地表水

This standard is applicable to water samples of various types with COD value greater than 30mg/L, with the upper limit being 700mg/L for water samples having not been diluted.This standard does not apply to saline water with a chlorides concentration greater than 1000mg/L (after dilution).2. DefinitionThe quality concentration of oxygen consumed by the dissolvable matter and suspended matter in water corresponding to the amount of dichromate used for the oxidization treatment under given conditions.3. PrincipleA potassium dichromate solution of known amount is added into the water sample, let it boil and flow back in strong acid medium with silver salt as the catalyst, then, with ferroin as the indicator, the non-reduced potassium dichromate in the water sample is titrated with ferrous ammonium sulfate, and the amount of consumed ferrous ammonium sulfate is converted into the quality concentration of the consumed oxygen.With the presence of acidic potassium dichromate, it is difficult to oxidize the aromatic hydrocarbon and pyridine, hence a low oxidization rate. Under the catalytic action of silver sulfate, the straight chain aliphatic compounds can be effectively oxidized.4. ReagentUnless otherwise noted, all reagents used in the test shall be AR quality conforming to the national standards, and the water used in the test being distilled water or water with equivalent purity.4.1 Silver sulfate (Ag2SO4), chemical pure.4.2 Mercury (HgSO4), chemical pure.4.3 Sulfuric acid (H2SO4), ρ=1.84g/mL.4.4 Silver sulfate-sulfuric acid reagent: add 10g silver sulfate (4.1) into 1L sulfuric acid (4.3), lay it up for 1~2 days so that it can dissolve, mix it evenly, and stir it carefully before use.4.5 Standard potassium dichromate solution4.5.1 Standard potassium dichromate solution at a concentration of C(1/6 K2Cr2O7)=0.250 mol/L: dissolve 12.258g of potassium dichromate which has been dried at 105℃for 2 hours into water, and dilute it to 1000 mL.4.5.2 Standard potassium dichromate solution at a concentration of C(1/6 K2Cr2O7)=0.025 mol/L: dilute the solution obtained in 4.5.1 by 10 times.4.6 Standard ferrous ammonium sulfate titration solution4.6.1 Standard ferrous ammonium sulfate titration solution at a concentration of C[(NH4)2Fe(SO4)2·6H2O]≈0.10 mol/L: dissolve 39g ferrous ammonium sulfate [(NH4)2Fe(SO4)2·6H2O] in water and add 20mL sulfuric acid (4.3). Dilute it to 1000mL after it is cooled.4.6.2 The concentration of this solution (4.6.1) must be accurately calibrated with the standardpotassium dichromate solution (4.5.1) each day before use.Take 10.00mL standard potassium dichromate solution (4.5.1) and put it into a conical flask, dilute it with water to about 100mL, add 30mL sulfuric acid (4.3), stir it well, add 3 drops (about 0.15mL) of ferroin indicator (4.7), and titrate it with ferrous ammonium sulfate (4.6.1). The color of the solution will change from yellow to bluish green and then to red brown, which will be the termination. Record the amount of ferrous ammonium sulfate consumed (mL).4.6.3 Calculation of the standard ferrous ammonium sulfate titration solution concentration:where: V – the ferrous ammonium sulfate solution consumed during titration in mL.4.6.4 Standard ferrous ammonium sulfate titration solution at a concentration of C[(NH 4)2Fe(SO 4)2·6H 2O]≈0.010 mol/L: dilute the solution obtained in 4.6.1 by 10 times, and calibrate it with standard potassium dichromate solution (4.5.2), the procedures of titration and concentration calculation being similar to those in 4.6.2 and 4.6.3.4.7 Standard potassium hydride phthalate solution, C(KC 6H 5O 4)=2.0824 m mol/L: weigh 0.4251g potassium hydride phthalate (HOOCC 6H 4COOK)which has been dried at 105℃ for 2 hours, and dilute it to 1000 mL and mix it well. With potassium dichromate as the oxidizer, the COD value when the potassium hydride phthalate is fully oxidized will be 1.176 g O 2/g (which means 1.176 g oxygen is consumed by 1g potassium hydride phthalate), so the theoretical COD value of this standard solution is 500mg/L.4.8 1, 10-phenanathroline monohydrate indicator solution: dissolve 0.7g green vitriol (FeSO 4·7H 2O) into 50mL of water, add 1.5g of 1,10-phenanathroline monohydrate, stir it to allow it dissolve, and dilute it with water to 100mL.4.9 Glass beads to prevent explosive boiling.5. InstrumentsCommon laboratory instruments and the following.5.1 Reflux device: all-glass reflux device with a 250mL conical flask with 24# standard ground opening. The reflux condensing tube is 300~500mm long. If the sampling is over 30mL, an all-glass reflux device with a 500mL conical flask can be used.5.2 Heating device.5.3 A acid titration tube of 25mL or 50mL.6. Sampling and specimens6.1 SamplingThe water sample will be collected in a glass bottle and analysis should be done as early as possible. If the analysis cannot be done immediately, the sample should be added with sulfuric acid (4.3) to a pH<2, and be kept below 4℃. The keeping time must not exceed 5 days. The volume of collected water sample must not be less than 100mL.V V O H SO Fe NH C 50.2250.000.10]6)()[(22424=⨯=⋅6.2 Preparation of specimensStir the sample sufficiently homogeneous, and take 20.0mL as the specimen.7. Procedures7.1 For a water specimen with a COD value less than 50mg/L, it should be oxidized with low concentration standard potassium dichromate solution (4.5.2), then heated and refluxed, before being titrated back with low concentration standard ferrous ammonium sulfate solution (4.6.4).7.2 The upper limit of determination is 700mg/L for water specimen having not been diluted. When this limit is exceeded, the determination can be made only after dilution.7.3 For water samples with serious contamination, both the specimen and reagent can be taken at 1/10 of the required volume and put them into a 10×150mm hard glass tube. Stir it well and heat it to boil for several minutes with an alcohol burner. Observe if the solution has turned bluish green. If so, add little specimen and repeat, until the solution will no longer turn bluish green. The proper times of dilution can thus be determined for the water sample to be tested.7.4 Take the specimen (6.2) and put it into the conical flask, or take an appropriate amount of specimen and add water to 20.0mL.7.5 Blank test: with the same procedures, perform a blank test with 20.0mL water in p。