FR2581193A1 - PROCESS FOR THE ANALYSIS OF ORGANIC ACIDS IN SOLUTION AND DEVICE FOR CARRYING OUT THIS PROCESS. - Google Patents

Abstract

THIS PROCESS FOR THE ANALYSIS OF ORGANIC ACIDS IN A TEST SOLUTION CONTAINING INORGANIC ANIONS AND ORGANIC ACIDS CONSISTS OF VEHICULATING THIS TEST SOLUTION WITH A MOBILE ACID PHASE UP TO A SEPARATION COLUMN7 IN ORDER TO BE FRACTIONED INTO A FRACTION OF ACIDS ORGANIC AND A FRACTION OF INORGANIC ANIONS, ORGANIC ACIDS BEING SEPARATED AT THE SAME TIME FROM ONE OF THE OTHER BY CHROMATOGRAPHY. THE SEPARATE ORGANIC ACIDS ARE THEN SUBJECTED TO CATIONIC EXCHANGE WITH A SUPPRESSOR8 AND THE ELECTRICAL CONDUCTIBILITY OF THE RESULTING PRODUCT IS FINALLY DETERMINED WITH THE AID OF A DETECTOR9.

Description

This invention relates to a method for analyzing a solution to

  test containing organic acids via separation by chromatography and to an analysis device implementing this method. As means of analysis using a separation by chromatography of a test solution containing interesting constituents and ions, the

  The following techniques are known from the prior art.

  The first technique performs this analysis by supplying with a suitable eluent, a separation column packed with an anion exchanger with a low ion exchange capacity of the order of 20 to 40 peq / g, separating the organic acid of the solution to be tested by

  ion exchange and then determining the contents

  organic acids using a detector such as, for example, an electrical conductivity meter (this technique is designated below:

  "first conventional technique").

  The second technique performs the analysis by magnetizing with a mobile phase formed of an acid such as, for example, phosphoric acid, a separation column packed with a strongly acid cation exchange resin with an ion exchange capacity. of the order of 4 meq / g, by separating the organic acids from the solution to be tested by deionization or adsorption and then detecting the contents of organic acids with a detector such as, for example, an apparatus for measuring electrical conductivity (this technique is called

  o30 below: "second conventional technique").

  The third technique performs the analysis by feeding with a mobile phase formed of hydrochloric acid, in a similar manner to the second technique, by passing the fluid from the column through a column packed with a cation exchange resin in silver form (Ag). and then determining the contents of organic acids with a detector such as, for example, an apparatus for measuring electrical conductivity (this technique is hereinafter referred to as "third conventional technique"). The first conventional technique has the disadvantage, however, that in the separation column, the inorganic ions and the organic acids are eluted at one and the same position or at very similar positions and the qualitative or quantitative analysis cannot be carried out. precisely. The second conventional technique has the disadvantage of the fact that organic acids are detected with low sensitivity and that organic acids, when they are

  present in a minimal quantity, escape detection.

  The third conventional technique has the disadvantage of the fact that the ion-exchange resin in Ag form reacts with the chloride ion present in the test solution, giving rise to solid silver chloride (AgCl), the resin resin. ion exchange in Ag form is then likely to be clogged and the resistance of the column to a flow of fluid

  increases to affect ease of use.

  This invention was produced with respect to the conventional techniques described above. An object of this invention is to provide a method which allows analysis with a high sensitivity of a test solution of organic acids even when the test solution contains foreign inorganic anions and, a device implementing this method. This invention has achieved the above-mentioned object with a method and a device which performs the analysis of the test solution of organic acids by conveying the test solution with a mobile phase formed from an acid solution to a appropriate separation column, by dividing inside the latter, the test solution into a fraction of organic acids and a fraction of inorganic anions and, at the same time, by separating the organic acids by chromatography, then subjecting organic acids to a cation exchange with a suppressor using a cation exchange membrane or a suppressor column) packed with a cation exchange resin and, by detecting electrical conductivity. The invention will be better understood on reading

  the description which follows, made with reference to

  annexed drawings given solely by way of example, in which: FIG. 1 is a schematic representation of an embodiment of this invention; FIGS. 2 and 3 are chromatograms obtained in accordance with this invention; -Fig. 4 is a schematic representation of another

  embodiment of this invention.

