Reagent combinations

Abstract

The present invention relates to a reagent combination for determining the chemical oxygen demand (COD) in a liquid medium, wherein the reagent combination comprises a first reagent, wherein the first reagent is an acidic solution of mercuric sulfate (HgSO4); a second reagent, wherein the second reagent is an acidic solution containing potassium dichromate (K2Cr2O7); and a third reagent, wherein the third reagent is an acidic solution, characterized in that at least the second reagent or at least the third reagent also contains potassium aluminum sulfate (KAl(SO4)2·12H2O).

Description

[0001] The invention relates to a reagent combination, in particular a reagent combination for determining the chemical oxygen demand (COD) in a liquid medium. The present invention also relates to an analytical device and a method for measuring the COD using the reagent combination.

[0002] The chemical oxygen demand (COD) reflects the degree of pollution of the water by reducing substances; this indicator is also considered one of the comprehensive indicators for the relative content of organic matter.

[0003] The dichromate method is currently the primary method used to measure the organic matter content of a water sample. The term chemical oxygen demand (COD) refers to the mass concentration of dichromate, which corresponds to the oxygen (O₂) consumed by soluble substances and suspended solids in a liquid medium sample (e.g., a water sample) when oxidized by potassium dichromate (K₂Cr₂O₇) under specific conditions. This value is typically expressed in milligrams per liter (mg / L). Measuring the chemical oxygen demand (COD) allows for the quantification of the amount of organic matter in the water. The principle of the dichromate method is described in the National Environmental Protection Standard of the People's Republic of China HJ828-2017.A known amount of potassium dichromate solution is added to the sample, and silver sulfate (Ag₂SO₄) is used as a catalyst in a strongly acidic medium. After refluxing, 1,10-phenanathroline monohydrate is used as an indicator, and ammonium iron sulfate ([(NH₄)₂Fe(SO₄)₂·6H₂O]) is used to titrate the unreduced potassium dichromate in the water sample, calculating the mass concentration of oxygen consumed from the amount of potassium dichromate consumed. Under acidic potassium dichromate conditions, aromatic hydrocarbons and pyridine are difficult to oxidize, and their oxidation rates are low. Straight-chain aliphatic compounds can be effectively oxidized under the catalysis of silver sulfate.

[0004] The influence of inorganic substances in water samples on the determination of chemical oxygen demand (COD) has always been a difficult problem to solve. The main interferer in the above method is chloride, which at high concentrations can react with potassium dichromate (K₂Cr₂O₇) and cause the chemical oxygen demand (COD) of the sample to become too high. The reaction of chloride with potassium dichromate follows the equation below: 6CI−+Cr2O72−+14H+→3Cl2+2Cr3++7H2O

[0005] Interference from chloride in the sample can be eliminated by adding a mercuric sulfate (HgSO4) solution. After reflux, chloride ions can combine with mercuric sulfate to form soluble mercury chloride complexes. However, this method is not suitable for determining COD in water with chloride concentrations above 1000 mg / L. Samples exceeding this concentration must be diluted before measurement.

[0006] The Chinese invention patent with publication number CN104515769A describes a method for the rapid determination of COD in wastewater with high chloride content, in which the sample is diluted to reduce the chloride concentration to below 2000 mg / l.

[0007] The Chinese invention patent with publication number CN108956509A discloses a reagent and a detection method for the rapid determination of the COD of highly chlorinated wastewater, whereby the chloride concentration must be below 4000 mg / l, otherwise the sample must be diluted.

[0008] European Invention Patent No. EP3591395A1 discloses a method for determining the chemical oxygen demand (COD) of highly chlorinated samples using pre-prepared reagents and a photometer. The chloride concentration in the sample can range from 1500 mg / l to 20,000 mg / l. If the chloride concentration of the sample exceeds 20,000 mg / l, the sample must be diluted.

[0009] To overcome the problem that it is difficult in the prior art to directly measure the COD value in wastewater with a high chloride content, the present invention provides a reagent combination as well as an analysis device and a method for measuring the COD using a reagent combination, in particular a reagent combination, an analysis device and a method for measuring the chemical oxygen demand (COD) in a liquid medium with a high chloride content.

