Discussion on water quality inspection analysis method
Abstract: With the development of China's national economy, China's water quality testing has matured after several periods of development, and has made remarkable achievements. Especially since 1990, with the continuous improvement of people's living standards and the continuous improvement of water quality requirements, China's water quality testing level has made a qualitative leap in this period. China has established several national monitoring stations, each of which is equipped with advanced large-scale instruments, such as gas chromatograph, low background radioactivity measuring instrument and mercury measuring instrument. A few of them are equipped with GC-MS-computer system. With these instruments, the equipment level of water quality monitoring stations in China can almost rival that of international cities.
Keywords: Discussion on international water quality analysis methods
Water is the source of our human existence, and the quality of water is related to people's life and the smooth progress of industrial production. With the continuous improvement of water quality requirements in China, water quality detection technology has also made great progress, especially since 1990, the living standards of the broad masses of people have been continuously improved, and the requirements for water quality have also been continuously improved, which has made a qualitative leap in the water quality detection level in China during this period. In this paper, the common methods of water quality detection and their respective application scopes are discussed, and finally the objects and processes of each method are determined, which provides a series of references for water quality detection and industrial water use and has far-reaching significance. Below, we will elaborate and discuss from the following aspects.
Method for determining dissolved oxygen in water
1. Required instruments and instruments with portable dissolved oxygen detector and dissolved oxygen electrode.
2. Secondly, step analysis. First, turn the measuring/zeroing power switch of the instrument to? Measurement? Set the dissolved oxygen/temperature measurement selector switch to dissolved oxygen, and turn the salinity adjustment knob to the left to the end. 2. Preheat the instrument for 5 minutes, and then put the electrode into the freshly prepared 5% sodium sulfite solution for 5 minutes. After the reading is stable, adjust the zero adjustment knob to make the instrument display zero. Because the residual current of the electrode is very small, if there is no sodium sulfite solution, just put the electrode in the air, then turn the measuring/zeroing power switch to zeroing, adjust the zeroing gear, and adjust the zeroing knob to make the instrument display zero. Thirdly, take the electrode out of the solution, rinse it with distilled water, carefully absorb the moisture on the surface of the film with filter paper, put it in the air until the reading is stable, and adjust the scale knob to make the reading indication value be the saturated dissolved oxygen value of pure water at this temperature. Fourthly, after calibration, the electrode is immersed in the measured liquid, and the reading of the instrument is the dissolved oxygen value of the measured water sample. The determination of dissolved oxygen in water usually adopts iodometry and its correction method and membrane electrode method. Clear water can be directly determined by iodometry. The water sample is colored or contains redox substances, algae, suspended matter, etc., which interferes with the determination. Oxidizing substances can make iodide contain no iodine, resulting in positive interference; Some reducing substances can reduce iodine to iodide, resulting in negative interference; Organic matter (such as humic acid, tannic acid, lignin, etc. ) may be partially oxidized to produce positive interference. Therefore, most polluted surface water and industrial wastewater must be determined by improved iodometry and membrane electrode method.
3. Iodometric method. Manganese sulfate and alkaline potassium iodide are added to the water sample, and the dissolved oxygen in the water oxidizes low-valent manganese into high-valent manganese, resulting in brown precipitation of tetravalent manganese hydroxide. After adding acid, hydroxide precipitates and dissolves and reacts with iodine ions to release free iodine. With starch as indicator, the content of dissolved oxygen can be calculated by titration with iodine released by sodium thiosulfate.
Second, the electrochemical probe method
1. The oxygen-sensitive thin film consists of two metal electrodes in contact with the supporting electrolyte and a selective thin film. The film can only penetrate oxygen and other gases, but water and soluble substances can't. The oxygen permeating the membrane is reduced at the electrode, resulting in a weak diffusion current, which is proportional to the dissolved oxygen content in the water sample at a certain temperature.
