Standard Methods; 2130 A, 2130 B

Turbidity of water is due to suspended solids such as clay, plankton, silt, finely divided organic matter, microscopic organisms and similar materials. These solids will deflect (or scatter) light as it passes through the sample. Turbidity is a measurement of the scattered light as compared to the amount of light scattered by a standard. The more light that is deflected the higher the turbidity of the sample. The measuring device used in today’s laboratories is called a nephelometric meter. This type of meter does not measure all of the deflected light, only that which is deflected at a right angle (90° ) from the sample and light source. Turbidity is read as nephelometric turbidity units (NTU).

• Turbidimeter: with a readout device to indicate intensity of light scattered at 90° . Due to the nature of the test, the meter must not allow any outside light to enter the detector area. The turbidimeter must be free from drift and be able to detect differences of 0.02 NTU or less for samples with turbidity of less than 1 NTU. The instrument should detect in the range of 0 to 40 NTU.

Design differences will cause variations in measured values for turbidity. To minimize these differences, select an instrument that meets the following criteria:

• Light source: Tungsten-filament lamp operated to meet U.S. Environmental Protection Agency NPDES requirements.
• Distance traversed by incident light and scattered light within the sample tube should not exceed 10 cm.
• Detector: The angle of the light acceptance detector must be centered at 90° to the incident light path. The angle of the light acceptance detector must not exceed ± 30° from the required 90° . The detector and filter system shall have a spectral peak response between 400 and 600 nm (nanometers).
• Sample Tubes are made of a clear colorless glass. Dust, dirt scratches and fingerprints deflect light. Therefore, keep the tubes scrupulously clean at all times. Touch tubes only around the top to avoid touching where light must pass. Throw away scratched or etched tubes.

1. Turbid free water - if the turbidity of the laboratory grade water is 0.05 NTU or higher, pass the water through a membrane filter having precision-sized holes of 0.2 m m. Rinse collecting flask twice with filtered water. Discard the next 200 mL of filtered water, then start collecting filtered water to prepare standards. Commercially prepared water can be substituted when its turbidity is lower than what is available in the laboratory.
2. Stock Turbidity Suspension Solution 1 - Weigh 1.00 gram of hydrazine sulfate, (NH₂)₂·H₂SO₄. Dissolve the 1.00 gram of hydrazine sulfate in a 100 mL volumetric flask. Use turbid free water to fill to the 100 mL line on the volumetric flask Prepare stock solution monthly.

Caution: hydrazine sulfate is a known carcinogen: avoid inhalation, ingestion and contact with skin.

3. Stock turbidity Suspension Solution 2 - Weigh 10.00 grams hexamethylenetetramine, C₆H₁₂N₄. Dissolve the 10.00 grams of hexamethylenetetramine in a 100 mL volumetric flask. Use turbid free water to fill to the 100 mL mark on the volumetric flask. Prepare stock solution monthly.
4. 400 NTU Turbidity Solution - Mix 5.0 mL each of Solution 1 and Solution 2 in a 100 mL volumetric flask. Let stand for 24 hours at 25 ± 3° C, then fill to the mark with turbid free water and mix.
5. 40 NTU Standard Solution (working solution) - Measure 10.00 mL of stock 400 NTU Turbidity solution. Add the 10.00 mL of stock solution to a 100 mL volumetric flask. Fill to the 100 mL mark with turbidity free water. Prepare this solution daily.
6. Alternate Standards - Commercially prepared standards are available.

The turbidity test should be determined on the day the sample is taken. If this is not possible, refrigerate the sample at 4° C for up to 24 hours. Remember to vigorously shake all samples before examination.

The following must be recorded on the data sheet:

• Source identification (source, name, and date(s) of collection)

• Analyst(s)
• Raw data

• Final results with correct units (reported to nearest NTU)

• Description of unusual sample characteristics

• Replicates are to be listed in an orderly cluster

• If a pre-calibrated scale is used, at least one standard must be analyzed in each range to check the operation of the instrument
• A series of standards should be used to calibrate the instrument in each range of interest
• Sample cells should be free of contamination, etches, and scratches

Calibration

Always follow the manufacturer’s instructions for calibration of your particular meter. If the instrument does not have a prepared calibration curve, make one by using various values of turbidity standards. Plot turbidity reading vs. the standard concentration to obtain curve.

Check the accuracy of the instrument against bubble-free prepared standards, make adjustments in readings according to manufacturer’s instructions. Run a minimum of one standard for each range used during the test, making sure that the meter gives stable readings in all sensitivity ranges used.

Sample Analysis

Measurement of turbidities less than 40 NTU.

1. Thoroughly shake sample. Wait until air bubbles disappear before pouring sample into turbidity tube. When necessary, immerse turbidity tube in an ultrasonic bath for 1 to 2 seconds to dislodge bubbles. Letting the sample stand for a period of time to allow air bubbles to dissipate will also allow solids to settle thus changing the characteristics of the sample being evaluated.
2. Wipe outside of tube to remove fingerprints, dust dirt, and water droplets. Place tube in turbidimeter.
3. Read turbidity from instrument direct reading scale or convert from calibration curve.

Measurement of turbidities greater than 40 NTU

1. Thoroughly shake sample. Dilute samples using turbid free water until sample falls between 30 and 40 NTU.
2. Wait until air bubbles disappear before pouring sample into turbidity tube. When necessary, immerse turbidity tube in an ultrasonic bath for 1 to 2 seconds to dislodge bubbles. Letting the sample stand for a period of time to allow air bubbles to dissipate will also allow solids to settle thus changing the characteristics of the sample being evaluated.
3. Wipe outside of tube to remove fingerprints, dust, dirt, and water droplets. Place tube in turbidimeter.
4. Read turbidity from instrument direct reading scale or convert from calibration curve.
5. Calculate the turbidity of the original sample using turbidity of diluted sample and the dilution factor.

Where:

Example:

If 40 mL of sample is diluted to 200 mL with 160 mL of turbid free water and the diluted sample showed a turbidity of 30 NTU, what is the turbidity?

The turbidity of the original sample is 150 NTU.

Report turbidity readings as follows:

• Collect sample
• Prepare raw data sheet
• Calibrate the turbidimeter
• Add sample to turbidity tube
• Read turbidity of sample from turbidimeter and record result