Method for measuring the acidity of an aqueous acidic solution

Abstract

The invention relates to methods for measuring the acidity of aqueous solutions. These methods are capable of measuring the total acidity of aqueous solutions containing a strong acid or a mixture of strong acids, and, if the solution contains one or more hydrolysable cations, the free acidity as well. Applications: any industrial and scientific research field where it can be necessary to measure the total acidity or the free acidity of aqueous solutions of strong acids.

Description

Technical Field

[0001] This invention relates to the field of analytical chemistry.

[0002] More specifically, the present invention relates to a method for measuring the acidity of acidic aqueous solutions.

[0003] These methods can measure solutions containing strong acids or mixtures of strong acids, and if the solution contains one or more hydrolyzable cations, i.e., those that can be hydrolyzed by OH-. - hydroxide ions and H+ + The total acidity of an aqueous solution of one or more cations that react with water to form protons can also be measured as free acidity.

[0004] This invention can be applied to all industrial and scientific research fields where it may be necessary to measure the total acidity or free acidity of aqueous solutions of strong acids.

[0005] However, the special benefit is:

[0006] On the one hand, for measuring the total acidity of aqueous solutions containing high concentrations of strong acids or mixtures of strong acids, conventional methods for measuring total acidity, such as pH meters, are unsuitable at such high concentrations.

[0007] On the other hand, it is used to monitor the free acidity of aqueous solutions containing one or more hydrolyzable cationic strong acids in laboratories or industrial production lines, wherein the aqueous solution is used in industrial processes where the free acidity represents a key parameter.

[0008] Examples of these methods include hydrometallurgical methods for processing or extracting metal elements of interest, particularly from ores, and methods for treating metal surfaces by acidic chemical pickling. Background Technology

[0009] According to the acid-base proton theory ( (Lowry theory) An acid is a substance that can release one or more H+ ions according to the following equation. + The chemical substances (ions or molecules) that make up a proton:

[0010]

[0011] Therefore, the acidity of the aqueous solution is determined by the H+ present in the solution. + It is characterized by the amount of protons.

[0012] When the aqueous solution contains not only acid but also other substances, it will release H+ through reaction with water. + When protons are hydrolyzable cations, the total acidity and free acidity of the solution can be distinguished.

[0013] Therefore, the total acidity is determined by the H+ present in the aqueous solution. +The total amount of protons is given, while the free acidity is given only by the H + The amount of protons is given. In other words, the free acidity does not take into account the H + protons.

[0014] The free acidity is a key parameter for many industrial processes involving acidic aqueous solutions carrying or potentially carrying hydrolysable cations.

[0015] Many methods for measuring the free acidity of acidic aqueous solutions containing hydrolysable cations are described in the literature. These methods have in common that the hydrolysis of the hydrolysable cations is first eliminated by a method such as precipitation, complexation or immobilization on a solid resin support, then the free acidity is measured. A recent document on these methods is published by T.G. Srinivasan and P.R. Rao in Talanta 2014, 118, 162-171, hereinafter cited as [1].

[0016] Recently, J. Néri-Quiroz et al. proposed a method for determining the free acidity of aqueous nitric acid solutions by sequential injection analysis (SIA). (Talanta 2016, 159, 330-335, hereinafter cited as [2]).

[0017] This method, which allows the free acidity measurement of 10 μL of sample in-line and only 1.5 mL of waste per analyzed sample, uses potassium oxalate for the complexation of the hydrolysable cations, then a titration with NaOH, after which the determination is made by colorimetry (in this case with Congo red).

[0018] As part of their work, the inventors have successfully developed a method that allows the measurement of the total and free acidity of acidic aqueous solutions with high precision depending on the type of acidity to be measured.

[0019] For the measurement of the free acidity, the present invention has not only the same advantages as the method of reference [2] (reduction of the volume of sample analyzed, reduction of the volume of waste produced, possibility of automation), but it is even easier to implement and more economical in terms of reagents since it does not require an acid-base titration reaction and therefore does not use an alkali titrant such as NaOH. SUMMARY

[0020] The subject of the present invention is therefore a first method whose aim is to measure the total acidity of an aqueous solution A1 containing a strong acid or a mixture of strong acids (with or without hydrolysable cations), this method comprising at least the following steps:

[0021] a) providing an aqueous solution A5 by mixing:

[0022] V1 volume of solution A1,

[0023] V2 volume of aqueous solution A2 having a pH value of pHA2, said aqueous solution A2 comprising a compound selected from a weak acid, a weak base, a weak acid salt or a weak base salt, and

