Process for preparing a liquid mixture

Abstract

The invention relates to a method for preparing a liquid mixture having a predetermined pH value without using a pH meter, the method comprising adding and reducing a controlled amount of a buffer component, preferably in liquid form, to the liquid mixture; and stopping the addition and reduction of the buffer component when a specific conductivity value is obtained, wherein the reduction and addition of the buffer component is performed in a way to maintain the buffer concentration constant and wherein the specific conductivity value corresponds to the predetermined pH value of the liquid and wherein the specific conductivity value is related to the predetermined pH value and the buffer concentration by a mathematical model. The mathematical model is a surface model equation of the form pH=A+B*concentration+C*concentration^2+D*conductivity or pH=A+B*concentration+C*concentration^2+D*conductivity+E*conductivity^2, wherein A, B and C and D and E are constants specific for the buffer system.

Description

[0001] This application is a divisional application with the same title as the parent invention, whose Chinese application number is 201480053772.X. The international application number is PCT / SE2014 / 051109, and the application date is September 26, 2014. Technical Field

[0002] This invention relates to a method for preparing liquid mixtures, and more particularly to the preparation of liquid mixtures, such as buffer solutions, wherein the conductivity of the liquid mixture is measured and the pH is indirectly determined. Background Technology

[0003] Buffer solutions are typically aqueous solutions composed of a weak acid and its conjugate base, or a weak base and its conjugate acid, and possess the property that the pH of the solution changes very little when a small amount of a strong acid or base is added to it. Therefore, buffer solutions are used in a wide variety of chemical applications to maintain a nearly constant pH value, including, for example, chromatography, filtration, and so on. Typically, a buffer solution can consist of more than one weak acid and its conjugate base. For example, a wider buffer region can be generated by mixing two buffers that have overlapping individual buffer regions.

[0004] A buffer solution having the desired pH, and optionally, the desired ionic strength, can be prepared by calculating the necessary amounts of the buffer components and mixing them. Although it is often necessary to solve several different equations in order to calculate the pH of a relatively simple mixture of a weak acid (or base) and a weak base (or acid) based on their relative concentrations, commercial software exists that can be used to perform such calculations.

[0005] Another possibility is to use feedback control, as in WO / 2011 / 162666, where the accuracy of the pH measurement limits the accuracy of the buffer solution's pH. However, pH meter calibration is time-consuming and often cumbersome. pH meters may not be very accurate, react slowly, and are easily contaminated, for example, by salts. pH measurements are also temperature-dependent, and different buffers have various temperature constants. Typically, pH meter readings cannot be corrected to a standard temperature.

[0006] Therefore, there remains a need in the field to improve pH measurement and to provide reliable instruments for this purpose. Summary of the Invention

[0007] This invention provides a reliable method and tool for measuring pH, which avoids the disadvantages of the prior art.

[0008] This invention relates to a method for indirectly determining the pH of a buffer solution, the method comprising, if its conductivity and concentration are known, using a surface model equation of the following form: pH = f(conductivity, concentration) or pH = f(conductivity, buffer concentration, salt concentration) The surface model equations were previously obtained by fitting such equations to previously measured pH values ​​used in the training dataset. The training dataset includes measured or controlled concentration values ​​and measured conductivity values ​​of solutions containing buffers at various concentrations and conductivityes and optionally at different temperatures, and the surface model equations are obtained by fitting the resulting data to the described formula.

[0009] In a first aspect, the present invention relates to a method for preparing a liquid mixture having a predetermined pH value without using a pH meter, the method comprising adding and reducing controlled amounts of a buffer component, preferably in liquid form, to the liquid mixture; and stopping the addition and reduction of the buffer component when a specific conductivity value is obtained, wherein the reduction and addition of the buffer component are carried out in a manner that maintains a constant buffer concentration and wherein the specific conductivity value corresponds to the predetermined pH value of the liquid, and wherein the specific conductivity value is related to the predetermined pH value and the buffer concentration by a mathematical model, such as a surface model equation implemented on a particular buffer system.

