METHOD OF DETERMINING CHEMICAL OXYGEN DEMAND (COD) FOR HIGH CHLORIDE SAMPLES

Abstract

The present invention provides a method of determining chemical oxygen demand (COD) for a sample with a high concentration of chloride. The method includes obtaining the sample, determining a concentration of chloride in the sample to obtain a known concentration of chloride in the sample, dosing an amount of the sample, an acid and an oxidizing agent into a container to obtain an analyte, heating the container containing the analyte, photometrically determining a preliminary chemical oxygen demand (COD) of the analyte in an analytic device, and correcting for the high concentration of chloride using a chloride correction to obtain the chemical oxygen demand (COD).

Claims

1. A method of determining chemical oxygen demand (COD) for a sample comprising a high concentration of chloride, the method comprising: obtaining the sample; determining a concentration of chloride in the sample to obtain a known concentration of chloride in the sample; dosing an amount of the sample, an acid and an oxidizing agent into a container to obtain an analyte; heating the container containing the analyte; photometrically determining a preliminary chemical oxygen demand (COD) of the analyte in an analytic device; and correcting for the high concentration of chloride using a chloride correction to obtain the chemical oxygen demand (COD).

2. The method as recited in claim 1, wherein the high chloride concentration is from ≥1,500 mg/l to 20,000 mg/l, preferably ≥2,000 mg/l to 20,000 mg/l.

3. The method as recited in at least one of claims 1 and 2, further comprising: diluting the sample if the chloride concentration is determined to be ≥20,000 mg/l.

4. The method as recited in at least one of claims 1-3, further comprising: cooling the container containing the analyte.

5. The method as recited in at least one of claims 1-4, wherein, the acid is sulfuric acid (H.sub.2SO.sub.4); the oxidizing agent is potassium dichromate (K.sub.2Cr.sub.2O.sub.7); the heating of the container is to a temperature of from 120 to 180° C., preferably from 140 to 170° C., preferably 148° C. or 170° C., for 15 to 150 min.; and the analytic device is a photometer.

6. The method as recited at least one of claims 1-5, wherein the container further comprises mercury (II) sulfate (HgSO.sub.4), preferably in an amount to complex at least 2,000 mg/l of chloride.

7. The method as recited in at least one of claims 1-6, wherein: the dosing of at least one of the acid, the oxidizing agent and the mercury (II) sulfate into the container occurs prior to the dosing of the amount of the sample into the container.

8. The method as recited in at least one of claims 1-7, further comprising: at least one of manually inputting and manually confirming the chloride correction by a user.

9. The method as recited in at least one of claims 1-8, wherein the chloride correction is based on at least one of a table, a graph, or a mathematical formula.

10. Use of the chloride correction as recited in claim 1 in an analytic device to determine chemical oxygen demand (COD).

Description

[0022] The present invention is described in greater detail below on the basis of embodiments and of the drawings in which:

[0023] FIG. 1 shows a suite of calibration curves for different chloride contents which can be used as the basis for determining the chloride correction of the present invention;

[0024] FIG. 2 shows a graph of the calibration curve factors' dependence on chloride concentration;

[0025] FIG. 3 shows a sample workflow for a method of the present invention;

[0026] FIG. 4 shows a first screen when performing the method of the present invention on a spectrophotometer;

[0027] FIG. 5 shows a second screen when performing the method of the present invention on a spectrophotometer.

[0028] A “high chloride concentration” as used in the present invention is understood to be a chloride concentration of ≥1,500 mg/L to 20,000 mg/L, preferably ≥2,000 mg/L to 20,000 mg/L. The method of the present invention can also be used for chloride concentrations which exceed 20,000 mg/L, however, the sample will then need to be diluted, preferably with deionized water or with another diluent whose chloride concentration is known and/or whose chemical oxygen demand (COD) has previously been determined. The container containing the analyte is preferably cooled prior to photometrically determining the preliminary chemical oxygen demand (COD) of the analyte.

