CH645990A5 - METHOD FOR MEASURING WITH REDOX OR ION SENSITIVE ELECTRODES. - Google Patents

Description

The invention is illustrated by the following examples:

Example 1: Continuous process

A solution with a CN ~ concentration of 12 mg / l and a proportion of catalyst (specified in DE-PS 2 352 856) of 0.02% by volume is used as the partial liquid flow to be examined. The solution has a temperature of 95 ° C and a pH of 11.5.

A flow of approximately 1001 / h of the starting solution is continuously conveyed into the reaction zone 6 through the lines 5.5a and 5b according to FIG. After the reaction section 6, the redox potential of the starting solution is measured with the electrode 7 in the 1st cycle. For the subsequent second cycle, the valve 10 opens over a period of about 5 minutes, and 0.51 per time cycle of a 3.5% by weight FLCh solution flows from the container 9 via the lines 8a and 5b into the reaction zone 6. Oxygen liberated by the decomposition of H2O2 is discharged from the measuring apparatus via the vent line 5c. During the second bar, this is ver5

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changed redox potential measured, which corresponds to the CN ~ zero concentration due to the complete conversion of the cyanide into cyanate. The measurement of both redox potentials results in a potential difference of 336 mV for the starting solution. Due to the clock-controlled valve 10, which takes into account the backflow of the reaction path, the measuring process requires a time period of approximately 10 minutes.

2. Example: discontinuous process

In Fig. 3 the discontinuous experimental apparatus with the clock-controlled valves is shown. A solution with a CN concentration of 120 mg / l and a proportion of catalyst (specified in DE-PS 2 352 856) of 0.02% by volume is used as the partial liquid flow to be examined. The solution has a temperature of 90 ° C. and a pH of 11.3.

When the valve 14a is closed and the valve 14b is open with a delay, the 1000 ml measuring volume 14 is filled with the solution. After the valve 14b has been shut off and the opening of 14a delayed, the amount of solution flows into the stirred tank 12 in the first cycle 20. The redox potential of the starting amount is measured with the electrode 15. In the meantime, from the measuring volume 18 of 30 ml filled from the container 16 via the line 17, a 3.5% by weight H2Ch solution is added by closing the valve 18b 25 and delayed opening of 18a. The stirrer 13 ensures in the second

Clock for the good mixing of both liquids. The H2O2 present in excess of the stoichiometric amount converts the existing amounts of cyanide completely into cyanate, and a redox potential corresponding to the CN zero concentration 30 is measured. There is a potential difference of 458 mV for the starting solution. After the measurement, the container 12 is emptied in the 3rd cycle. The measuring process takes about 2.5 minutes. To equalize the pressure, containers 12 and 16 are vented via lines 20 and 21, respectively.

3rd example

The measuring method was tested in various test series with wastewater of different CN ~ concentrations. The non-detoxified starting solution had a concentration of 120 mg / 1 CN-. This solution was gradually diluted to 1.2 mg / 1 and the potentials were registered. In addition, solutions with cyanide concentrations of 1 mg / 1.0.1 mg / 1 and 0.01 mg / 1 were prepared in the laboratory. The pH was adjusted to 12; the amount of activator of 0.02% by volume prescribed in DE-PS 23 52 856 was added to the solution. Fig. 4 shows the relationship between potential jump and CN ~ concentration in the range of 0.01-100 mg / 1. With this simple logarithmic representation, a linear relationship is obtained over a wide concentration range.

In order to determine whether the potential jump of 10 mV measured at 0.01 mg / 1 is due to the CN ~ concentration, a test was carried out with waste water without CN ~. A potential jump was observed, the amount of which is certainly less than 5 mV. This value cannot be entered in this curve in FIG. 4 because of the logarithmic representation of the CN ~ concentration. If one defines this 5 mV measured at a CN ~ concentration of zero as the zero point uncertainty of the method, then CN concentrations can be detected with sufficient certainty up to about 0.01 mg / 1.