[0024] V4 volume of solution A4, said solution A4 comprising a pH-sensitive dye, said pH-sensitive dye having an acid form and a base form, and a transition range between a first pH and a second pH, said second pH being higher than said first pH but lower than pHA2;

[0025] b) measuring the UV-Vis absorption spectrum of the dye present in solution A5 using solution A2 as a measurement blank;

[0026] c) determining the concentration of at least one of the acid form and the base form of said dye in solution A5 from the absorption spectrum obtained in step b);

[0027] d) determining the pH value of solution A5, noted pHA5, from the concentration measured in step c); then

[0028] e) determining the total acidity of solution A1 from pHA5 measured in step d);

[0029] and wherein pHA2 is such that the mixture of volumes V1 and V2 has a pH value in the transition range of the dye.

[0030] Thus, this first method is based on the use of the transition range of a pH-sensitive dye, also known as pH color indicator or acid-base indicator.

[0031] However, this use is different from the use in the prior art, since according to the present application, the transition range of the pH-sensitive dye is neither used to measure the pH value range in which the acidic aqueous solution is, nor to determine the chemical equivalent of the acid-base titration, but to measure the concentration of H + protons in the acidic aqueous solution by absorbance of the pH-sensitive dye.

[0032] In the present context, the terms "strong" and "weak" for an acid or a base have the meaning that is given to them by the acid-base proton theory, i.e. an acid or a base that is completely dissociated in water is considered strong, while an acid or a base that is not completely dissociated in water is considered weak.

[0033] According to the present application, the choice of the compound (weak acid, weak base, weak acid salt or weak base salt) present in solution A2 and the choice of the dye are not particularly limited.

[0034] Thus, the compound present in solution A2 can be selected from the many compounds known as weak acids (in particular carboxylic acids and polycarboxylic acids) or weak bases (in particular ammonia, amines and polyamines) and their salts.

[0035] In a similar way, the dye can be chosen from among the many compounds conventionally used as pH color indicators, such as bromocresol green, congo red, bromophenol blue, methyl yellow, methyl red, etc.

[0036] However, as mentioned above, the pH A2 corresponding to the pH of the solution A2 must be such that the pH of the mixture of volumes V1 and V2 is in the transition range of the dye, which can be easily verified beforehand by preparing this mixture and determining its pH by using pH paper, pH strips, pH probes, or even by calculation if the order of magnitude of the total acidity of the solution Al is known.

[0037] The concentration of the said compound in the solution A2 and the dye are therefore chosen accordingly. It will be understood that, if necessary, the pH A2 can be adjusted by adding a strong acid or a strong base to the solution A2, or one and / or the other of the volumes V1 and V2 can be changed so that the pH of their mixture falls within the transition range of the dye.

[0038] According to the application, the measurement of the ultraviolet-visible absorption spectrum of the dye present in the solution A5 (or step b)) preferably comprises:

[0039] obtaining the light intensity spectrum of the solution A2 in the ultraviolet-visible range;

[0040] obtaining the light intensity spectrum of the solution A5 in the ultraviolet-visible range; and

[0041] calculating the base-10 logarithm of the ratio of the light intensity values of the obtained spectrum of the solution A5 to the light intensity values of the obtained spectrum of the solution A2;

[0042] The light intensity spectra of the solutions A2 and A5 are advantageously obtained by means of a charge transfer camera, better known as a CCD (charge-coupled device) camera.

[0043] The absorption spectrum thus obtained shows two absorbance peaks (or maxima) located at two different wavelengths, one of which is characteristic of the acid form of the dye and the other of which is characteristic of the base form of the dye, and which vary on either side of the isosbestic point as a function of the pH value pHA5 of the solution A5.

[0044] In step c), the concentration of at least one of the acid and base forms of the dye in solution A5 is advantageously determined by applying the Beer-Lambert law, according to which the absorbance measured at a given wavelength on a solute in solution is directly proportional to the concentration of this solute in the solution, i.e. in practice, by comparing the maximum absorbance of this type shown by the absorption spectrum obtained in step b) with the absorbance values of a standard curve previously established from aqueous solutions containing the dye at variable concentrations and at variable pH values.

[0045] In step d), pHA5may be determined, for example, by the following equation (1):

[0046]

[0047] where:

[0048] K HC is the dissociation constant of the acid form of the dye in solution A5;

[0049] [HC] is the concentration of the acid form of the dye in solution A5, in mol / L; and

[0050] [C - ] is the concentration of the base form of the dye in solution A5, in mol / L.