[0029] The acid of the present invention can be any a molecule or ion capable of donating a hydron (proton or hydrogen ion H.sup.+) or which is capable of forming a covalent bond with an electron pair (i.e., a Lewis acid) without interfering with the determination of chemical oxygen demand (COD). Sulfuric acid (H.sub.2SO.sub.4) is preferably used as the acid in the present invention. The oxidizing agent can be any strong oxidizing agent which fully oxidizes organic compounds to carbon dioxide under acidic conditions. The oxidizing agent is preferably a chromate salt containing the chromate anion, CrO.sup.2−, or dichromate salts containing the dichromate anion, Cr.sub.2O.sub.2.sup.−7. Potassium dichromate (K.sub.2Cr.sub.2O.sub.7) is preferably used as the oxidizing agent in the present invention. The sample is preferably heated to a temperature from 120 to 180° C., preferably from 140 to 170° C., very preferably to about or exactly to 148° C. or to about or exactly to 170° C., for 15 to 150 min. The analytic device is preferably a photometer. Examples of preferred photometers include the DR6000™ UV VIS Spectrophotometer with RFID Technology and/or the DR3900™ Benchtop Spectrophotometer with RFID Technology, each from Hach.

[0030] The container used in the present invention is preferably a vial or a cuvette. If a cuvette is used, it is preferable if the cuvette and/or cuvette batch has previously been calibrated in order to additionally minimize any potential sources of measurement error. Cuvettes such as the LCK 1414 from Hach can preferably be used. The LCK 1414 cuvettes from Hach are provided with pre-dosed reagents so that only the correct amount of sample, or sample diluted with, for example, water, needs to be dosed therein. A precise and reliable measurement is thereby provided. The LCK 1414 cuvettes from Hach are additionally coded with a barcode so that a photometer can automatically recognize the cuvette and adjust any measurement result via a calibration value which has previously been determined for the cuvette/batch of cuvettes. This also provides for a more precise measurement result, for example, a more precise determination of chemical oxygen demand (COD).

[0031] The container can preferably include mercury (II) sulfate (HgSO.sub.4). The dosing of the container with the acid, the oxidizing agent and/or the mercury (II) sulfate preferably occurs prior to the dosing of the amount of the sample into the container. The container is preferably provided with the acid, the oxidizing agent and/or the mercury (II) sulfate as pre-dosed reagents in the container.

[0032] An important aspect of the present invention is the use of the so-called “chloride correction”. The chloride correction is preferably individually determined for and saved in a photometer, for example, as a previously-determined range for the photometer which then only needs to be applied (for example via reading the result, by manually inputting, and/or by manually confirming the result) by a user based on the total chemical oxygen demand (COD) measured. The chloride correction is preferably based on at least one of a table, a graph, or a mathematical formula. The present invention also provides for the use of the chloride correction in an analytic device to determine chemical oxygen demand (COD). One such graph of a chloride correction is shown in FIG. 1 and is based on the concentration of chloride as measured in the DR6000™ UV VIS Spectrophotometer with RFID Technology from Hach. A so-called “calibration curve” is thereby obtained. The calibration curves in FIG. 1 show that the measurement error based on a high chloride concentration will be higher for a low chemical oxygen demand (COD) and lower for a higher chemical oxygen demand (COD). It is believed that a reason therefor might be that large amounts of the oxidizing agent are consumed by the oxidation of organic substances at high COD concentrations. The error induced by reaction with chloride is therefore less because only a small amount of oxidizing agent remains available (mass action law). The use of multiple calibration curves to determine the chloride correction is therefore preferable in order to eliminate error.

[0033] FIG. 2 shows the dependence of the slope (F1) and axial intercept (F2) of the calibration curve on the chloride concentration. It is possible to determine the slope (F1) and intercept (F2) of the calibration curve for any chloride concentration with the formulas derived from the graphical fits. By inserting the formulas of the graphical fits into F1 and F2, it is possible to express the COD concentration in terms of the measured absorption and chloride concentration. An exemplary formula is set forth below based on the above data where:

[0034] C(COD)=Concentration of chemical oxygen demand (COD);

[0035] F1=Slope of calibration curve;

[0036] F2=Axial intercept of the calibration curve;

[0037] Abs=Absorption (=extinction); and

[0038] C(Cl)=Chloride concentration in the sample.