[0010] The reagent combination used by the present invention to solve the technical problem comprises: A first reagent, wherein the first reagent is an acidic solution of mercuric sulfate (HgSO4); a second reagent, wherein the second reagent is an acidic solution comprising potassium dichromate (K₂Cr₂O₇); and a third reagent, wherein the third reagent is an acidic solution, characterized in that at least the second reagent or at least the third reagent further comprises potassium aluminum sulfate (KAl(SO4) 2· 12H2O).

[0011] In the above reagent combination, the role of mercuric sulfate (HgSO4) is to eliminate chloride interference in the liquid medium. Potassium dichromate (K2Cr2O7) is a strong oxidizing agent in acidic environments, its function being to oxidize organic substances in a liquid medium. Potassium aluminum sulfate (KAl(SO4)3) was used as a catalyst. 2·12H2O). Potassium aluminum sulfate is also known as potassium alum, potassium aluminum alum, potassium alum, white alum, crude alum, federal alum, mica alum, potassium aluminum sulfate dodecahydrate, and is simply called "alum." Alum crystals (potassium aluminum sulfate) are colorless or white octahedral crystals that are readily soluble in water. Its crystal structure belongs to the heteropolar trigonal crystal system. Its aqueous solution is safe, non-toxic, colorless, and acidic. Potassium aluminum sulfate has the advantage of being inexpensive and readily available. When silver sulfate (Ag2SO4) is used as a catalyst and the liquid medium contains a high concentration of chlorine, silver ions tend to react with chloride ions to form silver chloride precipitates (AgCl), which are not readily soluble in water, thus reducing the catalytic effect and distorting the test results.In contrast to silver sulfate or other silver salts, potassium aluminum sulfate (KAl(SO4)) 2· 12H2O) not for the precipitation of chloride ions (Cl - ).

[0012] The acidity of the first reagent can be achieved by adding sulfuric acid (H₂SO₄). Likewise, the acidity of the second and third reagents can be achieved by adding sulfuric acid (H₂SO₄). It is understood that the acidity of the above reagent combination can also be achieved by adding other strong acids.

[0013] In some embodiments, the acidity of the first reagent is achieved by adding 10-15 volume percent concentrated sulfuric acid, the acidity of the second reagent is achieved by adding 10-15 volume percent concentrated sulfuric acid (concentration 96%), and the third reagent consists of 100 volume percent concentrated sulfuric acid (concentration 95-97%).

[0014] In some embodiments, the concentration of mercuric sulfate (HgSO4) in the first reagent (104, 204) is 150 g / l to 180 g / l. High concentrations of mercuric sulfate are used to eliminate interferences caused by high concentrations of chloride.

[0015] In some embodiments, the concentration of potassium dichromate (K2Cr2O7) in the second reagent (105, 205) is 20 g / l to 30 g / l.

[0016] In some embodiments, the second reagent or the third reagent is potassium aluminum sulfate (KAl(SO)4(2·12H2O) in concentrations of 5 g / l to 10 g / l.

[0017] The present patent also solves the technical problem by means of an analysis device comprising: a. Reagent combination according to one of the embodiments described above, b. reaction vessel, c. Processing system used to provide a liquid sample and the first reagent, the second reagent and the third reagent to the reaction vessel; d. Temperature control device, which serves at least to control the temperature of the reaction vessel and the processed sample contained therein; e. Measuring sensor for recording at least one measured value of the processed sample contained in the reaction vessel, wherein the measured value is related to a measured quantity; f. Control unit for controlling the processing system; as well as g. Evaluation unit for determining a COD value from the measured values ​​recorded by the measuring sensor.