[0051] In one variant, pHA5may be determined, for example, by the following equation (2):

[0052]

[0053] where:

[0054] K HC as defined previously; and

[0055] R is the ratio of the concentration of the base form of the dye in solution A5 to the concentration of the dye (acid form + base form) in this solution.

[0056] In step e), the total acidity of solution Al, expressed as H + protons, denoted [H + ], can be determined, for example, by the following equation (3):

[0057]

[0058] where:

[0059] V1and V2, pHA2and pHA5are as defined previously, V1and V2being expressed in L; and

[0060] Y is determined by the following equation (4):

[0061]

[0062] wherein:

[0063] K HA is the acid dissociation constant of the weak acid in solution A5 if solution A2 comprises a weak acid or a salt of a weak acid or the conjugate acid of the weak base in solution A5 if solution A2 comprises a weak base or a salt of a weak base;

[0064] [A - ] is the concentration of the conjugate base of the weak acid in solution A2 or the concentration of the weak base in solution A5 in mol / L;

[0065] [HA] is the concentration of the weak acid in solution A2 or the concentration of the conjugate acid of the weak base in solution A5 in mol / L; and

[0066] V2 and pHA5 are as defined above, V2 being in units of L.

[0067] It is noted that the acid dissociation constant K HA can be determined beforehand from literature data or, preferably, by experiment.

[0068] In the latter case, a volume V1 of a strong acid aqueous solution of known concentration C1, for example a commercially available standard solution, is added to a volume V2 of solution A2, and the pH of the resulting mixture M is then measured with a pH probe, K HA is determined from the following equation (5):

[0069]

[0070] wherein:

[0071] pH corresponds to the pH of the mixture M;

[0072] C1 and C2 are as defined above, in mol / L;

[0073] V1 and V2 are as defined above, in L; and

[0074] n is the number of moles of acid or base used (where applicable) to adjust the pH of solution A2.

[0075] The dissociation constant K Hc of the acid type dye in solution A5 can also be determined from literature data or, preferably, by experiment.

[0076] In the latter case, the UV-visible absorption spectrum of the previously defined mixture M is determined by using solution A2 as the measurement blank, and then the absorption spectrum of the mixture M' resulting from the addition of V4 volumes of solution A4 to the mixture M is determined by using the mixture M as the measurement blank.

[0077] Referring to the previously established calibration curve, it is then possible to measure the concentration of at least one of the acid and base forms of the dye in the mixture M' from the peaks of the acid and base forms of the dye visible on the absorption spectrum obtained from the mixture M', and to determine the dissociation constant K of the dye by means of the following equation (6) HC :

[0078]

[0079] wherein:

[0080] pH is the pH of the mixture M; and

[0081] R is the ratio between the concentration of the base form of the dye in the mixture M' and the concentration of the dye (acid form + base form) in the mixture.

[0082] The subject of the present application is also a second method, the aim of which is to measure the free acidity of an aqueous solution Al containing a strong acid or a mixture of strong acids and one or more hydrolysable cations, the method comprising at least the following steps:

[0083] a) providing an aqueous solution A3 by mixing V1 volumes of solution Al and V2 volumes of an aqueous solution A2 having a pH of pHA2, said aqueous solution A2 comprising a compound chosen from a weak acid, a weak base, a weak acid salt or a weak base salt, and a C2 concentration of a complexing agent of the hydrolysable cations;

[0084] b) measuring the UV-visible absorption spectrum of the hydrolysable cations present in solution A3, using solution A2 as the measurement blank;

[0085] c) determining the concentration C3 of the hydrolysable cations in solution A3 from the absorption spectrum obtained in step b);

[0086] d) providing an aqueous solution A5 by mixing solution A3 and V4 volumes of a solution A4 comprising a pH-sensitive dye, said pH-sensitive dye having an acid form and a base form, and a transition range between a first pH and a second pH, said second pH being higher than said first pH but lower than pHA2;

[0087] e) measuring the UV-visible absorption spectrum of the dye present in solution A5, using solution A2 or solution A3 as the measurement blank;

[0088] f) measuring the concentration of at least one of the acid and base forms of the dye in solution A5 according to the absorption spectrum obtained in step e);

[0089] g) determining the pH of solution A5, noted pHA5, according to the concentration measured in step f); then

[0090] h) determining the free acidity of solution Al according to the concentration C3 measured in step c) and the pHA5 measured in step e);

[0091] and wherein:

[0092] pHA2 and the volumes VI and V2 are such that solution A3 has a pH value in the transition range of the dye, noted pHA3;

[0093] the concentration C2 and the volumes VI and V2 are such that the complexing agent is in excess with respect to the hydrolysable cations in solution A3.

