CN1141584C - Stop-flow-Sequential Injection-Cerium Oxidation Online Monitoring Method of COD in Water - Google Patents

Abstract

The present invention discloses a method for automatically monitoring chemical oxygen demand (COD) in environment water by a stopped flow-sequential injection-cerium oxidation method in an on-line mode. The methods of sequential injection, stopped flow, sandwiched sampling, etc. are used. An SI method of the present invention has the characteristics of on-line automatic monitoring, rapidity, good reproducibility, economical operation, etc. An oxidizing agent is a cerium oxidizing agent which has the advantages of little pollution and mild reaction condition, the reaction adopts a stopped flow method and a sandwiched sampling method, and the reaction stability and the sensitivity are obviously enhanced. The methods are combined to monitor COD in an on-line mode, and thus, the method has the characteristics of simple structure, automation, rapidity, wide linear range, good reproducibility, high stability, high sensitivity, low operation cost, etc. The present invention can be used for monitoring domestic sewage, water in rivers and lakes and industrial sewage.

Description

Stop-flow-sequential injection-cerium oxidation method for on-line monitoring of chemical oxygen demand in water

1. Field

The invention relates to an automatic on-line monitoring method of chemical oxygen demand (COD) in environmental water, and further relates to a method for on-line monitoring of chemical oxygen demand (COD) in environmental water combined with stop-flow-sequential injection and cerium oxidation.

2. Background technology

At present, the automatic monitoring methods for COD in environmental water in domestic and foreign literature and patents mainly include: coulometric automatic titrator based on titration and photometric detector based on flow injection (FI). Potassium chromate, potassium permanganate and cerium oxidation. The coulometric automatic titration method consumes a lot of samples and reagents and takes a long time; the FI method has relatively high measurement speed and sampling frequency, and the main problem at present is the poor stability of long-term operation, especially the used The peristaltic pump must frequently replace the pump tube, and the consumption of reagents during operation is relatively large, and causes secondary pollution that cannot be ignored.

Thirteen closely related documents were found from 16 databases at home and abroad, such as Dialog, U.S. Patent, and Chinese Patent, among which the document (Trends in analytical chemistry, 1997, 16, 419-424) provided the sequence injection (SI) method was proposed to monitor multiple monitoring indicators (including COD) of water species, but no experimental program was proposed, nor was any relevant experimental process and results reported. In the relevant domestic literature, the invention patent "Method and device for continuous determination of chemical oxygen demand" (application number: 94112025) discloses a method and device for continuous determination of COD that can avoid secondary pollution. Reverse-phase FI method of flow solution and reagent as injection solution. A utility model patent (application number: 912017272) discloses a measuring instrument for flow injection chemical oxygen demand. The document "CTL-12 Type Chemical Oxygen Demand Measuring Instrument" (Petrochemical Environmental Protection, 1996, Phase 2) uses potassium dichromate high-temperature catalytic oxidation method to measure COD. The literature (Journal of Ningxia Agricultural University, 1998, Volume 1, Issue 1) introduces a method for the rapid determination of COD in environmental water by using cerium as an oxidant and stop-flow-reversed-phase flow injection analysis. Literature (Environmental Monitoring Management and Technology, Volume 12, Issue 1, 2000) gave a brief introduction to the determination of COD by the Smart Field Analyzer.

Although the above literatures mentioned about sequential injection, chemical oxygen demand, stopped flow, cerium oxidant, etc., most of the literatures mainly used the FI method, even the combination of stopped flow-reverse flow injection analysis and cerium oxidation, but There are no domestic and foreign literature reports on continuous COD determination using the stop-flow-sequential injection (SI) system.

The National Environmental Protection Agency once organized a study on China's 2020 environmental protection strategy and countermeasures, and put forward new requirements for the prevention and control of industrial and domestic wastewater pollution in the next 20 years. In order to meet this demand, it is of far-reaching significance to carry out online automatic monitoring research on water pollution indicators for timely grasping and controlling water pollution indicators. It is very necessary to develop an online monitoring instrument with low cost, good stability and little secondary pollution by adopting the latest analysis method and principle.

3. Contents of the invention

The object of the present invention is to provide a method for on-line monitoring of chemical oxygen demand (COD) in environmental water combined with stop-flow-sequential injection and cerium oxidation.