  In these diagrams, the references la to If each denote a reservoir, the references 2a to 2c each denote a liquid supply pump, the reference 3 denotes a damping member, the reference 4 a pressure gauge, the reference 5 a valve sampling, the references 6 and 7 each designate a column, the reference 8 a suppressor, the reference 8 'a column forming a suppressor, the reference 9 a detector and, the references 10 and 10' each designate a

constant temperature.

  Fig. 1 is a schematic representation of an embodiment of the present invention. In this diagram, the reference designates it a storage tank for an eluent formed, for example, of H2SO4 2mN, the reference lb a tank of storage of an elimination liquid formed, for example, of 10 mN KOH (designated below: "first elimination liquid"), the reference lc a storage tank for a solution to be tested, the references 2a to 2c denote c6acne a liquid supply pump, the references 1d to If each denote a tank of rejected liquid, the reference 3 a damping member intended to avoid pulses in the eluent, the reference 4 a pressure gauge, the reference 5 a sampling valve provided, on the one hand, with a first to a sixth connection orifice designated by the references 5a to 5f and, with a measurement tube 5g of an internal volume of for example, 100 1l and adapted to have its internal flow path alternately switched between the connection state illustrated in solid lines and the connection state illustrated in dotted lines, the references 6 and 7 respectively designate a guard column (for example of 8 * mm of internal diameter and of 5 cm in length) filled with a resin strongly acid cation exchanger having an ion exchange capacity of, for example, 4 to 5 meq / g and a separation column (for example 6 mm in internal diameter and 25 mm in length), the reference designates a double suppressor tube whose interior is divided into an internal chamber 8b and an external chamber 8c

  with a Sa tube made for example, with a membrane.

  cation exchanger, the reference 9 designates a detector such as for example, an apparatus for measuring the electrical conductivity and the reference 10 designates a bath at constant temperature intended to house the precolumn 6, the separation column 7, the suppressor 8 and the detector 9 in order to maintain them at a prescribed temperature (for example 40C). The guard column 6 is not an essential component. It can be omitted when the

separation column 7 is sufficient.

  In the embodiment of this invention, carried out as described above, the speed of rotation of the pump 2a is adjusted so that the eluent from the reservoir la is conveyed at a flow rate of 1 ml / min to pass successively from this pump 2a, in the damping member 3, the pressure gauge 4, the first and second connection orifices 5a, Sb of the sampling valve 5, the guard column 6, the separation column 7, the internal chamber 8b of the suppressor 8, the detector 9 and finally in the tank the rejected liquid. The speed of rotation of the pump 2c is adjusted so that the solution to be tested from the reservoir lc is conveyed at a volume flow rate of for example, 1 ml / min to pass successively from this pump 2c, in the fourth connection orifice 5d from the sampling valve 5, the third connection orifice c, the measuring tube 5g, the sixth connection orifice 5e and finally into the tank If of rejected liquid. The speed of rotation of the pump 2b is adjusted to convey the liquid for eliminating the reservoir lb at a volume flow rate of for example, 2 ml / min so that this liquid passes successively from this pump 2b, into the external chamber 8c of suppressor 8 and finally in

  the tank ld of rejected liquid.

  The ion exchange group of the tube 8a in the suppressor 8 is therefore in the H + form. The liquid for removing the reservoir 1b is then replaced with water, the external chamber 8c of the suppressor 8 is filled with water and the pump 2b is stopped. At this point, the output of detector 9 is set to 1200 pS / cm and this value acts as the base value. A sample containing 50 ppm of malonic acid, 10 ppm of hydrofluoric acid is introduced into the abovementioned measuring tube 5g. 50 ppm formic acid, 500 ppm acetic acid and 1000 ppm propionic acid (sample hereinafter referred to as "standard sample") and, at this stage, the sampling valve 5 is turned to switch the path internal flow from the connection state illustrated in solid lines to the state

  connection illustrated in dotted lines.

