Spectrophotometric Method
Spectrophotometric method is widely used in water quality monitoring because of its simplicity, rapidity, and accuracy, and in recent years, many applied researches on the rapid determination of spectrophotometric method have been reported. Studies have shown that the method has the advantages of fast testing speed, small sampling volume, convenient operation and low cost. The principle of its determination is that in acidic solution, the reducing substances in the test solution react with potassium dichromate to produce trivalent chromium ions, which have a great absorptive capacity for light with a wavelength of 0.5 mm, and the relationship between its absorbance and the concentration of trivalent chromium ions obeys the law of Langberg's law of Beer, which relates the trivalent chromium ions with the amount of the reducing substances in the test solution. Thus, the value of the test solution can be measured indirectly by determining the absorbance of trivalent chromium.
Sealed digestion method
The microwave sealed rapid method is the same as the national standard reflux method, which adopts sulfuric acid and potassium dichromate digestion system. Catalyzed by silver sulfate, high energy electromagnetic waves are used to heat the reaction solution. Under the action of high-frequency microwave, the molecules of the reaction liquid produce high-speed friction movement, so that its temperature rises rapidly, and the use of sealed digestion, so that the internal pressure of the digestion tank rises rapidly, and under high temperature and high pressure to achieve the purpose of rapid digestion. The excess of potassium dichromate after elimination is used to try ferrous spirit as an indicator, and ferrous sulfate is used to drop back according to the standard solution, and the value is calculated according to the consumption of ferrous sulfate money. The advantage of the method is that the amount of reagents is small, fast, simple, without special instruments, in a constant temperature oven to dissolve the sample, even if very simple laboratory can also be carried out, easy to popularize.
Electrochemical method
Coulomb method is a trial method in China, the theoretical basis of the method is Faraday's law. In the acidic medium edge a few, as an oxidizing agent, the sample digested coincidentally, with the electrolysis of ferrous ions produced as a coulometric titrant on the remaining amount of, for coulometric titration, according to the consumption of electricity calculation. The study shows week, the method does not need to use a standard solution, simple operation, fast, high oxidation rate, wide range of determination, can basically realize the analysis of semi-automatic.
Quick meter
In recent years, a variety of models of quick meter continues to develop successfully, and put into the market, has been applied in many monitoring stations, laboratories. Lanzhou Refining and Chemical Environmental Protection Instrument Research Institute and Qingdao nagging mountain electronic instrument factory produced a and a type of intelligent tachymeter. Quick meter State Environmental Protection Administration Institute of Monitoring Instruments applied to sewage charges monitoring, research shows that the instrument is extremely suitable for value greater than the charges monitoring. Developed an automatic analyzer that can measure the value of a sample per hour.
High chloride wastewater determination
Chloride ions on the determination of chemical oxygen demand, mainly for the low concentration of chloride ions. Determination, one is one of the main inorganic interferences, the standard method using a step slightly in addition to the effect of one. The impact of chlorine on the environment is very serious. For high chloride wastewater such as chlorinated ethylene oxide, propylene oxide and ammonia oxide propane wastewater, natural gas mining wastewater and industrial wastewater in coastal areas using seawater containing chloride ions are generally high. Its determination method mainly adopts the absorption method, sealing method, silver nitrate method and iodine amount method.
COD conventional determination method)
Two, potassium dichromate method of COD principle
In a strong acidic solution, accurately add an excess of potassium dichromate standard solution, heating reflux, the water samples of reductive substances (mainly organic) oxidation, an excess of potassium dichromate to try ferrous spirit as an indicator, with ammonium ferrous sulfate standard solution back to the drop, according to the amount of consumption of the standard potassium dichromate solution. Potassium dichromate standard solution amount of chemical oxygen demand calculation.
Cr2O72-+14H ++6e 2Cr3++7H2O (oxidation of water samples)
Cr2O72-+14H ++6Fe2+ 2Cr3 ++6Fe3 ++7H2O (titration)
Fe2+ + ferroferrous spirit (indicator) → reddish-brown (end point)
Three, equipment
1. 250mL all-glass reflux device;
2. four adjustable electric furnace;
3. 25 or 50ml acid buret, conical flask, pipette, volumetric flasks and so on.