[0010] The present invention also relates to a method for indirectly determining the pH of a buffer solution, the method comprising measuring its conductivity and measuring or controlling the concentration of the buffer, wherein the specific conductivity value is correlated with the predetermined pH value by means of a mathematical model.

[0011] When using the surface model equation, it can take the form pH = A + B*concentration + C*concentration^2 + D*conductivity, where A, B, C, and D are specific constants of the buffer system.

[0012] Alternatively, the surface model equation can take the form of pH = A + B*concentration + C*concentration^2 + D*conductivity + E*conductivity^2, where A, B, C, D, and E are specific constants of the buffer system.

[0013] In an alternative implementation, a surface function that predicts pH as a function of conductivity and concentration is obtained by regression modeling of a training set of the mixture, the mixture having known concentration, conductivity, and pH.

[0014] The concentration of the buffer can be measured by any of the following: IoR, IR, and UV absorbance at several wavelengths.

[0015] One alternative method for measuring or controlling buffer concentration is to prepare stock solutions of acid and base separately to known concentrations, for example, by diluting a weighed buffer salt to a measured volume and subsequently controlling or measuring the flow rates from the stock solutions and the flow rates of water to the common point of the buffer mixture. Another method for controlling or measuring buffer concentration is to measure the conductivity or another property of the stock solution, such as IR absorbance or other absorbance or IoR signal, and then, using a pre-determined conductivity (or IR or IoR) versus concentration curve, use those signals to inversely calculate the concentration of the stock solution. Using the information about the stock solution concentration obtained in this way and the information about the measured or controlled flow rates to the mixing point, it is possible to calculate the concentration of the mixed buffer.

[0016] With knowledge of the buffer concentration and the conductivity of the buffer solution, it is possible, according to the present invention, to determine the pH using a pH surface model equation that is a function of the conductivity and concentration of the buffer solution. For each type of buffer solution, such a surface model equation can be pre-determined by: (i) using a training dataset belonging to a relevant region of the concentration and conductivity under consideration; (ii) measuring the pH of the solution belonging to the training dataset; and (iii) using a numerical regression method such as multiple linear regression (MLR) or partial least squares (PLS) to obtain the surface model equation as a regression model from the data.

[0017] Preferably, the method of the present invention is executed by a computer.

[0018] In a second aspect, the present invention relates to the use of the methods described above for controlling buffer formulation systems or online dilution systems. The methods can also be used in screening experiments in which pH and concentration are used as Design of Experiment (DoE) parameters.

[0019] In a third aspect, the present invention relates to an apparatus comprising a computer program product, the computer program product including instructions for causing a computer to perform the methods described above.

[0020] The device may include a conductivity sensor and a concentration sensor, as well as a tool for calculating pH from the measured conductivity using the methods described above. The device may also allow input of buffer concentration and include a tool for calculating pH from the measured conductivity using the steps described above.

[0021] The invention will now be described in more detail by way of example only, with reference to the accompanying drawings. Attached Figure Description

[0022] Figure 1 The results show the scaled and centered pH coefficients based on the experimental details in Example 1 below.

[0023] Figure 2 A graph showing the observed pH values ​​versus predicted values ​​based on the experimental details in Example 1 below.

[0024] Figure 3 The graph shows the measured pH values ​​obtained from buffer solutions prepared in two different ways. Both values ​​are compared to the target value, as shown in Example 2 below, by formulation and by "pH via conductivity-feedback".

[0025] To aid in understanding the invention disclosed herein, several terms will be defined below.

[0026] definition buffer solution As used herein, a buffer solution is an aqueous solution typically composed of a weak acid and its conjugate base, or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of a strong acid or base is added to it. Buffer solutions are used in a wide variety of chemical applications as a means of maintaining a nearly constant pH value.

[0027] Electrical conductivity (electrolytic) The conductivity (or conductivity coefficient) of an electrolyte solution is a measure of its ability to conduct electricity. The SI unit for conductivity is "Siemens" per meter (S / m).

