4. determining the absorbance spectrum of the hydrolysable cation, namely UO.sub.2.sup.2+ in the case of uranium(VI) and Pu.sup.4+ in the case of plutonium(IV), at the same wavelengths as above by calculating the decimal logarithm of the ratio between the light intensity values of the acquired spectrum in point 2 above and the light intensity values of the acquired spectrum in point 1 above; [0127] 5. determining the absorbance spectrum of the dye on the same wavelengths as above by calculating the decimal logarithm of the ratio between the light intensity values of the acquired spectrum in point 3 above and the light intensity values of the acquired spectrum in point 2 above; [0128] 6. determining, from the maximum absorbance observed for the base form of the dye on the absorbance spectrum obtained in point 5 above and by reference to previously established standard curves, of the ratio R between the concentration of the base form of the dye present in the mixture solution A2/solution A1/solution A4 and the total concentration of the dye in this mixture; [0129] 7. determining the pH of the mixture solution A2/solution A1/solution A4 by means of the equation (2) defined above, which in this case becomes:

[00008]$\mathrm{pHA}5=-\log \left(10^{-4,55}\left(\frac{1}{R}-1\right)\right);$

and [0130] 8. determining the free acidity of the solution A1, expressed as the molar concentration of H.sup.+ protons, denoted [H.sup.+], by means of the equation (3) as defined above, which in the present case becomes:

[00009]$\left[H^{+}\right]=\frac{10^{-p\mathrm{HA}5}\times \left(5.10^{-5}+10^{-3}\right)-10^{-5,7}\times 10^{-3}+Y}{5.10^{-5}}$

and wherein: [0131] pHA5 is the pH of the mixture solution A2/solution A1/solution A4 as determined in point 7 above, and

[0132] Y is determined by means of the equation (7) as defined above, which in the present case becomes:

[00010]$Y=\frac{\begin{array}{c}10^{-p\mathrm{HA}5}\times \left(\left[A^{-}\right]\times 10^{-3}-\frac{n}{m}C3\times 5.10^{-5}\right)-\\ {10}^{-3,8}\times \left[HA\right]\times 5.10^{-5}\end{array}}{10^{-3,8}+10^{-p\mathrm{HA}5}}$

wherein: [0133] m is equal to 1 and n is equal to 3 as, in strong oxalic excess, the complexing reactions of UO.sub.2.sup.2+ and Pu.sup.4+ cations are written as:

UO.sub.2.sup.2++3C.sub.2O.sub.4.sup.2−<=>UO.sub.2(C.sub.2O.sub.4).sub.3.sup.4−

Pu.sup.4++3C.sub.2O.sub.4.sup.2−<=>Pu(C.sub.2O.sub.4).sub.3.sup.2−

[0134] C3 represents the concentration in mol/L of the UO.sub.2.sup.2+ or Pu.sup.4+ cation in the mixture solution A2/solution A1/solution A4.

[0135] In the present tests, the concentration C3 was not obtained experimentally but was introduced into the equation (7) by taking the concentration of uranium(VI) or plutonium(IV) in the analysed solution A1.

[0136] Table I below specifies for each analysed solution A1: [0137] its concentration, expressed in g/L, of uranium(VI) or plutonium(IV), [0138] its concentration, expressed in mol/L, of nitric acid, [0139] the pH theoretically presented by the mixture solution A2/solution A1/solution A4 obtained in point 3 above (referred to as “theoretical pH”), [0140] the pH of this mixture as determined in point 7 above (referred to as “measured pH” in Table I), [0141] the free acidity of solution A1, expressed in mol/L, as determined in point 8 above (referred to as “measured free acidity” in Table I) as well as [0142] the relative difference, expressed in %, between the concentration of nitric acid in the solution A1 and the free acidity of this solution.

TABLE-US-00001 TABLE I Measured Solutions A1 free Relative [Cation] [HNO.sub.3] Theoretical Measured acidity difference (g/L) (mol/L) pH pH (mol/L) (%) U(VI) 33 1.0 4.17 4.19 0.96 +4.00 66 4.12 4.11 1.01 −1.00 133 4.00 4.00 1.00 0 U(VI) 33 1.5 3.94 3.93 1.51 −0.66 66 3.88 3.88 1.50 0 133 3.74 3.72 1.54 −2.66 U(VI) 33 2.0 3.75 3.72 2.08 −3.8 66 3.68 3.70 1.96 +2.0 133 3.50 3.51 1.99 +0.50 Pu(IV) 20 3.02 4.10 4.11 3.04 −0.66 3.93 3.95 3.98 3.85 +2.03 4.99 3.79 3.81 5.00 −0.20

[0143] This table shows that the method of the invention makes it possible to determine the free acidity of acidic aqueous solutions comprising a hydrolysable cation with a bias with respect to the true value of this acidity of at most 4% and this regardless of the concentrations of acid and hydrolysable cations of these solutions.