Four, reagents
1. Potassium dichromate standard solution (C = 0.2500mo1 / L): weighing the benchmark or high-quality pure potassium dichromate dried at 120 ℃ for 2h 12.258g dissolved in water, transferred to a 1000mL volumetric flask, dilute to the standard line, shake well.
2. Try ferrous spirit indicator: weigh 1.485g o-phenanthroline (C12H8N2.H2O), 0.695g ferrous sulfate FeSO4.7H2O) dissolved in water, dilute to 100ml, stored in a brown bottle.
3. Ammonium ferrous sulfate standard solution (c ≈ 0.1mol / L): weigh 39.5g of ammonium ferrous sulfate dissolved in water, stirring while slowly adding 20mL of concentrated sulfuric acid, cooled down and moved to 1000ml volumetric flask, add diluted to the standard line, shake well. Before use, calibrate with potassium dichromate standard solution.
Calibration method: accurately suck 10.00ml of potassium dichromate standard solution in a 500mL conical flask, add diluted to about 110ml, slowly add 30mL of concentrated sulfuric acid, mix well. After cooling, add 3 drops of test ferrous finger test solution (about 0.15mL), titrate with ammonium ferrous sulfate solution, the color of the solution from yellow to reddish-brown through blue-green is the end point.
In the formula; C - the concentration of ammonium ferrous sulfate standard solution (mol / L);
V a standard solution of ammonium ferrous sulfate dosage (ml).
4. Sulfuric acid a silver sulfate solution: in 500mL concentrated sulfuric acid add 5g silver sulfate. Leave l-2d, shaking from time to time to make it dissolve. 5. Mercury sulfate: crystal or powder. 6. Sample to be measured
V. Measurement steps
1. 20.00 mL of well-mixed water samples (or appropriate amount of water samples diluted to 20.00mL) in a 250mL reflux conical flask, accurately add 10.00mL of potassium dichromate standard solution and several small glass beads or zeolites, connected to the mouth of the reflux condenser tube mill, the mouth of the condenser tube from the upper mouth to add slowly! 30mL of sulfuric acid and silver sulfate solution, gently shake the conical flask, so that the solution shaking, heating reflux 2h (since the beginning of boiling time). For high COD waste water samples, you can first take the volume required for the above operation 1/10 of the waste water samples and reagents in 15 × 150mm hard glass test tube, shake well, after heating to observe whether the green. If the solution is green, and then reduce the amount of wastewater samples, until the solution does not change, so as to determine the volume of wastewater samples should be taken when analyzing. Dilution, the amount of wastewater samples taken shall not be less than 5 ml. If the chemical oxygen demand is very high, the wastewater sample should be diluted several times. Wastewater chlorine ion content of more than 30mg / L, should be the first 0.4g mercuric sulfate into the reflux conical flask, and then add 20.00mL of wastewater (or the appropriate amount of wastewater diluted to 20.00mL), shake well.
2. After cooling, rinse the wall of the condenser tube with 90ml of water and remove the conical flask. The total volume of the solution should not be less than 140mL, otherwise the titration endpoint will not be obvious due to too much acidity.
3. The solution is cooled again, add 3 drops of test ferrous spirit indicator solution, titration with ammonium ferrous sulfate standard solution, the color of the solution from yellow to reddish brown by blue-green that is the end point, record the amount of ammonium ferrous sulfate standard solution.
4. Determination of water samples at the same time, take 20.00mL distilled water, according to the same operation blank experiment. Record the amount of ammonium ferrous sulfate standard solution when titrating the blank.
Calculation
where c a ferrous sulfate standard solution concentration (mol/L);
V0 - titration blank when the amount of ferrous ammonium sulfate standard solution (mL);
V1 - titration of water samples, the amount of ferrous ammonium sulfate standard solution (mL);
V1 - titration of water samples. - dosage of ammonium ferrous sulfate standard solution for titration of aqueous sample (mL);
g - molar mass of oxygen (l/2) (g/mL).