[0028] Ionic strength The ionic strength of a solution is a function of the concentrations of all ions in the solution (half the sum of the concentrations multiplied by the square of the ionic charges of all ions). Ionic strength is usually expressed in mol / dm³. 3 Provided.

[0029] Surface model equations The surface model equation used here is a function of the form f(x,y), where x and y are two independent variables representing the properties of the buffer solution. More specifically, in this context, x (or y) refers to the conductivity of the buffer solution and y (or x) refers to the buffer concentration. Invention Details As mentioned above, this invention relates to the preparation of liquid mixtures, and more specifically, the preparation of liquid mixtures, such as buffer solutions, wherein the conductivity of the liquid mixture is measured and the pH is indirectly determined if the buffer concentration is known. Furthermore, this invention relates to providing a method for preparing a liquid mixture having a predetermined pH value, the method comprising using conductivity as a feedback control parameter. Methods for predicting pH, determining surface model equations, and calculating pH are described below.

[0030] Predict pH Therefore, one object of the present invention is to indirectly determine the pH of a buffer solution if its conductivity and concentration are known. Compared with pH measurement, conductivity measurement is faster and more reliable, and can be temperature-corrected to, for example, standard room temperature of 25°C. Furthermore, calibration on an annual basis is generally sufficient for the conductivity meter. Therefore, measuring conductivity rather than pH should be advantageous.

[0031] This invention also provides a method for preparing a buffer solution with a given buffer concentration to a predetermined pH value without using a pH meter. Other methods exist that can achieve this, for example, a buffer solution with the desired pH, and optionally, the desired ionic strength, can be precisely prepared by calculating the necessary amounts of the buffer components and mixing them, as previously described in EP2269055 B1. However, unlike those methods, the method described in this invention does not use a formulation and instead uses measured conductivity as a feedback control parameter. Using feedback control to obtain a buffer solution with desired properties is not novel. For example, one such method is described in WO / 2011 / 162666. However, there are significant differences and advantages compared to that method (where the measured pH must be used as a control parameter). In this case, the correct buffer solution with the correct pH and buffer concentration is obtained by combining conductivity measurement and buffer concentration control through some means.

[0032] Buffer concentration can be measured or controlled in different ways. For example, buffer concentration can be measured or controlled by using refractive index (IoR) techniques, infrared (IR) spectroscopy, UV absorbance, or other methods.

[0033] One alternative method for measuring or controlling buffer concentration is to prepare stock solutions of acid and base separately to known concentrations, for example, by diluting a weighed buffer salt to a measured volume and subsequently controlling or measuring the flow rates from the stock solutions and the flow rates of water to the common point of the buffer mixture. Another method for controlling or measuring buffer concentration is to measure the conductivity or another property of the stock solution, such as IR absorbance or other absorbance or IoR signal, and then, using a pre-determined conductivity (or IR or IoR) versus concentration curve, use those signals to inversely calculate the concentration of the stock solution. Using the information about the stock solution concentration obtained in this way and the information about the measured or controlled flow rates to the mixing point, it is possible to calculate and control the concentration of the mixed buffer solution.

[0034] Therefore, since it is possible to calculate pH by monitoring the conductivity of the buffer solution according to the present invention, as long as the buffer concentration is calculated or controlled, it is also possible to determine the amount of each component, acid and base using conductivity feedback control.

[0035] Whether the methods described herein are used to predict the pH of a buffer solution or to generate a buffer solution with the correct pH and buffer concentration using conductivity feedback control, the central feature of this invention is a surface model equation for determining pH as a function of conductivity and buffer concentration—in other words, as a function of the form f(x,y), where x (or y) refers to the conductivity of the buffer solution and y (or x) refers to the buffer concentration. How this equation can be obtained will now be described.

