Chemistry: analytical and immunological testing – Oxygen demand
Patent
1991-04-12
1994-06-28
Warden, Robert J.
Chemistry: analytical and immunological testing
Oxygen demand
436146, 422 79, 210746, 210760, G01N 3318
Patent
active
053246667
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a process for the determination of oxidizable substances contained in water, in particular the chemical oxygen demand (COD) or the organic carbon content (TOC--total organic carbon) of an aqueous sample liquid by the oxidation of organic substances dissolved therein, wherein the sample liquid is diluted in a controlled manner with dilution water.
Processes for the determination of aqueous sample liquids, in particular sewage water, are known. Primarily in the monitoring of sewage and in their discharge into waters and at the inlet and outlet of sewage treatment plants, the determination of the COD values has become increasingly important.
In the known wet chemical processes oxidation of the organic substances dissolved in the sample liquid takes place by reaction with oxidizing agents added. These oxidizing agents and the reaction products generated, are, however, overwhelmingly toxic or at least harmful to the environment, so that their elimination is difficult. In addition, these processes cannot be carried out continuously.
In a known process for the determination of the quantity of organic carbon in a sample liquid (DE-A-24 58 143) a continuous mode of operation is possible. But this process, wherein the quantity of gaseous oxidation products is carried out, is very costly, both relative to the process and the necessary apparatus.
In another known process (DE-A-23 62 773) for the determination of organically bound carbon in water by the oxidation of organic substances, again the carbon dioxide formed by the oxidation is determined. Oxygen or air is used as the oxidizing agent. Under ultraviolet irradiation photochemical oxidation of organic substances takes place, in which a platinum catalyst is used to destroy the ozone generated by the irradiation of oxygen before it is able to contribute to the oxidation. This process does not operate continuously and is therefore not suitable for process control or automatic surveillance. Furthermore, the process is relatively expensive.
In a known process of the aforementioned generic type, electrochemical oxidation of organic substances is effected in a potentiometric measuring layout. In the oxidation of dissolved organic substances at an electrode, electrons are released. Between this electrode and a counter electrode a current flows through a measuring solution, consisting of the sample liquids, diluting water and an electrolyte. This current flow represents at a constant electrode potential a measure of the quantity of the organic substances oxidized. The ability of the electrodes to operate is detrimentally affected if the chemicals contained in the sample liquid form deposits. It is therefore necessary to regenerate the electrode from time to time. For this, the measuring process must be interrupted and a regenerating electrolyte introduced. No uninterrupted, continuous mode of operation is thus possible. In order to determine the necessity or effectiveness of a regenerating process, it is necessary to introduce a standardized calibrating liquid. The efficiency and thus the measuring accuracy of the known process also depends on how strongly the dissolved organic substances tend to be electrochemically oxidized. For this a high electric potential must be maintained between the electrodes, which in turn favors the formation of deposits.
It is the object of the present invention to create a process for the determination of the oxidizable content of water, in particular the chemical oxygen demand (COD) or the total organic carbon (TOC), that may be carried out without the use of dangerous chemical substances and without the difficulties encountered in electrochemical oxidation, rapidly and with an adequately high accuracy, using a moderate array of measuring instruments.
In a continuous process this object is attained according to the invention by that ozone is introduced into the sample liquid for oxidation, that following mixing and oxidation the residual ozone content or the consumption of ozone is determined and that the addition of
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Siepmann Friedrich W.
Teutscher Michael
Tran Hien
Warden Robert J.
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