[0018] In some embodiments, the processing system comprises a sample pump, a valve assembly, a metering unit, and a distributor, wherein the sample pump, the valve assembly, and the metering unit are used to transport a predetermined quantity of a liquid sample from a sample reservoir through a flow path to a reaction vessel, wherein the valve assembly, the metering unit, and the distributor are also used to supply the reaction vessel with a predetermined quantity of a first reagent, a predetermined quantity of a second reagent, and a predetermined quantity of a third reagent, wherein the valve assembly and the distributor are also used to supply air to the reaction vessel, such that the predetermined quantity of sample and the predetermined quantity of the first reagent supplied to the reaction vessel are completely mixed by the air, and the predetermined quantity of sample, the predetermined quantity of first reagent,The specified quantity of the second reagent and the specified quantity of the third reagent supplied to the reaction vessel are completely mixed by the air, with the control unit configured to control the sample pump, the valve group, the dosing unit, and the distributor.

[0019] In some embodiments, the measuring sensor is a photometer and the measured quantity is the absorption of the processed sample.

[0020] In some embodiments, the analysis device further comprises at least one first storage container (216) containing a first reagent (104, 204), at least one second storage container (217) containing a second reagent (105, 205), and at least one third storage container (218) containing a third reagent (106, 206), wherein the first storage container (216), the second storage container (217), and the third storage container (218) are each connected to the reaction vessel (101, 201) by flow paths.

[0021] In some embodiments, the analysis device further comprises a transparent shielding hood, wherein the transparent shielding hood is arranged between the operator and the reaction vessel and covers at least the reaction vessels and the measuring sensor.

[0022] In some embodiments, the processing system also includes a dilution module for diluting the liquid sample before it is fed into the reaction vessel.

[0023] The present invention also solves the technical problem by a method comprising the following steps: a. Providing an analysis device (100, 200) according to one of the embodiments described above; b. Providing a predetermined quantity of a sample of a liquid medium to the reaction vessel via the processing system; c. Adding a predetermined quantity of a first reagent to the reaction vessel via the processing system and mixing a predetermined quantity of a sample with a predetermined quantity of the first reagent; d. Adding a predetermined amount of a second reagent to the reaction vessel by the processing system to obtain a predetermined amount of a first analyte; e. Maintaining the first analyte at a first temperature by means of a temperature control device and determining and recording a first measurement value of the first analyte by means of a measuring sensor; f. Adding a predetermined amount of a third reagent to the reaction vessel by the processing system and mixing a predetermined amount of the sample with a predetermined amount of the first reagent, a predetermined amount of the second reagent and a predetermined amount of the third reagent to obtain a second analyte; g. Maintaining the second analyte at a second temperature using a temperature control device and determining and recording a second measurement of the second analyte using a measuring sensor; and h. Determining the Chemical Oxygen Demand (COD) of the sample using the evaluation unit and based on the first measurement and the second measurement recorded by the measuring sensor.

[0024] When the potassium dichromate method is used to determine COD, the oxidizing agent itself is consumed during the chemical reaction of Cr 6+ to Cr 3+ The reduction was reduced. Simultaneously, the color of the solution changed from orange to green. The reduction and color change of chromate are proportional to the concentration of the oxidizable material in the sample. Therefore, optical quantification of chromate reduction can be used as an indicator of the chemical oxygen demand of the sample. An aqueous potassium aluminum sulfate solution is colorless and does not interfere with optical quantification.

[0025] In some embodiments, the sample quantity used is 1 ml to 3 ml.

[0026] In some embodiments, the specific amount used of the first reagent is 1 ml to 3 ml.

[0027] In some embodiments, the specific amount of the second reagent is 1 ml to 3 ml.

[0028] In some embodiments, the specific amount used of the third reagent is 1 ml to 3 ml.

[0029] In some embodiments, in step e) the first temperature is 50 °C to 60 °C and the first analyte is held at the first temperature for 3 minutes to 10 minutes before the first measurement is taken.

[0030] In some embodiments, in step g) the second temperature is 100 °C to 175 °C and the second analyte is held at the second temperature for 3 to 120 minutes before the second measurement is taken.

[0031] In some embodiments, the process also includes rinsing the reaction vessel and the processing system with a liquid medium before step b).