[0036] Determination of surface model equations The form of the surface model equation of the buffer solution pH = f(conductivity, concentration) or pH = f(conductivity, buffer concentration, salt concentration) This can be generated by fitting pH data obtained from a finite number of points in the (conductivity, concentration) space to the equation above. A design of experiment (DoE) can be used advantageously but is not necessary. The step of fitting the equation to the measured data used for the training set is not difficult for anyone skilled in the art. The resulting equation is valid to a certain degree of accuracy in the region covered by a discontinuous number of points in the training set.

[0037] Calculate pH Finally, the pH is calculated as the predicted pH of the desired buffer solution using the surface model equations described above and the measured values ​​of conductivity along with the corresponding measured or controlled values ​​of buffer concentration.

[0038] The method of the present invention can be implemented by software running on an electronic data processing device, such as a computer. This software can be provided to the computer on any suitable computer-readable medium, including recording media, read-only memory, or electrical or optical signals that can be transmitted via electrical or optical cables, radio waves, or other means.

[0039] The predicted pH described above has not been described elsewhere before. Previously, pH prediction or buffer formulation at a given pH has been described, for example in US20110039712 A1 and EP 2269055 B1, but those methods or similar methods did not use conductivity and required formulation. On the other hand, another method describes predicting conductivity from its components (PCT SE2011 / 051513), but this method does not describe how pH can be obtained from conductivity measurements without calculating molar conductivity for each ionic class of the various compounds using the following formula: Л = Л0 – Κ x Sqrt(c) Where Л is the molar conductivity, Л0 is the molar conductivity at infinite dilution, c is the concentration of the ion class, and K is the Kohlrausch coefficient, where K and Л0 are obtained from a dataset containing predetermined values ​​of K and Л0 for each ion class.

[0040] The predicted pH described here only requires calibration of surface equations of the following form: pH = f(conductivity, buffer concentration) or pH = f(conductivity, buffer concentration, salt concentration) It has an effective range of accuracy for a given quantity and only measures conductivity and controls or measures buffer concentration and, if necessary, salt concentration. The predicted pH described herein does not require calculating molar conductivity for every class of ions, or Л0 is the molar conductivity or Kohlrausch coefficient for infinite dilution.

[0041] The predicted pH described herein can be used for several purposes. Exemplary applications include controlling buffer formulation systems or online dilution systems. Such predicted pH can also be used in screening experiments where pH and concentration (but not conductivity) are used as parameters of the Design of Experiments (DoE). Another application is determining the exact concentration of a stock solution or buffer by measuring its pH and conductivity.

[0042] The methods described above can be used to obtain specific surface models for specific buffer systems bound to a particular concentration of a non-buffered salt (e.g., NaCl). The surface equations can also be generalized to include the salt concentration, i.e., f(x,y,z), where z is the salt concentration. Alternatively, the NaCl concentration can be kept constant, resulting in a model for each NaCl concentration.

[0043] The invention will now be described in more detail by way of example only through the following non-limiting embodiments. Example

[0044] Example 1: Surface model equation for pH as a function of concentration and conductivity In this experiment, conductivity and buffer concentration were used to indirectly determine pH. Modde version 9.0 was used to obtain the surface model equation as a regression model incorporating model coefficients. The training set (table) included three different concentration levels of citrate buffer and six different conductivity values ​​for each concentration. Buffer mixing was performed using a lab-scale Äkta Probe system (GE Healthcare Biosciences AB) with 5 pumps and Unicorn 5 control software (GE Healthcare Biosciences AB), where concentrations were controlled by manually setting the stock solutions from the acid (0.5 M citrate) and the base (0.5 M trisodium citrate) and the flow rate from water. Conductivity was measured offline using a linear temperature correction with a constant of 2.1, and pH was measured offline using a calibrated HANNA pH meter.

[0045] Table 1: Partial least squares regression analysis was used to obtain the surface model equation as the relationship between concentration, conductivity, and pH. Figure 1 ).

[0046] The model's predictive ability was tested using a test set with seven different citrate buffers (Table).