[0094] The second method is based on the same principle as the first method, but differs from the latter in that, on the one hand, solution A2 comprises a complexing agent of the hydrolysable cations (s) present in solution Al of which the free acidity is to be determined, and, on the other hand, the method comprises the determination of the concentration of hydrolysable cations by spectrometry, this concentration being taken into account for determining the concentration of H + protons provided only by the acids in solution Al.

[0095] According to the application, the choice of complexing agent is not particularly limited, provided that this complexing agent is capable of forming a stable water-soluble complex with the hydrolysable cations known to be present or assumed to be present in solution Al.

[0096] Thus, the complexing agent can be, in particular, a polycarboxylic acid of glycine, citric acid, tartaric acid, oxalic acid or succinic acid type, a hydroxycarboxylic acid of gluconic acid, idonic acid or galactonic acid type, a hydroxydicarboxylic acid of glucaric acid, galactaric acid or mannaric acid type, an aminopolycarboxylic acid of iminodiacetic acid (IDA), nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) type, or a salt thereof, for example an alkali metal salt such as sodium or potassium, an alkaline earth metal salt such as calcium or magnesium, or even a transition metal salt.

[0097] Preferably, the compound present in solution A2 is also used as complexing agent, in particular if the compound present in solution A2 is chosen from the complexing acids mentioned above and their salts, in particular if solution A2 is a solution of oxalic acid or of a salt thereof.

[0098] According to the application, step b) preferably comprises:

[0099] the light intensity spectrum of solution A2 in the ultraviolet-visible range is obtained;

[0100] the light intensity spectrum of solution A3 in the ultraviolet-visible range is obtained; and

[0101] the logarithm to the base 10 of the ratio of the light intensity values of the obtained spectrum of solution A3 to the light intensity values of the obtained spectrum of solution A2 is calculated;

[0102] The light intensity spectra of solutions A2 and A3 are advantageously obtained by means of a CCD camera.

[0103] The absorption spectrum thus obtained is characteristic of the hydrolysable cations present in solution A3 and therefore in solution Al.

[0104] In step d), the concentration C3 of the hydrolysable cations in solution A3 is advantageously determined from the absorption peak visible on the absorption spectrum of solution A3 obtained in step c), also by applying the Beer-Lambert law, i.e. in practice by comparing the maximum absorbance shown by the absorption spectrum obtained in step c) with the absorbance values of a standard curve, previously established from aqueous solutions containing the hydrolysable cations known to be or assumed to be present in solution Al in variable concentrations.

[0105] According to the application, step e) preferably comprises:

[0106] the light intensity spectrum of solution A5 in the ultraviolet-visible range is obtained; and

[0107] the logarithm to the base 10 of the ratio of the light intensity values of the obtained spectrum of solution A5 to the light intensity values of the obtained spectrum of solution A3 is calculated;

[0108] The light intensity spectrum of solution A5 is also advantageously obtained by means of a CCD camera.

[0109] Step f) serves to measure the concentration of at least one of the acid and base forms of the dye in solution A5 from the absorption spectrum obtained in step e), step f) being advantageously carried out in the same way as step c) of the first method.

[0110] In a similar manner, in step g), pHA5 is advantageously determined by one of the equations (1) and (2) defined above in step d) of the first method.

[0111] In step h), the free acidity of solution Al, expressed in molar concentration of protons, is denoted [H + In step i), the total acidity of solution Al, expressed in molar concentration of protons, is denoted [H +] is determined in consideration of the concentration C3 determined in step c), for example by equation (3) defined by step e) of the first method above, but wherein Y is determined by applying the following equation (7):

[0112]

[0113] wherein:

[0114] m is the stoichiometric coefficient of the hydrolysable cation in the complexation reaction of the cation with the complexing agent;

[0115] n is the stoichiometric coefficient of the complexing agent in the complexation reaction of the hydrolysable cation with the agent; and

[0116] V1, V2, K HA , [A - ], [HA] and C3 are as defined above, V1 and V2 are expressed in units of L, and [A - ], [HA] and C3 are expressed in units of mol / L.