[0032] The advantageous effect of the present invention is that the present invention provides a reagent combination and a method for the direct measurement of the COD value in wastewater with high chloride content without dilution of the sample.

[0033] For the purposes of this specification, "high chloride content" means that the chloride concentration in the liquid medium is high. The reagent combination, the analytical apparatus, and the method disclosed in the present invention can measure at least the COD value of a liquid medium containing chloride in an amount of up to 20,000 mg / l. The reagent combination and the method of the present invention are not only suitable for liquid media with very high chloride concentrations but can also be used in liquid media with very low chloride concentrations. It is understood that the reagent combination, the analytical apparatus, and the method of the present invention are suitable at least for determining the COD value of a liquid medium with a chloride concentration from 0 mg / l up to and including 30,000 mg / l. If the chloride concentration of the liquid medium is higher than 30,000 mg / l, the COD value can be determined at a lower concentration.If the COD value is 000 mg / l, its COD value can be determined indirectly by diluting the liquid medium.

[0034] The reagent combination, the analytical device, and the method disclosed in the present invention can measure at least the COD value of a liquid medium with a COD value of up to 4000 mg / l. It is understood that the reagent combination, the analytical device, and the method of the present invention are suitable at least for measuring the COD value of a liquid medium with a COD value between 0 mg / l and 4000 mg / l (inclusive).

[0035] The term "liquid medium" in the specification also includes industrial wastewater, domestic wastewater and coastal seawater.

[0036] The present invention is described in more detail below with reference to the figures and specific embodiments. Fig.1 is a fourth embodiment of an analytical device for measuring the COD value of a liquid medium using the reagent combination of the present invention; Fig. 2 is an embodiment 5 of an analysis device; Fig. 3 is an embodiment 6 of an analysis device; Fig. 4 is an embodiment 7 of an analysis device;

[0037] In the illustration, 100, 200 represent: Analysis device; 101, 201: Reaction vessel; 102, 202: Processing system; 103, 203: Sample; 104, 204: First reagent; 105, 205: Second reagent; 106, 206: Third reagent; 107, 207: Temperature control device; 108, 208: Sensor; 109, 209: Control unit; 110, 210: Evaluation unit; 211: Sample pump; 212: Valve assembly; 213: Dosing unit; 214: Distributor; 215: Air; 216: First reservoir; 217: Second reservoir; 218: Third reservoir; 219: Transparent shield. 220. Dilution module. Example 1

[0038] In embodiment 1, the reagent combination for determining the chemical oxygen demand (COD) of a liquid medium comprises a first reagent, a second reagent and a third reagent, wherein the second reagent is a mixed acidic solution of potassium dichromate (K2Cr2O7) and potassium aluminum sulfate (KAl(SO4) 2·12H2O) and the third reagent is concentrated sulfuric acid, the specific components of which are listed in Table 1. Table 1. First reagent Second reagent Third reagent analysis HgSO4 H2SO4 K2Cr2O7 KAl(SO4)2·12H2O H2SO4 H2SO4 concentration 150-180 g / l 10-15 % 20-30 g / l 5-10 g / l 10-15 % 100 %

[0039] As shown in Table 1, the acidity of the first, second, and third reagents is achieved by adding sulfuric acid (H₂SO₄). The acidity of the first reagent is achieved by adding 10–15% by volume of concentrated sulfuric acid, the acidity of the second reagent is achieved by adding 10–15% by volume of concentrated sulfuric acid (96% concentration), and the third reagent consists of 100% by volume of concentrated sulfuric acid (95–97% concentration).