VI. Determination results
Time Sample number to be tested Sampling volume (ml) COD / mg / L ambient temperature (℃) Reporter
Note
1. The use of 0.4g of mercuric sulphate complex ions of the highest amount of up to 40 mg, such as the use of 20.00mL of water samples, that is, up to 2000mg / L of chlorine ions can be complexed water sample If 20.00mL water sample is taken, it can complex 2000mg/L chloride ion water sample. If the chloride ion less mercury sulfate, the concentration of lower, can also make to keep mercury sulfate: chloride ion = 10:1 (W / W). If a small amount of mercuric chloride precipitation, and does not affect the determination.
2. The volume of water samples can be taken in the range of 10.00-50.00mL, but the amount of reagents and the concentration of reagents need to be adjusted according to the following table, but also can get satisfactory results.
3. For the chemical oxygen demand less than 50mg / L of water samples, should be 0.025mol / L heavy liquid potassium chromate standard solution. Sulfur back drop with 0.01mol/L ammonium ferrous acid standard solution.
4. After the water sample heating reflux, the remaining amount of potassium dichromate in solution should be 1/5-4/5 of the amount added is appropriate.
5. Check the quality of reagents and operating techniques with potassium hydrogen phthalate standard solution, because each gram of potassium hydrogen phthalate theoretical CODcr value of 1.176g, so dissolve 0.4251g of potassium hydrogen phthalate in redistilled water, transferred to a 1000mL volumetric flask, diluted to the standard line with redistilled water to make it 500mg / L CODcr Standard solution. Newly prepared when used.
6. CODcr measurement results should be retained three digits.
Each time the experiment, should be ammonium ferrous sulfate standard solution for titration, room temperature higher especially pay attention to its concentration changes.
COD, basically the most important water quality indicators. It is defined as follows: under certain strict conditions, a variety of organic substances in the water with the addition of a strong oxidant (such as K2Cr2O7, KMnO4) effect of the amount of oxidant consumed, the results of the number of mg / L of oxygen to express. According to the strengthened oxidant of different, they are called potassium dichromate oxygen consumption (that is, common CODcr - chemical oxygen demand) and potassium permanganate oxygen consumption (that is, customarily referred to as OC oxygen consumption)
Why should we measure the COD, that is because of the water of the organic material is a variety of species, complex composition, and often The content is low, so it is difficult to determine the various organic substances separately. In environmental engineering practice, in addition to the necessary, designated organic compounds (such as benzene) for a single direct measurement, generally use indirect methods, that is, the determination of a number of comprehensive indicators to reflect the relative content of organic substances in water.
But I want to explain is that CODcr is generalized, it is the principle of determination is in the water samples such as a certain amount of potassium dichromate and catalyst silver sulfate, in a strong acidic medium heating reflux for a certain period of time, part of the potassium dichromate is oxidizable in the water samples can be reduced by the substance, with ammonium ferrous sulphate titration of the remaining potassium dichromate, according to the amount of consumption of potassium dichromate calculation of the value of COD.
In this case, the majority of organic substances in water can be said to oxidize, but for benzene, toluene, and other aromatic compounds are more difficult to oxidize. Strictly speaking, the chemical oxygen demand also includes the presence of inorganic reducing substances in water. Not all organic matter. Usually due to the amount of organic matter in wastewater is much more than the amount of inorganic reducing substances, so generally used in chemical oxygen demand to represent the total amount of organic matter in wastewater.
For chemical wastewater, CODcr as a total comprehensive indicators, for some other national standard regulations in the restriction of substances, but also separate
Potassium permanganate method for determination of COD in wastewater
Objective of the experiment
1. Mastery of potassium permanganate titration principle and operation.
2. To learn the method of determination of COD in wastewater by potassium permanganate method.
2. Experimental principle
Potassium permanganate index refers to the amount of oxygen consumed when treating water samples under certain conditions with potassium permanganate as oxidizing agent, expressed in mg/L of oxygen. Some organic matter and reducing inorganic matter in water can consume potassium permanganate. Therefore, the potassium permanganate index is often used as a comprehensive indicator of the degree of water pollution by organic matter.