  The standard sample present in the measuring tube is conveyed with the eluent in the precolumn 6 and the separation column 7 to be separated therein inorganic anions on the one hand and in organic acids on the other hand. At the same time, the organic acids are separated and eluted by chromatography. In the precolumn 6 and the separation column 7, the anions are removed (that is to say they are not retained in the columns) and are elected and, the

  weak anions are retained and eluted in the columns.

  Since inorganic anions are strongly ionic and organic acid ions are weakly ionic, these two classes of ions are separated from each other and

  the organic acid ions are eluted by chromatography.

  The solution leaving the separation column 7 is conveyed to the internal chamber Sb of the suppressor 8 where it is brought into contact through the middle of the tube 8a with the elimination liquid present inside the external chamber 8c to undergo an ion exchange. In this case, since the eluent is sulfuric acid and the ion exchange group of the cation exchange membrane is in the H + form, the eluent passes through the suppressor while remaining intact, in the same form in which he entered the suppressor. The eluent is then conveyed to the detector 9 for the determination of physical constants (such as for example, the electrical conductivity). The output (the detection signal) of the detector 9 is routed, for example, to a recorder (not shown) to> form a chromatogram there. Fig.2 shows a chromatogram prepared in this way. If the chromatogram is obtained when the eluent is caused to avoid the abovementioned suppressor 8, the peak heights of the different ions are increased by 20 to 30% Y and the retention times of the different ions are each advanced by about 1 minute

  while the baseline value remains unchanged.

  In the meantime, the liquid for eliminating the reservoir 1b is switched to the aforementioned second liquid for elimination and it is made to flow at a rate

  volume of 2 ml / min for about 30 minutes.

  When the above standard sample is analyzed in the same way as above, the chromatogram in fig. 3 is obtained. The comparison of the chromatograms of fig. 2 and 3, obtained as described above, reveals that the absolute value of the baseline (LB) of the chromatogram of fig. 3 drops to 0.4 times that of the chromatogram in fig. 2 and the peak heights of the various organic acids in the chromatogram in fig. 3 increased to 2 to 80 times those of the chromatogram in fig. 2. The ion F, although it is an inorganic anion, is not completely removed but is also retained in the separation column 7 above and is eluted. The anions, Cl, No ;, PI, SO 2 and Br, are removed completely and

3 4.

  are eluted at a position 'corresponding to the plunging peak

some water.

  Table 1 shows the results of the analysis of the chromatograms in Figs. 2 and 3. This table compares the two series of data each corresponding to a respective elimination liquid present in the tank lb, the basic absolute value detected by the detector 9, the pH of the mobile phase and the peak heights of the different ions. It simultaneously shows the baseline decrease ratios calculated from the variations of the baseline absolute value, the peak height increase ratios and the S / N increase ratios (suppressor system / non suppressor system ) calculated from variations in the peak heights of different ions. It will be noted from this table that in the process which provides the chromatogram of FIG. 3, the S / N ratio of the malonate ion increased to be worth 4.6 times, the F ion to 8 , 1 time, the formate ion up to 15 times, the acetate ion up to 140 times and the propionic acid ion up to 200 times, the corresponding S / N ratios obtained

  by the process giving the chromatogram of fig. 2.

  These increases in the S / N ratio occur for the following reason. For explanatory purposes, an acetic analysis (CIH3COO) will be simulated in a test solution using 2 mN sulfuric acid as the mobile phase and 10 mN potassium hydroxide as the elimination liquid. The sulfuric acid and the acetic acid are subjected after elution in the separation column 7 above, to a cation exchange in the suppressor 8 and, are consequently converted respectively into I SO 4 and CH 3 COOK. The absolute value of the baseline thus decreases from 1200 PS / cm to 480 YS / cm and the pH of the mobile phase increases from 2.7 to 8.5. As a result of this increase in pH, the degree of dissociation increases and

  the aforementioned increase in the S / N ratio is obtained.