[0040] To prepare, for example, 1000 ml of the first reagent, 100 to 150 ml of concentrated sulfuric acid (96% concentration) and 600 to 750 ml of ultrapure water are thoroughly mixed in a beaker. Then, 150 to 180 g of mercuric sulfate (HgSO4) are added and allowed to dissolve completely. After the temperature of the reagent in the beaker has returned to room temperature, the reagent is transferred to a volumetric flask, and ultrapure water is injected into the flask up to the 1000 ml mark to obtain the first reagent. Example 2

[0041] In embodiment 1, the reagent combination for determining the chemical oxygen demand (COD) of a liquid medium comprises a first reagent, a second reagent, and a third reagent, wherein the components of the second and third reagents differ from those in Example 1, wherein the second reagent is an acidic solution of potassium dichromate (K₂Cr₂O₇) and the third reagent is an acidic solution of potassium aluminum sulfate (K₂Al(SO₄)₂). 2· 12H2O), whose specific components are listed in Table 2. Table 2. First reagent Second reagent Third reagent analysis HgSO4 H2SO4 K2Cr2O7 H2SO4 KAl(SO4)2·12H2O H2SO4 concentration 150-180 g / l 10-15 % 20-30 g / l 10-15 % 5-10 g / l ≥90 % Example 3

[0042] In embodiment 3, the reagent combination for determining the chemical oxygen demand (COD) of a liquid medium also comprises a first reagent, a second reagent and a third reagent, wherein potassium aluminum sulfate (KAl(SO4)2·12H2O) is contained in both the second and the third reagent; the specific components are listed in Table 3. Table 3. First reagent Second reagent Third reagent analysis HgSO4 H2SO4 K2Cr2O7 KAl(SO4)2·12H2O H2SO4 KAl(SO4)2·12H2O H2SO4 concentration 150-180 g / l 10-15 % 20-30 g / l 5-10 g / l 10-15 % 5-10 g / l ≥90 %

[0043] As shown in Table 3, Example 3 differs from Example 1 and Example 2 in that the second and third reagents are both potassium aluminum sulfate (KAl(SO4)₂). 2· 12H2O). That is, the second reagent is a mixed acidic solution of potassium dichromate (K2Cr2O7) and potassium aluminum sulfate (KAl(SO4)2·12H2O). It is understood that when using the reagent combination in embodiment 3 for the determination of COD, the concentration of potassium aluminum sulfate (KAl(SO4) 2·12H2O) in the second and third reagents can be reduced accordingly if the amounts of the sample, the first reagent, the second reagent and the third reagent are the same as in embodiment 1 or embodiment 2. Example 4

[0044] In embodiment 4, an analysis device 100 for determining the chemical oxygen demand (COD) in a liquid medium comprises a reaction vessel 101, a processing system 102, a temperature control device 107, a measuring sensor 108, a control unit 109 and an evaluation unit 110.

[0045] As in Fig.As shown in Figure 1, first, 1 ml to 3 ml of a liquid medium sample 103 are introduced into a reaction vessel 101 of an analytical device 100, and the specific introduction method can be carried out via a processing system 102. Second, 1 ml to 3 ml of the first reagent 104, for example, an acidic solution of mercuric sulfate (HgSO4), wherein the concentration of mercuric sulfate (HgSO4) is 150 g / l to 180 g / l, are introduced into the reaction vessel 101 so that the sample 103 is mixed with the first reagent 104. Subsequently, 1 ml to 3 ml of a second reagent 105, for example, an acidic solution of potassium dichromate (K2Cr2O7), and the concentration of potassium dichromate (K2Cr2O7) is 20 g / l to 30 g / l, are introduced into the reaction vessel 101. In this way, the mixed liquid in reaction vessel 101 is the first analyte.The reaction vessel 101 and the first analyte are maintained at a first temperature, in particular 50 °C to 60 °C, by a temperature control device 107 for 3 to 10 minutes. Afterward, a first measurement of the first analyte is determined by a measuring sensor 108, wherein the measuring sensor 108 can be a photometer and the first measurement is an absorbance value. The purpose of maintaining the first temperature of the first analyte is to eliminate the influence of the ambient temperature on the investigation in order to obtain a stable first measurement.