In the method for on-line monitoring of chemical oxygen demand (COD) in environmental water combined with stop-flow-sequential injection and cerium oxidation announced by the present invention, the SI method adopted is the third generation technology of FI. In addition to the advantages of FI, It also has the following characteristics: 1) The system hardware is simple and reliable, and the computer control is simple and convenient. It can truly realize single-channel analysis, and it is easy to realize integration and miniaturization; 2) The same device can be used to complete the analysis of different items without changing the flow path settings, which is especially suitable for It is suitable for process analysis and complex analysis operations; 3) The consumption of samples and reagents is very small, suitable for long-term monitoring and analysis with expensive reagents and limited sample sources. It can be automatically calibrated with single or multiple standards and automatically diluted for samples , truly automatic analysis. The adoption of the cerium oxidation method can replace potassium dichromate or potassium permanganate, and cerium with strong oxidizing ability is used as an oxidant, which has low toxicity and will not cause secondary pollution to the environment; in addition, cerium (C _e ⁴⁺ /C _e ³⁺ ) oxidation potential is less affected by pH and acidity, and the concentration of acid can be greatly reduced during use, prolonging the life of each pipeline. Since the reaction between cerium and COD is a slow reaction, for this slow reaction system, the stop-flow technology is used in the SI system to increase the residence time (ie, the chemical reaction time), so that the sample strip and the reagent strip have enough time to mix and reaction to produce enough reaction product to be detected. Stop the sample in front of the flow cell to increase the retention time, and the dispersion will not increase during this increased time, which will help improve the sensitivity of the measurement, that is, the same chemical system and the same experimental parameters can be obtained than when the sample is not stopped. When the signal peak is higher, the peak width does not increase.

In order to achieve the above object, the technical scheme adopted in the present invention is: the sample, the cerium oxidant and the sample (i.e. the sandwich type sampling method) are sucked into the storage tube sequentially through the syringe pump and the multi-channel directional valve (hereinafter referred to as the pump and the valve). Then, from the other valve position of the multi-channel directional valve, the injection pump rotates forward for a certain period of time to push the solution in the storage tube into the reaction tube in the water bath for digestion. After stopping the flow for a certain period of time (such as 600s), the injection The pump rotates forward for a certain period of time to push the mixed flow in the reaction tube to the detector, and finally the software completes data collection and processing, and gives the measured value.

In the SI system, a sandwich (sandwich) sampling method is also used, that is, a certain volume of Ce ⁴⁺ is added between the COD bands of the two samples, and it can fully react with the COD, and a very stable peak signal is measured, which improves the Sensitivity of detection. Moreover, this sandwich-type sampling method can be used in SI to improve the stability and sensitivity of the determination of other reactions that utilize the decrease in the absorbance of the assay reagent itself. The COD determination disclosed in the present invention is based on the combination of cerium oxidation method and stop-flow-SI. The above method can be embodied by the following specific operation steps, but is not limited to the specific operation steps.

See Table 1 for the specific operation steps:

Table 1 Operation sequence of COD determination controlled by computer

Time(s) Pump Valve Description

                                                                                       

1 The water load valve is in the water load liquid position

                                                                         

32 Stop Pump stop

34 Sample Valve in sample position

35 reverse reverse suction sample

36 Stop Pump stop

38 Cerium valve in cerium position

39 Inversion of cerium absorption solution

39.5 Stop Pump stop

41.5 Sample Valve in sample position

42.5 reverse rotation suction sample

43.5 Stop Pump stop

45.5 UV detector Valve in UV detector position

46.5 Forward rotation The pump is turning forward to push the mixing to the water bath

49.5 Stop The pump stops, during which the sample belt and reagent

Have sufficient time to respond

649.5 Forward rotation The pump pushes product to the detector before turning forward

664.5 Stop The pump and the valve are in the stop and start position, like

Another cycle of Pinding begins

When the instrument just started measuring, the pump and valve were in a stopped state; at the first second, the valve turned to the water-carrier liquid 2 position; from the second to the 32nd second, the pump reversed to absorb the water-carrier liquid and then stopped; at the 34th second, the valve Turn to the position of sample 1; from 35 seconds to 36 seconds, the pump reverses to suck the sample and stops; at 38 seconds, the valve turns to the position of cerium solution 3; from 39 seconds to 39.5 seconds, the pump stops after it reverses to suck the cerium solution ; At 41.5 seconds, the valve turns to the sample 1 position; from 42.5 seconds to 43.5 seconds, the pump reverses to absorb the sample and stops; at 45.5 seconds, the valve turns to the UV detector 5 position; from 46.5 seconds to 49.5 seconds, the pump Push the mixed sample to the reaction tube of the water bath before turning forward and stop; from 49.5 seconds to 649.5 seconds, it is the stop flow period, during which the sample belt and the reagent fully react in the water bath; from 649.5 seconds to 664.5 seconds, The pump pushes the product to the detector before turning and stops, while the software completes the data acquisition and processing; at this point, the pump and valve are stopped and in the initial position, and another cycle of sample determination begins.