  TABLE 1 (results of analyzes of fig. 2 and 3) -15 (N) Rarrtc system (s) from RaqxtPa-rt non 5. te de ingldtan detadrai rlR - ata,! Supresseur sure e hazr dei S / N Liauide d ' elimination '10 MN

I | XOH _ _ _

  Absolute value of 480 base _ 1200.uS/a s / cm 0.4

pH 2.7 8.5 - - -

  dai maknoe 9.75 18 ppm [(ps / cmiuS / cm] 1.85 4.63 F 10 ppm12 3925 Height tp) s / cm] [IS / cm] - 3.25 8.13 of Formic acid 15 87.5 Pic 5 (PPM (ls / cm] '. [FS / cm] D Peak 50 ppm (ps / friend E} is / cmi - 5.83 14.6 Acetic acid 9.3 520 500 ppm [Ps / cm] [yS / cM 9 140 Proplo- acid 7.2 565 78.5 196 ni. as [lsc] ElV I 1,000 ppm Is / Cm] s /] 1 As described in detail above, since this invention basically comprises the fractionation of a test solution into a fraction of organic acids and a fraction of inorganic ions and, the separation by chromatography of the organic acids from each other as well as the determination of their contents, it has the advantage of allowing analyze the organic acids in the test solution with high sensitivity even when the test solution contains foreign inorganic anions. In addition, since the weak ion species with small dissociation constants (this is say high pKa values), are easily dissociated due to the increase in the pH of the mobile phase, this invention has the advantage of the fact that the determination of organic acids can be obtained with a higher sensitivity than with the

  conventional techniques mentioned above.

  Fig.4 is a schematic representation of another embodiment of this invention. In fig. 4, the numerical symbols which have their equivalent on

  Fig. 3, designate identical or similar parts.

  The description of these equivalent parts is omitted here

  to avoid unnecessary repetition. In this diagram, the reference 8 'designates a column forming a suppressor (for example 6 mm in internal diameter and 25 cm in length) packed with a strongly acidic cation exchange resin of the Kt type and the reference 10' designates a temperature bath. constant to house the precolumn 6, the separation column 7, the column 8 ′ forming a suppressor and the detector 9 by holding them at a

  appropriate temperature (for example 40'C).

  In the present embodiment of this invention as described above, the speed of rotation of the pump 2a is adjusted so that the eluent from the reservoir is sent at a volume flow rate for example of 1 ml / min so that this liquid passes through through this pump 2a successively in the damping member 3, in the pressure gauge 4, in the first and second connection orifices 5a, 5b of the sampling valve 5, in the precolumn 6, in the column of separation 7, in the column 8 ′ forming a suppressor, in the detector 9 and finally in the tank the liquid rejected. The speed of rotation of the pump 2c is adjusted so that the solution to be tested from the reservoir lc is conveyed at a volume flow rate of for example, 1 ml / min so that this liquid passes through the pump 2c successively in the fourth connection port 5d of the sampling valve 5, in the third connection port 5c, in the measuring tube 5g, in the sixth connection port 5f, in the fifth connection port 5e and finally in

  the tank If of rejected liquid.