[0117] In the first method or the second method, solution A2 preferably comprises a weak acid or a salt of a weak acid.

[0118] Preferably, solution A2 comprises oxalic acid or a salt thereof, preferably sodium oxalate.

[0119] Furthermore, the dye is preferably bromocresol green, which has a transition range between pH 3.8 and pH 5.4.

[0120] In this case, it is particularly preferred that solution A2 is a solution comprising 0.27 mol / L of sodium oxalate, which initially has a pH of 8, which is adjusted by the addition of a strong acid, for example sulfuric acid, so that its pH is higher than 5.4, while being less than 8, for example between 5.5 and 6.

[0121] For solution Al, an aqueous solution of nitric acid is preferably used, with or without a hydrolysable cation.

[0122] However, solution Al can also be an aqueous solution other than nitric acid, for example an aqueous solution containing or not containing a hydrolysable cation comprising hydrochloric acid, sulfuric acid, hydrofluoric acid, etc. or mixtures thereof.

[0123] Other features and advantages of the present application will be apparent from the following further description, which relates to tests for verifying the present application, and is given with reference to the attached drawings.

[0124] However, it goes without saying that this further description is given merely as an illustration of the present application and should not be interpreted in any way as a limitation of the subject matter. BRIEF DESCRIPTION OF DRAWINGS

[0125] Figure 1 An example is shown of the light intensity spectrum (denoted I and expressed in counts) of solution A2 containing 0.27 mol / L sodium oxalate, pH 5.7, obtained by a CCD camera in the 400 nm-750 nm wavelength range.

[0126] Figure 2 An example is shown of the light intensity spectrum (denoted I and expressed in counts) of solution A3, obtained by a CCD camera in the 400 nm-750 nm wavelength range, by mixing 1000 μL of solution A2 (the spectrum of which is shown in Figure 1 ) and 50 μL of sample A1 containing 2 mol / L nitric acid and 0.56 mol / L of uranium (VI) in water.

[0127] Figure 3 An example is shown of the light intensity spectrum (denoted I and expressed in counts) of solution A5, obtained by a CCD camera in the 400 nm-750 nm wavelength range, by mixing solution A3 (the light intensity spectrum of which is shown in Figure 2 ) and 150 μL of 0.02 wt% bromocresol green aqueous solution A4.

[0128] Figure 4 An example is shown of the absorption spectrum (denoted A and expressed in arbitrary units) of the uranyl cations present in solution A3, obtained by using the light intensity spectrum shown in Figure 2 as the measured value and the light intensity spectrum shown in Figure 1 as the measured blank.

[0129] Figure 5 An example is shown of the absorption spectrum (denoted A and expressed in arbitrary units) of the bromocresol green present in solution A5, obtained by using the light intensity spectrum shown in Figure 3 as the measured value and the light intensity spectrum shown in Figure 2 as the measured blank. DETAILED DESCRIPTION

[0130] The method of the application has been tested in order to verify whether it can obtain the free acidity, i.e. the acidity related to the presence of nitric acid alone, of an aqueous solution containing, in addition to nitric acid at a known concentration, also uranium (VI) and / or plutonium (IV) at a known concentration, with an acceptable deviation, ideally less than 5%.

[0131] These tests have been carried out both manually and by automation.

[0132] Example I - Manual Testing :

[0133] The manual tests were carried out at ambient temperature (20-25°C) in a nuclear glove box, using:

[0134] Solution A2: a saturated aqueous solution of sodium oxalate containing 0.27 mol / L of Na2C2O4, the pH of which was adjusted to 5.7 by addition of sulfuric acid;

[0135] A1 solution to be analysed: a series of aqueous solutions comprising:

[0136] nitric acid in a concentration ranging from 1.0 mol / L to 4.99 mol / L, and

[0137] 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;

[0138] Solution A4: an aqueous solution of 0.02% of bromocresol green; and

[0139] Spectrophotometer: SpectraPro SP500i UV-Visible spectrophotometer (Roper Scientific) equipped with a cell with an optical path equal to 10 mm.

[0140] Each test was carried out using a volume of 1000 μL of solution A2, a volume of 50 μL of solution A1 and a volume of 150 μL of solution A4.

[0141] The acid dissociation constant K of oxalic acid under the test conditions HA and the dissociation constant K of the acid form of bromocresol green HC were determined beforehand by experiment.

[0142] Under the test conditions, K HA was equal to 3.8 and K HC was equal to 4.55.