The SI method used in the invention has the characteristics of automatic, fast, good reproducibility and economical operation of on-line measurement. The oxidizing agent adopts cerium oxidizing agent with less pollution and mild reaction conditions, and the reaction adopts the stopped-flow method and the sandwich injection method, which significantly improves the reaction stability and sensitivity. The combination of these makes the system for online determination of COD have the characteristics of simple structure, automatic, fast, wide linear range, good reproducibility and stability, high sensitivity and low operating cost. In addition, the present invention uses the SI method to enable the same device to complete a number of different monitoring indicators (such as COD, ammonia nitrogen, total phosphorus, total carbon, etc.) without changing the setting of the flow path, laying a foundation.

4. Description of drawings

Fig. 1 is a flow diagram of measuring COD by the present invention.

5. Specific implementation

The present invention will be described in further detail below in conjunction with the accompanying drawings and embodiments.

Referring to Figure 1, the symbols in the figure represent respectively: 1': syringe pump; 2': storage tube; 3': multi-channel selector valve; 4': ultraviolet detector; 5': reaction tube; 6': Back pressure tube; 7': water carrier liquid; 8': sample; 9': standard solution; Ce ⁴⁺ : cerium oxidant; 10': water bath; 11' waste liquid.

in:

Syringe pump: It is the most stable and excellent infusion power component in the SI system. The primary purpose is accurate and highly reproducible pipetting and pushing of solutions. It is connected to the multi-channel selector valve through a polytetrafluoroethylene tube.

Multi-channel directional valve: It is the core part of the SI system, which can be directly connected with samples, reagents, cleaning solutions and detectors. Suck a certain volume of samples, reagents, and cleaning solutions from different channels to the storage tube in sequence, and then reverse the flow direction of the syringe pump to push the solution in the storage tube to the detector. It can only be effectively controlled by microcomputer programming. It is connected with the syringe pump and the detector through a polytetrafluoroethylene tube.

Storage tube: used only in SI system, the main function is to store the sample and reagent zone for quantitative inhalation. Generally, it is a polytetrafluoroethylene tube with an inner diameter of 0.8 to 1.5 mm. When the storage zone is drawn in and successively pushed out to the reaction tube and the detector, the reagent and the sample will partially penetrate each other and react, therefore, the storage tube also partially functions as a reaction pipeline.

Water-borne liquid, sample, standard liquid and Ce ⁴⁺ oxidant: their flows are controlled by a computer program (as shown in Table 1) in each connecting channel (such as polytetrafluoroethylene tube) of the multi-channel directional valve, reaction pipeline and detector.

Heating device: water bath or oil bath, microwave digestion, polymer resistance film, etc. can be used for COD digestion. It is connected with the multi-channel directional valve and the detector through polytetrafluoroethylene tubes.

Reaction tube: generally a polytetrafluoroethylene tube with an inner diameter of 0.5-0.8mm. Placed in a water bath, the stop-flow reaction was performed again.

Back pressure tube: generally a polytetrafluoroethylene tube with an inner diameter of 0.25-0.5mm, mainly to increase the pressure in the pipeline to eliminate air bubbles.

Ultraviolet detector: used for COD detection (λ=320nm), it is connected with the multi-channel selector valve and the heater through a polytetrafluoroethylene tube.

The following is a specific embodiment given by the inventor, but it is not limited to this embodiment, because the water quality in reality is varied, and the regulatory agency and personnel can respond to the instrument's response conditions, measurement periods, alarm values, etc. After setting, you can check the measurement records and modify the input mode of the instrument.

Embodiment: obtain from several blowdown outlets such as factory, moat, after filtering with 800 ^# sieve cloth, filtrate is divided into two parts, and one part is sent to Shaanxi Provincial Environmental Monitoring Station to detect, and one part is injected with stop-flow-order of the present invention The online monitoring method of chemical oxygen demand (COD) in environmental water combined with cerium oxidation is used for monitoring.

