Method for precise ph measurement and use of such a method

Abstract

The invention relates to a method for precisely measuring the pH of a liquid to be measured using a measuring device which has a pH measuring element with at least one sensitive surface region. The method has the following steps: (a) calibrating the pH measuring element using a calibration solution, and (b) carrying out at least one pH measurement on the liquid to be measured using the pH measuring element in order to generate at least one valid pH measurement value. In order to increase the measurement precision, a measurement preparation step (i) is carried out between step (a) and step (b), wherein during the measurement preparation step, at least the at least one sensitive surface region of the pH measuring element is in contact with a measurement preparation liquid, the pH value of which deviates from that of the liquid to be measured by maximally 1 pH value, preferably by maximally 0.5 pH value, for a duration of at least 30 seconds, preferably at least 5 minutes, more preferably at least 10 minutes, more preferably at least 30 minutes.

Description

[0001] Method for precise pH measurement and use of such a method

[0002] Description

[0003] The invention relates to a method for pH measurement according to claim 1 and to a use of such a method according to claim 11.

[0004] Continuous or periodically repeated pH measurement of liquids is widespread in process measurement technology and is used, for example, to monitor the pH of electrolytes. The pH measurement can be part of a control loop to keep the pH of a specific liquid within a predetermined tolerance range and can also be used to document the process parameters. Electrical pH measuring elements, which are usually in the form of so-called pH electrodes (often also referred to as "glass electrodes"), are used for pH measurement. The basic structure is as follows: Such a glass electrode has at least two chambers, one of which is filled with a buffer solution surrounding a measuring electrode, and the other chamber is filled with an electrolyte (which is different from the buffer solution) surrounding a reference electrode.Part of the wall of the first chamber is formed as a so-called glass membrane, and part of the wall of the second chamber is formed as a diaphragm. There are numerous designs of such pH electrodes, but they all follow the same basic principle.

[0005] During pH measurement of the liquids to be measured, both the glass membrane and the diaphragm must be in contact with the liquid to be measured; these two areas form the sensitive surface areas of the pH electrode. The actual pH measurement is a measurement of the voltage between the measuring electrode and the reference electrode. Since the pH value is highly temperature-dependent, the measurement is usually carried out under defined temperature conditions, for example, in a temperature bath. It is also known that such a pH electrode (and this applies to every pH measuring element) must be calibrated at specific intervals using a calibration solution (a pH buffer).

[0006] The pH measuring element is usually housed in a temperature-controlled reservoir, to which the necessary fluids can be added and removed again. The necessary fluids are the fluid to be measured, the calibration solution, and usually deionized water for rinsing.

[0007] A typical complete cycle (including calibration) is as follows: If necessary, after rinsing with deionized water, calibration solution is added to the reservoir, which is then used to calibrate the pH measuring element. After another rinse with deionized water, the actual measuring cycle (a sub-cycle of the overall cycle) is started. In each case, the liquid to be measured (e.g., a process bath) is poured into the reservoir, the pH measurement is performed, and then the liquid to be measured is removed from the reservoir and either discarded or returned. This addition, measurement, and removal is then repeated at predetermined intervals until recalibration is necessary.To give an example: For example, a measurement can be taken every half hour, while calibration is only necessary every one or more days. During calibration, there is a certain amount of "dead time," which can be tolerated in some cases. If this is not possible, two such measuring devices must be maintained.

[0008] Based on this, the present invention seeks to improve a pH measurement method, particularly one such as the one just described, so that improved accuracy can be achieved. This object is achieved by a method having the features of claim 1.

[0009] In many processes, the pH value does not need to be determined with great precision, and a measurement accuracy of, for example, ± pH 0.5 is often sufficient. In this case, a method as described above is completely adequate. However, it has been found that if you want or need to know the pH value more precisely, especially within an accuracy of ± pH 0.2 or even less, the method described above does not provide reliable, i.e., valid, pH measurements. It has also been discovered that this is due to the inertia of the pH measuring elements used (usually the pH electrodes). The influence of this inertia has so far been underestimated or played no role in the desired measurement accuracy. The reason for this inertia lies primarily in the relatively low "migration speed" of the ions.

[0010] To obtain a "perfect" pH measurement after calibration, the system, which consists of the pH measuring element and the liquid to be measured, must have reached its complete equilibrium state. Over time, the system approaches this equilibrium state according to a decreasing E-function, meaning it is never fully reached. However, by providing a specific "waiting time" between the calibration step and the generation of the first valid measured value, the equilibrium state can be approached sufficiently to achieve the desired measurement accuracy. During this waiting time, which defines a separate measurement preparation step, at least one sensitive area of ​​the pH measuring element must be in contact - preferably continuously - with a measurement preparation liquid whose pH value deviates by no more than 1 from the pH value of the liquid to be measured, with a deviation of no more than pH 0.5 being preferred.