[0143] The tests were carried out according to the following protocol:

[0144] 1. The volume of solution A2 was added in a spectrophotometric cuvette, the optical intensity spectrum in the UV-Visible range (here 400 nm-750 nm) was obtained;

[0145] 2. The volume of solution A1 was added in the spectrophotometric cuvette, this volume was mixed with the solution A2 already present in the cuvette and the optical intensity spectrum at the same wavelengths as above was obtained;

[0146] 3. The volume of solution A4 was added in the spectrophotometric cuvette and this volume was mixed with the mixture of solution A2 / solution A1 already present in the cuvette and the optical intensity spectrum at the same wavelengths as above was obtained;

[0147] 4. By calculating the base-10 logarithm of the ratio of the light intensity value of the spectrum obtained in point 2 above to the light intensity value of the spectrum obtained in point 1 above, the absorption spectrum of the hydrolyzable cation at the same wavelength is determined, which for uranium (VI) is UO2. 2+ And for plutonium (IV) it is pu 4+ ;

[0148] 5. The absorption spectrum of the dye at the same wavelength is determined by calculating the logarithm to base 10 of the ratio of the light intensity value of the spectrum obtained in point 3 above to the light intensity value of the spectrum obtained in point 2 above.

[0149] 6. Based on the maximum absorbance of the basic dye observed on the absorption spectrum obtained in point 5 above, and referring to the previously established standard curve, determine the ratio R between the concentration of the basic dye present in the mixture of solution A2 / solution A1 / solution A4 and the total concentration of dye in the mixture;

[0150] 7. The pH value of the mixture of solution A2 / solution A1 / solution A4 is determined using the equation (2) defined above. In this example, the equation (2) defined above becomes:

[0151] as well as

[0152] 8. Determine the free acidity of solution A1 using equation (3) as defined above, in terms of H+. + The molar concentration of protons is expressed as [H]. + In this example, equation (3) defined above becomes:

[0153]

[0154] And among them:

[0155] pHA5 is the pH of the mixture of solution A2 / solution A1 / solution A4 as determined in point 7 above, and

[0156] Y is determined by equation (7) as defined above. In this example, equation (7) becomes:

[0157]

[0158] in:

[0159] When m equals 1 and n equals 3, and oxalic acid is in excess, UO2 2+ and Pu 4+ The complexation reaction of cations is written as:

[0160] UO2 2+ +3C2O4 2- UO2(C2O4)34-

[0161] Pu 4+ +3C2O4 2- <=> Pu(C2O4)3 2-

[0162] C3 represents the concentration of UO2 in the mixture solution A2 / solution Al / solution A4 2+ or Pu 4+ in mol / L.

[0163] In the present test, the concentration C3 is not obtained experimentally, but by introducing into equation (7) the concentration of uranium (VI) or plutonium (IV) in the analytical solution Al.

[0164] The following table I specifies each analytical solution Al:

[0165] the concentration of uranium (VI) or plutonium (IV) in g / L,

[0166] the concentration of nitric acid in mol / L,

[0167] the pH value theoretically assumed by the mixture solution A2 / solution Al / solution A4 resulting from point 3 above (called "theoretical pH value"),

[0168] the pH value of the mixture determined according to point 7 above (called "measured pH value" in table I),

[0169] the free acidity of solution Al in mol / L, determined according to point 8 above (called "measured free acidity" in table I), and

[0170] the relative deviation in % between the concentration of nitric acid in solution Al and the free acidity of this solution.

[0171] Table I

[0172]

[0173] This table shows that the method of the present application makes it possible to measure the free acidity of acidic aqueous solutions containing hydrolysable cations, with a deviation of at most 4% with respect to the true value of this acidity, and this independently of the concentrations of the acid and of the hydrolysable cations of these solutions.

[0174] Example II - Automated Testing :

[0175] The automated tests were carried out at ambient temperature (20-25°C) using equipment comprising:

[0176] the same spectrophotometer as used in the manual tests described in example I above;

[0177] an apparatus for dispensing into the cuvette of the spectrophotometer the volumes of solution A2, A1 and A4 required for the test, by means of a micropipette; and

[0178] software for controlling the dispensing apparatus and software for processing and analysing the data provided by the ultraviolet-visible spectrum obtained by the spectrophotometer.