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4 sheets of drawings

Claims (4)

645 990 1. A method for measuring the concentration of a particular dissolved component with redox or ion-sensitive electrodes, in which the zero point of the potential is subject to changes during the measurement operation, characterized in that the component concentration in question is measured in such a way that on the one hand, the potential corresponding to the current concentration of the component in question is determined, and on the other hand a reagent is added which completely converts the component whose concentration is to be determined to a compound which does not or only negligibly influences the potential of the measuring chain, so that the electrode shows the potential corresponding to the zero concentration of the component in question, the difference between the two potentials indicating the actual concentration of the component in question. 2. The method according to claim 1, characterized in that the reagent is added in at least the equivalent amount which is necessary for the reaction of the component to be measured with the reagent, but preferably in an excess. 2nd PATENT CLAIMS 3rd 645 990 Measurement of the zero potential and concludes with the zero potential after the end of one or more sequences. The reagent that completely converts the component to be measured to a specific compound, so that this component is no longer present in the reaction medium at the time of the zero point measurement, must be appropriately selected in such a way that neither the reagent nor the compound formed with the component in question Change the potential of the measuring chain. While for a whole series of components to be measured the knowledge of their generally known chemical properties as well as that of the reagent to be added and the resulting compound can be used to select the reagent in question, in other cases a manual test with the one is in prospect reagent recommended. As a suitable reagent, e.g. For the measurement of hydrogen ions in acidic and alkaline areas, the usual buffer salt solutions of strong bases / weak acids or weak acids / strong bases have proven to be suitable. Even when determining the smallest amounts of pollutants in the so important wastewater treatment area, the selection of the reagent in question does not generally pose any problems. In particular, the determination of the smallest amounts of cyanide ions with the help of hydrogen peroxide at a drifting zero point or other interfering influences on the potential can be determined without any problems. For a more detailed explanation of the invention, this latter method will be discussed in more detail, but the invention is not limited to this! It is a well-known phenomenon that is particularly noticeable in the continuous execution of processes that the zero point of a measuring chain shifts parallel to the original calibration curve during the measurement, i.e. that the zero point drifts, see M. Hofton, Continuous Determination of Free Cyanide in Efflu-ents Using Silver Ion Selective Electrode, Environmental 10 (1976) 3, 277/280. The disturbances that cause this phenomenon are not known in many cases. Correct potentiometric concentration measurements are no longer possible in such systems. According to the process of DE-PS 23 52 856, cyanide- or nitrile-containing wastewater can be reduced with hydrogen peroxide and a special catalyst to amounts below 0.1 mg / 1 cyanide ion. The reaching of the final concentration of cyanide ions is determined potentiometrically. It turned out that the zero point of the redox potential is slowly shifted with the continuous detoxification of this waste water. By using the method according to the invention, in which hydrogen peroxide is used as the reagent in this case, the shifting of the zero potential in continuous operation could be compensated and the measurement of the cyanide ion concentration in continuous wastewater detoxification could be carried out perfectly. The redox potential was not disturbed by hydrogen peroxide itself or by the cyanate formed. Although the method according to the invention can also be carried out in the actual conversion containers, measurements on side streams are preferred. Here you go e.g. so that the side stream of the wastewater to be investigated is passed through a reaction zone without adding the reagent - here hydrogen peroxide - and the corresponding potential of the measuring chain is determined, whereupon after a certain period of time the reagent is continuously introduced into the reaction zone during a second period and thereby the zero potential of the measuring chain is measured. The hydrogen peroxide addition is then interrupted. The current concentration of the type of ion to be measured - here cyanide ions - is then automatically adjusted again, whereupon the cycle starts again with the addition of the reagent - here hydrogen peroxide. The reagent can be used both in the stoichiometric amount for the reaction, but preferably in excess. When calculating the stoichiometric amount of reagent, the maximum of the expected amount of the type of ion to be measured should preferably be used. In the event of unexpected concentration fluctuations in the solution to be measured, no error results can occur. This amount can easily be determined by a manual test. The method according to the invention is temperature-independent insofar as no specific temperature range is necessary for the implementation, but the measuring temperature depends on the respective reactions. In general, however, the reaction will proceed faster at a higher temperature. The electrometric force of the measuring chain is temperature-dependent in a known manner and can be compensated - as is usual with redox and ion-sensitive electrodes - e.g. automatically via resistance thermometer. The time span between measuring the zero potential and the cyanide ion concentration that has reestablished itself after some time may preferably always be the same for control reasons. However, the success of the method itself is not impaired by irregular periods between the different concentration measurements. The appropriate periods of time are determined by a preliminary test; you can e.g. be 1 to 2 minutes. The advantage when using regular time spans is the possibility to