[0047] Table 2: Test set for the predictive ability of this model The measured pH values ​​for all runs were within the predicted range, thus the model has been shown to allow for indirect determination of pH. Figure 2 ).

[0048] Example 2: Preparation of buffer solution at a given pH using conductivity feedback control.

[0049] One object of the present invention is to provide a method for preparing a liquid mixture with a predetermined pH value by using conductivity as a feedback control parameter. In this case, no formulation is required, and no online pH meter is needed. A total of 21 different buffer solutions were used to compare buffer preparation using this method (Table 3) with a second method in which the flow rate into the solution is controlled and buffer solutions are prepared using a formulation. The target conductivity value of the first method (which is the method described in the present invention) was obtained by measuring the temperature-corrected conductivity from the results file of the second method. According to the results, buffer preparation using conductivity feedback showed no difference in pH compared to buffers prepared by formulation and pH meter. Figure 3 ).

[0050] Table 3: Buffer solutions prepared in two different ways (by formulation and by conductivity feedback)

Claims

1. A method for preparing a liquid mixture having a predetermined pH value without using a pH meter, the method comprising adding and reducing controlled amounts of a buffer component, the buffer component being in liquid form, to the liquid mixture, wherein the liquid mixture is a buffer solution; And when a specific conductivity value is obtained, the addition and reduction of the buffer component are stopped, wherein the reduction and addition of the buffer component are carried out in a manner that maintains the buffer concentration constant, and wherein the specific conductivity value corresponds to a predetermined pH value of the liquid, and wherein the specific conductivity value is related to the predetermined pH value and the buffer concentration by a surface model equation of the following form: pH = A + B*concentration + C*concentration^2 + D*conductivity Where A, B, C, and D are specific constants of the buffer system. or pH = A + B*concentration + C*concentration^2 + D*conductivity + E*conductivity^2 Where A, B, C, D, and E are specific constants of the buffer system. The surface function that predicts pH, which is a function of conductivity and concentration, is obtained by regression modeling on a training set of mixtures with known concentrations, conductivity, and pH. The surface model equation can be predetermined by: (i) using a training dataset belonging to a relevant region of the concentration and conductivity under consideration, wherein the training dataset includes measured or controlled concentration values ​​and measured conductivity values ​​of solutions containing buffers at various concentrations and conductivityes and optionally at different temperatures; (ii) measuring the pH of the solutions belonging to the training dataset; and (iii) using a numerical regression method, multiple linear regression or partial least squares method, to obtain the surface model equation as a regression model from the obtained data.

2. The method of claim 1, wherein the concentration of the buffer solution is measured by any one of IoR, IR, and UV absorbance at several wavelengths.

3. The method of claim 1, wherein the concentration of the buffer solution is calculated from the known flow rate of a stock solution of known concentration and the flow rates of other components, water and non-buffered salts or additives, to the mixing point of said buffer solution.

4. The method of claim 1, wherein the concentration of the buffer solution is calculated from the measured conductivity and the known flow rate of the reserve solution and the flow rates of other components, water and non-buffered salts or additives, to the mixing point of the buffer solution.

5. The method of any one of claims 1-4, wherein the method is performed by a computer.

6. The method of any one of claims 1-5 is used for controlling the use of a buffer formulation system.

7. The method of any one of claims 1-5 is used for controlling an online dilution system.

8. Use of the method of any one of claims 1-5 in a screening experiment in which pH and concentration are used as experimental design parameters.

9. An apparatus comprising a computer program product, the computer program product comprising instructions that cause a computer to perform the method steps of any one of claims 1-5.

10. The apparatus of claim 9 for measuring pH, comprising a conductivity sensor and a concentration sensor and a tool for calculating pH from the measured conductivity using the steps of any one of claims 1-5.

11. The apparatus of claim 9 for measuring pH, comprising a conductivity sensor and a tool for allowing input of buffer concentration and calculating pH from the measured conductivity using the steps of any one of claims 1-5.

Images