  At this stage, the sampling valve 5 is turned and the flow path inside it is switched from the connection state illustrated in solid lines to the connection state illustrated in dotted lines . The test solution present in the measurement tube 5g is conveyed with the eluent Up to columns 6 and 7. In columns 6 and 7, the test solution is fractionated into a fraction of inorganic anions and a fraction of organic acids, at the same time, organic acids are separated from each other by chromatography. The solution which flows out of the separation column 7 is conveyed to the suppression column 8 in which the eluent and the abovementioned organic acid ions undergo the ion exchange reactions corresponding to the formulas ( 1) to (6) below:

  2R-503 3.K + H2SO4 - 2R-S03.H + + K2SO4 (1)

  2R-SO 3-K + + CH2 (COOH) 2 2R-SO3-.H + + CH2 (COOK) 2 (2)

R-SO3.K + HF R-SO3.H + KF (3)

  R-SO3.K + HCOOH - R-SO3.H + HCOOK (4)

  R-S03 - K + CH3COOH - R-SO3.H + CH3COOK (5)

  R-S03.K + + C2H5COOH - R- SO- H + + C2H COOK (6)

  Since the eluent undergoes the exchange reaction

2581 193

  it ionic corresponding to the formula <1) and flows through column 8 'forming suppressor, the conductivity

  electric drop from 1200 PS / cm up to 480 PS / cm.

  In addition, due to the cation exchange reaction of this nature, the pH of the eluent varies from about 2.5 to about 2.0. When the aforementioned control sample which contains 50 ppm of malonic acid (H C.COOH), 10 ppm

2 \ COOH

  hydrofluoric acid (HF), 50 ppm formic acid (HCOOH), 500 ppm acetic acid (CH3COOH) and 1000 ppm propionic acid (H5COOH), is used as a test solution, these acids are subjected to exchange reactions

  ionic corresponding to the above formulas (2) to (6).

  The constituents (the ionic species) which have undergone these ion exchange reactions are conveyed to the detector 9 for the determination of the electrical conductivity. The output signal subsequently produced by the detector, causes the recorder (not shown) to form a chromatogram. Table 2 shows the dissociation constants (the -log (Ka) value of Ka, that is to say, the pKa value) of the abovementioned acids, taken from the literature ("Analytical Chemistry Data Book"., 3rd edition revised, compiled by the Japan Analytical Chemistry Society and published by Maruzen). Table 3 compares the chromatogram prepared by directly conveying the eluate from the separation column 7 to the detector 9 instead of using the aforementioned suppression column 8, with the chromatogram prepared using the column 8 'forming a suppressor as described above and shows the peak height increase ratios and the S / N increase ratios calculated from these chromatograms. TABLE 2 Constants for dissociation of acids (pKa) Name of compound Temperature (C3 Coen tratlon (M Ka

HF 20 0.1 (KNO3) 2.35

  Formic acid 25 0 3.25 Acetic acid 25 0.1 4.53

Malonic acid 25 0.1 (KNO3 2.61.

  Propinnic acid 20 0.1 (NaCiO4) 4.66 TABLE 3 Analysis results Name of agent5 ax.pcort dS'aNatî Id

_ _ _ _ _ _

Malonic acid 1.85 4.63

F 3.25 8.13

  Formic acid 5.85 14.6 Aclde acéti.ue55j 9 140

78, 5 196

  Proionic acid 785 196 It can be deduced from Table 2 that the dissociation constants of the acids tpKa) increase in proportion to the increase in the pH of the solution. This increase in the dissociation constants of the acids implies that the degrees of electrical conductivity of these acids also increase and that, consequently, the determination of the contents of the organic acids in the solution to be tested by the

  detector 9 is carried out with great sensitivity.

  Table 3 shows that the ratios of increase in the heights of peak as well as the ratios of increase in the S / N ratio, increase

  in proportion to the pKa increase values.

