Help for high scores. Does anyone work in a municipal sewage treatment plant? How are COD and ammonia nitrogen measured?

This is the ammonia nitrogen determination method of Huaqing, Sichuan

This method can be used for: measurement of surface water, industrial wastewater, domestic sewage and other water quality.

Ammonia nitrogen (NH3-N) exists in water in the form of free ammonia (NH3) or ammonium salt (NH4+). The composition ratio of the two depends on the PH value and water temperature of the water. When the pH value is high, the proportion of free ammonia is high. On the contrary, the proportion of ammonium salt is high and the water temperature is the opposite. The main sources of ammonia nitrogen in water are the decomposition products of nitrogen-containing organic matter in domestic sewage affected by microorganisms, some industrial wastewater, such as coking wastewater and synthetic ammonia fertilizer plant wastewater, and farmland drainage. In addition, in an oxygen-free environment, nitrite present in water can also be reduced to ammonia by microorganisms. In an aerobic environment, ammonia in water can also be converted into nitrite, and even continue to be converted into nitrate. Measuring various forms of nitrogen compounds in water helps to evaluate the pollution and self-purification status of water bodies. Fish are sensitive to ammonia nitrogen in the water. High levels of ammonia nitrogen will cause fish death.

This instrument uses the Nessler colorimetric method to measure ammonia nitrogen in water. This method is easy to operate and has high sensitivity. The principle is: Ammonia nitrogen existing in the form of free ammonia or ammonium ions reacts with Sodium reagent to form a yellow-brown complex. The color of the complex is proportional to the content of ammonia nitrogen. Metal ions such as calcium, magnesium and iron, sulfides, aldehydes and ketones, color, and turbidity in water all interfere with the determination and require corresponding pretreatment.

Preparation method of ammonia nitrogen reagent:

1. Ammonia-free water (prepared according to GB7479-87) or fresh distilled water

2. Nessler's reagent: weigh 16g Sodium hydroxide, dissolve in 50 mL of water, cool to room temperature.

Weigh 7g potassium iodide and 10g mercury iodide, dissolve them in water, then slowly add this solution to the sodium hydroxide solution while stirring, and dilute to 100mL. Store in a brown bottle and seal tightly with a rubber stopper. Stored in a dark place, the validity period can be up to one year (the ratio of mercuric iodide to potassium iodide in Nessler's reagent has a great impact on the sensitivity of the color reaction. The precipitate generated after standing should be removed).

3. Potassium and sodium tartrate solution: Weigh 50g of potassium and sodium tartrate, dissolve it in 100mL of water, heat and boil to remove ammonia, fully cool and dilute to 100mL.

4. Ammonium standard stock solution: Weigh 3.819g (dry at 100°C for 2 hours) of ammonium chloride (excellent grade pure), dissolve it in water, transfer it to a 1000ml volumetric flask, and dilute to the mark. This solution contains 1000mg ammonia nitrogen per liter.

5. Ammonium standard solution: Transfer 5.00ml of ammonium standard stock solution into a 500ml volumetric flask, and dilute with water to the mark. This solution contains 10 mg of ammonia nitrogen per liter. (Prepare now for immediate use)

Analysis steps:

1. Determination of water samples

Take 5.00 mL of ammonia-free water (blank) and 5 ml of flocculated precipitation respectively. The pretreated water samples (so that the ammonia nitrogen content does not exceed 10.0mg/L) are added to special colorimetric tubes respectively, and 0.1ml of potassium sodium tartrate solution is added to each (2 drops can be added with a rubber-tipped dropper), and then 0.15ml is added to each. Nessler's reagent (use a rubber-tipped dropper to add 3 drops) and mix well. After leaving it for 10 minutes, use the standard curve measured in the above steps to measure the ammonia nitrogen concentration of the water sample on the instrument.

2. Working curve

Put 0, 0.20, 0.50, 1.00, 2.00, 4.00 and 5.00ml ammonium standard solution into a 10ml special colorimetric tube, and fill it up with ammonia-free water. 5ml (corresponding ammonia nitrogen values ??are: 0, 0.40, 1.00, 2.00, 4.00, 8.00, 10.00mg/L), add 0.1ml potassium sodium tartrate solution and mix well. Add 0.15ml Nessler's reagent and mix. After leaving it for 10 minutes, perform curve calibration on the instrument.

COD rapid determination method

At present, the potassium dichromate method (CODcr method) is generally used to measure COD value, which is according to the GBll9l4-89 standard in the "Common Method Standards for Environmental Monitoring" Method, heat to reflux, reflux for 2 hours starting from the time when the solution begins to boil, then cool to room temperature, and then add an indicator for titration.

Although the measurement results are reliable, they are time-consuming and each experiment requires at least 3 hours to complete. This article conducts comparative experiments with the standard method by shortening the heating time, and finds that heating for 3Omin can achieve basically the same effect as the standard method.

1 Instruments and reagents

Instruments: All-glass reflux device with 25Oml Erlenmeyer flask with #24 standard ground mouth, reflux condenser length is 300 ~ 5OOmm; 5Oml acid type Buret.

Reagents: potassium dichromate standard solution (0.25mol/L); ferrous sulfate standard solution (0.25mol/L); AgZSO4-HZSO4 solution; test ferrous indicator.

2 Methods and steps

Take 2Oml of evenly mixed water sample (or 20ml of diluted water sample) and place it in a 250ml ground reflux conical flask, and accurately add 10ml of K2Cr2O7 standard Solution and several small glass beads, connect the ground reflux condenser tube, slowly add 40mlAg2S04-H2S04 solution from the condenser tube, gently shake the Erlenmeyer flask to mix the solution evenly, and heat to reflux for 3Omin (time from the start of boiling). After cooling, rinse the wall of the condensation tube with water, remove the conical flask and add water to dilute to 15Oml, add 3 drops of ferrous sulfate indicator, and titrate with ferrous sulfate standard solution until reddish brown is the end point. Carry out two parallel experiments; at the same time, do a blank experiment using 2Oml of distilled water according to the same steps.

3 Comparison of experimental results

The results of measuring the above samples according to the standard method are shown in Table 1.

As can be seen from Table 1, by shortening the heating reflux time when measuring CODcr, although the value is lower than the standard method, it is still within the error range. This measurement method not only meets the requirements of water quality monitoring, but also saves time and reduces energy consumption.

After measuring the CODcr, multiply the CODcr value by the water sample coefficient (optional 0, 0.75, 0.043) to get the dilution factor. The BOD5 value can be further determined based on the dilution factor.

Note: Calculation method

The chemical oxygen demand of water sample in mg/L, the calculation formula is as follows:

C(V1-V2)×8000

COD(mg/L)=────────

V0

Where: C——ferrous ammonium sulfate standard titration solution The concentration of (4.6), mo1/L;

V1——The volume of ferrous ammonium sulfate standard titration solution consumed in the blank test (7.4), mL;

V2—— The volume of the ferrous ammonium sulfate standard titration solution consumed in the sample measurement (7.8), mL;

8000---The conversion value of the molar mass of 0.25O2 in mg/L

V0--the volume of the sample, mL;

The measurement results generally retain three significant figures. For water samples with small COD values ??(7.1), when the calculated COD value is less than 10 mg/L , should be expressed as "COD＜10mg/L"