EXAMPLE II—AUTOMATED TESTS

[0144] The automated tests were performed at ambient temperature (20-25° C.) by using a device comprising: [0145] the same spectrophotometer as the one used in the manual tests described in example I above; [0146] a device making it possible to distribute into the cell of the spectrophotometer, by means of micropipettes, volumes of solutions A2, A1 and A4 necessary for carrying out the tests; and [0147] software for controlling the distribution device as well as software for processing and analysing the data provided by the UV-visible spectra acquired by the spectrophotometer.

[0148] These tests were also carried out using: [0149] the same solution A2 and the same solution A4 as those used in the manual tests described in example I above; and [0150] as solutions A1 for analysis: aqueous solutions, referred to in the following as solutions 1, 2, 3 and 4, comprising: [0151] nitric acid in a concentration ranging from 1.08 mol/L to 10.02 mol/L, [0152] uranium(VI) in a concentration ranging from 45 g/L to 75 g/L and plutonium(IV) in a concentration ranging from 4 g/L to 7 g/L.

[0153] The composition of these solutions is specified in Table II below.

[0154] This table also specifies, for each solution Al to be analysed, the used volume of this solution as well as the used volumes of solution A2 and A4.

TABLE-US-00002 TABLE II Vol. solution Vol. solution Vol. solution Solutions [U(VI)] [Pu(IV)] [HNO.sub.3] A1 A2 A4 A1 (g/L) (g/L) (mol/L) (μL) (μL) (μL) 1 75 4 1.08 30 770 100 2 65 5 3.23 20 780 100 3 55 6 7.04 10 790 100 4 45 7 10.02 10 790 100

[0155] All of the solutions A1 were analysed in duplicate following the same procedure as described in the example I above, except that the concentrations of the uranyl UO.sub.2.sup.2+ and Pu.sup.4+ cations in each of the mixtures solution A2/solution A1/solution A4 were determined from the absorbance spectrum obtained in point 4 of this protocol, by reference to previously established standard curves.

[0156] As above, the free acidity of solutions A1, expressed as a molar concentration of H.sup.+ protons, denoted [H.sup.+], was determined by means of the equation (3) as defined above.

[0157] Table III below shows for each solution A1 analysed and each duplicate of this solution: [0158] the concentration of uranium(VI) and the concentration of plutonium(IV) in the solution A1 as determined from the absorbance spectrum obtained in point 4 of the operating protocol, these concentrations being expressed in g/L, [0159] the pH theoretically presented by the mixture solution A2/solution A1/solution A4 obtained in point 3 of the operating protocol (referred to as “theoretical pH”), [0160] the pH of this mixture as determined in point 7 of the operating protocol (referred to as “measured pH” in Table III), [0161] the free acidity of solution A1, expressed in mol/L, as determined in point 8 of the operating protocol (referred to as “measured free acidity” in Table III), as well as [0162] the relative difference, expressed in %, existing between the concentration of nitric acid in the solution A1 and the free acidity of this solution.

TABLE-US-00003 TABLE III Measured Measured Measured free Relative Solutions [U(VI)] [Pu(IV)] Theoretical Measured acidity difference A1 (g/L) (g/L) pH pH (mol/L) (%) 1 81.5 5.3 4.333 4.342 1.066 +1.28 82.9 5.1 4.344 1.063 +1.54 2 73.6 7.2 4.015 4.023 3.193 +1.14 76.3 7.0 4.018 3.220 +0.30 3 52.8 8.1 4.057 4.054 7.081 −0.58 55.8 7.9 4.052 7.100 −0.85 4 44.1 9.3 3.829 3.833 9.981 +0.39 40.9 9.5 3.839 9.889 +1.31

[0163] This table shows that the method of the invention, when implemented in an automated manner, makes it possible to determine the free acidity of solutions of aqueous acids comprising hydrolysable cations with a bias with respect to the true value of this acidity of less than 2% and this regardless of the concentrations of acid and hydrolysable cations in these solutions.

REFERENCES CITED

[1] T. G. Srinivasan and P. R. Rao, Talanta 2014, 118, 162-171

[2] J. Néri-Quiroz et al., Talanta 2016, 159, 330-335