METHOD AND DEVICE FOR CHEMICAL-FREE DETERMINATION OF THE CHEMICAL OXYGEN DEMAND (CSB) IN AQUEOUS SAMPLES

Abstract

The present invention relates to a method and a device for the chemical-free determination of the chemical oxygen demand (CODs) in aqueous samples. The object of the invention, to develop a method that compensates for the disadvantages of the standard method and at the same time is at least as good as this, in that it can determine the COD quickly and with a high measurement frequency without chemicals and is cheap and has a low personnel requirement and should be simple to automate, is achieved in that the chemical oxygen demand of aqueous samples is determined by non-specific oxidation of water components at an electrode (11), assisted by (ultra)sound from a sound source (15) in a frequency range in which no significant quantities of oxidative species are formed, i.e. below the cavitation threshold.

Claims

1. A method for determining the chemical oxygen demand of aqueous samples comprising: non-specific oxidation of water components at an electrode (11) supported by (ultra)sound from a sound source (15) in a frequency range in which no significant quantities of oxidative species are formed below a cavitation threshold.

2. The method according to claim 1, comprising using a three-electrode arrangement with a working (11), counter (13) and reference electrode (12).

3. The method according to claim 1, comprising determining the chemical oxygen demand from a current strength measured during the oxidation of the water components by comparison with a calibration.

4. The method according to claim 1, characterized in that the chemical oxygen demand is determined from the a charge which flowed during complete oxidation.

5. The method according to claim 1, comprising applying a linearly increasing or a linearly increasing and decreasing potential and obtaining the chemical oxygen demand from a difference between a current-voltage line obtained during the measurement and of a pure electrolyte solution (2).

6. The method according to claim 1, comprising operating a sound source (15) during the measurement of the chemical oxygen demand in a frequency range from 8 kHz to 50 MHz, and at a power below the cavitation threshold.

7. The method according to claim 6, comprising operating the sound source (15) for cleaning in a frequency range of 8 kHz to 50 MHz, and at a power above the cavitation threshold.

8. The method according to claim 1, comprising arranging electrodes (11, 12 and/or 13) and the sound source (15) in a flow cell.

9. The method according to claim 8, comprising additionally using the sound for degassing an electrolyte (2) and a sample solution (1).

10. A device for determining chemical oxygen demand, comprising: a working electrode (11), a counter (13) and a reference electrode (12) in a three-electrode circuit, an electrolyte, means for measuring and controlling a potential between a working electrode and a reference electrode and a current between the working electrode and the counter electrode, one or more sound sources (15) for activating different powers and one or more pumps (3, 4) for transporting the electrolyte (2) and a sample solution (1).

Description

[0043] A possible setup is shown in FIG. 1.

[0044] FIG. 1: Basic setup of a device for determining the COD.

[0045] FIG. 2: Basic setup of the electrochemical measurement cell.

[0046] FIG. 3: Calibration of the method for determining the COD.

[0047] FIG. 4: Improvement of the current signal through the use of ultrasound.

[0048] The device for determining the chemical oxygen demand shown in FIG. 1 contains the electrochemical measurement cell 6, which is described in more detail in FIG. 2. With the help of pumps 3 and 4, the sample solution 1 and the electrolyte solution 2 are pumped through the cell.

[0049] In this embodiment, there is an inlet for the electrolyte and an inlet for the sample solution. The valve 5 can be used to switch between the two flows so that first, the electrolyte flows into the electrochemical cell and the sample flows past it. This is shown in FIG. 1 by the differently marked lines. After a lead time, the valve 5 is then switched so that the sample now flows through the cell and the electrolyte flows past it. The sample must be dissolved in an electrolyte solution in order to have sufficient conductivity. After passing through the cell, the solutions flow into the waste container 8. There is also a measuring and control unit 7 for current and voltage. A potentiostat, for example, is used for this purpose.

[0050] FIG. 2 shows the electrochemical cell in more detail. Inlet 9 and outlet 10 for the solutions are shown here. The three-electrode arrangement consists of a boron-doped diamond electrode as the working electrode 11, the reference electrode 12 and the counter electrode 13. The three electrodes are connected to the measuring and control unit 7 via the contacts 14. A sound source 15 is also indicated symbolically.

[0051] In this embodiment, the amperometric method described above is used. A BDD electrode is used as the working electrode 11. 0.001 M Na.sub.2SO.sub.4 with 0.001 M H.sub.2SO.sub.4 is used as the electrolyte. An ultrasonic bath is used as the sound source 15. This introduces ultrasound at a frequency of 35 kHz and a power of 0.3 W/cm.sup.2 into the cell, whereby no cavitation is generated. A voltage of 2.8 V is applied. First the electrolyte flows through the measurement cell. A constant background current has been established after 30 s. The switch is made to the sample flow. Measurements are continued for a further 1 minute so that a constant current can be established again. The COD can be determined from the difference between the sample current and the background current. The current signal shows a linear dependence on the COD of the sample. By calibrating the method with standard samples of known COD, the COD can then be determined from the measured current signal. Such a calibration is shown in FIG. 3.

[0052] The current difference can be increased by introduction of ultrasound into the measurement cell, as shown in FIG. 4. Here, the current-time curves measured in the experiment were supplemented with curves representing the ideal progression. In principle, a current-time curve for the investigated amperometric method runs in such a way that after application of the voltage at 0 s, the current initially falls sharply and then, after a certain time, a state of equilibrium with a constant current is reached. After the change to the sample solution after 30 s, the current increases sharply due to the oxidation. A constant current is also restored after a waiting period. The difference between the investigations with and without ultrasound is that, firstly, the background current is established more quickly after the voltage is applied, which means that the measurement can be shortened. In the example shown, one could switch from the electrolyte to the sample solution after just 5 s and would thus reduce the measurement by 25 s to 65 s. Secondly, the signal current is higher in the case of the ultrasound-supported measurement, which lowers the detection limit of the measurement. In the example shown, the current difference increases by 30%, which means that the detection limit could be reduced from 10 mg/L to 7 mg/L.

[0053] When the coulometric method is used, the charge that flows until a sample is completely oxidized is measured instead of the current. The COD can be calculated directly from the charge. No calibration is necessary here.

LIST OF REFERENCE NUMERALS

[0054] 1 Sample solution

[0055] 2 Electrolyte solution

[0056] 3 Pump

[0057] 4 Pump

[0058] 5 Valve

[0059] 6 Measurement cell

[0060] 7 Measuring and control unit

[0061] 8 Waste container

[0062] 9 Inlet

[0063] 10 Outlet

[0064] 11 Working electrode

[0065] 12 Reference electrode

[0066] 13 Counter electrode

[0067] 14 Contacts

[0068] 15 (Ultra)sound source