switch on a clock phase generator. The length of the time between the determination of the instantaneous concentration and the addition of the hydrogen peroxide in excess should be kept as short as possible, since the reaction of the hydrogen peroxide with the cyanide ions present also takes a certain amount of time. In the following the invention is explained for example with reference to the drawings. Show it: 1 shows a diagram of the time course of the measuring chain voltage; Figures 2 and 3 are schematic representations of arrangements for the continuous or discontinuous execution of the inventive method. Fig. 4 is a diagram of the relationship between potential jump and CN ~ concentration. If a clock phase generator is used, the time periods until the measurement of the instantaneous concentration of the component of the zero potential are best the same. 1 shows the time course of the electrode tension with the same time periods for measuring the component concentration - in this case the cyanide ions - and for measuring the zero potential - in this case by adding hydrogen peroxide in excess. The potential of the measuring chain is shown on the ordinate and the time on the abscissa. The periods in which the hydrogen peroxide is added are shown as bars 2. In this case, the setting 5 10th 15 20th 25th 30th 35 40 45 50 55 60 o5 645 990 3. The method according to claim 1 or 2, characterized in that this measurement process, namely the determination of the instantaneous concentration and the production and determination of the zero potential, is repeated cyclically. 4. The method according to any one of claims 1 to 3, characterized in that a current to be examined with the component to be measured is passed through a reaction path without adding a reagent and the corresponding potential of the measuring chain is determined, whereupon the reagent continuously after a period of time a second period of time is introduced into the reaction zone and the zero potential of the measuring chain which is established is measured. 5. The method according to any one of claims 1 to 3, characterized in that a stream in the form of a defined amount is conveyed discontinuously into a reaction container and the instantaneous concentration of the component is determined with a measuring electrode, after which a defined amount of the reagent to be used is determined after a period of time is metered into the reaction container so that the component reacts completely and the measuring chain determines the potential corresponding to concentration 0, after which the container is emptied in a further period of time. 6. The method according to any one of the preceding claims, characterized in that a component is measured in aqueous solution. 7. The method according to any one of the preceding claims, characterized in that the cyanide ion content of a solution is measured. 8. The method according to any one of the preceding claims, characterized in that the cyanide ion content is measured using hydrogen peroxide as a reagent. 9. The method according to any one of the preceding claims, characterized in that the cyanide ion content is measured in a solution detoxified with hydrogen peroxide. 10. The method according to any one of the preceding claims, characterized in that one carries out the measurement of the potential of the instantaneous concentration of the component to be measured and the zero potential on a side current of the system to be measured. 11. Application of the method according to one of claims 1-10 for monitoring and controlling the regulation of continuously executed chemical processes on the basis of the measured values obtained. It is known that during measurements, especially with redox or ion sensitive electrodes, changes to the electrodes, e.g. due to interference ions, and cause potential shifts. A very unpleasant phenomenon for the continuous implementation of chemical processes with potentiometric monitoring is e.g. the drifting of the potential of the measuring chain, which normally does not play a role in the potentiometric monitoring of discontinuous processes due to the one-time, short measuring duration. However, the possibility of being able to carry out potential measurements on continuous processes is of very great importance for chemical processes that are monitored, controlled and regulated by these potential measurements, especially since many technical processes involve systems in which Interfering ions or other interfering variables, such as electrode coatings, occur, for example in wastewater detoxification. Monitoring such systems by taking samples and checking them by chemical analysis in the laboratory is generally extremely complex. In addition, this monitoring method as a control for the safe implementation of the chemical process is not harmless, since unforeseen operating fluctuations are usually recognized too late. It is an object of the present invention to provide a potentiometric method by means of which the continuous implementation of chemical processes in which unknown interfering influences on the measuring chain must be expected can be controlled and monitored. It has now been found that chemical processes can be carried out continuously, especially those involving changes in the potentiometric measuring chain. Interference can be calculated and monitored and / or controlled potentiometrically in a simple and safe manner by using redox or ion-sensitive electrodes, if the measurement of the component concentration in question, preferably on a side stream of the system, is carried out in this way that on the one hand the potential corresponding to the current concentration of the component in question is determined, and on the other hand a reagent is added which completely converts the component whose concentration is to be determined into a compound which does not or only negligibly influences the potential of the measuring chain, so that the electrode shows the potential corresponding to the zero concentration of the component in question, the difference between the two potentials indicating the actual concentration of the component in question. The reagent should expediently be added in at least the equivalent amount which is necessary for the reaction of the component to be measured, preferably in an excess. The method can of course also be used on pH