  The increase in the S / N ratio of malonic acid is therefore approximately 5 times and that of propionic acid approximately. 200 times! which shows that the present embodiment of the invention makes it possible to determine the organic acids with a very high sensitivity. Many variants are of course conceivable without departing from the scope of the invention. Thus, the packing charge of the column 8 ′ forming a suppressor can be, for example, a strongly acidic cation exchange resin in Na "form, a strongly acidic cation exchange resin in Li + form or I0 a resin exchange resin. strongly acidic cations in NHI form. When the ion-exchange resin present in the column 3 'forming a suppressor is completely converted into the H + form, it then becomes necessary to replace the column 8' with another fresh column or regenerate the exhausted column 8 'To meet this need, switching valves (not shown) can be provided before and after the suppressor column 8' so as to pass a regenerant such as N2 04, L2 S04 (NH4) 2804 or K2SO4 through column 8 ′ forming suppressor and converting the filling charge present in the suppressor., For example, in Na form. As described above, the present invention basically comprises, the fractionation of the test solution into organic acids on the one hand and into inorganic anions on the other hand and, at the same time, the separation by chromatography of the organic acids from each other and the determination of the organic acid contents of the solution to test. This invention has the advantage of making it possible to analyze the organic acids in the solution to be tested with a very high sensitivity even when this solution arrives at

  test for foreign inorganic anions.

  Since the weak ionic species with small dissociation constants (large pKa values) can be easily dissociated by increasing the pH of the mobile phase, the present invention performs the analysis with greater sensitivity than in the

  conventional techniques mentioned above.

258 1,193

Claims (6)

  1. Process for analyzing organic acids in a test solution containing organic acids and inorganic anions, characterized in that a fixed quantity of said test solution is transported with a mobile phase formed by an acid solution up to a separation column '7) packed with a strongly acidic cation exchange resin, the said test column is fractionated in said separation column into a fraction of said organic acids and a fraction of said inorganic anions and is separated at the same time by chromatography said organic acids from each other, the eluate from said separation column is then subjected to a cation exchange with a cation exchange membrane (Sa), and after which the contents of the organic acids in said solution to be tested are determined by through a measurement of electrical conductivity. 2.An organic acid analysis device in a test solution containing organic acids and inorganic anions, characterized in that it comprises a sampling valve (5) for collecting a prescribed quantity of the test solution, a column (7) packed with a strongly acidic cation exchange resin and suitable for fractionating the incoming test solution into a fraction of inorganic anions and a fraction of organic acids and, to separate the said organic acids by chromatography at the same time from each other, a suppressor Y8) with double tube whose interior is divided with a tubular membrane (Sa) cation exchange in an internal chamber (Sb) and an external chamber <(8c) to thereby induce a cation exchange between the eluate from said column, conveyed into said internal chamber and a suitable elimination liquid flowing through said external chamber, and a detector (9?) suitable for determining the high conductivity of the liquid conveyed from said chamber interior of said suppressor.   3. Device according to claim 2, characterized in that said column comprises a   precolumn (6) and a separation column (7).   4. Process for analyzing organic acids in a test solution containing organic acids and inorganic anions, characterized in that a fixed quantity of said test solution is transported with a mobile phase formed from an acid solution up to in a separation column (7) packed with a strongly acid cation exchange resin, said solution column is fractionated in said saparation column into a fraction of organic acids and a fraction of inorganic anions and at the same time, said organic acids are separated by chromatography from one another, the cations contained in the eluate from said separation column are then subjected to an ion exchange with a column (8 ′) forming a suppressor packed with an exchange resin cations, and the contents of the organic acids in said test solution are then determined by measuring the electrical conductivity of the eluate   from said suppressor column.   5.An organic acid analysis device in a test solution containing organic acids and inorganic anions, characterized in that it comprises a sampling valve (5) for collecting said test solution according to a prescribed quantity, a column (7) packed with a strongly acidic cation exchange resin and suitable for fractionating the incoming test solution into a fraction of inorganic anions and a fraction of organic acids and, at the same time, separating said organic acids by chromatography others. a suppressor column (8 ′) packed with a strongly acidic cation exchange resin and suitable for exchanging cations on the incoming eluate from said separation column, and a detector (9) intended to measure the conductivity   eluate from the suppressor.   6. Device according to claim 5, characterized in that said cation exchange resin lining said column (8 ') forming a suppressor is a cation exchange resin in Na + form, a cation exchange resin in Kt form, a cation exchange resin in Li + form or an exchange resin of cations in NH * form.