[0179] These tests were also carried out using:

[0180] the same solution A2 and the same solution A4 used in the manual tests described in Example I above; and

[0181] the solution A1 for analysis: an aqueous solution, hereinafter referred to as solutions 1, 2, 3 and 4, containing:

[0182] nitric acid in a concentration range of 1.08 mol / L - 10.02 mol / L, and

[0183] uranium (VI) in a concentration range of 45 g / L - 75 g / L and plutonium (IV) in a concentration range of 4 g / L to 7 g / L.

[0184] The composition of these solutions is specified in Table II below.

[0185] This table also specifies the volume of each solution A1 to be analysed, the volume of this solution used and the volume of solutions A2 and A4 used.

[0186] Table II

[0187]

[0188] All the solutions A1 were subjected to repeated analysis according to the same method described in Example I above, with the difference that the concentration of the uranyl cation UO2 2+ and of the Pu 4+ + cation in each solution A2 / solution A1 / solution A4 mixture was determined by reference to the previously established standard curve from the absorption spectrum obtained in point 4 of the protocol.

[0189] As above, the free acidity of the solution A1, expressed as the molar concentration of H + + protons, is denoted by [H + ] and is determined by means of equation (3) defined above.

[0190] The following table III shows each analysed solution A1 and each repetition of this solution:

[0191] the concentration of uranium (VI) and the concentration of plutonium (IV) in the solution A1, expressed in g / L, determined from the absorption spectrum obtained in point 4 of the protocol,

[0192] the pH value of the mixture resulting from point 3 of the operating protocol (referred to as "theoretical pH value"),

[0193] the pH value of the mixture determined at point 7 of the operating protocol (referred to as "measured pH value") in Table III,

[0194] the free acidity of solution Al, expressed in mol / L, determined at point 8 of the operating protocol (referred to as "measured free acidity") in Table III, and

[0195] the relative deviation, expressed in %, existing between the concentration of nitric acid in solution Al and the free acidity of this solution.

[0196] Table III

[0197]

[0198] This table shows that the method of the application, when implemented in an automated manner, makes it possible to measure the free acidity of aqueous acid solutions containing a hydrolysable cation with a deviation of less than 2% with respect to the true value of this acidity, and this independently of the concentrations of the acid and of the hydrolysable cation of these solutions.

[0199] Cited references

[0200] [1] T. G. Srinivasan and P. R. Rao, Talanta 2014, 118, 162-171.

[0201] [2] J. Néri-Quiroz et al., Talanta 2016, 159, 330-335.

Claims

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

2. The method according to claim 1, wherein step b) comprises: obtaining the light intensity spectrum of solution A2 in the UV-visible range; obtaining the light intensity spectrum of solution A5 in the UV-visible range; and calculating the base 10 logarithm of the ratio of the light intensity values of the obtained spectrum of solution A5 and the light intensity values of the obtained spectrum of solution A2. The concentration of at least one of the acid form and the base form of said dye in solution A5 is determined by applying the Beer-Lambert law. pHA5 is determined by equation (1): wherein: [HC] is the concentration of the dye in the acid form in solution A5 in mol / L; and pHA5 is determined by equation (2): wherein: R is the ratio of the concentration of the dye in the base form in solution A5 to the total concentration of the dye in the base form and the dye in the acid form in solution A5. wherein: V1 and V2 are expressed in units of L; and Y is determined by equation (4): wherein: if solution A2 comprises a weak acid or a weak acid salt: [HA] is the concentration of the weak acid in solution A2 in mol / L; and if solution A2 comprises a weak base or a weak base salt: [HA] is the concentration of the conjugate acid of the weak base in solution A5 in mol / L; and V2 is expressed in units of L.

7. A method of measuring the free acidity of an aqueous solution Al, said aqueous solution Al comprising a strong acid or a mixture of strong acids and one or more hydrolysable cations, said method comprising at least the steps of: a) providing an aqueous solution A3 by mixing V1 volume of said solution Al and V2 volume of an aqueous solution A2 having a pH of pHA2, said aqueous solution A2 comprising a compound selected from a weak acid, a weak base, a weak acid salt or a weak base salt, and a C2 concentration of a reagent complexing said hydrolysable cations; ​ ​ ​ 3. The method of claim 2, wherein, ​ 4. The method of claim 1, wherein, ​ （1） ​ K HC is the dissociation constant of the dye in acid form in the solution A5; ​ is the concentration of the dye in base form in the solution A5 in mol / L.