[0046] Subsequently, 1 ml to 3 ml of a third reagent 106, for example an acidic solution of potassium aluminum sulfate (KAl(SO4)2·12H2O), are introduced into the reaction vessel 101, and the concentration of the potassium aluminum sulfate (KAl(SO4) 2·The COD (dilution of 12H₂O) is 5 g / L to 10 g / L. At this point, sample 103 is completely mixed with the first reagent 104, the second reagent 105, and the third reagent 106 in reaction vessel 101, and the mixed liquid is the second analyte. After obtaining the second analyte, the temperature control device 107 is used to maintain the reaction vessel 101 and the second analyte contained therein at a second temperature, in particular 100 °C to 175 °C, and the reaction can continue for 3 to 120 minutes. When the color of the second analyte no longer changes, its second measurement can be taken by the measuring sensor 108, where the second measurement can be an absorbance value. The duration of the reaction depends on the COD value of sample 103; the higher the COD value, the longer the reaction time.

[0047] Finally, the chemical oxygen demand (COD) of sample 103 is determined using the evaluation unit 110 based on the first measured value and the second measured value determined and recorded by the measuring sensor 108.

[0048] If necessary, the reaction vessel 101 and the processing system 102 can be rinsed with the liquid medium before the liquid medium sample 103 is introduced into the reaction vessel 101. Example 5

[0049] Exemplary embodiment 5 is an example of the use of the Liquiline System CA80 200 COD analyzer from Endress+Hauser for measuring the chemical oxygen demand (COD) in a liquid medium. The analyzer 200 comprises a reaction vessel 201, a processing system 202, a temperature control device 207, a measuring sensor 208, a control unit 209, and an evaluation unit 210.

[0050] As in Fig.As shown in Figure 2, sample 203 is introduced into the reaction vessel 201 of the analytical device 200 by the processing system 202. The processing system 202 further comprises a sample pump 211, a valve assembly 212, a metering unit 213, and a distributor 214. Sample 203 is introduced into the analytical device 200 by the sample pump 211, and the valve assembly 212 and the metering unit 213 are used to quantify the liquid sample 203 and transport it through the flow path to the reaction vessel 201. The valve assembly 212, the metering unit 213, and the distributor 214 are also used to supply a predetermined amount of the first reagent 204, a predetermined amount of the second reagent 205, and a predetermined amount of the third reagent 206 to the reaction vessel 201.

[0051] The valve assembly 212 and the distributor 214 serve not only to supply the sample 203 and the reagent combination, but also to supply air 215 to the reaction vessel 201, so that a predetermined quantity of sample 203 supplied to the reaction vessel 201 and a predetermined quantity of mercuric sulfate (HgSO4) in the first reagent 204 are completely mixed by the air 215, thereby eliminating interference from chloride in the liquid medium. Likewise, a predetermined quantity of sample 203, a predetermined quantity of the first reagent 204, a predetermined quantity of the second reagent 205, and a predetermined quantity of the third reagent 206 supplied to the reaction vessel 201 can also be sufficiently mixed by the air 215.

[0052] The control unit 209 is configured to control the processing system 202, which includes a sample pump 211, a valve group 212, a dosing unit 213 and a distributor 214. Example 6

[0053] As shown in Fig. 3, the analysis apparatus 200 of Example 6 further comprises at least one first storage container containing a first reagent, at least one second storage container containing a second reagent, and at least one third storage container containing a third reagent, wherein the first storage container, the second storage container, and the third storage container are each connected to the reaction vessel by flow paths. Example 7

[0054] As in Fig.As shown in Figure 4, the analysis device 200 from embodiment 7 further comprises a transparent protective hood 219. The transparent protective hood 219 is located between the operator and the reaction vessel 201 and covers at least the reaction vessel 201 and the measuring sensor 208 to protect the operator. Since high temperature and high pressure prevail in the vicinity of the reaction vessel 201 during the measurement process, the presence of the transparent protective hood 219 can effectively protect the operator, especially if the reaction vessel 201 bursts due to improper handling.