[0011] In principle, it is advantageous if the difference between the pH value of the liquid to be measured and the pH value of the measurement preparation liquid is as small as possible. For this reason, among others, it is generally preferable for the liquid to be measured and the measurement preparation liquid to be taken from the same liquid or from a common liquid reservoir, i.e., they are chemically identical. The only difference between these two options is that in the first case, the storage container is not emptied again before the first measurement, whereas in the second case, the liquid (or in other words, the amount of liquid) used for measurement preparation is discarded.

[0012] It has been found that the waiting time should be at least 30 seconds, although significantly longer waiting times of at least 5, especially 10 minutes, are preferred. Only after this waiting time has elapsed is at least one valid measurement value (i.e., one suitable for further processing) generated.

[0013] Accordingly, the method according to the invention comprises the following steps:

[0014] Calibrating the pH electrode using a calibration solution, carrying out a measurement preparation step in which at least the at least one sensitive surface area of ​​the pH measuring element is in contact with a measurement preparation liquid for a period of at least 30 seconds, the pH value of which deviates by at most 1 from the pH value of the liquid to be measured,

[0015] Carrying out at least one pH measurement on the liquid to be measured using the pH measuring element to generate at least one valid pH measurement value.

[0016] As a rule, at least one rinsing step is performed between the calibration step and the measurement preparation step, in which the pH electrode is rinsed at least partially using deionized water. However, such a rinsing step is generally not performed between the measurement preparation step and the performance of the measurement, as this could at least partially negate the effect of the measurement preparation step. As already mentioned, such a method is generally used for process monitoring. It is therefore preferred that, after measurement preparation has been completed, a plurality of valid measured values ​​are generated before a new calibration takes place. Alternatively, a valid measured value continuum can be generated.

[0017] The procedure is generally carried out according to two cycles, namely an overall cycle, which includes the regular performance of a calibration, and a plurality of measurement cycles within the overall cycle, each of which follows the measurement preparation step of the overall cycle.

[0018] The measuring device usually has a control unit, so the process, including the generation of valid pH measurements, runs fully automatically. In this case, in particular, the measuring device and the process it performs can be part of a control loop designed to keep the pH value of the liquid being measured, which can particularly be a process bath, constant within narrow limits.

[0019] The measuring device used is usually separate from the container for the liquid to be measured and therefore has - as already mentioned - a storage container which surrounds the pH measuring element at least in sections and which has at least one access such that liquid can be fed into and removed from the storage container.

[0020] The invention will now be described in more detail using preferred embodiments with reference to the figures. The figures show:

[0021] Figure 1 is a highly schematic representation of a setup for precise pH measurement of a liquid to be measured stored in a container, Figure 2 is a highly schematic, but slightly enlarged representation of a part of the setup shown in Figure 1, namely the pH measuring electrode (glass electrode),

[0022] Figure 3 is a flow chart in which the measuring method according to the invention is illustrated using a first example and

[0023] Figure 4 shows a variant of the flowchart shown in Figure 3.

[0024] As already mentioned, the measuring method according to the invention serves to precisely measure the pH value of a liquid to be measured, in particular within a measurement accuracy of pH 0.2. Figure 1 shows – highly schematically – the elements used to implement this method. These are known in themselves; the invention lies exclusively in a novel process sequence that allows the measurement of the pH value of the liquid to be measured with greatly increased accuracy.

[0025] The actual measurement of the pH value is carried out with a pH measuring element, which in the embodiment shown, and according to current knowledge also corresponds to the preferred embodiment, is carried out with a so-called pH electrode (also called a “glass electrode”). Three types of pH electrodes are known in the prior art in a variety of designs, but are always based on the same measuring principle, which is why a very simple embodiment of such a pH electrode 10 is shown in Figure 2. This pH electrode 10 has two separate chambers, with the measuring electrode 12 being arranged in one chamber and surrounded by an internal buffer 11 (i.e. an aqueous solution). Part of this first chamber is designed as a glass membrane 13. A reference electrode 15 is arranged in a second chamber and is surrounded by an electrolyte 14. Part of this chamber is designed as a diaphragm 16.To measure the pH of a liquid, the pH measuring electrode must be immersed in the liquid to be measured in such a way that at least the glass membrane 13 and the diaphragm 16 are surrounded by the liquid to be measured. Thus, the diaphragm 15 and the glass membrane 13 form the sensitive surface areas of the pH measuring electrode.

[0026] The pH of the liquid in which the pH measuring electrode 10 is immersed is measured by measuring the voltage between the measuring electrode 12 and the reference electrode 15 by means of a voltmeter 17, as described in the prior art and as is familiar to the person skilled in the art.