[0039] The chemical oxygen demand (COD) can therefore be calculated as follows:

C(COD)=F1.Math.(Abs−F2)[1]

[0040] where

F1=[0.0256.Math.(C(Cl)).sup.2]−[0.9897.Math.C(Cl)−54.318]; and

F2=[0.0128.Math.(C(Cl)).sup.2]−[0.4047.Math.C(Cl)−86.797].

[0041] Inserting F1 and F2 into [1] therefore yields:

C(COD)=[−0.48.Math.C(Cl)−55.54].Math.Abs+0.15.Math.C(Cl)−87.41.

[0042] FIG. 3 provides a sample workflow for determining chemical oxygen demand (COD) in a high chloride sample, for example, seawater. This sample workflow can be used for samples (or diluted samples) with chloride concentrations of 1.5-20 g/L.

[0043] The workflow provides that the sample is provided in a cuvette with a screw top to ensure that no additional contaminants will enter the cuvette. The cuvette containing the original sample is vigorously shaken in a first step to bring all sediment into suspension. A vortex shaker can, for example, be used to perform the shaking. Failure to bring all sediment into suspension before pipetting the sample can result in a high bias. A specific amount of the sample, for example, 1.8 ml, is then pipetted into a test cuvette containing a pre-dosed reagent, such as the sulfuric acid (H.sub.2SO.sub.4), potassium dichromate (K.sub.2Cr.sub.2O.sub.7), and optionally the mercury (II) sulfate (HgSO.sub.4). This test cuvette is then closed and shaken, cleaned, and thereafter heated, for example, for two hours at 148° C. The heated test cuvette is then removed, inverted twice, and transferred to a rack to cool. The heated test cuvette can alternatively or additionally remain in the thermostat unit it has cooled. It is thereby important for the sediment to completely settle after cooling. If this is not the case, the test cuvette should be centrifuged, for example, for 2 min. at 4,000 rpm. The cooled test cuvette is then cleaned and inserted into the measuring chamber of the photometer.

[0044] FIGS. 4 and 5 show the screen of a DR6000™ UV VIS Spectrophotometer as used when testing a sample having a high chloride concentration. As set forth above, the chloride concentration must first be determined. This can occur, for example, using the LCK 311 test system from Hach which can determine a chloride concentration of 1 to 1,000 mg/L or via the QUANTAB Chloride Test Strips from Hach which can determine a chloride concentration of 300 to 6,000 mg/L. Other tests to determine chloride concentration are also commercially available. A skilled person will know to dilute a sample once or more than once if the initial chloride concentration exceeds the upper limit of the chloride test used.

[0045] FIG. 4 shows a first screen showing the evaluation methods/programs which is based on the thermostat used. Listed are a first program “1814 COD (HT)” which uses a thermostat set at 170° C. for 15 min. or at 148° C. for two hours, and a second program “1814 COD (LT)” which uses a thermostat set at 148° C. for two hours. FIG. 5 shows the result upon inserting the test cuvette and choosing the 1814 COD (LT). The user obtains three readings of 27.5 mg/L COD when the chloride concentration is in the range of 1,500-5,000 mg/L, 25.8 mg/L when the chloride concentration is in the range of 5,000-10,000 mg/L, and 23.2 mg/L when the chloride concentration is in the range of 10,000-20,000 mg/L. The user then only needs to select the true chemical oxygen demand (COD) based on the known chloride concentration.

[0046] It is of course possible to provide a fully automated test where, in a first step, the chloride concentration is determined, and, in a second step, the true chemical oxygen demand (COD) is determined as corrected by an applicable chloride correction or vice versa.

[0047] The present invention is not limited to embodiments described herein; reference should be made to the appended claims.