[0055] The processing system 202 of the analytical device 200 also includes an optional dilution module 220 for diluting the sample 203. The dilution module 220 may also include water and a valve (not shown) for dilution, and the control unit 209 is also configured to control the processing system, including the dilution module 220. The reagent combination, the analytical device 200, and the method disclosed in the present invention can directly measure the COD value of the sample 203, so that the dilution module 220 is not required. However, if the chloride content of the sample 203 exceeds the range that can be directly measured by the analytical device 200, a dilution module 220 can be added to indirectly measure the COD value of the sample 203 with an excessively high chloride ion concentration. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] CN 104515769A

[0006] CN 108956509A

[0007] EP 3591395A1

[0008]

Claims

[1] Reagent combination for determining the chemical oxygen demand (COD) in a liquid medium, the reagent combination comprising: a first reagent (104, 204), wherein the first reagent (104, 204) is an acidic solution of mercuric sulfate (HgSO4); second reagent (105, 205), wherein the second reagent (105, 205) is an acidic solution comprising potassium dichromate (K2Cr2O7); and a third reagent (106, 206), wherein the third reagent (106, 206) is an acidic solution, characterized by , that at least the second reagent (105, 205) or at least the third reagent (106, 206) furthermore potassium aluminum sulfate (KAl(SO4) 2· 12H2O). [2] Reagent combination according to claim 1, wherein the acidity of the first reagent (104, 204), the second reagent (105, 205) and the third reagent (106, 206) is achieved by adding sulfuric acid (H2SO4), wherein the acidity of the first reagent (104, 204) is achieved by adding 10-15 volume percent concentrated sulfuric acid (concentration 96%), the acidity of the second reagent (105, 205) is achieved by adding 10-15 volume percent concentrated sulfuric acid (concentration 96%) and the acidity of the third reagent (106, 206) is achieved by adding not less than 90 volume percent concentrated sulfuric acid (concentration 95-97%). [3] Reagent combination according to claim 1 or 2, wherein the concentration of mercuric sulfate (HgSO4) in the first reagent (104, 204) is 150 g / l to 180 g / l. [4] Reagent combination according to at least one of claims 1 to 3, wherein the concentration of potassium dichromate (K2Cr2O7) in the second reagent (105, 205) is 20 g / l to 30 g / l. [5] Reagent combination according to at least one of claims 1 to 4, wherein the concentration of potassium aluminum sulfate (KAl(SO4) 2· 12H2O) in the second reagent (105, 205) or in the third reagent (106, 206) is 5 g / l to 10 g / l. [6] Analytical device (100, 200) for determining the chemical oxygen demand (COD) in a liquid medium, the analytical device (100, 200) comprising: a. Reagent combination according to at least one of claims 1 to 5, b. Reaction vessel (101, 201), c. Processing system (102, 202) used to provide a liquid sample (103, 203) and the first reagent (104, 204), the second reagent (105, 205) and the third reagent (106, 206) to the reaction vessel (101, 201); d. Temperature control device (107, 207) which is used at least to control the temperature of the reaction vessel (101, 201) and the processed sample contained therein; e. Measuring sensor (108, 208) which is used at least to record a measured value of the processed sample contained in the reaction vessel (101, 201), wherein the measured value is related to a measured quantity; f. Control unit (109, 209) for controlling the processing system (102, 202); and g. Evaluation unit (110, 210) for determining a COD value from the measured values ​​recorded by the measuring sensor (108, 208). [7] Analytical device (100, 200) according to claim 6, wherein the processing system (101, 202) comprises a sample pump (211), a valve assembly (212), a metering unit (213) and a distributor (214), wherein the sample pump (211), the valve assembly (212) and the metering unit (213) are used to transport a predetermined quantity of a liquid sample (103, 203) from a sample supply area through a flow path to a reaction vessel (201), wherein the valve assembly (212), the metering unit (213) and the distributor (214) are further used to supply the reaction vessel (101, 201) with a predetermined quantity of the first reagent (104, 204), a predetermined quantity of the second reagent (105, 205) and a predetermined quantity of the third reagent (106, 206) to supply, wherein the valve group (212) and the distributor (214) also serve to supply air (215) to the reaction vessel (101, 201), so that a supply to the reaction vessel (101,201) supplied a predetermined quantity of sample (103, 203) and a predetermined quantity of the