5. The method of claim 1, wherein, ​ （2） ​ K HC is the dissociation constant of the dye in acid form in the solution A5; and ​ 6. The method of claim 1, wherein, The total acidity of the solution A1, expressed in moles of protons, is denoted by ] and is determined by equation (3): ] （3） ​ ​ ​ （4） ​ ​ K HA is the acid dissociation constant for the weak acid in the solution A5; is the concentration of the conjugate base of the weak acid in the solution A2 in mol / L; ​ ​ K HA is the acid dissociation constant of the conjugate acid of the weak base in the solution A5; is the concentration of the weak base in the solution A5 in mol / L; ​ ​ ​ ​ b) measuring the UV-visible absorption spectrum of the hydrolysable cations present in the solution A3 using the solution A2 as a measurement blank; c) determining the concentration C3 of the hydrolysable cations in the solution A3 from the absorption spectrum obtained in step b); d) providing an aqueous solution A5 by mixing the solution A3 and V4 volumes of a solution A4, the 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 the pH A2; e) measuring the UV-visible absorption spectrum of the dye present in the solution A5 using the solution A2 or the solution A3 as a measurement blank; f) determining the concentration of at least one of the acid form and the base form of the dye in the solution A5 from the absorption spectrum obtained in step e); g) determining the pH value of the solution A5, noted pH A5, from the concentration measured in step f); then h) determining the free acidity of the solution Al from the concentration C3 measured in step c) and the pH A5 measured in step e); and wherein: pH A2 and the volumes VI and V2 are such that the solution A3 has a pH in the transition range of the dye, noted pH A3; the concentration C2 and the volumes VI and V2 are such that the complexing agent is in excess with respect to the hydrolysable cations in the solution A3.

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

9. The method according to claim 7, wherein step b) comprises: obtaining an optical intensity spectrum in the UV-visible range of the solution A2; obtaining an optical intensity spectrum in the UV-visible range of the solution A3; and calculating the base-10 logarithm of the ratio of the optical intensity values of the obtained spectrum of the solution A3 and of the obtained spectrum of the solution A2.

10. The method of claim 9, wherein, The concentration C3 of the hydrolysable cations is determined by applying the Beer-Lambert law.

11. The method according to claim 9, wherein step e) comprises: obtaining an optical intensity spectrum in the UV-visible range of the solution A5; and calculating the base-10 logarithm of the ratio of the optical intensity values of the obtained spectrum of the solution A5 and of the obtained spectrum of the solution A3.

12. The method of claim 11, wherein, The concentration of at least one of the acid form and the base form of the dye in the solution A5 is determined by applying the Beer-Lambert law.

13. The method of claim 7, wherein, pH A5 is determined by equation (1): （1） wherein: K HC is the dissociation constant of the dye in acid form in the solution A5; [HC] is the concentration of the dye in acid form in the solution A5, in mol / L; and is the concentration of the dye in base form in the solution A5 in mol / L.

14. The method of claim 7, wherein, pH A5 is determined by equation (2): （2） wherein: K HC is the dissociation constant of the dye in acid form in the solution A5; and R is the ratio of the concentration of the dye in base form in the solution A5 to the total concentration of the dye in base form and of the dye in acid form in the solution A5.

15. The method of claim 7, wherein, The free acidity of solution A1, in order to The molar concentration of protons is expressed as [ ], determined by equation (3): （3） wherein: VI and V2 are expressed in L; and Y is determined by equation (7): （7） wherein: m is the stoichiometric coefficient of the hydrolysable cations in the complexation reaction of the hydrolysable cations with the complexing agent; n is the stoichiometric coefficient of the complexing agent in the complexation reaction of the hydrolysable cations with the complexing agent; if the solution A2 comprises a weak acid or a salt of a weak acid, then: K HA is the acid dissociation constant for the weak acid in the solution A5; is the concentration of the 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 HA is the acid dissociation constant of the conjugate acid of the weak base in the solution A5; is the concentration of the weak base in the solution A5 in mol / L; [HA] is the concentration of the conjugate acid of the weak base in the solution A5 in mol / L; and V1 and V2 are in units of L and C3 is in units of mol / L.

16. The method of any one of claims 1 to 15, wherein, The solution A2 comprises a weak acid or a salt of a weak acid.

17. The method of any one of claims 1 to 15, wherein, The solution A2 comprises oxalic acid or a salt thereof.

18. The method of claim 17, wherein, The solution A2 comprises 0.27 mol / L sodium oxalate.

19. The method of any one of claims 1 to 15, wherein, The dye is bromocresol green.

20. The method of any one of claims 1 to 15, wherein, The solution A1 is an aqueous solution of nitric acid.

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