After adding sulfuric acid to make the water sample acidic, a certain amount of potassium permanganate solution is added and heated in a boiling water bath for a certain period of time. The remaining potassium permanganate is reduced by adding excess sodium oxalate solution, and then the excess sodium oxalate is dripped back with potassium permanganate solution, and the permanganate index is calculated.
Three, apparatus
1. water bath device
2. 250mL conical flask
3. 50mL acid buret
Four, reagents
1. Potassium permanganate solution (C (1/5 KMnO4) = 0.1mol/L): weigh 3.2g of potassium permanganate dissolved in 1.2L of water, heating Boil to reduce the volume to about 1L, leave overnight, filter through a G-3 glass sand core funnel and store the filtrate in a brown bottle.
2. Potassium permanganate solution (C(1/5 KMnO4)=0.01mol/L): 25mL of the above potassium permanganate solution was sucked up, diluted to 250mL with water, and stored in a brown bottle. Calibrate before use and adjust to 0.01mol/L accurate concentration.
3. 1 + 3 sulfuric acid
4. Sodium oxalate standard solution (C (1/2Na2C2O4) = 0.1000mol / L): weigh 0.6705g of sodium oxalate in 105-110 ℃ drying one hour and cooling sodium oxalate dissolved in water, transferred to a 100mL volumetric flask, diluted with water to the standard line.
5. Sodium oxalate standard solution (C (1/2Na2C2O4) = 0.0100mol / L): absorb 10.00mL of the above sodium oxalate solution into a 100mL volumetric flask, dilute with water to the mark.
V. Experimental steps
1. Take 100mL mixed water sample (such as permanganate index is higher than 5mg/L, it is less discretionary, and diluted with water to 100mL) in 250mL conical flask.
2. Add 5mL (1+3) sulfuric acid, shake well.
3. Add 10.00mL of 0.01mol/L potassium permanganate solution, shake well, and immediately put into a boiling water bath and heat for 30 minutes (from the time the water bath reboils). The boiling water bath level should be higher than the level of the reaction solution.
4. Remove the conical flask, add 10.00mL of 0.0100mol/L sodium oxalate standard solution while it is still hot, shake well, and immediately titrate with 0.01mol/L potassium permanganate solution until it is slightly red, and record the amount of potassium permanganate solution consumed.
5. Calibration of the concentration of potassium permanganate solution: Heat the above titrated solution to 70℃, accurately add 10.00mL of sodium oxalate standard solution (0.0100mol/L) and then titrate with 0.01mol/L potassium permanganate solution to micro red color. Record the consumption of potassium permanganate solution, according to the following formula to obtain the correction factor of potassium permanganate solution (K):
K=
Equation: V-consumption of potassium permanganate solution (mL). If the water sample is diluted, another 100mL of water should be taken at the same time, and the same water sample procedure for the blank experiment.
Precautions:
1. In the water bath heating is completed, the solution should remain light red, such as lightening or all faded, indicating that the amount of potassium permanganate is not enough. At this time, the water sample should be diluted to increase the number of times and then measured.
2. Under acidic condition, the reaction temperature of sodium oxalate and potassium permanganate should be kept at 60-80℃, so the titration operation must be carried out while it is hot, and if the temperature of the solution is too low, it needs to be heated up appropriately.
VI. Calculation
1. The water sample is not diluted
Potassium permanganate index (O2,mg/L)=
Where: V1-consumption of potassium permanganate solution when titrating the water sample (mL);
K-correction factor;
M -concentration of potassium permanganate solution (mol/L);
8-molar mass of oxygen (1/2 O2).
2. Water sample diluted
Potassium permanganate index (O2,mg/L) =
Where, V0 - consumption of potassium permanganate solution in the blank experiments (mL);
V2 - amount of water sample (mL);
C -the ratio of water content in the diluted water sample, e.g., 10.00mL water sample is diluted to 100mL with 90mL water, then C=0.90.