first reagent (104, 204) are completely mixed by air (215), and a predetermined quantity of sample (103, 203), a predetermined quantity of a first reagent (104, 204), a predetermined quantity of a second reagent (105, 205) and a predetermined quantity of a third reagent (106, 206) supplied to the reaction vessel (101, 201) are completely mixed by air (215), wherein the control unit (109, 209) is configured to control the sample pump (211), the valve group (212), the metering unit (213) and the distributor (214), [8] Analysis device (100, 200) according to claim 6 or 7, wherein the measuring sensor (108, 208) is a photometer and the measured quantity is the absorbance of the processed sample. [9] Analysis device (100, 200) according to at least one of claims 6 to 8, further comprising: at least one first storage container (216) containing a first reagent (104, 204), at least one second storage container (217) containing a second reagent (105, 205), and at least one third storage container (218) containing a third reagent (106, 206), wherein the first storage container (216), the second storage container (217), and the third storage container (218) are each connected to the reaction vessel (101, 201) by flow paths. [10] Analysis device (100, 200) according to at least one of claims 6 to 9, further comprising: Transparent protective hood (219), wherein the transparent protective hood (219) is arranged between the operator and the reaction vessel (101, 201) and at least the reaction vessel (101, 201) and the measuring sensor (108, 208). [11] Analysis device (100, 200) according to at least one of claims 6 to 10, wherein the processing system (102, 202) further comprises a dilution module (220) for diluting the liquid sample (103, 203) before it is fed to the reaction vessel (101, 201). [12] Method for determining the chemical oxygen demand (COD) in a liquid medium, the method comprising the following steps: a. Providing an analysis device (100, 200) according to at least one of claims 6 to 11; b. Providing a predetermined quantity of a sample of a liquid medium (103, 203) to the reaction vessel (101, 201) by the processing system (102, 202); c. Adding a predetermined amount of a first reagent (104, 204) to the reaction vessel (101, 201) through the processing system (102, 202) and mixing a predetermined amount of a sample (103, 203) with a predetermined amount of the first reagent (104, 204); d. Adding a predetermined amount of a second reagent (105, 205) to the reaction vessel (101, 201) by the processing system (102, 202) to obtain a predetermined amount of a first analyte; e. Maintaining the first analyte at a first temperature by means of a temperature control device (107, 207) and determining and recording a first measurement value of the first analyte by means of a measuring sensor (108, 208); f. Adding a predetermined amount of a third reagent (106, 206) to the reaction vessel (101, 201) through the processing system (102, 202) and mixing a predetermined amount of the sample (103, 203) with a predetermined amount of the first reagent (104, 204), a predetermined amount of the second reagent (105, 205) and a predetermined amount of the third reagent (106, 206) to obtain a second analyte; g. Maintaining the second analyte at a second temperature using a temperature control device (107, 207) and determining and recording a second measurement of the second analyte using a measuring sensor (108, 208); and h. Determining the Chemical Oxygen Demand (COD) of the sample (103, 203) using the evaluation unit (110, 210) and based on the first measurement and the second measurement recorded by the measuring sensor (108, 208). [13] Method according to claim 12, wherein the specific sample quantity used (103, 203) is 1 ml to 3 ml. [14] Method according to claim 12, wherein the specific amount of the first reagent (104, 204) is 1 ml to 3 ml. [15] Method according to claim 12, wherein the specific amount of the second reagent (105, 205) is 1 ml to 3 ml. [16] Method according to claim 12, wherein the specific amount of the third reagent (106, 206) is 1 ml to 3 ml. [17] Method according to at least one of claims 12 to 16, wherein in step e) the first temperature is 50 °C to 60 °C and the first analyte is held at the first temperature for 3 to 10 minutes before the first measurement is determined. [18] Method according to at least one of claims 12 to 17, wherein in step g) the second temperature is 100 °C to 175 °C and the second analyte is held at the second temperature for 3 to 120 minutes before the second measurement is determined. [19] Method according to at least one of claims 12 to 18, wherein this method further comprises: The reaction vessel (101, 201) and the processing system (102, 202) are rinsed with a liquid medium before step b).

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