Definition of Turbidity

Turbidity is an optical effect that results from the interaction of light with suspended particles in a solution, most commonly water. Suspended particles, such as sediment, clay, algae, organic matter, and other microbial organisms, scatter light passing through the water sample. The scattering of light by suspended particles in this aqueous solution produces turbidity, which characterizes the degree to which light is hindered when passing through the water layer. Turbidity is not an index to directly characterize the concentration of suspended particles in a liquid. It indirectly reflects the concentration of suspended particles through the description of the light scattering effect of suspended particles in the solution. The greater the intensity of scattered light, the greater the turbidity of the aqueous solution .

Turbidity Determination Method

Turbidity is an expression of the optical properties of a water sample and is caused by the presence of insoluble substances in the water, which cause light to scatter and absorb rather than pass through the water sample in a straight line. It is an indicator that reflects the physical properties of natural water and drinking water. It is used to indicate the degree of clarity or turbidity of water, and it is one of the important indicators to measure the goodness of Water Quality.

The turbidity of natural water is caused by fine suspended matter such as silt, clay, fine organic and inorganic matter, soluble colored organic matter, and plankton and other microorganisms in the water. These suspended substances can adsorb bacteria and viruses, so low turbidity is conducive to water disinfection to kill bacteria and viruses, which is necessary to ensure the safety of water supply. Therefore, centralized water supply with perfect technical conditions should strive to supply water with as low turbidity as possible. The turbidity of the factory water is low, which is beneficial to reduce the odor and taste of the chlorinated water; it is helpful to prevent the reproduction of bacteria and other microorganisms. Maintaining low turbidity throughout the water distribution system favors the presence of an appropriate amount of residual chlorine.

The turbidity of tap water should be expressed in scattered turbidity unit NTU, which should not exceed 3NTU, and should not exceed 5NTU under special circumstances. The turbidity of many process waters is also important. Beverage plants, food processing plants, and water treatment plants that use surface water generally rely on coagulation, sedimentation, and filtration to ensure a satisfactory product.

It is difficult to have a correlation between turbidity and the mass concentration of suspended matter, because the size, shape, and refractive index of particles also affect the optical properties of the suspension. When measuring turbidity, all glassware in contact with the sample should be kept in clean conditions. After cleaning with hydrochloric acid or surfactant, rinse with pure water and drain. Samples were taken in glass vials with stoppers. After sampling, some suspended particles can precipitate and coagulate when placed, and cannot be restored after aging, and microorganisms can also destroy the properties of solids, so it should be measured as soon as possible. If storage is necessary, it should avoid contact with air, and should be placed in a cold dark room, but not more than 24h. If the sample is stored in a cold place, return to room temperature before measurement.

At present, the following methods are used to measure the turbidity of water:

(1) Transmission type (including Spectrophotometer and visual method): According to Lambert-Beer’s law, the turbidity of the water sample is determined by the intensity of the transmitted light, and the negative logarithm of the turbidity of the water sample and the light transmittance is in the form of Linear relationship, the higher the turbidity, the lower the light transmittance. However, due to the interference of yellow in natural water, the water of lakes and reservoirs also contains organic light-absorbing substances such as algae, which also interferes with the measurement. Choose 680rim wavelength to avoid yellow and green interference.

(2) Scattering turbidimeter: According to the Rayleigh (Rayleigh) formula (Ir/Io=KD, h is the intensity of scattered light, 10 is the intensity of human radiation), measure the intensity of scattered light at a certain angle to achieve the determination of water samples purpose of turbidity. When the incident light is scattered by particles with a particle size of 1/15 to 1/20 of the wavelength of the incident light, the intensity conforms to the Rayleigh formula, and particles with a particle size greater than 1/2 of the wavelength of the incident light reflect light. These two situations can be represented by Ir∝D, and the light at an angle of 90 degrees is generally used as the characteristic light to measure the turbidity.

