Measurement of solution turbidity by spectrophotometry
Turbidity
Turbidity is a measure of the degree to which suspended particles in water obstruct the passage of light. Water turbidity is caused by suspended matter present in the water, such as silt, colloidal matter, organic matter, plankton and microorganisms. The degree of turbidity is related not only to the presence of particulate matter in the water, but also to the size, shape and surface area of these particles. Natural water is made clear through processes such as coagulation, sedimentation and filtration.
1. Spectrophotometry 2. Visual turbidity comparison 3. Turbidity meter measurement
1. Spectrophotometry: Using a 1 cm cuvette, measure the absorbance of a standard turbidity solution at a wavelength of 660 nm. Plot an absorbance-turbidity standard curve, then measure the absorbance of the water sample and determine the corresponding turbidity from the standard curve. When turbidity exceeds 100 NTU, the sample must be diluted before measurement; the result should be multiplied by the corresponding dilution factor during calculation. A certain quantity of hydrazine sulphate is polymerised with 6-methylenetetramine to form a white polymer, which serves as a turbidity standard solution for comparison with the turbidity of the water sample under specific conditions. This method is suitable for the determination of natural water, drinking water and highly turbid water, with a minimum detectable turbidity of 3 NTU.
2. Visual turbidity method: The turbidity of a water sample is determined by comparing it with a standard turbidity solution prepared using diatomaceous earth (or white clay). This method is suitable for the determination of low-turbidity waters such as drinking water and source water, with a minimum detectable turbidity of 1 NTU.
3. Turbidimeter measurement: Measurement is carried out using various types of turbidity meters.
Commonly used turbidity units: FTU and NTU. Although named after different individuals, the numerical values are equivalent.
Part I: Spectrophotometric Method
2 Principle
At a suitable temperature, hydrazine sulphate polymerises with hexamethylenetetramine to form a white polymer, which is used as a turbidity standard solution and compared with the turbidity of the water sample under specified conditions.
3 Reagents
Unless otherwise specified, analytical reagents of analytical grade conforming to national or professional standards, and deionised water or water of equivalent purity, shall be used for the analysis.
3.1 Turbidity-free water
Filter distilled water through a 0.2 μm filter membrane and collect the filtrate in a flask that has been rinsed twice with filtered water.
3.2 Turbidity Standard Stock Solutions
3.2.1 1 g/100 mL Hydrazine Sulphate Solution
Weigh 1.000 g of hydrazine sulphate [(N₂H₄)H₂SO₄], dissolve in water, and make up to 100 mL.
Note: Hydrazine sulphate is toxic and carcinogenic!
3.2.2 10 g/100 mL hexamethylenetetramine solution
Weigh 10.00 g of hexamethylenetetramine [(CH₂)₆N₄] and dissolve in water, then make up to 100 mL.
3.2.3 Turbidity standard stock solution
Pipette 5.00 mL of hydrazine sulphate solution (3.2.1) and 5.00 mL of hexamethylenetetramine solution (3.2.2) into a 100 mL volumetric flask and mix thoroughly. Allow to stand at 25 ± 3 °C for 24 hours. After cooling, dilute to the mark with water and mix thoroughly. The turbidity of this solution is 400 NTU. It may be stored for one month.
General laboratory apparatus and
4.1 50 mL stoppered cuvettes.
4.2 Spectrophotometer.
Samples should be collected in stoppered glass bottles and analysed as soon as possible after sampling. If storage is required, they may be kept in a cool, dark place for no longer than 24 hours. Before testing, the samples must be vigorously shaken and allowed to return to room temperature.
All glassware coming into contact with the samples must be clean; it may be washed with hydrochloric acid or a surfactant.
6.1 Preparation of the Calibration Curve
Pipette 0, 0.50, 1.25, 2.50, 5.00, 10.00 and 12.50 mL of the turbidity standard solution (3.2.3) into 50 mL volumetric flasks and make up to the mark with water. After mixing thoroughly, a standard series with turbidities of 0.4, 10, 20, 40, 80 and 100 NTU is obtained. Measure the absorbance at a wavelength of 680 nm using a 30 mm cuvette and plot a calibration curve.
Note: When measuring at a wavelength of 680 nm, the presence of pale yellow or pale green colours in natural water does not cause interference.
6.2 Determination
Pipette 50.0 mL of the mixed water sample [free from bubbles; if turbidity exceeds 100 NTU, reduce the volume as appropriate and dilute to 50.0 mL with non-turbid water (3.1)] into a 50 mL cuvette. Determine the absorbance following the procedure for plotting the calibration curve (6.1), and read the turbidity of the water sample from the calibration curve.