According to the method of the present invention, the specific operation steps of the present invention are:

When the instrument just started measuring, the pump and valve were in a stopped state; at the first second, the valve turned to the water-carrier liquid 2 position; from the second to the 32nd second, the pump reversed to absorb the water-carrier liquid and then stopped; at the 34th second, the valve Turn to the position of sample 1; from 35 seconds to 36 seconds, the pump reverses to suck the sample and stops; at 38 seconds, the valve turns to the position of cerium solution 3; from 39 seconds to 39.5 seconds, the pump stops after it reverses to suck the cerium solution ; At 41.5 seconds, the valve turns to the sample 1 position; from 42.5 seconds to 43.5 seconds, the pump reverses to absorb the sample and stops; at 45.5 seconds, the valve turns to the UV detector 5 position; from 46.5 seconds to 49.5 seconds, the pump Push the mixed sample to the reaction tube in the water bath before turning forward and then stop; from 49.5 seconds to 649.5 seconds, the sample belt and the reagent fully react in the water bath during the stop flow; from 649.5 seconds to 664.5 seconds, the pump is turning Push the product to the detector before stopping, and at the same time, the software completes the data collection and processing; at this point, the pump and valve stop and are in the initial position, and another cycle of sample determination begins.

According to the above-mentioned technical scheme of the present invention, it is used for the detection of COD in environmental water, and the actual measurement results are as follows.

The water samples sent to the Environmental Monitoring Station of Shaanxi Province were tested by the standard method, and the test results are shown in the table below.

Experiments show that the two methods have a good corresponding relationship, and the COD deviation is less than or equal to 10%.

Table 2. The method of the present invention and the comparison of COD value in the measured water and waste water of national standard (GB11914-89) Sample No. COD The method of the present invention (mgl ^-1 ) National standard (mgl ^-1 ) Relative deviation(%) 12345678 11016444.110683.7182196.597.1 11015647.597.279.616719588.5 105.17.29.15.2108.79.7

Advantages of the present invention:

1) The sequential injection method is adopted, which makes the system hardware simple and reliable, easy to calculate and control, and the volume, mixing degree, and reaction time of the sample and reagent zone can be completely controlled by software; easy to realize integration and miniaturization, and minimize artificial Intervention has truly realized the automation and integration of the instrument.

2) Using a syringe pump, a). It can deliver highly reproducible pulsation-free liquid flow in a wide range of flow rates, with good stability and reproducibility in both long-term and short-term; b). The flow rate is basically not affected by the flow The influence of resistance changes in the system; c). The ability to resist strong alkali, high-concentration strong acid and organic solvents is very strong; d). No pump tube is required, and the loss of the injection pump during operation is lower than that of the peristaltic pump, which reduces the frequent on-line monitoring. The trouble of replacing the pump tube of the peristaltic pump ensures the reproducibility of the test results.

3) The oxidizing agent adopts cerium oxidizing agent with strong oxidation ability, low toxicity and no secondary pollution, and the reaction is carried out under mild conditions, which prolongs the life of each pipeline.

4) The stop-flow method is used to increase the time of the flow, so that the sample strip and the reagent have enough time to mix and react, and the sensitivity of the measurement is improved.

5) The sandwich-type sampling method is adopted to improve the stability and sensitivity of the reaction.

6) The consumption of samples and reagents is small, the operating cost is low, and it is suitable for long-term monitoring.

Claims (2)

1. One kind arrhea-sequential injection-cerium oxidation on-line monitoring water in the method for chemical oxygen consumption (COC), it is characterized in that:

   1) at first adopts the sandwich sampling system, successively sample, cerium oxygenant and sample are drawn in the memotron by syringe pump and hyperchannel selection valve;

   2) just changeing certain hour from another valve position of hyperchannel selection valve by syringe pump then promotes the reaction tube that solution enters in the water-bath the memotron forward and digests, after arrheaing certain hour, again by syringe pump just changeing certain hour forward the mixing in the driving a reaction pipe flow to detecting device, finish data acquisition and processing (DAP) by software at last, and provide measured value;

   3) mensuration of chemical oxygen consumption (COC) be based on the cerium oxidizing process and arrhea-sequential injection combines and carries out.
2. 2. the method for chemical oxygen consumption (COC) in according to claim 1 arrheaing-sequential injection-cerium oxidation on-line monitoring water is characterized in that: described sandwich sampling system is, with the cerium oxygenant Ce of certain volume ⁴⁺Be added between the two sample chemical oxygen consumption (COC) bands, just can react fully with chemical oxygen consumption (COC); The reaction of other type that the absorbance of utilization mensuration reagent own reduces records highly stable peak signal, improves the sensitivity that detects.