(3) Scattering-transmission Turbidity Meter: use Ir/It=KD or Ir/(Ir+It)=KD (Ir is the intensity of scattered light, It is the intensity of transmitted light) to measure the intensity of transmitted light and reflected light And, to measure the turbidity of the sample. Because the intensity of transmitted and scattered light is measured at the same time, it has higher sensitivity under the same incident light intensity.

Among the above three methods, the scattering-transmission turbidimeter is better, with high sensitivity, and the chromaticity in the water sample does not interfere with the measurement. However, due to the complexity of the instrument and the high price, it is difficult to promote and use it in G. The visual method is greatly influenced by subjectivity. G In fact, the measurement of turbidity mostly uses a scattering Turbidity Meter. The turbidity of water is mainly caused by particles such as sediment in the water, and the intensity of scattered light is greater than that of absorbed light. Therefore, the scattering turbidity meter is more sensitive than the transmission turbidity meter. And because the scattering-type turbidimeter uses white light as the light source, the measurement of the sample is closer to reality, but the chromaticity interferes with the measurement.

The turbidity is measured by the scattered light measurement method. According to the ISO 7027-1984 standard, the turbidity meter that meets the following requirements can be used:

(1) The wavelength λ of the incident light is 860nm;

(2) The incident spectral bandwidth △λ is less than or equal to 60nm;

(3) Parallel incident light does not diverge, and any focus does not exceed 1.5°;

(4) The measurement angle θ between the optical axis of the incident light and the optical axis of the scattered light is 90±25°

(5) The opening angle ωθ in water is 20°～30°.

and mandated reporting of results in formazin turbidity units

① When the turbidity is less than 1 formazin scattering turbidity unit, it is accurate to 0.01 formazin scattering turbidity unit;

②When the turbidity is 1-10 formazin scattering turbidity units, it is accurate to 0.1 formazin scattering turbidity units;

③ When the turbidity is 10-100 formazin scattering turbidity units, it is accurate to 1 formazin scattering turbidity unit;

④ When the turbidity is greater than or equal to 100 formazin scattering turbidity units, it shall be accurate to 10 formazin scattering turbidity units.

1.3.1 Turbidity-free water should be used for dilution standards or diluted water samples. The preparation method of turbidity-free water is as follows: pass distilled water through a membrane filter with a pore size of 0.2 μm (the filter membrane used for bacterial inspection cannot meet the requirements), rinse the flask for collection with filtered water at least twice, and Discard the next 200 mL. The purpose of using distilled water is to reduce the influence of organic matter in the ion-exchange pure water on the determination, and to reduce the growth of bacteria in the pure water.

1.3.2 Hydrazine sulfate and hexamethylenetetramine can be placed in a silica gel desiccator overnight before weighing.

1.3.3 When the reaction temperature is in the range of 12-37°C, there is no obvious effect on the generation of (formazin) turbidity, and no polymer is formed when the temperature is less than 5°C. Therefore, the preparation of formazin turbidity standard stock solution can be done at normal room temperature. But the reaction temperature is low, the suspension is easily absorbed by the glassware, and the temperature is too high, which can cause the standard value of high turbidity to drop. Therefore, the formation temperature of formazin is best controlled at 25±3°C. The reaction time of hydrazine sulfate and hexamethylenetetramine was nearly completed in 16 hours, and the turbidity of the product reached the maximum after 24 hours of reaction, and there was no difference between 24 and 96 hours. the

1.3.4 For the formation of formazin, when the pH of the aqueous solution is 5.3-5.4, the particles are ring-shaped, fine and uniform; when the pH is about 6.0, the particles are fine and dense in the form of reed flowers and flocs; When the pH is 6.6, large, medium and small snowflake-like particles are formed.

1.3.5 The standard solution with a turbidity of 400 degrees can be stored for one month (even half a year in the refrigerator), and the standard solution with a turbidity of 5-100 degrees will not change within a week.