2. The scope of application of this method. The lower limit of the electrode method depends on the instrument used, and it is generally suitable for water samples with dissolved oxygen greater than 0.1mg/l/L. When the water sample is colored and contains organic substances that can react with iodine, the iodometry and its correction method should not be used for determination, but the electrode method should be used. However, the gas or vapor containing chlorine, sulfur dioxide, iodine and bromine in the water sample may interfere with the determination, so it is necessary to change the membrane or calibrate the electrode frequently.
Three, potassium permanganate correction method
Potassium permanganate calibration is suitable for the determination of dissolved oxygen in water samples containing suspended substances such as activated sludge. Adding potassium fluoride can eliminate the interference in the determination. Sulfite, thiosulfate, polysulfate and organic matter will still interfere with the determination.
Four. ion chromatography
1. The first is the method principle. This method uses the principle of ion exchange to make continuous qualitative and quantitative analysis of various anions. Water samples are injected into carbonate and bicarbonate solutions and flow through a series of ion exchange resins, and the anions to be detected are separated from each other according to their different relative affinities to low-capacity strongly basic anion resins (separation columns). When the separated anion flows through the strongly acidic cation resin (inhibition column), it is transformed into the acid type with high conductivity, and carbonate-bicarbonate is transformed into carbonic acid with weak conductivity (clearing the background conductivity). The anions converted into corresponding acid types were measured by conductivity detector and compared with the standard. According to the retention time, anions are qualitative and peak height or peak area is quantitative.
2. Interference and elimination. Any substance with the same retention time as the anion to be detected will interfere with the determination. The ion concentration to be measured can be accurately quantified on the same order of magnitude. The ion concentrations in similar elution positions are too different to be accurately determined. When the concentration difference between Brˉ and NO3ˉ ions is more than 10 times, it cannot be quantified. Quantification can be achieved by proper dilution or addition of standard methods. High concentration of organic acids will interfere with the determination. Water can form a negative peak or decrease or incline the peak height, which often happens between f and c. Using eluent to prepare standards and dilute samples can eliminate the interference of negative peak of water.
Five, stannous chloride reduction spectrophotometry
1. Let's talk about the method and principle of this method first. Under acidic conditions, orthophosphate reacts with ammonium molybdate to form phosphomolybdate heteropoly acid. When reducing agent stannous chloride is added, it becomes a blue complex, usually called molybdenum blue.
2. Interference and elimination. When the content of chloride ion is above 0. 15%, the color development is weakened (as are other halide ions); When the content of sulfate ion is above 1%, the chromaticity will increase; Iron (Fe3+) ion has oxidation effect, and when the content reaches 40mg/L, it can affect the color development. When the copper (Cu2+) ion content is greater than 1mg/L, a negative deviation may occur. Silicic acid does not interfere under this color condition; Arsenic acid can develop color like phosphoric acid; Its chromaticity is about 1/20 of that of phosphoric acid. When the content of permanganate and hexavalent chromium in water is high, it will affect the color development of phosphomolybdenum blue. In this case, an appropriate amount of sodium sulfite solution can be added to reduce it, and then the residual sulfite ions can be removed by boiling.
3. Scope of application of this method, the minimum detection concentration of this method is 0.025mg/L, and the upper detection limit is 0.6mg/L. It is suitable for the determination of orthophosphate in surface water.
Conclusion of intransitive verbs
The continuous development of water quality detection technology, the diversification of water quality detection means and the continuous improvement of detection accuracy have provided strong technical support for the improvement of water quality and the continuous development of industry in China, and the owners have broad application prospects. Secondly, the continuous development of water quality detection technology also provides a guarantee for our human production and life, and promotes economic development.
refer to
[1] Liang Na. Discussion on water quality analysis method. Science and technology information, 20 1 1.03.
[2] Guli Mina. Analysis of several water quality detection methods. [J]。 China industry, 20 10.06.
Did you get a look at him? Chemist's thesis topic? People still see:
1. Title of physical examination paper
2. Reflections on the evaluation system of editors' titles in university publishing houses.
3. Look at the disadvantages of talent evaluation in colleges and universities from the evaluation of professional titles
4. Is there a word requirement for the title of the thesis, deputy high school?
5. The title of the thesis was searched by a senior teacher.