electrodes as a special case of ion-sensitive electrodes. The sequence of the method according to the invention is not necessarily limited to the sequence “measuring the potential of the instantaneous concentration - measuring the zero potential”. Of course, the opposite way can also be used, i.e. one begins with the setting of the zero potential and, after a predetermined period of time, measures the instantaneous concentration of the component in question which has now been formed. The sequences are preferably repeated cyclically, although they do not necessarily have to be carried out to their end, i.e. you start e.g. with the 5 10th 15 20th 25th 30th 35 40 45 50 55 60 o5 4th development of zero potential 1, i.e. with the addition of an excess of hydrogen peroxide over the amount equivalent to the cyanide ions present. The time periods were measured in minutes. As can be seen from FIG. 1, the potential of the measuring chain returns to the set zero point each time the system has been supplied with the appropriate amount of hydrogen peroxide. The difference between the potentials at measurement times 3 and 4 is a measure of the cyanide ion concentration. At this point it should be emphasized again that the potential of the measuring chain is practically not influenced by the excess hydrogen peroxide; this was determined before the measurements were carried out. The concentration of the reagent to be used - generally dealing with aqueous solutions - depends on the concentration of the type of ion to be measured. If this concentration is high, more concentrated reagent solutions can be used. If this concentration is lower, less concentrated solutions for indexing the end product are preferable. The process according to the invention can be carried out either continuously or batchwise. This should also be explained using the example of cyanide detoxification of waste water with hydrogen peroxide. The arrangement shown in Fig. 2 works as follows: A small partial stream of the wastewater to be investigated, which contains cyanide ions, flows via lines 5 and 5a through a reaction zone 6. a silver / thalamide measuring chain measured at point 7. At the entry of the reaction zone, i.e. at the entry of line 8a into line 5a, alternately diluted hydrogen peroxide solution is added at 5b from container 9 via line 8a and via a valve 10 controlled by a timer and thus the potential corresponding to the cyanide ion concentration of zero at 7 measured. The current cyanide ion concentration results from the difference between the two measurements. The second measurement, i.e. that of zero potential is carried out with the same measuring chain at 7. The waste water is continuously taken off via line 11 and discarded in this special case. In contrast, when carrying out chemical processes, the measured partial flow is preferably returned to the main flow. The system is vented via line 5c. While the measurement results of the continuous implementation of the method according to the invention are flawless, in practice there have been technical difficulties in carrying out the measurements, e.g. due to the back pressure in the reaction zone. As a result, effective mixing of the solution to be examined and the reagent supplied cannot always be guaranteed. For this reason, it is preferred that the process according to the invention be carried out batchwise. This works as follows: In contrast to the continuous method, the measurement is not carried out in a container through which the solution to be measured flows, but - see. Fig. 3 - in a reaction container 12 with stirrer 13. This reaction container is combined with 2 measuring vessels, one of which is intended for the solution to be examined, the other for the reagent to be added. In a first time step, a measured amount of the relevant solution from the measuring vessel 14, which is delimited by the valves 14a and 14b, is placed in the reaction container 12, added. (The valves 14a and 14b are preferably controlled by compressed air for reasons of explosion safety.) At point 15 there is the electrode which contains the currently relevant ion concentration, e.g. the cyanide ion concentration. Preferably, the electrode at point 15 should be able to determine this concentration immediately after entering the solution from measuring vessel 14. From the storage container 16, the reagent - in the case of cyanide ion measurement - ie the hydrogen peroxide - is fed via line 17 into the measuring vessel 18 for the reagent. Like measuring vessel 14, measuring vessel 18 is equipped with corresponding valves 18a and 18b, which are likewise preferably controlled by compressed air. In the time step following the measurement of the ion concentration, the hydrogen peroxide is now introduced into container 12, the reaction is waited for a certain period of time, and then the zero potential now obtained is measured with the electrode at 15. Then valve 19 is opened and container 12 emptied. The sequence is repeated accordingly. The valves 14a and 14b and 18a and 18b are controlled in such a way that a smooth operation of the method described above is ensured. The lines 20 and 21 are vent lines. The discontinuous implementation of the method according to the invention is characterized in that the above-mentioned difficulties of continuous implementation are avoided here and the discontinuous implementation also works satisfactorily in technical systems. The time required for the measurement can also be reduced in the case of discontinuous implementation compared to the continuous measurement. the time for a measuring cycle in the determination of the cyanide ions with hydrogen peroxide is 2 to 3 minutes, compared to 10 minutes in the continuous procedure. The reason for this is the spontaneous reaction of e.g. Cyanide ions with hydrogen peroxide in the reaction vessel, which - as mentioned - is equipped with a stirrer so that the reactants are mixed immediately. Even in spite of the additional cycle of emptying during the discontinuous implementation, there is still a time saving compared to the continuous implementation. In addition, the apparatus as such is much simpler and more reliable, since volumes are mixed instead of liquid flows and therefore no flow meters are required. The measurement method according to the invention can also be used for potentiostatic arrangements.