[0027] Figure 1 shows the pH measuring electrode 10 of Figure 2 just described in a configuration designed to measure the liquid stored in a container 40 ("liquid to be measured"), which may be, for example, a process bath. For linguistic simplicity, the "liquid to be measured" is referred to below as the "process bath," although it should be noted that the invention is not limited to the pH measurement of process baths (although this is an important application).

[0028] The measurement is carried out outside the bath in a fully automated process using a measuring device which includes, among other things, a pH measuring element, for example one as just described.

[0029] The measuring device just mentioned comprises, in addition to the pH measuring element 10, a reservoir 20 in which the pH measuring element 10 is arranged such that it is surrounded by a liquid that has been supplied to the reservoir 20, at least to the extent that the glass membrane 13 and diaphragm 16 (the sensitive surface areas) are surrounded by the respective liquid. For the sake of simplicity, the reservoir 20 is shown here as an open-topped container, although this is not usually the case in practice. An agitator 24 can be arranged in the reservoir 20.

[0030] The storage tank 20 is in turn surrounded by a tank 30 for a temperature control bath 32. A temperature control device 34 is provided, by means of which the temperature control bath 32—and thus also the storage tank 20 and the liquids contained therein—can be heated to a desired temperature. This is also known in the prior art and need not be described in detail here. For the sake of simplicity, the tank 30 for the temperature control bath is also shown as an open-topped tank, although this is also not usually the case in practice. Units consisting of a pH measuring element, storage tank 20, and temperature control bath tank 30 are available on the market.

[0031] As already mentioned several times, the measuring device briefly described above serves to determine the pH value of a liquid to be measured. This liquid to be measured can, as mentioned, be, for example, a process bath stored in a container 40.

[0032] To carry out the process, additional liquids are required, namely a calibration solution for the pH measuring element and, as a rule, distilled or otherwise deionized water. Accordingly, a container 50 for the calibration solution and a container 60 for the deionized water are also provided. The deionized water can, of course, also be drawn from a suitable supply, if available.

[0033] The aforementioned fluids must, of course, be able to be fed into the reservoir 20 and removed from it again. In the illustrated embodiment, the controllable valves 22, 42, 52, and 62 serve this purpose. Of course, pumps or similar devices could be used instead of or in addition to the valves, depending on the geometric conditions. The only important thing is that all of the aforementioned fluids can be fed into the reservoir and removed from it again.

[0034] In the illustrated embodiment, all liquids are discarded after their "use" in the measuring device, for which purpose a waste container 70 or an access point for wastewater treatment is provided. It would also be possible to recirculate at least some of the liquids, but this is not shown here. Particularly with regard to the process bath, it might be quite sensible to return them to container 40 after the measurement. Filter devices can also be provided, particularly between container 40 and the storage container 20, although these are also not shown here.

[0035] A control unit 80 is used to control the measuring device (including the controllable valves 22, 42, 52, 62 valves) and to record the measured data. This control unit usually also has a data output 82 so that the measured data can be automatically recorded, stored and further processed.

[0036] Figure 3 shows a first embodiment of a method sequence according to the invention based on a complete overall cycle. A new overall cycle always begins with the calibration of the pH measuring element 10. In a first step 1, the storage container 20 and thus also the pH measuring element 10 are generally rinsed with deionized water. This can be done by filling and draining the storage container 20 once or several times. In the next step (step 2), calibration solution (i.e., a pH buffer solution suitable for the pH measuring element) is added to the storage container, and after the temperature has been adjusted, calibration is carried out as is known and customary in the art (step 3). In the next step (step 4), the calibration solution is removed from the storage container 20, and then the container (and thus, of course, the pH measuring element) is rinsed with deionized water (step 5).Now (step 6) the liquid to be measured is fed from the container 40 - here referred to as the process bath - to the storage container 20.

[0037] According to the invention, a waiting step (step 7) now follows, which forms the measurement preparation step. As the name "waiting step" suggests, during this step one simply waits, namely until the system consisting of the pH measuring element (here the pH measuring electrode) 10 and the surrounding liquid is in a state that sufficiently corresponds to an equilibrium state. It has been found that this system is much more sluggish than previously considered, which is why a significant improvement in measurement accuracy results from giving the system enough time to sufficiently approach its equilibrium state. However, since mathematically speaking, the approach to the equilibrium state follows a declining E-function, the equilibrium state can never be fully reached.However, it has been shown that a significant improvement can also be achieved after a "finite" waiting time, whereby this waiting time is at least half a minute, preferably at least 10 minutes.

[0038] During the waiting period, pH measurements can of course be carried out in principle, but this does not result in valid measured values ​​according to the definitions given here, and even by carrying out several measurements during the waiting period, no valid measured value can be generated, since in this state the individual measured values ​​do not show a normal distribution.