Where: A — turbidity of the diluted water sample, °NT;
B — volume of dilution water, mL;
C — volume of the original water sample, mL.
The accuracy requirements for test results across different turbidity ranges are as follows:
Turbidity range (°NT) Accuracy (°NT)
1–10 1
10–100 5
100–400 10
400–1000 50
Greater than 1000 100
Part II Visual Turbidity Method
8. Principle
The water sample is compared with a turbidity standard solution prepared using diatomaceous earth. It is defined that the turbidity produced by 1 mg of diatomaceous earth of a specific particle size in 1000 mL of water is equivalent to 1 degree.
9. Reagents
Unless otherwise specified, analytical reagents conforming to national or professional standards, deionised water or water of equivalent purity shall be used for analysis.
9.1 Turbidity Standard Solution
9.1.1 Turbidity Standard Stock Solution: Weigh 10 g of diatomaceous earth (passed through a 0.1 mm sieve) into a mortar, add a small amount of water to form a paste and grind to a fine consistency, then transfer to a 1000 mL volumetric flask and make up to the mark with water. Stir thoroughly and allow to stand for 24 hours. Carefully transfer the upper 800 mL of suspension to a second 1000 mL volumetric flask using a siphon. Add water to the mark, mix thoroughly, and allow to stand for 24 hours. Draw off the upper 800 mL of suspension containing finer particles and discard it; dilute the lower solution with water to 1000 mL. After thorough mixing, store in a stoppered glass bottle; the diatomaceous earth particles in this solution have a diameter of approximately 400 μm.
Place 50.0 mL of the above suspension in a pre-weighed evaporating dish, evaporate to dryness on a water bath, and dry in an oven at 105 °C for 2 hours. Cool in a desiccator for 30 minutes, then weigh. Repeat the above procedure—i.e. drying for 1 hour, cooling, and weighing—until a constant weight is reached. Determine the weight (mg) of diatomaceous earth contained in 1 mL of the suspension.
9.1.2 Standard solution of 250 turbidity units: Pipette a suspension containing 250 mg of diatomaceous earth into a 1000 mL volumetric flask, make up to the mark with water, and mix thoroughly. The turbidity of this solution is 250 degrees.
9.1.3 Standard solution of 100 degrees turbidity: Pipette 100 mL of the standard solution of 250 degrees turbidity (9.1.2) into a 250 mL volumetric flask, dilute with water to the mark, and shake well. The turbidity of this solution is 100 degrees.
Add mercury chloride to each standard solution to prevent the growth of bacteria.
Note: Mercury chloride is highly toxic!
10 Apparatus
General laboratory apparatus and
10.1 100 mL stoppered colorimetric tubes.
10.2 250 mL colourless stoppered glass bottles; the glass quality and diameter must be consistent.
11 Analytical Procedure
11.1 Water samples with turbidity below 10 NTU
11.1.1 Pipette 0.1, 1.0, 2.0; 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0 mL into 100 mL colourimetric tubes, dilute with water to the mark, mix thoroughly, and prepare standard solutions with turbidities of 0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0 NTU.
11.1.2 Place 100 mL of the shaken water sample into a 100 mL cuvette and compare it with the standard solutions (11.1.1) described above. Observe vertically from top to bottom against a black background, select the standard solution that produces a similar visual effect to the water sample, and record its turbidity value.
11.2 Water samples with a turbidity of 10 degrees or higher
11.2.1 Pipette 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 mL of the 250-degree standard solution (9.1.2) into 250 mL volumetric flasks, dilute with water to the mark, and mix thoroughly. This yields standard solutions with turbidities of 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 NTU. Transfer these to a set of 250 mL stoppered glass bottles, adding 1 g of mercuric chloride to each bottle to prevent microbial growth.
11.2.2 Take 250 mL of the well-mixed water sample and place it in a set of 250 mL stoppered glass bottles; place a white cardboard sheet with a black line at the back of the bottle as a reference mark. Observe from the front of the bottle towards the back and, based on the clarity of the target, select the standard solution that produces a similar visual effect to the water sample; record its turbidity value.
11.2.3 Where the turbidity of the water sample exceeds 100 degrees, dilute it with non-turbid water (3.1) before measurement.
12 Presentation of analytical results
The turbidity of the water sample can be read directly.