[0039] After the waiting period has ended, at least one pH measurement is performed in step 8, thus generating a valid pH measurement within the meaning of this patent application. This valid pH measurement can be generated by a single pH measurement or by averaging several pH measurements (which, however, are all performed after waiting step 7). This valid pH measurement is then further processed, for example, stored and / or used to correct the pH of the liquid being measured.

[0040] After completion of the measurement or measurements according to step 8, the process bath (generally: the liquid to be measured) is removed from the storage tank.

[0041] In the event that only a valid pH measurement is to be generated, the procedure just described can now start again from the beginning.

[0042] However, since the calibration step usually takes a relatively long time, it is generally not practical to generate only one valid pH measurement per calibration step, which is why the process variant in Figure 4 has significantly greater practical relevance. As a rule, the pH value of the liquid to be measured fluctuates only relatively slightly and also quite slowly, which is why, after the waiting step has been performed once, a large number of valid pH measurements can be performed (on newly added liquid to be measured). Unless a waiting step (measurement preparation step) is to be performed before each measurement that leads to a valid pH measurement, rinsing with deionized water is generally not performed between these measurement steps.

[0043] The described measuring device and the measuring method automatically carried out with its aid can be part of a control loop with the aid of which the pH value of the process bath is kept constant within narrow limits. In this case, at least one dosing unit (usually at least two dosing units) is provided, and if a valid pH measurement value or several consecutively generated valid pH measurements or the average of several consecutively generated valid pH measurements deviates from a target pH value by a predetermined difference, a corresponding agent for shifting the measured pH value towards the target pH value is supplied to the process bath via the at least one dosing device. The data output from the data output 82 of the control unit is used at least indirectly to control the at least one dosing unit.

[0044] List of reference symbols

[0045] 10 pH measuring electrode (pH measuring element)

[0046] 11 internal buffers

[0047] 12 measuring electrode

[0048] 13 Glass membrane

[0049] 14 Electrolyte

[0050] 14 Reference electrode

[0051] 15 Diaphragm

[0052] 16 voltmeters

[0053] 20 storage containers

[0054] 22 controllable valve

[0055] 24 agitator

[0056] 30 containers for temperature bath

[0057] 32 Tempering bath

[0058] 34 T emperier device

[0059] 40 containers with liquid to be measured (process bath)

[0060] 42 controllable valve

[0061] 50 containers with calibration solution

[0062] 52 controllable valve

[0063] 60 containers of de-ionized water

[0064] 62 controllable valve

[0065] 70 waste bins

[0066] 80 Control unit

[0067] 82 Data output

Claims

Patent claims 1. A method for precise pH measurement of a liquid to be measured by means of a measuring device having a pH measuring element (10) with at least one sensitive surface area (13, 16), comprising the following steps: (a) Calibrating the pH measuring element (10) using a calibration solution, (b) Carrying out at least one pH measurement on the liquid to be measured by means of the pH measuring element (10) to generate at least one valid pH measurement value, characterized in that between step (a) and step (b) a measurement preparation step (i) is carried out, in which at least the at least one sensitive surface area (13, 16) of the pH measuring element (10) is in contact with a measurement preparation liquid for a period of at least 30 seconds, preferably at least 5 minutes, more preferably at least 10 minutes, more preferably at least 30 minutes, the pH value of which deviates by at most 1 from the pH value, preferably by at most 0.5 from the liquid to be measured.

2. Method according to claim 1, characterized in that between step (a) and step (i) at least one rinsing step is carried out, in which the pH measuring element (10) is rinsed at least in sections by means of distilled and / or deionized water.

3. Method according to claim 1 or claim 2, characterized in that the liquid to be measured and the measurement preparation liquid are taken from the same liquid or from a common liquid reservoir.

4. Method according to one of the preceding claims, characterized in that step (b) comprises generating a plurality of valid measurement values te and / or the generation of a valid measured value continuum.

5. Method according to one of the preceding claims, wherein the measuring device further comprises a storage container (20) which at least partially surrounds the pH measuring element (10) and which has at least one access such that liquid can be supplied to and removed from the storage container.

6. Method according to claim 5, insofar as it refers back to claim 4, characterized in that during step (b) the storage container is repeatedly filled with liquid to be measured and emptied again and at least after some of the fillings, preferably after each filling, at least one valid measured value is generated.

7. Method according to claim 6, characterized in that the time period between two fillings is between 5 minutes and one hour.

8. Method according to one of the preceding claims, characterized in that the method steps (a), (i) and (b) are repeated cyclically, the cycle time preferably being at least one day.

9. Method according to one of the preceding claims, characterized in that the method is carried out fully automatically by a control unit (80).

10. Method according to one of the preceding claims, characterized in that the liquid to be measured is a process bath, in particular an electrolyte.

11. Use of the method according to claims 8 to 10 as part of a control circuit for keeping the pH value of a process bath constant.

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